Limit speed of 4 chassis CIM motors

[microman1934]

Hi there,

I'm from team 4939 and for this year's competition, our robot turned out quite tall. So in order to decrease tipping of the robot, I was wondering if it was possible to limit the speed of the 4 chassis CIM motors.

Thank you,
Uday Sharma

[Qmcdonn]

You could certainly do this in software by setting a multiplier for the speed to 0.5 or something like that. I do not know of any electrical way to achieve the same result.

[DanielPlotas]

What language?

[nighterfighter]

You have a few ways to do this:

One way would to just multiply your inputs from the joystick by a constant, less than 1, and pass that value to your motor.Set() function.

Another way, would be to limit the actual speed of your robot. For this to work, you would need encoders on each side of the robot, to monitor your speed.

Then you have an if statement, comparing the current speed of your robot, to a predefined "maximum" speed. If you are approaching your maximum speed, lower the output of the motors by a certain amount.

I would suggest, for either solution, you have a toggle to turn it on and off. Maybe you will want to go full speed for some reason? Who knows.

[Ether]

Originally Posted by microman1934

So in order to decrease tipping of the robot, I was wondering if it was possible to limit the speed of the 4 chassis CIM motors.

You may want to limit acceleration.

[MrRoboSteve]

http://thinktank.wpi.edu/article/140

If you are underweight, you can also lower the center of gravity by adding ballast low in the robot.

[microman1934]

we are using java and we are underweight but by about 2 pounds

[MrRoboSteve]

http://www.chiefdelphi.com/media/papers/2211 could be translated to Java fairly straightforwardly.

[Ether]

What motor controllers are you using for your drivetrain motors? CAN or PWM?

[Ether]

Originally Posted by Ether

You may want to limit acceleration.

http://www.chiefdelphi.com/forums/sh...69&postcount=8

http://www.chiefdelphi.com/forums/sh...22&postcount=2

[GeeTwo]

Originally Posted by microman1934

So in order to decrease tipping of the robot, I was wondering if it was possible to limit the speed of the 4 chassis CIM motors.

Originally Posted by Ether

You may want to limit acceleration.

Originally Posted by nighterfighter

You have a few ways to do this:

One way would to just multiply your inputs from the joystick by a constant, less than 1, and pass that value to your motor.Set() function.

Another way, would be to limit the actual speed of your robot. For this to work, you would need encoders on each side of the robot, to monitor your speed.

Limiting acceleration using encoders would require writing some code to calculate the acceleration from the speed (just the change in speed divided by the elapsed time). I don't know of any libraries to do this, so unless you have some experienced programmers, you probably wouldn't want to attempt it at this late date.

On the other, hand, limiting the maximum value you send to the motors will reduce acceleration by itself. You can improve stability a bit more by combining this with a throttle change limiter - and that's rather easier. All you have to do is to determine what the maximum acceptable change in the throttle is for each poll of the joysticks and update of the motors. In java it would look like:

//in initializer
double maxSpeedDelta = 0.05; // larger values allow faster changes




// inside loop

//assumes that your current call looks like:
//     motor.set(joystick.getY())
newSpeed = joystick.getY(); // or your original argument to motor.set()
curSpeed = motor.getSpeed();
if (newSpeed > curSpeed + maxSpeedDelta)
     newspeed = curSpeed + maxSpeedDelta;
if (newSpeed < curSpeed - maxSpeedDelta)
     newspeed = curSpeed - maxSpeedDelta;
motor.set(newSpeed);

[nighterfighter]

Originally Posted by GeeTwo

Limiting acceleration using encoders would require writing some code to calculate the acceleration from the speed (just the change in speed divided by the elapsed time). I don't know of any libraries to do this, so unless you have some experienced programmers, you probably wouldn't want to attempt it at this late date.

I know what we used in previous years, on the cRIO using C++, we had a 2-speed transmission with automatic shifting, based on the speed, but we also used the timer class to prevent us from rapidly shifting from high to low gear within a certain period of time.

double time = GetTime();
double left_dist = leftSide.GetDistance();
double right_dist = -rightSide.GetDistance();
double left_rate = (left_dist - leftPrevDist) / (time- leftPrevTime);
double right_rate = (right_dist - rightPrevDist) / (time - rightPrevTime);
leftPrevDist = left_dist;
rightPrevDist = right_dist;
leftPrevTime = time;
rightPrevTime = time;
//printf("left: %f, right: %f, Speed! %f\n", left_rate, right_rate, absd((left_rate + right_rate) / 2));
if (!HighGear && absd((left_rate + right_rate) / 2) > 3.7) {
shift.Set(true);
HighGear = true;
} else if (HighGear && absd((left_rate + right_rate) / 2) < 2) {
shift.Set(false);
HighGear = false;
}

That's the relevant portion of what we did in 2011, and 2012. It was placed inside of our main tele-op loop.
The "shift" object was a solenoid, that shifted the pnuematic shifter in the gearbox.


While there probably isn't a library to do that, writing the code for it shouldn't be too difficult. (You could probably just modify that to do what you need it to...)

[zinthorne]

I'm driving this year and so we made a button that while held places the max output of all drive motors at 25% you could do something similar and keep you ability to use all power and the ability to go slow at the same time... The other thought I would have is possibly removing a 2 cims from your drivetrain If you are running 4 already. We dropped from 4 to 2 and still have more than enough speed. We also have a build up in acceleration and deceleration too.

[GeeTwo]

Oh, duh!

If you really want to limit acceleration, you can use the accelerometers built into the RoboRIO; just select the one oriented along the axis that you're driving, or a vector sum of them that points you that way. For example, if your RIO is laid out on a board 30 degrees from horizontal, with the USB end towards the front and elevated, your acceleration in the drive direction would be

driveAccel = accelYaxis.getAcceleration() * .866 + accelZaxis.getAcceleration() * 0.5;

[jblay]

Also, what might help is limiting deceleration. I would make sure my motor controllers are set to coast and not brake, this way you don't decelerate as rapidly so that you have less tipping issues when stopping.