Material Pickup Problem

  • Hello All,

    I am trying to figure out the best way to perform an operation with my Robot. The system has a vacuum lifting frame attached and it is loading and unloading sheets of PVC plastic into a thermo-former. The problem I am trying to solve is how should I go about adjusting to the diminishing stack of raw material as the process is going on. I figured I can either:

    A. program in an incremental drop every sheet and rely on the operator to reset the system when a fresh stack is loaded

    B. run with a proximity sensor that senses the height of the stack as the robot lowers to it

    B seems to be the more fail-safe option that does not require added input to prevent crashes, but being new to Kuka Robots I do not know how to tell the robot to move to a position over the stack, then lower in the Z direction until the proximity sensor activates. Any input, ideas, or program examples would be very much appreciated.

    For reference:

    Kuka KR210 L150

    Krc2 ED05 running 5.3.2

  • So I would have the robot moving towards the lowest spot of the stack(ex- From P2 -> LIN P3) and have an interrupt ready to catch the signal from a proximity sensor? That interrupt program would contain a Brake, activate my vacuum, and return to the main program to then lift and continue on?

  • Pretty much. A lot of operators mix the two types -- for speed, they use the calculated offset method, and have the sensor interrupt for finding the top of the stack (whenever an event, like the loading area light screen being broken, could indicate the stack has been meddled with), and as a "backup" in case the calculated offset is incorrect for any reason.

    One issue with using the search motion all the time is that the robot's stopping distance at high speed can be pretty long. So another approach might be to do a highs-speed dive to a "safe distance" above the calculated top of the stack, then switch over to a speed low enough to make the stop-on-interrupt work. Another approach would be to use two sensors -- one long range, the other short, and have the long-range sensor downshift the robot speed so that the short-range sensor can stop the robot at the right height.

    Be aware: most low-cost sensors have fairly large "error" ranges, so counting on the robot to stop at exactly the same distance from the plastic every time is probably not going to work. Spring-loaded vacuum cups are almost always used in conjunction with sensors in this kind of application. If you have a good vacuum sensor, you might use the "inexact" sensor to slow the robot down as described above, and use the vacuum-made sensor to actually stop the robot.

  • Thank you for that information, it has started to clear things up for me and I will continue to do some more digging as I just found the Expert programming manual. I have the spring loaded cups already so I have an inch and a half of wiggle room to work with, combined with a few other options you mentioned I think I can get it going. What is a typical stopping distance if I am running say 0.3 m/s? Or is it specific move dependent?

  • It's pose and inertia dependent. The robot stops slower when it reaches further out, or if the end effector is heavier. You'll probably have to run some tests to determine what the stopping distance is on your setup. 300mm/sec sounds a bit fast, though.

    Now, the robot's stopping distance should be pretty consistent. Usually I find the biggest issue to be variance in the trigger distance of the sensor. I've had cheap optical sensors hitting shiny surfaces whose trigger distance would vary by more than 100mm (sometimes 500-1000!) depending on whether the surface was smooth, rough, dusty, or clean.

    Bias for longer distance -- if your sensor trips the robot into slow motion too soon, you just lose some cycle time. If it trips too late, you'll collide hard and maybe break your end effector. Using a sensor that's immune to surface shininess (like an ultrasonic "sonar" transducer) will be more expensive but probably more reliable.

  • May I add that the System ingegrators manual may be better than the "Expert programming manual"
    You can download it from this thread... post nr 10.

    Kuka Downloads (software, cad...)

    Example on page 359.

    "The robot is to search for a part on a path. The part is detected by means of a

    sensor at input 15. Once the part has been found, the robot is not to continue
    to the end point of the path, but to return to the interrupt position and pick up
    the part...."

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