Skip to main content

Feedforward and Motion Profiles Together

Feedforward models predict the voltage a mechanism needs given a desired velocity and acceleration. A motion profile produces exactly that, a velocity and acceleration for every instant of a move. Put the two together, and a feedforward model finally has something genuinely useful to work with.

Closing the Loop Between Two Ideas​

Recall the feedforward equation from earlier in this section:

V=kS⋅sgn(v)+kV⋅v+kA⋅aV = k_S \cdot \text{sgn}(v) + k_V \cdot v + k_A \cdot a

Without a motion profile, vv and aa have to come from somewhere, and the honest answer, without one, is usually "a single, final target velocity, commanded all at once," which throws away exactly the smooth, moment-to-moment detail feedforward is built to use. A motion profile supplies vv and aa continuously, at every control cycle, so the feedforward term can predict the right voltage for right now, not just for wherever the mechanism will eventually end up.

In practice, a profile-following controller runs both pieces every single cycle:

  1. Ask the motion profile (trapezoidal or S-curve) what position, velocity, and acceleration the mechanism should be at right now, this instant, partway through the move.
  2. Feed that velocity and acceleration into the feedforward model, predicting the voltage needed to actually achieve them.
  3. Feed the position (and often velocity) error, the small difference between where the profile says the mechanism should be and where it actually is, into a PID controller.
  4. Command the feedforward voltage and the PID correction together, added on top of each other.

Why This Combination Works So Well​

This is the same principle raised on Closed Loop Control's landing page: a good feedforward model does most of the work predicting the right output, and the closed loop only has to clean up whatever small amount the prediction gets wrong. A motion profile makes that prediction dramatically better by giving feedforward a target that's always close to the mechanism's actual current state, rather than one far away. With the profile doing the heavy lifting and feedforward doing most of the pushing, the PID gains on top can often be kept small, gentle, and far less prone to the overshoot and oscillation problems that come from relying on PID to do all the work by itself.

See Comparing Motion Profile Shapes for when a trapezoidal profile is enough and when the extra smoothness of an S-curve earns its cost.