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sirisha - July 27, 2007 - 10:50 AM
Post subject: problem in implementing my PID controller

Hi!

I have a PID control for temperatue of Honeywell make. Model is UDC 3500. I want to give set point as 2000. Process value from field is normally 1500 or more. The output of PID is coming in %. But i want the output in Engineeering units. Because i want to feed the output of PID i.e temperature which is controlled as an input to another loop.

prabakaran - July 28, 2007 - 12:49 AM
Post subject:

what is the maximum value coming from the UDC when it shows 100%. Equate that value to the coming percentage value for getting the required output in 3500.

can i information what project ur handling

instruite - July 31, 2007 - 07:57 AM
Post subject: Re: problem in implementing my PID controller

sirisha wrote:

The output of PID is coming in %.

That is what display of PID Controller says right??
The actual output will be in voltage normally.
So all you have to do is just attenuate or/and amplify the output voltage of PID controller such that it will represent the input required for your next loop

sirisha - Aug 02, 2007 - 11:12 AM
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I m using a PID function block in my HC900. SP to it is always 2200. PV will be varying between 2000 to 2500. My PID function block is giving output in % between 0 to 100. But if i can get that value in eng units i can feed it to the next loop.

instruite - Aug 02, 2007 - 12:34 PM
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sirisha wrote:

I will like to have a look at the user manual or product specification of HC900 and UDC 3500
can you point me to its online website/link?
wil save me search time...

Carl_E - May 30, 2010 - 10:20 PM
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Yeah, it's an old thread, but sometimes people hit old threads from Google with the right combination of search terms, so here goes:

1) >The output of PID is coming in %. But I want the output in Engineeering units.

You won't get an output in any range/units other than 0-100% (with the minor exception that some controllers, like the UDC 3500 let your underrange or overrange slightly, from -5.0% to 105.0% in order to make up for miscalibrations in the output circuit). That applies to any PID controller, regardless of make or model.

2) >Because I want to feed the output of PID i.e temperature which is controlled as an input to another loop.

If you want to feed the output back into an input, like in cascade, then you need to use input conditioning to produce the engineering units and scale needed by the 2nd controller.

On the UDC 3500 you do a y=mx+b calculation on the input where
ratio 1 = m
Bias IN 1 = b

3) >what is the maximum value coming from the UDC when it shows 100%.

What comes from a UDC depends on which kind of output is has.

If it has a current output ranged 4-20mA, then 100% = 20mA (0% = 4mA)

If it has a relay output, running time proportional, then the relay is closed 100% of the cycle time; 0% = relay open for entire cycle time

4) >PID function block in my HC900. <snip> My PID function block is giving output in % between 0 to 100. But if i can get that value in eng units i can feed it to the next loop.

If this is for cascaded loops, here's the example from the book:

If cascade is not the intended purpose, then run the output into the remote SP (RSP) of the PID block

The RSP settings let you do the y = mx + b scaling on the output signal:

Carl_E - May 31, 2010 - 01:58 PM
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I forgot the HC900 math function block, with which any scaling can be achieved:

The inputs (letters) on the left can be one or more analog values, constants, or variables.

nita - June 09, 2010 - 11:32 AM
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Great information Carl.thanks and keep on enlightening us with such knowledge.
Nita

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