Page 51 - Instrumentation and Measurement
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FIGURE 4.1 On-off control.
An example of such a controller is the bimetallic thermostat (Figure 4.2) used with a simple
temperature control system. The bimetallic strip consists of two strips of different metals, e.g. steel
and copper or steel and brass, which expand at different rates. The two strips are joined together
throughout their length and so, as a consequence, a rise in temperature causes one strip to expand
more than the other and as they are joined together the only way this can occur is if they curve. This
curving can be used to close an electrical switch. Thus, if the actual temperature is above the required
temperature, the bimetallic strip is in an off position and the heater is switched off; if the actual
temperature is below the required temperature, the bimetallic strip moves into the on position and the
heater is switched on. The controller output is thus just on or off and so the correcting signal on or
off.
FIGURE 4.2 The basic principles of a metallic thermostat.
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