Butterfly valve and PLC

I would like to know how to control a motor driven butterfly valve from a PLC.
Thanks.

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With out the rest of the system or process it would only be a guess. A plc will do what you tell it to. So you must have inputs to tell the PLC what is happening. The you write a program to do your process. Then you will connect to the outputs to get the valve to open and close. There are different kinds of inputs and outputs analog which is scaleable and digital which is open and closed. Be glad to help need more information
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My problem is that i don't know exactly how a motor driven butterfly valve works. I suppose there is a DC motor to turn the valve. I suppose you have to put an encoder in the shaft to know the position of the valve, and from the PLC you order the motor to stop or to run. However, how do you know the number of revolutions the motor must run from a wide open position to a close one? As I said, it's the valve+motor, not the PLC, what I don't understand.
Thanks.
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It need not be a valve position signal used as feedback to the valve position control. Valve opening can be modulated by measuring the controlled medium's resultant flow, pressure or temperature.
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MOV (Motor Operated Valves) come in all sizes and shapes. Usually a butterfly valve is *not* meant for throttling/process control (their flow characteristic is very non-linear). For process control, typically a form of globe valve with specially designed 'trim' is usually called for (the 'trim' may be linear, 'equal-percentage', or 'quick-opening' depending on system design).
In many industrial cases, the operator is a gear train with an auxilary shaft that turns as the valve itself is moved. Various schemes of limit switches are arranged on the auxilary shaft to stop the motor at the full open and full closed positions. A reversible AC motor drives the whole scheme with a couple of contactors ('42' devices) and motor overloads. PLC not really necessary unless just to replace the manual control switch contacts to initiate opening or closing.
If you *do* want to stop/control the valve in some intermediate position, you would need a position signal. RVDT (rotory variable differential transformer) or LVDT (linear variable differential transformer) are common, but so is plain and simple potentiometers.
daestrom

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On 8/20/04 12:00 PM, in article snipped-for-privacy@ono.com,

A set up like you describe is used on a flow control for agricultural sprayers. The ones I saw operated on 12VDC. This application has the motor directly turning the valve with no gear reduction. A flow meter tells the controller to open or close the valve to regulate flow. It does not completely stop the flow. This valve is just a flap inside the valve. It looks like a choke inside a carburetor. Hiniker is one brand. A ball valve was used once on irrigation systems to shut off water for the end sprinkler. It was 120 VAC. That had gear reduction. A couple of cams mounted directly on the valve shaft tripped a couple of micro switches. The switches would tell the motor to stop turning when the valve reached the fully closed or open position. Jamesbury is one brand.
Dean
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