Feedforward

Feedforward predicts the actuator effort needed for a desired motion. Feedback corrects the remaining error. Used together, they produce faster and more consistent control.

Common model

voltage = kS·sign(velocity) + kV·velocity + kA·acceleration

kS overcomes static friction, kV models velocity demand, and kA models acceleration demand. Elevators and arms also need gravity terms.

double ffVolts = feedforward.calculate(setpoint.velocity, setpoint.acceleration);
double pidVolts = controller.calculate(measuredPosition, setpoint.position);
io.setVoltage(MathUtil.clamp(ffVolts + pidVolts, -12.0, 12.0));

Why feedforward helps

A feedback-only controller must wait for error before reacting. Feedforward can request approximately the right effort immediately, leaving feedback to handle model error, friction changes, battery voltage, and load variation.

Characterization

Estimate constants from measured voltage, velocity, and acceleration rather than guessing. Characterize the real mechanism in both directions where gravity or friction is asymmetric.

Validation

Plot requested velocity, measured velocity, feedforward voltage, feedback voltage, and applied voltage. A large continuous feedback term suggests an incorrect model, units problem, or unmodeled load.