Evolution and future of torque measurement technology

by Dr. Wilfried Krimmel

1. Strain Gauge (SG) Measurement Technology

Tension strut stressed under force FThe historical evolution of torque measurement technology began in the year 1678. In this year, Robert Hooke described the proportionality between the material extension and the associated material tension in the well-known Hooke's law.

Hooke's law

For further evolution, only year 1833 has a relevancy again.
The bridge circuit was then described by Hunter-Christie, by which smallest voltage changes can be measured. Nevertheless the switching received the name of the second inventor Wheatstone later, the actual fame belongs to Hunter Christie.

Bridge circuit formula by Hunter Christie

In 1856, Thomson, who later was called Lord Kelvin (the temperature scale was named after him), discovered the coherency between mechanical strain e of a resistance wire and its resistance change.

Coherency between mechanical strain e of a resistance wire and its resistance change

Coherency between mechanical strain e of a resistance wire and its resistance changeThereafter, experiments with resistance wires were consistently carried out; e.g. in 1917 Nernst experi-mented with them in order to measure pressure on combustion engines.

However, the first model of an easy to apply SG was not available until year 1938. Then, the first SG was developed by Prof. A.C. Ruge. Already three years later, the first industrially produced wire-SG was available on the market and spread very fast.

In 1952, the new foil-SG guaranteed the acceptance of industrially manufactured SG-sensors. It was etched by foils, coated with resistance material. Today, SG's are manufactured in the same way. The first foil-SG's for torque measurement were offered in the same year.

Therewith, stationary SG-torque sensors were manufactured. These sensors helped to solve many problems in development and experiments by means of reaction momentum measurement. However, measurements in the rotating shaft line are the most important and also the most frequent applications for torque sensors. Here, the evolution still lasted some more years in order to be able to offer serviceable SG-torque sensors on the market.

Principle for an angle-measuring torque sensor
Principle for an angle-measuring torque sensor

2. First Rotating Torque Sensors

If a shaft is loaded below an axial torque, it twists to an angle, proportional to the torque. This angle can be measured with an angle measuring system. According to this principle, the first rotating torque sensors with inductive measuring systems were already produced and offered on the market in 1945. Carrier frequencies of hundreds of kHz were used to supply the sensors. Therewith, the necessary coil systems were small. The amplitude of the AC voltage- measurement signal was proportional to the rotation angle of the measuring system and had the same frequency as the supply voltage.

The first transfer of the measurement signals of a SG-bridge which was applied on a rotating shaft was carried out with slip rings in 1952.

Principle sketch for a transformator turning transducer
Principle sketch for a transformator turning transducer

The transfer of the supply voltage and output voltage through slip rings requires certain wariness. The slip rings must be isolated from the shaft and from each other, even smallest insulation faults can cause considerable measurement errors. The surface pressure of the sliding contacts must be chosen in such a way, that on the one hand a preferably small transition resistance, sufficient safety against lift-off due to vibrations and out-of-roundness of the slip rings is available, but on the other hand that too strong heating and too strong abrasion does not occur. Next to the material selection, thorough processing of the surfaces is arbitrative.

Difficulties are to be expected in particular at high circumferential velocities. Some sensors are equipped with lift-off fixtures for the brushes which are attached for the measurement only. Disadvantage of this technique: the slip rings and coal brushes wear out and hence need to be renewed.

In order to retain a sensor with stable and maintenance-free signal transmission, a torque sensor was developed which enables slipring-free transfer of the measurement signals of a SG-bridge. Through AC supply of the bridge, an amplitude modulated AC voltage, proportional to the torque is obtained at the output of the bridge. Both, the AC voltage necessary for supply, the SG bridge and the measurement signal can be transmitted via a rotating transformer.

Sectional View through a Slipring Sensor
Sectional View through a Slipring Sensor

Therewith the triumphal procession of the rotating torque sensors on SG basis could not be stopped anymore.

In 1971, by electronics becoming invariably smaller, it was possible to integrate an amplifier onto the rotating shaft which was used for the supply of the SG-bridge and for the processing of the measurement signal. A rotating transformer was used for the supply of the sensor; the second one for the frequency-modulated transfer of the measurement signal.

In the meantime, SG-technology was advanced as well. Today, the sensors are produced temperature-compensated and creep-compensated. The great advantage of SG-technology consists of the fact that the compensation of the disturbance signals can be carried out at the point of measurement directly. The temperature dependence of the elasticity modulus of the used materials e.g. steel is approx. 3% per 100K temperature change. Since this disturbance signal enters into the characteristics of the sensor directly, it must be compensated accordingly.

Block Circuit Drawing for Rotating Sensors with AC-Supply
Block Circuit Drawing for Rotating Sensors with AC-Supply

At angle-measuring sensors a compensation, if at all, is only carried out the in the amplifier, thus a thermal behaviour of the characteristics is always reckoned. Furthermore, angle-measuring sensors have to fight the problem that they require relatively large angles of twist for the measurement of torque, which leads to torsion-supple arrangements which only allow slow measurement procedures.

By the electronics becoming continously smaller and hence improved transmission possibilities of the measurement signals, the market has changed to that effect that today many torque sensors are provided with integrated measuring amplifiers.

3. Modern Rotating Torque Sensors

The first torque sensors normally had analog signal outputs. At these interfaces, disturbances by adjacent power modules and drives are possible, especially in case of long feed lines and high dynamics For this rea-son in the past, the signal level of the sensor was increased; signal levels of ±5V and/or ±10V are usual. However, for many applications the interference immunity is not sufficient. The solution is a digital sensor electronics. Its principle mechanical assembly is shown in the following sectional picture.

Modern Torque Sensor with Integrated Electronics
Modern Torque Sensor with Integrated Electronics

Transformer Turning Transducer with ElectronicsLocated on the shaft is an area, reduced at the diameter, to which the SG-bridge is applied to. The rotating part of the rotating transformer and the rotating electronics is attached on the shaft as well. The fixed part of the rotating transformer and additional electronics are located in the housing. For the connection of the sensor a connector plug is implemented in the housing.

The integrated electronics has a µC, both in the stator and in the rotor with an appropriate memory. The measured value consumption occurs on the rotor by means of SG, the signal is immediately amplified and digitalized there. This digital signal then proceeds into a µC which prepares it for the transfer to the stator in the form of a serial word with checksum. The data signal gets conditioned In the stator and then converted for a serial RS 485 interface in a µC.

By the use of µC's, data such as serial number, calibration values, measuring range, calibration dates etc. can be stored and read out on demand from the shaft as well as from the stator.

The supply of the sensor takes place by a power supply unit, monitored by the µC, which can switch to a calibration control for the check-up of the sensor as well. A very high reliability of the measuring device is achieved by storing and read-out of the sensor data and the direct digitalization of the measurement signal at the point of origin.

Block diagram for a digital measurement signal transfer with integrated processors:

Block diagram for a digital measurement signal transfer with integrated processors

4. Areas of Application for Torque Sensors

Today we cannot imagine to proceed without torque sensors in many areas. Here is a small extract of areas of applications.

Education Development Testing Production, Quality assurance, Product monitoring Other applications

Electro motors

  • Characteristics
  • Performance
  • Torque

Combustion Engines

  • Characteristics
  • Performance
  • Torque

Test stands

  • Reel test stand
  • Electro motors
  • Combustion engines
  • Transmissions
  • Generators
  • Fans
  • Pumps

Measuring units

  • Torque sensor in versatile measurement configurations
  • Sliding roof
  • Windowlifter

Rehology

  • Rheometer

Torque sensors built in automobiles

Control, regulation

  • Tightening moment of screwcaps

Alignment

  • Bearing pre-load
  • Spring pre-load
  • Axial displacement

Screw technique

  • Tightening moment
  • Loosening moment

Examination of function

  • Crank torque
  • Generator
  • Pump
  • Pressure intensifier
  • Fan
  • Conveyor belt
  • Electro motors
  • Combustion Engines
  • Bearing friction
  • Shrink fit
  • Sealings (friction)

Rheology

  • Rheometer

Test stands for endurance tests

  • Hand-held tools
  • Household appliances
  • Automobile
  • Combustion engines

Wind force plants

  • Performance
  • Torque

Bore tower

  • Torque in bore rods

Positioning drives

  • Locking characteristics of valves

Agriculture

  • Torque controlled machines

Ships

  • Ship shafts

As you can see, torque sensors are used in all fields, from training to product design, from production, quality assurance to product monitoring. Even in farming, torque sensors can be found in the machinery. For the traceability verification of measurements, reference torque sensors are used more and more in order to check the production equipment on site, directly.

Basic Set-up for Motor Test Stand
Basic Set-up for Motor Test Stand

4.1 Application Case Motor Test Bench

For engines and driven hand tool applications, a torque sensor and a load unit is required. The average performance data in the continuous operation is acquired there. These data provide information about the correct function of the components, e.g. homogeneity of the magnetic rotor fields in the electric motor. However, information about controlling characteristics of the drive can also be determined by dynamic load.

4.1.1 Application case Test Stand for Combustion Engine
Torque Sensor with coupled Joint Shaft
Torque Sensor with coupled Joint Shaft

Here the sensor is coupled to the brake directly. The coupling of the combustion engine occurs via a joint shaft. By this, the alignment of the specimen is considerably simplified. Furthermore, the oscillations of the engine are not transmitted onto the sensor as strong. As you can see, there is a burst protection and a protection against contact positioned around the sensor and the joint shaft in order to assure sufficient protection from contingently detaching parts.

Test Stand for Combustion Engine
Test Stand for Combustion Engine

5. Future of Torque Sensors

The SG technique will be the primary solution for torque sensors in the future. By the electronics becoming smaller and electrically more stable, the sensors can be designed for higher spring rates which leads to im-proved dynamics of the measurement. The continuous progress in the amplifier technique allows to boost smallest measurement signals exactly and nearly error-free. The improved measurement signal conditioning can be used for a higher accuracy of the testing equipment layout as well.

The future also belongs to intelligent sensors with stored metrological data, whereby the measurements be-come more and more reliable and the data for quality assurance can be recalled from the sensor directly.

Association for Sensors and Measurement AMA

Authorised Economic Operator AEOF

DAkkS-Calibration Laboratory ILAC und DAkkS

Quality Management ISO 9001