3 thermostat temperature(average of 3) need to control one motorised valve actuator

pkpk89

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i need a solution, there are 3 different room with 3 conventional thermostats, if each room temperature should be maintained 30 degree celcius, taking average of all the 3 rooms temperature and then if the temperature in more than average of temperature in any one of the rooms, then actuator valve should be activated to release the chilled waterflow. how can i do this? using PLC or i can do the solution using PID controller?

Please guide me.
 
What PID controller are you talking about? There may be something in the market that does what you want... but I haven't seen one yet.

A PLC would do that... but it may be a bit expensive especially if you have thermostats instead of temperature probes.
 
please see the schematic, its ok if its expensive, please help me on this. need the plc ladder logic diagram and program ,
condition is below if set point in each room is 30 degrees= 30+30+30/3= 30

if above 30 degrees= valve actuator should be actuated(chilled water flow will be more)
If the average valve is 30 degrees, valve should be closed ot return back to its old position.

temperature control using thermostat.jpg
 
Try using the LIM (limit) indtruction.

If high temp switch on cooling
If low switch on heating
If within limits switch both off.

You will still have to do your averaging though

Thanks
Brian

Just reread your post. It seems like your three rooms are seperate with a common heating/ cooling supply.
if one room goes low and the other two are in limit will cause either cooling or heating to these rooms as well.

I would then have three seperate control systems (valves etc) One for each room.

You didnt mention how critical the temp must be

More info, better answers

Thanks
Brian
 
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one valve is used there, its an existing system, so i need to control the one valve using three different thermostats.

t1+t1+t3/3 = 30 degrees temp= actuator position should be (NC) which makes the chiller water flow more
t1+t2+t3/3= 34 degrees temp= actuator position should be (NO)
after some time when the temperature @ the three room becomes t1+t2+t3/3=30 degrees, valve position should be (NC)
For each temperature set point = 30 degree
if the temperature at any one room is increased to 34, then valve actuator should be NO.
 
It's a simple PID....just average the three temperatures together and that becomes the PV...SP is 30...CV is your valve. Any PLC with a PID function can do this. The question becomes what kind of signal are the temperature probes? And is the valve simply full open or full closed or modulated in between?
 
its actually temperature thermostat, which will give output as 4-20 mA, valve is actually modulated inbetween, if needs to maintain 30 degrees temperature of all the 3 rooms , then the valve actuator position like 50%, when temperature of rooms are above 30, then valve position should be 70% open, its actually modulated valve, please suggest a PID and controller, also ladder logics etc. thanks.
Actuator will be having 4-20mA or 0-10V output.
 
You've got to put some effort into this yourself. Noone here is going to design and code your system for you. It's not hard to spec a plc that has a PID function block...most do. Call your local vendors and tell them what you need and they'll be happy to provide you a Bill of Material. Then write the program the best you can after reading the help in the plc on PID function. Then show us your work and specific issues and we will be happy to help. But at this point you're asking for free engineering work and that's doubtful someone will do that.
 
temperature thermostat, which will give output as 4-20 mA
Typical residential thermostats do NOT have 4-20mA output. There might be a commercial thermostat that does, but not residential.

A Honeywell UDC3500 loop controller with two additional, optional analog inputs (total of three inputs) and the optional math options will average three RTD inputs and provide modulating (4-20mA) cooling (direct action) control to an actuator with a 4-20mA input. This control scheme would use a 100 ohm RTD (Pt100) as a room temperature sensor.

A controller like that is configured from various parameter selections, it is not programmed from a blank slate like a PLC.

Actuator will be having 4-20mA or 0-10V output.
Every actuator needs a control signal to tell it where to position its output shaft. An actuator might have an optional 4-20mA retransmit output that provides position feedback.
 
@mr.danw thanks for the your kind suggestion, so it means that i dont require a PLC or PID etc for my application?

I can go with ths 3 PT sensors(4-20)mA and one Honeywell UDC3500 loop controller (math program can be done in loop controller)
 
@mr.danw thanks for the your kind suggestion, so it means that i dont require a PLC or PID etc for my application?

I can go with ths 3 PT sensors(4-20)mA and one Honeywell UDC3500 loop controller (math program can be done in loop controller)

Not exactly....The UDC3500 is a PID controller. But that is its only function, whereas a PLC can do the PID an much more. You will still need to set up the PID control in the UDC3500. Also, pay attention to Dan's note that the UDC requires three RTD sensors....not 4-20ma. You may be able to buy a UDC that uses 3 Analog 4-20ma sensors, but I haven't used UDC's beyond RTD inputs, so don't know for sure.
 
Correct.

First input is universal, which includes Pt100. Need two optional universal inputs.

The math creating PV1 is
IN1 (ratio = 0.33) + IN2 (ratio = 0.33) + IN3 (ratio = 0.33)
for the average

IN1 TYPE = 100 LO

CONT ALG = PIDA
INP1 ALG = SUMMER (math function)
ALG1 INA = IN1
ALG1 INB = IN2
ALG1 INC = IN3
OUT ALG = CUrrent

PWR MODE = ALSP (starts in automatic mode (A) on power up; factory default is manual mode)
ACTION = DIRECT ('Direct' action for cooling)

The standard 4-20mA output is the control output.

It is a panel mounted device with open screw terminals on the back.

Like any modulating loop, you'll have to tune it. No, I don't what the tuning constants are. It includes an autotuning function.

DC3500-Ex or 3500-Ex is for modulating or ON-OFF control that includes a standard 4-20mA output with alarm relay(s)
DC3500-Cx is modulating only and excludes ON-OFF control.

DC3500-00-0A000-YYY-00000-00-0 where Y is either 1 or 2 will do PID cooling by averaging three RTD inputs. Other options at your discretion.

In reply to RobertMee's observation: The UDC analog inputs are universal. The #2 model code includes a 250ohm resistor for using 4-20mA inputs (#1 does include the resistor).
So the UDC universal analog input works with either 4-20mA or RTD.
 
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I would also wonder are these three rooms insulated from each other, or is there any danger that one room is cold while one room is hot? If you do just a standard average, one room could be at 20 degrees, one room at 30 and one room at 40, but the average would be 30, and your temperature requirement satisfied.
 
@mr.Danw, thanks for your kind suggestions and guidance.

my question is for , please kindly clear my doubt in rtd?

Also what would be the output range of the rtd? is it analog current or vlotage(4-20mA or 0-10V).

how can i loop the rtd to honeywell loop controller? directly can i connect the (3 wire or 2 wire) from pt100 rtd to UDC3500 controller or in between i need to use any temperature transmitter?[/quote]

can i use pt1000 rtd instead of pt100 rtd? pt1000 is available in honeywell.


attached the schematic i made for your kind info sir.

please see the link

http://i64.tinypic.com/2cwlyzm.png
 

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