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Torque Sensors Help Keep Vessels on Course

A UK-based developer and supplier of hydraulic drive units for marine steering and autopilot systems often uses wireless torque sensors in its test rigs

Developing hydraulic pump assemblies for marine steering and autopilot systems in yachts and other small pleasure crafts breaks from the norm in that most systems are powered by a 12- or 24-V storage batteries with limited capacity. This means the entire steering system must efficient to prevent draining the batteries. Yet, the pump must be capable of delivering enough hydraulic power to actuate the steering system steering system reliably in heavy seas and high winds.

Hydraulic systems often monitor pressure for closed-loop electrohydraulic control. However, Hydraulic Projects Ltd. (Hy-Pro), a UK-based developer and supplier of hydraulic drive units for marine steering and autopilot systems, often uses wireless torque sensors in its test rigs. The TorqSense sensor, manufactured by Sensor Technology Ltd., is a crucial part of Hy-Pro’s evaluation process to accurately measure the torque delivered by the electric drive motor to the pump under varying load conditions and over a wide range of speeds.

“We considered different ways of measuring the torque in the motor shaft on the test rig,” explains Barry Wynn, Senior Design Engineer at Hy-Pro, “but all were either inconvenient or were incapable of delivering the accuracy and fast response that we needed. We concluded that TorqSense sensors offered the most reliable, easy-to-use, and cost-effective solution that would meet all of our requirements.”

TorqSense sensors use surface acoustic wave (SAW) technology. They are comprised of two thin metal electrodes, in the form of interlocking “fingers”, on a piezoelectric substrate such as quartz. When a specific radio frequency is applied to the transducer, surface acoustic waves are set up, and the transducer behaves as a resonant circuit. The key feature, however, is that if the substrate is deformed, the resonant frequency changes. When the transducer is attached to a drive shaft, the deformation of the substrate—and, hence, the change in resonant frequency—is related to the torque applied to the shaft. In effect, the transducers become a frequency-dependent strain gauge.

Because the transducers operate at radio frequencies, the signals are easily coupled wirelessly. Therefore, the TorqSense sensors can be used on rotating shafts, and can provide data continuously without the need for brushes or slip rings that are often found in traditional torque measurement systems.

In the Hy-Pro application, a TorqSense sensor measures the torque applied to the pump over a speed range of 500 to 4,000 rpm. The results are displayed in real time, for convenient monitoring, and are also captured and stored for more detailed analysis later with Labview software.

Wynn adds, “The data it has provided us with has played an important role in helping us to refine our systems by ensuring an optimum match between the characteristics of the pump and motor over the full operating range. In fact, the insights we’ve gained during our tests have enabled us to further enhance the performance and reliability of our steering and autopilot systems, which, of course, means big benefits for our customers and a boost to our competitive position.”

For more information, contact Sensor Technology Ltd at www.sensors.co.uk.

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