Simple Temperature Control System

[tyguy303]

Im a mechanical engineering student that knows a little bit about control systems (PID, control valves, feedback systems, etc.) but I need some help with a project that I am building.

I am trying to build a fairly simple control system for a 30 gallon water bath. The goal is to design a system that keeps the water at around 60 F +/- 1 degree. My idea is to use a water cooler/heater (like one you might find in the waiting room of a doctors office) and replace the hot and cold water manual valves with automatic solenoid valves that I control with a PID controller that gets its feedback from a thermocouple. I want the valves to open briefly (a few seconds at a time) to let in just enough water to affect the temperature depending on the changes of the water bath. I dont necessarily want the water to be constantly flowing. The excess water will overflow to a holding tank to be pumped back up to the water jug that sits above the heater/cooler unit to keep the water loop closed.

I have a basic idea of how I want everything to work, but I dont know exactly which controller I should buy, which power supply, and which thermocouple/thermocouple cable. I've been looking at automationdirect.com at some micrologics plc controllers as well as the single loop temperature controllers, but Im afraid I will get the wrong one for the job. I want to use automatic sprinkler solenoid valves ($15 each) which require 24V AC to actuate.

I should note that I have a bit of programming experience with C++ and a little bit with Allen-Bradley 'ladder logic' so I understand at least the logic and basics of programming controllers.

Can anyone please help me to decide which controller, thermocouple, couple wire, and power source I need to buy for this pretty simple design?

 

[waross]

Electric heater, cheap PID controller, solid state relay and cheap temp Xmtr.
If this is a real world application this may be a simpler and cheaper way to go.
If this is a problem posed by your instructor, sorry. We don't do homework.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tyguy303]

Thanks Bill but this is not a homework problem, if it was I could would have someone (prof) to ask. This is kind of out of the curriculum for mechanical engineers but its a project that I figured I should be able to take on. Im interested in learning about control systems and was looking for some expert advice.

As far as the electric heater goes, this will not really work because the bath needs to be a cold-water bath, that sits below room temp. That is why I have the water heater/cooler, so that I can add cold when it rises above 60 and hot if it drops below.

Is a PLC too much? I might also eventually like to add a level sensor and a signal to the pump, but that doesn't need to happen right away.

Is a thermocouple too much? Can I get a cheap (but relatively accurate) thermometer to send analog signals to the controller?

 

[waross]

Check out Omega.com
You might want to look into those solid state heater-cooler chests. A solid state heat pump controlled by a simple PID controller may be the way to go.
Google "Peltier Effect" The Peltier Effect is reversible by simply reversing the direction of the current.
Here is a link to one example.

http://www.thermonamic.com/TEC1-01703-NET1.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sreid]

Google "Split Range Control." Many PLC Temperature Control Blocks contain this function.

 

[danw2]

The action in the single loop temperature controllers is typically called heat/cool; one relay output for heat, another for cooling.

Single loop controllers already have their HMI built in, shows you the temperature and setpoint. An HMI is a separate device oon a PLC, in many cases different software, 2nd learning curve.

>Is a thermocouple too much? Can I get a cheap (but relatively accurate) thermometer to send analog signals to the controller?

I suppose you can shop the 'net for an oddball "thermometer with analog output, but will the analog output be a conventional industrial signal (4-20mA, 0/1-5vdc)? Stand alone controllers can take thermocouples direct, analog cards for PLCs are usually specific to thermocouple or RTD or current/volts. Buy a $15 thermocouple and be done with it (type T is best for near ambient, spend an extra buck for ungrounded and an extra buck for limit-of-error)