fan and air mixer combined PIDE (Guardlogix) with PowerFlex drive

[bgtorque]

Hi All
I'd be interested to hear approaches hear on what options you might consider for setting up of the loop control. I'll describe the setup.

We have a test cell that has a basic air 'conditioning' setup. There is a fan with a fairly big, inefficient motor run from a PowerFlex 525. The duct work has vents from the fan into the front of the cell and a vent pulling it back out from the rear of the cell back to the fan. We also have a mixing arrangement where there is a full intake and full exhaust externally of the building.

As well as controlling the fan speed via the Powerflex (min 10Hz to max 50Hz) we have a paddle in the mixing arrangement controlled by an electro-pneumatic actuator that can block the paths to ensure that the cell is fully exhausting to the outside work and thus pulling completely fresh in from outside, or the paddle can be blocking off the outside connections to fully recirculate the air in the cell and with the benefit of the inefficient motor provide a pretty dirty, but effective cell heating capability. Obviously the paddle can opertate anywhere between these two conditions and is controlled with a 4-20mA from the PLC (the PowerFlex will be speed controlled via EtherNet connection between the PLC and PowerFlex).

In 99% of occasions we want to keep the fan speed running as slow as posible for two reasons, more economic from a power point of view and much, much quieter from a noise point of view. Thus we would tend to rely of modulating the mixing paddle accordingly. However, if we were dumping a lot of heat into the room the paddle may go to fully exhausting and then we still need to then increase the speed of the fan. However as soon as we do that then the paddle may act and start modulating. I'd be interested to hear about how you guys might set this up.

A secondary means of operation is the option to fully override the control and do max fan speed with full exhaust to evacuate the cell of any fumes in the case of a failure with then potentially shutting off the fan and moving the paddle to fully cell recirc to prevent providing oxygen in the case of a fire. These cases need to be fully defined by the appropriate risk assessment and H&S desing reviews.

Any thoughts on how you might link up these two PIDE loops for the fan motor speed and the paddle position would be gratefully received.

thanks,
-BG-

[Mispeld]

While the following thread does not exactly match the application described, it may offer some ideas to help develop a control strategy.

Sanity Check - Process Control

[bgtorque]

Many thanks, I did have a quick lread through and there is some interesting stuff in there. It seems the images are not hosted correctly anymore, not that it matters too much.

I'm inclined at the minute to run the mixer valve paddle on a much faster loop than the fan motor speed. It would need to be at least as fast to see the PV change on the fast loop and act before the motor speed loop reacted to work I guess.

[bgtorque]

Hi all. Just following up on this I thought that i'd add a bit of an image to supplement the description in my first post.


I am showing two seperate PIDE instructions that are working off the same process variable (PV) from the cell RTD and the same setpoint (a temperature set by the operator). The fan has to run at a minimum of 10hz to keep the motor happy, but I would like the mixer paddle to control through its position (vertical is max cooling as full intake/exhaust to the outside world, horizontal is max heating through full recirculation and heat input from the inefficient fan motor).

The air mixer paddle should be the predominant control and only when it gets close to full intake/ehaust at the vertical position (so CV =100%) should the fan motor PIDE 2 begin to function and look to ramp the speed up from 10Hz up to its max of 50Hz. Equally, when the cell gets too cool it should bias the opposite way to do everything to reduce the fan speed down to 10Hz before really looking to contorl through rotating the air mixing paddle towards the horizontal (CV=0%).

[cardosocea]

Wouldn't the fan have the inverse control variable of the mixer paddle?

" The fan has to run at a minimum of 10hz to keep the motor happy, but I would like the mixer paddle to control through its position (vertical is max cooling as full intake/exhaust to the outside world, horizontal is max heating through full recirculation and heat input from the inefficient fan motor)."

There's some added math for scaling, but if you command your mixer to 30% and fan to 70%, would it do what you're after?

[drbitboy]

Is external air always cooler than the temperature setpoint? I would think it would have to be else cooling would not be possible.

What are the external loads i.e. what else is cooling or heating the sensor?

When the paddle is closed (horizontal in the image; 0%), will the fan ever need to be increased above 10Hz to heat (I think this is what @cardosocea is getting at)?

Either way, I think a single PID with a split range output may be called for:

• 0% to M% output;
  • Heating control by motor; max heating at 0%
  • M > 0
  • Paddle at 0% (closed; horizontal); full recirc
  • Motor goes from 60Hz at 0% to 10Hz at M%
• M% to N%
  • Paddle control
  • N > M
  • Motor at 10Hz
  • Paddle goes from 0% (closed) at M% to 100% (open; vertical) at N%
• N% to 100%
  • Cooling control by fan
  • 100 > N
  • Paddle at 100% (open; vertical); full intake/exhaust
  • Motor goes from 10Hz at N% to 60Hz at 100%

Picking M and N would be key here to maintain the system gains across the three split ranges with respect to the single PID output, so a single controller gain can be used, and even if that is right the time constants will probably be off, so off-paddle control might be dodgy. That said, different PID tuning parameter sets for the three ranges would be another option.

Also transitions at N% and M% might get messy, so perhaps some overlap - or gap (i.e. deadband) - at those transitions makes sense. Noise in the PV may cause problems here, requiring filtering.

I suspect this going to be a slow process; an Integral-only control system might work. There may be some deadtime on the intake air, but it may be small compared to the system time constant. Varying intake air temperature may make the tuning a challenge; perhaps M and N could be a function of outside air temperature.