Flow Valves

We have to pistons, one on each side of our robot, that lifts our climber and intake mechanism up. When it retracts and goes down back into the robot, the pistons retract at a very high velocity and the mechanism ‘smashes’ into the robot. We still want the pistons to push the mechanism up very quickly, but we don’t want them to retract as fast. I heard that a flow valve could fix this problem.

Which flow valve would I need to make the piston retract slower than it rises?

This one: http://www.andymark.com/product-p/am-2897.htm
or this one: http://www.andymark.com/product-p/am-2690.htm

Also, how many of these flow valves would I need per piston?


Both would do the same thing.

am-2897 lets you put the flow control directly onto the cylinder. Great if you want to fine tune the flow per side. You’d want one per cylinder if you only want to control the flow rate to one direction of motion.

am-2690 is an inline flow control valve that you can put anywhere you have tubing run. This is nice because you can put it on the output of your solenoid and then put a Y after this, effectively controlling the flow control to both cylinders with one part.

I’d recommend am-2690 for most applications as the 1/8" npt to tubing version can take up quite a bit of space around the cylinder where the inline version allows you to put the flow control wherever you please.

Does the Flow Control, In-Line, 1/4" tube, press-in (am-2690) restrict flow both ways? If so, it might slow down filling of the piston when it is extending by restricting the flow on the exhaust side.

The 1/8 NPT Male to 1/4 Tube, Elbow Meter-Out Flow Valve (am-2897) says “while the opposite flow is not restricted”. Assuming it can be plumbed in the correct direction, it would meter air going into the piston, and unrestricted flow of air going out of the piston. If not, it might be able to be used on the other side, and restrict air going out, and unrestricted air going in.

Also, would having two pistons run off the same solenoid produce less upwards force on the mechanism than having the two pistons run off of different solenoids?

Both parts restrict the flow in only one direction.

By the way, we generally plumb them so we are controlling exhaust flow out of the cylinder.

They would produce the same force (just the product of the pressure and the surface area and the pistons), though one solenoid would take about twice as long to fill two cylinders.

As GeeTwo stated the force is the same, but you may have some unexpected motion. If you have the cylinders moving independent mechanisms, you may see that one mechanism moves first, then the second mechanism will move.

Depends on static friction, hose distance from each cylinder to valve, mechanical differences.

There are times when it pays to save the money on the second valve, and times when it pays to have the second valve. Depends on quickness, and control needed.

… and you can get both of these for free at AutomationDirect using the voucher in the KOP …

Go to AutomationDirect.com and search on “Flow Control?” There are about 100 options …

If the hoses are cut to the same exact lengths, the two cylinders’ motion should be in sync.

Hose length is only one of the variables. Load, Static Friction, Internal friction of the cylinders, flow control settings for each cylinder, mechanical mounting of each cylinder (variance give one cylinder a mechanical advantage than the other), fittings used on both sides, etc.

Once one cylinder over comes static friction, it will finish it’s stroke first. With flow controls you can typically force the cylinders to break away relatively balanced in a FRC application, but at the expense of speed.

Much of the info on the page they talk about hydraulic and pneumatic in the same instance, but the with the compressablility of air, syncing pneumatic circuits are much harder than hydraulic.

The simplest circuit uses only flow controls to build resistance to hold the fast cylinder back. The accuracy of flow-control synchronizing is only fair to poor.

One solenoid will take the same amount of time as two because they are limited by the supply line to the solenoids, the air for both cylinders has to come from the same place (pressure regulator) it does not matter when it splits off, whether before or after the solenoid(s), having two solenoids will not make a noticeable difference. There are other ways to get around this if you absolutely need it to be faster.

Putting a low side volume alleviates the issue of the regulators CV (mostly).

Getting 2 solenoids to fire 2 actuators at the same speed is tricky unless you use flow controls (Which I Highly recommend).

Have you looked into pneumatic shock absorbers or cylinders with end of stroke cushions?

Both products will allow you to maintain the same retract speed without the end of stroke smashing. They can also be used in conjunction with flow controls.

Check out: http://www.bimba.com/Products-and-Cad/Actuators/Inch/Round-Line/Non-Repairable/Original-Line-Adjustable-Cushions/

The shock absorbers are externally mounted and your tooling would contact them and be slowed down.

Check out: http://www.parker.com/literature/Literature%20Files/pneumatic/Literature/PDN1000US_Products/PDN1000-2_Products/PDN1000-2US_Shock%20Absorbers.pdf

Not sure of the cost.


Alternatively to using flow valves, you could always think about adding some kind of foam or other absorbent material to take the impact of the robot lifting up, or you could even consider using compression springs to counteract the force of the pistons, and slow down the upwards movement of the robot.

In 2014, we ended up using two solenoid valves to speed up the lift on our intake which was raised by two pneumatic cylinders. It is possible that we were using solenoid valves with M4 fittings, which only allow about 40% of the airflow of the permitted 1/8" NPT. In any case, I don’t recall seeing a rule which specifies the maximum airflow of a pressure regulator.

If the regulator is your bottle neck, this is an easy fix.

We didn’t have a problem with timing or speed, but this was a cRIO robot, which (IIRC) supported splitting the signal from the solenoid breakout to two solenoid valves. In any case, a spike is still a legal solution to drive multiple solenoid valves per R53.