The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or may impinge on a retroreflector target that is in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest. Exemplary systems for determining coordinates of a point are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. A coordinate-measuring device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface.
A second set of coordinate measurement devices belongs to a class of instruments that measure coordinates using one or more cameras to view points of light attached to a probe. Cameras view these points of light and from the images determine the position of the probe tip. In most cases, two or more cameras are used. These cameras may be rigidly connected, or they may be set up individually according to the particular measurement requirements. If the cameras are not rigidly connected, a reference length must be measured by the cameras either before or during the measurement to establish the scale. An exemplary system for determining the coordinates of a point using cameras and points-of-light is described by U.S. Pat. No. 5,196,900 to Pettersen.
It is also possible to find the coordinates of a probe tip by using a single camera to view points of light attached to the probe. Generally such devices are regarded as not very accurate but appropriate when low cost or ability to see into narrow openings is important. An exemplary system for determining the coordinates of a probe tip using a single camera is described by U.S. Pat. No. 5,440,392 to Pettersen, et al.
To get around the limitations in radial distance accuracy in the single-camera system, methods have been devised for combining a camera with a rangefinder. In one embodiment, a rangefinder comprises a distance meter and one or more steering mirrors. The rangefinder measures the distance to a retroreflector mounted on a probe. Simultaneously, a camera positioned near the rangefinder measures the angles to point light sources located on the probe. The combination of distance and angle information gives the coordinates of the probe tip. In a second embodiment, a rangefinder comprises a distance meter and one or more steering mirrors. The rangefinder directs light to a retroreflector mounted on a probe. Part of the laser light returning from the retroreflector travels to a distance meter and another part splits off to a camera. The camera also measures the angles to point light sources located on the probe. The combination of distance and angle information gives the coordinates of the probe tip. An exemplary system for determining the coordinates of a probe tip using a camera in conjunction with a rangefinder is described in U.S. Pat. No. 5,973,788 to Pettersen, et al.
None of these coordinate measurement devices is ideal for many of the problems in an automated factory. Laser trackers, though highly accurate and fast, are too expensive for many common operations such as precisely positioning robot end effectors to drill holes. In addition, laser trackers measure only three-dimensional coordinates rather than all six degrees of freedom. Because of this, laser trackers must in some cases measure three or more targets to find all six degrees of freedom. Measuring multiple targets in this way requires additional time and targets.
Camera-based coordinate measurement devices based on two or more cameras are also expensive and also have geometrical limitations. To be accurate, individual cameras must be spaced relatively far apart compared to the measurement range. Consequently, camera systems are limited in their ability to see into narrow openings. Camera systems also have a limited field-of-view, which means that time-consuming relocation procedures may be necessary to view all targets within the measurement volume.
Systems that contain only a single camera can be made inexpensively but are not accurate enough in measuring radial distance when a small target appropriate for robotic control or factory automation is used. Adding a rangefinder improves accuracy but is too expensive for most robotic and factory automation applications.
In view of these limitations, there is a need today for a device that is low cost and accurate and that does not have the geometrical limitations of today's camera-based systems.
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