The implementation of a compact continuous-time optical transient sensor with commercial CMOS technology is presented. In its basic version, this sensor consists of a photodiode, five transistors and a capacitor. The proposed circuit produces several output signals in parallel. These include a sustained, logarithmically compressed measure of the incoming irradiance, half-wave rectified and thresholded contrast-encoding measures of positive and negative irradiance transients, and a signal that shows a combination of the sustained and the bidirectional transient response. The particular implementation reported in this work responds to abrupt irradiance changes with contrasts down to less than 1% for positive transients and 25% for negative transients. Circuit modifications leading to more symmetric contrast thresholds around 5% are also described. Due to their compactness these transient sensors are suitable for implementation in monolithic one-or two-dimensional imaging arrays. Such arrays may be used to sense local brightness changes of an image projected onto the circuit plane, which typically correspond to moving contours.Index Terms-Active pixel, analog VLSI, CMOS image sensor, focal-plane sensor, neuromorphic sensor, temporal processing.
This paper discusses some of the fundamental issues in the design of highly parallel, dense, low-power motion sensors in analog VLSI. Since photoreceptor circuits are an integral part of all visual motion sensors, we discuss how the sizing of photosensitive areas can afSect the peformance of such systems. We review the classic gradient and correlation algorithms and give a survey of analog motion-sensing architectures inspired by them. We calculate how the measurable speed range scales with signal-to-noise ratio (SNR) ,for a classic Reichardt sensor with a f i e d time constant. We show how this speed range may be improved using a nonlinear filter with an adaptive time constant, constructed out of a diode and a capacitor, and present data from a velocity sensor based on such a filter. Finally, we describe how arrays of such velocity sensors can be employed to compute the heading direction of a moving subject and to estimate the time-to-contact between the sensor and a moving object. I. INTRODUCTION Various applications in automotive navigation, robotics, and remote sensing require sensors for processing visual motion that are small, consume little power, and work in real time. Considering the type of environments humans are typically exposed to, we shall use the term "real time" in its common anthropocentric meaning, i.e., for time delays not exceeding a few tens of milliseconds. Since motion-sensing algorithms have a large computational overhead, most realtime machine-vision applications require special-purpose parallel hardware for computing motion across the entire image. Parallel hardware implementations are particularly attractive if image acquisition and motion computation can be integrated on a single silicon chip. Such smart-vision
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