On Tue, 23 Sep 2014 22:43:50 -0500, Richard
wrote:

On 9/23/2014 6:51 PM, Tim Wescott wrote:
On Tue, 23 Sep 2014 07:37:36 -0500, Richard wrote:

On 9/22/2014 9:23 PM, RogerN wrote:
I've seen calculations for PID in a microcontroller but I'm thinking
something is missing.. or perhaps I'm misunderstanding something.

So if Derivative = current_error - previous_error then it's just
effective for one cycle. It seems like it should be the change over a
period of time,
not necessarily one pass through the loop.

One way would be to have an array of error[xx] and take the difference
in change over a period of time involving several of times through the
loop.

Also, I've seen some PID that the I adjustment is in units of time.

This might not be a big deal if there is a real high resolution, but if
you have a temperature control that is heating for a 5 degree error,
chances are the derivative term isn't going to have much effect if it's
calculated every millisecond. If it's calculated over a longer period,
for example how much change in the last 10 minutes, it would be more
meaningful.

I know a way to do this but I'm just looking for better ideas, perhaps
more efficient memory usage.

RogerN



Three is an excellent Wiki page on the subject.

http://en.wikipedia.org/wiki/PID_controller

It gets a bit sketchy in parts -- entire books could be (and have been!)
written about PID controllers alone. But as Wikipedia articles go, I
guess it's about average.



I thought it was very helpful.
At least I learnt what PID was for.

Btw...I have (2) Arduino books in ebook format if anyone needs them

give me an email address and Ill send them to you.

Gunner

"At the core of liberalism is the spoiled child,
miserable, as all spoiled children are, unsatisfied,
demanding, ill-disciplined, despotic and useless.
Liberalism is a philosophy of sniveling brats."
PJ O'Rourke