The PID Controller – Part 4 of 4

← Back to Part 1 – Introduction and some math
← Back to Part 2 – Some more math 🙂
← Back to Part 3 – The PIDController class
↓ Download the source code (Please register before proceeding to download)

As an example of the application of the PIDController class, here I present you the MediumMotorPositionPID class whose main feature is the angular position regulation of the Ev3 Medium Motor. From the Lego shop website:

The Lego Mindstorms Ev3 Medium Motor

The EV3 Medium Servo Motor is great for lower-load, higher speed applications and when faster response times and a smaller profile are needed in the robot’s design. The motor uses tacho feedback for precise control within one degree of accuracy and has a built-in rotation sensor.

Tacho feedback to one degree of accuracy

240-250 RPM

Running torque of 8 N.cm (approximately 11 oz.in)

Stall torque of 12 N.cm (approximately 17 oz.in)

Auto-ID is built into the EV3 software

The MediumMotorPositionPID class is derived from the PIDController ; it has a Motor property that you can use to assign the output port of the Ev3 Brick to which you have connected the medium motor; also, there are the Start() and the Stop() methods that you use to control the PID thread. When the thread is stopped, the motor is stopped too and brake is turned on. The OutputSignal protected setter is overridden to assign the output of the PID controller to the motor raw (force) output.

You can compile the code I propose in this page using Xamarin Studio 5.10.1 (Build 6), with Mono 4.2.1 as active runtime and the Monobrick Firmware Library 1.2.0.39486 to let it work on board the Lego Mindstorms Ev3 with the Monobrick firmware.

/// <summary>
/// EV3 Medium motor position PID
/// 
public class MediumMotorPositionPID : PIDController
{
	#region Properties
	/// 
	/// Gets or sets the Motor to be used for regulation
	/// 
	/// The motor.
	public Motor Motor { get; set; }

	/// 
	/// Get the output signal of the PID controller
	/// 
	/// The output.
	public override sbyte OutputSignal 
	{
		get {
			return outputSignal;
		}
		protected set {
			if (outputSignal != value) {
				outputSignal = value;
				Motor.SetPower (outputSignal);
			}
		}
	}
	#endregion

	#region Constructors
	/// 
	/// Initializes a new instance of the  class.
	/// 
	public MediumMotorPositionPID() : base()
	{
		// Fields initialization
		init();
	}

	/// 
	/// Fields initialization
	/// 
	private void init()
	{
		// Default constants
		Kp = 5f;
		Ki = 0.5f;
		Kd = 0f;
		SampleTime = 10;
	}
	#endregion

	#region Public Methods
	/// 
	/// Start the PID
	/// 
	/// Motor must be assigned before operating the PID
	public override void Start()
	{
		// Check for motor validity
		if (Motor == null) {
			InvalidOperationException ex = new InvalidOperationException ("Motor must be assigned before operating the PID");
			throw (ex);
		}

		// Stop motor
		Motor.SetSpeed(0);
		Motor.Brake ();
		Motor.ResetTacho();

		// Start the PID
		base.Start ();
	}

	/// 
	/// Stop the PID
	/// 
	/// Motor must be assigned before operating the PID
	public override void Stop()
	{
		// Stop the PID
		base.Stop();

		// Check for motor validity
		if (Motor == null) {
			InvalidOperationException ex = new InvalidOperationException ("Motor must be assigned before operating the PID");
			throw (ex);
		}
	
		// Stop motor
		Motor.SetSpeed(0);
		Motor.Brake ();

	}
	#endregion
}

/// <summary>

/// EV3 Medium motor position PID

///

public class MediumMotorPositionPID : PIDController

{

#region Properties

///

/// Gets or sets the Motor to be used for regulation

///

/// The motor.

public Motor Motor { get; set; }

///

/// Get the output signal of the PID controller

///

/// The output.

public override sbyte OutputSignal

{

get {

return outputSignal;

}

protected set {

if (outputSignal != value) {

outputSignal = value;

Motor.SetPower (outputSignal);

}

#endregion

#region Constructors

///

/// Initializes a new instance of the class.

///

public MediumMotorPositionPID() : base()

{

// Fields initialization

init();

}

///

/// Fields initialization

///

private void init()

{

// Default constants

Kp = 5f;

Ki = 0.5f;

Kd = 0f;

SampleTime = 10;

}

#endregion

#region Public Methods

///

/// Start the PID

///

/// Motor must be assigned before operating the PID

public override void Start()

{

// Check for motor validity

if (Motor == null) {

InvalidOperationException ex = new InvalidOperationException ("Motor must be assigned before operating the PID");

throw (ex);

}

// Stop motor

Motor.SetSpeed(0);

Motor.Brake ();

Motor.ResetTacho();

// Start the PID

base.Start ();

}

///

/// Stop the PID

///

/// Motor must be assigned before operating the PID

public override void Stop()

{

// Stop the PID

base.Stop();

// Check for motor validity

if (Motor == null) {

InvalidOperationException ex = new InvalidOperationException ("Motor must be assigned before operating the PID");

throw (ex);

}

// Stop motor

Motor.SetSpeed(0);

Motor.Brake ();

}

#endregion

}

An application of this class is the Sup3r Car, where a PID controller has been used on the steering motor.

The source code happily shared above is subject to the Code Project Open Licence (CPOL) 1.02, hoping that you will enjoy it!

The PID Controller – Part 4 of 4

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