Pneumatic Valves - Open Center/Closed center??

So how do you go about controlling the open center or open center valves? To get to that closed or open part, I assume you need to have both solenoids in the off state, so how do you typically design that? Do you just drop the outputs from the PLC or do you remove the power to the entire valve bank?

Typically, the valve will spring back to the center position with power removed from both solenoids. The design of the valve would determine what state the center position is.

What exactly are you wondering?

it's hard to tell but it looks like you are trying to stop a pneumatic cylinder in mid stroke

if that's the case then you need to energize both for and rev solenoids putting full pressure on both sides of the cylinder at the same time.

GaryS said:

it's hard to tell but it looks like you are trying to stop a pneumatic cylinder in mid stroke

if that's the case then you need to energize both for and rev solenoids putting full pressure on both sides of the cylinder at the same time.

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That won't work for a single rod cylinder due to the differences in piston area on the rod end vs. the cap end, and if both solenoids are attached to the same spool in a directional control valve it will cause erratic results and could fry the solenoid that loses the tug of war.

With hydraulic it is easy to do just turn off both solenoids and the cylinder stops in place. The key is that oil dose not compress so the volume in both sided of the cylinder stays where it is when the ports are closed. When you move the cylinder you open one port to the pump and the other to tank return then when you push the rod from one port you have to push the oil out of the other port so the volume in the cylinder is always the same no matter where it is in the stroke travel.

With air we know that it is compressed when we use it to move a cylinder when moving the cylinder from one side to the other you open one valve to the atmosphere while feeding compressed air into the other port the difference in air pressure moves the cylinder. The problem is when you close the ports ( generally directional valve open one port to atmosphere at the same time you open the other port to the compressed air supply ) the volume in the one side is greater than the other. When you close the supply port the pressure in both sided of the cylinder will try to equalize. Generally that means the cylinder rod will continue to travel to the end.

The trick is to use 2 - 3 way solenoid valves, one on each cylinder port when the solenoids are not energized both cylinder ports are vented to the atmosphere (equal Pressure) To move the rod energize one solenoid to supply line pressure to the one side of the cylinder ( rod moves) When you want to stop the rod energize the other solenoid while leaving the original solenoid energized ( both on) this will supply equal line pressure to both side of the rod stopping the rod. Time out the stop time and de-energize both solenoids and drain the pressure from both sides.
To control the travel speed of the rod install an adjustable flow control valves on the vent outlet on each end (Not the feed line)
You will not be able to stop with great accuracy but with a little work you can get it fairly close.

I think you will find the difference between the surface areas on the front and back of the rod piston on an pneumatic cylinder is not noticeable unless the load is very close to the max for the cylinder. If that’s the case them the system is grossly under sized and need to be resized correctly.

I would advise not venting both rod and piston to atmosphere unless the application calls for it.

What you end up with is a cylinder that’s going to slam in both directions unless you meter the air going into it. OR keep at least one of the solenoids on at all times. Keeping the cylinder retracted or extended. When you need to move the cylinder, turn on the other solenoid and turn the other off.

I thought for open center or closed center valves, you just turned both solenoids off to get to the center. I guess I was wrong from what you guys are saying.

The question I have is when going into a cage and trying to make it safe. Should I just dump air? If I did, the actuators will fall or drift I imagine.

How should you handle preventing things from moving?


Thanks!

Timeismoney08 said:

I thought for open center or closed center valves, you just turned both solenoids off to get to the center. I guess I was wrong from what you guys are saying.

The question I have is when going into a cage and trying to make it safe. Should I just dump air? If I did, the actuators will fall or drift I imagine.

How should you handle preventing things from moving?


Thanks!

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I believe you are correct. Turning both off will make the solenoid assume its centre position, which depending on choice of valve, could be ‘all ports blocked’ or ‘all ports vented’

The application should guide your choice of this.
Is it a vertical cylinder, is there a lot of weight, is it a horizontal cylinder, do you want to be able to move the cylinder by hand when you are inside the cage? Etc.

‘All ports blocked’ will lock it where it is (not fully if air leaks or piston seals passing)

‘All ports to atmosphere’ will leave the actuator free to move under gravity or physically, for example, by hand.

Timeismoney08 said:

The question I have is when going into a cage and trying to make it safe. Should I just dump air? If I did, the actuators will fall or drift I imagine.

How should you handle preventing things from moving?

Click to expand...

Thats one I have struggled with in the past also... Zero Energy State (ZES) is regulated for safety and there are air valves that do this with a block and bypass (venting)

I guess what are the valves moving, I have built a lot of mechanical devices that had to be installed with a pin and limit switch before anyone could enter the cage

I am by no means a safety expert but you may want to have this conversation with yours if you have one... I had many with our plant safety personal trying to come up with a safeway of locking out the machine and the more eyes you have looking at it the better because you may miss one and thats all it takes

The key to any safety application is to evaluate the risk, severity and frequency of each moving part. This is the purpose of the risk assessment. Sometimes removing pressure can lead to a higher risk (the effect of gravity needs to be controlled as well). Zero (powered) energy state can be achieved by using closed center valves, or by dumping pressure. Drift may occur if it is a vertical application and like mentioned previously, dumping pressure can lead to uncontrollable movement when pressure is reapplied. If it is truly a risk of injury, you may want to look at pneumatic rod locks for the cylinders. These items are truly fail-safe, meaning if they lose electrical or pneumatic power, then the brake clamps down on the cylinder rod. There are also brakes for rotary and linear applications where a rod lock will not fit. The brake should be sized to overcome the energy if the cylinder is fully pressurized.

A 'closed' center is more likely to keep the position - but also keep the energy trapped. If an operator walks up to it, they will likely not be able to move it. However, if a hose comes loose or something happens to the air - sudden movements can happen.

An 'open' center will dump all the air but might cause unwanted jerking motion during an e-stop state if the cylinder has a heavy load or is at an odd angle. The operator can walk up and move the cylinder by hand if it is small enough.

If you have ANY load in a vertical application, I highly recommend a rod-lock. However, be aware that rod-locks DO wear out and need to be replaced. Most maintenance people will not replace them until they break - and at that point it is too late in a safety application!

I got it. I'm just trying to figure out should I drop all of the 24V completely and control reliably, or should I just turn off the outputs in a non-safety rated PLC?

in any pneumatic application, it all depends on what you are trying to do.

there are multiple variations of valves.
I can't list them all and explain them, there is not enough room to post.

to hold a cylinder in place, I typically use double solenoid, blocked center.
you can also use a blocked center with the mid position having the pressure side going to the a & b ports, BUT !! that's very risky. when you loose air pressure, then next time air is applied, the cylinder will be a rocket when it is energized.

as far as the zero energy state of loto, you must do a hazard analysis and see what the risks are. as I said, loosing air pressure to both sides of a cylinder is dangerous when the cylinder has the air reapplied.

james

Timeismoney08 said:

I got it. I'm just trying to figure out should I drop all of the 24V completely and control reliably, or should I just turn off the outputs in a non-safety rated PLC?

Click to expand...

Turning off the outputs (via plc logic) will not suffice.
Dumping the whole 24vdc not recommended.

Think about interrupting the 24vdc to the output card (some cards have groups that can be isolated) or perhaps consider hardwired interrupting the individual wiring to the valves (both above via the safety relay)

You really have to do a risk assessment for anyone to tell you a good answer. Basically what kind of damage could the actuators cause if it catches an operator? If the risk is minor (worst case scenario a small bruise or similar) dropping 24V to the solenoids may suffice.
Most industrial machines I've seen and obviously I don't know your application requires at least a Performance level D (basically a machine that could maim you, but not kill you where your exposure isn't very high, I'm guessing entering a cage isn't high frequency) . In this case killing solenoid power isn't sufficient. You would need to remove all energy (air) using a dual channel safety air valve connected to a safety relay.
Again a risk assessment is needed in order to determine the correct method.

*Edit. Obviously my example above is only an example. There may be some applications that dumping the air to a certain actuator is more dangerous.