SIDEWALL TORSION SENSOR (SWT)

The sidewall torsion sensor (SWT) was designed and developed by Continental Inc. in Germany. It provides information about the forces on a tire that can help the driver handle the dynamics of a vehicle [1]. The tire sidewall is magnetized along its entire circumference with alternating north and south poles. Both the longitudinal and lateral forces acting between the tire and the road surface cause deformation. This deformation and the resulting forces can be estimated based on the alternation of the magnetic poles monitored by two sensors attached to the chassis, one is located near the zenith of the tire; the other is placed at the level of the magnetized tire sidewall.

If there are no longitudinal forces applied to the tires alternation between the magnetic poles occurs simultaneously at both sensors. Thus the time difference between the signals emitted by both sensors is zero. However, if longitudinal forces, such as braking or accelerating, are present; the maximum magnetic fields pass the sensors at different times, resulting in a phase difference between the two sensor signals. This phenomenon is the same during acceleration and deceleration, except that the polarity of the readings are reversed. The magnitude of the longitudinal force acting on each of the four tires depends on the phase difference and their relationship is approximately linear. The information provided by SWTs can be used to improve control systems such as ABS and TCS. The wheel velocity can also be determined from the information delivered by the two sensors. Furthermore, the sensor system is insensitive to the effects of dirt [1]. SWT is not capable of measuring the vertical force and friction force acting on the tire.

References

[1] Jim Gill, "Continental Teves and Team of Developers Working Hard to Ready 'Intelligent Tire' for Mass Production," PRNewswire, Aug. 24, 1999.