Temperatue control by using NI4351, TBX-68T and OPTO-22

[DSONG]

I am using LabVIEW6.0(windows 98)  and  PID toolkit(hardware:NI4351, TBX-68T and OPTO-22(G4ODC5MA))  to control temperature. I connect the heater power to the output of OPTO-22 and keep the output on/off to maintain the temperatue constant.

The output of PID algorithm is a continuous range, while the OPTO-22 only accept TRUE OR FALSE. So I have to transfer the output of PID algorithm to a TRUE/FALSE. I set a Trip level and hysterisis.  If the output of PID is larger than or equal to Trip Level, I will write a TRUE to Opto-22 and turn on the heater; if the output of PID is less than the difference between Trip level and hysterisis, I write a False to OPTO-22 and shut off the heater(the above idea comes from an example code titles as "PID_Control_Digital.llb".

I have no idea of how to determine Trip level and Hysterisis. Pratically I have tried to set  output of PID is from 0 to 100 and Trip level 50, Hysterisis 0 and tried different values for Kc, Ti and Td. It sounds it never arrives at steady-state.

I don't know where the problem is. I think I already have all the hardware.

Thanks,

[Brian_Beal]

I've never done this with your setup, but you want to time proportion your output.  If PID output is 20, then you want your relay to be on 20% of the time.  You need to decide what the time interval is(i.e. 20% of one minute), and you will also want to put in a minimum on time.  If your heater controls are all solid-state then you may not even need that minimum on time.

[.avi file]

Greetings!

1) Leave the PID compoinents as default. A helpful link on finding how these effect the steady state error is in the link below

http://digital.ni.com/public.nsf/websearch/6F1B2FC721FC62F986256CF4005D1A48?OpenDocument

2) The hysteresis value determines the relay switch points. For instance, if the hysteresis is set to 2, and the set temperature to 21 degrees, the relay will go ON at 19 degrees, and OFF at 23 degrees. You can leave the trip level as 50.

3) Are you getting any error? or How close to steady state does it acheive

4) Hopefully setting the on time (minimum) as mentioned helped

Good luck.Thanks and hope this helps
Avi Harjani