Bridge Controlled Error Detection:

Bridge Controlled Error Detection – Whenever a bridge is unbalanced, a potential difference exists at its output terminal. The potential difference causes current to flow through the detector (say, a galvanometer) when the bridge is used as part of a measuring instrument. When the bridge is used as an error detector in a control circuit, the potential difference at the output of the bridge is called an error signal, as in Fig. 11.14.

Passive circuit elements such as strain gauges, temperature sensitive resis­tors (thermistors) and photo resistors, produce no output voltage. However, when used as one arm of Wheatstone’s bridge, a change in their sensitive param­eter (heat, light, pressure) produces a change in their resistances. This causes the bridge to be unbalanced, thereby producing an output voltage or an error signal.

Resistor R_ν in Fig. 11.14 may be sensitive to one of many different physical parameters, such as heat or light. If the particular parameter to which the resis­tor is sensitive, is of a magnitude such that the ratio R₂/R_ν equals R₁/R₃, then the error signal is zero. If the physical parameters changes, R_ν also changes. The bridge then becomes unbalanced and an error signal occurs. In most control applications the measured and controlled parameter is corrected, restoring R_ν to the value that creates a null condition at the output of the bridge.

Since R_ν varies by only a small amount, the amplitude of the error signal is normally quite low. It is therefore amplified before being used for control pur­poses.