Hi all,
I have KUKA KR8 R2100 ARC HW mounted at wall (90 deg, it's configured correctly in WorkVisual).
I have some problems with accuracy (absolute accuracy, not repeatability). Recently I've checked mastering and realized that it's not const for J1. It depends on J2 and J3 angles. I rotated J2 and J3 and mastering for J1 changed. The max difference that I've found is 0.04 deg.
I didn't checked but I think that the same problem happens for J2 and J3.
Of course I've done final mastering for J1 when J2 and J3 are also in positions for mastering. But I understood that gravity has effect on robot accuracy. The total error may be ... 3 mm at bad position. May be larger because mastering position for J1 is only ~20 deg. Gravity error will be larger for J1 = 90 deg.
Does KUKA have gravity compensation for standard robot? Or it's included only in additional option (ABS, HA)?
I think that gravity compensation can use only known parameters: kinematics, weights of links, centers of mass, mount angle. There are no required to do accurate measurements with laser tracker.
I think that for floor or ceiling mounted robots mastering for J1 will be the same for different J2, J3.
KUKA KR8 gravity compensation
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Vlad222 -
November 19, 2019 at 11:27 AM -
Thread is Unresolved
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there are many things in MADA that affect static and dynamic behaviour of the robot. many such parameters are intentionally not documented as changes there are only done in consultation with KUKA. for example elasticity compensation etc can be switched on/off etc.
"I think that for floor or ceiling mounted robots mastering for J1 will be the same for different J2, J3."
why do you think that?
even without changing mounting option, if you change the load on the robot arm or change arm position, gravity will cause different torque on joints. although small, such elastic effects are measurable (link flex, gearbox compression etc.). certainly with high accuracy mastering as used with KUKA those effects can be observed. this is why for best results, all joints need to be brought to a repeatable position before mastering. mechanical zero or premastering position is a good way to ensure that repeatability.
changing mounting of the robot will also mean that for for best results mastering should be repeated.
and if robot is not positionally accurate, it is quite reasonable to see deviations over large travel (not repeatability).
positionally accurate robots can be measured to optimize accuracy in certain space (part of robot working envelope). this means that even positionally accurate robots may have deviations larger in some places of that working envelope.
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why do you think that?
Because gravity affects J1 of wall mounted robot. But not affect (has much smaller affect) J1 of floor or ceiling mounted.
Quotefor example elasticity compensation etc can be switched on/off etc.
Can I find this parameter in $robcor.dat or $machine.dat? Or it is only for HA robots? Is it affect position (static) accuracy?
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as mentioned, there a many settings without descriptions and reason is that users are not supposed to mess with them, should only follow recommendations after consulting KUKA. i mentioned elasticity just as an example. i don't know if and what effect it would have in your case and - it is something to discuss with KUKA. everything that users may need to be aware is in user documentation, other things are not disclosed for a reason and i don't have insight or into about things that are not disclosed. there is a lot of things in $DYN_DAT[] for example. btw. one of those values is used to enable or disable elasticity compensation. who knows what others may do...
mounting robot in different positions is not that rare, including offsets (tilted mount to preload A1 for example). therefore i would expect that gravity vector is used in any models that KSS uses. and i don't think that they have three different calculations (floor, ceiling, wall) for each of the models. i know i would not want to triple my work, it would make any support and improvements a complete nightmare.