Sensors and shielding

Thursday, March 27, 2014

Tags :  test-equipment

A test bed is only as accurate as the sensors it uses to collect data. In the case of engine dynamometers, the test environment is a very ‘noisy’ place, with many sources of vibration and electromagnetic interference (EMI) present, all of which can skew sensor readings.

This is a particular issue when it comes to sensors such as strain gauges and thermocouples, which have very low output signals. Whereas sensors like pressure transducers can have an output voltage of 1-10 V, the output of strain gauges and thermocouples will be only a few milli- or microvolts. That puts them in the same range as much of the electronic interference found in a racecar, particularly the new generation of hybrid Formula One and sports cars with high-voltage electrical circuits.

As a result the background ‘noise’ can have a considerable impact on the recorded output of a sensor, as the ‘signal-to-noise’ ratio is very low. This ratio is the level of a particular signal’s strength compared to the level of background noise. So if, for example, a sensor had a 5 V output signal from a sensor and a background level of signal noise of a few microvolts, the signal-to-noise ratio would be very high; however, if the sensor output is only in the millivolt range, the signal-to-noise ratio would be much lower, so it would be much harder to distinguish the signal from the noise.

While a lot can be achieved using complex signal processing algorithms to ‘clean up’ noisy signals, effective circuit design and shielding will go a long way towards improving sensor performance, reducing the need for such measures. The most obvious solution is to keep the cable length between a sensor and its signal amplifier as short as possible; long cables act as antennae, picking up electrical and magnetic interference. Twisted cable pairs also help to prevent signal noise by reducing induction between the wires.

Adding shielding around cables is also imperative. Shielding a cable simply involves enclosing the insulated signal cables in a conductive layer, normally braided metal wire or copper tape, that acts as a Faraday cage around the signal cables, reducing the impact of electrical or magnetic interference.   

Installation considerations must also be taken into account in order to reduce signal noise. For example, strain gauges are often mechanically and electrically attached to the component being measured, which can result in ground-loop currents forming that will contribute to signal noise. These occur where a difference in potential develops between the sensor ground and the signal amplifier. However, careful attention to detail when it comes to earthing the sensor, shielding and the signal amplifier can prevent them.

Things get a little trickier though when dealing with very high voltage systems, such as those in hybrids with powerful electric drive components and energy storage systems. The electromagnetic fields these produce can create problems not only for sensors used to measure their operation but any other sensors that may be close by. Fortunately, the frequencies of these fields can be identified and accounted for in the signal processing, but where very strong fields are present, the only solution is to increase the level of shielding on the sensor circuits, otherwise the EMI can actually be sufficient to damage the circuitry itself.

One interesting solution to the problem of measuring in very high EMI environments is the use of optical rather than electrical sensors. Where conventional electrical sensors use transducers to covert physical phenomena into electrical signals, optical sensors use light, and fibre optic cables instead of wires. Such sensors are immune to EMI, although they are still nowhere near as common as electrical sensors, and we will take a more in-depth look at their potential applications in a future RET-Monitor article.

Written by Lawrence Butcher

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