Indian Journal of Science and Technology

Abstract

Background/Objectives: Eddy current sensors are noncontact displacement sensors of high resolution providing measurement of the absolute position or change in position of any electrically conductive target. They are most suitable in dusty, smoky, dirty industrial environments where most other sensors would fail. This paper presents a design method and fabrication of a low cost eddy current sensor used in real time servo-control feed-back in Active magnetic Bearing. Method/Statistical Analysis: The primary functional piece of the eddy-current sensor is the sensing coil. This is a coil of wire near the end of the sensor probe. This piece of coil forms a part of an oscillator circuit there by carrying an ­alternating current through it which creates an alternating magnetic field. This field creates an eddy-current to flow on the target material whose distance is to be measured. This variation in field is used to sense the distance to the target. The coil is encapsulated in plastic and epoxy and housed in threaded stainless steel housing. Findings: The designed sensor has a measuring range of 0-3 mm and displacement resolution of 7 microns at a speed of 20 kHz sampling rate. This sensor was designed for the purpose of servo control feed-back in Active Magnetic Bearing System for position control. In this feedback control system we have employed a novel method of using time to digital conversion which converts the time period of the eddy current sensor’s square wave signal into digital counts using the 32bit timer counter of the enhanced capture module available in Texas Instruments C2000 controller. The digital counts give us the direct displacement value that can be used as feedback to generate the required error signal in the PID control loop. Application: The output frequency of the sensor is in the range of 700 kHz. We scale down this signal to 20 KHz by using the built-in divide by counter in the C2000 controller before feeding it to the capture module to capture the time period in a 32bit timer counter. This method of time period to digital count converter or displacement measurement helps reduce noise and errors in signal processing usually associated with ADC chips which converts analog voltage signal to digital counts.

Keywords: Active Magnetic Bearings, Eddy Current, Position Control, Servo Control