Torque Feed-forward

The torque feed-forward tells the controller what forces is required to move the axis in an arbitrary trajectory.

Here are the major features of torque feed-forward:

• Torque feed-forward results in virtually instantaneous response of the system.
• Feedback control loops (using PID loop or similar) take a finite amount of time before reacting.
• Torque feed-forward relies on an imperfect model of the system. This means that the feed-forwards need help from the feedback control loop in order to get accurate motion.
• Torque feed-forward can make the bulk of the move very quickly, while the feedback control loops correct the small errors that remain. As a result, a faster settling time can be achieved than if torque feed-forward was not used.
• There is a common misconception that torque feed-forward is similar to control loops and result in instability. Torque feed-forward is open loop, so it cannot suffer from closed loop instability.
• Torque feed-forward is typically less sensitive to being misadjusted than closed loop parameters.
• Feedback control systems can be excited into instability by grossly misadjusted torque feed-forward. However, the amount of misadjustment in the torque feed-forward necessary to cause such instability is very rare.