Pressure control with VSD

TamHo

Member
Join Date
Feb 2004
Location
Toronto
Posts
15
I am trying to control the pressure in an enclosed spraybooth cabin using VSD's. The system uses Benshaw's variable speed drives and are controlling a 15Hp intake AC motor and a 15HP exhaust AC motor.

The pressure is being monitored using a Setra pressure transmitter
http://www.setra.com/tra/pro/p_hv_265.htm
which provides a 4-20mA signal which is then fed back to an analog card (FP0-A21) on a NAIS FP Sigma PLC. The output of the analog card is then fed into the analog input of the Exhaust VSD which will control the speed of the exhaust motor. The Intake drive will be running at a constant speed, and only the exhaust drive speed will vary according to the speed of the intake drive and filter clogging.

At all times, the cabin pressure is to be balanced (no positive and no negative pressure).

Any thoughts or ideas will be extremely helpful.


TamHo
 
Iam not familiar with your equipment.but in most of this app we use with VFD with 2 analog inputs one for reference and one from the sensor.
The VFD have PID ability and keep the pressure according to your reference.
You can do it also by the PLC.
 
If the components you mentioned are installed and wired the way you say, the basics are already done. What exactly do you need help with? As Arik said you'll need to control the exhaust fan, probably, with a PID loop using the info from the Sentra. I read the Sentra page and they should be some help on the component side of the process but what is your ultimate question?.
 
Hi Arik, thanks for your help.

The VFD that I am using does indeed have 2 analog inputs.
But I would much prefer to be able to process the signal coming from the pressure transmitter in the PLC. This way I have some flexibility
of making the system a bit positive or a bit negative (some of my customers prefer running it negative and some prefer a bit positive).

Tam Ho
 
Rube,

Yes the components are wired and installed correctly.
The ultimate question would be:

What is the procedure to get the automatic pressure control working?

My understanding is what I will need to do is obtain the value when the cabin is at max. negative and the value when the cabin is at max. positive. Then it would be a matter of scaling the values. Am I correct?


Tam Ho
 
The result came from your reference. it much esay to let the VFD to do the PID loop.You set it once to auto tune and that it.
You may use analog output from your PLC for reference to the VFD.
 
Assuming the drive has PID function parameters this is practical since the setpoint will be fixed at 0. The selection or "playing" tuning parameters would be a bear. I hope that the input can be filtered either in the drive or transmitter since the static pressure would not be as tame as lets say level in a tank control loops. I would have prefered a single loop controller since they can be auto-tuned.
 
What do you mean by "single loop controller"?
Do you mean something like Red-Lion C48 or Omron E5CK.
Most of the VFDs have it inside with auto tune too.
 
First, question for HamHo. Do you understand how a PID setpoint controller works. If you do, good. If not, you need to learn about that first.

Second, there is a subtlety here that I think is being overlooked. It appears that the setpoint pressure is to be at ambient atmospheric pressure or slightly plus or minus. For that, you will need a sensor/transmitter that sends 4ma somewhere at negative pressure and 20ma somewhere at positive pressure. This would be an unusual device for sure. If the transmitter is capable of that, good. Otherwise, the system design is in some trouble, I would say. There might be a temptation to use an absolute pressure transmitter but that would cause plus or minus error each time the barometric pressure changed.

Assuming that the sensor problem can be solved, you will need a PLC with an internal setpoint controller. It is true that many VFD's have an external setpoint control capability but it sounds like the PLC is being used for other things as well as pressure control (like maybe an operator interface screen). Either way, the exhaust fan motor needs to be speed controlled by the setpoint controller error output, not be any direct speed signal.
 
Dick is right about learning the PID routine before continuing--PIDs can cause a lot of unneeded anxiety.
If you need to satisfy the rooms pressure, take the 4-20ma signal from the Setra unit and use it as the reference for PID control to the exhaust fan. Your setpoint will be the desired room pressure. Then you can tune the PID for as tight control of pressure as you need (can) with the motor/fan combo.
Dick--He needs a sensor that will read from right below his desired pressure to right above it. I assume this is what you meant by "negative" pressure
 
Rube, check TamHo's second post where he stated that the room pressure may be adjusted to slightly negative compared to ambient pressure to slightly positive.

That's why I say that he needs a transmitter that will cover both negative and positive swings from the neutral or zero differential point.
 
My bad--I read "negative" to mean less than zero (vacuum). You're correct and I meant to imply that the lower setting would be greater than zero but less than 14.7 PSI. Sorry for the confusion.
 
Of course the transmitter needs to read below zero or vacuum if the setpoint is to be at or near zero. I would expect that the scale would be in inches of water not PSI. A differential transmitter with the low side connection outside of the room would be a better choice.

A single loop controller I guess is an old fashion term now. They tipically have an analog input and analog output with a setpoint dial being set by the operator. If you have a PLC with a PID function block avaiable use that.

I still think loop tuning parameters will be a bear in this situation.
 
I reread the data sheet on the Senta control he says he's using and as Kevin submitted the scales are in WC inches. I used psi before I read that part. I also noticed the Senta does not sense for a vacuum.

I still, respectfully, say if you know what the pressure in the room is, from the Senta, and you know what you want it to be, you have what's necessary to control the exhaust fan, IOW you shouldn't need any other components.
 
Rube, if the pressure in the chamber is to always be positive with respect to the ambient, then you could take the transmitter and connect it as your setpoint with a slight positive trim or offset to get the drive PID loop to regulate there. But, if the transmitter is already at 4ma at ambient temperature, how do you trim below 4ma to get less pressure in the chamber compared to ambient.

As I think further about this, the drives I'm most familiar with do have the software option of setting up the input for 0ma to 20ma. That would place 4ma up the range a ways and give us a little room to drop below to, say 3.9ma, to get a lower pressure than ambient.

I suppose the best solution would be a transmitter that sent 4ma at -15" WC differential and 20ma at +15"WC differential. Then you could regulate anywhere from about one-half atmosphere vacuum to positive one-half atmosphere compared to ambient and changes in ambient pressure would automatically be reproduced inside the chamber to keep the differential the same. I have no idea if such a transmitter exists.

This seems strangely tough to do. I wonder if someone else can offer an easier solution here.
 

Similar Topics

Can I get the logic for controlling pump speed using pressure in the line? When pressure is high the speed should slow down and vice versa. I am...
Replies
20
Views
5,502
Hi All, I could do with some advice on a hydraulic control system. It is necessary for me to accurately position a vertical hydraulic ram with...
Replies
34
Views
1,708
Hi I am a beginner and therefore have problems with M221 PLC programming. I want to program the PLC so that the pressure increases slowly (I...
Replies
7
Views
2,220
Hello all, I am new to pid loops and control. I was wondering how to control a system with two process variables. This is a water system with a...
Replies
7
Views
1,953
I want to share a trick and see if anyone can do better! With hydraulic presses there is a point at the beginning of the actuator movement when...
Replies
10
Views
3,872
Back
Top Bottom