Are there solenoid valves available that are designed to operate under vacuum? All the KOP valves require at least some positive pressure to actuate, unfortunately. I’ve found several manufacturers online, but they all look to be producing for precision industry- so not real cheap…or available.
Secondly, the rules this year allow for vacuum devices, and the pnuematics rules allow for the purchase of any solenoid valve rated for 125 psi, but would the solenoid in a vacuum valve break these rules? It likely would not be rated to 125 psi, but it is a vacuum device, and solenoid valve.
Even if it was allowed in the rules, there is no reason to have a vacuum valve. I would even say it is a bad idea. The way venturis operate requires airflow to create vacuum. If you could somehow find a valve, you would be constantly running air through the vacuum generator. If you want a way to control the vacuum generator, you should use a pressure rated solenoid valve somewhere on the pressure side of the vacuum generator.
A few years ago (in the bad old days when you were limited to only the provided Pneumatics Kit parts) on one of my previous teams I found a way to generate a legitimate “extra valve”. (Bear in mind that the vent was NOT required that year.)
In essence, we took a chunk of (then provided) 2" x 3" x 1/8" extrusion, which the vent valve JUST fit inside of, a kit servo, and a slice of smaller box, and made a new, NON-piloted valve out of the vent valve! Of course, for THIS contest, you’ll need to buy a spare vent valve to make it.
A “vent valve”, identical to the one in the kit
A Kit Servo
A chunk of of 2" x 3" x 1/8" box extrusion (It used to be provided, in the “pre-Frame Kit” days) - length ~= 3 to 5 inches longer than the length of the servo.
A 1-1/2" slice of 1/2" x 3/4" x 1/16" box extrusion (or 3/4" x 3/4" x 1/16", or similar box extrusion - see below)
Misc hardware to attach the small extrusion to the servo’s horn.
I can describe the construction procedure in detail, if necessary.
Essence: The valve JUST fits sideways inside of the big extrusion. The little extrusion is “slotted” on one side with a Dremel, and acts as the “hand” that turns the knob. This “hand” is attached to the servo’s horn with small hardware. The servo is mounted pointing INTO the big extrusion, and the two extrusions are “aligned” with each other. Once that is mounted: Close the valve THEN insert it with the two ports pointed toward the SIDES of the big extrusion. The valve handle SLIDES INTO the slit small extrusion. There are NO modifications to the valve, whatsoever! Now mark, disassemble, and drill holes through the extrusion’s sides to accept the SMC pushon connectors. The whole box is then reassembled, and mounted onto the robot via holes drilled near the ends of the big extrusion. (On each end, I cut three sides of the large extrusion back about an inch, to form “mounting tabs”.)
Programming: You find the two PWM values that represent the open and closed positions on the servo, and make DEFINE VALVE_OFF and VALVE_ON constants of them. Now use PWM output commands to set the valve on or off.
NOTE: Be sure when you set it up and program it that the servo does NOT hit a valve stop (nor the end of the servo travel) in either the On or OFF position, or it will draw excessive current and possibly blow a fuse. (I checked the servo’s draw current to make sure it hadn’t stalled, by making a cut in the red wire of a spare PWM cable, and adding connectors for a cheap ammeter. Ask me for a drawing, if you need it.)
NOTE: This valve is SLOW. BUT it IS “non-piloted”, AND can handle vacuum as well as pressure. HOWEVER, the force that the valve requires to turn it is VERY close to the limit of the kit servo’s strength, so it takes significant time to toggle positions. Therefore, I’ve normally only used it to turn on or isolate entire pneumatic subsystems, not as a high speed valve.
I haven’t seen this year’s rules about servos, but if you are allowed to use a stronger one, I’d advise it. Alternatively, you can try to find another valve that takes less force to turn, but I like this one because it fits perfectly into the extrusion, with no slop.
ANOTHER SOLUTION (if you need faster response): COTS “Externally Piloted Valves” DO exist. This is where the valve’s pilot port has its OWN air connection to provide the required force amplification to operate the valve. THOSE valves can run near zero differential pressure, yet can still be driven from low solonoid currents, like the ones in the kit.
What you need is a non-air piloted valve. MAC Valves 35 series would work well for what i suspect you are doing.
I havn’t looked at this year’s rules, but in previous year’s i don’t remember anything prohibiting this setup.
The problem with the valves in the kit is that they are air piloted, that is they use a small internal valve to actuate a larger valve that actually controls the flow of air. Unfortunately, the pilot will not work off vacuum. Some piloted valves allow for the pilot to be driven from a dedicated positive pressure supply. I do no beleive that any of the valves in the kit can be adapted for external pilot supply. For an animation of how an air-piloted valve works, see the front page of www.macvalves.com
Thanks guys! I guess I just needed to be pointed towards the correct terminology. What we’ll likely use is called a direct acting valve, which is the same as a non-air piloted valve. They are available from simply McMaster-Carr, and pressure rated as well! Cool, thanks for the help!
In 2004 we used a vacuum & suction cup to pick up the ball. For a valve, we just used a servo & the tubing. We attached the tubing to the servo arm so that when the servo rotated, it kinked the tubing. Rotating back opened it up again. It was light, simple, and very effective, and lasted the whole season.
I’m curious- will direct acting valves spec’d out for 24 VDC run on 12 VDC? We’ve used piloted valves meant for 24 VDC on 12 VDC without issue before, so will a direct acting one operate? Especially because it will be under vacuum, so there should be far less forces acting on it than when it would be used under pressure.
I hate to burst your bubble here guys, but those “Direct Acting Valves” are not legal for 2007 FIRST robots. <R46>, as well as this question in the Q&A pretty much cover the use of non-air operated solenoid valves.
A non-air operated solenoid valve uses an electronic solenoid, which is considered to be a motor in the eyes of the GDC. What Keith has suggested is to make your own non-air operated solenoid valve using kit parts/motors. His solution of adapting a servo to rotate a manual shut-off valve is the perfect solution to this problem. Other motors could be used (globe, window) but they would also require a spike, limit switches, etc… The servo trumps all of that in terms of weight and simplicity.
We had a hard time dissecting the rules to determine which solenoid valves were legal and which were not. Through a few online searches, we were unable to find low-cost, low-weight, pressure rated air-operated solenoid valves. More than likely, we’ll be using the servo solution.
01-18-2007, 03:00 PM
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http://forums.usfirst.org/images/icons/icon1.gif **Direct Acting Valve in Vacuum?**
We have a direct acting solenoid valve rated for 100 psi that we wish to use expressly in our vacuum loop.Though not rated for 125 psi, it is more than adequate for the vacuum it will be operating in. Is it acceptable to use?
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http://forums.usfirst.org/images/statusicon/post_old.gif 01-19-2007, 09:52 AM
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http://forums.usfirst.org/images/icons/icon1.gif **Re: Direct Acting Valve in Vacuum?**
No. Under Rule <R105>, all "off the shelf" pneumatic devices must be rated for at least 125 psi. There is no exception for devices that are used for vacuum applications.
What this implies is that *any *off the shelf pneumatic solenoid valve (meaning used to control the flow of air in a mechanical system, not provide mechanical actuation directly), may be used so long as it is rated to 125 psi. A direct acting valve serves the same role as a piloted valve, just with more power draw.
Too bad we can’t use the one we have laying around though…