Automotive Powertrain & Chassis Torque Sensor Technology Joseph Gierut Dr. Ray Lohr Honeywell Transense Technologies 2520 South Walnut Road 66 Heyford Road, Upper Heyford Freeport, IL 61032 USA Bicester Oxon, OX25 5HD, UK www.honeywell.com http://www.transense.co.uk/ Surface Acoustic Wave (SAW) Torque Sensing is an emerging technology for automotive, transportation, rail and other similar industries for use in powertrain and chassis applications. Significant research and development efforts have allowed for mass-production of SAW torque sensors at a cost-effective price (See Figure 1). Certain engine, transmission, driveline and chassis processes can often be controlled more precisely using SAW torque technology. Used in appropriate applications, complex control algorithm and system development, test, evaluation and qualification time can often be significantly reduced with real-time torque sensor measurement that can provide feedback for closed-loop control. SAW Torque Sensor Features: o Battery-less, wireless operation o Nominal resonant frequency 433MHz o High measurement bandwidth up to 1 khz o Small, lightweight design o Enhanced accuracy and resolution o Immunity to electromagnetic interference o High temperature operation up to 150 C o Robust packaging, durable design o Operates in many harsh environments o Long-term stability o Established manufacturing processes Figure 1: SAW Torque Sensor Numerous SAW Torque Applications Enabled by Flexible Mounting: Improving engine, transmission, driveline and chassis performance is a high priority for many major vehicle OEMs. Replacement of hydro-mechanical control systems with sophisticated control algorithms and sensor technology has been taking place for some time. Industry drivers for improving powertrain and driveline systems include: shift quality, fuel efficiency, weight reduction, emission standards and vehicle safety. Industry drivers for improving Chassis Electronic Power Assist Systems (CEPAS) include cost & system complexity reductions by decreasing the number of mechanical components to a single shaft. Indicative SAW Torque Sensor Applications and Potential Benefits: Electronic Transmission Control: Input/Output Shaft Torque & Transmission Component Applications Hybrid Vehicle Powertrain Control Potential to Improve Shift Smoothness, Vehicle Performance & Fuel Economy Electronic Driveline Control: Torque Management Systems (4WD & AWD Vehicles) Potential to Improve Vehicle Performance, Stability & Safety Engine Management: Direct Measurement of Engine Torque (Crankshaft & Powertrain Component Applications) Potential to Improve Vehicle Performance & Fuel Economy Steering Torque Management: Measurement of Torque in the Steering Column Chassis Electronic Power Assist Systems (CEPAS) Applications Potential to Reduce System Complexity
Surface Acoustic Wave (SAW) Technology: Surface Acoustic Wave (SAW) technology enables wireless, battery-less, non-contacting strain measurement often suitable for the measurement of torque, pressure, temperature and other parameters. The SAW propagation mode is characterized by velocities typically 5 orders of magnitude below electro-magnetic waves with amplitudes in the order of nanometers and wavelengths in micrometers. Most energy is confined to within one wavelength of the surface. These characteristics have made SAW devices often ideal for the design of delay lines and filters widely used in radar, TV and the mobile telecommunications industry. SAW devices are typically designed to operate within the frequency range 30MHz to 3GHz. However, unlike filters for telecoms, the SAW torque sensor technology presented here utilizes the influence of strain, mechanical and thermal, on the resonant frequency of a single port SAW resonator the first order effects being: i) a reduction in resonant frequency with increase in surface strain (distance the wave has to travel) and ii) a change in the wave propagation velocity. SAW torque sensor technology utilizes the principal tensile and compressive strains, which act at +/- 45 to the axis, on the surface of a shaft in torsion for the measurement of torque (See Fig 2). To RF coupler s xx s yy M Figure 2: Single quartz die featuring a pair of nominal 433MHz single port SAW resonators Typically, two SAW devices are used in one sensor and differential measurement of resonant frequency is performed in order to achieve temperature compensation and eliminate sensitivity to shaft bending (See Fig 3). S 11 f F1 F2 Figure 3: Difference frequency measurements from two diametrically opposed sensor pairs SAW devices operating at 433MHz are bonded to metallic packages in order to enhance protection from surface contamination. However, again unlike filters in mobile phones, it is a requirement to couple the devices into the external strain field rather than to isolate them. As a result the bondlines must be highly elastic with no micro-plasticity over the operational temperature range in order for the sensors not to exhibit unwanted creep or hysteresis. The development of this know-how has been an essential element in the development of successful sensors.
The choice of piezo-electric material for SAW sensors is principally driven by performance, availability and cost. Single crystal quartz has been chosen because it is typically widely available in standard 4 inch wafers, can be processed routinely and at low cost in high volume, using standard photo-lithographic techniques. Furthermore, quartz has a characteristic anisotropy, which can be often utilized by variations in cut angle and propagation direction in order to promote the maximization of sensitivity and provide first order temperature compensation. SAW Based Technology How it Works: Piezo-electric SAW sensors utilize an oscillatory electric field to generate an acoustic wave that propagates on the substrate surface, then transforms back to an electric field for measurement. SAW sensors use piezo-electric material to generate and sense the acoustic wave (See Fig 4). Single crystal quartz substrate Inter-digital transducer Reflecting arrays of metal strips: Number ~ 200 Period ~ 2micron Al metallization layer thickness ~0.1 micron Figure 4: SAW Single Port Resonator Each SAW resonator is interrogated by a short RF burst transmitted through the rotary coupler to the sensing element. The transmit pulse lasts long enough such that its spectrum is narrow so that only one SAW device is efficiently excited at one time. A centrally placed Interdigital Transducer (IDT) converts the pulsed electrical input signal from the interrogator to a mechanical wave through the piezoelectric effect. These waves propagate from the resonator to the reflectors and back. This propagation continues to build energy until a resonance exists as a standing wave. The wavelength is roughly double the IDT spacing. SAW resonators typically continue to oscillate for a noticeable amount of time after the transmit pulse has been switched off. It is this decaying oscillation that is re-transmitted from the sensing element through the coupler and picked up by the transmit/receive electronics of the SAW Interrogation Board (See Fig 5).
Interrogation Pulse Sensor Response Sampling Time Figure 5: SAW Torque Sensor Electrical Characteristics The resonant frequency is a function of strain in the IDT in the direction of the resonator axis. Once the input signal stops, the IDT absorbs the acoustic energy and transmits the new frequency back to the receiver in the interrogation system. SAW sensor interrogation involves the measurement of frequency difference shifts between active and reference SAW devices to provide common mode rejection. In addition, coherent accumulation and averaging typically improves signal to noise ratio, while the measurement algorithm and monitoring of statistical variances promotes valid data. This system uses patent protected methodology for sensing and reading/interrogation. Wireless Operation through Rotary Coupler: SAW Torque Technology utilizes antennas and non-contacting rotary couplers to transmit RF power in and RF signals out of rotating items such as shafts and discs. RF signals are then typically transmitted via coaxial cables, or they may be locally interrogated and the results fed via CAN, to a central display or to a system controller (See Fig 6). Rotary Coupler Torsion Load Stationary Coupler & Electronics Key Highlights: - Enhanced Accuracy - Compact Design - Flexible Mounting - Cost Effective Assembly Figure 6: SAW Torque Sensor Assembly The SAW Torque sensor utilizes a rotary coupler design working in the 400-500 MHz frequency range. This design feature enables contact-less, battery-less, operation between the rotating SAW torque sensor and the stationary interrogation assembly.
The SAW Torque Sensor is connected to the input terminals of the rotary coupler. The rotary coupler design is optimized for each individual application. Many coupler configurations are possible depending on torque measurement location and available space. A typical gap between the stator and the rotor rings of the coupler is 1 mm. Figure 6 illustrates a typical torque sensor assembly module. A key advantage of SAW torque sensors is mounting flexibility allowing for numerous applications. SAW Torque Sensors can be mounted to virtually any component in a system that experiences torsional load. This key benefit of the technology allows design engineers many options when incorporating torque-sensing measurement into existing system applications. Overall, in its envisioned applications, SAW technology often offers the ability to provide high bandwidth wireless measurement of torque in generic shaft and disc components, typical of the requirements of the automotive powertrain, over the full-specified operational temperature range. Enhanced resolution and accuracy coupled with low hysteresis and drift contribute to the constructor s ability to derive high quality signals suitable for closed loop control of modern autotransmission, driveline and chassis systems. Automotive Powertrain Torque Sensing System Control Optimization Modern electronically controlled transmissions typically utilize torque as the basis for transmission shift control. Today s systems often rely on empirically based look-up tables for torque measurement estimates. However, it is often difficult to accommodate all the potential factors that can affect the efficient operation of the powertrain system. New engine technologies, such as, cylinder de-activation and variable valve timing, sudden acceleration/deceleration, HVAC system on/off usage and various environmental effects all contribute to this issue, while service wear also affect both engine and transmission characteristics over time. Changes in acceleration during a power-up or downshift can often be significant. Power-off shifts are typically characterized by an audible noise in the driveline system. Achievement of both smoothness and durability in a transmission system design is typically expensive and space consuming. Powertrain Torque Control can often improve responsiveness and fuel efficiency without the "shift shock" and sluggish driving characteristics of many automatics. Significant fuel economy and performance gains are also expected benefits of more efficient automatic transmission ratio changing. It appears that more and more of the torque transfer that occurs during an upshift or downshift in today s passenger car automatic transmission is electronically controlled. Sensors that can detect pressure, temperature, position and speed have seen a rapid deployment, both internal and external to the automatic transmission housing. Electronic transmission control is a seemingly absolute necessity as passenger car automatic transmissions migrate from 3 and 4 speed planetaries towards either 5 to 7 speed gearsets or other new forms of efficient and compact gear trains. In addition to increased forward ratios the emergence of Advanced Automatic Transmission Systems such as Continuously Variable Transmission (CVT), Infinitely Variable Transmission (IVT), Automated Manual Transmission (AMT), Double Clutch Transmission (DCT), and Hybrid Electric Vehicles (HEV) often require significant sensor content for proper operation. The addition of real-time torque measurement promotes the optimization of power train system management. As 4WD/AWD vehicle systems become more sophisticated and fully active systems become more prevalent there is a pressing need for improved torque sensing capability in driveline applications (Including wheel corners). Increasingly accurate, active torque management can provide significant improvement over passive, electro-mechanical or hydraulic approaches. Torque management is expected to be a key factor regarding improved vehicle performance, stability and safety.
Real time torque sensor An attractive sensor for an automatic transmission would be a torque sensor that is capable of measuring real time torque at millisecond or better update rate. This sensor would be located at one or two essential points on the automatic transmission, i.e. at the input shaft or the output shaft or possibly both. Knowing the real time torque, the transmission ECU would have the information necessary to continually update the transmission algorithms to promote smooth and or fast shifts. All other material information can be typically derived from the engine sensors. With one or two torque sensors properly placed, the automatic transmission ECU would no longer need to infer shift timing from remote sensing points. The troublesome real time delays that take place between the transmission ECU, the pressure sensors and solenoid valves would no longer be a significant issue for the transmission designer. Existing torque sensors principally include resistance strain gauges (requiring slip rings for interrogation) and non-contacting sensors based on the principles of magnetostriction. However, as of this writing, torque sensors within the automotive world have been primarily used for the laboratory testing and evaluation of engine and transmission systems and their control algorithms. Over the past few years, research and development has been applied to the provision of an increasingly reliable and accurate, low cost torque sensor technology for vehicle OEM applications. Surface Acoustic Wave SAW sensors provide wireless interrogation for applications such as: transmission input shaft torque, transmission output shaft torque and driveline torque for 4-wheel-drive (4WD) and All-wheel drive (AWD) vehicles. Essentially, a torque sensor module can often be integrated within the powertrain, and torsional strain monitored, for proportional real time torque measurement and corresponding system adjustments. Design integration Transmission sensors often find application in a variety of challenging locations and environmental conditions. Many sensors are found outside of the transmission case, submerged in the oil cavity area or nestled near the rotating members. The internal sensors are typically subject to the same conditions and specifications as the friction devices, transmission oil and planetary gearsets. The operating environment can, in certain instances, have a thermal range of 40 C to 150 C with simultaneous shock, vibration, EMC, humidity, galvanic corrosion and exposure to hostile fluids. SAW torque technology offers a robust package configuration for proper operation in many such environments. A key factor regarding successful sensor design is knowing how the automatic transmission is constructed. There are three principal assembly directions: drop in component from the pump or torque converter opening, sensor installed along the outside of the case, or sensor placed in the ECM valve body package via the oil pan. Where possible the sensor design should not interfere or detract from the assembly pattern of an automatic transmission. With SAW torque technology a compact package design typically provides ease of integration with existing system configurations. Various sensor mounting options are available allowing engineers flexibility for system integration. Precise knowledge of the peak torques sustained by any element of an automotive powertrain will enable mechanical designs to be optimized in terms of the dimensions of shafts and disc components thereby offering weight and real cost savings. Conclusion Torque sensors and wireless interrogation based on surface acoustic wave (SAW) technology have been developed over the past decade to promote robust, accurate and cost effective monitoring and feedback for the enhanced control of tomorrow s automotive transmissions.