Reliable torque sensing at low speed or low capacity

In the selection of a rotary torque sensor for a given application, the datasheets for particular products are often a good place to start, detailing the maximum torque capacity and maximum speed for the technologies under consideration. Those figures on the datasheet won’t necessarily guide you towards the right product for your application, but they’ll give you an indication of where to start a conversation with a supplier.

However, while figures on maximum speed and maximum capacity can give you an indication of performance at one end of the dynamics scale, they tell you next to nothing about performance at the opposite extreme.

At low speed or low torque values, the mechanical design of the rotary torque sensor has a much greater impact on factors such as accuracy, resolution, hysteresis and linearity, and environmental factors such as temperature really come into play.

Consider, for example, the strain gauge torque transducer employing slip rings to make the electrical connections from the casing to the rotating shaft. The strain gauge itself is bonded directly to the shaft, and connection is made via brush contacts.

In many applications, this will provide a useful and reliable solution, particularly as the principle disadvantages of the system are well known and generally reported within the product datasheets. In particular, while at low speed the electrical connection between the rings and the brushes are relatively noise-free, at increasing speeds electrical noise gradually has a greater impact on the output, eventually dominating and so decreasing the reliability of the output signal.

This maximum speed criterion for slip ring systems is a well known limitation, making them generally unsuitable for use in highly dynamic applications.

What is less well understood, however, is the suitability of slip ring based systems for very low torque applications. While the datasheet might hint at a torque capacity from zero to some upper torque limit, for very low capacity measurements the friction of the brushes themselves on the slip ring comes into play, and is an important limiting factor on the reliability of the output and the accuracy of the system.

Suppose instead that strain gauge is connected via a rotary torque transformer. As a non-contact implementation, this overcomes the wear of the brushes in applications towards the higher end of the speed limit, even if it doesn’t particularly extend that maximum speed, due to the need for bearings and the fragility of the transformer cores. At the higher end of the speed range, it is also recognised that noise and errors included by the alignment of the coils come into play.

What is less often discussed, however, is the potential for difficulties at the opposite end of the performance curve, in low speed applications. The physics of the construction and the limitations of the required signal conditioning technology can quickly impact on resolution at the very lowest speeds of operation, potentially making the device inherently unsuitable in a given application. Further, while linearity errors can be ignored at or near full capacity, they are an important limitation for low capacity torque measurement.

A torque measurement technique that has found favour in highly dynamic applications – with Formula 1 being a prime example – is the magneto-elastic torque sensor, built on the principle that the magnetic field of a material changes as it twists. Within its defined operational range, the resulting changes in magnetic field are proportional to the applied torque and can be measured by magnetic field sensors enabling the torque value to be derived.

However, at low speeds the change in magnetic field is decidedly non-linear, limiting the usefulness of the technology in such applications. Further, at low speeds, the impacts of environmental factors such as temperature are much more pronounced.

One technology that does lend itself to low rotational speed or low capacity applications is the surface acoustic wave transducer. This is a relatively new technology that measures the resonant frequency change of surface acoustic wave (SAW) devices in a non-contact manner when strain is applied to a shaft to which SAWs are fixed. The applied torque causes a deformation of the quartz substrate of the SAW device, which in turn causes a change in its resonant frequency.

Practical torque sensors such as those produced by Sensor Technology use two tiny SAWs made of ceramic piezoelectric material containing frequency resonating combs. These are glued onto the drive shaft at 90 degrees to one another. As the torque increases the combs expand or contract proportionally to the torque being applied. In effect the combs act similarly to strain gauges but measure changes in resonant frequency.

The adjacent RF pickup emits radio waves towards the SAWs, which are then reflected back. The change in frequency of the reflected waves identifies the current torque. This arrangement means there is no need to supply power to the SAWs, so the sensor is non-contact and wireless.

Easily embedded within a system design, and with complete freedom from brushes or complex electronics, the SAW based rotary torque sensor eliminates the problems of alternative solutions not only in the most dynamic applications, but also in the lowest speed and lowest capacity tasks. Not only is its output inherently linear and stable at low speed or low torque, but it is also completely immune to environmental factors such as temperature, magnetic field, vibration and electrical noise.

Low speed or low capacity tasks form an important subset of applications for torque sensors. For example, a radio telescope may rotate at speeds as low as one revolution per day, and the large scale solar panel arrays that track the sun rotate at similarly low speeds.

The mixing of many non-Newtonian liquids is not only carried out at low speed, but requires speed to vary reliably as torque changes to ensure consistent quality.

Sensor Technology SAW-based rotary torque measurement sensors excel in these low speed or low torque capacity applications. As well as offering performance that extends to the highest levels of required torque and speed, they can also be reliably used at rotational speeds from 0Hz upwards, and from zero torque to just a few mNm.

As such, surface wave acoustic torque measurement technology is opening up potential in a whole host of demanding low speed and low torque applications, delivering new levels of accuracy and stability.