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To be useful, industrial robots must meet positioning accuracy requirements for their given applications. Most industrial robots can return repeatedly to the same location in space quite precisely; they typically meet published repeatability specifications on the order of 0.5 mm. On the other hand, most industrial robots cannot move as precisely to a specified (x, y, z) position in space; they typically meet published accuracy specifications roughly an order of magnitude higher (typically 10 mm or worse) (Owens, 1994). In many cases, published repeatability specifications meet positioning accuracy needs in industrial robot applications such as spot welding, spray painting, and assembly. However, published positioning accuracy specifications often do not meet industry needs, when using off-line programming rather than manual teaching methods. As a result, to meet application positioning accuracy requirements, most robot users turn to off-line calibration to bring positioning accuracy close to robot repeatability levels (Owens, 1994). Off-line calibration generally consists of the following five steps: 1. Move the robot into several poses (positions and orientations). 2. Measure and record the precise 3D workspace coordinates of the robot tool center point (TCP) at each pose. 3. Read and record the corresponding position of the robot, from the robot controller, at each pose. 4. Use the differences between measured 3D workspace coordinates and corresponding positions read from the robot controller to correct the parameters in the kinematic model used by the controller to position the robot. 5. During robot operation, use the corrected kinematic model to compute adjusted positions in space and then command the robot to move to the adjusted positions (which causes the robot to move to the actual desired positions). The number of calibration poses used and the corresponding link positions for each pose must be selected to provide the information needed to accurately compute the kinematic model parameters (Robinson, Orzechowski, James, & Smith, 1997). For example, Owens (1994) used 25 different poses, while Rocadas and McMaster (1997) used 50 different poses. To measure pose positions precisely enough to complete off-line calibration, robot manufacturers generally use expensive measurement devices, such as theodolites, coordinate measurement machines, or laser tracking systems (Mayer & Parker, 1994; Nakamura, Itaya, Yamamoto, &

A Non-Contact Method for Detecting On-Line Industrial Robot Position Errors