[ATR142] Control heating process in PID with SSR output

user497 · Post by **user497** » 06/01/2014, 14:55

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?

Post by **Pixsys tecnico 1** » 07/01/2014, 8:54

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 · Post by **user497** » 13/01/2014, 16:39

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

Post by **Pixsys tecnico 1** » 14/01/2014, 8:41

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

user497 · Post by **user497** » 14/01/2014, 9:17

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?

Post by **Pixsys tecnico 1** » 14/01/2014, 17:28

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

user497 · Post by **user497** » 15/01/2014, 6:32

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

Post by **Pixsys tecnico 1** » 15/01/2014, 11:28

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 · Post by **user497** » 16/01/2014, 10:41

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)

Post by **Pixsys tecnico 1** » 16/01/2014, 10:48

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.

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