SCHOTT solutions no. 2/2009 > Sensor Technology
On the way to greater driving comfort and lower fuel consumption: It will soon be possible to equip automobiles with up to ten of the new saw sensors.
Photo: SCHOTT T. Bauer
Photo: SCHOTT T. Bauer
Sensors Help Save Fuel
A unique hermetic sensor housing for SAW technology allows for more environmentally friendly automobiles.
Automotive manufacturers have been waiting for these for quite some time: torque sensors that allow for exact metering of driving power, shifting operations, and steering movements and thus increase driving comfort and reduce fuel consumption. However, in the past, no sensor has ever been able to meet the demands with respect to performing exact measurements and remaining hermetic over its operating life – at least a quarter of a million kilometers. In working together with the British sensor technology company Transense Technologies plc, SCHOTT Electronic Packaging has now developed an entirely new three-part housing for a SAW (Surface Acoustic Wave) sensor that is capable of meeting all of these demands.
Automatic transmissions are becoming progressively smoother. However, despite the duplex clutch, passengers still feel a noticeable jolt while shifting gears and reaccelerating. New transmissions attempt to reduce this by using a clever electronic control system, nevertheless erratic changes in torque can still occur during shifting, especially as the vehicle and transmission ages so that the original calibration becomes less and less accurate. The reason is that it has not yet been possible to produce sensors that are capable of directly measuring the torque inside the drivetrain. The attempts made by various manufacturers to produce torque sensors from two housing parts have failed because, in order to perform exact measurements, this housing must be both elastic, as well as completely hermetic.
For the first time ever, the new sensor housing from Transense and SCHOTT consists of three parts: a base, a ring and a cover.
”The solution that Transense and SCHOTT have now developed is a unique housing that combines a metal with a high elastic limit for transmitting the torque to the sensor with an annealed metal suitable for hermetic glass-fritted elec- trical feedthroughs,” explains David Vile, Business Manager of Torque Systems at Transense plc. Unlike conventional housings, for the first time ever, this unit consists of three parts: a base, a ring and a cover.
The round base disk consists of a stainless steel that is not hardened by heat treatment, but rather only by mechanical processing. This, in turn, ensures that the metal remains elastic, even when it is subjected to high stresses. As a result, the linear strain is transferred to the torque sensor that is bonded to its inner surface. The second component of the housing made of annealed stainless steel contains openings for the two connector pins that are hermetically sealed by glass fritting inside an oven. The ring is actually welded to the base before the sensor is added. Then, once the sensor has been added, the housing is enclosed with a stainless steel cover welded in place.
The heart of the torque sensor is a strain sensitive SAW device. SAW stands for Surface Acoustic Wave and refers to the principle by which high frequency mechanical vibrations can exist on a solid surface – basically analogous to waves on top of the sea. ”The Transense sensor consists of a preformed piezo-electric quartz die on which up to 3 resonators are deposited using well-established photo-lithographic technology,” explains David Vile. In response to a nominal 433 MHz interrogation signal, this passive sensor returns signals at the natural frequencies of the resonators, which are directly related to the mechanical and thermal strains on the quartz die from which torque and temperature can be derived. Transense is also taking a new route with respect to data transmission, because, in order for the sensor to be able to measure torque accurately, it has to be welded or bonded directly to gear shafts or disk components and therefore be exposed to oil at high temperature. Sliding electrical contacts are unreliable in this environment; therefore Transense is successfully relying on non-contacting signal transmission that uses radio frequency couplers.
Transense Technologies and SCHOTT Electronic Packaging have been cooperating on an ongoing basis since 2002. SCHOTT and Transense started working together to develop the three-part housing for a torque sensor that is now ready for mass production back in 2004. Transense is currently negotiating with automobile manufacturers and Tier 1 and 2 suppliers and is quite confident that vehicles can soon be equipped with up to ten SAW sensors. ”These would not only allow for smooth and gas-saving shifting of gears, but also engine monitoring, increased traction by torque vectoring and improved control of electrical power assisted steering (EPAS),” notes Graham Storey, Commercial Director of Transense plc. <|
Established in 1991, the British company Transense Technologies is breaking into the automotive market with its innovative saw technology. Photo: Transense Technologies