Laser triangulation systems with stripe or spot locators have found applications in the industrial environment. In particular, laser cross locator systems have been found to provide more rapid, effective, and accurate hole location than conventional systems. A laser vision sensor has been developed consisting of a laser, cross-generating optics, a laser driver, a high-powered LED array, an LED driver, a high-resolution camera, a communication link, laser safety functions, and a temperature monitor. The system has the ability to locate and measure both conventional and countersunk holes in a variety of surface materials. Countersunk holes are more difficult to locate because the surface material is usually highly reflective and the countersunk hole area is strongly absorbent.
In the laser cross locator system, a laser generates a cross pattern larger than the hole size, while an annular array of LEDs provides hole illumination. A CCD camera captures the reflected signal, which undergoes processing to yield hole size and provide centering data for computer-numeric-controlled processing.
In laser cross locators, the laser system generates a cross pattern larger than the hole size. The LED array, in a circular pattern around the camera objective lens aperture, provides hole illumination at a spectral wavelength matched to the camera's bandpass filter. The bandpass filter minimizes any effect from the change in ambient lighting conditions that could degrade sensor operation.
Preliminary analysis of the laser cross image provides an initial estimate of the hole location parameters. The four arms of the cross strike the hole edges, providing the information necessary to determine hole radius, position, and the normal axis to the plane of the surface material. A second laser-cross image-analysis mode uses the capabilities of the integrated megapixel camera. Leveraging the hole laser-cross location parameter information, direct gray-scale analysis of the laser-cross image provides high-accuracy measurements of the hole.
The laser vision sensor can identify the position feature of a work piece to an accuracy of a few micrometers. This positional information uses a feedback digital signal to send offset information to the control driving mechanism of a robot or computer-numeric-controlled (CNC) machine tool. The sensor determines the exact location of the work-piece features, as well as the orientation of the surface complex (configuration) at a particular point.
When the sensor locates the center of a hole, in addition to hole position, the data available to the machine tool control includes hole diameter, countersink size, work-piece angle, and sensor height above the work piece. Sensor height information can speed productivity by overriding the CNC program, providing faster, more rapid infeed information to the next work-piece cut.
The design offers a 10-mm field of view, and at a standoff distance of 30 mm can measure height to ±4.3 µm, centering to ±3.6 µm, tool angle to ±0.3°, hole diameter to ±7.6 µm, and countersink depth to ±22 µm. oe
Roger Reiss is an independent consultant affiliated with Minotech Engineering Inc., Andover, MA.
Michael Hoffman is owner of North End Machine Co., Townsend, MA.