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LabVIEW PID controller ...please help

I need to control a relay switched heater (the heater
is either on or off - no ramping of voltage up or down)
with a LabVIEW PID routine. So my PID output will be the time
controlling on cycle of the heater, switched using a relay.

Can anybody recommend ways I could go about using LabVIEW
to translate the PID output of a duty cycle time to
controlling the heater using a SCXI-1161 relay card.
Any help is appreciated.
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Hallo, Ian,

Du meintest am 07.09.99 zum Thema LabVIEW PID controller ...please help:

> I need to control a relay switched heater (the heater
> is either on or off - no ramping of voltage up or down)
> with a LabVIEW PID routine.

Try a 2-point-control. Switches off if the temperature is too low,
switches on if it is too high. With some hysteresis between "low" and
"high".
You need some minutes to build it, and it works.

Viele Gruesse!
Helmut
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This response does not answer the question of how to use PID with duty cycle control. It only presents an alternate (possible) solution.
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If your PID output is between -100% to 100%, you can try to do something
like this :

Compare output with Zero, and wire as case box

If greater put your first relay ( for heating) to ON (or off depend on
your process), and wait a time of X.Pid output
(for example PID = 50% waiting = 500 ms, PID = 100% waiting 1000 ms, and so
on ...)
After waiting invert the digital ouput

If lower make the same action with the second relay (for cooling if exist)
and wait the absolute value of Pid.

You can have better result with a third state case, for example :
Less than -10 : R1 On, wait Pid.X, R1 Off
Between -10 to +10 : Nothing, the system is stable and the control loop can
terminate
greater than +10 : R2 On, wait Pid.x, R2 Off

good luck

Ian Paul Barber a
écrit dans l'article
<7r38ci$1tg$1@hecate.umd.edu>...
> I need to control a relay switched heater (the heater
> is either on or off - no ramping of voltage up or down)
> with a LabVIEW PID routine. So my PID output will be the time
> controlling on cycle of the heater, switched using a relay.
>
> Can anybody recommend ways I could go about using LabVIEW
> to translate the PID output of a duty cycle time to
> controlling the heater using a SCXI-1161 relay card.
> Any help is appreciated.
>
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