Question

A request has been received from an external customer for recommendations for linear position control sensors to be used in an explosive environment. For this reason unsafe electrical contacts must be avoided. The sensors will be installed in locations that are hazardous to access except when the plant is shut down annually for maintenance. The span required is 5 cm. The sensor case must not permit any ingress of dust or of water.

State what type of position sensors you would recommend and give your reasons. Sketch both the recommended sensor and circuit, and explain its operation. Comment on how signal conditioning requirements can now be met. Identify the durability required in respect of dust and water ingress.

Answer 1

I recommend linear variable differential transformer (LVDT), which is a most commonly used sensor for measurement of displacement in the range of millimeters to centimeters.

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Working Principle:

The principle is illustrated by the circuit diagram shown in the Figure below.

Further the signal conditioning circuit can be used to obtain processable signal.

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The device consists basically of three fixed coils, one of which is connected to an AC supply. The other two coils are connected to a phase-sensitive detector, and as a core of ferromagnetic material moves in the coil axis, the output from the detector will be proportional to the distance of the core from one end of the coils.

As the name suggests, the output from the phase-sensitive detector will be fairly linearly proportional to distance, and there are considerable advantages as compared to other types of distance sensors, as follows.

1. An LVDT transducer is that there is no physical electrical contact across the transducer position sensing element.
2. An LVDT outputs clean data, has an infinite resolution and a very long life cycle.

For example, AC-Operated LVDT Position Sensors of industry grade standards are hermetically sealed, constructed entirely of stainless steel for environmental robustness, they feature a through-bore design which makes the LVDTs’ cores accessible from both ends for better mechanical support and core guidance and facilitates cleanout in dusty or greasy locations.

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