The precise measurement of the torque acting on rotating parts plays a central role in the development and operation of a wide variety of devices - from huge underwater turbines for tidal power plants to miniature electric motors for razors or automatic heating valves. Regardless of the size, the measuring principle is usually the same: the deformation caused by a force acting on a mechanical element such as a gear shaft is measured and the torque is calculated from this. However, conventional methods have some disadvantages.

Acoustic surface wave

This is why the British company Sensor Technology has developed a patented measuring solution based on a completely new principle. The measuring element is a transducer for detecting surface acoustic waves (AOW), a phenomenon that is often referred to by the abbreviation SAW (surface acoustic wave).

Quality assurance on screw caps for bottles in the pharmaceutical industry: The torque sensor is used to monitor and document the closing of each bottle in the range of 1 Nm - at a cycle rate of one cap per second

© Althen

At the heart of the transducer are two electrodes that interlock in the manner of interlaced fingers. The electrodes made of a thin metal film are attached to a piezoelectric substrate, such as quartz. The transducer is attached to the mechanical element that is to be examined, for example a gear shaft. If an electrical signal of a certain frequency in the radio range is applied to such a transducer, this triggers acoustic vibrations that propagate on the surface of the substrate, the so-called surface acoustic waves. The transducer thus forms an oscillating circuit consisting of electrode and substrate.

The torque acting on the transmission shaft mechanically deforms it. This deformation is transferred to the piezoelectric substrate of the transducer, which changes the resonant frequency of the resonant circuit. The change in the resonant frequency therefore depends on the torque acting on the shaft. Due to this relationship, such a transducer can be used as a frequency-dependent force sensor.

Because AOW transducers operate in the radio frequency range, they are predestined for the wireless exchange of signals. This is a major advantage over strain gauges, as the transducer can be placed on shafts and other rotating parts without them being affected by cables or slip rings, for example.

Advantages over other methods

This provides the prerequisites for much more accurate measurements and uninterrupted data transmission, as the signal is much less impaired during radio transmission than with slip rings and other (electro)mechanical transmission methods. This also affects the processing speed of the signals. With torque measurement using AOW converters, the measurement data is available in real time for the first time. This proves to be a great advantage in test environments because the effects of changing conditions can be understood immediately. The development of AOW transducer technology has enabled a new, more compact and robust design for torque measuring devices. The Torqsense family of devices offered by Sensor Technology, which Althen distributes, can accurately measure torques from 1 Nm to 13,000 Nm. Special designs for even higher measuring ranges are available on request. Use at correspondingly high rotational speeds is only possible because the measuring electronics are connected wirelessly to the sensor and the inertia of the actual sensor on the moving part is negligible.

Although torque measurement using surface acoustic waves is a relatively new technology, Torqsense devices have already proven their advantages in a wide range of applications.

The areas of application

One important area is the testing of electric pumps of various sizes - from small appliances on pleasure boats to systems used in steelworks and other industrial plants. Industrial pumping systems must be regularly maintained. Once the maintenance work has been completed, the various characteristic curves are tested. Measuring the torque shows whether the pump reaches a certain speed and also delivers the corresponding pumping capacity. Users in the industrial sector rely on TorqSense for such applications because the system has proven to be significantly less susceptible to faults than previous solutions with strain gauges.

However, there are many other test applications in the development and production of various industries in which TorqSense devices are used. The suitability for speed measurements on motors is obvious. The devices can also be used to determine the stability of stud bolts or other mechanical components. For example, bolts are screwed together in a defined manner in corresponding material test benches. At the same time, the TorqSense device measures the torque acting on the bolt in real time. The tightening force is successively increased until the bolt breaks. This determines the limit value for the torque up to which a particular component can be used. An additional advantage for such test set-ups is that the TorqSense devices log the recorded data. It is also possible to export the data to the LabView software for subsequent detailed analysis.

In other applications, the torque measurement is used to draw conclusions about a completely different variable. In university research laboratories, for example, the viscosity of certain substances is determined using stirrers with a torque sensor attached to the shaft. If the viscosity of the substance under investigation changes, the forces acting on the shaft also change. This is because when the substance becomes more viscous, the resistance that counteracts the rotation of the agitator increases.

In general, measurement using surface acoustic waves can be considered one of the most versatile methods of torque measurement because it saves space, is not tied to a laboratory environment and can be mounted very flexibly thanks to wireless signal transmission. This makes TorqSense an ideal platform for customer-specific solutions.

Author: Thomas Richter is Sales Area Manager at Althen.

Conventional measuring methods

Design of the torque transducers with AOW technology. The unassembled measuring shaft is shown on the right. One of the electrodes on the measuring axis can be seen on the left, below it the pickup for determining the angle of rotation.

© Althen

Many different methods have been developed over time to measure torque. A large number of these methods are based on the deformation of the shaft. In addition to optical methods, such as the so-called shadow moiré technique or laser-based approaches, there is a whole range of electrical methods, including capacitive, inductive, piezoelectric and resistive methods. However, many of these often very costly procedures can only be used under laboratory conditions, which is not always practical in an industrial environment.

A more economical alternative is force measurement using strain gages. The strain gauge is connected to a power source and is applied directly to the mechanical element that is subjected to the torque. The mechanical deformation of the strain gauge changes its electrical resistance, which is reflected in a change in the output signal.

In many practical applications, however, this method has to contend with two difficulties. Firstly, the changes in resistance in the strain gauge are quite small, so the measurement can be very easily affected by noise. The measurement accuracy therefore leaves much to be desired. Secondly, it can be extremely difficult to connect the strain gauge, which rotates with the component under test (and often very quickly), to the power source. Auxiliary constructions with slip rings or inductive coupling using transformers are possible, but the time and cost involved can quickly negate the economic advantages of strain gauge-based measurement. An all-round satisfactory solution for torque measurement was therefore not available for a long time.