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I'm trying to find some youtube videos which are showing robot arms with some malfunction in it. The problem is, that the keyword is unclear. Should i search for “unstable”, “tremble”, “jerking” or “vibrate” as additional word?

I'm sure, that especially lowcost robot arms have often this kind of behavior. If the mechanic contains only of cheap plastic and the servo motor isn't accurate, the construction will result into an underactuated system which is difficult to control. But i like this behavior, because this provides the challenge to develop a motion controller which can overcome the issue with software.

Are any example videos available, in which the robot arm is quite inaccurate and makes it hard to position the endeffector to the target?

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The phenomena you're looking for is called "stick slip."

Static friction is the friction that hold an object stationary. Dynamic friction is the sliding friction between surfaces you get while in motion. In the transition from static to dynamic frictions, it's basically like flicking the load with a force equal to the difference between the two friction coefficients.

The problem comes when that impulse is large enough that it accelerates the load faster than the actuator/actor can keep up. If the load accelerates so much that it has the chance to come to a stop before the actuator catches up to the load, then you re-enter the static friction regime. This happens over and over; stick, slip, stick, slip, and typically happens fast enough that it enters the audible range.

I found a pretty entertaining video on YouTube that gives lots of good examples along with a more detailed engineering explanation. I skipped around a bit, but I don't think they showed a robot arm.

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  • $\begingroup$ The term “stick slip” doesn't fit quite accurate to the issue but it directs into the right direction. Under the term “damping” some more examples are given in which the robot moves uncontrolled. $\endgroup$ Feb 24 '19 at 12:26

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