How to calculate effort/torque values of a joint

I have a robotic arm with 6 DOF. For each joint, I get get the Joint Wrench, which is composed by the force and torque of the parent and child links of the joints. In other works, for joint 1 I can get ParentLinkForce, ParentLinkTorque, ChildLinkForce and ChildLinkTorque. Each of one is a 3D vector with x, y z.

My question is, how can I transform this to the current effort of the joint in N.m? So I can directly get the effort value of the torque after applying a torque action in the joint.

The wrench characterizes the forces and torques acting on the respective linkages. Part of this is motor torque, the other forces and torques are loading the mechanical structure (and motors down the chain).

If you have followed the Denavit Hartenberg convention in defining the coordinate systems and you have a rotational joint the joint torque is the z component of the torque part either the parent of the child wrench (depending on the motor orientation).

If you did not follow the DH convention, you should locate the axis which is co-located with the motor axis and use the torque component from the wrence corresponding to the co-located axis. If you do not have such an axis, you need to define a new coordinate system which has such an axis and make the coordinate system transformations form the parend/child coordinate system to the new one. Afterwards transform the wrench to the new coordinate system atteched to the motor and select the torque component corresponding to the co-located axis.

Please make sure the regard any gearboxes which the motor might have when calculating any actual motor torques.

• Hi,Please forgive my ignorance on this, my background is in AI and now I am working in robotics as well. Then I understand that if I follow the Denavit Hartenberg convention, the torque of a rotational joint is the Z component of the torque of the parent or the child? I am not sure what is the motor orientation since is a simulation :( Commented Nov 12, 2018 at 0:12
• Its just + or - the same value
– 50k4
Commented Nov 12, 2018 at 5:06