[ATR142] Control heating process in PID with SSR output

[user497]

Hello,
we have ATR142, that we use with PID to control heating process with SSR output.
But we have problem, that all the time controller is holding higher temperature as in setpoint.
If setpoint is 37, it is holding 40...42C.
We did already a lot of Autotune, but no results.
Parameters:
18 Pb = 7
19 ti = 999,9 !!!!!!! (MAX number in this parameter
20 td = 249,9 !!!!!!! ( And this looks like 1/4 from ti parameter )

IT looks very strange, that it is taking so big numbers in 19. and 20. parameter!
Where can we look for some problem? What is it indicating with 999 and 249,99 ???
And does in this case parameter 21. tc (Cycle time) take some effect on PID?

[Pixsys tecnico 1]

Hi, [T.c.] parameter is very important in PID action because is the cycle time of the output. In your case, using SSR as output, you have to set it as 1 Second. Then try to relaunch the auto-tune function (@ about 20°C if you work around 37°)

[user497]

Hello,
we still have some problems, that it is going over the setpoint for some 3...4C .
After Auto tune the parameters are like this:
Pb= 8;
Ti= 100;
td= 251
tc=1

What can we try to change, to have smaller overheat? ( ~ 0,5C is the best!!)

Thank YOu

[Pixsys tecnico 1]

Hi, Td value is very strange, try to set it as "25" (normally is 1/4 of Ti).

[user497]

Hello ,

thank You!
I changed this value to 25, but still, it is going over the set point! (Set point was 25C, and the highest was 29,2 C ).
The problem is, that the heating element is very small ~ 120W.

Which parameter can we change, to decrease overshoot?

[Pixsys tecnico 1]

Temperature is stabilized 4-5°C over the setpoint or goes up and down?

[user497]

It goes up 4-5C over setpoint and then slowly comes back to setpoint, as You can see in attached file some posts back!

[Pixsys tecnico 1]

Increase "Ti" to 125, "Td" to 1/4 of "Ti". Try to limit the power output with p.22 [o.po.L.] to 80-90%.


If it doesn't work:
c.HY 0.0 (if it is different)
Pb 12
Ti 600
Td 150
o.po.L. 100
Wait 2 hours (recording the process) and post the results.

[user497]

Hello,

i still have no changes in PID. I did autotune one more time and again the same!

I seted up also:
12
600
150
but the result was the same, some 5 degree ofer setpoint.

Maybe the problem is that, the heater is with very big inertia?
Because after autotune it used:
Pb=6
Ti=850
Td=215

And how can it know after autotune, what is important for us: time-speed ( to reach 98% of setpoint) or overshoot, what he can allow?

Because of cours, if the time is more important it will give more overshoot. But in our case the time is not important, but overshoot is more important!

P.S.
I`m using ATR121-B ( i choosed this topic, because there was PID controll already)

[Pixsys tecnico 1]

Remember to disable [TunE] parameter before set manually PID parameters. PID parameters calculated (12/850/215) are correct but your problem is that the process is very slow (overshoot came when temperature reach the setpoint and also after 1-2 hours). Disable autotuning, set PID parameters as I said (12/850/215) and wait 2 hours recording the data. Then post me the recorded graphic.

[user497]

Hello,

so, i made a test:

Pb=12
TI=600
Td=150
Tune: disabled

But the same problem.
In data, temperature are little bit higher, because it is closer to heater.
But the same problem with overheat!!!
The red dot line is alarm temperature : + 38C ( + 37,5 + 0,5 C ).



P.S.
We use ATR121-B

[Pixsys tecnico 1]

Ok, so set:

Pb: 15
Ti: 800
Td: 200
rEg: H.o.o.

[user497]

Here it is!
Not 2 hours, because it was already working day evening, but still overshoot is to big, over 40C.
Is there some more options to do?

[Pixsys tecnico 1]

With ATR121 we are so limitating in PID parameters so try:

Pb: 20
Ti: 999
Td: 500
rEg: H.o.o.

The first overshoot there will still be, but after that it would be more stable.

[user497]

Hello, thank You, i will try this on next working day.

But is it possible to do with ATR142?
I can still change it, because in this case it is more important to disable that overshoot!

Thank You.

[Pixsys tecnico 1]

With ATR142 we can use softstart to avoid overshoot when reaching the sepoint

[user497]

Hello,
so i tested it and now it is better, but still just little bit to high (max was ~ 38C, Setpoint: 37.0) (The red dot line is alarm line( 38C) - in attachment)
we need just a little bit less : max 37,5.
Can i try to change something more: bigger td ????

For information:
And in ATR142 case i need to change output power limit? or other parameter?

[Pixsys tecnico 1]

With ATR142 you will use 51 [GrAd] parameter to reach the setpoint slowly.

Now, with ATR121 you can try to increase Pb to 25 and Td to 600.

[user497]

Hello, so i changed controller to ATR142-ABC
Reg = Ho.o
Pb = 25
Ti = 999.9
Td = 600
tc = 1.0
tune = disabled
oPNo = PrcY
Grad: 10.0

Now its almost good, but still as You can see in graph, it is with bigger overshoot as it must be!
Setpoint: 37C ( +- 0,5C ) , but here max is 38C!
Is there some other parameter, which we can change?

[Pixsys tecnico 1]

Try limit the output power to 80% with parameter P22 [o.Po.L] as "80".

You have to leave [oPNo] = "cont" if you use only [Grad] parameter (we are not using pre-programmed cycle but only a fixed gradient to reach the setpoint).