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Controling two solenoid valves with a PLC

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SiNa

New Member
Hello everyone,

I have two 3/2 solenoid valves and a pneumatic cylinder. I want the piston to go quickly to its fully extended position and stays there for 20 milli-seconds before retracting. The solenoids work with 110 volts AC.
I have never worked with PLCs and valves before. Do you know what kind of PLC should I buy and how to do the wiring and programing?
Omron for example, has a huge variety of PLCs which look very complicated to program for a beginner.

Thanks a lot for your help.
 

Boncuk

New Member
Hi,

I wouldn't bother using a PLC.

If I understand correctly, the piston is a two-way piston being driven by two solenoid valves (type 3 1/2)

Depending on the applied air pressure and the valve cross section the piston will stay at the fully extended position for the required 20ms if you reverse pressure to the opposite side of the piston. Reversing pressure doesn't necessarily mean that the piston will move instantaneously. It has to bleed off the pressure on one side first before it moves.

If the piston moves normally at e.g. 1bar and you supply 4bar of pressure, the excessive pressure (3bar) has to bleed off before the piston moves in the opposite direction. The speed of bleed off depends not only on the pressure, but also on the cross section of the pneumatic system (valves, hoses and air supply for the cylinder)

Boncuk
 
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SiNa

New Member
Thank you Boncuk,
How should I charge and discharge the solenoids in the proper sequence then? I should charge one solenoid to extend the piston and after a delay charge the other solenoid to retract it. Can I do it by simply connecting and disconnecting the power?
Do you have any information about PLCs, to have more precise control?
 

Boncuk

New Member
Hi SiNa,

what is so important about the time the piston has to be at the extended position?

You might use an SPDT-switch (with center position OFF) to activate the solenoid valves alternating.

If retraction of the piston has to made semi-automatic you might use a timer circuit (NE555-chip) being triggerd by a microswitch when the piston is extended, and activate the valve for retraction after the preset time.

No reason to complicate such a simple thing.

Boncuk
 

SiNa

New Member
Thanks a lot Boncuk, your response is very helpful.

We use the pneumatic cylinder to collect soot from within a flame. The samples are collected when the piston is in the fully extended position. If it stays there for a long time, the sampling grid becomes oversaturated. If stays there for a short time, we do not have enough samples to analyze.

Thanks again.
 

Boncuk

New Member
Hi SiNa,

before you dig into an (overkill) electronic solution I suggest to make some experiments before.

There are lots of pneumatic control parts on the market, such as chokes (throttles).

Using a choke you'll be able to control the piston extend time accurately to 100µs (0.5%) at desired extend time of 20ms. This of course means precisely controlled air pressure in the range of +/- 0.05bar.

Check out Festo Pneumatic for chokes.

The rest can be done by using a monoflop activating the 'piston extend' function for the preset time, after which it will return to the 'retract' position without any more switching.

Please consider the piston travelling time in your calculation, depending on:

- air pressure
- pneumatic system cross section and hose (or pipe) length, including the air inlets of the cylinder.

Boncuk
 

Boncuk

New Member
Hi SiNa,

here is a circuit which will suit your needs. The time delay is adjustable from 19.78 to 30.76ms to compensate for piston travelling time.

The operation is very simple. To take a probe just push the button (S1). No matter how long it is depressed the timing won't change.

I used the CMOS version of the NE555 = ILC555, which can handle higher frequencies.

If the time is passive the retract solenoid is always powered. When it's active the relay activates the extend valve.

You might omit the LED (D3) and it's current limiting resistor (R7) if you don't require active indication of the circuit.

The relay used is a FINDER type S30 which can handle 24VDC at 2A, much more than the solenoids need. It can be placed into a modified DIL-16 socket and changed very quickly that way.

Boncuk
 

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SiNa

New Member
Thank you very much Boncuk! The information you provided is very useful. I will let you know about our progress!
Thanks again.
 

Boncuk

New Member
Hi SiNa,

please be aware that a one-shot of 20ms duration is almost impossible using a normal timer-chip (NE555).

During simulation that chip did not trigger the output with a distinct negative going pulse on the trigger input (pin2), with the timing components set to 20ms.

Using a CMOS timer it should work properly. 20ms are achieved with the discharge resistor 18.205K and a timing capacitor of 1µF.

Use low tolerance resistor and capacitor.

Boncuk
 
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