Hello…
Last year someone posted a link for information on programming for mecanum wheels usage. Does anyone remember what it was. Our team
is concidering this this year. Any help will be greatly appreciated.
Thank YouWe’re considering it this year; I’d PM or email the main programmer. His forum name’s cprogrammer.
well…
and then use the search tool to look for keywords like omni wheel and holonomic and kiwi There are numerous posts and good info out there 
SPAM built an omni wheel 'bot for the 2004 game. It was successful, but you will trade off agility for traction and pushing power. Do you wish to run with the big dogs? Or dance around them!
Eric
This is what we use for a simple open-loop mecanum drivebase. Default_Routine() computes internal speed, turn rate, and slide (sideways travel) values from the joystick inputs, then computes motor speeds from the internal values.
The p1 joystick is on the left, and p2 is on the right.
pwm01 is the right front motor, pwm02 is the left front, pwm03 is the right rear, and pwm04 is the left rear.
rc_dig_in01 is a “deadman” switch which sets the motors to neutral unless it is closed. This was intended to be used by people “riding” the robot.
/*******************
function: deadband()
inputs: inp, input value
center, value of input that represents neutral
band, size of deadband to consider neutral
output: scaled and deadband-compensated output value
inputs below center will yield negative output
inputs above center will yield positive output
inputs within deadband either side of center will yield 0
usage: typically call with joystick value (0-254) as input,
joystick neutral (127) as center, and a small number (5)
as the size of the deadband. The white POS joysticks
should probably have a much larger band (10) instead.
*******************/
int deadband(int inp, int center, int band)
{
int oup;
if (inp < center-band)
oup = inp - center + band;
if (center-band <= inp && inp <= center+band)
oup = 0;
if (center+band < inp)
oup = inp - center - band;
return oup;
}
/*********************
function: pwm_limit()
inputs: inp, input value
output: scaled and range-limited output value
usage: call with signed value (+/- 127) as input,
will return a valid pwm value
*********************/
unsigned char pwm_limit (signed int inp)
{
if(inp<-127)
inp = -127;
if (inp>127)
inp = 127;
inp = inp+127;
return (char)inp;
}
/*******************************************************************************
* FUNCTION NAME: Default_Routine
* PURPOSE: Performs the default mappings of inputs to outputs for the
* Robot Controller.
* CALLED FROM: this file, Process_Data_From_Master_uP routine
* ARGUMENTS: none
* RETURNS: void
*******************************************************************************/
void Default_Routine(void)
{
signed int speed, turn, slide;
signed int front_r, front_l, back_r, back_l;
long int l_y, r_y, l_x, r_x;
l_y = deadband(p1_y,127,10);
r_y = deadband(p2_y,127,10);
l_x = deadband(p1_x,127,10);
r_x = deadband(p2_x,127,10);
l_y = l_y/2;
r_y = r_y/2;
l_x = l_x/2;
r_x = r_x/2;
speed = l_y+r_y;
turn = l_y-r_y;
slide = -l_x-r_x;
front_r = speed-turn+slide;
front_l = speed+turn-slide;
back_r = speed-turn-slide;
back_l = speed+turn+slide;
if (rc_dig_in01 == 0)
{
pwm01 = pwm_limit(front_r);
pwm02 = pwm_limit(front_l);
pwm03 = pwm_limit(back_r);
pwm04 = pwm_limit(back_l);
}
else
{
pwm01 = 127;
pwm02 = 127;
pwm03 = 127;
pwm04 = 127;
}
This is pretty similar to what Team 95 has used on holonomic/omni test robots in the past (we haven’t competed with one since around '97 or so, however).
It’s easy for things like variations in the carpet and motor differences to cause your drive mechanism to drive slightly off. You should consider adding a gyroscope and feedback loop so your bot can drive 100% straight (and there’s good code examples out there for doing this).
And, with the USB Chiklet available this year, it’s really easy to use a three-axis joystick, so it’s not hard to control an omni-robots three degrees of freedom direct from one joystick.
It looks like we aren’t the only team who is going to be trying out mecanums this year… just a couple of tips when searching for info, so far I have found results by searching for a couple of common spellings:
meccanum
mechanum and
mecanum
All produce results.
FIRST has info at http://www.usfirst.org/uploadedFiles/Community/FRC/Team_Resources/Team_Resources_Assets/Mecanum_Omniwheels_Hine.ppt
and the CD white papers have a great power point presentation called “strafing”, I believe that runs you through the derivation of the formulas used for a variety of omni-drive systems.
Good luck, and have fun going sideways.
Jason
Thanks guys…Hmmmm…will send this along to our programmers
Our team’s programmer (<–Yes singular!!) worked out all the math to use the omniwheels, and the mechanum twice, with the wheels oriented differently. Turns out, since we have no money, that we won’t be using any holonomic drive this year.
I am trying to use the code posted above but I am not able to make it strafe using the code. Is there as simple way of doing this. Help is appreciated.
If by “strafe” you mean travel sideways, the code I posted does that when the joysticks are pushed sideways.
I am using your code but when I move both joysticks to one side the wheels spin but the robot stays in place. All the wheels either spin inward or all wheels spin outward but never one side spins inward and the other spins outward.
You’re using terms that I don’t understand. How does a wheel spin “inward” or “outward”?
This code is for a mecanum drive base, with four mecanum wheels all mounted so they spin either forward or backward. The “handedness” of the wheels is important, and it’s possible to set things up in one of two ways. One way has the angled rollers contacting the floor so that the robot can very nearly spin in place without any of the wheels turning. This code assumes the other way, where the angled rollers resist spinning in place when the wheels are stationary.
The “meat” of the code is these four lines:
front_r = speed-turn+slide;
front_l = speed+turn-slide;
back_r = speed-turn-slide;
back_l = speed+turn+slide;
All wheels drive at the same rate based on the (forward) speed. The left motors drive more and the right motors drive less based on the (right-hand) turn. The motors on one diagonal drive more in one direction, and the motors on the other diagonal drive more in the other direction, based on the sideways slide.
You might need to change the sign of some of these parts based on how your motors are wired.
I got your code to work and it works great. One problem: speed controls orange light goes off or blinks about every 5 minutes and we have to recalibrate the speed controls. Is there any way to alleviate or completely fix this. Help is greatly appreciated.
Are you using the white joysticks from the Kit of Parts? The mechanical trim adjustment on the ones I know from prior years has a habit of moving by itself when you push the joystick far from the center. If that’s your problem, try taping it down or using a spot of hot melt glue to keep it in place.
We’ll try that. That’s what I think the problem is but our drivers think it’s all because of the code. We are using black joysticks from a while ago. They have a square base.
If you use tank drive the OMNI wheels will deffinately help, but they are a pain to build if you are… 1535 used them for the past 2 years when they were known as 1596, dont know if we’re ghoing that way again.
We’re going mecanum for our bot, we’re rookies. I’m the programmer, I came up with this code, haven’t tried it yet but it’s simple and should work perfectly. (cause we don’t have a bot yet - only 2 weeks into the six) I also have a program I made that simulates mecanum movement with my drive programming. Here’s a section of the user_routines.c with the default drive commented out and my drive put in. It shows which pwm’s are which motors, and has a spot for inverting motor outputs if needed.
/*---------- 1 Joystick Drive ----------------------------------------------
*--------------------------------------------------------------------------
* This code mixes the Y and X axis on Port 1 to allow one joystick drive.
* Joystick forward = Robot forward
* Joystick backward = Robot backward
* Joystick right = Robot rotates right
* Joystick left = Robot rotates left
* Connect the right drive motors to PWM13 and/or PWM14 on the RC.
* Connect the left drive motors to PWM15 and/or PWM16 on the RC.
*/
/*
p1_x = 255 - p1_y;
p1_y = 255 - pwm05;
pwm13 = pwm14 = Limit_Mix(2000 + p1_y + p1_x - 127);
pwm15 = pwm16 = Limit_Mix(2000 + p1_y - p1_x + 127);
*/
/* here's my mechanum drive, ignore all of the above default code
here's which pwms are which motors
14 13
15 16
*/
double motor[4] = {127.0,127.0,127.0,127.0};
// actual axis mixing
motor[0] = p1_y - p1_x - p1_wheel + 255.0;
motor[1] = p1_y + p1_x + p1_wheel - 255.0;
motor[2] = p1_y - p1_x + p1_wheel;
motor[3] = p1_y + p1_x - p1_wheel;
// limit motor outputs to 0 - 254
for (int n=0; n<4; n++)
{
if (motor[n] > 254.0)
motor[n] = 254.0;
if (motor[n] < 0.0)
motor[n] = 0.0;
}
// invert motor outputs here. Ex:
// motor[0] = -1.0 * motor[0] + 254;
// add .5 so type cast to int (pwm_13 = motor[0];) is rounded,
// not always rounded down
for (int n=0; n<4; n++)
motor[n] += .5;
pwm_13 = motor[0];
pwm_14 = motor[1];
pwm_15 = motor[2];
pwm_16 = motor[3];
EDIT:
And of course, since this is c and not c++, you have to move the variable declarations to the beginning of the subroutine.
In other words, put
double motor[4] = {127.0,127.0,127.0,127.0};
int n=0;
right after
void Default_Routine(void)
{
and change the for loops from
for (int n=0; n<4; n++)
to
for (n=0; n<4; n++)
there’s my edit
Use the following program, copy and paste to a compiler, to see what my mecanum drive does. It gives you the total diagonal vectors (as that’s how the mecanum wheels push, and they roll freely in the other diagonal, each pair of wheels pushes in a different diagonal), then converts those into the horizontal/vertical force vectors, then converts that into the total movement vector and the angle of movement. It also gives the rotational vector force. Vector outputs are such that “where 100 = the result of one motor’s full, direct power” as is commented in the program. It also gives each motor’s output (keep in mind 127 is stopped, 254 is forward, 0 is backward) You give in input for each joystick axis where 100 = full forward or right and -100 = full backward or left. Y axis is joystick up/down, makes the bot move forward/backward. x axis is joystick left/right, makes bot strafe left/right. z axis is joystick twist or throttle wheel or a second joystick’s left/right, and makes the bot spin to the left/right. It converts the -100 - 100 input to 0-255 input. It’s all in double for accuracy, so feel free to input stuff like 75.678. Notice how when going full forward you have a certain forward force, but when you add to that a full spin right the forward speed halves. Then when you stop moving, the full spin to the right doubles in speed. That’s cause some of the motors’ force have to be redistributed from movement to turning, slowing the movement. That’s how my nifty mecanum drive code works 
. Normal 4-wheel, 4-wheel-drive does the same thing. When you’re going full forward and decide to turn at the same time, you will slow down a bit.
#include <iostream>
#include <cmath>
using namespace std;
#define PI 3.141592654
// converts degrees to radians
double rad (double deg)
{
return deg * ( (2.0*PI)/360.0 );
}
// converts radians to degrees
double deg (double rad)
{
return rad * ( 360.0/(2.0*PI) );
}
// actual program
main()
{
// declare variables
double x=127.5, y=127.5, z=127.5,
motor[4] = {127.0,127.0,127.0,127.0},
fwdright = 0.0, fwdleft = 0.0, turnright = 0.0,
fwd = 0.0, right = 0.0,
angle = 90.0, total = 0.0;
while(true)
{
// get joystick axis inputs
cout << "Y Axis: ";
cin >> y;
cout << "X Axis: ";
cin >> x;
cout << "Z Axis: ";
cin >> z;
// change axis inputs (-100 - 100) to 4-bit inputs (0 - 255)
x = (x+100.0) * (255.0/200.0);
y = (y+100.0) * (255.0/200.0);
z = (z+100.0) * (255.0/200.0);
// actual axis mixing
motor[0] = y - x - z + 255.0;
motor[1] = y + x + z - 255.0;
motor[2] = y - x + z;
motor[3] = y + x - z;
// limit motor outputs to 0 - 254
for (int n=0; n<4; n++)
{
if (motor[n] > 254.0)
motor[n] = 254.0;
if (motor[n] < 0.0)
motor[n] = 0.0;
}
// round really small motor numbers to 0
for (int n=0; n<4; n++)
if (motor[n] < .001)
motor[n] = 0;
// show motor outputs
for (int n=0; n<4; n++)
cout << "
Motor " << n << ": " << motor[n];
// change motor 4-bit outputs (0-255) to display (-100 - 100)
for (int n=0; n<4; n++)
motor[n] = (motor[n]-127.0) * (200.0/254.0);
// calculating movement vectors and rotational vector
// where 100 = the result of one motor's full, direct power
fwdleft = motor[0] + motor[2];
fwdright = motor[1] + motor[3];
turnright = motor[1] + motor[2] - motor[0] - motor[3];
// calculating forward and right movement vectors
fwd = sin( rad(135.0) ) * fwdleft +
sin( rad(45.0 ) ) * fwdright;
right = cos( rad(135.0) ) * fwdleft +
cos( rad(45.0 ) ) * fwdright;
// calculating total movement vector and angle
total = sqrt( pow(fwd, 2) + pow(right,2) );
angle = deg ( atan2(fwd, right) );
if (angle < 0)
angle += 360;
// round really small vector numbers to 0
if ( fabs(fwdleft) < .001)
fwdleft = 0;
if (fabs(fwdright) < .001)
fwdright = 0;
if (fabs(turnright) < .001)
turnright = 0;
if (fabs(fwd) < .001)
fwd = 0;
if (fabs(right) < .001)
right = 0;
if (fabs(total) < .001)
total = 0;
if (fabs(angle) < .001)
angle = 0;
// show all vectors
cout << "
Turn Right vector: " << turnright
<< "
Forward Left movement vector: " << fwdleft
<< "
Forward Right movement vector: " << fwdright
<< "
Vertical movement vector: " << fwd
<< "
Horizontal movement vector: " << right
<< "
Total movement vector: " << total
<< "
Total movement vector angle: " << angle << " degrees";
getchar();getchar();
cout << "
-----------------------------------------"
<< "---------------------------------------
";
}
}
hope this helps
oh yeah and if u want a gyro for true holonomic movement (as im planning to do) just turn the xy joystick inputs to magnitude and direction, then offset the angle by the gyro’s input, then convert back into the joystick xy input and run through the drive like nothing happened. simple trig. This makes it so that no matter what direction the bot is facing, forward always makes the bot move ‘up’ and backward always makes if move ‘down’ etc. even if ‘down’ is actually the bot’s forward-left.
The valid range of pwm control outputs is 0-254, with 127 being neutral. You should change all your 127.5 values to 127, and all your 255 values to 254.
thanks very much, didn’t know that - thought it was normal 4 bit. I haven’t tried it yet, guess it wouldn’t have worked. The joystick serial inputs are still 0-255 right?