P3.10 Photodetector Light source Reflecting surface I d Fig. 1 – Representation of an optoelectronic reflective sensor. MODULATION METHOD FOR LOW-COST IMPLEMENTATION OF OPTOELECTRONIC REFLECTIVE SENSORS Lino Marques, Urbano Nunes and Aníbal T. de Almeida Institute of Systems and Robotics Department of Electrical Engineering University of Coimbra, Pólo II 3030 Coimbra Portugal Phone: +351 239 796 277 Fax: +351 239 406 672 Email: {lino, urbano, adealmeida}@isr.uc.pt Abstract It is common to find in Industrial Automation, and particularly in Robotics, applications that need a large number of proximity sensors. In applications where the cost is a very important factor, the most common choice of proximity sensing is based in optoelectronic reflective sensors. This article describes a new modulation method that facilitates the construction of large arrays of these proximity sensors. The proposed method allows the utilization of off-the-shelf infrared (IR) digital receivers, like the ones used in IR data communications, by modulating the emitted light with a saw-tooth envelope. The output from each receiver will be a Pulse Width Modulated (PWM) signal whose duty represents the proximity to the target surface. The resulting architecture for such sensing system allows an easy increment of sensing elements with a small incremental cost. The paper also describes the implementation of a prototype with eight sensing elements and analyses its characteristics. Keywords: infrared sensing; proximity sensor; range sensing; robotic sensors. 1. INTRODUCTION The most common optoelectronic methods used to measure distance in industrial environments, namely in robotics, are based on telemetry, triangulation and amplitude detection (reflex). Telemetry is based on the time-of-flight of a pulse of light. Systems based on this method are expensive and are mainly used to measure large distances [1]. Triangulation sensors are geometric sensors composed by a camera and a collimated light source. The distance to the surface is measured by the image position of the interception spot between the light beam and the surface. Since these sensors are based on a geometric principle, they are very immunes to the environment light and to the reflecting properties of the surface. Their main utilization is the accurate measure of distance in a range up to one meter [2]. Reflective sensors use a light source and a photodetector placed side-by-side (see Figure 1). The energy absorbed by the photodetector depends on the distance, on the orientation and on the reflecting properties of the target surface. For parallel and close to each other emitter and detector, it can be shown that the absorbed energy (E) decreases with the inverse square of the distance (d) to the surface: ( ) 2 ; d e reflectanc n orientatio f E ≅ (1) This measuring method is mainly used for known target surfaces as well as for low cost applications. In our particular case, we use this type of sensors in several mobile robots and autonomous wheelchairs to measure Proc. II of the 10th International Conference Sensor 2001, Germany, P 3.10, 585-590.