Advanced Instrumentation• Course contents

• Sensors and types• Introduction to MEMS• Transducers• Signal conditioning circuits• Data acquisition and transmission• Biomedical instrumentation• Integrated, Intelligent and smart sensors• Electronic nose• Real world interfacing

Sensors and Transducers• Sensor

• Device that detects a change in a physical stimulus and turns it into a (electrical) signal which can be measured or recorded. Eg Bimetallic strip, Light Dependent Resistors, Hg in glass thermometer etc

• Actuator• Converts electrical or pneumatic signal to some

kind of motion. Eg, solenoid actuator

• Transducer• Device that transfers power from one system to

another in the same or in the different form. Eg. Weighing machine, electric motor

Classification of the sensors• Active and Passive

Active: Input excitation is required (Electrical, Pneumatic, Hydraulic etc) Example : potentiometer as displacement sensorPassive: Input excitation is not required

Example : Mercury Thermometer • Null and Deflection Type

Null : Balancing a bridge to find resistance Passive: Finding the defection due to resistance

change in bridge circuit

Classification of the sensors• Monitoring and Transmitting Type

Monitoring: eg; Mercury thermometerTransmitting: eg; ThermocoupleCurrent (4 mA to 20 mA) should be transmitted but not voltage. A transmitter is used to convert any signal into current signal.

• Analog and Digital TypeAnalog: Output varies continuously, infinite position for the pointerDigital: Output varies in discrete form. Finite number of positions

Static Characteristics of Instruments1) Span & Range

Range : High measurement possibleSpan : Difference between max. and min measurement possible E.g. Thermocouple (700 0C to 1200 0C) Ammeter (0 to 10 A)

2) Mean, Spread & Standard deviation Mean : Average values of the same measurement

n1 2 3 n i

meani=1

x + x + x …+ x xX = =n n

Spread : Deviation from mean

Standard deviation * :

Static Characteristics continued

i meand = (x x )i

2 2 2 2 2n1 2 3 n i

i=1

d + d + d …+ d dσ = =n -1 n -1

* http://en.wikipedia.org/wiki/Bessel%27s_correction

3) AccuracyIt is the closeness with which an instrument reading approaches the true value of the quantity measured

It is specified in terms of inaccuracy or limits of error1) Point accuracy. e.g. nonlinear scale ammeter2) Percentage of Full Scale (10 V ± 5% voltmeter)3) Percentage of true value (Default)

Instrument classification: Class-n means ±n% accuratePrecision class instruments: Class 0.01, 0.05, 0.1, 0.5Laboratory instruments: Class 1 and Class 2

Commercial class instruments: Class 5 and Class10Manufacturer also guarantee the instruments for time duration

Static Characteristics continued

Static Characteristics continued4) Precision: The degree to which repeated measurements

show the same results

Repeatability — the variation arising when all efforts are made to keep conditions constant by using the same instrument and operator, and repeating during a short time period

Reproducibility — the variation arising using the same measurement process among different instruments and operators, and over longer time periods

Example for Precision & Accuracy

Accurate but less Precise Precise but less Accurate

Targeting a bull's-eye

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Significant digits/figuresPrecision are commonly expressed by means of significant figures

Rules1)(a) Non zero digits (b) zeros in between non zeros (c) zeros with decimal point are significant2)Trailing zeros (without decimal), leading zeros are not significant

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Assignment-1

1) How many significant number does each of the following numbers have

0.00305 3.0500 305000 305.

3.05×105 100.12 100 1.00

2) Multimeter reads 1.24 kΩ and other 0.80 kΩ

Resolution and Sensitivity• Resolution: The smallest detectable incremental

change of the input parameter that can be detected in the output signal. Eg; Scale, Multi range meters

Expressed either as a proportion of the readingOR

Absolute values

• Sensitivity: For an instrument or sensor with input x and output y. Sensitivity = dy/dx

ORMinimum input of physical parameter that will create a detectable output change. Eg; Ammeter(2A,100 div, 90o)

Practical and Theoretical Parameters

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Mistakes and ErrorsNo mistake in direct measurement. Data processing causes mistakes

E.g. 1) Measuring resistance passing 3 A current and voltmeter reads 1 V

2) Adding three reading of ammeter with different significant figures after decimal

• Static Sensitivity: The slope of static calibration curve evaluated at the input value

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Rounding (IEEE Standard 754)Rounding to nearest

1)Round to nearest, ties to even – rounds to the nearest value; if the number falls midway it is rounded to the nearest value with an even (zero) least significant bit, which occurs 50% of the time; this is the default algorithm for binary floating-point and the recommended default for decimal

2)Round to nearest, ties away from zero – rounds to the nearest value; if the number falls midway it is rounded to the nearest value above (for positive numbers) or below (for negative numbers)

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Assignment-2

1) Round off 0.25 to one significant digit using the two methods suggested in IEEE Standards. Compute the result obtained by dividing by 2. Compare the computational errors

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Calibration

It is a procedure that involve a comparison of the particular instrument with either1) a primary standard2) a secondary standard (higher accuracy than the

instrument to be calibrated)3) known output

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Dead zone/space and Dead time

Dead space: The largest of a measured variable for which the instrument does not respondCause: friction in mechanical measurement system

Dead time: The time before the instrument begins to respond after the measured quantity has been changed.E.g: Camera, Data acquisition card, Ammeter

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HysteresisIt refers to the differencebetween upscale sequenceof calibration and downscalesequence of calibration

he=(yupscale-ydownscale)x=x1

Hysteresis error = [he(max)/FSOR] ×100FSOR : Full Scale Output Reading

Fig. Internal force verses extension of a rubber band follows hysteresis. External force is in opposition to internal force

E.g. Potentiometer used as displacement sensor with significant losses

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Linearity• If input-output relationship is a straight line passing

through origin• Calibration easy• Uniform sensitivity

Y ∆y

Measured quantity

Out

put r

eadi

ng

• Nonlinearity cause lot of problem during signal conditioning even though it is more accurate in some cases

• E.g. LVDT (linear) Thermistor (Non-linear)

Δy(Max. deviation)%Nonlinearity = ×100Y(FSOR)

Tolerance Maximum deviation of a manufactured componentfrom some specified value.

Tolerance Colour±1% brown

±2% red

±5% gold

±10% silver

Input Impedance At the input of each component in a measuring

system, there exists a variable xi1 and at the same point, there is associated with another variable xi2 such that the product has the dimensions of power

VZI

For electrical parallel measurements

For electrical series measurements. E.g Ammeter

For other measurements. E.g. Tachometers,

Thermometer Similarly Output Impedance

Bias A constant error exists over the full range of

measurement of an instrument.This can removed easily E.g. Zero Error

Drift Change of some static characteristics of a measuring

system due to variations in ambient temperature, pressure etc.Instruments are calibrated under controlled conditionsE.g. Sensitivity drift, Zero drift.Zero drift of voltmeter due to temperature change can be expressed in volts/0C