Eight test rigs share one TorqSense transducer

1st Pass Ltd started out as a regional systems integrator for National Instruments' LabView software. However, the company soon realised that there was a considerable need for a wider service encompassing all aspects of test rig design and manufacture. Founder and Managing Director, Chris Turner, explains: "Engineers involved in either research or quality control need test rigs, but designing them is not their forte. They tend to focus only on the immediate requirements so tend towards inflexible, mono-functional designs. Six months later, when new requirements emerge, they have to build another rig.

"We saw this as a gap in the market and decided to offer a full package whereby we would interview the engineers to establish likely future requirements, then design and build a test rig or test station that addresses all of these. This includes electrical and mechanical design, software, firmware, support systems, etc. We will also install, calibrate and maintain the rigs."

Recently Turner was asked to visit a major West Country manufacturer of vacuum cleaners to assess its needs for life-testing of electric motors. The company has a considerable research and development function and it was decided that eight separate rigs were required. These were to be based on the well-proven concept of the motors driving against a brake, with dynamic performance data collected from around the rig and analysed using LabView.

With the rigs' workload expected to be high, it was sensible to go for a solid design that would last through many years of punishing duty. However, this stretched the budget, so Turner applied some lateral thinking. He says: "We needed to be able to calibrate and recalibrate the rigs on a regular basis, and we wanted the calibration process to be as simple as possible. I was able to use a TorqSense with an RS232 output for this - a sensor that can monitor the torque in a rotating shaft without actually touching it - as it opened up the potential for foolproof, robust yet sensitive calibration."

Measuring torque

TorqSense technology uses a radio frequency (RF) transmission link. This picks up resonant frequency signals from two piezo ceramic combs fixed onto the shaft, which vary their frequency as the combs deflect under the effects of applied rotational torque. So, as torque increases, the combs change their resonant frequency in proportion to the change in applied torque caused by the rotating shaft.

The SAW (surface acoustic wave) devices function very much as strain gauges and the RF coupling used to supply power to the SAW devices is transmitted as a radio frequency, which eliminates interference (being piezo-based, the SAW devices consume less than 1mW).

Turner continues: "Initially we assumed that each rig would have its own on-board calibration unit, but I got to thinking that, as they were only used at the beginning of each test run, they would actually be idle the vast majority of the time. So I began to wonder about developing a demountable unit that could be shared around all the rigs."

TorqSense's non-contact operation and lack of flying leads make demountable installations an attractive possibility, so Turner knew he had to come up with an easy way of achieving accuracy in the transducer's positioning. He called Sensor Technology and arranged a brainstorming session, where sliding the TorqSense unit up against end stops was identified as the best option.

"To do this we had to find a way to move the brake away from the motor so that we had room to slip the calibrator in," says Turner. "It was fairly obvious to mount the brake on a slide too; this was a simple solution that required little re-designing of our original concept."

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