Common practice to use temp controller?

Is it common practice to interface temperature controllers such as this unit here, to a PLC system for monitoring and controlling temperature based inputs? Or am I missing something or a possible easier configuration? New to all this and just trying to familiarize myself with ideas!

Thanks!

Hi Joe,

You don't need that hunk of junk, you've got yourself a perfectly good PLC! You could simply code the same functionality as that temperature controller into your PLC, and probably with a lot more smarts too.

All that controller is doing is taking an reading (from a thermocouple or some other temperature measuring device) and comparing it to a setpoint, lets say 30 degrees celcius (sorry we work in Celcius here!).

A very, very basic controller might use some comparison logic to check if the measured value was less than the setpoint and if it is, turn on an output to drive a heater (via relay and contactor or something like that). When the temperature exceeds the setpoint, it turns the heater off. (very basic, and you need some deadband and heater protection in there too).

Later on you can check out what's called PID control - a more sophisticated way of controlling a process. But for now the above is probably sufficient.

What you need is some form of measuring device that you can connect to your PLC. I seem to remember you had an SLC with a bunch of I/O lying around. I've never used one, but I believe Rockwell make thermocouple and RTD input cards. coupled with an appropriate RTD / thermocouple, you'd have a temp reading.

Failing that, you could "roll your own" with a PT100 temperature sensitive resistor and a few additional resistors. It'll behave a bit like a potentiometer that's sensitive to temperature, the only problem is that they don't have a linear response. I vaguely remember having to do this in a university lab.

You'd then be able to monitor the temperature in your PLC and turn on an output to drive a heater / fan / whatever.

I don't do a lot of work with temperature measurement, mostly flow / pressure / and water quality related measurement. Maybe someone else can point you in the direction of a cheap PLC friendly temperature sensor.
http://www.ebay.com/itm/2-wire-tran...392?pt=LH_DefaultDomain_0&hash=item256e163258

It depends. If you don't want to pay for an RTD or thermocouple input board, then maybe a stand-alone temp controller makes sense.

Stand-alone PID temperature controllers still find wide use because they do a specific job well and the HMI part (display and keypad) are part and parcel of the unit, the HMI doesn't have to be programmed, it's just there.

There are some control strategies that let a PID (modulating) temp controller do its thing and the PLC sends the temp controller a setpoint via digital communications, when required.

The device you linked to is a pretty simple thermostat, ON/OFF control for heat and/or cooling, and its range goes to 100°C. There's lots of temp control that goes a lot higher that that particular unit could not handle.