Torque sensor

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A torque sensor is a sensor for recording the physical measured variable torque . The torque indicates how strongly a force acts on a rotatably mounted body. The torque is given in the unit Nm. Torque sensors are divided into two different classes:

  • Static torque sensors
  • Dynamic (rotary) torque sensors

The torque measurement on a measurement object is usually always an indirect measurement. Most of the measuring principles are based on the detection of the strain or tension properties of a material. The strain or stress characteristics of materials are generally proportional to the torque at a known working point or working window. If you leave this working point or the defined window, it may happen that this proportional relationship no longer exists and meaningful measurement is no longer possible. There are various technological options for acquiring a torque on a measurement object:

  • Strain gauges (DMS)
  • passive magnetoelastic strain measurement ( magnetostriction )
  • active magnetic-inductive strain measurement (inverse magnetostriction)
  • fiber optic strain measurement

Static torque sensors

A static torque sensor records the torques on a measuring body or a measuring point which - when a force is applied - does not move. In these applications, the torque sensor can be mechanically coupled directly to the measurement object. In the case of static torque sensors, the measurement setup is relatively simple compared to dynamic torque measurement, since the sensor can be supplied with energy through simple cabling and the measurement signals can be tapped.

For the measurement of static torques and the calibration of torque sensors, there are standards and regulations for the measurement setup and the measurement process, but not for dynamic torque sensors in which the measurement setup changes constantly due to rotation. This means that the systems available on the market are tested and calibrated according to the manufacturers' internal guidelines.

Dynamic (rotary) torque sensors

Dynamic (rotary) torque sensors record the torques on measurement objects that rotate when a force is applied . Rotating the measurement object presents the user with the challenge of recording the mechanical stresses or strains of the measuring point on the one hand and supplying the measuring point with the necessary energy for the measurement and forwarding the measurement data on the other. Two basic options have been developed to implement such a measurement:

  • contacting measuring systems (DMS)
  • contactless measuring systems ( magnetoelastic sensors)

Contacting measuring systems are mechanically connected to the measuring point by screwing or gluing. The energy supply is realized either via slip rings and sliding contacts or via wireless energy and information transmission. These systems have the disadvantage that they are often expensive and complicated to integrate, but they deliver a reliable measured variable due to their technological maturity. Contactless measuring systems interact with the measuring point either via magnetic fields or acoustics . Both technologies offer the possibility of interacting with matter by generating a stimulus . The interaction consists in evaluating the response of the stimulus due to the change caused by the matter (measuring point) and extracting the desired information from it.

Technologies for torque sensors

Strain gauges

The strain gauge (DMS) is a technology that has established itself in industry and measurement technology over the past 30–40 years. Its constant further development has meant that the system is very reliable and is used in many applications - often without alternative. Strain gauges are also often used to measure force . The disadvantage of the strain gauge in torque measurement is the necessary reliable mechanical coupling to the measuring shaft. In most applications, the strain gauge is bonded to the measuring point with a special adhesive. This connection is necessary because the expansion of the measuring point on the metal structure, the resistive change of which is recorded, has a strong influence on the accuracy of the measurement. Changes in this connection over time lead to the measurement error or the offset and the sensitivity changing and thus the measurement becoming inaccurate.

This weak point and the fact that the sensor stuck to the measuring point has to be supplied with energy either via a complicated radio system or a slip ring, is the reason why torque sensors are usually only found in test bench applications today. Most applications in series products depend on inexpensive and robust systems that cannot be implemented with this technology.

To measure the torque, in the preferred arrangement, four strain gauges are glued to the surface of the shaft, which is usually used as the measuring object, two of them at an angle of 45 ° upwards and two at an angle of 45 ° downwards. Depending on the direction of rotation, two strain gauges are always stretched and two strained, whereby the strain gauge resistances change accordingly in opposite directions. The four strain gauges are interconnected in a full bridge for signal evaluation, which is supplied by an appropriate voltage. The bridge output voltage is then largely proportional to the torque within the usual measuring ranges.

Active magnetic-inductive torque sensors

The first patents were registered as early as 1960, describing a technology that is capable of measuring torque without contact with the measuring point. An alternating magnetic field is coupled into the measuring point via an inductance . The susceptibility of the measuring point changes when the force acts on the measuring point. This change affects the permeability of the material and thus the magnetic conductivity. This change in magnetic conductivity can be detected with secondary inductances and converted into a measurement signal that is proportional to the applied torque.

This technology did not make its breakthrough until the 2000s, since high-performance electronics are required to generate and measure high-frequency alternating fields. The need in electromobility, especially for pedelecs or e-bike applications, for torque sensors is one of the drivers for the constant further development of this technology.

Web links

Individual evidence

  1. ↑ DAKKS: Calibration of torque sensors . DAKKS, accessed in 2010 .
  2. ↑ Development and investigation of force transducers with thin-film strain gauges . August 30, 2016 ( ptb.de [accessed January 23, 2018]).
  3. ↑ Jörg Böttcher: Online Compendium Measurement Technology and Sensor Technology: Torque Sensors. Retrieved August 25, 2019 .
  4. ↑ A. Schwersenz, P. Cörlin, C. Leiser, T. Kitzler, T. Senkbeil: P3.5 - Contact-free electro-magnetic reactance based mechanical tension sensors . In: Proceedings Sensor 2017 . May 30, 2017, doi : 10.5162 / sensor2017 / P3.5 ( ama-science.org [accessed January 23, 2018]).
  5. ^ Nahum Kipnis: Chance in Science: The Discovery of Electromagnetism by HC Oersted . In: Science & Education . tape 14 , no. 1 , January 1, 2005, ISSN  0926-7220 , p. 1–28 , doi : 10.1007 / s11191-004-3286-0 ( springer.com [accessed January 23, 2018]).
  6. ↑ FREDERICK T. CALKINS, ALISON B. FLATAU AND MARCELO J. DAPINO: Overview of Magnetostrictive Sensor Technology * .