Absolute Accuracy Robot Calibration

A robot is generally capable of repeating the same move towards a specific point in space, over and over, within a high level of tolerance (referred to as ‘unidirectional Repeatability’), generally well below 0.1mm. But this does not mean that that particular point in space is well known relative some global coordinate frame (such as the robot base frame): the level of ‘Absolute Accuracy’ achieved by an ‘off-the-shelf’ industrial robot is generally in the several millimeters and more. This is generally due to manufacturing tolerances for every robot unit, incorrect joint zero-mastering, mechanical flexibility, gear backlash, etc.

That problem is further compounded by the additional components forming the complete robot-cell. The end-effector attached at the end of the robot might have its own mechanical flexibility; furthermore, its actual built – and therefore its ‘point of interest’ (e.g. the tip of a drill) – probably differs from the design intent. The same holds true for the fixture located on the floor intended to hold the part. The situation gets even worse when involving one or more external axes along which the robot travels, and/or if the part is held on a one- or multi-axis positioner.

To achieve the highest possible Absolute Accuracy for a given robotic operation, it is therefore imperative to ‘close’ as much as possible the entire ‘metrological chain’ formed by the different components linking the point of interest on the robot’s end-effector (the TCP), on one end, and the point on the part the robot is operating on, on the other end. Sometimes, the just described general process is somewhat inversed, where the robot holds the part through a ‘gripper’ and the tool (TCP) is fixed on the floor.

The DynaCal™ robot calibration system not only allows robot calibration but the calibration of the entire robot-cell in one single step: the robot, the end-effector’s TCP, the fixture’s location (alignment), and optionally the external axes and/or positioner are calibrated altogether. This is achieved by moving the robot to a variety of positions in space, each time measuring the robot at (or at a point in a known location relative to) the TCP, expressed in the part’s coordinate frame. This can be performed with the DynaCal proprietary measurement hardware, or with any external (large-range) measurement equipment (such as a laser tracker).

Full Absolute Accuracy robot calibration as just described is used in virtually every kind of robotic application (except for very low-accuracy applications, such as painting), e.g. in arc-welding, waterjet cutting, drilling and routing, assembly, spot-welding, gauging, etc. The purpose of robot calibration is generally to reduce manual programming time, improve process quality, and/or to enable standard robots to achieve accuracies of much more expensive equipment (i.e. CNC machines, CMMs, etc.). Robot calibration enables ‘Off-Line Programming’, ‘Cloning’ of existing robot-cells, ‘Mirroring’ across production lines, ‘Swapping’ of robots, etc. (see “Off-Line Programming with No Touchup”, “Duplicate Robot Programs with No Touchup”).

Robot calibration, including calibration of all its peripherals, for Absolute Accuracy with the DynaCal robot calibration system is generally done once at the installation of the robot-cell, but it can of course be repeated at any time throughout production, for reasons such as robot maintenance, robot replacement, etc. However, during normal production operation, it is generally recommended to use a dedicated in-line recovery tool instead – such as the roPOD™ or the AccuBeam™ system – in order to automatically detect and correct as promptly as possible any accuracy issue within the robot-cell, thus insuring that your robot-cell will maintain throughout production the level of Absolute Accuracy achieved at installation with the DynaCal robot calibration system (see “Maintain your Robot’s Accuracy throughout Production”).