All torque and no contact

A non-contact torque sensor claims to offer significant price savings over proprietary designs. Mark Fletcher reports

A torque sensor concept, if mass produced, could be up to 90 per cent cheaper than other technology currently on the market.

Instead of a strain gauge, the new design from Siemens in Germany uses light to measure torque on a rotating shaft.

The researchers use a laser source, concave mirrors and detectors to arrive at a simple concept which is unaffected by contamination and will easily retrofit to existing machinery.

It is estimated that 30 per cent of UK energy consumption is by electric motors in some shape or form. This means that there is a lot of rotating machinery out in the world of industry. Improved control of these machines would benefit from some kind of torque feedback. The applications base for this technology is therefore very large and widespread and, with the potential to offer 90 per cent cost savings, it will interest a great many designers.

The method of operation of the sensor is fairly simple in concept but the design has had to take into account the high speeds and accuracies demanded by modern equipment.

Two small concave mirrors are attached to the shaft to be measured. A few tens of millimetres from the shaft are sited two laser diodes which transmit a constant light directly onto the mirrors. As the shaft rotates, the mirrors reflect this laser light towards two detectors.

The two-element photodiode detectors are split into two equally large parts by a desensitised section a few micrometres wide. Signal processing is used to ascertain the torque value. This is achieved by signal processing hardware which creates a time signal between the instances at which the two elements receive the same quantity of light.

"Using a zero-point detector, that point in time can be determined to within nanoseconds," researcher Dr. Reinhard Maier explains. "The advantage of this measurement technique is that it works independently of fluctuations in light intensity. As a result, the accuracy of the measurements is not affected by dirt in the optical system or the ageing of the laser diodes."

To start with, a measurement is taken at no load with the staggered reflectors creating two signals slightly apart from each other. This time interval is constant in a no-load situation and is used as the basis for under-load measurements. When the shaft is under load it will exhibit a twisting motion which will alter the relative positions of the mirrors to each other. By using the differences in time intervals between no-load and under-load conditions, it is possible to determine the torque.

"Since the rotation amounts to only a few hundredths of a degree, the measuring instrument has to be very sensitive," Maier says. "This is achieved through the use of concave mirrors." The mirrors can turn a small movement into a much larger one if the system is appropriately calibrated. Maier explains the phenomenon as being similar to the shadow cast by an object nearer a light source being bigger than one further away.

The sensor can be installed without much effort, it measures even the smallest torsion (0.0003 degrees) and it can withstand temperatures up to 700 degrees C. It is also impossible to overload it, unlike strain gauges which can break under high load conditions, and it does not require expensive telemetry equipment. Siemens is currently looking to take the design further either in house of by licensing it to an outside party.