A General Guide on Torque Transducers

What is a torque transducer?

A torque transducer, also known as a torque sensor, is a device used to accurately measure the amount of torque being applied. This torque could be from a torque wrench or torque screwdriver. There are a variety of designs of torque transducers measuring via different methods. There are also different types for different tasks, such as In-Line, Annular and Rotary, each being specifically tailored to a different use.

Static Torque Transducers

Static torque transducers are used to measure stationary torque, also known as static or non-rotational torque. One face of the static transducer is secured in a fixture, whilst the rotational element of the equipment under test (i.e. the drive on a torque wrench) is placed into the other face. Typical uses for static torque transducers include the calibration and verification of manual torque tools and verifying the torque output of rotary torque tools with additional various types of simulation fixtures.

Rotary Torque Transducer

In contrast, rotary torque transducers are used to measure rotational torque. The torque measuring shaft can rotate freely in the housing, and these rotary transducers will be rated to a maximum number of RPM (revolutions per minute). These use many of the techniques that we will mention in a minute. For strain gauge types they do require power and signal output to be transmitted and this can require rotary contact devices such as slip rings. Alternatively, they can use inductive coupling to energise them, usually by radio frequency (RF) techniques. For example, they are typically used for measuring motor and gearbox drives and power tools, such as impulse and continuous power tools.

How do Torque Transducers work?

So how do torque transducers actually work? There are several methods used within torque transducers to measure the amount of torque applied.

Strain Gauged Torque Transducers

AWS Torque Transducers make use of pairs of strain gauges within the transducer's housing to convert mechanical strain into a differential voltage signal. These voltage signals are then interpreted by a display to show your torque readings. AWS offer our Professional Transducer Display, which contains a chip which can display your torque readings via canbus.

The advantages of strain gauged torque transducers are that they use a very well tried and much proven technology with infinite resolution. They are also less prone to failure than other types of transducer due to their simplicity and robustness. Those types, including AWS products, that use a full wheatstone bridge configuration, are usually wired in such a way that they are able to only read torque, not bending, tension or compression. They are also very temperature stable when using compensated gauges for the strain material. Strain gauged torque transducers are usually highly accurate with long term stability. The design of the input and output mechanical interfacing influences the performance of the transducer as in many other technologies.

Optical Torque Transducers

Optical Torque Transducers use a light source and an aligned pair of gratings. As torque is applied, a small twist on the shaft causes a change in light amplitude by a shuttering effect. The detected light intensity then produces an electrical signal which in turn is interpreted by a display to indicate your torque reading. These transducers have a lower stiffness and increased torsional spring figure as they require a greater twist (angle deformation) within the shaft to alter the shutter setting, than with strain gauges. A diagram showing the layout of an Optical Torque Transducer is shown on the left.

 

Optical Reflective Phase Detecting Torque Transducers

Optical Reflective Phase Detecting torque transducers use timing/phase differences caused by sensing shaft twist. They are more commonly used in rotary applications, which usually require less resolution.

Inductive Type Torque Transducers

Inductive Type Torque Transducers use RF inductors/coils for non contact signal coupling and function by shaft twist altering the inductive coupling and therefore phase and amplitude changes for detecting the torque value. The inductive system usually uses radio signal technology. Because of the need for inductive coil attachments, they have a larger mass on the shaft and therefore a greater inertia, which influences the sensitivity of the system being measured. This is especially so in rotary torque transducers.

Magnetic Reluctance Torque Transducers

Magnetic Reluctance Torque Transducers use a particular magnetic property of a material when stressed. They are usually used in very defined areas of measurement in control systems, being low cost and not needing very accurate requirements. They are usually built into transport systems.

Surface Acoustic Wave Torque Transducers

Surface Acoustic Wave Torque Transducers use acoustic wave sensors, which are bonded to the shaft. When torque is applied to the shaft, the substrate of a polished piezoelectric crystal is stressed, causing the resonant frequency of this substrate to change. This resonant frequency change between the substrates can then be detected by a RF couple also found within the transducer. This can then be interpreted by a display to show the torque reading. These types are usually used in rotary torque transducers. An diagram of an example of a Surface Acoustic Wave Torque Transducer can be seen on the left.  

How often do torque transducers need calibrating?

Much like all other measuring equipment, torque transducers will need calibrating frequently to ensure they are providing accurate readings. According to BS 7882:2017, torque measuring devices such as torque transducers should be recalibrated at least every 2 years to ensure your torque readings are accurate and reliable. On top of this, if a transducer has been impacted (e.g. dropped), misused (e.g. overloaded), or repaired, it should be recalibrated as soon as possible, as it may have drifted from its calibration as a result.

During the calibration of a torque transducer, ambient temperature can have an effect on the stability of the readout torque values, so it is important to ensure that before use, the torque transducer has been powered in advance for up to 30 minutes for thermal stability. BS 7882 states that the temperature must not change by more than ± 1°C during the calibration. This will greatly improve the accuracy and stability of the transducer's readings.

By failing to calibrate your torque transducers at regular intervals, you are potentially compromising the accuracy of all tools tested and verified on the transducer. For this reason, having calibrated torque transducers is crucial. If your torque transducer is out of calibration, so are your tools calibrated by this transducer. Therefore, you cannot guarantee the torque values used to bolt critical joints are correct.

If you’re looking to bring the torque transducer calibration process in-house, we recommend you take a look at our Universal Torque Calibration Machine, which applies the torque using a motor and zero backlash gearbox rather than using beams and weights, reducing operator fatigue and time needed to complete a calibration.