Mechanum wheel programming in C++

[johncap100]

Has anyone programmed the mechanum wheels using C++?/
if so could you share?

thanks

[mikets]

The WPI library provides the RobotDrive object that includes a method:

void RobotDrive::MecanumDrive_Polar(float magnitude, float direction, float rotation)

You can call this method to drive the mechanum wheels. It is using polar coordinate system where you specify the magnitude and direction you want to go. Rotation will specify the heading you want the robot to face.

To calculate the magnitude and direction from joystick reading, you can do:

MecanumDrive_Polar(joystick->GetMagnitude(), joystick->GetDirectionDegrees(), 0.0);

[johncap100]

my understand is that we need to specify each motor/wheel assembly , ie all four individually as each wheel is connected indepedently to a single motor.

so then do we use the above statements for each motor/wheel assembly?

thanks

have you actually written some code just to move the bot with mechanum wheels?

[Ether]

Originally Posted by johncap100

my understand is that we need to specify each motor/wheel assembly , ie all four individually as each wheel is connected indepedently to a single motor.

so then do we use the above statements for each motor/wheel assembly?

thanks

have you actually written some code just to move the bot with mechanum wheels?

You take the commands from the joysticks and you process them (it's called inverse kinematics) to create the 4 wheel speeds.

The drive interface could be a single 3-axis joystick, or two 2-axis joysticks, or any other input device(s).

Mecanum has three completely independent degrees of freedom:

fore/aft
strafe (right/left)
rotate (turn, yaw)

A mecanum vehicle can perform all three of these motions simultaneously.


Here's some reference C code for you:

// 3-axis joystick interface to a mecanum drive

// define your driver interface,
// in this case a 3-axis joystick:

forward = -Y;
right = X;
clockwise = Z;

// put any drivability adjustments here for forward, right, and clockwise,
// for example gain, deadband, etc:

// if rotate gain is too hot, tweak it down:

clockwise /= 2;  

// add deadband so you don't get strafe when you don't want it:

if ((right>-0.1)&&(right<0.1)) right = 0; 


// now apply the inverse kinematic tranformation:

front_left = forward + clockwise + right;
front_right = forward - clockwise - right;
rear_left = forward + clockwise - right;
rear_right = forward - clockwise + right;


// finally, normalize so that no wheel speed command
// exceeds magnitude of 1:

max = abs(front_left);
temp = abs(front_right);
if (temp>max) max = temp;
temp = abs(rear_left);
if (temp>max) max = temp;
temp = abs(rear_right);
if (temp>max) max = temp;

if (max>1) 
  {front_left/=max; front_right/=max; rear_left/=max; rear_right/=max;}


// you're done. send these four wheel commands to their respective wheels

Questions? Fire away.

[tomy]

Originally Posted by Ether

You take the commands from the joysticks and you process them (it's called inverse kinematics) to create the 4 wheel speeds.

The drive interface could be a single 3-axis joystick, or two 2-axis joysticks, or any other input device(s).

Mecanum has three completely independent degrees of freedom:

fore/aft
strafe (right/left)
rotate (turn, yaw)

A mecanum vehicle can perform all three of these motions simultaneously.


Here's some reference C code for you:

// 3-axis joystick interface to a mecanum drive

// define your driver interface,
// in this case a 3-axis joystick:

forward = -Y;
right = X;
clockwise = Z;

// put any drivability adjustments here for forward, right, and clockwise,
// for example gain, deadband, etc:

// if rotate gain is too hot, tweak it down:

clockwise /= 2;  

// add deadband so you don't get strafe when you don't want it:

if ((right>-0.1)&&(right<0.1)) right = 0; 


// now apply the inverse kinematic tranformation:

front_left = forward + clockwise + right;
front_right = forward - clockwise - right;
rear_left = forward + clockwise - right;
rear_right = forward - clockwise + right;


// finally, normalize so that no wheel speed command
// exceeds magnitude of 1:

max = abs(front_left);
temp = abs(front_right);
if (temp>max) max = temp;
temp = abs(rear_left);
if (temp>max) max = temp;
temp = abs(rear_right);
if (temp>max) max = temp;

if (max>1) 
  {front_left/=max; front_right/=max; rear_left/=max; rear_right/=max;}


// you're done. send these four wheel commands to their respective wheels

Questions? Fire away.

So with that code I can just pop it into my code and it should work or will it have to be modified.?

[Ether]

Originally Posted by tomy

So with that code I can just pop it into my code and it should work ...?

That was not the intent; it is reference code. It lacks window dressing (variable declarations etc), and you need to decide whether or not you want to shape the joystick response curves (gain, deadband, etc). I recommend that you study and understand it, then tailor it for your specific application.

[ekapalka]

Originally Posted by Ether

Questions? Fire away.

What about field-centric? My team tried this out a few times but got really weird results. Would the following code snippet be correct?

[...] gyro->Reset(); float theta = gyro->GetAngle(); [...]

yVal2=(float) (yVal*Math.cos(theta) - xVal*Math.sin(theta));
xVal2=(float) (yVal*Math.sin(theta) + xVal*Math.cos(theta));

When we tried using this in our mecanum code method the robot could drive straight but would jitter and twitch if we tried to rotate it (like some robots do when their power draw exceeds the operating power of the motor controllers when your battery is low).

[Ether]

Originally Posted by ekapalka

What about field-centric?

WPILib now has mecanum code, with field-centric option. Take a look at that (use just use it as-is).

[mikets]

Originally Posted by johncap100

my understand is that we need to specify each motor/wheel assembly , ie all four individually as each wheel is connected indepedently to a single motor.

so then do we use the above statements for each motor/wheel assembly?

thanks

have you actually written some code just to move the bot with mechanum wheels?

You have 4 individual motors one for each wheel but you don't have to deal with them yourself, the WPI library will take care the rest for you. So when you instantiate the RobotDrive object, you will need to initialize it with four motors in its constructor. Once you define the RobotDrive object with four motors, you can make the MecanumDrive_Polar call to drive it around. The RobotDrive object will figure out the power and direction for each wheel for you.

Either:
RobotDrive(UINT32 frontLeftMotorChannel, UINT32 rearLeftMotorChannel,
	UINT32 frontRightMotorChannel, UINT32 rearRightMotorChannel,
             float sensitivity = 0.5);
Or:
RobotDrive(SpeedController *frontLeftMotor, SpeedController *rearLeftMotor,
	SpeedController *frontRightMotor, SpeedController *rearRightMotor,
	float sensitivity = 0.5);

And yes, we were using Mecanum wheels last year so we did write some code for them.

[jhersh]

Originally Posted by mikets

The WPI library provides the RobotDrive object that includes a method:

void RobotDrive::MecanumDrive_Polar(float magnitude, float direction, float rotation)

You can call this method to drive the mechanum wheels. It is using polar coordinate system where you specify the magnitude and direction you want to go. Rotation will specify the heading you want the robot to face.

To calculate the magnitude and direction from joystick reading, you can do:

MecanumDrive_Polar(joystick->GetMagnitude(), joystick->GetDirectionDegrees(), 0.0);

Note that the MecanumDrive_Cartesian is much more suited to driving with a joystick. It was designed to directly pass 3 joystick axes in.

void MecanumDrive_Cartesian(float x, float y, float rotation, float gyroAngle = 0.0);

This also allows you to simply pass in the current reading from a gyro to accomplish field-oriented-control.

-Joe

[johncap100]

so does one need only two joysticks or do you need to use 4 with two drivers. seems to me two joysticks would work. Did you find any brand or type better than another. I am looking at the Logitech Extreme 3D Pro. It has the 3 axis capability.

thanks

[mikets]

For driving mecanum, two joysticks would work. Even one would work if the joystick has at least 3 axes (e.g. X, Y and Twist). I don't have the Logitech Extreme 3D Pro, so I can't comment on it.

[jhersh]

Originally Posted by mikets

For driving mecanum, two joysticks would work. Even one would work if the joystick has at least 3 axes (e.g. X, Y and Twist). I don't have the Logitech Extreme 3D Pro, so I can't comment on it.

Our team used the 3D Pro to drive a mecanum bot last year and it was very natural. The kids picked up how to control it very quickly.

[johncap100]

Originally Posted by jhersh

Our team used the 3D Pro to drive a mecanum bot last year and it was very natural. The kids picked up how to control it very quickly.

so Joe would you have an example of code we might use just to see how we might drive our bot? I have some kids who can program but they are just starting out and it would help alot if we had some code we knew worked and then could build on it

Did you use two joysticks?

thanks John

[ChiefDelphi]

Yes, It would be extremely helpful if the sample C++ Code for Mechanum Wheels is provided with a good explaination

Thanks