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The Inherent Benefits of SAW SensorTechnology

Introduction:

Surface Acoustic Wave (SAW) resonant sensor systems have been the subject of development by Transense Technologies over the past 10 years for a variety of measurement applications. First it may be useful to review the basic technology and physical characteristics of the system:

SAW Technology:

SAW devices, as deployed by Transense, are realised as crystalline quartz substrates (dies) typically 6 x 4 mm by 350 microns thick. On each die up to 3 resonators, with natural frequencies around 433MHz, are laid down in thin film aluminium, using photo-lithographic techniques. Each resonator comprises a central inter-digitated transducer (IDT) with a series of reflecting strips distributed on either side. Overall resonator dimensions are 2 to 3 mm long x 0.3 to 0.4 mm wide. The spacing between individual features in the IDT and reflectors is of the order of 2 microns so you need a microscope to resolve the fine detail. SAW resonators respond to both mechanical and thermal strain by changing their natural frequency of vibration.

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In practice, Transense SAW dies are either bonded directly to structural components to sense local strains or packaged inside small stainless steel buttons. The buttons, typically 11 mm diameter by 3 mm thick, are then either bonded or welded to structural components. Because the resonators would be adversely affected by dust or contamination, it is preferable to use hermetically sealed button packages in most automotive or industrial sensing applications.

The piezo-electric nature of crystalline quartz means that an oscillatory electrical input to the IDT will induce a mechanical vibration (SAW) on the surface of the die. Further, when the electrical input signal is stopped, vibration of the SAW resonator at its natural frequency persists for perhaps 20 micro seconds and a portion of that mechanical energy will be converted, due to the piezo-electric effect, back into an electrical signal at the IDT.

Transense have developed SAW Interrogation Units (SIU) comprising patented electronic hardware and embedded software. The SIU generates a radio frequency (RF) pulse which is transmitted to the IDT of the SAW sensor, exciting the resonator into mechanical vibration. When the interrogation signal is paused, the SAW resonator "rings" at its natural frequency. The returning (back scattered) electrical signal is received by the SIU, which analyses it determining the natural frequency of the SAW sensor and hence the surface strain on the die.

Interrogation can be via wired connections or by non-contacting means - either a broadcast radio signal or by use of RF couplers. A coupler comprises a stator and a rotor which may be a pair of disks or a pair of co-axial cylinders. The rotor is mounted directly to the structural component and rotates with it. It carries a 360° microstrip which is wired to the SAW sensor. The stator is mounted to the chassis side and, as its name suggests, does not rotate. It too carries a 360° microstrip, which faces its partner on the rotor, and is wired to the SIU. Electromagnetic coupling allows transmit / receive signals to pass between the couplers so the SIU can interrogate the SAW sensor.

By mounting 3 resonators in specific locations and directions on a single SAW die, it is possible to determine both mechanical strain and temperature independently. The mechanical strains may be directly converted, via traceable calibration standards, into engineering parameters, such as pressure or force or torque, depending on the design of the sensor button and its mounting orientation on the structural component, eg shaft or disk. The accurate sensing of temperature, on the same die, enables these parameters to be temperature compensated over the range -40°C to 125°C and significantly higher for specialist applications.

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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.


Conclusions:

This article has reviewed resonant SAW sensing technology as developed and applied by Transense Technologies plc.

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.