Torque starting concepts

1. Torque Measuring
Torque fundamental concepts

2. Outline
 Basics
 Physical definition of torque
 Use of torque measuring
 Mechanical basics
 Different methods to measure torque
 Principal assembly of strain gage torque transducer
 Strain gage and measure bridge
 Function
 Compensation

3. Definition of Torque
 Torque:
The vector product of position
vector and force called torque
sin

FrM
FrM
r
F

x
y
z

4. Uses of Torque Measuring
 Test Rigs (electric, combustion)
 Power
 Efficiency factor
 Torque over angle rotation measuring
 Etc.
 Screw application
 Detent torque
 Calibrate torque key
 Etc.

5. Stress and Strain
 Strain is caused by torque
Elastic strain
If balanced forces act on a solid body from outside, a change in form
and volume occurs, which can be measured.
Once these forces stop acting the distortions go back completely,
provided that the deformation has not exceeded a certain limit
(adherence to the elastic range).
Otherwise the body of measurement becomes useless.

6. Stress and Strain
 Strain ε is defined as the
relative change of length Δl / l
l
l


7. Hook’s Law
With torsion applies to shear stress :
The modulus of elasticity E
is the proportionality factor.
It is a material constant
with the unit [N / mm²].
It influences the sensor performance
regarding its sensitivity. The module
itself is influenced by temperature.
At one axle stress condition the following
relations apply :
 = Normalspannung
shear stress
shear module
slip
normal stress
modulus of elasticity
Hook’s range of the curve can be written as:
µ = poisson’s ratio
(0,3 by metal)

8. Stress and Strain
 The elastic range is influenced
by strain and the modulus of
elasticity. There is an overstrain
in the plastic range, making the
body of measurement useless
elastic range
plastic range
Hook’s
straight line
B = break strength
S = elastic limit
= break stressB

9. Conversion English/Metric
Symbol When you know Multiply by To find Symbol
lb-ft Pound-feet 0.1383 kilogram meters kgm
0z-in Ounce-inches 720 gram millimeters gmm
kgm Kilogram meters 7.233 gram millimeters lb-ft
gmm Gram millimeters 0.0014 Ounce-inches oz-in
lb-ft Pound-feet 192 Ounce-inches oz-in
Nm Ounce-inches 0.00521 Pound-feet lb-ft
Nm Newton-meters 8.85 Pound-inch lb-in
Kpm Kilopond-meters 7.233 Pound-feet lb-ft
Kpm Kilopond-meters 86.8 Pound-inch lb-in

10. Different Methods to Measure
Torque
mechanic
(oscillating string)
resistive
strain gage
surface wave
resonator
piezo electric
resistive
thick film sensor
magneto elastic
Ring magnet
optical
Pattern
recognition
Photo elasticity
capacitive
capacitive
strain gage
angle difference shield
optical
phase shift
Differential
transformer
yoke
Eddy-current
coil slotted sleeves
eddy current
segment disks
absolute
capacitive
differential
transformer
resonators
Mechanical Strain Torsion Angle

11. Torque Measurement via
Differential Transformer
 Due to torsion a coil leaves
its original position and like
that the induction/coupling
changes.
 The illustration shows an
idealized coil system, in
practice the system is
positioned on a radius, i.e.
the signal is not linear.
Mt = 0
Mt ≠ 0

12. Torque Measurement via
Differential Transformer
 Applied for over 50 years
 Very small measuring ranges
possible
 No active components, low
inductivities
 High temperature range
 Accuracy is less than strain
gage transducer
 Mechanical construction is very
complex

13. Torque Measurement with
Magneto-Flexible Toric Magnet
 Passive system
 Low cost
 Less accuracy (0,5-1%)
A magnetic field is generated by a ring of magneto elastic material, which is magnetized
in circumferential direction. Influenced by stress a magnetic reorientation occurs,
and an extern magnetic field develops, which is measured with magnetic field sensors.

14. Torque Measurement using
Torsion Angle
In the range of elastic strain the following
correlation between the torsion angle  and
torque M applies :
At circular-cylindrical shaft
applies:
Polar moment of inertia
Shear modulus 80.000 N/mm² at steel).
Length of the torsion sector

15. Principal of Torque Transducer
Torque measuring
body
strain
strain-
gage
mV/V V
ADC transmission processing conversion
V Digit Digit Digit
Voltage
Frequency
RS-232C
Field Bus

16. Strain Gages
 In the strain gage method the
electric resistance of a wire is
changed by the influence of
strain ε. The length l of the
wire is increased to the
length l+dl. The diameter D
is reduced by the amount dD.
The change of resistance of
a wire is proportional to the
strain of the wire.
Fig:
Standard strain gage
Cover
Connecting bands
measuring grid
Support
length of
measuring grid

17. Wheatstone-Bridge
 Minor changes of Resistance generate a output voltage
 KSM uses exclusively the full bridge. In a full bridge R1 and R4 must
be strained and R2 and R3 compressed.
UB US
R1
R2
R3
R4

18. Wheatstone-Bridge
 Bridge sensitivity EB is the quotient of bridge voltage UB and bridge
supply voltage US
 The bridge voltage UB is proportional to the strain, which is
measured
1000


k
U
U
E
S
B
B
Factorkk
m
m
VUmVU
V
mV
E SBB



][;][;][;][

19. Example for Strain Gage
Measurement
strain gage
width b
Ehb
Fl


 2
6

 On the upside and bottom side
of a bending beam (width b and
height h) strain gages are
arranged at l intervals from the
power flow. In the illustration the
upper strain gage is strained
and the lower strain gage is
compressed.
 The strain ε of the bending
beam at the position l is
calculated as follows:

20. Application of Strain Gage in
Industry
 The measuring technology with strain gage is universally
applied in practice. This method helps to obtain maximum
accuracies (0,1% or better)
 Advantages:
 Can be applied in calibration devices
 No drift, static measurement are possible
 Disadvantages:
 Application of strain gage and the evaluation of the signal are complex
 An energy supply is required
 The ambient temperature for STM torque sensors is maximally 100°C
 Strain gage application at STM is only performed by
specialists in a clean room

21. Strain Gage Application
 Marking lines on the measuring body to place the strain gage
 Pasting the strain gage on the measuring body
 Wiring the strain gage
 Cover the strain gage
 Make the compensation

22. Strain Gage Application and
Clean Room
Clean Room Cell Class 100
Particle size Particle no. / dm³ Particle no. / dm³
inside outside
1,0 µ 0 ~110.000
Fig:
Endoscope for quality
assurance +
humidity sensor
Fig:
Workplace for
strain-gage application

23. Pasting in the Clean Room
 The body of measurement is
cleaned carefully
 Glue is thinly brushed on the strain
gage and contact spot on the body of
measurement, then the product is
aire for 10 minutes
 The strain gage is precisely
positioned on the contact spot and
pressed on
 Soldering tags are affixed together
with the strain gage in the same
manner

24. Pasting in the Clean Room
 Strain gages are covered with Teflon foil and silicone rubber
pad.
 Contact pressure is applied
 The applied body of measurement is hardened in the oven
 The body of measurement is cleaned carefully again

25. Wiring
 The strain gage bridge is
interconnected with wires of the
same length and the leads
soldered On.
 The connecting leads are
covered with Coat.
 Wires must fit closely to the
body of measurement. After the
wiring, a temperature
compensation and a zero point
compensation are carried out.

26. Compensation
 Copper wire (positive
temperature coefficient) is
connect in series to one of the
resistor to compensate the
temperature influence
 Manganin wire (zero temperature
coefficient) is connect in series to
compensate Zero-Point
UB US
R1
R2
R3
R4

27. Application Examples