Design and implementation of a plug-in power metering device
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Design and implementation of a plug-in power metering device Carlos de Almeida Santos de Castro e Abreu Thesis to obtain the Master of Science Degree in Electronics Engineering Supervisors: Prof. Pedro Miguel Pinto Ramos Examination Committee Chairperson: Prof. Paulo Ferreira Godinho Flores Supervisor: Prof. Pedro Miguel Pinto Ramos Members of the Committee: Prof. S´ onia Maria Nunes dos Santos Paulo Ferreira Pinto September 2020
Transcript of Design and implementation of a plug-in power metering device
Design and implementation of a plug-in power meteringdevice
Carlos de Almeida Santos de Castro e Abreu
Thesis to obtain the Master of Science Degree in
Electronics Engineering
Supervisors: Prof. Pedro Miguel Pinto Ramos
Examination Committee
Chairperson: Prof. Paulo Ferreira Godinho FloresSupervisor: Prof. Pedro Miguel Pinto Ramos
Members of the Committee: Prof. Sonia Maria Nunes dos Santos Paulo Ferreira Pinto
September 2020
Declaration
I declare that this document is an original work of my own authorship and that it fulfills all the require-
ments of the Code of Conduct and Good Practices of the Universidade de Lisboa.
i
Declaracao
Declaro que o presente documento e um trabalho original da minha autoria e que cumpre todos os
requisitos do Codigo de Conduta e Boas Praticas da Universidade de Lisboa.
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Acknowledgments
Gostaria de expressar toda a minha gratidao e apreco a todos aqueles que, directa ou indirecta-
mente, contribuıram para que este trabalho se tornasse uma realidade.
Ao Prof. Pedro Ramos e Sr. Pina por toda a orientacao, apoio e disponibilidade ao longo deste
processo.
Aos meus amigos, Sebas, Afonso, Pedro e Hugo, por todas as discussoes tecnicas e menos
tecnicas.
Aos meus amigos, Pico, Joana, Rita e Bruno por todo o companheirismo e palavras de incentivo.
Ao grupo de sempre (TG) por nunca me deixarem esquecer que existe sempre um lado bom da
vida.
Ao Rodrigo, pelas distracoes e incentivo ao desenvolvimento de um novo hobby.
As minhas irmas, Mariana e Sofia, por sempre acreditarem em mim mesmo quando eu nao o fiz.
Aos meus pais, Paula e Carlos, por sempre primarem pela minha educacao e por toda a forca e
carinho que me tem transmitido ao longo da vida.
A Lara, uma companheira eterna para a vida que nos espera, pela paciencia angelical.
A todos voces, os meus sinceros agradecimentos.
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Abstract
This project focuses in developing and implementing a device capable of monitoring the electric
power consumption of an appliance connected to the 230 V, 50 Hz power grid.
The device presents a Schuko CEE 7/4 to connect to the grid and on the other end a Schuko CEE
7/3. The appliance maximum current is 16 A. A specific energy metering Integrated Circuit (IC) is used
and sensing circuitry is included for voltage and current, to attenuate the grid’s voltage to the Analog-
to-Digital Converter (ADC)’s input range of the IC and convert current drawn into a differential potential,
once again taking into consideration the input range.
The system includes a non-volatile memory where measured values are stored. The files format
will be readable by any modern device, for example a smartphone or a computer. The system also
comprises a wireless communication module, to offer system management (e.g. period between saved
values) and data analysis options (e.g. graphs) to the user without interrupting the data acquisition.
Avoiding the necessity to take the Secure Digital Card (SD Card) out of the metering device to view the
results, which requires the system to be halted.
The device is based on a Peripheral Interface Controller (PIC) microcontroller from Microchip, which
manages data processing and communication with the energy metering IC, SD Card and bluetooth
module.
Keywords
Energy metering, Single Phase, Embedded System.
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Resumo
Este projeto tem como foco o desenvolvimento e implementacao de um dispositivo capaz de moni-
torizar o consumo eletrico de um eletrodomestico ligado a rede eletrica de 230 V, 50 Hz.
O dispositivo apresenta-se ligado a rede eletrica atraves de um conector Schuko 7/4 e na outra
extremidade a um Schuko 7/3. A corrente maxima do eletrodomestico e de 16 A. Um IC especıfico para
a medicao de energia e usado bem como circuitos de condicionamento de tensao e corrente, com o
intuito de atenuar a tensao da rede para a gama de tensoes de entrada dos ADCs do IC e converter
a corrente consumida numa diferenca de potencial, mais uma vez tendo em consideracao a gama de
tensoes de entrada.
O sistema inclui uma memoria nao-volatil onde os valores medidos sao armazenados. O formato dos
ficheiros sera legıvel por qualquer dispositivo moderno, por exemplo, um smartphone ou computador. O
sistema tambem inclui um modulo de comunicacao sem fios, para disponibilizar ao utilizador a gestao
do sistema (ex. intervalo entre valores guardados) e opcoes de analise de dados (ex. graficos) sem
interromper a aquisicao de dados. Caso contrario, seria sempre necessario retirar o cartao de memoria
do dispositivo de medicao para visualizar os resultados, o que requer que o sistema seja interrompido.
O dispositivo e baseado num microcontrolador PIC da Microchip, que gere o processamento de
dados e a comunicacao com o IC medidor de energia, cartao de memoria e modulo bluetooth.
Palavras Chave
Contador de Energia, Monofasico, Sistema Embebido.
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Contents
1 Introduction 1
1.1 Purpose and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Goals and challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Electric Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Signal Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Document organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 State of the Art 11
2.1 Voltage Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1 Hall effect Voltage Transducer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.2 Voltage Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.3 Resistive divider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Current Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1 Shunt resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.2 Current Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.2.3 Hall Effect Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.4 Rogowski Coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3 Wireless Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4 Non-volatile memory - SD Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.1 Intelligent Electric Energy Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.5.2 Develop, implement and characterize an electric energy monitoring device . . . . 20
2.5.3 Electric energy meter for low voltage usage . . . . . . . . . . . . . . . . . . . . . . 21
2.5.4 A Low-cost Wi-Fi Smart Plug with On-off and Energy Metering Functions . . . . . 21
2.5.5 Data acquisition and control using Arduino-Android Platform : Smart plug . . . . . 22
2.5.6 Development of Embedded System for Making Plugs Smart . . . . . . . . . . . . . 23
2.5.7 Power-Efficient Smart Metering Plug for Intelligent Households . . . . . . . . . . . 24
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3 System Architecture 25
3.1 Microcontroller - Microchip PIC24F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Energy meter IC - ADE7753 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Signal conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.3.1 Voltage sensor - Resistive divider . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3.2 Current sensor - Shunt resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.4 Bluetooth module - Itead HC-05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5 Memory - SD Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.6 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.7 Mobile Application (APP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.8 Software - PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4 Results 51
4.1 Energy meter IC - ADE7753 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 SD Card and Real Time Calendar-Clock (RTCC) . . . . . . . . . . . . . . . . . . . . . . . 54
4.3 Bluetooth module - Itead HC-05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4.1 Voltage Root Mean Square (RMS) and Current RMS . . . . . . . . . . . . . . . . . 56
4.4.2 Active and Apparent Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.4.3 Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.5 Final Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
5 Conclusions 61
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List of Figures
1.1 Power vectors triangle from [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 Three examples of waveform sampling (6 samples) adapted from [11]. . . . . . . . . . . . 7
1.3 Replication of the signal spectrum to be acquired due to the signal sampling process [11]. 9
2.1 Operating principle of closed loop voltage transducer adapted from [15]. . . . . . . . . . . 14
2.2 Voltage sensor with resistive divider [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Implementation of a shunt resistor [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Implementing an integrator using an Operational Amplifier (OPAMP) [17]. . . . . . . . . . 17
2.5 System diagram adapted from [9]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.6 Application Screens [7]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.7 Network design [27]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.8 System block diagram [27]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.9 Voltage sensor circuit [18]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.10 Block diagram of the efficient smart plug [37]. . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1 Energy meter architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 PIC24FJXXXGA010 pin diagram [23]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3 PIC24FJXXXGA010 clock diagram [23]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.4 ADE7753 functional block diagram [2]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.5 ADE7753 channel 1 offset range with gain set to 1 [2]. . . . . . . . . . . . . . . . . . . . . 34
3.6 ADE7753 Active Power calculation [2]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.7 ADE7753 Active Power calculation block diagram [2]. . . . . . . . . . . . . . . . . . . . . . 35
3.8 ADE7753 Reactive Power calculation block diagram [2]. . . . . . . . . . . . . . . . . . . . 36
3.9 ADE7753 Apparent Power calculation block diagram [2]. . . . . . . . . . . . . . . . . . . . 36
3.10 HC-05 pin diagram [4]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.11 HC-05 module [4]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.12 SD Card Serial Peripheral Interface (SPI) interface [7]. . . . . . . . . . . . . . . . . . . . . 40
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3.13 SD Card folder structure (red: folder path, yellow: file name). . . . . . . . . . . . . . . . . 41
3.14 Power Supply Unit (PSU) diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.15 IRM-15-12 block diagram [46]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.16 LM2575 block diagram [47]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.17 MCP1825 block diagram [48]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.18 Application screen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.19 Data review feature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.20 Live data feature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.21 Main loop flowchart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.22 Data buffers management functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.23 ADE7753 interrupt function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4.1 Software reset request. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 Successful software reset. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.3 Printed Circuit Board (PCB)’s front view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4 PCB’s back view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.5 Alcor HS100. [50] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.6 Measured RMS voltage and RMS current from a resistive load. . . . . . . . . . . . . . . . 58
4.7 Measured apparent, active and reactive power from a resistive load. . . . . . . . . . . . . 58
4.8 Measured RMS voltage and RMS current from a rated 65 W laptop charger. . . . . . . . . 59
4.9 Measured apparent, active and reactive power from a rated 65 W laptop charger. . . . . . 60
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List of Tables
1.1 System specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Main characteristics of wireless technologies. . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1 Oscillator configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 PIC Universal Asynchronous Receiver-Transmitter (UART) error rate for various baud
rates for high-speed and standard mode [23]. . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 ADE7753 maximum input signal levels for channel 1 [2]. . . . . . . . . . . . . . . . . . . . 33
3.4 ADE7753 offset correction range for channel 1 and 2 [2]. . . . . . . . . . . . . . . . . . . . 33
3.5 HC-05 current consumption [43]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
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Acronyms
AC Alternate Current
ADC Analog-to-Digital Converter
AP Access Point
APP Application
ARM Advanced RISC Machine
ASCII American Standard Code for Information Interchange
BLE Bluetooth Low Energy
CPU Central Processing Unit
CRLF Carriage Return Line Feed
CSV Comma-Separated Values
CT Current Transformer
CTS Clear to Send
DC Direct Current
dsPIC Digital Signal Peripheral Interface Controller
EDR Enhanced Data Rate
FAT File Allocation Table
FRC Fast Internal Oscillator
GSM Global System for Mobile Communications
I2C Inter-Integrated Circuit
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IC Integrated Circuit
IDE Integrated Development Environment
I/O Input/Output
IDE Integrated Development Environment
IEC International Electrotechnical Commission
IoT Internet of Things
LAN Local Area Network
LCD Liquid Crystal Display
LSB Least Significant Bit
LPF Low-Pass Filter
MCUs Microcontroller Units
OPAMP Operational Amplifier
PIC Peripheral Interface Controller
PCB Printed Circuit Board
PF Power Factor
PGA Programmable Gain Amplifier
PLC Power-line Communication
PLL Phase-Locked Loop
PSU Power Supply Unit
RMS Root Mean Square
RS-232 Recommended Standard 232
RTCC Real Time Calendar-Clock
RTS Request to Send
SD Card Secure Digital Card
SoC System-on-a-chip
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SPI Serial Peripheral Interface
SMD Surface Mounted Device
UART Universal Asynchronous Receiver-Transmitter
USB Universal Serial Bus
VT Voltage Transformer
WAN Wide Area Network
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1Introduction
Contents
1.1 Purpose and motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Goals and challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Electric Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Signal Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Document organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1
2
Introduction
1.1 Purpose and motivation
The XIX century was marked by the spread of electric energy starting with street lighting and later
on in households, to power telegraphs, telephones, radios and televisions succeeded by modern appli-
ances, such as fridges and washing machines. The demand for electric energy will only tend to increase
as the world moves to a more interconnected society with data centers working around the clock [1].
This consumption of the modern era comes as a service and with it came the need to meter it for billing
purposes. Two centuries passed by and nowadays the need to monitor the electric energy consumption
goes beyond billing. This includes balancing the power grid based on demand in real time and patterns
from previously logged information. Furthermore, the need of better knowing the load patterns of an
equipment; where it can be a steady load or its consumption may be arranged in short bursts, lead to
the development of more advanced plug-in power meters.
The objective of the prototype to be designed is a compact and remotely accessed energy metering
device, where the measured power consumption is limited to one single mains appliance. This project
describes the design, development and implementation of a plug-in power meter. Aside from the basic
role expected from an energy meter, other functionalities characterise this device, meaning real time
monitoring, long term logging, control of relevant parameters, wireless connectivity for an improved user
experience.
The energy metering device will work as a man-in-the-middle; where the device is connected to the
mains, and the load to a Schuko connector present on the developed device. Data about voltage, current,
instant and accumulated power consumption will be acquired. A non-volatile memory; a SD Card, is
present for long term logging up to a month. Moreover, the user will resort to an application on a
smartphone to review data and adjust parameters.
3
1.2 Goals and challenges
The primary objective of this work is to develop a compact device able to monitor the electrical
power consumption of an Alternate Current (AC) equipment. The project must acquire data for a long
period of time and register on non-volatile memory, connect to the mains through a Schuko CEE 7/4
connection and pass-through to the metered equipment with a Schuko CEE 7/3 connection. A metering
IC [2], current and voltage sensing circuitry is required for the measurement. As for processing unit, the
embedded system must be based on a PIC [3]. A wireless interface [4] in conjunction with a smartphone
application shall be integrated for live monitoring and download of the acquired data, improving the user
experience. As such, the goals for this project are to develop:
• An embedded measurement system;
• Firmware capable of performing processing and routing of data;
• Metering IC conditioning circuit;
• Power supply;
• Integration of non-volatile memory;
• Wireless interface;
• Smartphone APP.
The system specifications goal is presented in Table 1.1.
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Table 1.1: System specifications.
Connector Schuko CEE 7/3 and 7/4
Input Voltage 230 V
Maximum Current 16 A
Wireless Communication Bluetooth or Bluetooth Low Energy (BLE)module
Non-volatile Memory Micro SD Card
Sampling Rate of Voltage and CurrentSensors 27.9 kSPS
Acquired Data active, reactive, and apparent power; currentand voltage RMS
Microcontroller Microchip 16-bit Microcontroller,PIC24FJ128GA010
Energy Meter IC Analog Devices single metering IC,ADE7753
1.3 Electric Power
In the XIX century a battle between two electric energy standards for mass distribution took place,
between Thomas Edison (Direct Current (DC)) and Nikola Tesla (AC). The high distribution cost of DC,
mainly linked to power losses in the line that forced the presence of a power plant no further than 1 mile
away from the end user, as well as the high cost and at the time lack of step-up and step-down voltage
technology for DC [5] encouraged three-phase AC to be implemented. [6]
A load with an impedance purely resistive is when all power consumed by the user is transformed
into transformed energy, light, mechanical, sound, etc. However, not all loads are resistive, and thus
may hold a reactive component, either capacitive and/or in most cases inductive (motors, transformers).
A reactive component generates a phase shift on the sine wave of current that feeds the load.
The power dissipated in terms of resisitive component is called Active Power (P ) expressed typically
in kilowatts (kW), and the reactive component, Reactive Power (Q) expressed in kilovolt ampere reactive
(kVAr). The later, is considered a loss, since it is not converted into other form of useful energy to the
load. Apparent power (S) expressed in kilovolt ampere (kVA) is the amount of electric energy required
to distribute a given (P ); thus S could be equal or higher than P . For a given sinusoidal voltage and
current, apparent power, described by active and reactive power (1.1 and 1.2) as represented in Figure
1.1, remains constant [7].
5
Figure 1.1: Power vectors triangle from [7].
~S = P + jQ. (1.1)
And
S2 = P 2 +Q2. (1.2)
For a P equal to S, the Power Factor (PF) is unitary; meaning there is no phase shift between
voltage and current. With a PF of one, all the work (P ) can be done with less current. Further is the
gap between voltage and current sine waves (lower PF), the lesser P is transferred in favor of Q. For a
given P , the lower the power factor, higher S is required to compensate Q, leading to a higher current
consumption [8].
Power factor is the ratio (1.3) between P and S,
PF =P
S. (1.3)
Where for sinusoidal waveforms, PF has a direct relationship with the phase shift (φ) between voltage
and current, given by
PF = cos(φ). (1.4)
S (1.5) is given by the RMS value of voltage and the RMS value of current,
S = V RMS × IRMS . (1.5)
6
From (1.4), a null power factor is characterized by 90 degrees phase shift either from a capacitive
load (-90) or an inductive load (90).
The energy meter should measure P and Q. The later is important specially for customers where
inductive loads are more common (AC motors and transformers) since these loads consume not only P
but also a considerable amount of Q. Since, in a case where the electrical energy distribution company
bills only for P instead of S, it would seem but deceitfully that the customer in question was drawing far
less energy. [9]
1.4 Signal Acquisition
Acquiring a signal is an analog to digital conversion of a time-varying signal, with a fixed sampling
rate. The acquisition of the power outlet will be done with uniform sampling, as the sole purpose is to
acquire a known signal, for example a test was conducted in Enschede (Netherlands) [10] where its
frequency rarely deviates more than 0.2 % from 50 Hz. This process will convert the continuous signal
into a discrete signal; result of the sampling with a small period (discrete time) defined by the sampling
rate and limited by the conversion time of the system.
An ADC deals with the discretization of the signal amplitude into binary code based on the analog
value at the input and resolution; the number of bits of the converter. This data is then processed and
turned into meaningful information through a Central Processing Unit (CPU).
On the other hand, the process of acquiring a waveform trough sampling loses any information be-
tween samples; the same discrete signal may characterize numerous continuous signal as demonstrated
in Figure 1.2.
Figure 1.2: Three examples of waveform sampling (6 samples) adapted from [11].
7
This phenomena can be explained by the Sampling Theory [11] also known as Nyquist Theorem,
Shannon Theorem or Nyquist-Shannon Theorem that was demonstrated in the first half of the XX cen-
tury [12] [13]. The baseline of this theory shows that for a signal with a limited bandwidth, if acquired
with a sampling rate of at least two times higher than its bandwidth, it is possible to rebuild the original
signal. This approach mitigates the event of sudden variations between samples [11].
The process of sampling corresponds to multiplying the signal desired to be acquired (x(t)) by a sum
of Dirac delta functions analogous to the sampling moments. This sum is also known as unit impulse
symbol is described by
d(t) =
∞∑n=−∞
δ(t− n∆t), (1.6)
where ∆t is the time interval between samples and corresponds to the inverse of the sampling rate
fs =1
∆t. (1.7)
Sampling a signal corresponds to multiply (1.6) by the signal x(t), where the signal value is only kept
at the sampling instants and between them all information is lost,
x(t)× d(t) = x(t)×∞∑
n=−∞δ(t− n∆t) =
∞∑n=−∞
x(n∆t)× δ(t− n∆t). (1.8)
In the frequency domain, (1.8) corresponds to applying a convolution function to the signal spectra
X(f) and the Dirac sum D(f),
TF [x(t)× d(t)] =X(f) ∗D(f)
2π= fs ×
∞∑k=−∞
X(f − kfs), (1.9)
with D(f) defined by
D(f) = TF [d(t)] = 2πfs
∞∑k=−∞
δ(2πf − k2πfs) = 2πfs
∞∑k=−∞
δ(f − kfs), (1.10)
Equation 1.9 describes that the sampled signal spectrum will correspond to the sum of spectral
replicas of the signal to be sampled, X(f). These replicas are centered at the on multiple frequencies
of the sampling frequency, as depicted in Figure 1.3.
In Figure 1.3 there is no overlap of the multiple spectral replicas of the signal to be acquired, since
there is enough margin between the end of the spectrum to be acquired and the start of of the first
spectral replica, present at fs − fmax. To ensure no superposition, fmax < fs − fmax thus leading to the
relation, defined by the Sampling Theory
8
Figure 1.3: Replication of the signal spectrum to be acquired due to the signal sampling process [11].
fs > 2× fmax, (1.11)
or the Nyquist frequency defined by
fN =fs2> fmax. (1.12)
1.5 Document organization
This document is organized as follows:
• Chapter 1 is a introduction about the project development, choices taken to tackle the the problem
and some considerations about electric power and signal sampling.
• Chapter 2 is the state of the art, in this chapter it is presented some of the work already done in
plug-in power meters with non-mechanical solutions.
• In Chapter 3 the architecture of the proposed system is shown and the selection of components is
detailed.
• Chapter 4 presents the project results.
• Chapter 5 presents the project conclusions and suggestions for future work.
9
10
2State of the Art
Contents
2.1 Voltage Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2 Current Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Wireless Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4 Non-volatile memory - SD Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.5 Previous Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
11
12
State of the Art
2.1 Voltage Sensor
The national residential electric grid has an RMS voltage value of 230 V, and ADCs require lower
input voltage; thus there is a need to create a conditioning circuit. There are three methods that could
be implemented:
• Hall effect voltage transducer;
• Voltage Transformer (VT);
• Resistive divider.
2.1.1 Hall effect Voltage Transducer
On an Hall effect based voltage transducer, a primary winding is fed by a current proportional to the
voltage to be measured, generating a magnetic flux. Separated by a small air gap there is a Hall effect
sensor which senses this flux, causing a potential difference at the output. The output can be connected
in a open loop or closed loop [14]:
• in open loop the output of the Hall Effect sensor is the system output; the core can be easily
saturated and is characterized with greater nonlinearity in terms of error with respect to the closed
loop;
• in closed loop the Hall Effect sensor output feds current to a secondary winding, amplified by a
high-gain OPAMP and translated into voltage by a resistor with exactly the same waveform of the
primary current (Figure 2.1);
13
Figure 2.1: Operating principle of closed loop voltage transducer adapted from [15].
This setup offers galvanic isolation between the high voltage line and the data acquisition system,
providing remarkable linearity and bandwidth compared to transformers [14]. Although, its working
principle makes it susceptible to false readings due to external magnetic fields [16].
2.1.2 Voltage Transformer
A voltage transformer offers a simple solution to step-down the voltage to a suitable range; based
on induction between a primary and a secondary winding. Galvanic isolation can be guaranteed but
considerable volume and high cost must be taken into account. The voltage at the secondary is
VS =nPnS
VP , (2.1)
where VP is the voltage in the primary (RMS 230 V), VS the output voltage in the secondary winding. nP
and nS the number of turns in the primary and secondary winding, respectively [7].
14
2.1.3 Resistive divider
A resistive divider is a series of resistors to create an attenuation network placed in parallel with the
source. Resistors should be high valued to pull the least current possible; diminishing its effect on the
source. This setup is described by the source voltage, the series of resistors and the desired output
voltage; as shown in Figure 2.2 and defined by
Vout =R2
R1 +R2× VAC . (2.2)
Figure 2.2: Voltage sensor with resistive divider [7].
This configuration is the cheapest of them; although it requires R1 to be a series of multiple resistors
to share the dissipated power across them. The occupied area is small, especially if Surface Mounted
Device (SMD) resistors are used but lacks the advantage of isolating the output from the mains.
2.2 Current Sensor
The load voltage does not vary much contrary to current; thus it is required a wider measurement
dynamic range and frequency range due to rich harmonic contents in the current waveform. There are
several current sensor topologies, such as [17]:
• Low resistance current shunt;
• Current Transformer (CT);
• Hall effect sensor;
• Rogowski coil.
15
2.2.1 Shunt resistor
A shunt resistor is the cheapest solution for current sensing; with highly stable low values available
in the range of µΩ to mΩ. This resistor should be placed in series with the load, as shown in Figure 2.3.
The voltage across the resistor is proportional to the current flowing through it
I =U
R. (2.3)
Figure 2.3: Implementation of a shunt resistor [7].
Two disadvantages to take into consideration is parasitic inductance and dissipated power. When
performing high precision current measurements even at line frequency, the inductance is generally in
the order of nH but its effect in the phase can be significant enough to cause an error in case of a low
power factor. The heat dissipated by the resistor is proportional to the square of the current flowing and
the smaller its value lesser the heat generated [7] [17].
2.2.2 Current Transformer
A current transformer translates the current flowing through the primary winding, which is connected
in series with the load, into a lower current in the secondary. Among high currents measurements, this
topology is the most common. It’s a low power sensor, having little impact on the measured current and
deals great with high currents. It presents a low phase-shift if calibrated although in extreme cases of
current or substantial presence of DC component it may saturate due to the core characteristics. After
the ferrite core is magnetized, it will show hysteresis with negative repercussions on its accuracy, until
demagnetized [17].
16
2.2.3 Hall Effect Sensor
As in Hall effect voltage transducers there are open-loop and closed-loop implementations. The first
being the option with lower cost. The system withstands measuring very large current and has a good
frequency response. On the other hand, the output of the Hall effect sensor is sensitive to temperature
and usually requires a stable external current source [17].
2.2.4 Rogowski Coil
A Rogowski coil is an inductor which has mutual inductance with the conductor carrying the current
to be measured. This phenomena makes that a change in the current passing through the wire causes
an induced electromagnetic force in the coil, due to the magnetic field generated. Rogowski coil typically
has a core of air, so in theory there is no hysteresis, saturation or non-linearity. Its low inductance allows
for fast response to current changes and the lack of an iron core makes it respond linearly even at high
currents. These characteristic as well as its smaller size and cost compared to a CT make it a better
option for high current applications, such as, in electrical power transmission. Its output is proportional
to the time derivative of the current
Vout(t) = − 1
RC
∫vin(t) dt, (2.4)
therefore requires an integrator with an OPAMP as shown in Figure 2.4 [17].
Figure 2.4: Implementing an integrator using an OPAMP [17].
17
2.3 Wireless Communication
Wireless communication offers further flexibility to the system by removing the need of a cable and
by making it more versatile in terms of to what it can connect to. Smartphones lack ethernet ports
and even some laptops nowadays. For the device in development, a short range wireless connection
is ideal for system management since all data is saved locally there is no need to constantly upload
it elsewhere. Embedded systems in general implement short range wireless communication such as
Bluetooth, Bluetooth Low Energy (BLE), WiFi or ZigBee [18]. Table 2.1 presents the main characteristics
of these wireless technologies [19] [20].
Table 2.1: Main characteristics of wireless technologies.
Standard WiFi ZigBee Bluetooth Bluetooth LowEnergy
NetworkTopology Star Star/Mesh Star Star
Data Rate (Mb/s) 54 0.25 3 1
Maximum PowerConsumption 700 mW 75 mW 100 mW 50 mW
Range 100 m 30 m 100 m 30 m
WiFi would be useful for an Internet of Things (IoT) approach since it would be the only standard
implementation that would easily connect to an existing Access Point (AP) connected to the internet.
Although it is the fastest option and with high range, there is no benefit to include the system in a Local
Area Network (LAN) if there is no desire to implement a server to make it accessible outside (Wide Area
Network (WAN)). The current implementation of this project looks to include a wireless interface but not
to make it remote accessible since it would increase the total system cost and complexity.
ZigBee is a low power, highly resilient wireless network type, with range that could match WiFi and
can be further expanded through a mesh topology. But it isn’t commonly offered on smartphones and
laptops, external modules would need to be acquired [21].
Bluetooth or BLE offers enough range for the purpose of the developed system, it is low cost, low
power and is available in virtually any device making it ideal in terms of versatility.
2.4 Non-volatile memory - SD Card
A non-volatile memory allows the metering system to be independent in terms of data logging.
SD Card offers a good price per GB ratio, enough capacity for long term logging, basic and fast com-
munication through SPI interface. Furthermore, its popularity made available a number of abstraction li-
18
braries to simplify the programming necessary for the interaction between the embedded system and the
SD Card. These abstraction layers also implement a filesystem such as File Allocation Table (FAT); mak-
ing the SD Card compatible with a broad range of devices (computers, smartphones, tablets, etc.) [22].
2.5 Previous Work
This section presents some related projects developed in the last decade.
2.5.1 Intelligent Electric Energy Counter
This project [9] consist of an energy metering system capable of measuring the power consumption in
real-time. Based on a PIC24 microcontroller [23], it resorts to the built-in ADCs for sampling the voltage
and current sensing circuits (Voltage transformer TZ111V and Current transformer TC174V [24]), then
the live information is presented on a Liquid Crystal Display (LCD) display, as depicted in Figure 2.5.
This device accounts with the following major characteristics: possibility to miniaturize it, low power
consumption and the main advantage for the final user, the possibility to visualize the results through the
display.
Figure 2.5: System diagram adapted from [9].
19
2.5.2 Develop, implement and characterize an electric energy monitoring de-
vice
The objective of this thesis [7] was to develop an energy metering device. Based on a PIC24 micro-
controller [23] and a specific IC from Analog Devices (ADE7753 [2]) for sampling the voltage and current
sensing circuits, voltage divider and shunt resistor, respectively. Live information is presented on a LCD
display and in the developped android APP either live measurements or the previously logged data, as
depicted in Figure 2.6. There is also the possibility to transfer the raw data from the SD Card to any
electronic device runing any modern operating system (e.g. Microsoft Windows, Linux, Apple MacOS,
Android).
Figure 2.6: Application Screens [7].
20
2.5.3 Electric energy meter for low voltage usage
The work presented in [25] consists on the development, implementation and design of a single
phase energy meter for low voltage use. This device can be used in installations up to 30 A and has as
a CPU a Digital Signal Peripheral Interface Controller (dsPIC) from Microchip, which communicates with
a a specific IC for energy metering, capable of reporting RMS current, RMS voltage, and instantaneous
as well as accumulated power consumption. The voltage sensor is a resistive divider and the current
sensor a shunt resistor. The information is displayed on a LCD and can be sent through Universal Serial
Bus (USB) and/or Recommended Standard 232 (RS-232) interface. A transformerless power supply
was used to power-up the device which draws less than 2 W, meeting the requirements for IEC62053
standard [26].
2.5.4 A Low-cost Wi-Fi Smart Plug with On-off and Energy Metering Functions
The paper presented in [27] consist of a device based on a ESP-WROOM2 [28], a microcontroller
with a built-in WiFi module, another part consists of a relay driver with a voltage quadrupler (from 3.3 V
to 12 V) and a relay to latch the supply from the load. It operates with devices on the same network; such
as a home AP; making it easy to interact with virtually any modern mobile device through a WebApp as
shown in Figure 2.7.
Figure 2.7: Network design [27].
The system communicates internally through SPI between the microcontroller and the energy meter
(STPM01 chip [29]) and a couple IO pins to flag the relay driver and voltage quadrupler, as shown in
Figure 2.8.
21
Figure 2.8: System block diagram [27].
The STMP01 energy meter is capable of calculating RMS voltage, RMS current, apparent power,
frequency, power and power factor from the sampled data. The device was tested with a single-phase
AC power supply, CAL-SOURCE 200 [30], set at 220 VAC at 50 Hz and a phase angle fixed at 0 degree;
varying the current from 1 A up to 10 A the system accuracy of measurement resulted in magnitude
error less than 0.5 % [27].
2.5.5 Data acquisition and control using Arduino-Android Platform : Smart plug
In [31], a paper describes the development of a smart plug for remote monitoring. Implemented
on an open source microcontroller, the Arduino Duemilanove (ATmega168 or ATmega328 [32]) which
communicates via SPI interface with an ethernet module (enc28j60 [33]) to connect to the internet. The
system resorts to an non-invasive technique for current sensing with a current transformer (SCT-013-
030 [34]). It is capable of sensing up to 30 A and outputs a maximum of 1 V, acquired by the included
ADC which makes part of the Arduino peripherals. Furthermore, a server was required to be setup and
connected to the AP, as well as an Android APP [31].
22
2.5.6 Development of Embedded System for Making Plugs Smart
The project presented in [18] tackles the multiple attempts to create a smart plugs, points out the
multiples approaches to the wireless and non-wireless communication techniques for remote manage-
ment: Global System for Mobile Communications (GSM), Bluetooth, Power-line Communication (PLC),
ZigBee [35] and Ethernet. The embedded system is described as a microcontroller, Arduino mega
2560 [32], linked to the internet via an ethernet cable connected to the AP through an external Arduino
module, Ethernet shield [36]. Current sensing is done with the SCT-013-030 [34], a split core current
transformer. Arduino does not have any AC capable inputs, neither accepts inputs above 5 V, thus an AC
to DC and step down circuit was developed to create the voltage sensor based on a full-bridge rectifier
shown in Figure 2.9. An Android APP was developed to display information such as Apparent power,
AC voltage, AC current and control up to 8 devices in the network (turn on or off) [18].
Figure 2.9: Voltage sensor circuit [18].
23
2.5.7 Power-Efficient Smart Metering Plug for Intelligent Households
In [37] a power-efficient solution for smart metering plugs is described; it gives emphasis on the fact
that most smart plugs that are wireless use WiFi, a fairly inefficient solution [38] compared to BLE [39].
The proposed solution resorts to BLE due to its compatibility with a wide range of devices while keeping
the power consumption low. The project was focused on the following requirements:
• compactness - built-in type of smart plug.
• low power - power efficient solution.
• easy integration - compatible with other devices.
• increased security - a secure element integrated.
• low price - reasonable price for large scale deployment.
The system is built around a System-on-a-chip (SoC), a power efficient Advanced RISC Machine
(ARM) Cortex M0 core extended by a BLE transmitter and peripherals for SPI, Inter-Integrated Cir-
cuit (I2C) and UART communication. The metering front-end of the smart plug is based on Infineon’s
dual channel on-chip isolated ADC chip that provides serial data from two sigmadelta ADCs via a SPI
interface. On the high voltage side of the ADC, both channels are connected as voltage and current
monitors using a voltage divider and a current shunt, respectively. The whole smart plug is powered
from the mains using an AC to DC converter in step down configuration with 3.3 V as the nominal output
voltage. The system only requires a 3.3 V supply and is characterised by a peak power consumption of
170 mW. The full system diagram is depicted in Figure 2.10 [37].
Figure 2.10: Block diagram of the efficient smart plug [37].
24
3System Architecture
Contents
3.1 Microcontroller - Microchip PIC24F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 Energy meter IC - ADE7753 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Signal conditioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4 Bluetooth module - Itead HC-05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.5 Memory - SD Card . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.6 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.7 Mobile APP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.8 Software - PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
25
26
System Architecture
The developed system is comprised by current and voltage sensors, an energy meter IC, a non-
volatile memory unit, a wireless communication interface and a PIC microcontroller.
The energy metering IC continuously acquires samples from both current and voltage sensors, and
the PIC gathers information from the IC when requested either via an interrupt or a bluetooth request. All
data received on the microcontroller regarding voltage, current and power are stored on an SD Card. SPI
is used to communicate between the PIC and the SD Card. The UART module is enabled to interface
with the bluetooth module. Communication with the energy meter is achieved through SPI. An overview
of the system architecture is represented in Figure 3.1.
Figure 3.1: Energy meter architecture.
27
3.1 Microcontroller - Microchip PIC24F
Microchip’s PIC24 family of Microcontroller Units (MCUs) features a 16 MIPS core and enhanced
on-chip peripherals [23]. It communicates through SPI1 module with the SD Card, SPI2 module with
the energy meter IC and UART with the Bluetooth module. There is also another UART and two I2C
modules available. This number of communications modules allow for various interface configurations
with minimum compromises. Another feature worth mentioning is the availability of a RTCC, five timers
and interrupts based on external inputs, timers or peripherals. Lastly, the program language used is C
and the program is uploaded to the unit via a PICkit3 programmer [40]. A pin diagram of the used PIC24
is depicted in Figure 3.2.
Figure 3.2: PIC24FJXXXGA010 pin diagram [23].
An Fast Internal Oscillator (FRC) feeds a clock source of 8 MHz and with the integrated Phase-
Locked Loop (PLL), it reaches 32 MHz, maximizing the peripherals clock at 16 MHz by following
FCY =FOSC
2. (3.1)
28
All clock sources and derivatives used in this project are described in Table 3.1.
Table 3.1: Oscillator configuration.
FOSC RTC PLL Multiplier FRCPLL FCY
8 MHz 32.768 kHz 4× 32 MHz 16 MHz
A simplified diagram of the oscillator system present in the PIC24FJXXXGA010 family is depicted in
3.3.
Figure 3.3: PIC24FJXXXGA010 clock diagram [23].
29
The system was designed so that by tweaking three properties, namely the UART clock, enable/dis-
able predefined CTS and RTS pins and customize the termination character, any UART capable blue-
tooth module can be used. PIC’s UART module has a register controlled baud rate generator (UBRGX ),
which values can be calculated depending on the selected mode set by BRGH. [3]
For BRGH = 1, high-speed mode is enabled (4 clocks per bit period), BRGH is given by
UBRGX = INT
(FCY
4× baudratedesired− 1
). (3.2)
Resulting in a actual baud rate of,
baudratecalculated =FCY
4× (UBRGX + 1). (3.3)
And for BRGH = 0, standard mode enabled (16 clocks per bit period), by
UBRGX = INT
(FCY
16× baudratedesired− 1
). (3.4)
Giving an actual baud rate of,
baudratecalculated =FCY
16× (UBRGX + 1). (3.5)
Lastly, the error is given by,
Error =baudratecalculated − baudratedesired
baudratedesired. (3.6)
The results are displayed in Table 3.2. The baudrate was set at 115.2 kHz delivering an error free
and more stable communication with the bluetooth module.
30
Table 3.2: PIC UART error rate for various baud rates for high-speed and standard mode [23].
Set Baud Rate
921600
460800
307200
230400
115200
57600
38400
19200
9600
Calculated Baud Rate(BRGH = 1) Error
1000000 0.085%
500000 0.085%
307692 0.001%
235294 0.021%
117647 0.021%
57971 0.006%
38461 0.001%
19230 0.001%
9615 0.001%
Calculated Baud Rate(BRGH = 0) Error
1000000 0.085 %
500000 0.085 %
333333 0.085%
250000 0.085%
125000 0.085%
58823 0.021%
38461 0.001%
19230 0.001%
9615 0.001%
In addition to the three serial peripherals used to access the SD Card, transmit/receive data over
Bluetooth and communicate with the energy meter IC, the integrated RTCC is set to append timestamps
to the acquired samples.
Furthermore, three interrupts are active, two of which are triggered by external inputs and the third
one by the UART module. These external inputs are configured as such to detect the insertion of
the SD Card leading to its initialization and to sense the zero-crossing output (ZX) from the energy
meter IC thus synchronizing the reading of the registers [2]. UART interrupts allows the system to
fetch the streamed data from the Bluetooth module to a software buffer as soon as it’s available without
polling; received strings can be later processed when the system is free of other more critical tasks (e.g.
retrieving samples from the IC, free buffer space by flushing samples to the SD Card).
31
3.2 Energy meter IC - ADE7753
The Analog Devices’s ADE7753 is an energy meter IC with a high accuracy, compliant with the fol-
lowing International Electrotechnical Commission (IEC) standards: IEC 60687/61036/61268 and IEC
62053-21/62053-22/62053-23.The IC is intended for single phase applications with all the signal pro-
cessing required to perform active, reactive and apparent power calculation. It can also measure RMS
voltage and RMS current. This energy meter resorts to two second-order Σ − ∆ ADCs to acquire the
analog inputs. The IC power consumption tops at 25 mW with a 5 V supply. [2]
The ADE7753 is compatible with SPI to read data and allows calibration for power, phase and input
offset with on-chip registers. Furthermore, it contains a Programmable Gain Amplifier (PGA) (up to
16×) to adapt for a smaller input range in case of a shunt or current transformer, along with an internal
integrator in channel 1 for use with Rogowski coil sensors [17]. The chip also provides a pulse output
frequency (CF ) proportional to the active power and a zero-crossing output signal. It is characterized
by less than 0.1% error in active power measurements over a dynamic range of 1000:1 at an ambient
temperature of 25C. The functional block diagram is presented in Figure 3.4. [2]
Figure 3.4: ADE7753 functional block diagram [2].
The ADE7753’s sensing inputs are comprised of two fully differential voltage input channels with a
maximum voltage range of ± 0.5 V. Both channels have a PGA with user defined gains of 1, 2, 4, 8 and
16. Furthermore, channel 1 also has a full-scale input range selection for its ADC. Table 3.3 summarizes
all possible setup combinations for the PGA and channel input range.
32
Table 3.3: ADE7753 maximum input signal levels for channel 1 [2].
Max Signal
Channel 1
0.5 V
0.25 V
0.125 V
0.0625 V
0.0313 V
0.0156 V
0.00781 V
ADC Input Range Selection
0.5 V 0.25 V 0.125 V
Gain = 1 - -
Gain = 2 Gain = 1 -
Gain = 4 Gain = 2 Gain = 1
Gain = 8 Gain = 4 Gain = 2
Gain = 16 Gain = 8 Gain = 4
- Gain = 16 Gain = 8
- - Gain = 16
At the ADC level, it is also possible to adjust offset errors in the range of 20 mV to 50 mV based on
the gain set and the Least Significant Bit (LSB) weight, as described in Table 3.4 and depicted in Figure
3.5.
Table 3.4: ADE7753 offset correction range for channel 1 and 2 [2].
Gain Correctable Span LSB size
1 ± 50 mV ± 1.61 mV/LSB
2 ± 37 mV ± 1.19 mV/LSB
4 ± 30 mV ± 0.97 mV/LSB
8 ± 26 mV ± 0.84 mV/LSB
16 ± 24 mV ± 0.77 mV/LSB
33
Figure 3.5: ADE7753 channel 1 offset range with gain set to 1 [2].
The zero-crossing detection circuit generates its output in regards to the channel 2 input; ZX signal
is logic high when a rising flank crosses zero and logic low on a falling edge crossing zero. This output
is used to generate interrupts and synchronize readings, as well as for calibration purposes.
The ADE7753’s channel 1 and 2 sample voltage inputs at a user selected rate depending on the
MODE register configuration, the offered options are 3.5 kSPS, 7 kSPS, 14 kSPS or the default and
used in this project, 27.9 kSPS. The RMS values are then simultaneously calculated for both channels
by
V RMS =
√√√√ 1
N×
N∑i=1
V (i)2 (3.7)
and saved in their corresponding registers (V RMS, IRMS). All conversions from the registers
values to Volts (channel 2) and to Amps (channel 1) must be done in the microcontroller; taking into
account the selected input range and gains for the ADCs, as well as the designed voltage and current
sensor circuits.
The average power over a defined integral number of periods n is given by,
P =1
nT
∫ nT
0
p(t)dt = V I, (3.8)
where T is the line cycle period, V is the RMS voltage and I the RMS current.
Active power 3.8, is equal to the DC component of the instantaneous power signal, V I. Extracted
from the multiplication of voltage and current signals by a Low-Pass Filter (LPF), specified in Figure
3.4 and 3.7 as LPF2; this process can be visualized in Figure 3.6 and corresponding block diagram is
depicted in Figure 3.7.
34
Figure 3.6: ADE7753 Active Power calculation [2].
Figure 3.7: ADE7753 Active Power calculation block diagram [2].
The average reactive power over a selected integral number of lines cycles (n) is given by,
RP =1
nT
∫ nT
0
Rp(t)dt = V I sin(φ), (3.9)
where φ is the phase difference between the voltage and current channel.
Reactive power 3.9, is the product of voltage and current drifted apart by 90, meaning the channel
1 input is shifted 90 from channel 2 and then the DC component of the instantaneous reactive power
signal is extracted by the LPF, as depicted in the block diagram of Figure 3.8.
35
Figure 3.8: ADE7753 Reactive Power calculation block diagram [2].
Apparent power, as defined in 1.5, is the maximum power that can be feed to a load; where V RMS
and IRMS are the effective voltage and current delivered. And is independent from the phase shift φ,
the drift between the current and voltage. Figure 3.9 shows the process of calculation of the apparent
power.
Figure 3.9: ADE7753 Apparent Power calculation block diagram [2].
3.3 Signal conditioning
The energy meter IC contains two ADC with a variety of user defined input gains, as it includes a
PGA in each channel; nonetheless it has a maximum differential input range of ± 0.5 V. A conditioning
circuit is present at the input to meet the IC input range, and if necessary it’s possible to amplify the
resulting signal to an ideal range avoiding loss of information due to the signal amplitude or the ADCs
resolution.
36
3.3.1 Voltage sensor - Resistive divider
The choice of voltage sensor was focused on low cost and reduced footprint thus leading to a resistive
divider setup. The attenuation network is composed of a single 1 kΩ resistor and two 499 kΩ resistors,
resulting in a ratio of
ratio =R2
(R1 +R2)=
1000
499000× 2 + 1000= 1× 10−3. (3.10)
For a voltage peak of 425 V (300 VRMS), the voltage sensor output is
Voutput = Vpeak × ratio = 425× 1× 10−3 = 0.425 V. (3.11)
And the maximum current is,
Ioutput =Vpeak
(R1 +R2)= 4.254× 10−4 = 0.425 mA. (3.12)
Resulting in a power consumption per resistor of,
Pmax = I2output ×R, (3.13)
P499k =(4.254× 10−4
)2 × 499000 = 90.3 mW, (3.14)
P1k =(4.254× 10−4
)2 × 1000 = 180 µW. (3.15)
The resistors of 1 kΩ (ERJP06F1001V) and 499 kΩ (ERJP06F4993V) used for the voltage sensor,
have a power dissipation characteristic of 500 mW. This large margin in terms of power dissipation allows
for cooler operation and a steadier temperature overall for the resistors, thus mitigating the resistance
drift that otherwise could occur with the temperature increase. [41]
3.3.2 Current sensor - Shunt resistor
Once again, the selection for the current sensor relies on the same attributes, low cost and small
footprint. A 10 mΩ Shunt resistor is chosen taking into consideration its power dissipation capabilities
and temperature coefficient. As detailed in Table 1.1, the system must handle to measure current up to
16 A, which leads to
Pshuntmax= I2max ×Rshunt = 2.56 W, (3.16)
37
of power generated as heat, that has to be dissipated by the Shunt.
In this case, sensed voltage in the IC input is
Voutmax= Imax ×Rshunt = 0.16 V. (3.17)
The voltage output of the selected current sensor is within energy meter IC range.
Lastly, a small temperature coefficient is required so that as current increases, thus also the heat
generated (3.16), the shunt resistor preserves its characteristics. Based on availability, the smallest
temperature coefficient is ± 50 ppm/ C. A 3 W current sense chip resistor of 10 mΩ from Bourns is
used as current sensor [42]. It complies with the requirement and handles a maximum working current
of
Ishuntmax=Pshuntmax
Rshunt=
3
0.01= 17.32 A. (3.18)
3.4 Bluetooth module - Itead HC-05
Wireless communication is carried by a Bluetooth module with the task to create an abstraction layer
from the wave driven transmission to the available serial communication, most of which the available
solutions resort to UART.
The Itead HC-05 [4] supports Bluetooth 2.0 with Enhanced Data Rate (EDR), delivering a maxi-
mum bit rate of 3 Mbps. Connects to the PIC via UART, it allows data to be streamed bidirectionally.
Any American Standard Code for Information Interchange (ASCII) string sent to the module, either via
Bluetooth from a smartphone or from the microcontroller via UART is echoed to the other device. The
termination character, declaring the end of transmission, is a Carriage Return Line Feed (CRLF).
Parameters can be user defined via UART, such as customizing the device name and the pairing pin
code, when powered in AT mode also known as command mode [4]. There is a dedicated pin (PIO11)
to enter this mode; it is required to pull-high the pin before powering the module. A pin diagram is shown
in Figure 3.10. The chip operates at 3.3 V and features a low power operation for its Input/Output (I/O)
(1.8 to 3.3 V). [4]
38
Figure 3.10: HC-05 pin diagram [4].
As for peak current consumption, the HC-05 caps out at 40 mA while pairing to a device, after which
the current drawn is 8 mA in any case; as described in Table 3.5. [43]
Table 3.5: HC-05 current consumption [43].
Parameter Min. Max. Units
Pairing 30 40 mA
TX mode Peak Current - 8 mA
RX mode Peak Current - 8 mA
The available module has six I/O’s of the thirty-four pins of the chip (Figure 3.10); V DD (5 V in this
specific version) and GND to power the device, TX and RX for UART communication, STATE (PIO1)
that drives an LED to offer a quick visual indicator of the bluetooth connection state and EN (PIO11)
which can also be triggered via an on-board push-button to enter in AT mode, as depicted in Figure
3.11.
39
Figure 3.11: HC-05 module [4].
The baud rate of the unit can be configured. An experiment was setup to check the stability of
the selected baud rate. A string was sent repeatedly via UART to the Bluetooth module and echoed
to a smartphone running a serial terminal, if any character was missing or incorrect; it was considered
unstable. Clear to Send (CTS) and Request to Send (RTS) pins are physically inaccessible in this version
of the module (Figure 3.11), all tests were conducted without flow control, from which the maximum baud
rate achieved for viable operation was 115.2 kHz.
3.5 Memory - SD Card
Non-volatile memory allows the system to store and keep the previously measured data even when
powered down. The SPI interface connection between the PIC and the SD Card is depicted in Figure
3.12 [7].
Figure 3.12: SD Card SPI interface [7].
40
All the acquired samples will be saved on this type of memory, following a specific file tree organiza-
tion. CD pin doesn’t take part of the SPI interface, it’s a switch present on the SD Card holder that gets
closed when a card is inserted. The microcontroller should have a pin set as input to sense the closed
circuit and initialize the SD Card when inserted.
To interface with the SD Card memory blocks a file system (FAT) and library module, FatFS, was
implemented. Offering an abstraction layer on the programming side to create/read/edit folders and files
tailored to small embedded systems, while keeping the SD Card interpretable by any device. [44]
The FAT file system was originally designed for small volumes and simple folder structures. In FAT,
memory sectors are combined into clusters, there is an allocation table keeping track of the starting
cluster of each folder/file and each cluster then points to the next one that comprises the file or a end-
of-file indicator. [7]
The chosen file structure is comprised of a main folder (year), a sub-folder (month), another sub-
folder (day of the month) and lastly a text file (hour) in Comma-Separated Values (CSV) format where
data is saved (e.g. /YEAR/MONTH/DAY/hour.csv). This setup minimizes the number of files per folder
which helps prevent hangups or even disk failure, while offering the user ease of navigation through
the acquired data. As an example, the system was initialized on the 8 of September 2020 at 17h, the
SD Card folder structure and file naming scheme is depicted in Figure 3.13.
Figure 3.13: SD Card folder structure (red: folder path, yellow: file name).
Each sample is a new line written into the appropriate file based on its timestamp and offers informa-
tion in the following order: number of sample, RMS voltage, RMS current, apparent power, active power,
reactive power and timestamp.
41
All files and folders, for a complete thirty-one day run, are created at startup to reduce write time
when flushing the acquired data into memory. But a dynamic function was also implemented, in such
way that if left more than thirty-one days running, there is no major data loss since files and folder will
continue to be created as needed; making the SD Card volume size the real limiting factor. Although,
due to the time required for these actions to complete (file: two seconds, folder: five seconds) some
data may be lost during the process of file/folder creation, depending on the selected acquisition rate.
Lastly, if the user requests to stop the run prematurely, all empty files and folder are deleted.
3.6 Power Supply
The system requires two different voltage rails to work, 3.3 V and 5 V. For the implemented microcon-
troller family (PIC24F) and SD Card, 3.3 V is needed and 5 V for the energy meter IC (ADE7753). The
bluetooth module (HC-05) can be powered with 5 V through the module or 3.3 V if connected directly to
the chip. The power source of the system is the mains AC outlet.
The total system power consumption is 235 mA on the 3.3 V rail and 84 mA for the 5 V rail. The
microcontroller running at the maximum frequency (Table 3.1) draws 35 mA, the SD Card can peak to
200 mA, the energy meter IC 4 mA and the bluetooth module up to 80 mA. [3] [2] [43] [45]
Figure 3.14 presents a diagram of the designed power supply setup.
Figure 3.14: PSU diagram.
A universal AC input (85 to 264 VAC [46]) switching power supply rectifies and attenuates the mains
AC voltage to DC 12 V. Other characteristic such as short circuit, overload and over voltage protection,
make the IRM-15-12 a compact, reliable and affordable solution to generate 12 V. This switching power
supply can deliver up to 1.25 A. The block diagram is represented in Figure 3.15. [46]
Figure 3.15: IRM-15-12 block diagram [46].
42
It is required to further step-down the voltage to the required levels. From 12 V to 5 V, there is a
7 V drop, power dissipation is a concern, but can be circumvented with a step-down voltage switching
regulator. The LM2575-5, when powered with 12 V, can deliver up to 1 A with a voltage output between
4.75 V and 5.25 V with 77 % efficiency. The power dissipated is low thus no heatsink is required. This
switching regulator shuts down when internal temperature reaches 150 C and has a thermal resistance
characteristic of 65 C/W. For the device to shut down, it would require an ambient temperature over
100 C, since generated power is low and can be calculated by [47]
Pmax = (Vinmax × IQmax) +
(VoutVinmin
)× Ioutmax × Vsatmax
= ((12× 1.05)× 0.011) +
(5
12× 0.975
)× 1× 1.3 = 0.694 W.
(3.19)
Leading to an internal temperature rise over ambient of
Trise = Pmax ×Rφ = 0.694× 65 = 45.12 C. (3.20)
The switching voltage regulator IC is presented in Figure 3.16. [47]
Figure 3.16: LM2575 block diagram [47].
43
Lastly, a linear voltage regulator is used to achieve the final step-down to 3.3 V, from the 5 V supplied
by the LM2575-5 output. The chosen MCP1825S variant is a SOT-223-3 package with a fixed 3.3 V
output, accepts inputs in the range of 2.1 V to 6 V, has a current limit of 500 mA and has integrated over
temperature and short circuit protection. This linear regulator shuts down when internal temperature
reaches 150 C and has a thermal resistance characteristic of 62 C/W. For the device to shut down, it
would require an ambient temperature over 80 C, since generated power is low and can be calculated
by
Pmax = (Vinmax − Voutmin)× Ioutmax
= ((5× 1.05)− (3.3× 0.975))× 0.5 = 1.016 W.(3.21)
Leading to an internal temperature rise over ambient of
Trise = Pmax ×Rφ = 1.016× 62 = 63 C. (3.22)
The MCP1825S’s functional block diagram is depicted in Figure 3.17. [48]
Figure 3.17: MCP1825 block diagram [48].
44
3.7 Mobile APP
A smartphone mobile APP was developed to complement the designed system. It offers the possibil-
ity to review previously acquired data (voltage RMS, current RMS, apparent, active and reactive power)
in form of graphs, as well as live samples. The user can start, pause or reset the system and instantly
change the acquisition rate from a list of predefined intervals. Moreover, the APP has the task to sync
the system, on connect the APP automatically updates the time and date of the PIC RTCC based on the
smartphone clock and calendar. The application screen is depicted in Figure 3.18.
Figure 3.18: Application screen.
Pressing Start triggers a chain of processes. The PIC will start scanning the SD Card for the files and
folders; if non-existent, the missing ones are created for a thirty-one day run. Furthermore, it requests
the user to select from a list an acquisition rate for the energy meter IC.
The Stop button holds the current state of the system, halts all sampling and cleans the log structure
by deleting the empty files and folders from the SD Card. The user can then remove the SD Card, view
or copy the data to another device. To proceed with the same run, the user inserts the SD Card back
into the slot and presses Start; since Stop holds the system state, the execution will continue then from
the previous state. Holding down the Stop button will start a complete reset of the system, all variables
are set back to their initial value and and all data present in the SD Card is deleted.
45
With the APP connected via bluetooth, Get Data button requests the user to chose a date and time-
frame, after which the corresponding data is downloaded and processed. Due to performance reasons,
fifty samples are retrieved at a time, taking up to ten seconds to receive and process each request;
the user is prompted to download more samples or halt the process to plot the downloaded data. This
feature is presented in Figure 3.19.
Figure 3.19: Data review feature.
46
Turning on the LiveMonitor switch will start a looping one second timer. Every second, the app
retrieves the last sampled data by the energy meter IC and displays the values at the bottom of the
screen. When live monitoring is disabled, a graph will be automatically plotted as a summary, as depicted
in Figure 3.20.
Figure 3.20: Live data feature.
3.8 Software - PIC
Microchip offers an Integrated Development Environment (IDE) to configure, develop and debug
embedded designs based on their MCUs, MPLAB X. A plugin can be installed to further assist the user,
MPLAB Code configurator, a graphical programming environment to generate libraries for the desired
peripherals. Both tools were used to create the C program that runs on the PIC. The setup is divided in
modules, each manages a functionality of the developed system: main loop, SD Card, energy meter IC,
bluetooth module, logging buffers and processing of bluetooth commands.
In Figure 3.21 a flowchart presents the main workflow of the system. At boot, the system initial-
izes and configures the I/O, RTCC, communication peripherals (SPI and UART), energy meter IC and
interrupts.
47
Figure 3.21: Main loop flowchart.
A structure variable datacell was created to hold the following data items: RMS voltage, RMS current,
apparent power, active power, reactive power, sample number, date and time. There are two buffers
each holding fifty datacell’s in order to avoid data overlap, since at the maximum acquisition rate (40
ms) a buffer is full every two seconds and flushing it to memory takes one second. The current buffer
saves the received data from the energy meter IC. When full, the ADE7753’s interrupt gains access to
the second buffer which is empty and the filled buffer is pointed to be transferred to the SD Card. Figure
3.22 is a snippet of the developed code to achieve this functionality.
48
Figure 3.22: Data buffers management functions.
Three interrupts are used in this project, each one set with different level of priority. The insertion
or removal of the SD Card triggers an external interrupt with the highest priority, updating its mount
status and altering accordingly the edge awareness (negative edge when inserted or positive edge
when removed) of CD pin from the SD Card holder (Figure 3.12). The ZX output of the ADE7753
(Figure 3.4) is detected to count line cycles and read the IC registers at the user defined rate, as well as
synchronise the registers access with the zero-crossing of the AC outlet voltage [2]. The energy meter
IC interrupt function is presented in Figure 3.23.
Figure 3.23: ADE7753 interrupt function.
49
The lowest priority is set to the UART peripheral. When data is received via bluetooth, an interrupt
request is produced to save the received bytes into a queue. In the main loop, a function is called to
parse and copy the result to a string buffer (commandlist) that can hold the last five commands. The
availability of a new command is then flagged as true. Upon processing all commands, the flag is set to
false.
50
4Results
Contents
4.1 Energy meter IC - ADE7753 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2 SD Card and RTCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3 Bluetooth module - Itead HC-05 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.5 Final Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
51
52
Results
4.1 Energy meter IC - ADE7753
The SPI1 peripheral was set at 4 MHz for the ADE7753 since this is the maximum frequency that
could be achieved while keeping consistent results. An Hantek 4032L data analyser was used to verify
clock, chip select behaviors and correct bytes where being transmitted [49].
Writing and reading is defined by the most significant bit of the address, 1 to write and 0 to read.
Figure 4.1 shows the request for the energy meter IC to reset by writing 0x0040 to the MODE register
(0x09). Figure 4.2 is the reply from the ADE7753 of a successful reset, confirmed by reading the
STATUS register (0x0C) and acknowledging the reset flag (0x0040). [2]
Figure 4.1: Software reset request.
Figure 4.2: Successful software reset.
53
4.2 SD Card and RTCC
Due to the high load that may occur on the SD Card in certain situations, such as: write measured
values and read request made over bluetooth to download the last couple hours of data; the card has
SPI2 peripheral [3] exclusively dedicated to it. The clock is set at the maximum available internally on
the PIC24FJ128GA010 rated at 16 MHz.
During an experiment, the microcontroller was set to continuously write to the SD Card, multiple text
files were created and filled with time stamps; every thousand program cycles a new file was created
named after the loop number. Files were created successfully and all characters were saved. Further-
more, on Windows file explorer, the file properties specify the correct time and date in created and last
modified variables.
4.3 Bluetooth module - Itead HC-05
A bluetooth communication has been successfully established with a smartphone; the app used to
confirm the result was a simple bluetooth serial terminal. All messages sent by the user were immedi-
ately acknowledged by the PIC24 thanks to the UART interrupt [3].
Main functionalities are accomplished, but the transmission speed is lacking at a mere 115.2 kHz,
due to the UART interface and limited clock fed to the PIC24 peripherals of 16 MHz. To solve this
situation, either the UART has access to a higher clock source or changing the bluetooth module for one
with a SPI interface would be preferable. Although the latest comes with the caveat of having to share
and manage one SPI peripheral with two devices, since both are already in use for the SD Card (SPI2)
and the energy meter IC (SPI1) [3].
54
4.4 Calibration
The PCB design is finalized as depicted in Figures 4.3 and 4.4.
Figure 4.3: PCB’s front view.
Figure 4.4: PCB’s back view.
For calibration purposes, a resistive load is applied to the PCB mains output. Two Alcor HS100 220 Ω
aluminium housed power resistor (Figure 4.5) were connected in series, for a total of a 440 Ω load. [50]
For a voltage RMS of 230 VRMS , the expected current RMS is,
Ioutput =VRMS
R=
230
220× 2= 0.523 A. (4.1)
From Equation (3.13), the resulting power consumption per resistor is given by,
P220 = (0.523)2 × 220 = 60.18 W, (4.2)
55
Figure 4.5: Alcor HS100. [50]
which is in the range of the 100 W rated power dissipation capabilities of these resistors. [50]
4.4.1 Voltage RMS and Current RMS
The 440 Ω load is mainly resistive thus it is expected a zero phase shift between both channels of
the energy meter IC. Current RMS (A) and voltage RMS (V) were also measured with a multi-meter
simultaneously with the read register values.
The calibration values for the voltage channel is given by
V/LSB =VRMSmulti−meter
V RMSregister=
235.7
1118093= 2.11× 10−4, (4.3)
and the current channel by
A/LSB =IRMSmulti−meter
IRMSregister=
0.561
30251= 1.85× 10−5. (4.4)
4.4.2 Active and Apparent Power
Active and apparent power are equal when current and voltage waves are in sync. For the same test
setup it is expected a null reactive power. The expected active and apparent power in this situation from
(1.2) and (1.5) are given by
P = S = VRMS × IRMS = 235.7× 0.561 = 132.23 W(VA), (4.5)
where VRMS and IRMS were measured with the multi-meter.
56
The following constants between register values and energy were obtained with accumulation mode
set to five line cycles (n) [2] and are calculated from
Wh/LSB =P ×
(n×period
3600
)LAenergyregister
=132.23×
(5×0.01917
3600
)25
= 1.41× 10−4, (4.6)
and
V Ah/LSB =S ×
(n×period
3600
)LV Aenergyregister
=132.23×
(5×0.01917
3600
)22
= 1.6× 10−4. (4.7)
Where the line cycle period is calculated in seconds by the IC and is available through PERIOD
register. [2]
4.4.3 Reactive Power
A different load was connected, a fully charged laptop running a benchmark. Since active and
apparent power are already calibrated, it is possible to obtain the reactive power based on the calibrated
values of active and apparent power (LV AENERGY and LAENERGY ) from the energy meter IC,
|Q| =√S2 − P 2 =
√65.3382 − 41.4392 = 23.899 VAR, (4.8)
Resulting in a reactive energy conversion defined by
V ARh/LSB =|Q| ×
(n×period
3600
)LV ARenergyregister
=23.899×
(5×0.01917
3600
)1
= 6.363× 10−4. (4.9)
4.5 Final Results
Tests were conducted to prove the functionality of the developed system. Data was logged at a rate
of 200 ms intervals. Despite there wasn’t always the expected amount of samples, which was nailed
down to the energy meter IC itself that sporadically creeps to respond the SPI requests when using a low
level of line cycles accumulation, information was correctly saved in the SD Card. Further investigation
is required into the limitations in terms acquisition rates but the finalized PCB increased the system
stability in terms of voltage supply and signal integrity versus previous prototypes. From the acquired
information, RMS voltage, RMS current, apparent, active and reactive power values show expected
results.
57
Figures 4.6 and 4.7 show the results of a twenty minutes execution with a constant 440 Ω load with
a measured voltage and current of 225 V and 0.512 A, respectively.
Figure 4.6: Measured RMS voltage and RMS current from a resistive load.
Figure 4.7: Measured apparent, active and reactive power from a resistive load.
58
The results from Figure 4.7 show an apparent and active power (4.5) overlap, as well as a near
absence of reactive power as expected from a resistive load. These traces of reactive power may be
linked to the need for a calibration based on a larger data set and/or the presence of capacitance and
noise.
Figures 4.8 and 4.9 present the acquired values over a thirty-five minutes run of a laptop in three
different states: idling, charging while in sleep mode and running a benchmark.
Figure 4.8: Measured RMS voltage and RMS current from a rated 65 W laptop charger.
In its turn, Figure 4.8 shows the RMS voltage and RMS delivered to the laptop charger. The three
different load states can be seen on the acquired current values. While idling, the current constantly
varies depending on the background tasks and services. In sleep, the charger delivers a steadier current
that tends to decrease due to the intrinsic resistance characteristic of a battery while charging, where it
increases as its being filled. Lastly, running the benchmark the laptop requests the most effort from the
65 W charger, this rated power value can be confirmed as active power in Figure 4.9.
59
Figure 4.9: Measured apparent, active and reactive power from a rated 65 W laptop charger.
The energy meter IC response limitations previously mentioned, can be perceived in these tests
given that some values mainly in the voltage measurement are off-track from the expected. Never-
theless, the acquired results offer good expectation for further development of this project, although
calibration needs further attention.
60
5Conclusions
61
62
Conclusions
For further insight in electric power consumption, an accurate and feature complete energy metering
device is required. The implementation of a system based on an IC designed specifically for the purpose,
not only adds an abstraction layer for easier integration but also assures a level of certified accuracy and
reliability [2]. This approach only requires signal conditioning which can be accomplished at low cost
without a trade-off in size by using a resistive divider and a shunt resistor, with the only concern being
power dissipation.
The embedded system has a non-volatile memory (SD Card) access via SPI and a working wireless
communication via bluetooth, integrated by UART. Although, with a compromise in data transfer speeds,
the asynchronous serial communication and its implementation in the program makes it easily adaptable
to a wide range of UART capable bluetooth modules available on the market. As for the data logging,
the FAT file system offers compatibility with all devices that include an SD Card reader and the chosen
file structure not only improves file access performance for the PIC but also makes it straightforward
for the end-user to navigate through the measured information. All files are in CSV format making it
compatible with spreadsheet programs for advanced information processing. A smartphone APP was
successfully developed with the ability to adjust acquisition rate, download sections of the acquired data,
which is presented in the form of graphs and a constantly updated view of the last measured values.
The user can pause the execution and take the SD Card for analysis and later insert it back to resume
measurement. The APP is also in charge of adjusting the PIC RTCC automatically upon connection
via bluetooth and offers the possibility to fully reset the system and clear the measured data from the
SD Card.
The project mostly fulfills the defined goals. An embedded measurement system with data logger
functionality and wireless capabilities was fully designed. Furthermore, an APP was developed to com-
plement the system and offer further versatility to view the data.
This work could be improved by focusing on the following aspects:
• Further smartphone APP development could lead to speed improvements when processing the
downloaded data;
63
• Implement a relay and exploit the IC’s capability to detect line anomalies;
• Incorporate a battery and power-path management IC in case of energy fault or the relay being
triggered;
• Find a more efficient format to save information and transmit data over bluetooth.
64
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