The present disclosure relates to a coordinate-measuring device having the ability to determine three orientational degrees of freedom (DOF). Such a coordinate-measuring device may be used in conjunction with a device having the ability to measure three translational DOF, thereby enabling determination of the position and orientation of a rigid body in space.
Some coordinate-measuring devices have the ability to measure the three-dimensional (3D) coordinates of a point (the three translational degrees of freedom of the point) by sending a beam of light to the point. Some such devices send the beam of light onto a retroreflector target in contact with the point. 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 (ADM) or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. The device may include a gimbaled beam-steering mechanism to direct the beam of light to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more beams of light it emits. A coordinate-measuring device closely related to the laser tracker is the total station. In many cases, the total station, which is most often used in surveying applications, may be used to measure the coordinates of a retroreflector. Hereinafter, the term laser tracker is used in a broad sense to include total stations.
Ordinarily the laser tracker sends a beam of light to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors that intersect in a common vertex point. For the case of a “hollow” SMR having a reflecting surface in contact with air, the vertex is located at the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to a surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of laser tracker contains only an interferometer (IFM) without an ADM. If an object blocks the path of the beam of light from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner, as described in more detail below. Some laser trackers contain only an ADM without an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges et al., the contents of which are herein incorporated by reference, describes a laser tracker having only an ADM (and no IFM) that is able to accurately scan a moving target. Prior to the '446 patent, absolute distance meters were too slow to accurately find the position of a moving target.
A gimbal mechanism within the laser tracker may be used to direct the beam of light from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest. The gimbal mechanism used for a laser tracker may be used for a variety of other applications. As a simple example, the laser tracker may be used in a gimbal steering device having a visible pointer beam but no distance meter to steer a light beam to series of retroreflector targets and measure the angles of each of the targets.
Angle-measuring devices such as angular encoders are attached to the mechanical axes of the tracker. The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
Several laser trackers are available or have been proposed for measuring six, rather than the ordinary three, degrees of freedom. Such laser trackers combine measurement of three orientational DOF with measurement of three translational DOF to obtain measurement of six DOFs. Exemplary six-DOF systems are described by U.S. Pat. No. 7,800,758 ('758) to Bridges et al., the contents of which are herein incorporated by reference, and U.S. Pat. No. 5,267,014 ('014) to Prenninger, the contents of which are herein incorporated by reference.
One method of measuring three orientational DOF of a retroreflector is to project light onto a retroreflector that includes marks. The marks, which are captured by a camera, are evaluated to determine the three orientational DOF. Prior art methods have included markings at the lines of intersection of the three mutually perpendicular reflectors of a cube-corner retroreflector. For the case of a cube-corner retroreflector made of glass, prior art methods have also included marks placed on the front face of the retroreflector. Although these markings and methods are suitable for their intended purpose, there is a need for improved markings and methods.