Programming Puzzle

[plnyyanks]

Originally Posted by Ether

This is from WPILib. Doesn't LabVIEW have something similar?

Not to my knowledge. Here is the manpage for the basic PID VI we're given. Also, none other more advanced PID VIs seem to indicate that this functionality exists, either.

Bummer, because it'd be nice...

Originally Posted by Ether

If not, I think this or this would work, where angle error is your desired canon angle (in degrees) minus your sensor reading (converted to degrees).

Yeah, we've done stuff like this in the past when using sensors that will cross zero. Or, I think the VI I linked in my last post could be good, as well. (edit: no, it won't. see my last post)

edit: here's a VI I quickly wrote up that demonstrates both of the methods Ether wrote up: zero_cross_test.vi

[Todd]

You could also try using a PID to minimize error, rather than reach a setpoint.

IE: Rather than

My setpoint is currently 4.5 my process variable is at .1

Say

My setpoint is always 0, currently my process variable is -.6 (where -.6 is the circular difference between your 'goal' and your 'actual') then when your goal changes, the frame of reference you use to calculate your process variable changes, and your (probably only proportional gain?) controller will move your motor.

[Chris Hibner]

I wrote a vi called HeadingWrap.vi that we've used for autonomous. You can use it to convert an angle outside of +/-180 degrees to be within the +/-180 degree range. You can use it to do what you're trying to do.

This is how it's used: Calculate your angle error into your PID (i.e. desired - actual) and feed the result into HeadingWrap.vi. The output will come out within the +/-180 range and will give you the shortest turn to your desired angle.

You can find it here: http://www.chiefdelphi.com/media/papers/2553 in the sample robot code zip file.

Here's more or less how it works:

Let's say you command your canon to 355 degrees and it overshoots so your sensor reads 3 degrees. For your next PID calculation, your error is 355 - 3 = 352 degrees. This is the problem you've had: it wants to command your canon to do another loop. To solve it, check your error to be within +/- 180. If not, subtract (or add, if the error is negative) 360 degrees. For this case, we subtract 360 so: 352 - 360 = -8 degrees. This will get your canon to head in the right direction by the correct number of degrees.

Here's the basic algorithm:

Error = desired - actual;
while (abs(Error) > 180)
{
   if (Error > 180)
      Error -= 360;
   else
      Error += 360;
}

You now just have to trick your PID: feed the above "Error" calculation into your setpoint and feed 0 into the process variable.

EDIT: For your application, the while loop is unnecessary. We use it in autonomous since we allow the heading integrator to grow past 360 in both directions (i.e. we don't wrap the heading integral - we just take care of the wrap on the error term as stated above).

[Chris Hibner]

While my above post solves your math issue, you may have a bigger problem.

A continuous pot has a gap in which the wiper will not contact the resister band (right where it will flip from 5V to 0V). When that happens, the A/D pin will be at a floating voltage. You may get weird voltage readings in that band depending on how the A/D pin is "wired". Is there a pull-up or pull-down resistor on your board? If not, you should definitely add one so when the wiper loses contact, you have a defined voltage that is either 0V or 5V (not somewhere in between).

[Ether]

Originally Posted by plnyyanks

edit: here's a VI I quickly wrote up that demonstrates both of the methods Ether wrote up: Attachment 12945

Thanks Phil. For any C programmers who may be lurking, here's how to do it in C:

error -= 360*floor(0.5+error/360);

That converts any angle to the equivalent minimum absolute value, with the correct sign.