It's safer than you thought to belt up

Sensor Technology Ltd: Car seatbelts are now proven to be more reliable than previously thought thanks to a chance discovery by test rig builder Mindready Solutions in Belfast.

When building a replacement rig for a global seatbelt manufacturer they used a highly sensitive transducer from Sensor Technology instead of a standard one, which highlighted previously undetected large torque oscillations within the spring retractor mechanism.

“The oscillations were so massive that at first we thought there was something wrong with our rig,” recalls Mindready’s Nick Beckett, “but we went through everything with a fine toothed comb several times and our results were proven valid. Further tests on the seatbelts themselves then established that they are actually more reliable than they were thought to be.”

The test rig is really quite simple. A motor rotates the reel spindle through 15 turns and back, simulating the extraction and retraction of a standard belt to its full extent, and the transducer records the changes in torque or 'force curve' throughout the motion. The spring force generated is expected to remain more or less constant throughout the travel excepting a critical ramp, section where the force rapidly changes over less than one turn. The rest of the equipment is concerned with control, data logging and analysis.

The new production rig is designed to run at 400rpm in order to maintain the required end of line throughput, hence the need for a high performance transducer. Beckett was responsible for specifying the transducer and considered several options before deciding on a TorqSense unit from Sensor Technology in Oxon.

TorqSense is based on the Surface Acoustic Wave (SAW) phenomenon and is in effect a frequency dependent strain gauge. The surface waves are produced by passing an alternating voltage across the terminals of two interleaved comb-shaped arrays, laid onto one end of a piezoelectric substrate. A receiving array at the other end of the transducer converts the wave into an electric signal.

The frequency is dependant upon the spacing of the teeth in the array and as the direction of wave propagation is at right angles to the teeth, any change in its length alters the spacing of the teeth and hence the operating frequency. Tension in the transducer reduces the operating frequency while compression increases it.

To measure the torque in a rotating shaft, two saw sensors are bonded to a shaft at 45deg to the axis of rotation. When the shaft is subjected to torque, a signal is produced which is transmitted to a stationary pick up via a capacitive couple comprising two discs, one of which rotates with the shaft, the other being static. The frequency of the oscillation used is typically 200MHz.

One of its great advantages is that it imparts no load to the machine it is monitoring, because the sensing head mounted on the rotating shaft uses wireless communications to download the constant data stream to the stationary pickup.

During the seatbelt tests the sensitivity of the transducer needed to be progressively damped to meet the established customer limit criteria.

Changes in test speed from 15rpm to 400rpm were investigated and the TorqSense output proved to be consistent over this range. Set to several different levels, damping was tried and the results were averaged over different numbers of runs.

“The results clearly showed that the TorqSense device was capable of delivering higher levels of accuracy and sophistication than had previously been applied to product design verification and test,” says Beckett.

“Test results have to be precise to ensure that the seat belts will operate with the required degree of safety. Our rig tests belts for global automotive majors, and they never settle for second best when it comes to safety.”