AB Heating PID/PIDE control

Hello everyone, I just want to start this with I have lots of knowledge in programming AB plc's from the 5's thru the 5000's, but I have zero knowledge of programming PID loops.

I am attempting to control the temperature in a vacuum chamber with 3, 1.5kw heaters. They are all controlled via SSR's right now and work but they aren't real tight. So I thought I would try a PID loop. There lies the problem and the literature I can find only confuses me more.

I am working with a 1769-L35e compact logix controller, v16. I have 1 thermocouple for the chamber temp and one output enabling the SSR's.

Any direction would be greatly appreciated, thanks in advance for your time

Duane

PS I attached a cleaned up version of what I have now in the zip file, it is RS5000

I like to scan my PID logic once every second, then take the % output and move it into a timer preset. I then fire the SSR while the timer is timing.

Another useful idea is to configure your HMI (if you have one) to allow you to change the P, I, and D values on the fly.

I do have an HMI, which is yet another obsticle I'm working through. Lots of practice with Rsview, but this newer version is taking some getting used to.

Good idea though on being able to adjust via the HMI, I will work to that end.

Thanks

You would do well to search here for posts by Ron Beaufort like these, this ones a good one on bumpless transfer.

your title mentions PID and PIDE ... which one are you going to use? ...

survival tip: for my first attempt at this, I'd definitely make it PID (Ladder Logic) and not PIDE (Function Block Diagram) ...

I only had time to BRIEFLY SKIM the file that you attached – but it LOOKS LIKE you're going to use Local:4:I.Ch2Data (Chamber Temp) as the analog input signal ... you'll need to keep the scaling on this in mind when you set up the PV ("Process Variable") signal for the PID ...

it also LOOKS LIKE you're using HV_RR:1:O.0 (Heater Contactor) as the digital output signal ... this seems to be set up on an Ethernet I/O connection to a 1794-OB16/A Flex-I/O DC output module ... so – you'll need to decide how you want to convert the ANALOG output of the PID (the CV "Control Variable") into a suitable DIGITAL output signal ... something along the approach recommended by demons is quite common ...

before you go too far with this, I'd highly recommend that you run the existing control through a couple of "tests" and trend (graph) the results ... I notice that you already have a trend set up for the "Chamber Temperature" so that's probably a good place to start ... if you could post a screen shot of the resulting trend, that would help the forum members get a better idea of "where you stand" – and make it easier to get "where you want to go" ...

good luck with your project ... I wish I had more time to help - this looks like it could be a lot of fun ...

Ron Beaufort said:

y

it also LOOKS LIKE you're using HV_RR:1:O.0 (Heater Contactor) as the digital output signal ... this seems to be set up on an Ethernet I/O connection to a 1794-OB16/A Flex-I/O DC output module ... so – you'll need to decide how you want to convert the ANALOG output of the PID (the CV "Control Variable") into a suitable DIGITAL output signal ... something along the approach recommended by demons is quite common ...

Click to expand...

HV_RR:1:0.0 is actually the Main power enable to the SSR's. HV_RR:1:0.1 Fires the SSR's a percentage based on a 1 sec timer. Until the chamber reaches 100deg c. the max allowed is 60% or 0-600 on the 1 sec timer,Control_Timers[2]. Then 100% is allowed until it begins to decrease the percentage once temp is within 25 deg of setpoint.

So I think this is basically the same logic that Demons was talking about. Only under the PID control it would be a more fluid, instead of the major steps I am now taking.

Attached is a screen shot of how it runs now. Control_In[5] is the setpoint, Heater_int[3] correlates to the percentage on time, and local:4:I.ch2data is chamber temp. I have to explain that once chamber temp is reached the Output to the SSR's is disabled but the percentage doesn't change. I have learned that once it stabilizes the heaters need 70% to maintain.

Thanks again for your assistance and expertise!!!!

can anybody kindly convert the 'heating.ACD' file to PDF please.

notice that this thread is several years old - but this is a PDF of what was originally posted ...

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Thank you so much Sir.