ML 1400 & PID with VFD & Positional Valve

Hi everybody,

I never write a PID instruction before, and I just think about PID instruction to adjust the VFD hertz base on the % opening of a positional Valve. Can I do that ? If yes, where I should start ?

Let's say VFD is from 0 Hz to 60 Hz, and a actuator with (4-20mA) signal to adjust the open gate from 0 - 100% open. So if the actuator is open 50% then the VFD has to be 30Hz, how can I do it with PID instruction ?

Thanks

leonlai said:

Hi everybody,

I never write a PID instruction before, and I just think about PID instruction to adjust the VFD hertz base on the % opening of a positional Valve. Can I do that ? If yes, where I should start ?

Let's say VFD is from 0 Hz to 60 Hz, and a actuator with (4-20mA) signal to adjust the open gate from 0 - 100% open. So if the actuator is open 50% then the VFD has to be 30Hz, how can I do it with PID instruction ?

Thanks

Click to expand...

You could use a PID Instruction, but from the information you have provided, this sounds like a a simple Scaling problem. A PID Instruction is usually used to Control something to maintain a Setpoint.

In this case, it sounds like you just want the VFD to follow the Valve Position. I think you would get better results just using Math instructions to Output the desired VFD Speed based on the Commanded Valve Position.

Stu....

Chapter 20 of "Rockwell Automation Publication 1766-RM001D-EN-P" explain PID instruction for ML1400. Please download it from www.ab.com
After PID is setup, scale your 0-100% output signal of PID instruction to be the input of a SCP instruction whose output will be your 4-20 mA analog signal connected to your VFD.
Previously your VFD has been configured to accept a 4-20 mA signal as the one to be used to drive the VFD.

Thanks for quick reply,
sthompson: Basicly I wanna control both Valve and Vfd base on a Level Sensor of a Tank(input 4-20mA) So that I think PID will help right ? but may be complicate >_<
widelto: Thanks, I never write a PID before, so any manual will count

Thanks guy

Basicly I wanna control both Valve and Vfd base on a Level Sensor of a Tank (input 4-20mA). So that I think PID will help right?

Click to expand...

The PID instructions requires 3 basic parameters: Setpoint (what level you want to run at), Control Variable (what you are controlling), and Process Variable (a feedback signal that tells the PID how close it is to the Setpoint). So you could make your Level Sensor 4-20 mA signal the Setpoint, and your Valve Position and Drive Speed the Control Variable. Then what do you use as the Process Variable - what do you have that can be fed back to indicate the state of the process? Do you have an analog signal from the drive that indicates its actual speed? If so, then you could use that as the Process Variable.

Otherwise, do what Steve said. A scaled output to the Valve and another for the Drive, each based on the Level Sensor, will work just as well, and be much easier to set up.

One more question: How do I know what Kc/Ti/Td to put in the Setup Screen ??
Do you have example code ?

about the Process Variable: for actuator, I dont have feedback
for VFD, it's a AB powerflex 4 series.
AB powerflex is control by RS485 network, and I do can read the out put frequency (but by RS485) will count like analog ? I think it'll be ok as long as there is a feedback to PID instruction right ?

How do I know what Kc/Ti/Td to put in the Setup Screen ??

Click to expand...

That is the age-old question. Entire libraries have been written, and many methods devised. Most of us base our settings on experience from previous jobs.

Or you could not use the PID - you don't need it (if you have explained the situation correctly).

I think it'll be ok as long as there is a feedback to PID instruction right?

Click to expand...

Perhaps, maybe, but the PID will gain you nothing that you will not have without the PID.

Does the valve and motor speed have anything to do with controlling the level in the tank that your Level Sensor is measuring? If yes, then it would be appropriate to try to set up a PID (or two) to control the level in the tank based on the valve position and the motor speed. In that case you would have an external operator Setpoint for Desired Tank Level, your Process Variable Tank Level Sensor measuremnet, and then PID Control Variable Output, scaled for Valve Position and Motor Speed.

If not, then don't use a PID.

Yes, I dont' need to use PID instruction; it's just because I want another way to control and play with PID to gain some experience. Thanks you guys !