PID.cs

class PID
{
    public double KP { get; set; }
    public double KI { get; set; }
    public double KD { get; set; }

    public double SetPoint { get; set; }

    public double MinSet { get; set; }
    public double MaxSet { get; set; }

    private DateTime lastTime;
    private double iTerm = 0;
    private double lastPV = 0;

    public PID(double minSet, double maxSet)
    {
        KP = 5;
        KI = 0;
        KD = 0;

        MinSet = minSet;
        MaxSet = maxSet;
    }

    public double UpdateController(double processValue)
    {
        DateTime now = DateTime.Now;

        double error = SetPoint - processValue;

        double pTerm = KP * error;
        double dTerm = 0;

        // if we have a previous time to calculate the integral
        // and derivative terms, do so
        if(lastTime != null)
        {
            // calculate the time difference
            double dt = (now - lastTime).TotalSeconds;

            // calculate the integral component
            iTerm += KI * error * dt;

            // calculate the derivative component
            // NOTE: use -ΔPV instead of ΔErr to eliminiate derivative kick
            dTerm = -KD * (processValue - lastPV) / dt;
            lastPV = processValue;
        }
        lastTime = now;

        // deal with integral windup
        // and clamp it to [MinSet, MaxSet]
        double result = pTerm + iTerm + dTerm;
        if(result > MaxSet) {
            iTerm -= result - MaxSet;
            result = MaxSet;
        }
        else if(result < MinSet) {
            iTerm += MinSet - result;
            result = MinSet;
        }
        return result;
    }
}