Quote:
Originally Posted by Andrew Blair
I have a very legitimate reason for needing a vacuum valve, and it's not to control the vacuum generator.
Basically, I need a valve that operates without a requirement for pressure. If the only option is to use a butterfly valve actuated with a cylinder, then the value is lost.
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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.
Required:
- 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.
I hope this helps!
- Keith