Data Acquisition System

Priyanka Goswami

Instrumentation and Control Engineering Department of Electrical Engineering

Institute of Technology, Nirma University

Abstract— The paper gives information about data acquisiton systems. It describes the various components

of the system and their working. The paper also discusses the applications of data acquisition system, its advantages and disadvantages. A simple Arduino based data acqiusition system using labview has been

described.

Index Terms—Acquisition, hardware, software, sensors, interface.

I.INTRODUCTION

Data acquisition is the process of sampling signals that measure real world physical conditions and

converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems (abbreviated with the acronym DAS or DAQ) typically measures an electrical or

physical phenomenon such as voltage, current, temperature, pressure, or sound and converts the analog waveforms into digital values for processing and analysis with the help of computer. Stand alone data

acquisition systems are called data loggers.

A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable

software. Compared to traditional measurement systems, PC-based DAQ systems exploit the processing power, productivity, display, and connectivity capabilities of industry-standard computers providing a

more powerful, flexible, and cost-effective measurement solution. Also a digital signal can be stored in volatile (RAM) or permanent (magnetic) memory from where it can be reproduced error-free or can be

imported into a computer for manipulation and analysis. A Data Acquisition System is comprised of three parts; an I/O sub-system, a host computer and the controlling software. Hence these systems are often referred to as embedded DAQ systems.

II.NEED OF DATA ACQUISITION SYSTEMS

Data acquisition systems (DAS) interface between the real world of physical parameters, which are

analog, and the artificial world of digital computation and control. With current emphasis on digital systems, the interfacing function has become an important one; digital systems are used widely because

they are low cost, accurate, and relatively simple to implement. In addition, there is rapid growth in the use of microcomputers to perform difficult digital control and measurement functions. Computerized feedback

control systems are used in many different industries today in order to achieve greater productivity in our modern industrial societies.

Data acquisition systems have evolved over time from electromechanical recorders containing typically from one to four channels to all-electronic systems capable of measuring hundreds of variables

simultaneously. They are now used by most engineers and scientists for laboratory research, industrial control, test, and measurement of input and output data to and from a computer. Industries that presently

employ such automatic systems include steel making, food processing, paper production, oil refining, chemical manufacturing, textile production, cement manufacturing, and others.

III.COMPONENTS OF DATA ACQUISITION SYSTEM

Each data acquisition system has unique functionality to serve application specific requirements. A data

acquisition and control system typically consists of the following components:

Sensors which measure physical variables such as temperature, strain, pressure, flow, force and

motion (displacement, velocity and acceleration).

Signal conditioning circuitry to convert sensor signals into a form that can be converted to digital

values.

Analog-to-digital converters, which convert conditioned sensor signals to digital values which can be used by the computer.

A computer with the appropriate application software to process, analyze and log the data to disk. Such software may also provide a graphical display of the data.

An output interface, to provide an appropriate process control response.

Hence the complete DAQ module can be divided into three categories:

Source and the system

DAQ hardware: It acts as an interface between the signal and a PC. DAQ cards often contain multiple components (multiplexer, ADC, DAC, TTL-IO, high speed timers, RAM). These are

accessible via a bus by a microcontroller, which can run small programs.

DAQ software: Specialized DAQ software may be delivered with the DAQ hardware. DAQ

software is needed in order for the DAQ hardware to work with a PC.

Fig.1.Block diagram of Data Acquisition System

A. Source And System

It consists of the input acquired via sensor and the signal conditioning circuit to make the acquired

signal suitable with the DAQ hardware. It consists of:

1. Sensors and transducers: In order to sense and measure physical variables such as pressure, flow and motion, it is necessary to use transducers or sensors, which convert physical variables into electrical

signals and transmit these signals to a signal conditioning device or directly to the data acquisition board.

Depending on the type of sensor, its electrical output can be a voltage, current, resistance, or another electrical attribute that varies over time.

The six distinguishable energy forms that are usually measured are:

Radiant: Light, infrared radiation, nuclear radiation

Mechanical: Displacement/velocity/acc, Force/Torque

Thermal: Temperature, Heat flux

Electrical: Resistance, Capacitance, Voltage, Current, Energy, Power

Magnetic: Magnetic Flux density

Chemical: pH, Chemical Composition

2. Signal acquisition and conditioning circuit: The signal conditioning device performs the following main functions:

Supplies power to the transducer when required

Amplify, filter and digitize the sensor signal

Provide an appropriate output signal that can be easily processed by the analog to digital convertor.

An amplifier boosts the amplitude of the transducer output signal to a useful level for further.

Transducer outputs may be microvolt or milivolt level signals, which are then amplified to 1 to 10 V levels. Furthermore, the transducer output may be a high impedance signal, a differential signal with

common-mode noise, a current output, a signal superimposed on a high voltage, or a combination of these. The amplifier, in order to convert such signals into a high-level voltage, may be one of several specialized types, such as the programmable gain instrumentation amplifier. The amplifier is frequently followed by a

low-pass active filter that reduces high-frequency signal components, unwanted electrical interference noise, or electronic noise from the signal. The amplifier is sometimes also followed by a special nonlinear

analog function circuit that performs a nonlinear operation on the high-level signal. Such operations include squaring, multiplication, division, rms conversion, log conversion, or linearization.

Fig.2. Signal conditioning steps

B. DAQ Hardware

The DAQ hardware turns the computer into a measurement and automation system. The terminal block and cable route the signals to the specific pins of the DAQ device. The main criteria to consider

while selecting a DAQ device for a specific application are:

If it provides connection to the internet

Availability of software support

Type of input channels: a).Single ended input channels or b).Differential input channels

Compatibility with variety of protocols

No. of analog input channels

Sampling rate

Resolution

Accuracy

Board size

Plug-in type data acquisition cards

Not all DAQ hardware has to run permanently connected to a PC. Intelligent stand alone loggers and oscilloscope can operate completely independent of the PC.

1. Analog to Digital convertor: The processed analog signal next goes to an analog multiplexer, which

switches sequentially between number of different analog input channels, in case signal from multiple sensors are present. Each input is in turn connected to the output of the multiplexer for a specified period of time by the multiplexer switch. During this connection time, a sample-hold circuit acquires

the signal voltage and then holds its value while an A/D converter converts the value into digital form. The sampling time is determined by the DAQ hardware. The resultant digital word goes to a computer

data bus or to the input of a digital circuit.

Analog-to-Digital Converters (ADCs) transform an analog voltage to a binary number (a series of 1’s

and 0’s), and then eventually to a digital number (base 10) for reading on a meter, monitor, or chart. The number of binary digits (bits) that represents the digital number determines the ADC resolution.

The different types of analog to digital convertors used in data acquisition systems are:

Successive-Approximation ADCs

Voltage-to-Frequency ADCs

Dual Slope Integrating ADCs

Sigma Delta ADCs

The Analog to digital convertor also incorporates the following capabilities

High speed data transfer to the computer

Noise and anti-aliasing filtering

Programmable gain type of instrumentation amplifier

Circuitry for hardware and software triggering

2. DAQ hardware interfaces: In telecommunications, communication protocol is a system of digital rules for data exchange within or between computers. Thus, a protocol must define

the syntax, semantics, and synchronization of communication. If the DAQ system is in the form of modules then it can be connected to the computer's ports (parallel, serial, USB, etc.) or if in form of cards then they are connected to slots (S-100 bus, ISA, MCA, PCI, PCI-E, etc.) in the motherboard.

Some of the major networking protocols, that are used in various industrial data acquisition systems are:

CAMAC - Computer Automated Measurement and Control

Industrial Ethernet

Industrial USB (Universal Serial Bus)

LAN eXtensions for Instrumentation

MODBUS

GPIB

I2C

C. DAQ Software

DAQ software is needed in order for the hardware to work with a PC. It is the most critical factor in

obtaining reliable, high performance operation. The software transforms the PC and the hardware into a complete data acquisition system which can analyze and display the parameters acquired from the source.

Specialized DAQ software may be delivered with the DAQ hardware. The different softwares available are broadly classified into two categories:

Programmable Software involves the use of programming languages such as C++, Visual C++, BASIC,

etc. the advantage of using these softwares is it provides flexibility but it increases the complexity of the code.

Data acquisition software packages do not require programming. They enable developers to design the

custom instrument best suited to their application. E.g. LabView, MATLAB, etc.

IV. DATA ACQUISITION PROCESSORS V.

For real time data acquisition and DAP boards are available which have on board processors. All data acquisition processor boards consist of:

Have onboard processors. Run DAPL, a multitasking, real-time operating system.

Have onboard memory. Support external expansion boards to increase the number of data channels.

All have onboard analog and digital inputs and outputs. Onboard processors distribute intelligence to the

Data Acquisition Processor. This frees data acquisition from the control of operating systems, such as Windows, which may not be available to the acquisition application when critically needed. By placing a

processor onboard, data acquisition and control is implemented by its own operating system, which is optimized for data acquisition. In any application the onboard intelligence has a set of tasks to do. To facilitate configuration and control of the DAP, software running on the PC communicates with the

onboard intelligence, instructing it on how to behave in an application. Some of the major advantages of using DAP boards are:

Each Data Acquisition Processor (DAP) board gives the system an additional processor running a real-

time operating system that can be controlled from a Windows application.

Two or more DAP boards, in the same PC or on a network, can work together as a single synchronized

system. This is highly advantageous for large scale data acquisition, like in process industries.

High quality measurement: DAPs can provide direct measurements with 15+ accurate bits in the best

case. This helps to produce accurate measurements for quiet signals, or unbiased measurements for

noisy signals. DAP onboard filtering and careful calibration can yield measurement accuracy even in the presence of random noise.

Many signal channels: DAP boards are designed with high-channel-count applications in mind, supported by an array of termination and expansion products.

High sampling rates: Data Acquisition Processors are built for speed. At the low, slow end of the DAP product line, sampling rates are 800,000 samples per second, without sacrificing sampling accuracy at

these speeds. Some other data acquisition cards sample fast, but not many can maintain accuracy at their maximum rates.

Signal generation: In addition to sampling, Data Acquisition Processors can also generate

signals: complex waveforms, digital alarm outputs, and digital or analog control commands. These processes do not interfere with sampling or reduce its capacity. Output clocking accuracy is the same as

the input clocking accuracy.

Real time response: The reserve of processing power necessary for high-speed data transfers also make

it possible for a DAP to respond to external events reliably. Delays of 10 milliseconds or much longer are common in a loaded PC workstation. But on a DAP, updates are available at 1 millisecond intervals

or faster that are never late.

Long term operation: DAP processing has no unplanned interruptions, no conflicting applications, no

monitors or processes to interfere.

Example of DAP board: The xDAP line of Data Acquisition Processor systems combines high performance analog interface with a speed-optimized USB host connection. Each xDAP has onboard storage for a minute of buffered data at maximum rates and autonomous sampling is controlled by a 2 GHz

Intel processor. xDAP 7420 doubles the maximum per-channel sampling speed to 2M samples per second.

VI.ADVANTAGES AND DISADVANTAGES OF USING DAQ SYSTEM

Some of the main advantages of using data acquisition systems are:

Reduced data redundancy

Reduced updating errors and increased consistency

Greater data integrity and independence from applications programs

Improved data access to users through use of host and query languages

Improved data security

Reduced data entry, storage, and retrieval costs

Facilitated development of new applications program

Some disadvantages of using data acquisition systems are:

Database systems are complex, difficult, and time-consuming to design

Substantial hardware and software start-up costs

Damage to database affects virtually all applications programs

Extensive conversion costs in moving form a file-based system to a database system

Initial training required for all programmers and users

VII.APPLICATION

A simple and cost effective data acquisition system can be easily designed using an Arduino Uno card and LabView software developed by National Instruments for experiments and project in laboratories. In

this project a potentiometer is interfaced with Arduino Uno card. Using LabView software the change in voltage and resistance can be displayed on a computer screen. It is a simple real time data acquisition

system. The following steps were implemented for interfacing:

Arduino software was used for interfacing the card with LabView. The Labview Interface For

Arduino (LIFA) file is available in LabView Device and Drivers.

After uploading the LIFA file in Arduino it can be interfaced using LabView.

In LabView using the Arduino function a module can be designed to acquire the data from the sensor that has been connected to the card. Here a potentiometer has been connected.

On changing the resistence there is a change in the voltage and that is displayed on the screen using a dial. The resistance can also be displayed after performing some mathematical

calculations.

Fig.3 Module for Arduino Interfacing

Fig.4 Display screen

VIII.CONCLUSION

Data acquisition systems basically acquire a analog signal through a transducer, convert it into digital

signal and then processes, stores, measures and analyses the acquired data with the help of various software algorithms. Real time data acquisition systems consist of a processor which is mounted on the

module and this facilitates fast data processing and analysis. Data acquisition systems are used in all

industries such as chemical process plants, in consumer goods such as cameras, mobiles, etc and in laboratories as well.

REFRENCES

[1] National Instrument, Data Acquisition System, www.ni.com/data-acquisition [2] United Electronic Industries, PC based Data Acquisition System, www.ueidaq.com

[3] Principles of data acquisition and conversion, Texas Instruments, www.ti.com [4] Data Acquisition Handbook, Third Edition, Measurement Computing Corporation

[5] Data Acquisition System Review, www.engineersedge.com [6] Data Acquisition Processor boards, Microstar Laboratories, www.mstarlabs.com