Limit speed of 4 chassis CIM motors

[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.

[Mr V]

Originally Posted by 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.

Yes tipping when abruptly getting off the throttle will also likely cause tipping. With 4 motors you can set one on each side to coast and one on each side to brake for a lower braking force. We did that on the rookie team I am working with this year and it has worked good in practice.

Personally I would remove a motor per side if it can be done w/o major disassembly.

[GeeTwo]

Originally Posted by 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.

Absolutely. Deceleration is a form of acceleration, and I included it implicitly. The following line was intended to reduce deceleration, and putting the motor controllers in coast would prevent a sudden unintentional change.

Originally Posted by GeeTwo

if (newSpeed < curSpeed - maxSpeedDelta)
     newspeed = curSpeed - maxSpeedDelta;