Thermocouple help needed

Working with controls for almost 20 years, but never used thremocouple relays - no need in metal cutiing
Now trying to help plastic manufacturer: replacing control with FX2N Mitsu PLC.
Customer asked to replace 12 themperature control relays with PLC control, leaving existing sensors on the machine - he needs preset from HMI.
Apparently I need thermocouple modules (2 FX2N-8AD or similar)
Relays I see on the machine now are Fuji, no any other data. They are about 8-12 years old.
Anything I should be aware, tell me now please, before I buy / promise something.

Thermocouple --> PLC

First thing you need to know is the type of thermocouple used (ie, Type J, K, etc). As I'm sure you know, your PLC input card will have to be compatible with that type thermocouple. Some cards will handle only a limited number of types.

Of course, it may not be too hard to change types, if standard thermowells are used on the machine...

I assume when you say "themperature control relays" you mean
themperature controler.
When you dealing with plastic machine you need usually accurate tmp each zone.

To get it you need to work with PID mode not On Off mode.

So you need in your PLC option to PID command.
You need thremocouple inputs module for 12 zones or thremocouple to 4-20mA converter. and 12 Analog in.
Other option to put 12 themperature controlers and connect them by RS485 from your HMI.
1st check if you need to work on off or PID and you have PID on your PLC.
2nd what type of inputs you going to use.Direct TC or analog
3rd you need transistor ouputs to control on Solid Stat Relays if you working in PID.
If you can use Omron CQM1 they have TC inputs and PID control and image of Tmp controler icon for your screen.A-B have for SLC same option

Very important put in your logic sycel to cheak if your TC damaged
or SSR damaged by check over Tmp and stop power to this zone by another output if your SSR shortcut nothnig will stop the tmp go up.
I hope I helped

Thank you for reply guys,

Like I said before we are replacing controls on existing plastic forming machine, this is not injection molding, just stamping the plastic sheet coming from the roll.

I am not sure about PID, unless PID is already part of existing hardware relays. I am pretty sure he needs On/Off control

Customer wants to replace 12 of these relays with PLC control to have ability to change presets from HMI. He wants to keep existing sensors and just remove controllers.
I was told that Mitsu FX2N-8AD module is capable to handle J,K,T types of probes. but I don't know what type is used with this Fuji relays now.

3rd you need transistor ouputs to control on Solid Stat Relays if you working in PID.

Click to expand...

I am not sure about this, what I suppose to control with solid state?
Customer told me he uses analog input only.

Typcal Temp control work like that:

Sensor-Temp Controler-Heaters

Sensor can be PT-100/1000 J K TC....

Temp controler can be PLC PC or Temp controler.

Heaters can operated by contactor, Solid stat relay,Mercury relay
or can be Gas brenner.

that your loop

When you working with PID the controler(Temp Controler or PLC) operated by pulses in order to get precise Temp. mecanical switch would take a part after short time.that the reason to use Solid Stat Relay Or Mercury relay.

With ON OFF Temp control you may have over temp .when you stop the power to the heaters the temp still increase and you may have over temp of 10-20 degrees.With PID you get the right Temp.
Trasistor output dont have any mechanical amortisation and can handel many pulses in second.

Your customer dont understand that the control system have input(sensor) and output(contactor or SSR) you need to switch off the power to heaters.

I hope it more clear now.


Regards

The first thing you need is a clear definition from your customer of his objectives, the control strategy he is looking for, how it differs from the existing strategy (if it does), how the existing system works, and what it's problems are. Talk to the machine operators and not just management. The operators may have a lot of information that isn't passed up the chain, and even if they aren't able to articulate cause and effect in engineering terms they sure know what works and what doesn't.

Make sure you check the capacity of the PLC if you are using PID controls, as many models only have four or two or so on PID loops available in a single CPU.

And you can use PID control with on/off control of heaters (timed proportioning control), so make sure you check that out.

What About the accuracy needed?

Contr_Conn
Using thermocouple with PLC is something i don't like i always prefer to use stand alone micropressor based temperature controllers(PID) .this provide many feature that can be used to accuratly control your process temperaturs such as
-auto tunning your PID parameter ,
-many types of control exist you can select the most suitable one to your process(On-off,P,PI,PID)
-some provide to control action heating & cooling
-universal inputs(one controller for many types of sensors)
-Teperature compensation
-availabilty to control your heater through contactor(relay o/p) or solid state relay(Logig o/p)
and more
Any how if you are going to control your temperature using plc I Prefer to use Temperature Transmitter to convert the thermocouple signal(mv) to a standared form of signal (0-10 vor 4-20mA) this will be better in accuracy specially if you have a long distance between your M/C and the control panel .In this case you will use also a standered analog inputs module instead of TC module in PLC .Also you will not use compensation cables or special terminal blocks (if you use TC directly to thermocouple module you have to use compensation cable and special terminal blocks according to youy TC type to avoid generating new thermocouples in the connection points)
best luck

Tom
What you meam by saying
"And you can use PID control with on/off control of heaters (timed proportioning control)".
Or you mean controling on heaters by pulses from the PLC?
Could you please wide my horizon.

I'll talk to the customer to get more details.

This is a ralatively small place and the owner knows every screw on each machine, he designs pressforms, process and takes care mostly of everything, including programming and repairs.
He needs some PLC help now: too busy to write ladder and screens - he feels above it.
My task now to replace old PLC5 controls: he hates AB for some reason - that gives me some work .
He pays per hour and I never had a problem to get payment from this guy. Plus I'll ebay his old PLC5

BUT: This is not my area of expertize: I am working with metal cutting machines (both CNC and PLC) and never used thermocouplers.

Old controls have 9 hardware termocoupler controllers and they are giving PLC descrete On/off signal.
All heaters controlled by regular relays (on/off).
I did not see any analog inputs/outputs.

He wants to add 3 more sensors.
That is why I don't see any need for PIDs at least for now.
As far as I know FX2N has few loops, so it should not be a problem.
I will price him 2 8-point input modules, hoping it will do the job.
Next week I'll try to get more info on this Fuji thermocouplers and hardware controls.

Thanks for info.

Contr_Conn

It look more clear now.

you need to make sure your PLC can get TC input. other wise you need
TC to 4-20 Transducer you need 12 its not cheap you have in the market 2 types Isolation and Nonisolation if you install the transducers neer the PLC you can use Nonisolation type (cheap)
if you put it neer the sensor use the Isolation type to avoid noises
If you need to extend to TC cable pay attention its special cabel with
polarity you need magnet to check it.and special terminals.
each zone need 2 outputs one for control and one for safety take it in your calculation.
I know it havey info for the first time.
I will be in US next week if you want I will send you my phone number in US feel free to ask what ever you want.I will try to guide you.

Regards

ArikBY asked: 'Tom What you meam by saying "And you can use PID control with on/off control of heaters (timed proportioning control)".'

There are a couple of ways of doing this. The hard way is to use a controller (Omega Engineering among others has them) that converts an analog signal to a timed pulse for controlling the current to the heaters. Essentially, the higher the analog signal ouput the longer the contactor stays on. This effectively controls the input energy to the heater.

The easy way is to use timed proportioning output, though, is to have the output of the PLC PID loop directly control the heater contactor. Some PLCs, like some single loop PID controls, have direct time proportional outputs. For most you have to "roll your own" proportional output using a timer. My rusty dusty Allen Bradley Instruction Set Reference Manual shows sample logic in Chapter 10, and my AutomatinDirect.com user manual shows sample logic in Chapter 8.

Peter's SMC post shows another way of controllling a heater with on/off control.

Of course an even easier technique is to use simple "thermostat" control with a deadband that turns the heater on below a set temperature, and keeps it on until the measured temperature rises above another value equal to set temperature plus deadband. This avoids PID limitations and tuning problems and for many processes works quite well.

Tom

If I understand you well. You mean PID loop but the output to heaters go in pulses( on-off).like evry PID temp controler.
Instead analog output like in proportional valve for instance.

Just came back from the meeting with customer - he decided that he will use standalong temperature controllers - this will simplify my task.
He indeed looking to have PID control on every circuit, so dedicated controllers will be easy to implement, especially with my "zero" experiense in this area

Alex

Go for Omron E5CN they will do the job very easy and very friendly.
Feel free to call if you need more help.

temperature does not normally need pid control.
and can be done very easy in plc just look at temp and switch heating on.
if needed i can write program
heck i will do it here.
ld temp
gt setpoint
out stop heating

ld temp
lt setpoint
out start heating