Technology

Measurement of Surface Strain

(Strain Gauge Sensors)

This has been the conventional method for measuring rotating torque by using a piezoresistive (a material that changes resistivity depending on strain) strain gauge attached to the rotating shaft. Changes in strain, because of torque, are recorded as variations in an electric signal.

Disadvantages

Reliance on torsional strain inhibits their application to shafts of arbitrary torsional stiffness, and to offer a frequency response above 1kHz
Strains are too small (at most a few parts of 1000) to be measured directly, so complicated workarounds must be established: conventionally, four gauges are arranged in a Wheatstone bridge circuit
Slip rings or local telemetry are required to feed a continuous current to the gauges and acquire the signal from the bridge circuit. The use of slip rings can act to limit the maximum RPM of the shaft, and wear can significantly reduce system reliability. The use of local wireless telemetry can limit the maximum bandwidth of the system and can either be prone to signal drop-out or create significant electromagnetic emissions that may interfere with other sensors, electronics, and telemetry on the vehicle.
Strain Gauges sensors are typically not robust or resilient to harsh environments

Measurement of the Twist Angle

(Displacement Sensors)

This method typically uses a pair of identical toothed disks attached at opposite ends of a portion of the shaft enables a ‘twist angle’ to be determined from the phase difference between them through an optical or magnetic measurement, which in turn enables torque to be calculated.

Disadvantages

Requires the shaft to be rotating
Requires a reduced diameter section of the shaft known as a torsion bar to enhance the twist angle (several degrees at most for a length-to-diameter ratio L/D = 5)
Sensitive to temperature

Measurement of Magnetic Fields

(Magnetoelastic/Magnetostrictive Sensors)

This method for torque readings is achieved through the measurement of changes in magnetic permeability or the magnetic field that occur on regions of the shaft surface because of torsional stress from the applied torque. These variations in permeability or the generated magnetic field are measured by encircling the shaft in some pattern of coils of wires and observing differences in induced voltages.

Disadvantages

Permeability does not depend exclusively on torque and will vary with frequency, temperature, and magnetization
The magnetostrictive effect is highly non-linear
Local variations in magnetic properties of typical shaft surfaces limit the attainable accuracy
Calibrated electronics are specific to each calibrated shaft and sensitive to variations of the gap between the shaft and pickup coils
Impact to the shaft can disrupt the magnetic properties on which the calibration is based.
Technology limited to the instrumentation of components manufactured from steel
Susceptible to the influence of stray magnetic fields (i.e. close proximity of motors)

	Strain Gauge Sensors	SAW Sensors	Displacement Sensors	Magnetoelastic/ Magnetostrictive Sensors
Non-contact	NO ₁	YES	YES	YES
Torsionally stiff	NO	YES	NO	YES
Independent of T without need for compensation	NO	NO	In some cases	NO
Radially Mountable	NO	NO	YES	NO
Package-able in motorsport race conditions	NO	YES	NO	YES
Suitable for use is liquid environments i.e. oil	NO	YES	NO	NO
Frequency response	1kHz	Possible up to 8Kz, normally 1.7 kHz	500 Hz	2 kHz
Operational Speeds	NK ₂	0 – 30,000 rpm	NK ₂	0 - 20,000 rpm
Operational Temperature range	NK ₂	-40°C to+150°C	NK ₂	+20°C to +130°C
Static and Dynamic measurement	NO	YES	NO	YES
Suitable for medium to high volume manufacture	NO	YES	YES	NO
Suitability to a variety of steels and non-ferrite materials	YES	YES	YES	NO

1 - To have a non-contact capability requires inductive coupling, slips rings or electronic shaft telemetry
2 - NK = Not Known

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