RS5000 PIDE w/2 TC's & SRTP???

Geared

Member
Join Date
Oct 2005
Location
Canada
Posts
6
I am setting up a heater which is much like a heat-gun, air blows through a tube with an element in it.
The response time of the heater is very fast so I am driving the heater off of a SSR.
The heater comes with 2 built in thermocouples, on the inlet and the output.
I have RS5000 Pro and want to put the outlet TC through a PIDE and a SRTP to my SSR.
I don't know what to do with the inlet T.C. (I am new to PID).
Any help would be much appreciated.

G
 
Quick reply

Thermocouple outlet is the Process Value, the value that is changing along with the output. The output will be your SSR which drives the heating. The incoming T.C. might be for a recir loop? Or perhaps an overtemp check?
 
No, this is actually a job, and I am actually an old man, I do alot of programming, just new to process control:)
 
So what are you trying to heat up? Are you going to recir the air? Was it designed to recir the air?
 
No recirc., it just blows compressed air at the product to heat it before assembly.
This is an assembly line application where the heater will be running continually.
Would the inlet T.C help with accuracy?
I have heard of a couple different contol methods; cascaded PID, and constant ratio, would either of them apply to this?
 
Knowing inlet air temperature might be helpful in calculating how much heater output is needed to achieve the desired output temperature. Where this differential figures into the PID equation, I have no idea... :confused:

🍻

-Eric
 
With the before heat and the post heat, you can preload some feed forward action if the inlet temperature is too low. Or back it off if it is too high. I've done things similiar to this when I can sense that a process will be undergoing a very abrupt change.
 
My 2 cents

The inlet T/C should be used for the feedforward term of the PID, and it should provide the bulk of the output value. From your description, the air flow is constant so some simple math should provide the FF gain. The gain units might be (% output)per(deg. rise)

Then use the outlet T/C as feedback to the PID to trim the output as necessary.
 
I found FF in the PIDE instruction, it states "FF is summed with CV after zero crossing deadband limiting is applied to CV"??, do I then wire my inlet T/C to FF (after the gain calculation) and my outlet T/C to PV and my output to the heater to CVEU? How would I calculate the gain using the equation that you provided? I need my hand held with this one, this process stuff is like black magic!
 
Feed Forward:

Do you understand how you are going to wire your output? Along with the TCs?

It sounds like you will have a discrete output, and your PID output is an analog signal, so how will you create an analog signal from the discrete output? Easy, time proportioning. Run two timers, one the master cycle timer, with a base of say 1 second. The second timer will be loaded with a preset value somewhere between 0-1 seconds, based on the value of the PID output.

PID has 3 terms, Proportional, Integral, and Derivative. See Ron's PID thread for an indepth discussion of how each functions.

The feed forward is preloading an amount of integral to make up for a known disturbance to the process. Let's say you have a SP of 150 C. Your PV is sitting nicely at 149.8 C. Your input air (first TC) is reading 27 C. Everything is going swimmingly. Now you get a cold blast of air in the infeed and what will happen? Without feedforward, your PV will drop, let's say to 143 C. The integral will build back up with respect to time, turning the output on and bringing the output TC (PV) back up to 150 C. But now let's say that you have feedforward going on, and you sense the drop in input air temp. You can preload that integral (feedforward) so that it doesn't suffer the dip down to 143, let's say it only drops to 149 (you've calculated very well) and your Process Value never suffers a dip or sag. This can only be done when you can reliably sense that your process will be suffering a disturbance.
 
Greetings Geared,



and welcome to the forum ...



if you’re new to this process control stuff, here’s a common sense approach that might help ...



forget about the inlet TC to begin with ... use the outlet TC as the PV ... have the PID’s output (the CV) control the temperature ... tune the loop and see how good a response you can get ...



then ... and only then ... start thinking along these lines:



if the inlet air temperature suddenly decreases a LARGE amount, then I know I’ll need more output from the PID ... I COULD wait for the outlet TC to tell me that the temperature is too low ... but ... if I wait for that, then the process is bound to suffer ... specifically, the PID won’t start to compensate for the lower inlet air temperature until the outlet TC notices and reports the “too low temperature” problem ...



from the other end of the lever ...



if the inlet air temperature suddenly increases a LARGE amount, then I know I’ll need less output from the PID ... I COULD wait for the outlet TC to tell me that the temperature is too high ... but ... if I wait for that, then the process is bound to suffer ... specifically, the PID won’t start to compensate for the higher inlet air temperature until the outlet TC notices and reports the “too high temperature” problem ...



so ... let’s use the inlet TC too ... this will tell me what the air inlet temperature is ... once I experiment and find a “baseline” or “normal” inlet air temperature, I’ll have the FF go to zero ... the PID will take care of the normal day-to-day temperature control ...



but ... if the inlet TC reports a sudden “too cold” condition, I’ll have the FF crank in a value of (let’s say) 15% ... this will instantly compensate for the abnormally “too cold” inlet air temperature ... AND ... I won’t have to wait for the process to see a “too low” temperature before the PID starts to respond ...



also ... if the inlet TC reports a sudden “too hot” condition, I’ll have the FF crank in a value of (let’s say) minus 15% ... this will instantly compensate for the abnormally “too hot” inlet air temperature ... AND ... I won’t have to wait for the process to see a “too high” temperature before the PID starts to respond ...



well ... that’s the basic idea of FF (Feed Forward - also called “bias”) control ... secret handshake: the PID’s output (CV) is made up of the sum of FOUR components ... (1) the Proportional action ... (2) the Integral action ... (3) the Derivative action ... and last (and frequently forgotten) number (4) the Feed Forward action ...



the FF is YOUR (the programmer’s) opportunity to instantly affect the final output of the PID ... you can program in an addition to - or a subtraction from - the PID’s normal CV calculated value ...



big question: so how do you know how much FF to add or to subtract? ... usually this is done by experimenting ... if the process can be accurately modeled with a mathematical equation, then the values can be calculated ... if you’re launching a space shuttle or something along those lines, that approach is undoubtedly the way to go ... but most people just use a “seat of the pants” approach and keep cranking in various values until things start to improve ... (the term “salt to taste” comes to mind) ... the first big issue is this: in your particular application, how much control do you have over the inlet air temperature? ... if the answer is “very little or none” then your experiments to find the proper FF values are likely to be very tedious and time-consuming ... point: how are you going to “experiment” if you can’t control the variables involved? ... unfortunately there’s usually no good answer to that one ...



but let’s move on ...



suppose that you’ve successfully found the ideal values for the FF action ... you have those values ready and waiting ... but right now, the inlet air temperature is “normal” and so the FF value is currently set at zero ... things have been simmering along for a “very long time” and the system is stabilized ... the birds are singing, the sun is shining, life is lovely ...



suddenly the inlet air temperature drops a large amount ... the inlet TC reports the difference ... and even before the outlet temperature starts to drop, your program instantly cranks in just the right amount of FF to compensate for the decrease in the inlet air temperature ... let’s say that the FF is +15% just to have a number ... good news: the PID doesn’t have to wait for the outlet respond before it calculates a new (increased) value for the CV ...



the additional +15% from the FF does the trick ... the outlet temperature stays right on track ... once again, the birds are singing, the sun is shining, life is lovely ...



BUT ... here’s where you have to watch yourself ... remember that you’re currently adding an “artificial” +15% to the PID’s output ... we’ll come back to that idea in just a minute ...



now suppose that the inlet air temperature slowly - but steadily - increases back to its “normal” value ... and suppose that your program doesn’t do anything at all with that +15% FF value ... suppose that we just let the normal action of the PID compensate for the slow - but steady - increase in outlet temperature that would result from these conditions ... the outlet temperature might stay very close to the required setpoint ... and no one would notice a problem ...



BUT ... here’s the rub ... what’s going to happen the NEXT TIME that the inlet air temperature takes a sudden nose dive? ... the TOO simple answer: the FF will just crank in another 15% addition ... do you see the trap? ... if this same pattern keeps repeating, eventually you’re going to have a FF which “maxes out” - and then the party’s over ...



and so ... in most cases, once the FF has been cranked in, it must be gradually cranked back out again ... usually this is done by programming a repetitive cycle to slowly - but steadily - bring the FF value back to zero ... while this is happening, the PID (bless its little heart) is taking care of the day-to-day business of controlling the outlet temperature - by adjusting the CV ...



next big question: so how fast can you “bleed off” the FF value? ... of course that depends on the application ... usually a substantial amount of experimenting is necessary to answer that one ...



so ... that’s the best I can do for an “off-the-cuff” discussion of FF ...


(continued in next post)
 
(continued from previous post)

now for the good news: you probably don’t need FF anyway ... you said:

The response time of the heater is very fast ...




well if that’s true, then more than likely the PID will be able to keep things close enough to “on track” without bothering with FF at all ... especially if the inlet air temperature is adequately stable ... so ... is it subject to sudden large variations? ... if the answer is “no” then I suggest that you ignore the inlet TC and get on with your life ... chances are that Feed Forward won’t buy you any improvement in the operation of the process ... think about just using the outlet TC as the PV for the PID ... why needlessly complicate things? ...



finally ... since you admit to being new ... a golden nugget of truth ...



the PID can NOT “keep a process on target” ... no matter how well it’s tuned ... all the PID can do is “push the process back toward the target” once a deviation has been noticed ... think about it this way: the PID does NOT have a crystal ball in its pocket ... specifically, it can NOT forecast the future and say something like: “I think that the process is about to drift downward away from the target - so I’ll increase my output to compensate for that” ... when you think about it that way, common sense will tell you that the process MUST deviate some amount (hopefully just a small amount) BEFORE the PID can possibly respond with any change to its output ...



BUT ... enter Feed Forward to the arena ...



IF (and that’s a big IF) there is some particular variable upstream of the process - something like your inlet air temperature for example - that can be adequately monitored, then it’s possible (at least in some processes) to use the Feed Forward effect to instantly crank in just the right amount of compensation to actually “keep the process on target” ... I’ve personally tinkered around with some experiments that have been able to “flat-line” the response of certain processes - in spite of some sudden and significant changes to conditions upstream of the PV ... but then again, I was just tinkering around in a lab ... specifically I wasn’t working out there in the real world with a pressing deadline and a demanding audience ...



hope this helps ...
 
Last edited:
Thank you very much gentlemen, I feel alot more confident now in what I have to do! Crocop; I am using RS5000 w/function block which has a Split Range Time Proportional function built in to drive my discete output, but thanks anyway for the timer tip!
Ron; I have read many of your posts and I was hoping that you would reply, you simplified it very nicely for me, Thanks again for the indepth help!
 

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