About Repeatability

From Wikipedia:
  • Accuracy – is how closely a robot can reach a commanded position. When the absolute position of the robot is measured and compared to the commanded position the error is a measure of accuracy. Accuracy can be improved with external sensing for example a vision system or Infra-Red. See robot calibration. Accuracy can vary with speed and position within the working envelope and with payload (see compliance).
  • Repeatability - is how well the robot will return to a programmed position. This is not the same as accuracy. It may be that when told to go to a certain X-Y-Z position that it gets only to within 1 mm of that position. This would be its accuracy which may be improved by calibration. But if that position is taught into controller memory and each time it is sent there it returns to within 0.1mm of the taught position then the repeatability will be within 0.1mm.
    Accuracy and repeatability are different measures. Repeatability is usually the most important criterion for a robot. ISO 9283 sets out a method whereby both accuracy and repeatability can be measured. Typically a robot is sent to a taught position a number of times and the error is measured at each return to the position after visiting 4 other positions. Repeatability is then quantified using the standard deviation of those samples in all three dimensions. diagrams
    ST Robotics uses a modified form of the ISO 9283 test with less motion between samples. Positions are taught that test X Y and Z dimensions. Each dimension is tested individually. The robot goes into position against an LVDT (electronic dial gauge) and readings are taken automatically by the controller via the analog I/O card.
    A standard test is conducted over a 24 hour period with a short test over 5 hour period. Within the test the position is measured by the LVDT 30 times at 1 minute intervals. The robot then calibrates and another 30 samples are taken over the half hour period. From the readings we get it can be seen that calibration itself introduces some variation or lack of repeatability. For applications requiring much higher repeatability we advise absolute calibration using what we call a nest. Instructions for doing this are in our manuals.
    This video shows one cycle of the test for the Y dimension on an R12. The gauge is at Y=350mm

    Note that any error in pitch also contributes to the drift. The graph below shows the a typical result from the above test. Vertical axis is times 0.001mm. Most of the drift is attributable to a change in temperature which varied from 15C to 35C over the period due to sunlight through the windows. The robot should be allowed to warm up for 30 minutes before use for best repeatability. Even so total drift is less than 0.2mm which is quite respectable for a low cost robot. Given a constant temperature a repeatability of better than +/- 0.05mm could reasonably be expected.
    X-axis drift (side to side) drift graph
    Y-axis drift (axes 2,3 and 4, still within 0.2) drift graph