Digital eddy current probe

Abstract

Devices which can sense the relative proximity of materials without physical contact are good sensors in the field of magnetic suspension, brushless D.C. motor commutation, weighting system, robotics, etc. The dynamic characteristics of these systems need a fast and robust controller. Hence, many of these applications still adopt analog controllers. But with the advent of the high speed logic it has become feasible to use digital control. The performance of the digital control systems is mainly decided by the front end design of the system. The front end converts information from analog real world into a digital medium, consisting mainly of the analog to digital converter (ADC) whose speed, accuracy, aperture delay, conversion time and dynamic range set the standard for the controller timing specifications. Further, if the ADC is multiplexed for more than one sensor input to the controller, it increases the controller delay causing the digital control system unsuitable for such applications. To overcome the timing problems of controller due to the ADC, a custom-built data capture system has been designed using gating logic technique, whose specifications can be set according to the system requirements. Further, the data capture system can be realised along with the other circuitry of the system using field programmable gate arrays (FPGA) reducing the size and cost of the system. In this paper, we propose three different types of gating logic with their merits and demerits to realise digital eddy current sensor without using the ADC which enables sensor amplitude measurement digitally at a faster rate with reduced size and cost.