Moving Intelligence Down the Cable (and into the sensor!) Sensors Expo June 9, 2005 Mike Edick,...

Moving Intelligence Down the Cable(and into the sensor!)

Sensors Expo June 9, 2005Mike Edick, Wilcoxon Research Sr. Electrical Design Engineer

Sensors Expo June 9, 2005

Mike Edick, Wilcoxon Research Sr. Electrical Design Engineer

The purpose of a sensor is to transform some localized energy disturbance into another measurable energy form, to transmit the energy some distance away from the measured site. The localized disturbance can be “DC” (non-changing) or “AC” (time-variable) in nature, or fractions there-of.

Application of sensors in an industrial setting requires unique observation of the localized energy disturbances, because of the potentially dangerous environment surrounding the equipment or process requiring monitoring. Extreme temperature, pressure, vibration, hazardous atmosphere, or electrical noise often requires sensitive analysis equipment to be great distances from the local energy disturbance being monitored by the sensor.

Accuracy of transmission of “DC” sensed signals is typically affected by resistive impedance of the transmission line, while accuracy of “AC” sensed signal is typically plagued by resistive and capacitive impedance of the transmission line. These impedances generate offset errors and allow noise injection into the transmission line.

(background)




Early accelerometers were both “AC” and “DC” type, both types are capacitive in nature:

• Parallel-plate “capacitor” accelerometers (DC, AC)

• Quartz “natural” accelerometers (AC)

• Piezoelectric “polled” accelerometers (AC)

(new MEMS accelerometers are capable of DC and AC, discussed later)

Early ‘charge-mode’ accelerometers contained no internal signal amplifiers, the sensor’s outputs were tied directly to the sensing element:

SensingElement

Sensor Electrical Output

(background, cont’d)




Accelerometers typically work under the principle of charge, where:

Q=C x V(charge = capacitance x voltage)

Local monitoring of the un-amplified sensor had minimal error, because of short length of transmission line.

When industrial-application of accelerometers is required, transmission-lines can be a few feet long up-to several hundreds of feet. Transmission-line amplifiers (also called “signal conditioners” or “LNA” low-noise amplifiers) generally use capacitive-feedback (opposed to resistive-feedback) for gain control and to reduce low-frequency noise. Gain of these amplifiers is a function of the sensor capacitance:

Gainamp = Csensor / Cfeedback

What’s interesting to note is, since the sensor’s charge-output is also a function of the capacitance, and the capacitance of the transmission-line is directly in parallel with the sensor’s capacitance, gain out of the amplifier remained constant no-matter how long the transmission line was!





Unfortunately, any continuously-changing capacitance of the transmission-line or “triboelectric” effect appears as a signal and is amplified. This signal is noise, and is our first reason for getting monitoring-intelligence closer to the sensor.

Frequency response, or the usable frequency-band, of a sensor is important for certain monitoring applications. Transmission-line capacitance, whether between the un-amplified transducer and signal-conditioner or between the amplified transducer and analyzing monitor, limits high-frequency response of any analog monitoring system.

High Impedance(allows RFI/EMI interference)

Un-AmplifiedSensor

Cable Capacitance(triboelectric interference,

low-pass corner)

AMP

Amplifier Output Impedance(100-150ohm typ, allows RFI/EMI

interference, allows low-pass)

Cable Capacitance(low-pass corner, couples

RFI/EMI interference)

Analyzer /Monitor

Amplifier





10,000

1,000

100

10

1


Typical IEPE Accelerometer Gain/Response versus Cable-length(Isupply=1.6mA, cable=30pF/ft)

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

8

1 10 100 1000 10000 100000

Hz

dB

1Ft

10Ft

100Ft

1000Ft

10000Ft



The Way It Was...

(the way it was...)

Motor

One, two, or hundreds of sensors mounted in a factory, with long analog transmission lines between sensor and data acquisition analyzer. All in an effort to reduce analyzer interference or reduce analyzer locations.

• long transmission lines create low-pass corners, limiting frequency response

• high sensor output impedance allows RFI to couple into capacitive / inductive lines

• EMI / triboelectric effects cause increased noise pickup in long lines

• multiple-runs of twisted-pair or coaxial cable plus installation cost more than sensors and acquisition system

• cables are susceptible to damage in industrial setting

• cable shielding is important, between signal-to-signal, signal-to-ground, and signal-to-power conductors

Sequential Processing




The Way It Evolved...

(the way it evolved...)

One, two, or hundreds of sensors mounted in a factory, with short signal lines between sensors and a junction box having built-in multiplexer and digitizer. Then a single digital transmission line between junction box and data acquisition analyzer is used, significantly reducing system installation costs.

• still a sequential process, many minutes between each sensor reading

• high sensor output impedance allows RFI to couple into capacitive / inductive lines

• EMI / triboelectric effects cause increased noise pickup in long lines

• requires human interaction during analysis

• sensor cables and local processor are susceptible to damage in industrial setting

• cable shielding is still important, between signal-to-signal, signal-to-ground, and signal-to-power conductors

Multiplexed Sequential Processing

MotorMotorMotor




The Way It’s Becoming...

(the way it’s becoming...)

One, two, or hundreds of sensors mounted in a factory, each having limited internal intelligence capable of self-deterministic alarm or process decisions. Sensors directly-operate machinery based on conditions, providing output to PLC, DCS, or network interface.

• local decisions are often enough, removing any need for off-site transmission of analog signals

• sensor signals are more digital in nature, much-less susceptible to transmission-line interference. PLCs are easier to configure.

• no-need for human interaction during analysis, “set-it and forget-it” operation

• redundant off-site monitoring for analysis is required , because frequency or time-domain information is lost by sensor’s internal processing

• general data-logging tasks may still remain, requiring inexpensive data-cable for main transmission lines

• cable shielding is not as important

Parallel Processing

MotorMotorMotor

PLCDCSPLC

NetworkHub





(the way it’s becoming, cont’d...)

Modern self-deterministic sensors include a sensing element, low-noise programmable amplifier, analog (real) band-pass filtering, an analog-to-digital converter, and some form of “brains” via programmable controller or DSP. With simple firmware (programming) changes, sensors are capable of self-calibration and a plethora of mathematical processing, capable of intelligent decision making. Outputs are digitally-accurate.

Housed within a shielded environment, modern sensors have virtually none of the transmission-line problems mentioned previously.

Although debatable, 10-bit to 12-bit of internal performance (200PPM) is often sufficient, since the signal is not acted-upon by outside sources that plagued “online” or remote data acquisition systems. Communication-friendly performance of future devices should consider 16 or 24-bits of output resolution.

Parallel Processing

ADC

MicroController

orDSP

DACFilterPGA

LNA

4-20mA &

Power Control

Modern "4-20" Sensor Construction

Outputs arestill ANALOG!






With multiple modern sensors executing decision-making in parallel:

• system response is markedly improved over one analyzer sequentially-processing multiple sensors

• data bandwidth requirements, if off-site data-logging is required, are negligible (many PLC, DCS, and modern sensors only read inputs or update outputs 1-to-10 times a second)

• there is virtually-no transmission-line interference altering calibration or noise-floor operation of the sensor. “Ones and zeroes” are highly immune to EMI, RFI, and transmission-line impedance (within reason, depending on data-rate)

Parallel Processing


SENSOR SENSOR SENSOR SENSOR

PLC / DCS

SENSOR SENSOR SENSOR SENSOR

PLC / DCS

NETWORK HUB(access point)




What’s Required Inside?

Amplifiers

•Low-noise front-ends, high-power back-ends

•FET-inputs better for high-impedance transducers, Bipolar-inputs better for higher-frequency lower-noise

•Use low-power rail-to-rail types for low-voltage systems, low-noise ±supply amps if power allows

•Players: TI/BB, National, Linear, Microchip, Analog, Intersil, Maxim






Filters

•Real (discrete), active programmable, switched-capacitor

•Real filters are a must! Combine with programmable for low-noise or switched-cap for high-order poles

•Players: TI/BB, National, Linear, Maxim






ADCs / DACs•8, 10, 12, 14, 16, “24”-bit resolutions, all with various sampling rates, pick the right one for the task

•Choose 12/16-bit for common, high-speed, or multiplexed-input sensors. More-speed means more processing, more-bits means better power-supply and reference requirements… don’t over-design!

•Choose Flash-types for fast, no-frills sampling

•Stay away from Delta-Sigma for multiplexed or switching inputs, but use for reduced up-front filtering. <16bit generally are FIR (finite-impulse response) with flat gain and phase-response, >16bit generally use Sinc-type decimation filters (a moving brick-wall window) that are not-friendly to stepped-response. are better-suited for very-low-frequency (DC) use

•SARs have decent performance, watch-out for more-severe anti-aliasing requirements

•Don’t believe for an instant that 24-bit ADC systems exist, ADC ENOB limitations along-with power-supply and reference limitations put today’s actual (usable) resolution around the 18.5-18.8 bits range

•Players: TI/BB, National, Linear, Microchip, Analog, Intersil, Maxim, Cirrus, Asahi Kasei (AKM)






Regulators & Power-Supplies

•References-IC’s are generally lower-noise than resistor-divider / buffer routine, more expensive too!

•Buy reference 10X-quieter than planned ADC-resolution, but don’t over-design ($$$)

•Amplifiers, filters, digital electronics are generally immune (>40dB of PSRR) to power-supply problems, but ADCs and DACs aren’t (unless you use a reference-input)

•Use reference-outputs for inputs to ADCs and DACs only, don’t use for general circuit biasing (added noise to reference-output, excessive loading of reference can cause noise and thermal problems). Try to shield regulators from temperature fluctuation

•Players: TI/BB, National, Linear, Microchip, Analog, Xicor(Intersil), Maxim






Microcontrollers

•8,10,12,14,16-bit Micros are plentiful, don’t over-design!

•Micros with built-in ADC/DAC uses less real-estate, but are generally higher-noise

•Determine if parallel (multiple) or sequential (multiplexed) ADC-inputs are required prior to design

•PC’s or PC/104’s offer huge computing-power, but aren’t very “embeddable” inside a sensor

•Devices with in-circuit emulation, JTAG, or UARTs are a must for all but the simplest designs. UARTs, even if they’re not required for the design, speed development 100X during testing

•More computing power or speed = more electrical power. Determine early in the design whether you’re forced to use every available instruction-cycle or need near-real-time execution… that will tell you what type of firmware programmers you need to hire (assembly or C)!

Players: TI, Microchip, Atmel, Rabbit, STMicro, SiliconLabs, Intel




(the System Evolutionary Chart...)

Valve(pneumatic)

Controls

Relay(electric)Controls

DCS

PLC

DCS

RR

R"distributedcontrol"

"remote digitalcontrol"

"DCS"

PR

OC

ES

SC

ON

TR

OL

Plant Operations

OnlineMonitoring

MultiplexedOnline Monitoring

PortableData Collector +Remote Analyzer

Local MultiplexedData Collector +Remote Analyzer

Local + RemoteInstallation

Online Monitoring

Local MultiplexedPortable Data

Analyzer

DY

NA

MIC

SIG

NA

LA

NA

LYS

IS

MUX

JunctBox

S

SS

S

SS

S

S MUX

SS

S S

JunctBox

S

SS

S

SS

S

S S

S

S

NetworkedSensor System




(the way it needs to be...)

“Wired” Network Sensor

The Way It Needs To Be... Distributed Parallel Processing

•Sensor output is Digital (noise-immune, digitally-accurate)

•Sensor can receive inputs from network for advanced algorithm operation

•Sensor can receive new operational programming, set-point changes from network

•Sensor receives electrical operational power through network connection

•Sensor has internal memory; capable of transmitting high-bandwidth time-domain information, low-bandwidth processed signal, or electronic calibration information

•PC or analyzer can directly-address individual sensor, removing need for human involvement

•Sensor’s pre-processed output signal can be very low in bandwidth, allowing large numbers of sensor-channels on one digital transmission line (time-multiplexed)


SENSOR SENSOR SENSORPLCDCS

PLCDCS

SENSOR SENSOR SENSOR

NETWORK HUB(access point)



(the way it needs to be, cont’d...)

“Wireless” Network Sensor

The Way It Needs To Be... Distributed Parallel Processing

•Sensor output is Digital (noise-immune, digitally-accurate)

•Sensor can receive inputs from network for advanced algorithm operation

•Sensor can receive new operational programming, set-point changes from network

•Sensor requires local-powering, battery, super-cap, or energy-harvesting to operate

•Sensor’s output must be pre-processed for low-bandwidth battery-powered applications

•Sensor has internal memory; capable of transmitting high-bandwidth time-domain information, low-bandwidth processed signal, or electronic calibration information

•PC or analyzer can directly-address individual sensor, removing need for human involvement

•Sensor’s pre-processed output signal is very low in bandwidth, allowing large numbers of sensors on one wireless channel (time-multiplexed)

SENSOR SENSOR SENSORPLCDCS

PLCDCS

SENSOR SENSOR SENSOR

NETWORK HUB/ REPEATER

( ( ( ) ) )( ( ( ) ) )( ( ( ) ) ) ( ( ( ) ) ) ( ( ( ) ) ) ( ( ( ) ) )

( ( ( ) ) )




(the way it needs to be, cont’d...)

Wired or Wireless Network Sensors

•Required bandwidth, transmission-distance, and/or available operating-power are the dominant drivers for device design.

•Some communication mediums are more efficient with contiguous blocks of data versus byte-transfer, some aren’t

•Anticipate total number of sensors, total data-size to be transmitted, transfer-efficiency, and distance at start of device design.

•Keep in mind signal resolution and bandwidth are converse properties in a networked environment. A wired 100BT Ethernet-hub can handle 1 sensor with 24-bit resolution updating at 3.75MHz, or it can support 255 sensors at 16-bit resolution updating at 20KHz!

Approximate (maximum) transfer efficiencies based on overhead requirements for various

wired and wireless transmissions*:

•802.11a/b/g/n: ~50% without interference

•10BT/100BT 802.3: ~90% (108B of 1500B)

•USB1.x: ~90% (144B of 1500B)

•USB2.x: ~96% (43B of 1500B)

•RS232/422/485/Profi: ~80% (2b of 10b)

•SPI: ~100% (0b of 8b)

•CAN: ~83% (11b of 75b)

*these are general values and certainly do not apply to every situation




(the way it needs to be, cont’d...)Available Wireless Technologies versus Data-Rate and Distance

GSMGlobal System Mobile Comm

33kbps

>1-

mil

eR

ang

e<

100-

met

erR

ang

e<

10-m

eter

Ran

ge

<10

0-m

eter

Ran

ge

<10

-fo

ot

Ran

ge

Low Data Rate High Data Rate

PA

N(p

erso

nal

-are

a n

etw

ork

)IE

EE

802

.15-

typ

e

LA

N(l

oca

l-ar

ea n

etw

ork

)IE

EE

802

.11-

typ

e

WA

N(w

ide-

area

net

wo

rk)

GPRS/3GGeneral Packet Radio Service

171kbps

LMDSLocal Multipoint Distribution Service

1.5Gbps

Cellular Voice Cellular Data Local Broadcast Type, 28-31GHz

802.11bWireless Ethernet11Mbps / 2.4GHz

802.11gWireless Ethernet54Mbps / 2.4GHz

802.11aWireless Ethernet54Mbps / 5GHz

Bluetooth2802.15 subset

1Mbps / 2.4GHz

Wireless USB802.15 subset

54Mbps / 2.4GHz

ZigBee802.15 subset

250Kbps / 2.4GHz

RFID802.11 subset

2Mbps / 2.4GHz

Bluetooth1802.15 subset

100Kbps / 2.4GHz

ZigBee802.15 subset

20Kbps / 915MHz

RFID802.11 subset2Kbps / 12MHz

ASK/FSK"Key FOB"

64Kbps / 315-450MHzLOW

POWER

Wilcoxon ResearchAvaliable Wireless Technologies

(Examined 01/04)




(summary...)Summary: Why do we need intelligence inside the sensors?

• Industrial Cabling Problems

- wire capacitance attenuates analog AC signals, wire resistance attenuates analog DC signals

- long wires means noise-injection into system (frequency-response, triboelectric, capacitive-coupling of RFI)

- multiple wires means multiple-expense, increases channel-to-channel cross-talk

- cables are expensive to install, often damaged by environment or contact

• Sequential Signal Processing

- multiple sensors sequentially monitored by one analyzer creates inherent delays, not good for process-control

- analyzer often some-distance from monitored-sensor, to reduce environmental hazards to man and equipment

- only one sensor can be monitored at a time




(summary, cont’d...)Summary: Why do we need intelligence inside the sensors?

• Multiplexed Sequential Signal Processing

- significant reduction in cabling expense for multiple sensors

- still have transmission-line length/noise problems

- still monitors only one sensor at a time

- usually requires human intervention, controlling switch-box or portable data collector

• Parallel Signal Processing

- significant reduction in cabling expense for multiple sensors

- significant reduction in latency, self-determination is basically one analyzer per sensor, continuous monitoring

- significant reduction in human involvement, “set-it and forget-it” sensor operation

- significant reduction in required output bandwidth, allows use of inexpensive data-cables for transmission

- sensor outputs are digital in nature, highly immune to noise, digitally-accurate





• Parallel Signal Processing (cont’d)

- intelligent sensors require digital and analog electronics (amplifier, filter, ADC, DAC, micro, DSP, reference)

- 10-12bit performance is sufficient for most applications, don’t over-design

- over-designing a circuit to use >16bit ADCs is costly or misleading, due to reference and power requirements

- more ADC-bits = more dynamic-range and lower noise-floor and is costly to implement, more front-end amplification gives same noise-floor performance as higher-bit systems at the expense of full-scale signal range

- available power sets the design, low power = low voltage & low speed, high-power = low-noise & high speed

- sensor outputs are digital in nature, but still analog in value, and must be interpreted by PLC or DCS





• Distributed Parallel Signal Processing

- the coming art and future of sensor designs

- sensor outputs are pure digital communication

- sensor receives power through communication interface, internal battery, or through energy harvesting

- sensor output can be high-bandwidth raw signal, low-bandwidth processed-signal, or calibration data

- sensor can receive digital input, altering programming or set-point operation

- sensor connects to network directly, sensor is individually addressable

- sensor’s digital output can be transmitted by wired or wireless methods at various data-rates or distances


Moving Intelligence Down the Cable (and into the sensor!) Sensors Expo June 9, 2005 Mike Edick,...

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