Burner Cascade PIDE

[jrhpuuk123]

Hello all,

we have an homogenizing oven with three zones, 2 burners in each zone, a soak (smaller) and a main (large). during ramp to the air temp setpoint the main burner does most of the work, and after it hits temp and the load temp gets to soak the smaller burner takes over and maintains temp. the company that did our controllogix migration did a very rough job of the conversion and the oven isnt performing quite as good as it used to or as it could be. id like to use the PIDE in controllogix but im also wondering if this two burner 1 zone thermocouple setup would benefit from a cascaded PIDE. any info is appreciated, thanks!

 

[drbitboy]

What measurement controls do you want to cascade? Obviously temperature is PV of the first POD, but what is the PV of the second cascade PID?

 

[jrhpuuk123]

thanks for responding drbitboy. i originally thought it would be temp as well, but after looking at it for the past hour and a half i think it would either have to be air temp and load temp or the air temp is primary and the secondary would be the main burner output since we would see a change in the main burner output before we'd see a change in the air temp, and the smaller burner could respond accordingly to where we wouldnt get as large of a temp drop during that crossover. am i thinking of that correctly?

 

[drbitboy]

I think what you are describing is akin to "split range," not "cascade," control. Split range control is when there are multiple systems to effect control of the Process Variable (PV) via a single feedback control loop (e.g. the PID), and the Controlled Variable (CV) output of the loop varies the output of each of the multiple control systems over a different portion of it's total range. One form of split range control is when one control system is heating and the other is cooling, and to control the system at any given time either might be required e.g. heat reactants initially (total output CV range of 50-100% corresponds to heating system output of 0-100%) then cool to remove heat from an exothermic reaction (total output CV range of 0-50% corresponds to cooling system output of 100-0%), or heat a building at night and cool it during the day.

In the case of this thread, 0-100% of the soak burner might correspond to 0-20% of the PID CV, and 0-100% of the main burner might correspond to 20-100% of the to PID CV, the soak burner would operate at 100% when the PID CV is above 20%, and the main burner would operate at 0% when the PID CV is below 20%. The actual breakpoint would not be 20%, but would depend on the relative process gains of the two burners.

I would think it would be easier to implement a simpler approach with two modes:

• Startup mode: run the main burner at 100% output until the measured temperature is "close" to setpoint,
• Soak mode: then switch to PID control using the soak burner.

This way the added complication of the relative gains of the two burners does not make the system tuning more difficult. Presumably the main burner is there to minimize the time it takes to add the necessary sensible heat to the initial materials at ambient temperature to get them to the soak temperature, in which case it makes sense to always run the main burner at 100%. The truck will be determining the temperature at which to transition from startup to soak mode: too low and time is wasted while the smaller soak burner completes the startup temperature rise; to high and the m main burner causes temperature overshoot.

Cascade control would have the measured temperature as the PV, plus a target setpoint (SP) to the primary PID, and the controlled variable (CV) output of that primary PID would connected , or cascaded, to the SP of a secondary PID, perhaps with measured fuel flow as its PV, and the secondary PID's CV output would be the valve position. That way any non-linearity in the valve characteristic would not affect the tuning parameters in the primary PID.

 

[MaxK]

Absolutely agree with drbitboy

@jrhpuuk123, could you please:
- draw process diagram
- explain the relative position of the zones and their purpose
- why do you need 2 heaters, why not just one with power control?
- what do you mean by the term “cascade control”?

The main question:
What is the final goal? What do you need be achieve in the end? What wrong with current system?

I hope you understand that the loop-loop cascade system is several times less reliable than the single-loop

 

[drbitboy]

...the company that did our controllogix migration did a very rough job of the conversion and the oven isnt performing quite as good as it used to ...

This is the most important thing to know: if it was better before the conversion to Logix, how was it done before? Was it a manual operation? Or manual on startup then switch to an analog PID?

If the "right" way to do this can be determined, perhaps by talking to the operators, who would also know of any quirks in the system, then perhaps the Logix PLC could be programmed to do that.

 

[jrhpuuk123]

thank you all, drbitboy i like the explanation of startup and soak mode, ill look at tuning the system to that. i appreciate all the help!