A Novel CMOS Sensor for Position Detection F. De Nisi, F. Comper, L. Gonzo, M. Gottardi, D. Stoppa and A. Simoni Centre for Scientific and Technological Research ITC-irst 38050 Povo Trento Italia denisi @itc.it J- A. Beraldin Institute for Information Technology National Research Council of Canada, Ottawa Canada, K1A 0R6. angelo.beraldin@nrc-cnrc.gc.ca Abstract A novel architecture of optical sensor developed for flying- spot active triangulation will be presented. The architecture implements a spot position calculation based on a two steps procedure allowing for increased readout speed and color detection. The proposed sensor has been fully integrated in standard CMOS technology and is currently under test. Preliminary experimental results will be presented. 1. INTRODUCTION Flying spot active triangulation 3D ranging camera [1] may take advances from VLSI integration especially for what concerns the optical sensors used for spot position detection. Integration of these sensors may in fact help to accelerate the deployment of these 3D measuring techniques in many fields like heritage, robotic guidance and industrial process automation. The block diagram of an integrated position sensing device is shown in Figure 1. The photodetector as well as analog signal conditioning, timing control, analog to digital conversion and digital processing can be integrated on the same silicon substrate using standard microelectronic technologies. Figure 1. Block diagram of an integrated position sensor. Two different types of photodetectors can be considered for integration: lateral effect photodiodes (LEP) and discrete response position detectors (DRPS), i.e. linear array of photodetectors. The former are basically analog detectors working as photoresistor and have been extensively studied in the past [2-4]. Although, they can be very fast and precise [5], they are mainly limited by the fact that the shape of the light distribution on the sensor surface is never known. This limitation influences the accuracy in measuring the light distribution when operated in presence of strong ambient illumination. From the VLSI point of view one has to stress the fact that standard microelectronic fabrication processes are optimized for electronic circuitry but not for optical sensors. Therefore the designer has only few fabrication layers at disposal which are suitable for LEPs and even those are not optimized and thus allow for LEPs with higher noise figures. Linear arrays of photodetectors, or DRPS, are a valid alternative to LEPs. These sensors are currently used in state of the art flying spot 3D camera rangers [6]. They allow for the recovery of the full shape of the light distribution on the photosensitive area and are therefore very accurate, but suffer for speckle limited spot position detection [7] and readout speed. The former is basically due to the continuously decreasing size of the single photodetectors. Commercial devices in fact are thought for spectroscopic applications [8] where only the number of photodetectors within the array is important leading therefore to a minimum size of photodetectors in the order of a few microns. Speed is the second issue of DRPS. They in fact are slower than LEP’s because all photodetectors have to be read out sequentially prior to the measurement of the location of the spot position. In this paper a novel architecture for a DRPS optimized for flying spot 3D ranging camera is presented. The sensor, fully integrated in CMOS standard technology, features random pixel access and in a particular topology also color detection. In section 2 the active triangulation technique is briefly reviewed and the main sensors specifications are considered. Section 3 reports on the architecture and principal building blocks of the proposed sensor. Section 4 will present some preliminary results and finally in section 5 conclusions will be drawn.