SAW based TPMS - Theory and Practice Dr V Kalinin and Dr R Lohr Transense Technologies plc tire technology EXPO 2004 Stuttgart, Germany 23-25 March 2004
Business Model Transense is a technology transfer company Develops IPR in surface acoustic wave (SAW) based wireless, batteryless, sensors for the automotive market Two current business streams: Tyre Pressure Sensor (TPS) Systems: in-tyre and on-valve Torque Sensor (TQS) Systems: EPAS and Driveline Licenses Application companies (Tier 1s and Tier 2s) and Supplier companies to create virtuous supply chains Provides engineering support to licensees through: Provision of prototype systems Assistance in manufacturing process development Derives revenue from licences, engineering support and ongoing royalties from all licensees
Legislation Firestone tragedy - brought home the importance of tyres and is driving legislation TREAD Act - US is first with legislation (low tyre pressure warning from Nov 2003), but Europe and Japan are following the standardisation route Run-flat tyre technology, dominated by Michelin (PAX system) and its licensees, already requires tyre pressure monitoring by law No batteries means environmentally friendly as huge volumes - more than one billion new tyres sold per year - create disposal problems
Technical Collaboration Universities: Oxford Brookes University (SAW design) Manchester University (Microsystems/Thin Film) Cranfield University (Gyro/Thin Film) Sheffield University (Electric Vehicles) Oxford University (Testing & Nanotechnology) Nottingham University (Gyro) St. Petersburg University Russia (SAW Research) Institutions, etc.: National Physical Laboratory Instron Transense Technologies plc
Intellectual Property Transense Technologies plc In a business where potential application customers may be numbered in 100s of millions per year, the protection of Intellectual Property is vital 8 granted Patents - Transense has exclusive automotive rights worldwide granted in Europe, US and Japan A further 40 Patent Applications filed mainly covering SAW sensor applications for tyre pressure and torque measurement Core Patent covers concept of using SAWs for non-contact measurement Further Patents refer to ASICs, Couplers, Antennas and special devices
Competitive technologies in TPMS ABS-Indirect measurement: low cost but many drawbacks, e.g bendy roads, slippery road surfaces, more than one tyre deflating at once. Recently this method has been legally rejected in USA for TREAD Act purposes. Piezo-resistive, battery powered: current technology, relatively expensive, battery life and disposal are real limiting issues Piezo-resistive, inductively powered (125 khz): one or two systems are being launched, somewhat bulky, cost still an issue Resonant Crystal (modulated 2.45 GHz carrier), batteryless: one system near market, complex, cost? Giant Magnetic Impedance, batteryless: one system known to be in development, temperature stability may be an issue, cost?
Existing Licences Licensee Date Type Description SmarTire Sep 1999 Appl On-wheel tyre pressure 3DMl Dec 2000 Appl Michelin Jun 2001 Appl TT electronics Oct 2001 Appl Honeywell Apr 2002 Manuf Tai-Saw Oct 2002 Manuf Temex Microsonics Feb 2003 Manuf In-wheel tyre pressure In-tyre tyre pressure EPAS torque sensor Pressure sensor packaging SAW devices SAW devices Melexis Nov 2003 Manuf ASIC
Surface Acoustic Wave Sensors In 1887, Lord Rayleigh discovered the SAW propagation mode and in a classic paper predicted the wave properties Rayleigh waves have a velocity typically 5 orders of magnitude slower than the corresponding electro-magnetic wave - the slowest to propagate in solids Wave amplitudes are typically 1 nm and wavelengths 1-100µm Most acoustic energy is confined within 1 wavelength of the surface so SAW sensors have the highest sensitivity of the various acoustic wave sensor types SAWs typically operate between 50 and 950MHz
SAW propagation and energy distribution λ SURFACE WAVE PROPAGATION Z-AXIS DISPLACEMENT Y-Z PLANE Y OUTPUT TRANSDUCER Z Y Z X Y Z SAW SUBSTRATE INPUT TRANSDUCER X PIEZOELECTRIC SUBSTRATE STRAIN ENERGY DEPTH λ
SAW Sensing Element Transense Technologies plc One-port SAW Resonator Reflecting array of metal strips Number ~ 1,000 Period ~ 2 µm Inter-digital transducer Reflecting grating IDT h A Quartz λ/2
Interrogation Method typical resonator response
Measurement Method (1) Aim is to measure (a) strain in the substrate (b) temperature. Idea: f 1 f 2 f 3 F 1 = f(s, T) F 2 = g(s, T) F 1 F 2 f Diaphragm
Measurement Method (2) However, by novel mechanical design, the variables can be separated directly T1SAW T2SAW F 1 = f(s) F 2 = g(t) PSAW Thin film
Sensor characteristics before package optimisation: 1.2 ractional variation of F1 1 0.8 0.6 0.4 0.2 0-0.2 0 50 100 150 200 Pressure, psi 25 C -40 C 0 C 25 C 50 C 100 C 25 C
Characteristics of the sensor after package optimisation: 1.2 0.6 Fractional variation of F1 1 0.8 0.6 0.4 0.2 0-0.2 0 50 100 150 200 25 C -40 C 0 C 25 C 50 C 100 C 25 C Fractional variation of F2 0.4 0.2 0-50 -0.2 0 50 100 150-0.4-0.6-0.8 0 psi 2 psi 10 psi 40 psi 70 psi 100 psi 130 psi 150 psi Pressure, psi Temperature, C
Typical FEA graphic of circular sensor
SAW TPMS sensor embedded into a valve
Tyre Pressure SAW systems measure pressure with temperature compensation Attached to Wheel (e.g. back of valve or to run-flat system) Embedded in Wheel Embedded/attached to Tyre
Future Applications Transense Technologies plc Development of Wheel Corner Sensors to enhance vehicle stability control system Suspension Braking Drive Shaft Torque Bearing Tyre Wear and Tread Adhesion Tyre feel for electric steering Overall system integration via CANbus
SAW Sensors - Summary Transense Technologies plc Transense SAW sensors comprise typically: Double SAW Torque Device or Triple SAW Pressure Device SAWs sense pressure, temperature and torque by detecting minute strain changes Wireless operation enables continuous interrogation and display SAW sensors operate at high frequency - typically 100s MHz SAW offers high bandwidth and high resolution measurement SAW Interrogation using digital techniques can lead to the digital sensor SAW devices are small, light weight, robust, environmentally friendly (no batteries) and offer low costs in volume manufacture Applications include tyre pressure, EPAS and driveline torque measurement