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Constraint Torque Output

Hi,


I am using Vortex Studio Essentials to include and simulate the model of a Mars rover. It is connected to Simulink for controlling and trajectory generation.


For the transmission between the CC motor and the wheel, i have connected them by a cylindrical constraint. I would like to see the torque that applies the constraint for reaching the desired velocity, but graphing the torques in the outputs page of this constraints, I get results that do not have sense to me.


What is the output that shows the actual value of the torque applied by the constraint?


Hi Pablo,


Sorry for the late reply. The sign of a joint control force depends on the direction of the axis and the order of the parts. So, it might simply be that the sign is negative because the force that is reported is the force that is applied to part 1 and not part 2 or vice versa.

In more detail, the control force is simply the Lagrange multiplier which is computed by the Solver and that satisfies the constraint equation that models the joint control. By multiplying the Lagrange multiplier with the transpose of the Jacobian of the constraint equation one recovers the generalized force that this constraint equations applies to the parts (F = J^t * lambda). Since the Jacobian contains entries for connecting both parts, but with a sign flip, the generalized force vector F contains the forces and torques that get applied to both parts, but flipped (according to Newton's third law, of action-reaction).

Since the control force is just a scalar (the Lagrange multiplier lambda), it can have either of the two signs (positive or negative), and this simply depends on the Jacobian of the joint, which depends on the attachment axes and the order of the parts.


Hope this helps,

Daniel




Hi Daniel, 


Thank you for the reply, it has been helpful. 


But, analyzing the response of the control torque from the joint which represents the motor, how is it possible that the establishment value of the torque is negative (as the image shows) if the vehicle is moving foward in a flat surface? Do I have to modify some parameter of the joint, or it has an offset applied...?


image



Thanks for the attention,


Pablo.

Hi Pablo,


The control force of the controllable coordinates of the Cylindrical joint are available in the outputs tab.

You can get the scalar force applied by the joint to achieve the desired velocity (in case you motorized the respective coordinate), and you can also get the total world space force and torque as applied by the joint to the two jointed parts.

The control force can be found under the Linear and Angular sub-sections.

And the world space force and torque applied to the parts can be found further up.


I marked all relevant fields in red here:

image


Note that the scalar control force values are only the force applied by the joint to drive the controllable coordinates. The world space force and torque applied by the joint to the parts contains this force and all the other forces which need to be added to keep the two parts attached.


Hope this helps,

Daniel

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