Torque Sensors Explained

An expert's guide to torque transducers and torque sensors. Types of torque sensors and how torque sensors work.

1. Transducer Specialists… APPLIED MEASUREMENTS LTD appmeas.co.uk | ([phone]

2. Torque Transducers & Torque Sensors Explained APPLIED MEASUREMENTS LTD An Expert’s Guide appmeas.co.uk

3. Torque is a twisting or turning force about an axis, that can be applied in a clockwise or counter-clockwise direction. WHAT IS TORQUE? appmeas.co.uk

4. APPLIED MEASUREMENTS The most common torque sensor measurement principle uses bonded strain gauge technology, where the strain gauges are bonded to a suitably designed shaft. HOW A TORQUE TRANSDUCER WORKS appmeas.co.uk

5. APPLIED MEASUREMENTS When torsion is applied to the shaft causing it to twist, shear stresses are induced. These are measured by bonding the strain gauges at 45° to the horizontal torque axis. HOW A TORQUE TRANSDUCER WORKS appmeas.co.uk

6. APPLIED MEASUREMENTS As the shear stress induced in the shaft is the same throughout its length, the strain gauges can be bonded at any point along its length. HOW A TORQUE TRANSDUCER WORKS appmeas.co.uk

7. APPLIED MEASUREMENTS Typically four strain gauges are bonded and connected into a Wheatstone bridge configuration with temperature compensation components included within the bridge circuitry. GETTING AN ELECTRICAL SIGNAL FROM THE TORQUE TRANSDUCER appmeas.co.uk

8. APPLIED MEASUREMENTS With an excitation voltage applied to the bridge and torque induced into the shaft, an electrical output linearly proportionate to that torque will result. GETTING AN ELECTRICAL SIGNAL FROM THE TORQUE TRANSDUCER appmeas.co.uk

9. APPLIED MEASUREMENTS The completed Wheatstone Bridge requires a stable DC supply to excite the circuit. This is usually 5Vdc or 10Vdc but can be any value from 1Vdc up to 18Vdc. WHEATSTONE BRIDGE CIRCUIT appmeas.co.uk

10. TORQUE SENSOR INSTRUMENTATION appmeas.co.uk These low-level millivolt signals are compatible with a vast range of bespoke strain gauge sensors and their associated instrumentation.

11. TORQUE SENSOR INSTRUMENTATION appmeas.co.uk These instruments include:

12. TORQUE SENSOR INSTRUMENTATION appmeas.co.uk These instruments include: Digital Displays

13. TORQUE SENSOR INSTRUMENTATION appmeas.co.uk These instruments include: Digital Displays Analogue Signal Conditioners

14. TORQUE SENSOR INSTRUMENTATION appmeas.co.uk These instruments include: Digital Displays Analogue Signal Conditioners Digital Amplifiers

15. APPLIED MEASUREMENTS Typical analogue amplifiers will generate a higher level voltage (0-5Vdc, 0-10Vdc) or current (0-20mA, 4-20mA) for onward processing. ANALOGUE SIGNAL CONDITIONERS appmeas.co.uk

16. EMPLOYEES: +/- 25 APPLIED MEASUREMENTS Digital amplifiers typically provide an RS232, RS422 or RS485 output using either the common ASCII protocol or one of a specialised, industry-specific protocols that have been developed such as Modbus. DIGITAL AMPLIFIERS

17. APPLIED MEASUREMENTS Torque measurements can be either Static/Reaction or Rotary. A static or reaction torque measurement involves little or no rotation of the item being measured, such as during torque spanner testing. STATIC / REACTION TORQUE VS. ROTARY TORQUE appmeas.co.uk

18. APPLIED MEASUREMENTS A rotary torque measurement would be subject to continuous rotation. This rotation could be constant, such as a system measuring the torque on a stirrer in a mixing vessel or periodical such as measuring the torque produced by an electric screwdriver. STATIC / REACTION TORQUE VS. ROTARY TORQUE appmeas.co.uk

19. APPLIED MEASUREMENTS Dynamic torque is subject to variation or change. For instance the torque on the motor drive shaft of an electric hover mower would be subject to dynamic torque. First, as you turn on the power to make the blade spin up, then as you move the mower around your lawn between areas of varying grass density or from cut to uncut grass. DYNAMIC TORQUE appmeas.co.uk

20. SLIP RING ROTARY TRANSFORMER (Inductive Loop) WIRELESS RADIO TELEMETRY POWER & SIGNAL TRANSMISSION IN ROTARY TORQUE SENSORS appmeas.co.uk

21. APPLIED MEASUREMENTS The first rotating strain gauge torque transducer employed slip rings to make the electrical connections from the casing to the rotating shaft. The normal procedure is to use coin silver for the slip rings and silver graphite for the brush gear. SLIP RING appmeas.co.uk

22. APPLIED MEASUREMENTS The conductive rings rotate with the sensor and use a series of sprung brushes to contact the rings and transmit the electrical signal. Slip rings only minor drawbacks in that the brushes and to a lesser extent, the rings, do wear and as such have a finite life. SLIP RING appmeas.co.uk

23. APPLIED MEASUREMENTS At low speed the electrical connection between the rings and brushes are relatively noise-free, however, at higher speeds electrical noise will eventually degrade their performance. The maximum speed (rpm) for a slip ring is determined by the surface speed at the interface between the brush and ring. SLIP RING appmeas.co.uk

24. APPLIED MEASUREMENTS The rotary transformer system consists of two coils. One static coil that is attached to the transducer’s housing and one rotating coil that is attached to the transducer shaft. ROTARY TRANSFORMER (Inductive Loop) appmeas.co.uk

25. APPLIED MEASUREMENTS There is no contact between rotor and stator. Both transmission of power to and signal back from the rotating strain gauge bridge circuit is incorporated. Rotary transformer type torque sensors offer typical inaccuracies of ±0.2% and are capable of speeds up to 50,000rpm. ROTARY TRANSFORMER (Inductive Loop) appmeas.co.uk

26. APPLIED MEASUREMENTS As this design incorporates bearings, the maximum rpm is more than the slip-ring design but limited by the maximum specified performance of the bearing. The system can also be susceptible to noise and errors, which are induced by the alignment of the transformer primary-to-secondary coils. Specialised signal conditioning is also required in order to produce a signal acceptable for most data acquisition systems. ROTARY TRANSFORMER (Inductive Loop) appmeas.co.uk

27. APPLIED MEASUREMENTS The avoidance of rotor and stator coils means that installing wireless torque sensors is a simplified process over other types as there is no cabling provision or coil alignment to consider. The measurement signal can be transferred over considerable distances. WIRELESS RADIO TELEMETRY (Battery Powered, 2.4GHz) appmeas.co.uk

28. APPLIED MEASUREMENTS The data stream provided by the wireless telemetry system is broadcast and so can be read by multiple receiver units with different functions such as digital displays, analogue outputs and PC-based USB acquisition making multi-featured systems quick and easy to set up. WIRELESS RADIO TELEMETRY (Battery Powered, 2.4GHz) appmeas.co.uk

29. A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power and protecting the connected equipment from the damaging effects of misalignment, vibration and shock overload. MECHANICAL COUPLINGS appmeas.co.uk

30. APPLIED MEASUREMENTS The torque transducer will accommodate only a small amount of misalignment, but ideally, no misalignment should exist between input and output shafts. Severe misalignment will lead to premature bearing failure due to the repeated stresses developed in the contact areas between the bearings and the raceway, even if the coupling is properly mounted and maintained. MECHANICAL COUPLINGS appmeas.co.uk

31. APPLIED MEASUREMENTS In correctly aligned shafts, the bearings will be subjected to loads less than the established rated loads for the bearings in which case will result in extended bearing life. Popular types used are disk (or plate) / bellows / diaphragm / elastomer / grid / gear and many other proprietary names given by different suppliers. MECHANICAL COUPLINGS appmeas.co.uk

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