Many mechanical systems use high accuracy position feedback in conjunction with servo electronics and mechanical actuators to obtain precision positioning of mechanical components. The accuracy of the position detector limits the system performance. Compensations for errors which have a repeatable nature are possible. Random errors must be made small compared to the expected system resolution. There are many well known technologies which can be used to detect the position of a moving element. This article examines those rotary applications where the servo-positioner must be capable of high accelerations. In such systems the moving parts of the sensor must have a low inertia compared to the total moving inertia. A low inertia sensor will allow for efficient use of the torque produced by the actuator. A sensor with a high inertia can cause the mechanical structure to have resonances which will limit the stable bandwidth of the servo-system. In these situations capacitive position sensing emerges as a practical sensing technique. Transducers with inertia ranging from .005 to 1.0 gm-cm2, and having useful angles of +/-45 degrees motion, can be made for systems with up to 20 kHz of bandwidth. The resolution will be lowest for the low inertia versions, typically around 100 (mu) radian. Capacitive sensors with an inertia of 1.0 gm-cm2 and an accuracy 0.5 (mu) radian are possible.
Brian P. Stokes, Brian P. Stokes,
"High-accuracy capacitive position sensing for low-inertia actuators", Proc. SPIE 1454, Beam Deflection and Scanning Technologies, (1 February 1991); doi: 10.1117/12.28033; https://doi.org/10.1117/12.28033