PID C++ implementation

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.DS_Store

pid.cpp

/**
 * Copyright 2019 Bradley J. Snyder <snyder.bradleyj@gmail.com>
 *
 * 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.
 */

#ifndef _PID_SOURCE_
#define _PID_SOURCE_

#include <iostream>
#include <cmath>
#include "pid.h"

using namespace std;

class PIDImpl
{
    public:
        PIDImpl( double dt, double max, double min, double Kp, double Kd, double Ki );
        ~PIDImpl();
        double calculate( double setpoint, double pv );

    private:
        double _dt;
        double _max;
        double _min;
        double _Kp;
        double _Kd;
        double _Ki;
        double _pre_error;
        double _integral;
};


PID::PID( double dt, double max, double min, double Kp, double Kd, double Ki )
{
    pimpl = new PIDImpl(dt,max,min,Kp,Kd,Ki);
}
double PID::calculate( double setpoint, double pv )
{
    return pimpl->calculate(setpoint,pv);
}
PID::~PID() 
{
    delete pimpl;
}


/**
 * Implementation
 */
PIDImpl::PIDImpl( double dt, double max, double min, double Kp, double Kd, double Ki ) :
    _dt(dt),
    _max(max),
    _min(min),
    _Kp(Kp),
    _Kd(Kd),
    _Ki(Ki),
    _pre_error(0),
    _integral(0)
{
}

double PIDImpl::calculate( double setpoint, double pv )
{
    
    // Calculate error
    double error = setpoint - pv;

    // Proportional term
    double Pout = _Kp * error;

    // Integral term
    _integral += error * _dt;
    double Iout = _Ki * _integral;

    // Derivative term
    double derivative = (error - _pre_error) / _dt;
    double Dout = _Kd * derivative;

    // Calculate total output
    double output = Pout + Iout + Dout;

    // Restrict to max/min
    if( output > _max )
        output = _max;
    else if( output < _min )
        output = _min;

    // Save error to previous error
    _pre_error = error;

    return output;
}

PIDImpl::~PIDImpl()
{
}

#endif

pid.h

/**
 * Copyright 2019 Bradley J. Snyder <snyder.bradleyj@gmail.com>
 *
 * 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.
 */

#ifndef _PID_H_
#define _PID_H_

class PIDImpl;
class PID
{
    public:
        // Kp -  proportional gain
        // Ki -  Integral gain
        // Kd -  derivative gain
        // dt -  loop interval time
        // max - maximum value of manipulated variable
        // min - minimum value of manipulated variable
        PID( double dt, double max, double min, double Kp, double Kd, double Ki );

        // Returns the manipulated variable given a setpoint and current process value
        double calculate( double setpoint, double pv );
        ~PID();

    private:
        PIDImpl *pimpl;
};

#endif

pid_example.cpp

/**
 * Copyright 2019 Bradley J. Snyder <snyder.bradleyj@gmail.com>
 *
 * 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.
 */

#include "pid.h"
#include <stdio.h>

int main() {

    PID pid = PID(0.1, 100, -100, 0.1, 0.01, 0.5);

    double val = 20;
    for (int i = 0; i < 100; i++) {
        double inc = pid.calculate(0, val);
        printf("val:% 7.3f inc:% 7.3f\n", val, inc);
        val += inc;
    }

    return 0;
}

pid_README

To compile library:
g++ -c pid.cpp -o pid.o

To compile example code:
g++ pid_example.cpp pid.o -o pid_example

Thanks a lot!

I would just suggest to check the Dt term as it might lead to a division per zero:

if(dt == 0.0)
    throw std::exception("Impossible to create a PID regulator with a null loop interval time.");

It works - nice :) :+1 . Now it's a good starting point for speed optimization like fixed point implementation for FPU-less MCUs.

Can I ask why you decided to create 2 classes for this?
I don't know the in and outs of c++, but I'm wondering :)

@Tbone2401 It is called PImpl. See https://en.cppreference.com/w/cpp/language/pimpl for more information :)

nice, it is a good tutorial to learn the C++ implementation of pid controller.

Hello, dt value is in ms ? or in second ? Thank you

@brztitouan That depends on your own situation. If you use s or ms, just scale all other parameters accordingly.

I am struggling to find the cmath header. Where could I find this file?

Hello, I did a fixed-point version:

My Gist

Hello,
May I ask the license of this library if you don't mind ?

Maybe "dt" should be passed in to calculate since it's likely to change at least slightly.

Hello,
May I ask the license of this library if you don't mind ?

added license to source files

what is dt

or how to calculate it

dt is the time between 2 latest PV reading (and/or Command setting), guess the first value then read a timer at every loop count

Very nice! thanks @bradley219

how to use this inside a class? how to initialize the pid object inside the constructor of another class?

Nice work! Some ideas for minor improvements:

• Add support for passing dt as an optional parameter to calculate - like mentioned by others.
• Add support for ReverseActing - e.g. where Brakes needs to be increased to decrease vehicle speed
  error = reverseActing ? (pv-setpoint) : (setpoint - pv)

Excellent solution. It worked really nice for our purposes. Thanks for your support to the community

A readme or function description as comments would make this perfect

as @ngjermundshaug said. support for ReverseActing is needed.

It is a good starting point. The above code implements the controller effort using zero-order-hold method. However, the recommended method from control theory is to use first-order-hold method. More details about the implementation can be found in the following link
https://www.analog.com/media/en/technical-documentation/application-notes/pi_ctrl.pdf
Pages 4 ad 5.

Can this be used to create more than 1 PID?

Thanks a lot!

I would just suggest to check the Dt term as it might lead to a division per zero:

if(dt == 0.0)
    throw std::exception("Impossible to create a PID regulator with a null loop interval time.");

dt variable as I could see is a double type.
Even inside an OS environment with a handle of division by zero exception, it won't complain!!.
in C++ this isn't a problem, only division by zero for integral values will trigger an exception.
At least this is how modern C++ compilers do because they use SSE instructions, that have no problem for such divisions.
After all, division by zero exception is a target specs in C++, unlike some other programming languages that initiate some checking before executing the division expression.