Dosing Pump and rotary Valve

MayadaAhmed

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Join Date
Apr 2021
Location
Khartoum
Posts
16
Hello,
I want to ask about two instruments, the dosing pump and the rotary valve.
I will be using the dosing pump to dose a liquid, and the rotary valve to feed powdered lime, my question is how to connect each one to the PLC, is it directly or do I need an intermediate device,
Also, how to control the rotary feeders through PLC, would I be manipulating the speed of the shaft or the motor, or does it has another way?
Thank you in advance, I am still a beginner and I searched everywhere for answers but didn't find anything.
Please help šŸ˜Š
 
Both are usually driven by an AC motor which in turn is driven by a VFD, a variable frequency drive. I am sure others are better at explaining exactly what a VFD does and how it does its' thing, but basically it comes down to feeding lower frequency AC to the motor if you want it to run slowly and feeding it a higher frequency if you want it to run fast. You want to control the speed from your PLC. The classic way to do this is to have an analog output from your PLC send a voltage (typically 0..10V) or small control current (typically 4..20mA) as a "speed reference" or "setpoint" to the VFD.

Additionally there is usually a digital output that when it is high serves as a RELEASE signal to the drive. So the drive requires a digital output as a RUN signal ("mr VFD you are hereby allowed to run your motor, but only as long as this signal remains HIGH. Do stop your motor when it goes low.").

Run your motor very fast by feeding the max voltage of 10V from the analog output of your PLC to the analog input in the VFD. Or run it super slow by feeding it for instance 0.5V. Or anything in between for speeds between the lowest and highest speed.



There are more ways to go about (field bus communication between PLC and VFD), but this is a brief version of the classic approach.
 
Both are usually driven by an AC motor which in turn is driven by a VFD, a variable frequency drive. I am sure others are better at explaining exactly what a VFD does and how it does its' thing, but basically it comes down to feeding lower frequency AC to the motor if you want it to run slowly and feeding it a higher frequency if you want it to run fast. You want to control the speed from your PLC. The classic way to do this is to have an analog output from your PLC send a voltage (typically 0..10V) or small control current (typically 4..20mA) as a "speed reference" or "setpoint" to the VFD.

Additionally there is usually a digital output that when it is high serves as a RELEASE signal to the drive. So the drive requires a digital output as a RUN signal ("mr VFD you are hereby allowed to run your motor, but only as long as this signal remains HIGH. Do stop your motor when it goes low.").

Run your motor very fast by feeding the max voltage of 10V from the analog output of your PLC to the analog input in the VFD. Or run it super slow by feeding it for instance 0.5V. Or anything in between for speeds between the lowest and highest speed.



There are more ways to go about (field bus communication between PLC and VFD), but this is a brief version of the classic approach.
Thank you very much, this was very informative šŸ™Œ
So by adjusting the frequency hence the speed of the motor, I can end up with different flow rates? Is this decided on by calibration? Or is it build in the device through its datasheet?
 
Thank you very much, this was very informative ļæ½ļæ½
So by adjusting the frequency hence the speed of the motor, I can end up with different flow rates?

Exactly!

Is this decided on by calibration? Or is it build in the device through its datasheet?
Various options there. Calibration is an option, but your control logic is then basically "blind folded" and will not compensate for any deviations from what you expect your flow rate to be.

Usually there is some sort of feedback loop in the control logic. By whatever means you measure the actual flow, compare to the desired flow and adjust the setpoint in order to move the actual flow closer to the desired flow. You may for instance dose your powders or liquid from a container which sits on loadcells. These can be used to electronically "weigh" the container - it is the heart of any digital weighing instrument (your kitchen scale, the one your wife has in the bathroom in order to keep your belly in shape, etc). This can also be connected to your PLC so you see a gradual and steady decline of the weight of your container. The declining weight over a time period is a way to measure the actual flow rate. Other methods are also used.

The difference between desired and actual flow rate is what we call the "error". The simplest way to adjust is to compensate for only a fraction of the error. One would then expect to have the actual flow rate move ever closer but ever slower towards the desired flow rate. This is called P control (for Proportional). More advanced tactics can be applied, which may work better (faster, more effectively) in many practical applications. A strategy that is often being used is known as PID. This is Proportional control, and additionally Integration and Differentiation. Very useful, one needs to go through a good deal of math to fully understand this. Mostly known in a branch of engineering called "Control theory". A whole can of worms, food for thought and a great deal of lengthy discussions on the internet.

Peter Nachtwey, a very respected member of this forum, has created a series of very informative video's on PID. Search for "Peter ponders PID" and be prepared to spend some time.
 
Last edited:
Exactly!

Various options there. Calibration is an option, but your control logic is then basically "blind folded" and will not compensate for any deviations from what you expect your flow rate to be.

Usually there is some sort of feedback loop in the control logic. By whatever means you measure the actual flow, compare to the desired flow and adjust the setpoint in order to move the actual flow closer to the desired flow. You may for instance dose your powders or liquid from a container which sits on loadcells. These can be used to electronically "weigh" the container - it is the heart of any digital weighing instrument (your kitchen scale, the one your wife has in the bathroom in order to keep your belly in shape, etc). This can also be connected to your PLC so you see a gradual and steady decline of the weight of your container. The declining weight over a time period is a way to measure the actual flow rate. Other methods are also used.

The difference between desired and actual flow rate is what we call the "error". The simplest way to adjust is to compensate for only a fraction of the error. One would then expect to have the actual flow rate move ever closer but ever slower towards the desired flow rate. This is called P control (for Proportional). More advanced tactics can be applied, which may work better (faster, more effectively) in many practical applications. A strategy that is often being used is known as PID. This is Proportional control, and additionally Integration and Differentiation. Very useful, one needs to go through a good deal of math to fully understand this. Mostly known in a branch of engineering called "Control theory". A whole can of worms, food for thought and a great deal of lengthy discussions on the internet.

Peter Nachtwey, a very respected member of this forum, has created a series of very informative video's on PID. Search for "Peter ponders PID" and be prepared to spend some time.
Okkk, I see
šŸŒŗ Thanks for clarification
 
So can't I connect them directly to the PLC, I mean does specific types have built in motors and drives?


No, a pump or dosing mechanism is not connected directly to a PLC. We have a number of major components here. A pump or solids dosing mechanism, a motor which drives this, a VFD which drives the motor and a PLC which controls the VFD.

There are devices that combine some of these in one box. E.g. there are motors with an attached VFD. There are VFDs with an internal PLC so some of the logic or even all of it can be programmed into the VFD. But at least conceptually you would distinguish these components as they each have very different tasks. It depends very much on the situation at hand if an integrated device can be used.
 
There are different types of dosing pumps and rotary valves.

Pneumatic dosing pumps run by a solenoid valve could be run by a CR wired directly to the PLC. There are also pulse dosing pumps that pulse every input signal.

Also, 2 position rotary valves that open when one lead is powered and close when the other lead is powered could be connected to one CR on a PLC output. Tie the Open wire to the N.O. contact and the Close wire to the N.C. contact.
 
What kind of dosing pump are you looking at? Something like a Prominent or Iwaki dosing pump you can interface directly with the I/O cards on a PLC. You usually need to give them a START (or in some cases a STOP) command, as well as a 4-20mA (or other analog) or pulse signal to set the rate of the dosing.
 

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