Interfacing - PC Based Instrumentation and Control - page 268

Physical Type of

parameter sensor Notes

Optical Available in diffuse and through scan types. Diffuse scan types

proximity require reflective targets. Both types employ optical transmitters and

switch* receivers (usually infra-red emitting LEDs and photodiodes or

phototransistors). Digital input port required.

Strain Resistive Foil type resistive element with polyester backing for attachment

strain to body under stress. Normally connected in full bridge

gauge configuration with temperature-compensating gauges to provide an

analogue output voltage. Analogue input port required.

Semiconductor Piezo-resistive elements provide greater outputs than comparable

strain gauge resistive foil types. More prone to temperature changes and also

inherently non-linear. Analogue input port required.

Temperature Thermocouple* Small e.m.f. generated by a junction between two dissimilar metals.

For accurate measurement, requires compensated connecting
cables and specialized interface. Analogue input port required.

Thermistor Usually connected as part of a potential divider or bridge. An

analogue output voltage results from resistance changes within the
sensing element. Analogue input port required.

Semiconductor Two-terminal device connected as a current source. An analogue

temperature output voltage is developed across a series resistor of

sensor* appropriate value. Analogue input port required.

Vibration Electromagnetic Permanent magnet seismic mass suspended by springs within a

vibration sensor cylindrical coil. The frequency and amplitude of the analogue

output voltage are respectively proportional to the frequency and
amplitude of vibration. Requires an analogue input port.

Weight Load cell Usually comprises four strain gauges attached to a metal frame.

This assembly is then loaded and the analogue output voltage
produced is proportional to the weight of the load. Requires
an analogue input port.

∗ Further details, interface circuits or photographs can be found later in this chapter.

Interfacing switches
and sensors

Sensors can be divided into two main groups according to whether they are
active (generating) or passive . Another, arguably more important distinction in
the case of PC-based process control systems, is whether they provide digital
or analogue outputs. In the former case, one or more digital I/O boards will be
required whereas, in the latter case one or more analogue input ports must be
provided.
We shall deal first with techniques of interfacing switches and sensors which
provide digital outputs (such as switches and proximity detectors) before exam-
ining methods used by interfacing sensors which provide analogue outputs .It
should be noted that the majority of sensors (of either type) will require some
form of signal conditioning circuitry in order to make their outputs acceptable
to conventional PC expansion cards.