Why do some motors wobble when they stop, and others don’t?

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To clarify, I don’t mean something like a car engine. I mean, something like I see electric arms that move, then when they get to the point where they need to stop, they stop instantly but then kind of wobble for a second. But then other motors don’t do this, and they just stop instantly without any wobble. I notice this with stuff like animatronic robots. Sometimes low-quality animatronics have the wobble that really takes away from the illusion of life.

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2 Answers

Anonymous 0 Comments

The low quality animatronic wobbling is because the “stop” is handled abruptly. The inertia of the moving parts needs to dissipate over time.

A great example would be your driving style. If you’re going 20mph and need to stop, let’s say you have two choices:

1) Slowly apply the brake and then slowly reduce brake pressure as you come to a comfortable stop. If you had an air freshener hanging from your review mirror, it won’t even wobble.

2) Just slam the brakes. This will certainly stop your car but your air freshener is going to swing all over the place for a couple seconds.

In both cases you stopped in the same spot after the same motion, but the way in which that motion was halted matters. Cheap animatronics act like slamming a brake in option 2. It takes a lot more software control and higher quality actuators to behave like option 1.

In addition to that, there are likely some mechanical fitment tolerance issues at play. Not everything fits together perfectly and there may be a little bit of slop in the joints and linkages, such that as it starts/stops moving, there’s a little bit of “play” in the joints that needs to be taken up first. That’ll manifest as a jerky appearance to the motion sometimes. Or a little bit of wobble in other joints and linkages as a particular motor starts or stops.

The biggest difference is cost. There are different types of actuators and motors, and the really robust and smooth ones cost a lot more money than cheaper parts with less precision or control. Same with linkages fitment strategies. The high quality stuff is really expensive, when a cheaper option is 90% as good and only costs 10% as much.

Anonymous 0 Comments

Most of these motors are step motors or motors with worm gears in them.

When the motor stops, the weight of what it holds(the arm and everything the arm holds) creates an impact that kicks on all the other motors freeplay(the loose between parts) necessary for the motors to work without burning themselves out.

The way to stop these wobbles is to program a deceleration in the movements or build those components with dampers or springs to absorb the shock. Both solutions are not necessary if the robot is still at an experimental phase, if it will not be used as a consumer product(as it could look cheap) or if it is not handling sensible objects.