Engine test cells benefit from non-contact torque sensors

In the technological race to develop a new generation of high-performance, low-emissions automotive powerplants, high-capability test beds have played an important role by simultaneously monitoring multiple parameters.

The automotive sector is pinning many of its hopes for the future on the development of electric/petrol and electric/diesel hybrid engines integrated with known performance-enhancing technologies such as fuel injectors and superchargers. Other projects are looking at marrying other "green' options such as regenerative braking and kinetic energy storage with hybrid engines.

Engine test beds now require a high degree of engineering design if they are to be able to monitor multiple parameters simultaneously. However, for them to be efficiently usable they have to be, at heart, as simple as possible. This need has been converted into a market opportunity for a specialist torque sensor firm, Sensor Technology, which produces TorqSense, an in-line digital torque sensor that is easy to use, accurate and low cost.

Tony Ingham, from Sensor Technology, explains: "Measuring an engine's torque is a fundamental business; older sensors often used slip rings, which wear over time thus increasing inaccuracies, or had analogue electronics that are difficult to interface with digital equipment. TorqSense does away with slip rings as it is non-contact, working through an RF couple and with modern digital RF electronics inside; it allows easy interfacing with digital equipment and its own TorqView software so the development engineers can concentrate on monitoring the key input and output parameters."

Efficient energy flows

In a hybrid powerplant electrical power and mechanical torque are essentially the same thing - an energy flow - which has to be used as efficiently as possible with minimum losses. Significantly the two directions of energy flow, to and from the power source, are equally important to optimise so that recharging is as efficient as running.

Ingham observes: "There are some really radical thinkers currently employed in advanced engine design. To them, electric motors, IC engines, generators and regen units are all essentially the same - torque producers that ultimately drive the road wheels! So is it any wonder that they like TorqSense?"

A typical test bench used in this sector is based around a TorqSense torque sensor. This is mounted in-line and used to monitor all rotary inputs and outputs within whatever combination of engine and electric machine is under scrutiny so that efficiencies calculations and mapping can be implemented.

Ingham says: "This configuration has definite advantages when it comes to the extended tests with multiple repeats essential for developing reliable engines. Belt load measurement of input torque is notoriously difficult while strain gauged belt pulleys are expensive and time-consuming to create. But TorqSense creates a plug-and-play type test rig and operating environment; the efficiency gains in the test environment are orders of magnitude."

How TorqSense works

TorqSense transducers use two tiny quartz piezoelectric combs, called Surface Acoustic Wave (SAW) devices fixed to the surface of the sensor's shaft. As the torque increases the spacing between the combs inside the SAW devices varies, allowing the resonant frequency to change proportionally to the applied torque of the rotating shaft under test. In effect the SAW devices are frequency-dependent strain gauges that measure changes in resonant frequency of the test shaft as the test programme is run and, with a wireless radio frequency (RF) coupling to transfer the data signal to a pick-up head, the sensor is non-contact.

The same RF coupling also supplies power to the SAW devices. This is possible because the devices are based on piezo technology so need less than one milliwatt of power. This arrangement eliminates the difficulties associated with fitting slip rings inside the sensor and maintaining their contact quality throughout an extended test run.

Ingham concludes: "TorqSense makes motor testing much more straightforward than any other method of measurement. Its digital outputs and ease of operation allows easy data capture, intelligent calibration and advanced analysis toolsets."

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