Laser tracker-based kinematic parameter calibration of industrial robots by improved CPA method and active retroreflector

Typical approaches to calibrate industrial robots are based on open- and closed-loop methods; the screw-axis measurement methods traditionally receive much less attention. Although the identification process does not guarantee the physical-mathematical link between the robot parameters in the first two groups of techniques, these techniques are generally more effective in reducing the global positioning error compared to the screw-axis methods. The third group of techniques acquires parameters based on the physical reality of the robot, effectively keeping the physical-mathematical link. This group is considered more appropriate than the previous two groups; however, it cannot reduce the overall error when considering the entire workspace of the robot compared to the previous groups. This paper presents a new technique to identify the kinematic parameters of an industrial robot based on a combination of techniques from the aforementioned categories. This new data acquisition technique uses a laser tracker with an active target, which maximises the angle covered by each joint and greatly simplifies the screw-axis measurement process. An identification procedure based on circle point analysis is also proposed, and the procedure evaluates the technique by obtaining initial values with a new formulation of the objective function of error based on mutual distances between the points captured in screw-axis measurements. This type of measurement also allows the eccentricity and backlash of each joint to be characterised independently such that local joint corrections could be made in combination with the identified parameters.