Focus detection system with zero crossing detection for use in o

Details Focus detection system with zero crossing detection for use in o Focus detection system with zero crossing detection for use in o

1989-05-22

1990-08-28

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

2502014, G01N 2186, G01J 120

Patent

active

049528165

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a focus detection circuit for detecting when the focal spot of a focussed light beam is in focus on a surface towards which the light beam is directed.
It is known in optical scanning probes to focus a light beam from a semiconductor laser onto a surface using a movable objective lens. The reflected light is re-focussed inside the probe onto a focus detector consisting of a pair of photo-diode detectors, the output of which controls a lens tracking mechanism which maintains the focal point on the surface. The design is such that a displacement of the object surface in the direction of the light beam results in a change in the distribution of light energy falling on the photo-diodes. This provides a focus error signal which is used to move the lens in a direction such as to reduce the focus error signal to zero, at which point the focal point of the light beam is back on the surface.
The above-described tracking mechanism relies on the fact that the focal point is always at the same distance from the lens, when using a collimated or coherent light source, so that the movement of the lens which is required to bring the light spot back into focus on the surface equates to the movement of the surface.
It is also known to provide an optical trigger probe (for example a probe described in an article in OEM DESIGN published in Oct. 1986) which may be moved towards a surface and provides a signal when a beam emitted by the probe comes into focus on the surface.


SUMMARY OF THE INVENTION

It is an object of the present invention to provide a focus detection system suitable for use with an optical trigger probe.
According to the present invention there is provided an optical measuring system comprising a probe having a housing, a light source and a focussing device mounted within the housing for producing a light beam focussed at a point outside the housing, and a focus detection system for receiving light reflected from a surface towards which the light beam is directed, charactrised in that the light source and the focussing device are mounted in fixed relationship within the housing, the focus detection system including detector means for producing two outputs indicative of the amount of said reflected light falling on different parts thereof and means receiving said two outputs and for producing a focus error signal which has a zero value when the light beam is in focus on the surface and a non-zero value when the light beam is out of focus on the surface, and wherein the focus detection system further includes a zero crossing detection circuit for detecting the zero crossing points of the focus error signal and for producing an output each time a zero crossing is detected.
In such a system the focal point is always a fixed distance from the lens, and provided that the lens position is accurately known within the probe body, and the probe position is accurately known from the scales of a measuring machine on which the probe is mounted, the position of the surface along the line of the light beam can be accurately determined by noting the machine scale readings when the output signal is produced.
The above described focus detection system however has a range limited to a few tens of microns. Outside this range the focus error signal tends towards zero, and any noise in the system due, for example, to vibration of the probe, or stray light within the probe, can give rise to large numbers of zero crossings when the reflected light level from the surface 12 is low or zero. Thus the zero crossing detector circuit may provide a large number of false trigger signals when operating outside of this range.
A probe with a focus detector system as described above can be used for detecting small variations of shape or in surface detail of a workpiece provided that the position of the surface remains within the working range of the probe.
In order to adapt such a probe for use with a co-ordinate measuring machine in which the probe is driven towards a surface from

REFERENCES:
patent: 3491240 (1970-01-01), Vyce
patent: 4363116 (1982-12-01), Kleuters et al.
patent: 4495407 (1985-01-01), Kodama et al.
patent: 4513408 (1985-04-01), Nomura et al.
patent: 4546460 (1985-10-01), Ando
patent: 4683561 (1987-07-01), Bierhoff et al.
patent: 4766305 (1088-08-01), Fahl
patent: 4815060 (1989-03-01), Nomura
patent: 4816665 (1989-03-01), Hsu
"Non-Contact Probe Uses Light to Measure Curved Surfaces", OEM Design, Oct. 1986, p. 11.

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