SAW Technology

Competing Technologies

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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 Sensors)

This method for torque readings is achieved through the measurement of changes in magnetic permeability that occur on regions of the shaft surface because of torsional stress from the applied torque. These variations in permeability 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
  • Permeability is neither an intrinsic property of the magnetic material nor a single-valued, structure sensitive property
  • 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

Magneto elastic 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