CoppeliaSim forums

I stumbled upon the following: in an single 3 links+2 revolute joint model the mass of the links determines successful inverse kinematics.
Image of the the model: https://www.dropbox.com/s/q1q2vflci55bazj/Capture.PNG

If the model is assigned a mass of 1.0 (kg?) for each link, the model would "collapse" and lay on the ground when ask to follow a target in its operation plane. However if the link is reduced to 1/10 of its weight the target is reached. I would believe the joint operation is independent of the weight but apparently not. Doesn't the ik mode + hybrid operation imply that the joint is basically forced to the position the IK determines?

Could you elaborate on what is really hybrid operation? The paragraph:

When the joint is in passive mode, inverse kinematics mode, dependent mode or motion mode, it can optionally also be operated in a hybrid fashion: hybrid operation allows the joint to operate in a regular way, but additionally, just before dynamics calculations, the current joint position will be copied to the target joint position, and then, during dynamics calculations, the joint will be handled as a motor in position control (if and only if it is dynamically enabled (...)).

is still not clear to me. What do you mean by operate in a 'regular way', is the 'target joint' position other that the joint itself? what is actually the target position?

Thanks.

Hello,

when you operate a joint in IK mode, and additionally have enabled the hybrid operation, then the joints will operate as dynamically enabled joints that work in position control. The position controller relies on maximum allowed torque, PID parameters and so on. You can adjust the joint dynamic properties here.

So what really happens for a dynamic manipulator when the joints are in IK mode with hybrid operation:

• IK will be computed. This computation results in target joint positions (or joint positions needed in order to satisfy the IK equations).
• the target joint positions will then be applied by the dynamics engine, as target values to reach (you cannot instantaneously modify a dynamic joint, as you would also not be ableto instantaneously reach a new joint position on a real robot)
• the dynamics engine (via the joint controller) will then try to match those desired values. But since the joints are dynamic, they need to follow dynamic rules such as: maximum exertable torque/force, maximum velocity, etc.
• the result of the dynamics computation are never exact, and will always slightly differ from the exact values computed via IK.

Cheers

A very detailed and clarifying answer. Thanks a lot!