Programming Mecanum

[Zrob]

Hi. Im new to the programming jig, but I'm planning to use Java to program the robot. Our team is going to use mecanum wheels this year, but I don't really know how to program that. Any advice/tips/etc?

[ericarseneau]

I dont know what mecanum wheels are, however you will likely connect the motors to the speed controls, the Jaguars, or to some digital or analog output. If you use Jaguars then there is a class that handles the Jaguars (Victors) as well. There is control for the analog and digital out, although I dont know what they are as I am not as familiar with WPILib.

[RyanCahoon]

Check out the holonomicDrive method of edu.wpi.first.wpilibj.RobotDrive. Holonomic Drive is just another name for the class of omnidirectional drives that work by vector addition (mecanum and quad-omnis).

--Ryan

[gaby1367]

if you are new go with LABview because dragging boxes and conecting wires is easier than typing code and having to worry about syntax

[Zrob]

Wait, Ryan, could you give me a sample of what that would look like?

And gaby, I programmed our bot last year, attempting Labview, but ended up going with Windriver.

[RyanCahoon]

I actually haven't programmed FRC in Java before, but using the information available at http://first.wpi.edu/FRC/frcjava.html, this is what I'd guess. Follow the instructions in the getting started guide to set up a basic robot project, the code they give you in the guide looks like this:

package edu.wpi.first.wpilibj.templates;
import edu.wpi.first.wpilibj.Joystick;
import edu.wpi.first.wpilibj.RobotDrive;
import edu.wpi.first.wpilibj.SimpleRobot;
import edu.wpi.first.wpilibj.Timer;
public class RobotTemplate extends SimpleRobot {
    RobotDrive drive = new RobotDrive(1, 2);
    Joystick leftStick = new Joystick(1);
    Joystick rightStick = new Joystick(2);
    public void autonomous() {
        for (int i = 0; i < 4; i++) {
            drive.drive(0.5, 0.0); // drive 50% fwd 0% turn
            Timer.delay(2.0); // wait 2 seconds
            drive.drive(0.0, 0.75); // drive 0% fwd, 75% turn
            }
        drive.drive(0.0, 0.0); // drive 0% forward, 0% turn}    }
        }
    public void operatorControl() {
        while (true && isOperatorControl() && isEnabled()) // loop until change
        {
            drive.tankDrive(leftStick, rightStick); // drive with joysticks
            Timer.delay(0.005);
        }
    }
}

This sets up the robot for a standard differential ("tank") drive. To switch to holonomic drive, refer to the WPIlibJ Javadocs (all of these links are available from that WPI Java resource page [first link]) for the Robot Drive class: Right now, they're constructing the RobotDrive object with two parameters which specify the PWM ports that the left and right motors are connected to. Since you have 4 motors, you should use the

RobotDrive(int frontLeftMotor, int rearLeftMotor, int frontRightMotor, int rearRightMotor)

constructor instead. Once you have this, you can switch from the drive() and tankDrive() methods to the holonomicDrive() method. You will need to actually use the methods of the Joystick class to get numeric values for the joystick axes, as the holonomicDrive method expects doubles. I would guess that your teleop would look something like this:

public void operatorControl() {
    while (true && isOperatorControl() && isEnabled()) // loop until change
    {
        drive.holonomicDrive(leftStick.getDirectionDegrees(), leftStick.getMagnitude(), rightStick.getX(GenericHID.Hand.kRight)); // drive with joysticks
        Timer.delay(0.005);
    }
}

Or if your joystick has a twist/Z axis, you can replace rightStick.getX with leftStick.getTwist()

Code is untested, so may require some adjustment.

--Ryan

[Zrob]

Alright, got it. Just what I needed. Thanks!

[Todd]

Has this worked out for you?
Our team hasn't tried it yet because the drive base is still being assembled, and we're coding in Labview but I'm pretty sure that the holonomic drive code vi uses the same algorithm as the java RobotDrive holonomicDrive function.

I was curious because I was under the impression that some of the mathematics of mecanum wheels were different from omni wheels, which I believe are what the stock algorithm was designed for.

[Kahn!]

Originally Posted by Todd

Has this worked out for you?
Our team hasn't tried it yet because the drive base is still being assembled, and we're coding in Labview but I'm pretty sure that the holonomic drive code vi uses the same algorithm as the java RobotDrive holonomicDrive function.

I was curious because I was under the impression that some of the mathematics of mecanum wheels were different from omni wheels, which I believe are what the stock algorithm was designed for.

I'm interested in the answer to this question as we have also gotten some conflicting information on the topic.

[Steve_Alaniz]

Not sure I understand where this thread is. I have the coding for mechanums in C and it would have to be translated to Labview or Java or used as C in Labview. But not sure if its needed.

Steve

[Bryan Herbst]

Here is the code that our team uses for mechanum (in C, but easily portable)-

velocity = (p1_y - 127); //Joystick 1 y
rotation = (p1_x - 127); //Joystick 1 x
strafe = (p2_x - 127); // Joystick 2 x

wFL = velocity - rotation - strafe; //Front Left wheel, etc...
wFR = velocity + rotation + strafe;
wRL = velocity - rotation + strafe;
wRR = velocity + rotation - strafe;

pwm01 = wFL + 127;
pwm02 = 127 - wFR;
pwm03 = wRL + 127;
pwm04 = 127 - wRR;

[jhersh]

Originally Posted by Todd

I was curious because I was under the impression that some of the mathematics of mecanum wheels were different from omni wheels, which I believe are what the stock algorithm was designed for.

The ambiguously named holonomic drive method in C++, LabVIEW, and Java was designed for use with mecanum wheels, not omni wheels.

[Daniel Jones]

The holonomic drive function will work with either mecanum or with a 4-wheel omni drive if the wheels are oriented 90 degrees from eachother, arranged like this:
/\
\/
It will not work with kiwi drive.

[aziobro]

We tried the above code almost exactly.
It apears to work, but the left-right control of the joystick is inverted. Same with the rotation.

What is the best way to flip to readings from the joystick?

We are currently using the method to get the magitude and direction from the joystick.

[Todd]

I'm not sure in the Java library but in labview direction is departure from 0 degrees forward. Negating your direction should flip it's left/right movement.