Optoelectronic Proximity Sensor Finds Edges
Seams are tracked for automatic control of welding.
NASA's Jet Propulsion Laboratory, Pasadena, Califomia
An optoelectronic system based on a relatively simple array of optical proximity sensors locates the edge of a plate. The system can be used to control automatic production machineryQfor example, to locate spot welds at the correct lateral position with respect to the seam between two overlapping metal plates or sheets. The system follows curved seams as well as straight ones, and neither special control programming nor precise initial positioning of a workpiece is necessary for accurate tracking of a seam. Facing the workpiece is an array of optoelectronic proximity sensors (see Figure 1) in a line across the seam. Each proximity sensor is calibrated for use with the particular workpiece material and configuration, so that the output of each sensor can be used to determine the distance to the workpiece surface. Each sensor is sampled in turn. The light-emitting diode is turned on, and the output of the detecting photodiode is measured. The digitized measurements are used to locate the exposed edge of the workpiece: the system computer finds the lateral position that corresponds to a previously calibrated sensor-output value between the values for the flat surfaces on either side of the edge. In effect, this amounts to a crude form of image analysis, in which the field of view of each sensor constitutes a picture element. For high definition of the position of the edge (high resolution), the sensor array should be dense and the fields of view should abut or overlap slightly. Although the light-emitting diodes and detecting photodiodes could be mounted in the sensing head as in Figure 1, it would be better to remove them from the vicinity of the weld to reduce electromagnetic interference. Therefore, in a practical system, the diode pairs are mounted on circuit cards with other electronic circuits and connected through glass optical fibers to the lens array in the sensing head. This arrangement also facilitates tracking by reducing the size and mass of the sensing head. Other subsystems and the overall system configuration are shown in Figure 2. Through a standard keyboard, the operator instructs the computer in the specifics of the task. The computer controls the sampling of the proximity sensors and displays the processed outputs in sequence on a video screen. The results of the sensor scans can also be sent to a printer or saved on a magnetic disk.
Point of Contact:
Antal K. Bejczy
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena CA 91109
818-354-4568
bejczy@telerobotics.jpl.nasa.gov
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