PID output to a variable

Technical C/C++
1

I am trying to use a PID loop from the WPI library to control my robot drive in the autonomous mode. I don’t know how to make the PID loop output to a variable that I can set to the drive speed. I tried making a bogus victor and setting a float variable equal to that value, but I got an error when I built the code saying “cannot convert Victor*' tofloat’ in assignment”. Any ideas out there?

Thanks in advance,
will

1 Like
2

The way the PID controller in WPI lib works is that you inherit the PIDOutput class and provide a PIDWrite callback function. The PID controller will call the PIDOutput:: PIDWrite function to run your motors. In essence, do something similar to this:


class MyRobot: public SimpleRobot, public PIDOutput
{
 
public:
    void PIDWrite(float output)
    {
        //program your motors accordingly with the output value.
    }
 
    MyRobot()
    {
        //Create the PID controller object.
    }
}
3

Thanks mikets. That looks very helpful, but I don’t know a whole lot of C++ and I’m not exactly sure about what I am supposed to do with that code that you gave me. Do I need to make a subclass? Do I put the code that controls the motors in the subclass?

So far what I have been trying is:

class IterativeDemo : public IterativeRobot

{
RobotDrive *myRobot;

PIDEncoder *leftencoder;
PIDEncoder *rightencoder;

float leftAutoDrive;
float rightAutoDrive;

public:
myRobot = new RobotDrive(1, 3, 2, 4);

leftencoder = new PIDEncoder(7,8,true,Encoder::k4X);
leftencoder->SetDistancePerPulse(0.04318);	//inches
leftencoder->Start();

rightencoder = new PIDEncoder(9,10,true,Encoder::k4X);
rightencoder->SetDistancePerPulse(0.04318);	//inches
rightencoder->Start();

leftAutoControl = new PIDController(0.1, 0.01, 0.001, leftencoder, bogusLeft);
rightAutoControl = new PIDController(0.1, 0.01, 0.001, rightencoder, bogusRight);

}

void AutonomousInit(void) {
	auto_periodic_loops = 0;				// Reset the loop counter for autonomous mode
	disabled_periodic_loops = 0;
	tele_periodic_loops = 0;
	
	leftAutoControl->Enable();
	rightAutoControl->Enable();
	
	//Initialization
	LoopsPerSec = 50;
	
}

void AutonomousPeriodic(void) {
	// feed the user watchdog at every period when in autonomous
	GetWatchdog().Feed();
	
	auto_periodic_loops++;
	dash_periodic_loops++;


	leftAutoControl->SetSetpoint(18);
	rightAutoControl->SetSetpoint(18);
	
	leftAutoDrive = bogusLeft;
	rightAutoDrive = bogusRight;
	myRobot->TankDrive(rightAutoDrive, leftAutoDrive);	 // drive with tank style

}

The error is in the “leftAutoDrive = bogusLeft” and “rightAutoDrive = bogusRight” parts.

4

Are you going to use the encoders to drive straight or do you also want to use the encoders to turn a certain angle? Your code suggested you have 4-motor drive system. Are you doing mecanum or just normal wheels? Where are the encoders mounted? These questions are important to determine how the code is written.

5

We are just trying to go straight for now. We are planning on using a 4-motor drive on our final robot, but the prototype chassis we have to play with our programming on only has two motors. We are just doing normal wheels. Thank you for taking time to give me such a specific answer.

6

The PID controller in WPIlib has the following constructor:


PIDController(float p, float i, float d, PIDSource *source, PIDOutput output, float period = 0.05);

It means it takes a PIDSource and a PIDOutput object pointers. In the PID controller object, it periodically calls the PIDGet() function from the PIDSource object to obtain the input value (your encoders) and applies the PID algorithm to calculate the output. Then it calls the PIDWrite() function from the PIDOutput object to write to the motors.
This means you need to provide the PIDSource and PIDOutput objects when creating the PIDController object. The easiest way to do it is to have your IterativeDemo class inheriting both PIDSource and PIDOutput. Essentially, you are saying IterativeDemo is both PIDSource and PIDOutput as well. Then in the IterativeDemo class, you need to implement PIDGet() and PIDWrite().
If you just need to go straight, you need only one PID controller. Something similiar to the following:


class IterativeDemo : public IterativeRobot, public PIDSource, public PIDOutput
{
    ...
    double PIDGet()
    {
        return (leftEncoder->GetDistance() + rightEncoder->GetDistance())/2;
    }
 
    void PIDWrite(float output)
    {
        myRobot->TankDrive(output, output);
    }
 
    //
    // Constructor
    //
    IterativeDemo()
    {
        leftEncoder = new PIDEncoder(7,8,true,Encoder::k4X);
        leftEncoder->SetDistancePerPulse(???);
        rightEncoder = new PIDEncoder(9,10,true,Encoder::k4X);
        rightEncoder->SetDistancePerPulse(???);
        driveControl = new PIDController(0.1, 0.01, 0.001, this, this);
        ...
    }
 
    void AutonomousInit()
    {
        driveControl->Enabled();
    }
 
    void AutonomousPeriodic()
    {
        driveControl->SetSetpoint(18);
    }
}

Now if you also want to do PID control turn, that’s a lot more challenging :slight_smile:
You then need two PID controllers and they are not independent of each other.

7