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> May I obtain also the code that you mentioned? I'm supposing the code
> was used at the HandyBoard?
Yep ... and no.
The following was applied to a pneumatic cylinder controlled via an Amplicon
Liveline I/O
card and an SMC proportional valve (ie not Lego).
code
*********************************************
pidloop:
'e = error, sp = set point (desired value), pv = process value
'(current value)
'x100 to convert to a percentage
'ie error is a percentage difference between your desired value and the
'current value
e = (sp - pv) * 100
'a deadband is used to avoid hunting
'ie if the percentage error is small then do nothing (or loop here)
'deadband - initially set to 1
IF (e ^ 2) < .25 THEN GOTO pidloop
'sum the errors
sum = sum + e
'this PID equn should work fine, there are a few different ones out
'there
'k = gain (speed of algorithm response), Dt = change in time (how
'often
'you run the PID algorithm), Ti = integral time (accuracy of response)
'Td = derivative time (stability of control), reqp = control effort
'as a %
reqp = (k * e) + ((k * (Dt / Ti)) * sum) + ((k * (Td / Dt)) * (e - Eold))
'clip the control effort to be a percentage between 0 and 100
ReqUnclip = reqp
IF reqp < 0 THEN reqp = .1
IF reqp > 100 THEN reqp = 100
Eold = e
GOTO pidloop
END SUB
*********************************************************
The output reqp is the value you should output from your controller to the
system. e.g.
Say you use a controller that has an output range of 0 to 10 V and reqp is
25%. Hence you out 2.5V.
The value of K, Ti and Td will be different for each system controlled. One
way to gain the values is
with Ziegler and Nichols methods. Simple empirical methods. (see Integrated
electrical
and electronic engineering for mechanical engineers, C Fraser and J Milne,
McGraw Hill, 1994, ISBN
0-07-707973-6). An example set of values for my pneumatic system were K = 5,
Ti = 0.04,
Td = 0.05. Set Dt, the time, to what you want, try 0.5 to 1 sec, depends on
your system.
I seem to remember I ran Dt at a few milliseconds for my app. Meant messing
with
the PC clock. Have code if you want it.
Remember to only run the loop every Dt increment, it does not run continuously.
I totally recommend the listed book. Superb sections on Basic and C
programming, control
theory and electronics. I am a bit biased as the authors were two of my Uni
lecturers.
I can also post details on Ziegler and Nichols methods if you want and
I think I have a QB prog that allows you to play with the values and see their
effect on the system. Also a pdf of my Masters that this was taken from that
has info on the theory of PID and some of the testing I did with it.
Just shout if you do not understand or want more help. It has been a couple of
years since I looked at this stuff and so my comments may be a bit cryptic.
Mike
ps apologies for the formatting, pasted it in from notepad
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Message is in Reply To:
 | | Re: PID Control
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| Hi Mike: May I obtain also the code that you mentioned? I'm supposing the code was used at the HandyBoard? Thanks in advance for your help... Victor G. Ruiz Aranda ruizvic@mx1.ibm.com IBM de Mexico S.A. Coordinador IAS Planta de Manufactura T/L (...) (25 years ago, 27-Jan-00, to lugnet.robotics.handyboard)
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