This paper describes an algorithm to regulate and control the acquisition of images of a computer vision system used to measure contact wire wear in railways. Real-time implementation of the algorithm is also shown. In this work it will be shown that is possible to base a control strategy on a simple measure derived from the image histogram. The system developed controls acquisition process varying the exposure time of CCD without the necessity to control other parameter of camera/digitiser combination as black and white reference levels of digitiser. In the control strategy proposed, histogram is used as exposure time indicator. A new architecture has been defined to implement the control system in real time. Implementation system has been divided in two processing .On the one hand, a hardware processing in which histogram values extraction is carried out by FPGAs (Field Programmable Gate Array).On the other hand, the calculation of new exposure time based on the histogram is carried out by a PC whose operating system is RTLinux.
The control system has been incorporated as a new module in the system used by RENFE (Spanish Railways Company) to measure the contact wire wear and measurement system precision has been increased considerably.