Mechanical measurement & control
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Transcript of 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.