[?]: PID with two actuators

kelertas · Post by **kelertas** » Tue Nov 13, 2007 9:15 am

I want to ask one question.
Is it possible that two actuators could control one setpoint and both actuators could not work by the same time.

Best regards.

dante · Post by **dante** » Tue Nov 13, 2007 9:25 am

actuators do not control anything. they are only executing.

kelertas · Post by **kelertas** » Tue Nov 13, 2007 9:37 am

Well, yes. I'll ask then in the different way. Could PID regulator could control the setpoint by executing two independent actuators?

dante · Post by **dante** » Tue Nov 13, 2007 9:42 am

It's not clear. The setpoint is not controlled. The setopint is the "desired" value and is to be set from outside PID. Do you need cascaded PIDs?

kelertas · Post by **kelertas** » Tue Nov 13, 2007 9:52 am

Well I'll tell yuo the situation. For example : In the room the desired temperature should be 20C. The sensor shows me 30C. I have two independent valves and two ducts. One duct suplly cold air and the second hot air. Is it possible to do PID regulator, wich first is closing hot air valve, when tihs valve fully closed, then it is opening cold air valve until riche the setpoint.

dante · Post by **dante** » Tue Nov 13, 2007 10:07 am

Course its possible. Is only a matter of how are you interpreting the output of the PID. You can, for instance, say:
- between 0% and 30% one valve is controlled, the other one will not change the position;
- between 31% and 100% the other valve is controlled,
all this have to be impletented AFTER the PID, between the PID output and the valves, B U T :
your problem is much, much complicated, you need to recirculate the air and to mix the recirculated with fresh air... need lot of experience.

kelertas · Post by **kelertas** » Tue Nov 13, 2007 12:01 pm

between 0% and 30% of what?

zenchek · Post by **zenchek** » Tue Nov 13, 2007 12:40 pm

For this application is necessary PID with heat - cool control algoritm.
ABB commander 351 or 355 have this algoritm and two outputs, one for heath and other fot cool.
zenchek

kelertas · Post by **kelertas** » Tue Nov 13, 2007 12:44 pm

where can i get this algorithm? Besides the valves is Discrete, so PID output I must modulate with PWM

dante · Post by **dante** » Tue Nov 13, 2007 12:51 pm

so everythig is WRONG; you cannot use discrete valves for this application, you must use regulation valves (valves driven with analog signals). You can use PWM with electrical heaters but not with valves.

kelertas · Post by **kelertas** » Tue Nov 13, 2007 2:19 pm

dante · Post by **dante** » Tue Nov 13, 2007 2:26 pm

what temperatures do you have on the cold and on the hot sources?

zenchek · Post by **zenchek** » Tue Nov 13, 2007 4:19 pm

kelertas wrote:where can i get this algorithm? Besides the valves is Discrete, so PID output I must modulate with PWM

See this topic, have links to download PID function blocks for Siemens PLC http://www.plc.net.ru/viewtopic.php?p=1 ... ght=#13103
zenchek

Umencho · Post by **Umencho** » Tue Nov 13, 2007 4:34 pm

zenchek wrote:
kelertas wrote:where can i get this algorithm? Besides the valves is Discrete, so PID output I must modulate with PWM
See this topic, have links to download PID funktion blocks for Siemens PLC http://www.plc.net.ru/viewtopic.php?p=1 ... ght=#13103
zenchek

Hi

I think that DANTE is right. You need valves with analog signal. With this algorithm you can't control the temperature in a target , I mean you always will be nearby but never exactly. For ducts you have only full open or full close. You can control only hot or cold. But you can try and post the result here.

.
I don't know maybe you can control with succes, but this depends from size of the ducts and how big is room.

regards

kelertas · Post by **kelertas** » Tue Nov 13, 2007 6:29 pm

I will use S7-200 PLC for this. My point for this algorithm is:
Lets say PID output is scaled from 0 to 100. that is equal to from 0s to 40s of PWM. So if my PID output is 50 the valve will be opened for a 20 seconds and closed for 20 s.
But I still dont know how to open, lets say cold valve, if hot valve is fully closed and temperature still higher than setpoint.

zenchek · Post by **zenchek** » Tue Nov 13, 2007 7:38 pm

Umencho wrote:
zenchek wrote:
kelertas wrote:where can i get this algorithm? Besides the valves is Discrete, so PID output I must modulate with PWM
See this topic, have links to download PID funktion blocks for Siemens PLC http://www.plc.net.ru/viewtopic.php?p=1 ... ght=#13103
zenchek
Hi

I think that DANTE is right. You need valves with analog signal. With this algorithm you can't control the temperature in a target , I mean you always will be nearby but never exactly. For ducts you have only full open or full close. You can control only hot or cold. But you can try and post the result here. .
I don't know maybe you can control with succes, but this depends from size of the ducts and how big is room.
regards

Sorry guys, i do not read that control valves are on-off, for this application is necessary modulating dampers for ducts.
Regards- zenchek

kelertas · Post by **kelertas** » Tue Nov 13, 2007 8:18 pm

what you mean zenchek, I did not understand?

zenchek · Post by **zenchek** » Tue Nov 13, 2007 9:29 pm

Hi kelertas.
This is some definitions.
Modulating Dampers
A damper, in HVAC terminology, is an air valve used to regulate the flow of air through the duct.
There are two general kinds of dampers (definitions are probably not in line with HVAC terminology, but hopefully understandable enough):
Simple, also known as bang-bang, dampers - these can only be in fully open or fully closed position;
Modulating dampers are able to stay in any position between fully open and fully closed.
First kind is, of course, cheaper, but has serious drawbacks - first, the
noise they make is quite noticeable (one can get used to it, though),
second, they provide only limited control and are not suitable to advanced
applications.
Modulating dampers, on the other hand, are silent by design and provide
adequate control, but require more sophisticated control systems in order to utilize the capabilities.

Go to follwing links for more information:

Actuators for Commercial HVAC Dampers
http://gfcportal.com/technical/tech_det ... /actuators

Belimo actuators:
http://www.belimo.eu/CH/EN/Product/Actuators/index.cfm
http://www.belimo.eu/pdf/e/overview_air_en.pdf
http://www.belimo.com.br/pdfs/novidades ... _set01.pdf
Belimo have actuators for control signal from the PLC in game of 0-10v 2-10v 4-20 mA or PWM.
Regards zenchek