Replacing temperature PID controllers with PLC

Hi everyone, Im planning to replace existing temperature controllers with a PLC.

The setup will be aprox 8 thermocuples, and the outputs are 8 solid state relays.

The actual controllers uses PID, so there I have two questions:

1) If i put the same parameters P,I,D that are in the controllers, will the PIDs of the PLC respond the same way as the ones in the controllers ?

2) How can I use the analog output of the PLC to control the solid state relays, should I use PWM ? is it really necessary to use a PID to achieve a constant temperature ?

Im planning to use the Micro800 series of allen bradley.

Any more info needed tell me..

Thanks!

Question 1: The only way the PLC response will match that of the existing controller is if the PLC uses the same implementation of the PID equation as the controller. Don't expect it to happen.
Question 2: Some solid state relays offer an analog input option. Lacking one, PWM is a viable solution. If you choose to use PWM, make sure your PLC output is a solid state device and not a relay contact output. You can often achieve acceptable temperature control without PID.

Not sure about the Micro800 series, but there is example code on how to do this for both CompactLogix and Micrologix/SLC on the Rockwell Knowledgebase. Will these be heating only or are they a heating and cooling application? If you search "Time Proportional" or "Split Range Time Proportional" output i think you will find something.

A Time Proportional output will take the analog control value from a PID and scale so that it is on for a period of time and off for a period of time.

I would very much like to get that sample code for Time Proportional on a MicroLogix...but I can't access the knowledgebase, I always get an error window, I wonder if its because I do not have a current support contract....I used to....can anyone post it here for me? It would be hugely appreciated.

Update, I found what I wanted here in the forums and attached it.

It is very unlikely that your P, I and D parameters will work on the PLC as they are on your controller. However, the order between the values and sign should give you a good starting point for your PID.

Does the Micrologix have a self tuning algorithm as the big brothers do (Compact and Control Logix)?

PWM for controlling solid state relays from a PLC is not the best way. Last time I checked, there are SSR's that have 4-20mA input and then do the smart bit of switching inside them.
It's my understanding that SSR's tend to fail closed, so have a relay controlled by your PLC to cut the power to the heater if the temperature control is not working.

I think it is possible to get the PLC to work as well or better than the temperature controllers but....

1) If i put the same parameters P,I,D that are in the controllers, will the PIDs of the PLC respond the same way as the ones in the controllers ?

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It depends on the units of the parameters. The problem I have with AB PIDs is that the PID gain itself is unitless. The gain is control counts output per temperature counts in or basically counts/counts. One take take into account the counts/degree of temperature and the counts/% control. Also, are the integrator and derivative gains real gains or are they time constants. Few on this forum specify the units of the gains and just provide numbers that don't mean anything. I know how the Micro Logix gains work.

2) How can I use the analog output of the PLC to control the solid state relays, should I use PWM ? is it really necessary to use a PID to achieve a constant temperature ?

Click to expand...

Can't you use a digital output?

I have an example ML1500 program in the misc download area called hotrod. I think I created in in 2005.

Peter Nachtwey said:

I know how the Micro Logix gains work.

Click to expand...

Peter the OP is considering using a Micro800, this is not the same as the MicroLogix... I have not used a PID in the CCW software on a Micro800 yet so not sure if they would be the same or how different they would be

joaco1993, not sure how much memory you will need with running 8 PID loops but you may want to consider a 870 or 850 but just dont sell yourself short on the memory

No to PWM. PWM is mainly used to control speed of DC motors. SSR is AC - a very different control method.
There should be something built in to the software to pulse an SSR. Similar to the controllers you took out. With controller, you choose SSR out, and controller handles the pulsing. The PLC software should have a smart box that closely duplicates a stand-alone controller.

The question of analog, PWM, or pulsing outputs should be determined by the heating method of your process.

For example if you’ve got a steam heat exchanger or short wavelength infrared lamps you need some sort of analog or PWM based control. There’s a wide variety of these available depending on your application.

If you’re using cartridge heaters or some other slow response element pulsing a zero-cross ssr is just fine.

Keep in mind that a PID loop in a PLC does not necessarily come with auto tune functionality. This seems to be the biggest advantage of standalone PID controllers. Update rates, output periods, etc all play a role in this too and you will likely have to figure this out on your own with a PLC. I prefer to get a comm option card and control a stand alone PID controller from the PLC. If all you are attempting to achieve is multi-zone PID control Watlow sells a multi-zone controller that has an Ethernet option.

Hi
i am having problems tuning my IPID ona Micro800 PLC.
My process is a baking furnace that bakes plane parts.
My themocouple is inserted into the parts ( composite material )
so we have a very very slow response process.
Can someone give me some tips

You could try the auto tune function here is a link to a video.
https://www.youtube.com/watch?v=-F_8BVAqORE