A Strain Gauge

for Measuring

1Faculty of Science, University of Sirte

2Faculty

Abstract-Torque measurement is paramount in engine and

transmission testing, turbine testing, pump testing and testing of

gear trains and power measurement within propulsion systems.

Monitoring torque can be critical to the performance of axles,

drive trains, gear drives, and electric and hydraulic motors. Other

in-plant applications include gas and steam turbines. Really,

torque is an important factor in anything that rotates or spins on

a shaft, spindle or axle. In this research, the shaft w

for the application of a “V shaped” strain gauge;

torque on a rotating shaft. The implementation of the transducer

will be executed. This paper also discusses

transducer with Spider8 measurement electronics,

diagram of the measuring chain, main points of the measuring

software and the measurement accuracy. T

tested with simulated loads. The structure

possible for measuring torque with strain gauges on it. This saves

wear on the drive system and increases product quality.

I. INTRODUCTION

Although torque is unquestionably an important mechanical

quantity in the construction of machines; its significance is not

confined to that area alone. The precise measurement of torque,

particularly in rotating components, places heavy demands on

manufacturers and users of test benches. The situation is

further complicated by the trend towards improv

mechanical performance of modern engines by increasing the

rotational speed; coupled with a desire for greater accuracy in

such areas as the measurement of efficiency [2].

There is no denying that signals play a major role in our life.

A signal can be a function of time, distance, position,

temperature, pressure, torque, etc., and it represents some

variables of interest associated with a system. For example

an electrical system the associated signals are electric cu

and voltage. In a mechanical system, the associated signals

may be force, speed, torque, etc [1]-[4]. It is

that the most important quantity of a rotating machine is the

acting mechanical power. There are many methods to measure

this. By definition, the most straightforward

by measuring both the torque directly on the rotating shaft and

the rotational speed. There are many widely used measuring

methods and more or less standardized instruments for

measuring rotational speed; however

measurement device is specific for each application, since

must be built onto the machine’s structure.

Based Torque Transducer (Sensor)

Measuring Dynamical Load In-

M. Hilal Muftah1 and S. M. Haris2

Faculty of Science, University of Sirte, Sirte, Libya

aculty of Engineering & Built Environment

Universiti Kebangsaan Malaysia

43600 UKM Bangi, Malaysia

Torque measurement is paramount in engine and

transmission testing, turbine testing, pump testing and testing of

gear trains and power measurement within propulsion systems.

Monitoring torque can be critical to the performance of axles,

drives, and electric and hydraulic motors. Other

plant applications include gas and steam turbines. Really,

torque is an important factor in anything that rotates or spins on

the shaft will be adapted

” strain gauge; to measure the

implementation of the transducer

is paper also discusses calibration of the

transducer with Spider8 measurement electronics, the block

the measuring chain, main points of the measuring

The transducer is then

of a grinder makes it

possible for measuring torque with strain gauges on it. This saves

product quality.

NTRODUCTION

ough torque is unquestionably an important mechanical

; its significance is not

confined to that area alone. The precise measurement of torque,

particularly in rotating components, places heavy demands on

benches. The situation is

further complicated by the trend towards improving the

dern engines by increasing the

with a desire for greater accuracy in

such areas as the measurement of efficiency [2].

ignals play a major role in our life.

can be a function of time, distance, position,

., and it represents some

of interest associated with a system. For examples, in

an electrical system the associated signals are electric current

echanical system, the associated signals

It is undoubtedly true

rotating machine is the

acting mechanical power. There are many methods to measure

straightforward method would be

the torque directly on the rotating shaft and

the rotational speed. There are many widely used measuring

methods and more or less standardized instruments for

however for torque, the

device is specific for each application, since it

nto the machine’s structure. For many years

metallic resistance strain gauges have been used as principal

sensors for measuring torque on a shaft. Therefore,

gauge torque transducer is capa

[3]-[5].

Torque is defined as a measure

object that causes it to rotate.

about an axis, which is call

“Distance (r)” from the pivot point to the point where the force

acts is called the moment arm. Note that

a vector, and points from the axis of rotation to the point where

the force acts, as shown in Fig.1 [1]

Fig.1.

More generally, the torqu

“Position (r)” in some reference frame) can be defined as

the cross product:

Where “particle's position (r)

fulcrum, and “Force (F)” is the

“Magnitude (τ)” of the torque is given by

Where “Distance (r)” is the distance

to the particle, “ Force (F)”

applied, and “Angle (θ)” is the angle between

force vectors [16]-[17]-[18].

II. MATERIALS AND

A. Strain Measurement CStrain gauges change resistance in response to the surface

strains that they sense. The relationship between strain and

Torque Transducer (Sensor)

-Rotation

metallic resistance strain gauges have been used as principal

sensors for measuring torque on a shaft. Therefore, the strain

e torque transducer is capable to measure dynamical loads

measure of the force acting on an

to rotate. “Force (F)” rotates the object

called the “Pivot Point (O)”. The

from the pivot point to the point where the force

acts is called the moment arm. Note that “Distance (r)”, is also

a vector, and points from the axis of rotation to the point where

the force acts, as shown in Fig.1 [1]-[7]-[8].

. Definitions

More generally, the torque on a particle (located at

in some reference frame) can be defined as

(1)

)” is the vector relative to the

is the force acting on the particle. The

of the torque is given by

(2)

distance from the axis of rotation

)” is the magnitude of the force

the angle between the position and

ATERIALS AND METHODS

Strain Measurement Circuit ges change resistance in response to the surface

they sense. The relationship between strain and

Development, Energy, Environment, Economics

ISBN: 978-960-474-253-0 75

resistance is expressed by the gauge factor (G.F.) of the strain

gage foil, which can range from (2.0 -4.0)

foil material is constantan, (a 55% copper

alloy) having a gauge factor of (2.0). The

are converted into voltage changes by passive

The voltage is then amplified for signal transmission or display

[11]-[18]-[20].

Excitation is usually supplied to such networks

regulated power supply. The output of networks used with

semiconductor strain gauges may be large enough, for some

applications to obviate the need for amplification [2]

Wheatstone bridge circuits are used for most types of strain

measurements. The Wheatstone bridge is well suited for

measurement of small changes in resistance and is

suitable for measuring resistance change

(SG). It is commonly known that the strain ga

applied strain into a proportional change of resistance.

The applied strains is defined by

Where ( ) is the change in length and (l) is the original length.

The relative change of the resistance of a strain ga

described by the equation

Where ( ) is the change in resistance and

resistance. The "factor (k)", also known as the ga

characteristic of the strain gauge obtained

The exact value is specified by the strain ga

In general, the gauge factor for metal strain gages is about 2.

In the following, the Wheatstone bridge circuit

described with respect to its application in strain ga

applications. Two different presentations are

is based on the original notation of Wheatstone, and

is an alternate notation that can be more easily understood

person without an electrical or electronic engineering

background. Both versions are, in fact, identical in their

electrical functions.

Fig.2. Wheatstone bridge circuit

The most common Wheatstone bridge circuit used as a

strain gauge bridge is the four arm bridge construction

which each of the four arms is active, which means that each

arm contains a gauge that senses strain [2]-

B. Design of the Torque Transducer with Strain G

ge factor (G.F.) of the strain

). The most common

a 55% copper and 45% nickel

changes in resistance

are converted into voltage changes by passive circuit networks.

The voltage is then amplified for signal transmission or display

supplied to such networks by a

regulated power supply. The output of networks used with

semiconductor strain gauges may be large enough, for some

applications to obviate the need for amplification [2]-[16].

circuits are used for most types of strain

The Wheatstone bridge is well suited for

in resistance and is therefore,

suitable for measuring resistance changes in a strain gauge

strain gauge transforms

strain into a proportional change of resistance.

(3)

is the original length.

change of the resistance of a strain gauge is

(4)

and (R) is the original

known as the gauge factor, is a

obtained experimentally [2].

strain gauge manufacturers.

ge factor for metal strain gages is about 2.

, the Wheatstone bridge circuit is

with respect to its application in strain gauge

. Two different presentations are shown, (Fig. 2: a)

is based on the original notation of Wheatstone, and (Fig. 2: b)

can be more easily understood by a

person without an electrical or electronic engineering

. Both versions are, in fact, identical in their

bridge circuit

Wheatstone bridge circuit used as a

bridge construction, in

which each of the four arms is active, which means that each

-[9]-[16].

Design of the Torque Transducer with Strain Gauge

Fig.3. 28mm diameter

Referring to Fig. 3, the indicated strain can be calculated

using the following data:

“Electrical Power (5KW)”

(22.08Nm)”, “Diameter (28mm1)”, “Young’s Modulus (202

(ν=0.28)” [6]-[16].

Then the calculations can be applied as

For a full bridge circuit (four active strain gauge

Where:-

The strain of a single strain gauge

C. Torque Transducer Calibration

There are various methods of transferring the measuring

signal from rotating shafts. For a slow rate of rotation with only

a few revolutions of the measured

and unwinds provides the simplest solution. It is applicable to

all types of circuits [15]-[17].

A second method is the transfer of the bridge supply

voltage and the measuring signal using sets of slip rings. Only

high quality versions are suitable due to the requirement

extremely low contact resistance between the slip ring and the

brush. Low wear at high speeds and very low thermal voltages

are also demanded [6]-[19]-[21]

The calibration will be done with Sp

output of the “Voltage (mV

computer for each weight

weights in the Spider8 channel set up window

bridge must be selected with zero

28mm diameter shaft

he indicated strain can be calculated

KW)”, with “Torsion Moment

mm)” of shaft, “Speed (3000min-

(202KN/mm2)”and “Poison’s Ratio

can be applied as follows:

full bridge circuit (four active strain gauges)

(5)

(6)

(7)

(8)

The strain of a single strain gauge is then:

(9)

alibration

There are various methods of transferring the measuring

signal from rotating shafts. For a slow rate of rotation with only

few revolutions of the measured object, a cable that winds

and unwinds provides the simplest solution. It is applicable to

A second method is the transfer of the bridge supply

voltage and the measuring signal using sets of slip rings. Only

suitable due to the requirement for

extremely low contact resistance between the slip ring and the

brush. Low wear at high speeds and very low thermal voltages

[21].

The calibration will be done with Spider8 setup (Fig. 4), the

mV/V)” will be taken from the

increasing and decreasing the

Spider8 channel set up window. In Ch0, the full

bridge must be selected with zero loads; the tare function will

Development, Energy, Environment, Economics

ISBN: 978-960-474-253-0 76

be set in “Voltage (mV/V)”, all the results will be organized in

tables.

Fig.4. Spider8 measurement electronics

HBM supplies this type of equipment as slipping assemblies

for fitting to rotating shafts of various sizes with

brushes for separate mounting. The set contains five slip rings;

four for connection of the strain gauges and the fifth

a ground connection with the rotating shaft to prevent

interference. Slip ring assemblies (Fig. 5)

free end of a shaft can be supplied with 6 slip

[13].

Fig5. SK 6, slip ring

III. RESULTS AND DISCUSSION

A. Choosing Strain Gauge

All the required information about the strain gauge

indicated:

Gauge length = 2mm, Gauge factor = 2.05±1%

Gauge resistance = 120Ω ± 0.4.

Even though good contact materials

amount of variation of the contact resistance (contact noise) is

unavoidable with slip ring transmitters.

HBM states a contact resistance between the

the brush of “40 m Ω” with variations of

the contact resistance itself is relatively unimportant, variations

in its value are reproduced in the measuring signal

torque transducer will be connected to Spider8 which

will be connected to the computer [12]-[13]

6.

, all the results will be organized in

measurement electronics

HBM supplies this type of equipment as slipping assemblies

for fitting to rotating shafts of various sizes with two sets of

brushes for separate mounting. The set contains five slip rings;

ges and the fifth to provide

a ground connection with the rotating shaft to prevent

for mounting on the

free end of a shaft can be supplied with 6 slip rings [11]-[12]-

ISCUSSION

information about the strain gauge rea

Gauge factor = 2.05±1%

are used, a certain

amount of variation of the contact resistance (contact noise) is

ce between the slip ring and

of” < 2mΩ”. Whereas

the contact resistance itself is relatively unimportant, variations

oduced in the measuring signal. And the

connected to Spider8 which in turn,

[13], as shown in Fig.

Fig.6. Schematic of the torque transducer and its connections

To calculate the torque,

measured. For this, measuring weights

various weights will be used, which are

2, 2.5, 3, 3.5, 4 and 4.5 kg.

Weights have to be converted from

Newton (N)” as follows:

“Force (N)” = “mass (kg)” * “9.

Torque is calculated using the following formula:

Where:

F = “Force (N)”

l = “Length of the Lever (m)”.

Data will be taken three times

three series. The average of the three series

“Strain ( )”

The strain for each resistance

then be compared to the indicated strain calculated

Testing the transducer in real

the measurement of electrical power and speed.

In the Lab, the grinder will be

circuits, all instrumentation

voltage, and Spider8 will be connected to

Firstly, calibration testing

the results will be measured three times

next step.

Testing the transducer during operation

with 4kg loads of the corn used

measure the electrical power and speed.

The calibration value of “T

approximately determined, “Voltage

the line “Current (A)”, the “Electrical P

“Speed (rpm)” which will be

tachometer, will be used to read the speed during operation

optical ray is beamed directly

mounted on the bearing to read

To calculate the mechanical power the formula as shown

below can be applied:

Schematic of the torque transducer and its connections

To calculate the torque, the first strain gauge has to be

measuring weights are needed; in this case

used, which are respectively 0.5, 1, 1.5,

Weights have to be converted from “Kilogram (Kg)” to ”

“mass (kg)” * “9.81(m/sec2)”

Torque is calculated using the following formula:

(10)

.

taken three times, which will be tabulated into

The average of the three series will be calculated.

he strain for each resistance will also be calculated. It will

compared to the indicated strain calculated from (9).

real operating condition is made by

measurement of electrical power and speed.

will be connected to the electrical

, all instrumentation connections are for current and

connected to a computer.

will be conducted without load;

measured three times in preparation for the

ansducer during operation will be conducted

loads of the corn used to represent real conditions to

measure the electrical power and speed.

The calibration value of “Torque (Nm)” will be

Voltage (V)” between two phases,

the “Electrical Power (W)” and the

will be measured with a special

used to read the speed during operation. An

directly onto a small reference piece

read the shaft speed [14].

cal power the formula as shown

Development, Energy, Environment, Economics

ISBN: 978-960-474-253-0 77

· (10)

B. Setting Catman®

Express Software:

This software is associated with computer-controlled

measuring equipment, and provides a user-friendly interface

for the system functions. The software is an integral component

performing many functions, affecting every aspect of the

measurement process.

In the I/O input and output channel window, channel names

and detected channels are displayed, therefore the properties

torque, weight and time can be chosen. The setup and units

menu also appears in this window and the bridge and gauge

factors can be entered (bridge factor is 4, gauge factor is 2.05).

The setup assistant window displays the Ch0 full bridge

torque and, Ch1 full bridge against load. In the two windows

the “Torque value (Nm)” and “Weight (kg)” against the “Strain

value (mV/V)” are displayed, the testing will be done and the

results obtained.

IV. CONCLUSION

Most torque transducers use strain gauge transduction, some

use reductive transduction, and in other relatively recently

developed designs, use the difference in angular displacement

between the two ends of a torsion bar to obtain either a phase-

difference measurement (photo electrically,

electromagnetically or inductively) or a variable illumination

measurement.

A very important quantity of rotating machines is the acting

mechanical power. It can be calculated from the torque and the

shaft speed. Several kinds of torque measuring methods were

presented. There are many widely used measuring methods and

more or less standardized instruments for measuring the

rotational speed. A digital hand tachometer was chosen for

measuring speed.

Torque measurement is more complicated compared to

speed measurement. It requires a specific solution for every

application and it must be built into the machine’s shaft.

Because of its the high accuracy, relatively simple operation

and high dynamical behavior, the strain gauge sensor was

chosen for measuring torque on the rotating shaft. During the

design, the shaft was adapted to enable the application of strain

gauges. To the shaft’s end, a HBM made slip-ring assembly is

attached, connecting the rotating shaft to the standing cable and

the Spider8 measurement electronics. The HBM CATMAN-

Express software will be used during calibration and in the

actual tests.

ACKNOWLEDGMENT

This work is supported by the People’s Bureau of the Great

Socialist People’s Libyan Arab Jamahiriya, Kuala Lumpur,

Malaysia and Faculty of Engineering & Built Environment

Universiti Kebangsaan Malaysia 43600 UKM Bangi, Malaysia

and they are gratefully acknowledged.

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[4] Case, John, Lord and Ross, T.F.Carl, Strength of Materials and Structures with an Introduction to Finite element Methods, 3rd edition,

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Development, Energy, Environment, Economics

ISBN: 978-960-474-253-0 78