PID question

[Guest bazzi]

I am wondering about how the  internal settings of the pid works. 

We have a process that needs to bee kept as close as possible to 85°C  with a maximum og 90° and minimum of 80° 

 this is a steam tupe heater  which has an internal pipe with some fluid pumped through.   The steam and the fluid enters at the same end and at the other end we have  our temperature sensor that gives us the process value the steam is controlled by a valve controlled by 4-20 mA. 

The process fluctuates around  8°   wich is considered to much.

there are some other variables that affect this process which is best described as the temperature drops by a few degrees with some minutes a part.

I am adding a new window where I can set the high and low value of the  pid output range . So hopefully that will minimise the fluctuation a bit.

 

But I keep thinking about the Input range.  (process value low limit and high limit)

I have these values set as 0 for the low limit and 1000 for the high limit = 0-100°C  same as the range of the pt100. 

Should I rather set these values as the lowest value and highest value that we want.  800-900 (80° - 90°)

 I would really appreciate some help here.

Best regards 

Bæring

 

[Joe Tauser]

Your process low and high limits are fine.

Heating with steam injection can be a beast.  What are the values of your tuning parameters?  How fast is the loop updating?  It should not be too fast.

Autotune doesn't always work so well.  Have you tried manually tuning the loop?  Start with I and D = 0 and adjust P to get a small continuous oscillation about some value, and make a note of how long the oscillation lasts.  This is your process response time.  Once you've gotten this, double the P value.  Don't worry if it's not at the setpoint yet. 

Start with a value of I equal to your process response time, and adjust it to get to your setpoint.  You probably won't need a value for D if there are no regular upsets to your system.

Joe T.

[Guest bazzi]

Thank you for the reply.  

I had the sample time set at 50 because the recommendet setting is 10-100 (0,1-1 sek).

I changed that to 100 and got a little better result.  Would you recommend to go further perhaps 200 ??

My first setup was with auto tune. I have been trying to get that to work for a few days now.  At best we got the output within 1 °  wich I was quite happy with.  

But when they run a different bash through the heater. The output starts to fluctuate around 8-9°.    Yesterday I changed the ladder so that it skips the autotune  I set it up so it has a 100% output till 80°C and after that the pid takes over.  I messed around with the PID settings but ditn´t get it to work well before we were out of material.

 

Next week I will be at it again.  But before that I have to make sure that the PID is completely without the auto tune so I am sure that there is nothing changing behind my back.

What I have done so far.

I disabled the nets with the autotune. And added a net that stores the PID values  and sets  MB34 (autotune done) high.  

With these settings the PID output goes to 4 (PID running) 

Is there something else I should disable??

 

Next week I will manually tune the loop using your methood. Any hints that anyone can offer is really appreciated.

[Joe Tauser]

If your products have different thermal response properties you may have to set up a recipe table that copies different PID values to the block based on what you are running.  It sounds like one size doesn't fit all.

Joe T.

[Guest bazzi]

Yes makes sense. Thank you