Load data vs. point reach accuracy

    Hello,


    Many of times I went into discussion if completing Load Data Determination is crucial in improving point reaching accuracy of robot.


    Just to be clear, I am aware of a neccessity of performing LDD in aspects of improved motion planning and lifetime of servos etc (and I always complete LDD), but I am pretty sure, that Load Data is only regarded in dynamics of robot, and has nothing to do when it goes about statics.


    Let's assume that we have two same robots with the same tool, f.e. 160kg servo gun. Both robots have tool offset teached. On the first robot load data determination was performed, and the second one is without LDD.


    Then we calibrate base with those heavy tools mounted on flange, using Indirect method (4 points, entering coordinates from plate). Are we gonna have difference in quality of those calibrations between those two robots, or not?


    Below I attach quote from KSS8.2 SI Manual:

    Quote

    5.11 Load data

    The load data are factored into the calculation of the paths and accelerations

    and help to optimize the cycle times. The load data must be entered in the robot

    controller.

    Yes. The resolver is located on the motor shaft. But the actual position of the robot is behind the gearbox on the shoulder.

    The logic is based on the fact that the rear drive is not 100% rigid, there is mechanical backlash.

    And in order to achieve repeatability of the robot's position in space, calibration is necessary

    also under load. This opportunity to offer a cuckoo and from practice I know that I have a real effect,

    on the accuracy of the point in space. The cogs are more resistant to mechanics,

    viable against wear than fanuk.Kteri calibration of the load does not know.

    Straight feel of 100% without mechanical clearance, throughout the lifetime

    Sorry, I did not understand your post at all.


    Quote

    And in order to achieve repeatability of the robot's position in space, calibration is necessary

    also under load.

    Calibration, which one? You mean Load Data Determination?

    No, calibration of the robot's actual kinematics, vs it's "ideal" model.


    Basically, every robot works by assuming it's internal ideal kinematic model is accurate. Various calibration methods to "correct" the model to the as-built condition of the robot, and include the "flex" in the various links due to gravity, exit. The SARCA product from New River Kinematics is one example.


    It's possible to create your own SARCA equivalent, but the math is non-trivial, and the calibration needs to be done using the actual working payload in order to be accurate. It also needs to be carried out in Joint space, not just Cartesian space (ie, moving the to same position with a different orientation, or the same position&orientation but with a different wrist/elbow config requires separate calibrations for each condition).


    By contrast, the payload data has relatively little effect on the robot's positional accuracy (unless this is an Absolute Accuracy robot). Payload data mostly effects speed, inertia, and gravity compensation. Mostly. For most milling operation, I suspect that the low speed, and the existence of "recoil" forces from the milling head, would make the accuracy effects of the payload data lost in the noise. Still, making your payload data as accurate as possible is always a good idea, and in this case would eliminate one potential contributor to accuracy issues.

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