Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-09-21
2002-05-21
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S222100, C356S138000, C359S199200, C700S108000
Reexamination Certificate
active
06392222
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to machine systems capable of being precisely positioned with respect to a workpiece and, more particularly, to machine systems having retroreflectors for permitting optical endpoint control and associated methods.
BACKGROUND OF THE INVENTION
Modem machine systems oftentimes include robots, machine tools or other mechanical positioning devices, including computer numerical control (CNC) machines, (hereinafter collectively referred to as “machines”) that must be precisely positioned with respect to a workpiece in order to appropriately machine the workpiece. For example, the CNC machines employed during the manufacture of aircraft, automobiles or other vehicles must be precisely positioned with respect to the workpiece such that the resulting parts are within the relatively strict tolerances demanded by the particular application. In order to determine the position of a machine tool or a portion of a machine tool, such as the end effector of a machine tool, some machine systems include optical end point control. See, for example, U.. Pat. No. 5,903,459 which issued May 11, 1999 to Thomas A. Greenwood, et al., and which describes a precision measuring system and method, the contents of which are incorporated by reference herein. See also, U.S. patent application Ser. No. 08/867,857 filed Jun. 3, 1997 by Thomas A. Greenwood, et al., which also describes a precision measuring system and method, the contents of which are incorporated by reference herein.
Machine systems that utilize optical endpoint control include one or more retroreflectors which are mounted upon the machine to serve as targets. For example, one or more retroreflectors can be mounted upon the end effector of a machine. Machine systems that include optical endpoint control also include a laser measurement system, i.e., a laser tracker, for illuminating the retroreflectors. By detecting the light reflected by each retroreflector, the laser measurement system can determine the distance and the direction to the retroreflector. Based upon the distance of the retroreflector and the direction to the retroreflector, the position of the retroreflector and, in turn, the position of the portion of the machine that is carrying the retroreflector can be precisely determined. Based upon the determination of the actual position of the machine, the machine system can accommodate any differences that are detected between the anticipated position of the machine and the actual position of the machine. These differences in position are attributable to a wide variety of factors including expansion and contraction of the machine and the workpiece as a result of thermal changes in the factory and mechanical misalignments of and between individual axes of the machine tool. By compensating for differences between the anticipated and actual positions of the machine, the machine system can fabricate the resulting part in a much more precise and repeatable fashion.
A variety of retroreflectors have been developed to receive incident light and to reflect the light in a direction substantially parallel to the incident light. However, conventional reflectors have a relatively limited field of view known as an acceptance angle that significantly limits the applications in which retroreflectors can serve to properly reflect incident light. More particularly, light received by a retroreflector within the acceptance angle will be properly reflected by the retroreflector. However, light outside of the acceptance angle will not be reflected and, therefore, cannot be utilized to position the retroreflector. As such, the acceptance angle defined by a retroreflector restricts the position and orientation of the retroreflector relative to the light source, such as the laser tracker. This limitation is particularly disadvantageous in applications in which the retroreflector is mounted upon a machine, such as a robot or other machine tool, that can move in multiple directions and about multiple axes relative to the light source and may frequently be positioned such that the retroreflector does not face the light source, thereby preventing the light emitted by the light source from falling within the acceptance angle defined by the reflectometer. Without adding additional light sources and/or additional retroreflectors which would, in turn, increase the complexity and cost of the machine system, the position of the machine cannot therefore be determined in instances in which the incident light does not fall within the acceptance angle defined by the retroreflector, i.e., in instances in which the retroreflector does not face the light source.
Various types of retroreflectors have been developed, although each defines a relatively limited acceptance angle. One common retroreflector is a trihedral prism reflector that is frequently referred to as a solid corner cube retroreflector. The trihedral prism retroreflector has three mutually orthogonal surfaces such that light incident upon the prism is reflected generally parallel to, but laterally displaced from the incident light. While trihedral prisms are relatively inexpensive and are fairly accurate with the incident and reflected beams being parallel to within 2.0 microradians, the lateral displacement of the reflected beam from the incident beam varies due to refraction based upon the angle at which the incident light strikes the retroreflector, i.e., the incidence angle. In order to maintain accurate retroreflector properties, the trihedral prism retroreflector is therefore limited to an acceptance angle of about +/−15°.
Another type of retroreflector is a hollow corner cube retroreflector that is constructed of three mutually orthogonal mirrors. Although the lateral displacement between the incident and reflected beams does not vary as a function of the incidence angle, a hollow corner cube retroreflector is generally relatively difficult to manufacture and is accordingly more expensive than a comparable trihedral prism reflector. In addition, hollow corner cube retroreflectors typically have an acceptance angle of +/−25°.
A third type of retroreflector is a cat eye in which several hemispherical lenses are bonded to form a single optical element. While a cat eye has a larger acceptance angle, such as about +/−60°, a cat eye is significantly more expensive than a trihedral prism retroreflector or a hollow corner cube retroreflector. While a cat eye has a much greater acceptance angle than a trihedral prism retroreflector or a hollow corner cube retroreflector, the acceptance angle of a cat eye is still insufficient in many situations, particularly in many high precision manufacturing operations in which the retroreflector will be mounted upon the end effector of a robot or other machine tool that will assume many different positions during the manufacturing process.
One attempt to overcome the limited acceptance angles of conventional retroreflectors is to group a plurality of hollow corner cube retroreflectors in a cluster. Unfortunately, the clustered retroreflectors do not form a single, large, continuous acceptance angle. Instead, the clustered retroreflectors form a plurality of distinct acceptance angles with gaps between each acceptance angle. As such, certain angular regions still do not fall within the acceptance angle of any of the clustered retroreflectors. In addition, clustered retroreflectors have not been able to be constructed so as to simulate a single target since the retroreflectors have not been able to be positioned such that their apexes are coincident.
Accordingly, although machine systems having optical endpoint control have been developed, these machine systems are limited by the somewhat restricted acceptance angles of conventional retroreflectors which prevent the machine system from measuring the position of the retroreflector when the machine has assumed a position in which the retroreflector cannot be illuminated within its acceptance angle. In this regard, although a variety of
Allen Stephone
Alston & Bird LLP
The Boeing Company
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