Torque Measuring

Torque fundamental concepts

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

Definition of Torque

Torque:

The vector product of position

vector and force called torque

sin

FrM

FrM

r

F

x

y

z

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.

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.

Stress and Strain

Strain ε is defined as the

relative change of length Δl / l

l

l

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)

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 stress

B

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

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

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

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

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.

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

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

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

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

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

UE

S

BB

Factorkk

m

mVUmVU

V

mVE SBB

][;][;][;][

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:

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

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

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

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

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

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.

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

Application Examples