Started with a copy-and-paste from an email from Les Watts to the Developer list:
"Sometimes there is terminology confusion with this stuff, so could we
define terms as follows? I know we are all familiar with the functions,
but might call them different names."
- Trajectory planner: Program for calculating a path based on typically G code input or cannonical functions. Normally deterministic, since the source g-code is time invariant.
- Interpolation: technique used by a trajectory planner to assign positions in between (gcode) waypoints.
- PID controller: Works by comparing current position, vel, etc vs time to the plan created by the trajectory planner. Must be real time, because perturbations are not known.
- FF: Terms teaching the contoller the dynamics of a machine. In the limit, if FF were a pefect represntation of a machine's system function, it could run open loop with no errors. If the machine dynamics are time invariant, FF is deterministic and does not have to be RT. I note that typically machine dynamics are not time invariant though in the general case because the load at the end effector has an unknown mechanical impedance.