This document discusses measurement and instrumentation in mechanical systems. It begins by defining measurement and classifying instruments as absolute or secondary. It then describes the generalized components of a measurement system including the sensing element, signal conditioning, and output display. Different types of inputs like desired, interfering, and modifying inputs are discussed. Examples of half, quarter, and full Wheatstone bridge circuits used with strain gauges are provided. Key characteristics like linearity, accuracy, precision, and hysteresis that are evaluated during static calibration of instruments are also summarized.

1. Mechanical Measurements and
Control
Presented By – Anand Patange

2. Introduction to Measurement
Measurement: Measurement is the comparison
between measurand and standard.

3. Module 1
I. 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.

4. • Function of engineering
i. Design of equipment and process
ii. Proper operation control and maintenance
of process.
Continue….

5. II. Classification of measurement instrument[1]
Absolute instruments: gives
magnitude of quantity under
physical constant
Secondary instruments: they are
calibrated by comparison
with Absolute instruments

6. III. Generalized Measurement System[1]

7. 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
Continue….

8. IV. Types of Input[1]
• Desired
Input
• Interfering
Input
• Modifying
Input
Fig. Generalized input-output configuration of
measurement systems

9. 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.
Continue….

10. You will see that it is very similar to the basic Wheatstone Bridge circuit, however
one of the resistors has been swapped for a strain gauge. You know that how each
strain gauge has a known resistance when zero strain is applied to it, if this
resistance is the same as the value of the other three resistors then when zero
stress is applied, Vout will also be zero. As the object that the gauge is attached
to has a force applied to it, the gauge undergoes a strain and as a result its
resistance changes.
Wheatstone quarter bridge with dummy strain gauge configuration
Fig. Wheatstone quarter bridge circuit.
 Interfering input
• Temperature.
• 50 Hz field nearby power
lines.
Continue….

11. Sometimes the quarter-bridge circuit encounters problems when there are
changeable temperatures. Unfortunately the resistance of the strain gauge can vary
as the temperature changes causing the readings to become unreliable. To overcome
this problem, a half bridge setup shown in fig. is often preferred. The half-bridge
configuration is where two out of 4 resistors are replaced with strain-gauges. So
temperature ranges will change resistance by the same proportion for both,
effectively canceling each other.
Wheatstone Half Bridge with Dummy Strain Gauge configuration
Continue….

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

13. 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

14.  Output is
Directly
proportional
to input
 Linearity : Linearity is the maximum deviation of any
calibrated point from strain line.

15. 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

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

17. Hysteresis : It is non coincident between input
and output curve
Ex. Strain Gauge

18. Drift : Environment Effect to the instruments
1. Zero Drift
2. Span/Sensitivity Drift
3. Zonal Drift

19. References
1. A.K. Sawhnay, Puneet Sawhnay, “Mechanical
Measurement and Instrumentation &
Control” Dhanpat Rai & co.(p) ltd.
Educational and technical publishers.
2. R.K. Jain, “Engineering Metrology” Khanna
Publishers.