Torque measurement: Laser system to save cost, space

January 20, 2015 // By Christoph Hammerschmidt
More efficient wind turbines, lighter racing cars and safer medical instruments are just three goals of an innovative measurement system currently under development by scientists from Hannover. The optical sensor will allow non-contact measurements of the position of a shaft and at the same time the torque applied to it. For practical tests the scientists are currently searching industrial partners.

Researchers from the IPH research institute and the Laser Zentrum (LZH), both located in Hannover, Germany, are developing a system that allows conclusions as to the efficiency of engines and generators. The efficiency of these systems can be measured by determining torque and shaft position. An example: If the driver of a car is pushing the accelerator pedal too rigidly, the wheels get spinning - the power of the engine is no longer transformed in thrust, the torque at the wheels falls down to almost zero. If the torque is measured constantly and the signal is processed in a feed-back loop to the engine control unit, it is possible to automatically reduce the torque slightly and keep the wheels from spinning. In motor sport, such as system is called launch control.

In wind turbines, the torque is zero if the rotor blades are standing still. If the wind pick up suddenly and a wind gust starts turning the wind turbine, the torque is rising quickly and strongly. Such a sudden increase of torque can damage the generator. The efficiency of the system is the highest if the torque applied is rather even. Permanently measuring and controlling can achieve through such an even torque. While systems to measure torque such as encoder disks or strain sensors are available, all these sensors have a major drawback: They need to be attached mechanically to the shaft, occupying valuable installation space. Non-contact measurement systems based on magnetic principles are very costly.

Therefore, the Hannover scientists are developing an optical system that works without mechanical contact and therefore does not need to be attached to the shaft. Thus, it also will be possible to be retrofitted to existing systems. The goal is to achieve a significantly higher resolution that existing systems while at the same time reducing its weight and size. One application where this would offer an advantage is motor sports where every gram counts.

The principle of the IntegrAD (Integrated optical absolute-value encoder and torque analyser) is rather simple: The shaft is encoded with optical absolute markers through a laser beam which creates very tiny and exact code lines. In the transmission, LEDs and photo diodes scan these codes. If a torque is applied to the shaft, it creates torsion - the shaft becomes somewhat twisted. The torsion can be measured through the absolute scale. Since the laser markings feature a very high resolution, it is possible to determine even very small changes, enabling the system to determine position and torque of the shaft in real-time and to forward these data to the engine control system.