Methods of torque measurement in drive train technology

Torque is an important mechanical quantity in many applications. Measuring torque precisely, in particular on rotating parts, places high demands on test bench manufacturers and users.

There are two different approaches to determining torque: the direct and the indirect method.

Direct method

The torque signal is determined through direct torque measurement in the drive train. Normally, torque flanges are used for contactless measurement, for example, the T12 digital torque transducer or T40B from HBM.

The direct method of torque measurement offers many technological advantages. The flange technology developed by HBM is characterized by an extremely short design and enables easy integration of high-quality torque transducers into test benches. Further advantages include higher measurement accuracy and the possibility to measure higher rotational speed.

Indirect method

Torque is indirectly determined through measurement of the electric motor's power at the converter in the drive train.

Torque can be calculated together with a rotational speed measurement. Modern test and measurement equipment makes it easy to determine electric power and rotational speed in electric machines. However, relatively large errors and thus measurement uncertainties may be caused when calculating torque, since the power loss and the machine's operating states are also used in the calculation. Calibration is also very difficult. The reaction forcemeasurement method is used for indirect determination of torque. The force applied to the end of the lever arm is measured using a force transducer. Torque is determined indirectly through measurement of certain auxiliaryquantities in the drive train. This includes all methods involving measurement either of the strain resulting from shaft torsion on its surface or of the shaft's torsion angle. In both cases, torque is then calculated using the determined auxiliary quantity.

Fig. 1: Reaction torque measurement using a force transducer

Fig. 2: Reaction torque measurement / Cradle dynamometer

Drawbacks of the indirect method

Torque measurement using a force transducer

Determining torque through reaction torque measurement using a force transducer - e.g. a self-aligning brake (see Fig. 1) - requires complicated mechanics. Interference effects such as variations in the self-aligning brake over time or expansion of the lever arm resulting from temperature variations need to be taken into account; otherwise they may cause measurement errors.

Furthermore, this method is not suitable for dynamic tests because of the large masses involved, since they virtually act as a "mechanical low pass" (see Fig. 2).

Determination via auxiliary quantities

When determining the measurand torque using auxiliary quantities, for example, strain and torsion angle, it is essential to allow for individual errors resulting from diameter and input shaft length tolerances or from a measurement error in the torsion angle. In addition, errors resulting from lacking or limited temperature compensation need to be taken into account when applying these measurement methods.