Subject: Mechanical Engineering Measurement. (I-Scheme III Sem. Diploma in Mechanical Engg.) Ch. no. 2. displacement, force & torque measurement. Department of Mechanical Engg. Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar. Prepared by Prof. Amol Yashwant Kokare Sir

1. Unit No. 2
Displacement, Force &
Torque Measurement
Department of Mechanical Engineering Prof. Kokare A.Y.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Subject- Mechanical Engineering
Measurement

2. Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.
Unit No.2- Displacement,
Force & Torque
Measurement (12 Marks)
CO b.- Use relative
instrument for
measurement force and
Torque.

3. SELECTION OF DISPLACEMENT SENSORS
1. The accuracy required.
2. The resolution required.
3. The size of the displacement.
4. Displacement type (linear or angular).
5. The cost and material made.
Applications of Displacement Transducers
Level measurement - Lift position control.
Machine alignment- Alignment measurement.
Assembly checking- Deformation measurement.
Close loop control- Expansion displacement.
Tool positioning- Contraction displacement.
Movement control-Deflection measurement.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

4. DISPLACEMENT MEASUREMENT
Capacitive Transducers- Advantages
1. It gives an accurate frequency response to both statia and
dynamic measurements.
2. It works on simple capacitance principle.
3. The power required to operate is small
4. It is inexpensive
5. It is easy to fabricate and have low maintenance cost.
Disadvantages
1. An increase or decrease in temperature to a higher level
may deviate the accuracy, sensitivity.
2. Proper insulation is required between metallic parts of
transducer.
3. Due to lengthy lead and cables, it can cause errors or
distortion in signals.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

5. Dielectric Constant
The capacitance, formed between the
central roc the surrounding tube, varies
as the changing liquid level varies the
dielectric constant.
Area
A change of capacitance by varying are
a finds application in torque measurement.
Distance Area
The most common application of
varying distance is for
displacement and motion
measurement. In a pressure
transducer, a fixed electrode
and a diaphragm act as a capacitor.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

6. Potentiometer
1. It gives an accurate frequency response to both static and
dynamic measurements.
2. It works on simple capacitance principle.
3. The power required to operate is small
4. It is inexpensive
5. It is easy to fabricate and have low maintenance cost.
Advantages
1. They are inexpensive, 2. Operating principle is very simple,
3. They are useful for measurement of large amplitude of
displacement, force, pressure, etc.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

7. Disadvantages
1. Poor dynamic response because of friction. 2.Resolution is poor.
3. The problems with wiper contacts are that they wear out,
become misalign and generate noise.
Application
1. For precise measurement of displacement.
2. To control or change the brightness of lamp or LED.
3. For measurement of voltage in thermocouple.
4. Potentiometers can be used as motion sensor or position sensor.
Resolution
Resolution for potentiometer is defined as the potential difference
between adjacent turns of coil divided by the input potential
difference to the potentiometer. R = 2 ld + δ
Noise
Potentiometer noise is defined as the fluctuations in output
voltage due to slider contact bounce, dirt and wear, variation in
voltage drop across the contact and effect of resolution.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

8. LVDT (Linear Variable Differential Transformer)
The principle is based upon modulation of the excitation
signal. In LVDT Motion of magnetic core changes the
mutual inductance of two secondary coils relative to
primary coil. LVDT (Linear Variable Differential Transformer)
is a popular transducer which is based on a variable-
inductance principle for displacement measurements.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

9. LVDT specifications
1. Input or excitation voltage – 3 to 15 V. (sinusoidal).
2. Stroke or core displacement - 0.1 mm to 100 mm.
3. Input frequency – 20 to 50 KHz.
4. Sensitivity - 0.5 to 30 mV/V input /0.05 mm.
5. Output impendence - < 50. 6. Resolution - 2 x 10-3 mm.
Advantages of LVDT
1. It serves as a primary detector transducers.
2. It cannot be overloaded. 3. It is an economical device.
4. It is insensitive to temperature changes.
5. It gives high output without any intermediate amplification.
Disadvantages of LVDT
1. It is designed for a specific input frequency.
2. The device becomes complicated, if the direction from null is
also required.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

10. Applications of LVDT
1. It is used as a transduction element.
2. It is used in the CNC machine for displacement measurement.
3. It is used in the CNC machine for displacement measurement.
4. It is used for different measurements like pressure, force,
thickness, etc. also in the control systems.
RVDT (Rotary Variable Differential Transformer)
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

11. RVDT (Rotary Variable Differential Transformer)
A modified from of differential transformer cab be used for
measuring angular rather than linear displacement. This type is
known as a rotary variable differential transformer of RVDT.
Although the RVDT is a continuous (360°) rotation device, the
range of most linear operations for a typical RVDT is only about
± 40°; operating range is better than 0.5 percent of full range.
Advantages of RVDT
1. It is used extensively to measure position and offer significant
advantages over other types of angular position transducers.
2. The foremost advantage is the lack of physical contact between rotor
and stator.
3. Another major advantage is the absence of brushes slip rings.
4. The bearing play is usually so negligible that RVDT exhibits practically
no mechanical hysteresis.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

12. Force Measurement
F = m x a 1 kg force is equal to 9.807N
Characteristics of force measurement system as follows:
1. Non linearity is the vertical deviation from the line OA i.e. best
fit line.
2. The difference of readings between upscale and downscale
readings at any given force is defined as hysteresis.
3. Any difference between observed value of force and true or
actual value is known as errors of measurement.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

13. Characteristics of force measurement system as follows:
4. Full scale output is the span which is the output at rated capacity
the output at zero reading.
5. The repeatability is the measure of agreement between the
results of successive
6. Sensitivity is the full scale output divided by the rated capacity
of a given force transducer.
Creep Curve for force Transducer
For creep and creep recovery, the
results may he depending on
how long the force applied has
been at zero or the rated value
before the change of force is
made.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

14. Load Cell
A load cell is a device that converts a force or load into a
measurable output.
Load cell is a force transducer which utilizes deflection of an elastic
member as a primary sensing device and strain gauge as a
secondary transducer. Types of mechanical load cells :
(a) Hydraulic load cell, (b) Pneumatic,& (c) Strain gauge load cell
Applications
Tool dynamometers, weighing machine vehicles, load
measurement in material handling device such as crane or AGV.
Bag filling machines, suspension bridges etc.
Types of Load Cell
a) Hydraulic load cell, b) Pneumatic load cell,
c) Strain gauge load cell, d) Piezoelectric load cell,
f) Capacitance load cell.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

15. Hydraulic load cell
The input force is applied to the
piston via loading platform.
Piston moves downward and
deflects the diaphragm. The
deflection in the diaphragm
increases the pressure in the
fluid in chamber.
This increase in pressure of fluid
is a function input force which is
to be measured. It is indicated
by a suitable pressure gauge
calibrated in terms of force
units.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

16. Pneumatic load cell
The working principle of
pneumatic load cell and
hydraulic load cell is same.
The pneumatic load cell
works on the force balance
principle.
It uses diaphragm of a
flexible material, here piston
is absent. Pressurized air is
supplied to one side of
diaphragm and allowed to
escape through a position
controlled bleed valve.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

17. Strain gauge load cell
It works on the principle of
the elasticity i.e. when axial
force is applied, its column
gets compressed and when
force is released it regains its
original position.
The deflection shown by
detector can be directly
calibrated to read axial
force.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

18. Advantages of load cell
1. Load cell is small and compact in size.
2. Accuracy is very good. 3. It is less expensive.
4. It has good sensitivity and low hysteresis.
Disadvantages of load cell
1. Its performance is affected by non axial force.
2. It requires temperature compensation network.
3. Excessive stress or force may damage load cell
permanently.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

19. Piezoelectric force sensor
Piezoelectric materials are materials that produce an electric
charge under mechanical stress. The electric charge obtained is
proportional to the applied mechanical stress.
A charge amplifier can convert this charge into voltage signal.
Hence the output voltage of the device is proportional to applied
mechanical force. In most of
the piezoelectric force transducers, artificial quartz is used, so it is
also known as quartz force transducers. Fig. 2.3.5 shows the
construction of piezoelectric force transducer.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

20. Inline and Reaction Torque Measurement
A torque sensor or transducer converts torque into an electrical
signal. The most common transducer is a strain gauge that converts
torque into a change in electrical resistance.
The strain gauge is bonded to a beam or structural member that
deform when a torque or force is applied.
Deflection induces a stress that changes its resistance. A
Wheatstone bridge converts the resistance change into a
calibrated output signal. The design of a reaction torque cell seeks
to eliminate side loading (bending) and axial loading and is
sensitive only to torque loading.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

21. Torque Sensors
Slip Ring
The principle is based on torsion part of shaft and its torsion.
For a torque measurement we must place these strain gauges in
this way. For pressure, tension and bending measurement the
position of strain gauges are different for the transfer of the signal
from moving rotor to stator terminals the slip rings are used in this
way the signal is partly interfered.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

22. Rotary Transformer
The transducer consists of two elements components, one is
stationary and other is rotating.
One transformer is used to transmit the AC excitation voltage to
the strain gauge bridge as shown in Fig. 25 and second transformer
is used to transfer the signal output to the non-rotating part of the
transducer and then to the indicating device.
It consists of pair of concentrically coils with one coil rotating
within or beside the other coil which is stationary.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

23. Infrared Sensor
This is non-contact type measurement. The sensor is designed for
applications such as dynamometers and test stands where a high
performance, high noise immunity and non-contact torque
measurement is required.
It is used to power a circuit on the rotating sensor. The rectified
regulated voltage is then sent to a strain gauge bridge which
senses applied torque.
FM Transmitter
FM transmitters can unite any sensor to a remote data acquisition
system by digitizing signals and transmitting them to FM receiver
for analog conversion. Here a strain gauge and a small transmitter
are mounted on rotating shaft.
This radio signal is picked up by a receiver and decoded into analog
voltage. This voltage is amplified and sent to indicting device.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

24. Dynamometers
A dynamometer is a device used to measure torque absorbed or
torque transmitted by rotating member.
Types of Dynamometer
1. Absorption Dynamometers
Absorption dynamometers measures the power output of the
engine to which they are coupled by absorption.
Example is Eddy current dynamometer.
2. Transmission Dynamometers
In transmission dynamometers the power is measured by
transmission principle which is transferred to the load coupled to
the engine and then it is indicated on indicator . These are also
called torque meters.
Example is strain gauge transmission dynamometer.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

25. Eddy Current Dynamometers
When the solid rotor moves in the field produced by stator
windings an emf is produced in it resulting in a large loss of power
due to eddy current.
Advantages 1. They are small in size. 2. The absorption
power can be changed by changing d.c. circuit. 3. It can
measure high power output at all speeds. 4. Torque developed is
smooth and continuous under all operating conditions.
Limitation 1. It cannot produce any torque at zero speed.
2. It produces only small torques at low speed.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

26. Strain Gauge Transmission Dynamometer
In this type of arrangement 2 strain gauges are subjected to tensile
stress and while other is subjected to compressive stress.
Strain gauge 1 and 3 must be diametrically opposite to strain
gauge 2 and 4.
Due to torsion, strain gauge senses compressive as well as tensile
deformation.
Further these strain gauges are connected to Wheatstone circuit.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

27. Advantages
1. It is sensitive to torque.
2. It has full temperature compensation.
3. It provides automatic compensation for bending and axial loads.
4. It gives an instantaneous result.
Disadvantages
1. The slip rings may wear out and causes maintenance problems.
2. The device is generally expensive.
3. Initial settings need skill and is time consuming.
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.

28. THANK YOU
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Department of Mechanical Engineering Prof. Kokare A.Y.