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
Sensors Help Save Fuel
A unique hermetic sensor housing for SAW technology allows for more environmentally friendly automobiles.
Bernd Müller
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.

For the first time ever, the new sensor housing from Transense and SCHOTT consists of three parts: a base, a ring and a cover. Source: SCHOTT
”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.

Source: Transense
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.

Established in 1991, the British company Transense Technologies is breaking into the automotive market with its innovative saw technology. Photo: Transense Technologies
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t.pfeiffer@schott.com
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