Simulate flywheel for linear LinearMotion

I’m attempting to simulate a PID controller for a shooter using the velocity of an encoder in ticks/sec. The relevant parts of the PID are here:

    def returnPIDInput(self):
        velocity = self.m_Motor1.getSelectedSensorVelocity(0)
        SmartDashboard.putNumber("Shooter Velocity", velocity)
        return velocity

   def usePIDOutput(self, output):
        SmartDashboard.putNumber("Shooter Output", output)
        return self.m_Motor1.set(output)

    def start(self):
        self.setOutputRange(0, 1.0)

My PhysicsEngine is configured to update the quad_position and quad_velocity like this:

class PhysicsEngine(object):

    def __init__(self, physics_controller):

        self.physics_controller = physics_controller

        self.shooter = motion.LinearMotion('Shooter', 3, 917)
    def update_sim(self, hal_data, now, tm_diff):

        # Simulate the Shooter
        s_motor = hal_data'CAN'][robotmap.SHOOT_MOTOR_1]'value']
        s_position = self.shooter.compute(s_motor, tm_diff)
        hal_data'CAN'][robotmap.SHOOT_MOTOR_1]'quad_position'] = s_position
        hal_data'CAN'][robotmap.SHOOT_MOTOR_1]'quad_velocity'] = getRate(hal_data'CAN'][robotmap.SHOOT_MOTOR_1]'quad_position'],

The getRate() function just returns the ticks/10msec:

def getRate(motor, last_pos, tm_diff):
    rate = abs(motor - last_pos) / tm_diff / 100
    return int(rate)

The net result is indeed a motor that returns a rate of 27.5 when published to a dashboard (1440 clicks/foot / (.5ft wheel * pi)) / 100msec = 27.5 clicks/10msec).

My “problem” is that there’s no actual flywheel motion so the PID instantaneously achieves full speed and then when the PID switches back to 0, instantaneously achieves 0 rate. My thought is that I need to simulate some sort of inertia but this is proving to be difficult. I can do it with a relatively simple class that only allows a decay at a certain rate but this doesn’t work when the motor is not “on”.

Has anyone simulated a flywheel in the physics module or have thoughts on how to simulate inertia (or maybe just what I’m doing wrong…). Full disclosure: I’m not a math major nor developer by trade so feel free to point out the obvious mistakes in logic or math as well.

Thanks in advance.

I’m not particularly good at math either… and as such, the LinearMotion helper doesn’t simulate inertia at all. In order to do that, I think you’d need to compute the actual velocity of the motor, then pass that in to compute() instead of the motor value.

Long story short, I “solved” this problem by moving from using PIDSubsystem to using ClosedLoopControl on the TalonSRX. I also had to disable braking on the motor. Maybe everyone else knew that but, I didn’t…