INSTRUMENTATIONCHARACTERISTIC

WHAT IS INSTRUMENTATION

• is a collection of Instruments and their application for the purpose of Observation, Measurement and Control. Reference: ISA std. S 51.1

• The key word is– Observation or measurement– control

Process ControlPriyatmadi 2008

3

Instrumentation

process

TT

TIC

I/P

4-20 mA4-20 mA

3-15psi

Set point

Cold water in

hot water outsteam in

Instrumentation

MEASUREMENT INSTRUMENTATION MODEL

What is sensor• Def. 1. (Oxford dictionary)

– A device giving a signal for the detection or measurement of a physical property to which it responds.

• Def. 2. – A sensor is a device that receives a signal or stimulus and

response with an electrical signal.

Electricalmechanical

MagneticChemical

OpticalRadiationThermal

• Passive and active sensors– Passive sensors are sensors which do not

provide energy to sense, they just absorb the energy form the measurand and convert it to electrical signal, e.g. pressure gauge, thermocouple

– Active sensors are sensors which provide energy in measurement process , e.g. radar

MeasurementsHeisenberg (1927): ”The momentum and position of a particle can not both be precisely determined at the same time.”Measuring activity disturbs the physical process (loading effect), produce errorMeasurement error:That is the difference between the measured value and the true value.error = measured value - true valueDeterministic errors:They are repeated at every measurement, e.g. reading offset or bias. Sucherrors can be reduced by proper calibration.Random errors:They are caused by several parameters and change in time in anunpredictable fashion. They can be quantified by mean errors, standardDeviation. Can be reduce by averaging several measurements

Sensor properties

Sensor properties

Ideally, the sensor characteristic is a straight line should take no time convert the input. But that is never the case.

input

output

SENSOR CHARACTERISTIC

Accuracy : Error measurementSensitivity: change in output for unit change in

inputResolution: the smallest change in the signal that

can be detected and accurately indicated by a sensor.

Linearity: the closeness of the calibration curve to a straight line.

Drift: the deviation from the null reading of the sensor when the value is kept constant for a long time.

SENSOR CHARACTERISTICHysteresis: the indicated value depends on

direction of the test (increasing and decreasing)Repeatability (precision): the maximum deviation

from the average of repeated measurements of the same static variable.

Dynamic Characteristics: A sensor may have some transient characteristic. The sensor can be tested by a step response where the sensor output is recorded for a sudden change of the physical variable. The rise time, delay time, peak time, settling time, percentage overshoot should be as small as possible.

ACCURACY

Accuracy• Accuracy is a degree of conformity of an indicated value

to recognized accepted standard value or ideal value• Measured accuracy is the maximum positive and

negative deviation observed during a testing a device under specified condition and procedure.

• Accuracy rating is a number or quantity that defines a limit that error will not exceed when the device is used under specific condition.

• When the operating conditions are not specified reference operating condition should be assumed.

• In specification sheet term accuracy should be assumed to mean accuracy rating.

• Accuracy rating includes the combines effect of conformity, hysteresis, dead band, and repeatability.

Accuracy

Upscale calibration

downscale calibration

Specified characteristic

Low permissible error limit

Low permissible error limit

Max actual negative deviationMax actual negative deviation

Accuracy, rating

0 Input 100%

output

Accuracy, rating

Accuracy rating can be expressed in number of form, e.g.:

1. In term of measured variable e.g.: ±2o C

2. In percent of span e.g.: ±0.5% of span

3. In percent of upper range e.g.: ±0.5% of upper range

4. In percent of scale length e.g.: ±0.5% of scale length

5. In percent of output reading e.g.: ±0.5% of output

-10 110

Range -10 to 110, upper range 110, lower range -10 Span = length = 120

Measuring Accuracy

Create calibration table by1. Set 50% input (the input must be

secondary standard source)2. Read the output3. Compute the percentage deviation and

write it down in the table4. Repeatedly increase the input until 100%

is reach then decrease until 0%, increase and decrease again and again.

CALIBRATION TEST TABLE

Input %

actual error %

up down up down up down up

0   -0.04   0.05   0.06  

10   0.14 0.04 0.15 0.05 0.16 0.06

20   0.23 0.08 0.26 0.09 0.26 0.13

30   0.24 0.09 0.25 0.10 0.26 0.11

40   0.13 -0.07 0.15 -0.04 0.17 -0.04

50 -0.18 -0.02 -0.16 0.01 -0.13 0.01 -0.13

60 -0.27 -0.12 -0.25 0.10 -0.23 -0.08  

70 -0.32 -0.17 -0.30 -0.16 -0.28 -0.12  

80 -0.27 -0.17 -0.26 -0.15 -0.72 -0.13  

90 -0.16 -0.06 -0.15 -0.05 -0.14 -0.04  

100 0.09   0.11   0.1    

Measured Accuracy

Input %

actual error % of span

up down up down up down up

0   -0.04   0.05   0.06  

10   0.14 0.04 0.15 0.05 0.16 0.06

20   0.23 0.08 0.26 0.09 0.26 0.13

30   0.24 0.09 0.25 0.10 0.26 0.11

40   0.13 -0.07 0.15 -0.04 0.17 -0.04

50 -0.18 -0.02 -0.16 0.01 -0.13 0.01 -0.13

60 -0.27 -0.12 -0.25 0.10 -0.23 -0.08  

70 -0.32 -0.17 -0.30 -0.16 -0.28 -0.12  

80 -0.27 -0.17 -0.26 -0.15 -0.72 -0.13  

90 -0.16 -0.06 -0.15 -0.05 -0.14 -0.04  

100 0.09   0.11   0.1    

Measured accuracy is the greatest positive and negative deviation of the recorded values. Measured accuracy is -0.32% to +0.26%

DEAD BAND

Dead band.

Dead band is the range through which an input can be varied without initiating observable response.

Dead band is usually expressed in percent of span

Dead band

Dead band. To measure dead band proceed as follows: 1. Slowly increase the input until a detectable output

change is observed2. Observe the input value3. Slowly decrease the input until a detectable output

change is observed4. Observe the input valueThe difference between step 2 and 4 is the dead band. Those steps is repeated for input from 0% to 100%.The highest number is reportedExample: the dead band is 0.10% of the input span

DRIFT, POINT

Drift, Point

To measure drift proceed as follows:1. Adjust the input to the desired values without overshoot

and record the output value. The test device should be permitted to warm up before recording the initial output value.

2. Maintain a fixed input value and fixed operating condition for the duration of the test.

3. Record the output value during the test.

Drift is change of input-output relation over a period of timePoint drift is the maximum change in recorded output

during the test period, expressed in percent of output span.Example: The point drift is 0.1% of output span for 24 hour

test

HYSTERESIS

Hysteresis A property of element evidenced by the dependence of the output value for the given excursion of input, upon the history of prior excursions and the current direction of the traverse.

Hysteresis

input

output

Hysteresis + dead band

Hysteresis

input

output

input

output

Hysteresis + dead band

Dead band

input

output

Hysteresis

• Hysteresis is usually determined by subtracting the value of dead band from the maximum separation between upscale going and down scale going indication of calibration report.

• This measurement is sometimes called hysteresis error or hysteretic error

HysteresisInput %

actual error %

up down up down      

0   -0.04   0.05   0.06  

10   0.14 0.04 0.15 0.05 0.16 0.06

20   0.23 0.08 0.26 0.09 0.26 0.13

30   0.24 0.09 0.25 0.10 0.26 0.11

40   0.13 -0.07 0.15 -0.04 0.17 -0.04

50 -0.18 -0.02 -0.16 0.01 -0.13 0.01 -0.13

60 -0.27 -0.12 -0.25 0.10 -0.23 -0.08  

70 -0.32 -0.17 -0.30 -0.16 -0.28 -0.12  

80 -0.27 -0.17 -0.26 -0.15 -0.22 -0.13  

90 -0.16 -0.06 -0.15 -0.05 -0.14 -0.04  

100 0.09   0.11   0.1    

Hysteresis + dead band = 0.22%If the dead band is 0.1% the hysteresis is 0.12%

LINEARITY

Linearity

The closeness to which a curve is approximates a straight line

input

output

a b

• The linearity of curve a is better then curve b.

• It is usually measured as a nonlinearity and is expressed as

linearity e.g. a maximum deviation between an average

curve and a straight line.• There are 3 type of linearity i.e. independent, terminal based, and

zero based linearity

Independent Linearity

input

output

• It is the maximum deviation of calibration curve (averaged of

upscale and down scale reading) from a straight line so positioned

as to minimized the maximum deviation.

Max ± deviation are minimizedAnd equal

Terminal based Linearity

input

output

• It is the maximum deviation of calibration curve (averaged of

upscale and down scale reading) from a straight line coinciding with the calibration curve at the upper

and lower range values

Max deviation

Zero based Linearity

input

output

• It is the maximum deviation of calibration curve (averaged of

upscale and down scale reading) from a straight line so positioned

as to minimized the maximum deviation and coincide with the

lower range value.

Max ± deviation are minimized

Measuring Linearity 1. Take the average deviation for

every input

2. Find the straight line for independent, terminal based,

and zero based linearity.

3. Compute the linearity

Input % dev % 0 -0.050

10 0.100

20 0.175

30 0.175

40 0.050

50 -0.075

60 -0.175

70 -0.225

80 -0.200

90 -0.100

100 0.100

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

12

3zero based straight line

terminal based straight line

independentstraight line

1. Independent linearity =.18%2. Terminal based linearity =.28%

3. Zero based linearity =0.21%

REPEATABILITY

Repeatability The closeness of agreement among number of consecutive measurement for the output of the same value of the input under the same operating condition approaching from the same direction. It is usually measured in non-repeatability and measured as repeatability in percent of span

input

output

Repeatability

Upscale calibration curve

Down scale calibration curve

RepeatabilityInput %

actual error % of span

up down up down up down up

0   -0.04   0.05   0.06  

10   0.14 0.04 0.15 0.05 0.16 0.06

20   0.23 0.08 0.26 0.09 0.26 0.13

30   0.24 0.09 0.25 0.10 0.26 0.11

40   0.13 -0.07 0.15 -0.04 0.17 -0.04

50 -0.18 -0.02 -0.16 0.01 -0.13 0.01 -0.13

60 -0.27 -0.12 -0.25 0.10 -0.23 -0.08  

70 -0.32 -0.17 -0.30 -0.16 -0.28 -0.12  

80 -0.27 -0.17 -0.26 -0.15 -0.72 -0.13  

90 -0.16 -0.06 -0.15 -0.05 -0.14 -0.04  

100 0.09   0.11   0.1    

Repeatability =0.05%

Typical specification

SENSORS

Motion sensors

• These transducers measure the following variables: displacement, velocity, acceleration, force, and stress.

• Such measurements are used in mechanical equipment such as servo-systems, robots, and electrical drive systems.

• Motion sensors include the following types of devices: potentiometers, resolvers, optical encoders, variable inductance sensors (displacement), tachometers (velocity), piezo-resistive sensors (strain).

POTENTIOMETER

CAPACITIVE SENSORS

d

WLC 0

d

XLWC

)(0

d

WLC 0

Resolver• Resolvers are used in

accurate servo and robot systems to measure angular displacement. Their signal can be differentiated to obtain the velocity.

• The rotor is connected with the rotating object and contains a primary coil supplied by an alternating current from a source voltage vref. The stator consists of two windings separated by 90o, with induced voltages

V01= K vref sin θV02= K vref sin θ

Tachometer

• The permanent magnet generates a steady and uniform magnetic field. Relative

• motion between the field and the rotor induces voltages, which is proportional

• to the speed of the rotor.• The inductance gives the

tachometer a certain time constant so that the

• tachometer cannot measure fast transient accurately.

Optical encoders• These are optical devices to

measure angular displacement and angular velocity.

• A disk of an optical encoder is connected to the rotating shaft.

• The disk has patterns (holes). • On one side of the disk there is

a light source and on the other photo-detectors. When the disk rotates the light is going through the holes and the photo-detectors generate series of pulses.

• There are two types of optical encoders: incremental and absolute.

Optical encoders

• The incremental encoder provides a pulse each time the shaft has rotated a defined distance.

• The disc of an absolute encoder has several concentric tracks, with each track having an independent light source and photo detector.

• With this arrangement a unique binary or Gray coded number can be produced for every shaft position.

LVDT

• The two secondary coils are connected in the opposite phase. When the core is in the middle there is no output voltage.

• Moving the core from the central position unbalances the secondary coils, developing an output.

displacement

Vout

LVDT

Strain gauge

Strain gauge

• When external forces are applied to a stationary object, stress and strain are the result.

• Stress is defined as

Strain gauge

• Strain is defined as the amount of deformation per unit length of an object when a load is applied.

Strain (ε) = ΔL/L• Typical values for strain are less than

0.005 inch/inch and are often expressed in micro-strain units:

1 μstrain = 106 strain

Strain gauge

• Strain may be compressive or tensile and is typically measured by strain gages.

• It was Lord Kelvin who first reported in 1856 that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance.

• This phenomenon was first put to practical use in the 1930s.

Strain gauge

• Fundamentally, all strain gages are designed to convert mechanical motion into an electronic signal.

• A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor.

Strain gauge

• If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the gage factor (GF), is given by:

GF = (ΔR/R)/(ΔL/L)

Strain gauge

• The ideal strain gage would change resistance only due to the deformations of the surface to which the sensor is attached.

• However, in real applications, temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal all affect the detected resistance.

Strain gauge

• Because most materials do not have the same properties in all directions, a knowledge of the axial strain alone is insufficient for a complete analysis. Poisson, bending, and torsion strains also need to be measured. Each requires a different strain gage arrangement.

Strain gauge

• The deformation of an object can be measured by mechanical, optical, acoustical, pneumatic, and electrical means.

• The earliest strain gages were mechanical devices that measured strain by measuring the change in length and comparing it to the original length of the object.

Strain gauge

• The most widely used characteristic that varies in proportion to strain is electrical resistance. Although capacitance and inductance-based strain gages have been constructed, these devices' sensitivity to vibration, their mounting requirements, and circuit complexity have limited their application.

• The photoelectric gage uses a light beam, two fine gratings, and a photocell detector to generate an electrical current that is proportional to strain. The gage length of these devices can be as short as 1/16 inch, but they are costly and delicate.

Strain gauge

• The first bonded, metallic wire-type strain gage was developed in 1938. The metallic foil-type strain gage consists of a grid of wire filament (a resistor) of approximately 0.001 in. (0.025 mm) thickness, bonded directly to the strained surface by a thin layer of epoxy resin

Strain gauge

Strain gauge

Application of Strain gauge

• Strain gages are used to measure displacement, force, load, pressure, torque or weight. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit.

• The strain-gage sensor is one of the most widely used means of load, weight, and force detection.

• As the force is applied, the support column experiences elastic deformation and changes the electrical resistance of each strain gage. By the use of a Wheatstone bridge, the value of the load can be measured. Load cells are popular weighing elements for tanks and silos and have proven accurate in many other weighing applications.

Application of Strain gauge

• Strain gages may be bonded to cantilever springs to measure the force of bending.

• The strain gages mounted on the top of the beam experience tension, while the strain gages on the bottom experience compression. The transducers are wired in a Wheatstone circuit and are used to determine the amount of force applied to the beam.  

Application of Strain gauge

• Strain-gage elements also are used widely in the design of industrial pressure transmitters. Using a bellows type pressure sensor in which the reference pressure is sealed inside the bellows on the right, while the other bellows is exposed to the process pressure.

• When there is a difference between the two pressures, the strain detector elements bonded to the cantilever beam measure the resulting compressive or tensile forces.  

Application of Strain gauge

•  A diaphragm-type pressure transducer is created when four strain gages are attached to a diaphragm.

• When the process pressure is applied to the diaphragm, the two central gage elements are subjected to tension, while the two gages at the edges are subjected to compression.

• The corresponding changes in resistance are a measure of the process pressure. When all of the strain gages are subjected to the same temperature, such as in this design, errors due to operating temperature variations are reduced.

Piezoelectric Materials

• Many polymers, ceramics, and molecules such as water are permanently polarized: some parts of the molecule are positively charged, while other parts of the molecule are negatively charged.

Piezoelectric Materials

• When an electric field is applied to these materials, these polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material.

Piezoelectric Materials

Furthermore, a permanently-polarized material such as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect.

Piezoelectric Materials

• Conversely, an applied electric field can cause a piezoelectric material to change dimensions.

• This phenomenon is known as electrostriction, or the reverse piezoelectric effect.   

• Piezoelectric Effect  Reverse Piezoelectric Effect