gripper finger force not balanced

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louxibai
Posts: 4
Joined: 10 Oct 2018, 22:36

gripper finger force not balanced

Post by louxibai » 11 Feb 2019, 19:24

Hi,

I build a Franka Emika panda in VREP for simulating object grasping, the original gripper fingers from their CAD file are not the same as the real ones, so I replaced them with a different finger, but when it grasps the Cuboid and lifts it, the two fingers keep oscillating, it is hard to describe, please check the scene I uploaded:

https://www.dropbox.com/s/kquw0c9lte0iu ... t.ttt?dl=0

I'm not sure how to fix this. I suspect it might be the position of the fingers are not balanced, but I don't know how to position them perfectly. Could you give some advice? Thanks a lot!

Best Regards,
X L

coppelia
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Posts: 6952
Joined: 14 Dec 2012, 00:25

Re: gripper finger force not balanced

Post by coppelia » 12 Feb 2019, 10:41

Hello,

this is typical, since the two joints exert a same but opposite force.
This can be though similarly as trying to carry a small cube with two fingers from different hands/arms: the operation will not be as stable as carrying the box with two fingers of the same hand.

In order to avoid this, you should have something like:

gripperBase --> positionalJoint --> leftFinger --> grippingJoint --> rightFinger

Actuation will happen by changing the joint velocity sign in grippingJoint (which is in force/torque mode).
positionalJoint (which is in position control mode) will adjust its position according to the position of the grippingJoint, i.e. by trying to keep the two fingers always centered. This can best be achieved with a joint callback function. Have a look how most of the grippers in the model library have been designed (e.g. the Baxter gripper.ttm).

Cheers

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