Theoretical and experimental aspects are discussed concerning design and experimental setup of a simple and robust controller for optimization of on-line measurement of coating thickness, using a PC-based approach. Solutions assumed for a feedback control system are described in order to satisfy the main requirements set by this application: accuracy of operation, dynamic bandwidth, modularity of settings depending on the specific coating, handiness of using and modifying when necessary, robustness of control, and global operating reliability. Experimental results confirm that an acceptable measurement uncertainty can be achieved in tin plate coating thickness measurements by this simple displacement control system of the fiber optic measuring head.