Sensor Technology Provides the Key to Learning
22 June 2010
Electrical machines at Dublin Institute of Technology (DIT) have effective teaching resources.To provide maximum flexibility, the demonstration equipment in the DIT machines laboratory employs modular construction. This means, for example, that experiments with both AC and DC machines can be carried out on the same test bed, as whichever machine is needed is simply clamped into place.
Whatever the configuration of the equipment, however, the experiments and demonstrations performed in the laboratory require the monitoring and collection of a number of key operating parameters, including torque, speed and power. “We had tried many methods for measuring these parameters, but had found it impossible to get results we could rely on, particularly in the case of the torque and power measurements,” said Terence Kelly, Technical Officer at Dublin Institute of Technology. “Then we heard about the Sensor Technology’s innovative products, and after we’d visited the company and seen the sensors in action, we knew that they we’d found the solution we were looking for.”
The sensors that attracted the attention of DIT depend for their operation on surface acoustic wave (SAW) transducers. These transducers comprise two thin metal electrodes, in the form of interlocking “fingers”, on a piezoelectric substrate such as quartz. When an RF signal of the correct 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 motor drive shaft, the deformation of the substrate and hence the change in resonant frequency is related to the torque applied to the shaft. In other words, the transducer, in effect, becomes a frequency-dependent strain gauge.
Since the transducers operate at radio frequencies, it is easy to couple signals to them wirelessly. Hence, TorqSense sensors that incorporate the SAW transducer technology can be used on rotating shafts, and can provide data continuously without the need for the inherently unreliable brushes and slip rings that are often found in traditional torque measurement systems. Other features of these novel sensors that made them particularly suitable for use in the DIT machines laboratory include a large safe overload margin, high accuracy and resolution, ability to operate equally well clockwise and anticlockwise, and integral temperature monitoring.
Also, as well as measuring torque, the sensors also provide speed and power data. “The biggest benefit of the TorqSense sensors is, of course, that they use a wireless connection between the transducer itself and the sensor electronics,” said Terence Kelly. “This solves all of the major problems at a stroke. But they also excel in terms of the amount of information they collect, and because they deliver measurements of all key parameters in real time. In addition, they are robust, which is particularly important in a teaching laboratory and, in the year that we’ve been using them, they have proved totally reliable.” DIT Machines Laboratory has purchased six RTW321 series TorqSense sensors. In addition to two conventional analogue outputs, these also provide data digitally via RS 232 and USB ports. In this application, the USB ports provide a convenient connection to a PC that is also used configure the operation of the sensor.
The sensors have an integral self-diagnostic feature that ensures the data they supply is trustworthy, and also warns users if the maximum speed or torque ratings are exceeded. Five of the sensors purchased by DIT are currently installed on demonstration equipment, while the sixth is held in reserve in anticipation of use in a future research project.
The five sensors currently in operation have a torque range of 0 to 15 Nm, but they are configured for a maximum reading of 10 Nm in order to discourage students from pushing them to – and possibly beyond – the limit of their range. The sensors monitor speed from 0 to 1,500 rpm, and power from 0 to 1.4 kW. “Since we installed the new sensors, we’ve had nothing but positive comments from the staff and students who use the machine laboratory,” said Terence Kelly. I think it’s fair to say that the sensors have greatly improved our facilities for demonstrating machine characteristics and for experimenting with them, and that they are, therefore, a great asset for our teaching programme.”
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