Direct And Indirect Torque Measurement Techniques In Drive Train Technology

torque measurement

Torque is a type of force that tends to cause rotation. It is one of the most important mechanical quantities in many industrial applications. Precise measurement of the torque on rotating sections, placing high demand on test bench users and manufacturers. There are a couple of ways to determine torque:

Indirect Method

Measuring the power of the electric motor in the drive train is the indirect method of torque measurement. Calculation can be done along with measurement of rotational speed. Modern measurement and test gear makes it straightforward to establish rotational speed and electric power in electrical machines. However, there’s a scope of uncertainties and errors in measurement when it comes to measuring torque. One of those reasons include the machine’s operating states and power loss. It is tricky to do calibration as well.

For indirect determination of torque, a reaction force measurement test is employed. A force transducer is employed for measuring the force that’s placed on the lever arm’s end. Indirect measurement is done through measuring some supplementary quantities in the drive train. This incorporates all the methods involving those of the tension resulting from shaft’s torsion angle or shaft torsion surface.

Direct Method

Following this method, the torque signal is determined using direct measurement in the drive train. For contact-less measurement, generally torque flanges are used. Technology-wise, there are many advantages to direct torque measurement. Enabling easy integration of high quality transducers into test benches, the flange technology is known by a short design. Further benefits include the possibility to measure high rotational speed and higher measurement accuracy.

Selection Criteria For Torque Transducers

When choosing a torque transducer, there are a few things to keep in mind:

  • Maintenance and measurement requirements
  • Torque size to be measured, from the viewpoint of quasi-static processes.
  • Dynamic torque from rotary retardation and acceleration.
  • Moment of inertia and mass of transducer
  • Maximum oscillating torque and speed of rotation.
  • Required accuracy and parasitic load in the viewpoint of various aspects
  • Environmental effects: EMC conditions, atmospheric humidity, temperature, chemicals, fluids, foreign bodies and dust.
  • Dynamic torque from parasitic loads and torsional vibrational from axial and bending vibrations.

What Are The Drawbacks Of Indirect Method?

A good example of torque determination through measuring reaction torque employing force transducer, is a self aligning brake. This particularly requires sophisticated mechanics. Effects of interference such as lever arm’s expansion due to temperature variations or differences in the brake over a period of time needs to be considered, else they may lead to errors in measurement. Since large masses are involved and they literally act as a ‘mechanical low pass’, this method is not ideal for dynamic tests, and hence one should refrain from using it!

Determination Through Auxiliary Quantities

When estimating the torque through secondary quantities, for instance, torsion and strain angle, it is imperative to permit for each errors resulting from a measuring fault in the torsion angle and from diameter and input shaft length tolerances. On application of these methods, errors that were a result of limited temperature compensation should still be taken into consideration.

by George Chitos