Editing Chart recorder (section)

==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 amplifier, 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.<ref>Béla G. Lipták ''Process control and optimization'' CRC Press, 2006 {{ISBN|0-8493-1081-4}}, page 820</ref>