Benefits of SAW Technology
The principal benefits of Transense resonant SAW sensing systems are the abilities to measure engineering parameters such as torque, pressure and temperature on rotating components wirelessly and passively – that is to say no power has to be separately applied to the sensor as it gets the energy to excite the SAW and transmit its response from the interrogating RF pulse.
- As a result, torque measurement in automotive powertrains and electric power assisted steering (EPAS) systems, or pressure measurement in car or truck tyres, or dynamic torque / force / pressure / temperature measurement in industrial applications, is straightforward in principle.
- Conventional strain sensing technologies, eg. foil strain gauges applied to components in order to sense torque, require either direct wired connections with slip rings to enable transmission of power and signal across the rotary / stationary boundary, or on-board electronics comprising typically a battery, local signal conditioner and receive / transmit radio components.
- Even when the battery is replaced by a kinetic energy harvesting device, there still needs to be energy storage (small rechargeable battery or capacitor) on the component with attendant weight and cost.
Further SAW benefits include:
- Low Mass: a typical Torque or TPMS button package weighs 2 grams. This is very beneficial in motorsport applications where every gram counts.
- Small Size: enables the addition of one or two buttons to existing components such as shafts or disks with only minimal intrusion or modification. Modern automobile engines, transmissions and drivelines are engineered to minimum size and weight so that free space is always a rare commodity.
- No Special Material required for the component: Again the structural materials in modern automotive designs are optimised for strength, fatigue life and cost. The only requirement for an effective SAW sensor is that the material be elastic, ie. free from plastic deformation in use, which is the normal specification for any structural component.
- No Sensitivity to Magnetic Fields: Many automotive applications require that torque sensing is required in proximity to electric motors and solenoids. In addition the earth’s magnetic field varies with altitude and proximity to mountains.
- Mechanically Rugged: In manufacturing environments, components may be knocked or dropped applying high shock loads. In service, high speed rotation involves very high centripetal forces often thousands of “g”. Also in service significant vibration levels may be present. SAW sensors have demonstrated considerable tolerance to these loadings.
- Good Dynamics: SAW sensors can be sampled at 2 kHz enabling for example, torque measurement in engines and powertrains every few degrees of shaft or disk rotation (eg. every 3 degrees at 1000 rpm).
- Good Measurement Accuracy: SAW torque sensors can deliver better than 1% of full scale accuracy over the temperature range -40°C to +125°C together with low hysteresis and drift.
- Cost Effective: A typical Transense SAW torque or TPMS button is an intrinsically low cost device. It contains no electronic components, just a stainless steel can and a quartz die.
The above resonant SAW sensor system benefits are not all achieved by the competing torque sensing magneto-elastic (M-E) technologies. In these systems, a shaft is magnetised or has a magnetic ring fitted tightly or deposited thereon. Surrounding the shaft, a sense coil detects changes in the magnetic field within the shaft or ring from which the torque in the shaft may be determined.
M-E sensors are susceptible to unwanted magnetic fields, however they can be shielded. Where the shaft itself is magnetised then there are special material requirements. Mechanical knocks can cause changes in the shaft’s magnetisation. There are also practical minimum coil size issues, axial lengths are typically 30 – 50 mm. Achieving low hysteresis and zero stability are significant measurement challenges.
In principle, RF pulses, circa 433MHz, excite SAW resonators deposited on a piezo-electric quartz die, which ring at a natural frequencies determined by the mechanical and thermal strain applied to it by the component on which it is mounted. The back scattered RF signals can be analysed to measure the frequencies and determine the mechanical strain and temperature.
Resonant SAW sensing systems are particularly relevant to measuring dynamic engineering parameters, especially on rotating components, eg. pressure in tyres and torque in powertrains.
There are a number of other benefits including small size and mass, good measurement performance over a wide temperature range and dynamic band and tolerance to extraneous loads and stray magnetic fields.
Transense resonant SAW sensing systems compare favourably with competing strain sensing technologies.