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-   -   Help me understand PIDController::OnTarget() (http://www.chiefdelphi.com/forums/showthread.php?t=103683)

wireties 23-02-2012 22:38
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by Joe Ross (Post 1133612)
I've never heard that before. Where did you hear it? I would have thought that Joe Hershberger would have mentioned the MD5 check instead of helping with instructions on how to rebuild the library in the following thread. http://www.chiefdelphi.com/forums/sh...ad.php?t=89131

I do agree it's better to override a method if possible.
My apologies - I could swear I tried this the first year we had the cRIOs and you got a library version error. When I asked about it, the WPI folks said you had to have special permissions (and be on the "build team") to access the svn repository and do a proper build. Obviously that is no longer the case (if it ever was). Sorry to steer anybody wrong!

wireties 23-02-2012 22:41
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by jwakeman (Post 1133655)
I think you're correct that there is no danger of segmentation faults. However, if access to the variables is not synchronized then the potential for logic errors exist.
... thus the "may not be desirable" caveat ;o)

wireties 23-02-2012 22:48
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by jwakeman (Post 1133659)
Can someone explain the difference between these two uses of the semaphore? I see both being used in the PIDController class.

Code:
CRITICAL_REGION(m_semaphore)
{
}
END_REGION;
Code:
{
    Synchronized sync(m_semaphore);
}

the first is a semaphore used for mutual exclusion (to protect the critical region), in this use-case the semaphore is taken before and given after a critical region of code and always in the same context - the semaphore includes the concept of ownership (by the thread/task) and recursion

the second is a semaphore used for synchronization, in this use-case the semaphore is taken in one context/thread/task and given or flushed in another context - for example given/flushed in a periodic timer callback function (or maybe an interrupt service routine) and taken in a task/thread

hth

jwakeman 24-02-2012 11:05
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by wireties (Post 1133810)
the first is a semaphore used for mutual exclusion (to protect the critical region), in this use-case the semaphore is taken before and given after a critical region of code and always in the same context - the semaphore includes the concept of ownership (by the thread/task) and recursion

the second is a semaphore used for synchronization, in this use-case the semaphore is taken in one context/thread/task and given or flushed in another context - for example given/flushed in a periodic timer callback function (or maybe an interrupt service routine) and taken in a task/thread
I figured. So in QNX parlance (which I am most familar with) these would be a Mutex and Condition Variable. However, I don't see where the semaphore is being flushed in the PIDController implementation. The periodic loop is accomplished with a Notifier calling the Calculate() method periodically. Is the sync actually being used somehow here?

jwakeman 24-02-2012 11:32
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by vamfun (Post 1133685)
What is maximumInput and minimumInput?
It is simply the minimum and maximum setpoint that the controller will accept. In the case of my shooter speed controller they are set to 0 and 100 respectively (units are feet per second in this case). For my turret position controller I am using -45 to 45 (degrees) and my turret speed controller accepts -250 to 250 (degrees/sec)...i think you get the idea.

Quote:
Originally Posted by vamfun (Post 1133685)
Now that you brought this code up, would someone mind explaining why it is there?
The code I posted actually falls within one more if-statement, if(m_continuous). I've also always wondered what this code is meant to represent. What is meant by continuous in this case? Also running some values though the code doesn't help me understand at all.

Using my example of a shooter speed controller with minimum input of 0 and maximum input of 100 fps. Say I step my setpoint from 0 to 100. My error would be equal to 100 on the first iteration. So 100 is greater than (100 - 0) / 2 and 100 is greater than zero so m_error ends up being set equal to 100 - 100 + 0. So if I step my speed controller from zero to maximum the output will be computed as zero?!

Another scenario, I step the setpoint from 0 to 60. The error of 60 is greater than 50 and greater than zero so the m_error becomes 60 - 100 + 0 = -40. Now my positive error became negative error..I'm confused.

Code:
if (m_continuous)
{
    if (fabs(m_error) > (m_maximumInput - m_minimumInput) / 2)
    {
        if (m_error > 0)
        {
            m_error = m_error - m_maximumInput + m_minimumInput;
        }
        else
        {
            m_error = m_error + m_maximumInput - m_minimumInput;
        }
    }
}

Ether 24-02-2012 11:39
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by jwakeman (Post 1134081)
What is meant by continuous in this case?

Using my example of a shooter speed controller with minimum input of 0 and maximum input of 100 fps. Say I step my setpoint from 0 to 100. My error would be equal to 100 on the first iteration. So 100 is greater than (100 - 0) / 2 and 100 is greater than zero so m_error ends up being set equal to 100 - 100 + 0. So if I step my speed controller from zero to maximum the output will be computed as zero?!

Another scenario, I step the setpoint from 0 to 60. The error of 60 is greater than 50 and greater than zero so the m_error becomes 60 - 100 + 0 = -40. Now my positive error became negative error..I'm confused.
I don't know for sure, but I thought it was for rotary position control for continuous-rotation devices. i.e. 360 degrees is the same as 0 degrees.

For example, if your setpoint is 10 degrees and your process variable is 350 degrees, you want to rotate clockwise 20 degrees, not counterclockwise 340 degrees.

RufflesRidge 24-02-2012 11:40
Re: Help me understand PIDController::OnTarget()
 
m_continuous is used to indicate if the range specified should be considered to be continuous (wraps from max to min) or not (the default is false). This is explained some in the documentation for the SetContinuous method.

So for something like a continuous rotation turret or swerve pod, you may set min as zero, max as 360 and continuous to true using SetContinuous(true). In this case the minimum and maximum represent the same value so the error for your hypothetical is indeed 0.

wireties 24-02-2012 11:50
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by jwakeman (Post 1134069)
I figured. So in QNX parlance (which I am most familar with) these would be a Mutex and Condition Variable. However, I don't see where the semaphore is being flushed in the PIDController implementation. The periodic loop is accomplished with a Notifier calling the Calculate() method periodically. Is the sync actually being used somehow here?
Sounds right, in VxWorks the take and give functions are the same (semTake and semGive) but the semaphores are created differently (semBCreate vs semMCreate vs semCCreate).

Jeez - I've got to stop giving advice from memory - sorry again. Despite the name it seems the Synchronized class is also used for mutual exclusion - weird. The destructor gives the semaphore - I missed that. The Notifier class uses a semaphore for synchronization and the DriverStation has a semFlush example.

jwakeman 24-02-2012 12:01
Re: Help me understand PIDController::OnTarget()
 
Quote:
Originally Posted by RufflesRidge (Post 1134086)
m_continuous is used to indicate if the range specified should be considered to be continuous (wraps from max to min) or not (the default is false). This is explained some in the documentation for the SetContinuous method.

So for something like a continuous rotation turret or swerve pod, you may set min as zero, max as 360 and continuous to true using SetContinuous(true). In this case the minimum and maximum represent the same value so the error for your hypothetical is indeed 0.

OK that's pretty neat. I'm learning a lot about the PIDController implementation on this thread.


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