Shooter

What Does a Shooter Do?

A shooter is a great solution when game pieces have to be scored into a high goal. A shooter can typically be made with just one or two motors to spin a set of top and bottom / left and right wheels.

Code Example

 private final TalonFX shooterTopMain; // Falcon
  private final TalonFX shooterBottomFollower; // Falcon
 
  private final MotionMagicVelocityVoltage motionMagic = new MotionMagicVelocityVoltage(0);
 
    public ShooterWheels() {
  
    shooterTopMain = new TalonFX(CANConstants.Shooter.kShooterTop);
    shooterBottomFollower = new TalonFX(CANConstants.Shooter.kShooterBottom);
    shooterTopMain.getConfigurator().apply(ShooterConstants.GetWheelsConfiguration());
    shooterBottomFollower.getConfigurator().apply(ShooterConstants.GetWheelsConfiguration());
    }
 
    public void setShootVelocityMotionMagic(double velocityTop, double velocityBottom) {
    motionMagic.Slot = 0;
    shooterTopMain.setControl(motionMagic.withVelocity(velocityTop));
    shooterBottomFollower.setControl(motionMagic.withVelocity(velocityBottom));
  }

• As seen previously in the Arm subsystem, TalonFX motors are instantiated, the motor that spins the top shooter wheels and the motor that spins the bottom shooter wheels.
• The lines below that applies a talonFX motor configuration. The method being called can be seen below:

 public static TalonFXConfiguration GetWheelsConfiguration() {
        TalonFXConfiguration configs = new TalonFXConfiguration();
 
         configs.Slot0.kP = 0.11; // An error of 1 rotation per second results in 0.11V output
         configs.Slot0.kI = 0.5;  // An accumulated error of 1 rotation per second increases output by 0.5V every second
         configs.Slot0.kD = 0.0001; // A change of 1 rotation per second squared results in 0.0001 volts output
        /*
         * Voltage-based velocity requires a feed forward to account for the back-emf of
         * the motor
         */
        configs.Slot0.kV = 0.12; // Falcon 500 is a 500kV motor, 500rpm per V = 8.333 rps per V, 1/8.33 = 0.12
                                 // volts / Rotation per second
 
        configs.MotionMagic.MotionMagicCruiseVelocity = 80;
        configs.MotionMagic.MotionMagicAcceleration = 160;
        configs.MotionMagic.MotionMagicJerk = 1600;
 
        // Peak output of 8 volts
        configs.Voltage.PeakForwardVoltage = 0.9 * 12;
        configs.Voltage.PeakReverseVoltage = -0.9 * 12;
 
        configs.TorqueCurrent.PeakForwardTorqueCurrent = 40;
        configs.TorqueCurrent.PeakReverseTorqueCurrent = -40;
 
        configs.MotorOutput.NeutralMode = NeutralModeValue.Coast;
 
        return configs;
    }

• While this looks like a lot, it becomes much more straightforward after each part is explained.

TalonFXConfiguration

• Think of this like a "container" for the settings of the motor controller. Inside, there are a few main categories of configuration. Slot0 Configs, Motion Magic Settings, Voltage Limits, Current Limits, and Neutral Mode.

Code Explanation

Slot 0 (PIDF Configuration)

    configs.Slot0.kP = 0.11; // An error of 1 rotation per second results in 0.11V output
    configs.Slot0.kI = 0.5;  // An accumulated error of 1 rotation per second increases output by 0.5V every second
    configs.Slot0.kD = 0.0001; // A change of 1 rotation per second squared results in 0.0001 volts output

What are Slot 0 configs?

• Slot 0 configs holds the PID and control loop parameters for motor configuration. While slot 0 is the default slot for these configurations, they can also be placed in slot 1, 2, and so on.

• Slot0.kP = 0.11;
  This sets the Proportional gain (kP) for the PID controller in Slot 0. The proportional term determines how much the controller output changes in response to the error. In this case, an error of 1 rotation per second would produce a 0.11V output change.

• Slot0.kI = 0.5;
  This sets the Integral gain (kI), which accumulates over time. The integral term helps eliminate steady-state error by adding a portion of the accumulated error to the output. With kI = 0.5, an error of 1 rotation per second will increase the output by 0.5V every second, as the error persists.

• Slot0.kD = 0.0001;
  This sets the Derivative gain (kD), which predicts the future error based on its rate of change. The derivative term helps prevent overshooting and smoothens the response. A change of 1 rotation per second squared will produce a 0.0001V output change.

Voltage-Based Velocity Control (Feedforward)

        /*
         * Voltage-based velocity requires a feed forward to account for the back-emf of
         * the motor
         */
    configs.Slot0.kV = 0.12; // Falcon 500 is a 500kV motor, 500rpm per V = 8.333 rps per V, 1/8.33 = 0.12
                             // volts / Rotation per second

• Slot0.kV = 0.12;
  This sets the voltage feedforward term (kV), which compensates for the back electromotive force (back-EMF) of the motor. Since the Falcon 500 motor is rated at 500 kV, this means the motor will spin at 500 RPM per volt applied. In this case, 0.12V per rotation per second is used to account for the motor's characteristics. The formula comes from converting motor RPM to rotations per second (RPS) and calculating the corresponding feedforward voltage.

Motion Magic Settings

    configs.MotionMagic.MotionMagicCruiseVelocity = 80;
    configs.MotionMagic.MotionMagicAcceleration = 160;
    configs.MotionMagic.MotionMagicJerk = 1600;

• Motion Magic is a control mode designed for smooth movement along a motion profile (e.g., for moving to a position smoothly).

• MotionMagicCruiseVelocity = 80;
  This sets the cruise velocity for the motion profile, which is the speed the motor will aim to maintain once it reaches a steady state in its motion. In this case, it's set to 80 units.

• MotionMagicAcceleration = 160;
  This sets the acceleration that the motor should use to reach the target velocity. Here, it's set to 160.

• MotionMagicJerk = 1600;
  This sets the jerk, which is the rate of change of acceleration. Jerk limits how quickly the acceleration can change, preventing sudden, jerky movements. It's set to 1600.

Voltage Limits

    configs.Voltage.PeakForwardVoltage = 0.9 * 12;
    configs.Voltage.PeakReverseVoltage = -0.9 * 12;

• PeakForwardVoltage = 0.9 * 12;

• This limits the maximum forward voltage output to 90% of 12V, or 10.8V.

• PeakReverseVoltage = -0.9 * 12;

• Maximum reverse voltage is set to the opposite.

Current Limits (Torque)

    configs.TorqueCurrent.PeakForwardTorqueCurrent = 40;
    configs.TorqueCurrent.PeakReverseTorqueCurrent = -40;

• PeakForwardTorqueCurrent = 40;

• This sets the maximum forward current allowed to be sent to the motor for torque production.
• It limits the current to 40A in the forward direction.

• PeakReverseTorqueCurrent = -40;
  • maximum reverse current is set to the opposite.

Neutral Mode (Motor Brake/Coast Mode)

    configs.MotorOutput.NeutralMode = NeutralModeValue.Coast;

• NeutralMode = NeutralModeValue.Coast;
  This sets the neutral mode of the motor to Coast. In Coast mode, when no power is applied, the motor will freely rotate and slow down gradually. If set to Brake mode, the motor would resist movement and apply braking to stop the motor more quickly.

  💡

  See an application of Brake mode on the Arm subsystem page.

Return the Configuration

    return configs;

• Finally, the method returns the configured TalonFXConfiguration object. This object contains all the settings specified for the motor.

setShootVelocityMotionMagic() Method

• Now, the Shooter motors are configured In order to run the wheels, the setShootVelocityMotionMagic() method was created, as seen below:

public void setShootVelocityMotionMagic(double velocityTop, double velocityBottom) {
    motionMagic.Slot = 0;
    shooterTopMain.setControl(motionMagic.withVelocity(velocityTop));
    shooterBottomFollower.setControl(motionMagic.withVelocity(velocityBottom));
  }

Code explanation

motionMagic.Slot = 0;

• First, the code above sets the slot's gains that the motionMagic control uses. Because we have configured the talonFX motors to slot0, that is also the slot that we use here.

shooterTopMain.setControl(motionMagic.withVelocity(velocityTop)); shooterBottomFollower.setControl(motionMagic.withVelocity(velocityBottom));

• Lastly, this code applies power to the motor using the setControl() method set to motion Magic using the velocity top and velocity bottom parameters.