mechanical measurement & control

Mechanical Measurements and Control

Presented By – Anand Patange

Introduction to Measurement

Measurement: Measurement is the comparisonbetween measurand and standard.

Module 1I. Significance Of Mechanical Measurement[1]

• As Science And Technology move ahead, new phenomenon and relationship are discovered

• Required to validate hypothesis and understanding

• Measurement holds: detection

-acquisition-control and analysis of data.

• Function of engineering

i. Design of equipment and process

ii. Proper operation control and maintenance of process.

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II. Classification of measurement instrument[1]

Absolute instruments: givesmagnitude of quantity underphysical constant

Secondary instruments: they arecalibrated by comparisonwith Absolute instruments

III. Generalized Measurement System[1]

Block diagram of Bourdon pressure gauge

Bourdon Tube acts as Primary sensing /Variable conversion element.Tube connected through linkage to gearing arrangement Gearing arrangement amplify small displacement and make pointer rotate.

Pressure

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IV. Types of Input[1]

• Desired Input

• Interfering Input

• Modifying Input

Fig. Generalized input-output configuration of measurement systems

Fig. Measurement of differential pressure with manometer

Fig. Acceleration as an interfering input

Fig. Angle of tilt as an interfering input

When Calibration Need• Change in ambient temperature change

length of calibrated scale.• Change in ambient temperature change

density of mercury.

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You will see that it is very similar to the basic Wheatstone Bridge circuit, howeverone of the resistors has been swapped for a strain gauge. You know that how eachstrain gauge has a known resistance when zero strain is applied to it, if thisresistance is the same as the value of the other three resistors then when zerostress is applied, Vout will also be zero. As the object that the gauge is attachedto has a force applied to it, the gauge undergoes a strain and as a result itsresistance changes.

Wheatstone quarter bridge with dummy strain gauge configuration

Fig. Wheatstone quarter bridge circuit.

Interfering input• Temperature.• 50 Hz field nearby power

lines.

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Sometimes the quarter-bridge circuit encounters problems when there arechangeable temperatures. Unfortunately the resistance of the strain gauge can varyas the temperature changes causing the readings to become unreliable. To overcomethis problem, a half bridge setup shown in fig. is often preferred. The half-bridgeconfiguration is where two out of 4 resistors are replaced with strain-gauges. Sotemperature ranges will change resistance by the same proportion for both,effectively canceling each other.

Wheatstone Half Bridge with Dummy Strain Gauge configuration

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Where possible, it is often preferable to replace all four of the resistors in thecircuit with strain gauges known as a Full Bridge circuit. This circuit consists oftwo complimentary pairs when each pair is like the causes in the half-bridgecircuit. The benefit that this give is that of a great sensitivity to slight changes inthe strain placed on the specimen., The main benefit of this circuit over theother two options is that it is linear whereas both the quarter and half bridgeconfigurations are not. This means that the output voltage on the full-bridge isdirectly proportional to the applied force and no approximation is necessary.

Wheatstone Full-Bridge Strain Gauge configuration

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a) All types of input are constant.b) Developed input-output relationship, so that transfer function of

instrument is constant not differential equations.c) Calibration affords opportunity to check instrument against

known standard.

Static Calibration: All static performance characteristics are obtained in one form or another by a process called static calibration

1.2 Static Characteristics

Output is Directlyproportional to input

Linearity : Linearity is the maximum deviation of any calibrated point from strain line.

Accuracy : Closeness to the true value

Precision : It is measure of reproducibility of measurement

Reproducibility : Closeness of output reading with same input when change in method of measurement

Threshold : When input in creases gradually from zero there will be some minimum value below which “no output change is detected”

Ex. Gear Backlash

Hysteresis : It is non coincident between input and output curve

Ex. Strain Gauge

Drift : Environment Effect to the instruments

1. Zero Drift

2. Span/Sensitivity Drift

3. Zonal Drift

References

1. A.K. Sawhnay, Puneet Sawhnay, “MechanicalMeasurement and Instrumentation &Control” Dhanpat Rai & co.(p) ltd.Educational and technical publishers.

2. R.K. Jain, “Engineering Metrology” KhannaPublishers.