ADAPTIVE MICROSYSTEMS WITH OPTICAL LINE SENSORS
Ingo Martiny, Rolf-Rainer Grigat
Technische Universität Hamburg-Harburg
Technische Informatik I, Harburger Schloßstr. 20, D-21071 Hamburg,
Tel. +49 40 7718 3125/2816, Fax +49 40 7718 2911
email martiny @tu-harburg.d400.de, grigat@tu-harburg.d400.de
KEY WORDS: Analog VLSI Vision Chips, Photoreceptors, Measurement of Angular Position, Color Sensor.
ABSTRACT:
Application specific optical sensors process contrast ratios up to 7 decades, variable frequencies per pixel from DC up |
to some kHz depending on illumination, application specific number and geometrical position of light sensitive elements
and optimized use of analog and digital signal processing on chip. Potential applications are precision speed |
measurement, parallel barcode readout, precision measurement of angular velocity and position, color analysis or
segmentation of tomographic pictures. For the time being eleven different vision chips with photoreceptors and signal
processing have been manufactured and analyzed. This article describes the basics of vertical bipolar transistors
designed in standard CMOS processes and used as photoreceptor elements and the differences to existing CCD |
solutions. The results of our investigations are presented together with three application examples in industrial
environment: Line cameras with linear and logarithmic output, angular position measurement of rotating disks and color
filtering with line cameras.
1. FEATURES OF VISION CHIPS
Today very compact sized optical sensorsystems can be realised. In particular line cameras for visible and infrared |
wavelengths may include signal processing and even bus interfaces on one silicon chip. Adaptive, robust and cost |
effective microsystems are feasible. A recent survey is given in the book of Koch and Li [1]. Vision Chips may be |
preferred to conventional CCD-cameras, if one or more of the following advantages hold.
e Large contrast ratio of up to seven decades of illumination. This ratio is comparable to the total sensitivity range
of the human eye. Common CCD-sensors do not exceed four decades, therefore cast shadows in sunlight easily
pose a problem. By processing currents of phototransistors or photodiodes sunlit objects and shadows or center of
a flame and peripheral areas may be observed at the same time.
e Variable pixel frequency up to continuous operation allows measurements comparable to high speed cameras.
The analog bandwidth of the photosensitive elements increases with the power density of the incident light. With |
photodiodes designed in a standard 1 p CMOS process we measured more than 500 kHz bandwidth for a high |
optical power density of about 2 mW/mm? with a laser diode. Phototransistors are slower by about a factor of their |
current gain, so we observed a bandwidth of 20 kHz with the same illumination. With HP High Power HLMP-8103
LEDs with 35 mW/sr of intensity we observed 200 kHz with a phototransistor. Both types of photosensitive
elements slow down significantly in darkness. The user has to take care for the bandwidth when covering 7
decades of intensity. Delbriick and Mead increased the analog bandwidth for dynamic signals with an active
feedback device by one order of magnitude [2]. The parallelism of the photosignals allows parallel processing of all
signals. Investigated examples are a parallel barcode reader and local adaptivity to the incident power density of
light.
e Number and geometrical positions of the photoreceptors can be chosen by the designer. Sometimes the user
needs a special arrangement of 32 by 32 or 512 by 3 elements. Even the geometry of the single elements can be
designed individually within the design rules of the silicon foundry. An absolute position encoder with 10 nm |
resolution has been developed by Engelhard and Seitz [3] with a sinusoidal photosensor structure of 20 um grid |
size. The dependency of the photosensitivity of an element on its geometry, manufactured in a standard CMOS |
process, is under investigation for our testchips. The photosensitive elements may be crossed by signal lines |
almost without effect. Standard aluminium wires in CMOS processes proved to be optically transparent within the |
visible region of light. Therefore the photoreceptors can be placed side by side without extra routing channels
between them and no microlenses are necessary for a spatially homogeneous aperture. Bóhm et al. realize
homogeneous apertures by threedimensional chips, where the photodiodes are layered upon the electrical circuits
in a thin film on ASIC technology [4].
e The optimal combination of analog and digital signal processing allows the realization of high speed ultra
compact low power systems. In the analog domain the preprocessing is performed, e.g. optical power adaption or
signal integration. In the digital domain parallel computing, error detection and transmission may be realized.
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995
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