PID Controller in C#

/*

  PID Controller
  - supports bumpless transfer
  - reset windup avoidance
  - tuning change bump mitigation


Copyright (C) 2011 by Greg Buehler

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
*/

using System;

namespace AbstergoPID
{
    public class PID
    {
        public enum OperationMode
        {
            Auto,
            Manual
        }

        public enum MovementDirection
        {
            Direct,
            Reverse
        }

        private long lastTime;
        private float iTerm;
        private float lastInput;

        public float Input { get; set; }
        public float Output { get; set; }
        public float Setpoint { get; set; }

        public float Kp { get; private set; }
        public float Ki { get; private set; }
        public float Kd { get; private set; }

        public int SampleTime { get; set; }
        public float OutputMinimum { get; set; }
        public float OutputMaximum { get; set; }

        public OperationMode Mode { get; set; }
        public MovementDirection Direction { get; set; }

        public void Compute()
        {
            // early exit if in manual control
            if (Mode == OperationMode.Manual) return;

            // early exit if called before sample interval
            long now = DateTime.Now.Ticks;
            long timeDelta = now - lastTime;
            if (timeDelta < SampleTime) return;

            // computer working error
            float error = Setpoint - Input;
            iTerm += (Ki * error);

            // clamp intergral in output ranges
            if (iTerm > OutputMaximum) iTerm = OutputMaximum;
            if (iTerm < OutputMinimum) iTerm = OutputMinimum;

            // calculate input error factor
            float iError = Input - lastInput;

            // calculate PID Output
            Output = Kp * error + iTerm - Kd * iError;
            if (Output > OutputMaximum) Output = OutputMaximum;
            if (Output < OutputMinimum) Output = OutputMinimum;

            // replace interval variables for next iteration
            lastInput = Input;
            lastTime = now;
        }

        public void SetTunings (float Kp, float Ki, float Kd)
        {
            // error check for settings less than 0
            if (Kp < 0 || Ki < 0 || Kd < 0) return;

            // scale sample interval and apply to tuning
            float SampleRate = (float)SampleTime / 1000;
            this.Kp = Kp;
            this.Ki = Ki * SampleRate;
            this.Kd = Kd / SampleRate;

            // if direction is reversed, invert tuning
            if (Direction == MovementDirection.Reverse)
            {
                Kp = 0 - Kp;
                Ki = 0 - Ki;
                Kd = 0 - Kd;
            }
        }

        public void SetSampleTime(int NewSampleTime)
        {
            // new sample time must be valid
            if (NewSampleTime > 0)
            {
                // adjust scaling of intergral and derivative for new time
                float ratio = (float)NewSampleTime / (float)SampleTime;
                Ki *= ratio;
                Kd /= ratio;

                this.SampleTime = NewSampleTime;
            }
        }

        public void SetOutputLimits(float Min, float Max)
        {
            // min must be less than max
            if (Min > Max) return;
            this.OutputMinimum = Min;
            this.OutputMaximum = Max;

            // immediatly clamp the output value and intergral term
            if (Output > OutputMaximum) Output = OutputMaximum;
            if (Output < OutputMinimum) Output = OutputMinimum;

            if (iTerm > OutputMaximum) iTerm = OutputMaximum;
            if (iTerm < OutputMinimum) iTerm = OutputMinimum;
        }

        void SetMode(OperationMode Mode)
        {
            // if moving from manual to auto, reinitialize for bumpless transfer
            bool movingToAuto = (Mode == OperationMode.Auto && this.Mode != Mode);
            if (movingToAuto)
            {
                Initialize();
            }

            this.Mode = Mode;
        }

        void SetDirection(MovementDirection Direction)
        {
            this.Direction = Direction;
        }

        void Initialize()
        {
            lastInput = Input;
            iTerm = Output;
            if (iTerm > OutputMaximum) iTerm = OutputMaximum;
            if (iTerm < OutputMinimum) iTerm = OutputMinimum;
        }

        public PID(float Input, float Output, float Setpoint, float Kp, float Ki, float Kd, MovementDirection Direction)
        {
            this.Input = Input;
            this.Output = Output;
            this.Setpoint = Setpoint;
            this.Kp = Kp;
            this.Ki = Ki;
            this.Kd = Kd;
            this.Direction = Direction;

            Initialize();
        }
    }
}