Pressure Sensor + PLC + Drive

rahatali

Member
Join Date
May 2007
Location
Lahore
Posts
9
I want to use Pressure Sensor with PLC and from PLC want to use 0~10VDC output to control the drive, so that Drive could maintain the speed of ac motor according to the pressure or the machine.
So can any body help me out in this regard?? PLC could be Delta or Siemens, so please assist me how I will connect Sensor and will get output for the Drive.
Warm regards!
 
It depends on how you want to do it, you could use the feedback from the transducer and subtract a setpoint to create an error signal. Then use the error as a command for the drive unit. If the response time is not critical you could even just use an on/off approach with a little null band included so the drive isn't cycling on and off too often.

Do you have more details on what you are tying to do?
 
Rahotali,

Ken is right, most PLCs have a PID function.

(P)roportional = The difference in command and feedback.

(I)ntegral = The accumulated time difference spent under and over the setpoint.

(D)erivitive = The rate of change in the feedback signal or your "Delta".


Use the proportional to set your scaling for the command to the drive, basicly deciding how fast the drive should run based on a particular difference in between the feedback and command pressures (called your error).

Then your Derivitive or "Damp" will act just like a mechanical damp pot, causing the drive to glide nicely up to the setpoint without overshooting (like a shock absorber).

The integral will be used to automaticly balance the system so you run closer to the setpoint over time.
 
No, he said that there is PID onboard on the drive, not the PLC.
And that is correct. Standalone pump control to maintain pressure is a typical VFD application.

Siemens MM430 are specialised for "HVAC" (pumps and fans).
Other manufacturers have similar product lines.
 
Pressure Sensor + PID Drive

Thank you so much dear!
Yes I have on drive which has PID function, but how i will connect pressure sensor wires with it, I have option DC0-10V and 4-20ma, do I still need any controller of sensor which could prive such voltage or only sensor can do so..!!
such as Pressure Sensor >> Controller >> Drive??
if you know any digital pressure sensor then could you please refer me such as catalog or pdf file for study!!
Thanks!
 
If the drive has PID onboard, then i'll bet that it has the necessary 24V on its terminal strip for supplying the sensor. At least that is the case with the MM430 that I mentioned before. Actually it has 2 analog inputs, so one could be connected to the pressure sensor, the other could be used to recieve a setpoint from an external source, a potentiometer or a PLC.
 
we dont' have 24V but I found 10VDC, would it be workable, and also could you plesae tell me any type of pressure sensor that could meet my requirment.
 
You don't need a "digital" pressure sensor. Any good loop powered 4-20 mA analog pressure transmitter will work.

You are confusing hte 10 VDC analog signals on your drive with the 24VDC power supply that the VFD has for discrete inputs etc. As Jesper points out, this probably has enough capacity to provide the 20 mA power for the pressure transmitter.

It appears to me that you are just starting to work in analog control. I suggest that you contact the tech supprt people from your drive supplier. They can probably sit down with you and in 15 minutes gie you the info that will take hours to get on your own or over the internet.
 
Sorry my bad.

Anyhow, what is the application?

Fluid? Air? Pressure you are trying to maintain? ect?

What drive and transducer do you have now?

From what I gathered thus far, you will first needs to produce a feedback signal that will output 0-10 volts, 10 volts being a bit higher than your highest potential set point pressure but below any dangerous pressure, zero being zero pressure (eg 10v = 1000psi). You may need to have a 24v supply to supply the transducer.

Then you will turn down all your settings, hook up the output from the tranducer to the feedback input. Apply 9 volts command voltage and start turning up your gain (proportional scaling value or KP)until the motor starts maintaining the setpoint pressure (in the example above that would be 900 psi). Then do your damp so it comes up to the setpoint nice without overshoot or oscillation. Then lastly your integral, after the rest of the tuning is done.

There is a little bit of art to tuning a close loop servo, espeacially a nested one, but you should be able to get it.
 
I hope you don't expect precision pressure control...

How precise do you need your pressure control to be?
What kind of a fluid pump are you using, fixed or variable displacement?
What is the size of the motor? Whats is its intertia?
What is the volume of fluid under compression?
What kind of load (ridgid or compressible) is on the actuator?

The answer to these question will determine to a large degree what kind of control you will have. A marginal degree of pressure control can be achieved using a motor/drive set up. But great pressure control is going to require somethimg more precise. Don't expect to get better than +/-150KPa apparent pressure control with an AC drive only set up, its likely you will get much worse depending on the aparatus. However, coupling the AC drive with other simple solenoid valves can result in significantly better control.

For the best pressure control you will need to use a servo valve. Using a directional control servo valve to control acutator force with a properly tuned PID loop you can expect to do better that +/- 10Kpa apparent pressure accuracy on typical systems.

------------------------------------

A note about my use of the phrase "apparent pressure." This is the measured pressure by your transducer. This apparent accuracy is the accuracy of your pressure control loop as measured by the transducer that provides feedback for the loop. The transducer itself has a certain degree of inaccurccy in its reading. The uncertianty in the controlled pressure is the accuracy of the transducer + the apparent accuarcy of the control loop. In many of the applications I deal with (pressures of up to 125,000KPa with apparent control accuracy of +/-10Kpa and overrall accuracy of +/-250KPa) this is an important consideration.
 
Dear Alric,



Can you help me to setting up one ac drive with 3 soft starters? I can set up drive but my problem is hydraulic consideration.



Regards,

Danesh
 
Dear Rahat use FC105 (scale block) for reading the sensor value by analog input and use FC106 (unscaled block) for giving the speed to drive by analog output.
U need 1 AI & 1 AO.
 
rahatali,

Try Danfoss VLT inverters, very easy to install & setup. Depending what size motor your running you could use the FC51 series (0.18 - 7.5Kw) these have a tiny footprint & pretty cheap. Or go for the VLT 5000 & 6000 HVAC they all have 2 analogue inputs and 1 output configurable for 0-10volts or 0-20ma.
Plus they have the free set up software if you want to backup or setup from your pc (setup is even easier)
 

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