Measuring Multiple Turns of a Shaft With a One-Turn Potentiometer

Abstract

Accurate measurement of multiple rotations of a shaft with a single-turn potentiometer is made possible by a nonlinear observer. Simulations illustrate the perfor- mance of the system in an open-loop and in a closed-loop configuration. I. INTRODUCTION An incremental shaft encoder is the standard sensor for accurate measurement of multiple rotations of a shaft. But shaft encoders are relatively costly and require additional supporting hardware (an up-down counter and a digital interface to a computer). There are applications in which the cost of this hardware is prohibitive. A potentiometer (pot) in a simple bridge circuit is a relatively inex- pensive way of measuring the rotation of a shaft, but a one-turn pot limits the rotation to less than one rotation. A multiple-turn pot extends the range to the number of rotations of the pot, but even so the number of rotations is limited. A one-turn pot can be used to measure multiple ro- tations by providing an independent means of tracking the total number of rotations. This can be accomplished in many ways (optical, magnetic, etc.) but these require additional hardware and circuitry. The present paper presents an alternative that uses only the output signal of the pot itself. The total number of turns of the pot is effectively tracked by use of a nonlinear observer that employs a model of the dynamic process that produces the shaft motion and a model of the output of a one-turn pot when the shaft has multiple rotations. The application is illustrated with the dynamics of a d-c servomotor, but it is applicable to other dynamic systems.