Feedforward control is a pro-active way of controlling mechanisms. Unlike PID, which reacts to errors, Feedforward uses a model of the physical system to pre-calculate the power needed to achieve a specific movement.

Why use Feedforward?

With PID alone, if you have gravity or friction (like an arm or linear slide), the mechanism will likely sag or drop. You would have to use a high K_i to compensate, which makes the robot slower to respond.

Feedforward calculates the exact amount of power needed to maintain a certain position (against gravity) or speed (against friction).

1. Velocity Feedforward (K_v)

This is the amount of power needed per unit of velocity.

• Output = K_v × target velocity

2. Static Friction Feedforward (K_s)

This is the “voltage jump” needed to overcome friction and get the robot moving.

• Output = K_s (sign stays consistent with direction)

3. Gravity Feedforward (K_g)

Crucial for arms and slides.

• For a linear slide: Constant K_g to hold the weight.
• For a rotating arm: K_g × cos(θ) where θ is the arm’s angle.

Implementation in FTC

Here is how you can implement a standard Feedforward calculation for a linear slide:

public class SlideFeedforward {
    private double ks, kg, kv, ka;

    public SlideFeedforward(double ks, double kg, double kv, double ka) {
        this.ks = ks;
        this.kg = kg;
        this.kv = kv;
        this.ka = ka;
    }

    public double calculate(double velocity, double acceleration) {
        return (ks * Math.signum(velocity)) + kg + (kv * velocity) + (ka * acceleration);
    }
}

Combined PID and Feedforward

In a real FTC robot, you should use PID + Feedforward. The Feedforward does the “heavy lifting” by getting you near the target, and the PID handles the small corrections.

double ff_power = feedforward.calculate(target_vel, target_accel);
double pid_power = pid.calculate(current_position);

motor.setPower(ff_power + pid_power);

Tuning Feedforward

If your robot has Roadrunner or Pedro Pathing, they come with built-in tuning tools for K_s and K_v for the drivetrain. For your custom mechanisms, start with K_s (the minimum power to move the mechanism), then K_v (the relation between power and steady velocity), and finally K_g (the power to hold it still).