How to use angle sensors to measure torque – Turck

It is difficult to measure torque on a continuously rolling axis, and there are several ways to do it. The most common method is to derive the torque from the energy required by the rolling shaft. In general, this means measuring the current of the motor that drives the rolling shaft. Although this is simple but not accurate, the current consumption is related to other factors, such as speed, supply voltage, bearing condition and temperature.

How to use angle sensors to measure torque – Turck

How to use angle sensors to measure torque – Turck

A more accurate approach would be to measure the deformation on the shaft using strain gauges, perhaps using an external acoustic wave (SAW) device. Although this approach is accurate, it requires either a sliding ring or a wireless method to transmit the strain gauge on the shaft to the outside world.

Strain gauges often have a large temperature coefficient and have a bad habit of loosening in harsh conditions. Measuring torque in a test chamber with strain gauges or SAW devices is generally fine, but it is not suitable for many industrial applications.

The torque of the engine is measured by changing the torque on the shaft and thus the phase displacement between two "multi-speed" resolvers mounted on the shaft and held in a straight line with the shaft being measured. Following the rolling of the shaft, two signals are generated per resolver, one according to the sinusoidal curve and the other according to the cosine curve.

If the loaded torque is zero, the signal from the two resolvers shows zero phase displacement. With the loading torque, it appears that the phase of one output signal is shifted relative to the other. The phase displacement and the loaded torque are directly proportional. If a multi-speed resolver is used with a large number of cycles, only a small change is required to cause significant phase shifts.

This makes the technology very sensitive and suitable for measuring changes at less than 1 degree or even less than 0.1 degrees. Two resolvers (one inside the other) can be arranged using a special flexible shaft or concentrically and the inside and outside of the shaft can be connected by a strong torsional tension spring.

With the development of sensor technology, more and more resolvers will be replaced by increasingly modern inductive encoder solutions. Inductive encoders operate on the same inductive principle as resolvers, but they use a printed circuit board instead of a wire-wound transformer structure. This is important to help minimize the volume, weight and cost of inductive encoders while maximizing their measurement performance. In addition, the inductive encoders offer easy-to-use electrical interfaces – DC power input and serial data output.

Inductive encoders are planned to have the same operational advantages as resolvers, as they are planned to use the same underlying physical properties. They are flat and have a large hole in the center, like a resolver, without the need for a slip ring. Inductive encoders can be used to measure torque and affirm viewpoints.

Since all the electronics of an inductive encoder are already in its stator, there is no need to specify and supply the electronics individually. The inductive encoder is also capable of supplying up to 4 million counts per revolution, so that even a small change in viewpoint can provide a very high resolution torque measurement. The thermal coefficient of an inductive encoder is smaller than that obtained by the strain gauge arrangement. Any dynamic distortion due to the high angular velocity of the shaft can be eliminated when the same clock signal is used to trigger the readings of both encoders.

With inductive encoders, there is no risk of damage to the equipment in the event of overloading or torque shocks. This technology also offers two types of measurement – viewpoint and torque – which are less expensive than measuring torque with strain gauges. Measuring torque with viewpoint sensors may be an old technology, but with the addition of modern inductive encoders, the use of sensing physics for viewpoint measurement has been revitalized, and this method is also very useful for torque and viewpoint sensing.

How to use angle sensors to measure torque – Turck

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