Getting Gyro Working w/ Mecanum Drive

[WillNess]

Originally Posted by Ether

Like this:

Sorry but that doesn't really help me. Like I understand how it works but I don't know how I would implement this in Java.

[Ether]

Originally Posted by WillNess

I don't know how I would implement this in Java.

Why? What part is not clear?

[WillNess]

Originally Posted by Ether

Why? What part is not clear?

The actual code I would write.

[GeeTwo]

I recommend using one of the WPIlib MecanumDrive methods, and just make corrections to the "twist" input when you want to go straight based on the error. Do not use the fourth "gyroscope" input to MecanumDrive_cartesian; this is only used for "field coordinates".

The correction could be a full PID, but because of the amount of friction in a mecanum drive system, you should be able to get away with a simple proportional correction; D will be mechanical, and I is usually not needed when seeking a position.

To do this, you will need to create a numeric variable to hold the desired heading, say azi.
In your joystick-reading loop:

• When the joystick values indicate that you do not wish to hold a constant facing, set azi to NaN, or some number outside of the valid range.
• When the joystick values indicate that you do wish to hold a constant facing:
  • If isnan(azi), set azi =gyro.getAngle()
  • otherwise, add a correction of the form prop * (azi - gyro.getAngle()) to the "twist" value. You will have to tune the value of prop.

Debugging tip: If the robot starts spinning faster and faster, do a quick shut down, change the sign of "prop" and try again.

[WillNess]

How do I set the robotdrive to the MecanumDrive method?

Originally Posted by GeeTwo

I recommend using one of the WPIlib MecanumDrive methods, and just make corrections to the "twist" input when you want to go straight based on the error. Do not use the fourth "gyroscope" input to MecanumDrive_cartesian; this is only used for "field coordinates".

The correction could be a full PID, but because of the amount of friction in a mecanum drive system, you should be able to get away with a simple proportional correction; D will be mechanical, and I is usually not needed when seeking a position.

To do this, you will need to create a numeric variable to hold the desired heading, say azi.
In your joystick-reading loop:

• When the joystick values indicate that you do not wish to hold a constant facing, set azi to NaN, or some number outside of the valid range.
• When the joystick values indicate that you do wish to hold a constant facing:
  • If isnan(azi), set azi =gyro.getAngle()
  • otherwise, add a correction of the form prop * (azi - gyro.getAngle()) to the "twist" value. You will have to tune the value of prop.

Debugging tip: If the robot starts spinning faster and faster, do a quick shut down, change the sign of "prop" and try again.

[GeeTwo]

Originally Posted by WillNess

How do I set the robotdrive to the MecanumDrive method?

mecanumDrive_cartesian() is a method within the standard RobotDrive class in both C++ and java WPIlibs. For what are hopefully obvious reasons, you must use one of the four-controller forms of the new RobotDrive() constructor in order to use the mecanum methods.

[WillNess]

Originally Posted by GeeTwo

mecanumDrive_cartesian() is a method within the standard RobotDrive class in both C++ and java WPIlibs. For what are hopefully obvious reasons, you must use one of the four-controller forms of the new RobotDrive() constructor in order to use the mecanum methods.

So in my teleopPeriodic would I put:

RobotMap.driveRobotDrive41.mecanumDrive_Cartesian(OI.joystick0.getX(), OI.joystick0.getY(), OI.joystick0.getZ(), 0);

or where should I do that? Also, should I set the fourth variable to my Gyro Angle or to 0?

[Ether]

Originally Posted by WillNess

should I set the fourth variable to my Gyro Angle or to 0?

As explained in earlier posts in this thread, passing the gyro angle to the fourth parameter gives you field-centric control. You said that's not what you want.

Originally Posted by Ether

What you described above will make the driver interface field-centric

Originally Posted by GeeTwo

Do not use the fourth "gyroscope" input to MecanumDrive_cartesian; this is only used for "field coordinates".

See the note "Optional Input for field-centric control of XY motion" in the code diagram linked in this post:

Originally Posted by Ether

if your goal is to make the bot drive straight when the driver is commanding zero rotation, you could do something like this:

http://www.chiefdelphi.com/forums/at...4&d=1420598437

[dawonn]

The code is not particularly well documented, but here's the code we used this year:

https://github.com/CopperBots/Copper...obot.java#L632

We pass in the rotation joystick value that has already been deadband'ed.
If the joystick is at zero rotation, we use a proportional controller to keep the robot straight.
If the joystick is not at zero, we update the current heading of the robot and pass the joystick value back to the drive system unchanged.

We also have the option to disable gyro mode at any time in case something goes wrong like a bad calibration, or a loose cable.
There is also an option to update the kP value from the smart dashboard and some diagnostic information sent back to help troubleshoot when things go wrong.

[WillNess]

Thanks, this helps a BUNCH! The problem is mixing mecanum drive with your code

Originally Posted by dawonn

The code is not particularly well documented, but here's the code we used this year:

https://github.com/CopperBots/Copper...obot.java#L632

We pass in the rotation joystick value that has already been deadband'ed.
If the joystick is at zero rotation, we use a proportional controller to keep the robot straight.
If the joystick is not at zero, we update the current heading of the robot and pass the joystick value back to the drive system unchanged.

We also have the option to disable gyro mode at any time in case something goes wrong like a bad calibration, or a loose cable.
There is also an option to update the kP value from the smart dashboard and some diagnostic information sent back to help troubleshoot when things go wrong.

[Ether]

Originally Posted by dawonn

If the joystick is at zero rotation, we use a proportional controller to keep the robot straight.
If the joystick is not at zero, we update the current heading of the robot and pass the joystick value back to the drive system unchanged

Sounds just like the note in the lower left corner of this code diagram

[WillNess]

Well what I'm asking is what do I put in for the fourth parameter, because I have to put something in it.

Originally Posted by Ether

As explained in earlier posts in this thread, passing the gyro angle to the fourth parameter gives you field-centric control. You said that's not what you want.





See the note "Optional Input for field-centric control of XY motion" in the code diagram linked in this post:

[Ether]

Originally Posted by WillNess

Well what I'm asking is what do I put in for the fourth parameter, because I have to put something in it.

Look at the WPILib code for mecanumDrive_Cartesian():

    public void mecanumDrive_Cartesian(double x, double y, double rotation, double gyroAngle) {
        if(!kMecanumCartesian_Reported) {
            UsageReporting.report(tResourceType.kResourceType_RobotDrive, getNumMotors(), tInstances.kRobotDrive_MecanumCartesian);
            kMecanumCartesian_Reported = true;
        }
        double xIn = x;
        double yIn = y;
        // Negate y for the joystick.
        yIn = -yIn;
        // Compenstate for gyro angle.
        double rotated[] = rotateVector(xIn, yIn, gyroAngle);

Now look at the WPILib code for rotateVector():

    /**
     * Rotate a vector in Cartesian space.
     */
    protected static double[] rotateVector(double x, double y, double angle) {
        double cosA = Math.cos(angle * (3.14159 / 180.0));
        double sinA = Math.sin(angle * (3.14159 / 180.0));
        double out[] = new double[2];
        out[0] = x * cosA - y * sinA;
        out[1] = x * sinA + y * cosA;
        return out;
    }

The cosine of zero is 1. The sign of zero is 0. So passing zero as the fourth parameter to mecanumDrive_Cartesian() will result in no rotation of the vector, which is what you said you want.

[WillNess]

Thank you Ether, that helps tons.

Originally Posted by Ether

Look at the WPILib code for mecanumDrive_Cartesian():

    public void mecanumDrive_Cartesian(double x, double y, double rotation, double gyroAngle) {
        if(!kMecanumCartesian_Reported) {
            UsageReporting.report(tResourceType.kResourceType_RobotDrive, getNumMotors(), tInstances.kRobotDrive_MecanumCartesian);
            kMecanumCartesian_Reported = true;
        }
        double xIn = x;
        double yIn = y;
        // Negate y for the joystick.
        yIn = -yIn;
        // Compenstate for gyro angle.
        double rotated[] = rotateVector(xIn, yIn, gyroAngle);

Now look at the WPILib code for rotateVector():

    /**
     * Rotate a vector in Cartesian space.
     */
    protected static double[] rotateVector(double x, double y, double angle) {
        double cosA = Math.cos(angle * (3.14159 / 180.0));
        double sinA = Math.sin(angle * (3.14159 / 180.0));
        double out[] = new double[2];
        out[0] = x * cosA - y * sinA;
        out[1] = x * sinA + y * cosA;
        return out;
    }

The cosine of zero is 1. The sign of zero is 0. So passing zero as the fourth parameter to mecanumDrive_Cartesian() will result in no rotation of the vector, which is what you said you want.

[WillNess]

Do you think this will work? (I only included the drive code)

public void teleopInit(){
	gyroMode = true;
}

public void teleopPeriodic() {
        Jx = OI.joystick0.getX(); //Drive Joystick X
        Jy = OI.joystick0.getY(); //Drive Joystick Y
        Jz = OI.joystick0.getZ(); //Drive Joystick Z
        rotationSpeedForError = RobotMap.GyroMod(Jz);
        RobotMap.driveRobotDrive41.mecanumDrive_Cartesian(Jx, Jy, Jz + rotationSpeedForError, 0);
}

public double GyroMod(double rotation){
	if (gyroMode == true) {
		double error = gyro.getAngle() - gyroHeading;
		double kP = .05;
		if (rotation == 0) {
			rotation = rotation + kP * error;
		} else {
			gyroHeading = gyro.getAngle();
		}
	}
	return rotation;
	}
}