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# Revolute joints for heavy links

I have a revolute joint joining two heavy links. 32 and 22 tons. The angle of the joint is controlled by a PD controller in order to simulate articulated steering. The front link (weighing 32 tons) is the child of the rear link (weighing 22 tons). When I set the PID to move the joint to a certain angle, the parent link is jerked by the motion. My P-gain is quite large (>500000). Going lower is not enough to let the joint rotate.

How can I move the front link while keeping the rear link relatively stationary?

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## Comments

I'm not fully familiar with your situation, but why doesn't this make sense? The Third Newton's law of motion should apply. If the parent link is not fixed in the place and it is not much heavier than the child link, I would expect it to move with respect to the map as the reaction to the joint actuation.

( 2019-07-24 02:05:48 -0500 )edit

@kumpari That is true, it would require a large force considering the mass of my links and the friction of the tires. The problem was that the contact of my tires with the ground plane was jittery which caused the rear link to jerk more than it should.

( 2019-07-24 12:47:22 -0500 )edit

## 1 Answer

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Answering my own question:

I managed to keep the rear link 'more stationary' by making sure that the rear wheels have good contact with the ground plane and setting proper friction coefficients. This was done by setting <implicitSpringDamper> to true and setting the values of <mu1>, <mu2>, <minDepth>, <kp>, <kd> to model the softness of the tires and allow them to slightly sink into the ground for better contact.

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Asked: 2019-07-23 13:36:46 -0500

Seen: 47 times

Last updated: Jul 24