Potentiometric (servo) Instruments
Analog chart recorders using a galvanometer movement to directly drive the pen have limited sensitivity. In a potentiometric type of recorder, the direct drive of the marking pen is replaced with a servomechanism where energy to move the pen is supplied by an amplifier. The motor-operated pen is arranged to move the sliding contact of a potentiometer to feed back the pen position to an error amplifier. The amplifier drives the motor in such a direct as to reduce the error between desired and actual pen position to zero. With a suitable signal processing amplifer, such instruments can record a wide range of process signals. However, the inertia of the servo system limits the speed of response, making these instruments most useful for signals changing over the span of a second or more.
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