# Revolute Joint: Actuation by Torque AND Motion

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Thomas Scheu on 1 Mar 2023
Commented: Joel Van Sickel on 18 Oct 2023
Hi,
for my model i want to actuate a revolute joint by a constant torque and a known motion. This is because I have a constant input rotational speed, but a variying torque.
It works by applying torque and motion separately, but in combination I get the following error:
"In the dynamically coupled component containing Revolute Joint 'Test_Shaft_Gear_3/Revolute Joint', there are fewer joint primitive degrees of freedom with automatically computed force or torque (0) than with motion from inputs (1). The prescribed motion trajectories in this component may not be achievable. Solve this problem by reducing the number of joint primitives with motion from inputs or increasing the number of joint primitives with automatically computed force or torque. Resolve this issue in order to simulate the model."
Is anybody out there to help me?

Joel Van Sickel on 10 Mar 2023
You cannot apply torque AND motion like this. If you apply torque, the model will calculate what the motion is for you. If you apply motion, the model will calculate the torque for you. So you need to pick only one of them.
Thomas Scheu on 18 Oct 2023
Is there an option for a joint which is fully specified?
My problem is I have a simple gear on a shaft and i want to add power to this shaft/gear (so I know the velocity AND the torque of the shaft).
Joel Van Sickel on 18 Oct 2023
You can know the velocity and torque of the shaft, but from a modelling perspective you can only use one of them as an input. In most cases, trying to apply torque and motion to a joint overspecifies the problem such that is is mathematically impossible to solve. If you want to apply a constant power, you can measure the speed, and then apply the appropriate torque.
However, you might be happier using inverse kinematics:
I

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