Controlling 40 Heaters

ataslaki · Mar 18, 2004

I've got a new machine to program that features 50 miniature heaters for 50 individual stations. I'm thinking of using a PLC to control the heat around 300°F ± 10°.

Can this be done with a PLC? Is there a PLC that can do the PID on its own? I know I can write my own code to try to control the heaters in a PLC, but it lacks the accuracy of a OMEGA controller.

Any suggestions?

JesperMP · Mar 18, 2004

I think you will have difficulty to find a modern PLC that does NOT have PID functionality.
Here are the ones I am sure of:
Allen Bradley SLC5/03+, PLC5, ControlLogix.
Siemens S7.

Others can tell you about other brands.
For 50 PID loops you will have to look at a medium sized PLC at least.

Sekeeley · Mar 18, 2004

I use a high density (32 point) DC output card on a SLC with Crydom or Higam Bay SSR's (these have integrated heat sinks) to control either open or closed loop heaters. Use a 1746-BTM to gain autotune of the PID loop over having to use a standard T/C input card.

ndzied1 · Mar 18, 2004

What type of heating elements are they? Perhaps induction coild you can simply turn on and off? Or do they require an analog signal?

What accuracy do you need to hold?

What type of feedback do you have/need?

The Moeller PLC's have PID functionality and also have an autotuning PLC block.

BTalbot · Mar 18, 2004

It's not a plc but if all your doing is temperature and PID you might want to look at his from Red Lion
HERE

Sekeeley · Mar 18, 2004

I haven't used that Redlion product before but I have used the DLC which cost about $100 per zone and worked really well, it communicated using Modbus.

Tom Jenkins · Mar 18, 2004

I don't think you are going to find many PLCs with 50 individual PID loops - most PLCs have some limits on how many PIDs you can use.

Do you really need to have a PID for each heater? Can't you just use a thermostat with a deadband of 5 or 10 degrees, and keep the temperatures within the ±10 degrees you need?

ataslaki · Mar 18, 2004

Some Details

I could use individual temperature controllers, but the idea of having a PLC is compact, and can do the job.

Yes, basically the PLC is going to only do the job of a heater controller.

30W individual flat round heaters from Watlow is what's being used.

I've done the same with a PLC, but only a few termocouples; 50 is alot, that's why I came to the forum to get some ideas if it's doable with a PLC.

Thanks all for the suggestions.

SAK-CO TECH · Mar 19, 2004

I have controlled as many as 75 RTD's (NR8's and NR4's) in a single controller without any problems with the SLC500 series. You would only be limited (as usual) by memory and available slot space.

In these particular applications, we did not use PID but had a deadband of +/- 2 degrees to open and close each channels respective solenoid valve.

Regards,
Chris

Lancie1 · Mar 19, 2004

The number of PID blocks will not be the limiting factor in the Allen-Bradley SLC-500 series. Your cost will be in buying enough analog outputs to control 50 heaters. If you use the NO4I, it would take a 13-slot rack. As for as the PID loops, you could do it with only one PID block, using "Indirect Addressing" to multiplex all 50 heaters. In heating applications you have plenty of time to "poll" each device, so with 50 it would not be unreasonable to do 1 each second, locking in the PID output computation for each heater until the next poll.

elevmike · Mar 19, 2004

If your desired temp is fixed, you may find it much simpler and less expensive to use a thermostat, or tempature switch. To directly control the heaters.

We use these temp switches to monator hyd oil temp. I dont see one for 300 deg. but I'm sure somebody makes them.

Actually we use a 75 deg switch. The one I posted is for 250 deg.

shooter · Mar 23, 2004

use the outputs of the plc directly as it is only 30 watts even on 24 volt it is under 2 amps.
for input you can use one analog input and switch it with outputs and relays or use lots of inputs.

for control i would use pulses
if wanted i can write simple program and send it no problem.

i can recommend wago for this nice small and compact.

Controlling 40 Heaters

ataslaki

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JesperMP

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Sekeeley

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ndzied1

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BTalbot

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Sekeeley

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Tom Jenkins

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ataslaki

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SAK-CO TECH

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Lancie1

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elevmike

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shooter

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