Joint Friction of Robot Dynamic Models

The robot’s dynamic model is required in the implementation of most advanced model-based control schemes. The dynamic model is crucial because it can be used to linierize the non-linier system in both joint space and task space. Since the robot’s dynamic parameters are normally not available for industrial manipulators, proper procedures should be carried out to identify these parameters.

One way to identify the dynamic parameter is to dismantle the robot and measure link by link. However, it is obvious that his approach is not always feasible in practice. Another problem, with dismantling approach is that it does not account for the effects of joint friction.

In order to account for joint friction, several methods were proposed. These methods can be roughly divided into two groups: to identify joint friction and rigid body dynamic separately or to identify joint friction and rigid body dynamics simultaneously. The former first identify the rigid body dynamic parameters using the identified friction parameters. Since friction parameters are identified joint by joint, non-linier dynamic friction models such as Stribeck and/or hysteresis effects can be considered.

The main drawback of this method comes from the fact that friction can be much time-varying. Moreover, friction forces/torques are always coupled to the inertial forces/torques, thus, one can not be precisely identified without the other.

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