Solution: Variable position of cylinder in compliance with <R74>

[GearsOfFury]

Roboman: I understand that's the rule, I'm asking "Why do we think the GDC has this rule in place?", and should it be reasonably challenged for next year's rules? The single vent rule was further emphasized first in Lunacy with "all" underlined. 2008 was the first year they broke out mention of the valve separately; before that it was mentioned as part of the Nason main relief rule and wasn't as explicit: "The Parker pressure vent valve must be connected to
a Clippard tank such that, when manually operated, it will vent to the atmosphere to relieve any stored pressure."

The fact that they've refined and emphasized the rule leads me to believe there were safety issues at some events (perhaps in 2007 and again in 2009). I'm just wondering if they were "emergency" occurrences that required quick relief by one and only one valve, or if we could build safe (and potentially more functional) robots that had multiple relief valves that together are capable of relieving all stored pressure.

[Roboman01]

Originally Posted by GearsOfFury

Roboman: Why do we think the GDC has this rule in place?

Stored pressure in a pneumatic system is inherently dangerous. If a part happens to fail, one must have a reliable method of quickly venting the system pressure, preferably from a single point. Requiring a single release valve is a much easier way of making sure all teams have this, rather than judging a pneumatic system's safety on a case-by-case basis. I actually believe this rule is a good one. However, I also believe that the GDC should allow a broader range of pneumatic devices, such as check valves, for next year.

Originally Posted by PAR_WIG13500

I understand, and always have understood, that you were describing a different system. I just didn't want people to perceive either system as being more complex (or confusing) than it really was by trying to match the system I proposed to the system you provided a diagram of, since that is impossible due to the fact the systems are far from similar.

Also, 'simpler' is a matter of opinion, I consider mine to be simpler due to the fact that it doesn't require very much attention in the code, all you need to do is set the regulator and let it find equilibrium with the spring on its own. Also, it isn't very complex physically either. I will draw a diagram and post it for clarity.

I understand your system, however, it seems a little jury-rigged, to be frank. Keep in mind that the force exerted by springs changes based on how much they are extended or compressed, unless you use a constant-force spring (obviously). Also, you're relying on a relieving regulator to vent excess pressure in the cylinder. While these are more common than non-relieving regulators, the latter are certainly not rare, and could easily be confused with a pressure-relieving regulator.

In addition, to implement your system, you would need to figure out some way of controlling the regulator. You mentioned hobby servos, which could work, but they are confined to <180 degrees. Sail winch servos are not allowed, and still would not rotate enough to open and close the regulator entirely. They are also much more coarse in their movements, which could limit your true control over the cylinder.

Your system would have a variable force in addition to the variable stroke, since you're varying the pressure, rather than the amount of air in either end. As you should know, reducing the pressure also reduces the force exerted, and the spring on the end that counteracts the cylinder's rod will cause the net force to be near zero, since it's stopping the travel mid-stroke. This is obviously not good for actuating an arm, or anything that will be exerting any sort of force.

[Al Skierkiewicz]

Guys,
You are reading a little too much into this. The vent valve is to relieve system pressure so that the robot cannot move while in transport. There is no rule (except in my mind and those teams that design safely) that says the robot cannot move when energized. So many teams design just such a robot. When the pump comes on, things begin to move. Because the pits, the queue and transport areas are so crowded, we don't want a accident to occur simply because a system has pressure and a team member inadvertently enables the robot. I have had my arm caught in a moving mechanism and seen a robotic arm swing out and knock over the pit table into the pit behind it at Champs In addition to the safety, many rules are in place to reflect good engineering practice (similar to wire color codes) and to prevent teams with no pneumatic mentorship from hurting themselves.

[PAR_WIG1350]

Originally Posted by Roboman01

I understand your system, however, it seems a little jury-rigged, to be frank. Keep in mind that the force exerted by springs changes based on how much they are extended or compressed, unless you use a constant-force spring (obviously). Also, you're relying on a relieving regulator to vent excess pressure in the cylinder. While these are more common than non-relieving regulators, the latter are certainly not rare, and could easily be confused with a pressure-relieving regulator.

In addition, to implement your system, you would need to figure out some way of controlling the regulator. You mentioned hobby servos, which could work, but they are confined to <180 degrees. Sail winch servos are not allowed, and still would not rotate enough to open and close the regulator entirely. They are also much more coarse in their movements, which could limit your true control over the cylinder.

Your system would have a variable force in addition to the variable stroke, since you're varying the pressure, rather than the amount of air in either end. As you should know, reducing the pressure also reduces the force exerted, and the spring on the end that counteracts the cylinder's rod will cause the net force to be near zero, since it's stopping the travel mid-stroke. This is obviously not good for actuating an arm, or anything that will be exerting any sort of force.

(emphasis added by me)

1) {response to bold text} These facts are what allows such a system to work. The entire operating principle of the design relies on a variable force being counteracted by a controlled variable force. The two forces cancel out at equilibrium. A constant force spring would not work in this system, the description specifically calls for a variable force device.

2) {response to underlined text} This was a problem with the system as originally described, but I edited the description to account for the force applied by the mechanism being actuated. The forces will still cancel out at equilibrium, assuming the correct bore and spring are chosen based on the direction and magnitude of the forces that will be applied by the mechanism.

3) {response to first line of quoted text} It is somewhat jury rigged. The system is meant to be implemented on a robot that had previously used multiple solenoids to control a single cylinder, to make it fit within the rules. Your system would be preferable in this situation, since it requires little to no change to the code and minimal changes to the hardware. however, if I were building the system from nothing, I would prefer my system based on the fact that it would appear easier to program (plus my team has been successful in positioning a rotary device with pots, but has little/no experience with linear positioning, as far as I am aware).

4){red paragraph} It wouldn't be terribly difficult to use a window motor or a RS-395 (with an appropriate transmission, possibly from a servo [minus the mechanical stops]) to control the regulator, assuming fairly accurate pots are used and/or the values used in the code are properly adjusted.

[PAR_WIG1350]

Originally Posted by GearsOfFury

May I pose a practical question? Why do we think the GDC has this rule in place? Why not allow the use of additional pressure relief valves in line with the cylinder rod/head supply lines, mounted on your bot right next to the "main" relief valve? You could use quick-release toggle valves (e.g. http://www.mcmaster.com/#pneumatic-t...valves/=b7a2dk) to make it super-easy to relieve pressure in an emergency... One could even devise some kind of mechanical toggle that connects to the valves and flips all the connected reliefs at once.

So, is it a safety / emergency thing for use at the end of the match or in an emergency situation, or does the ability to trap air at mid-stroke during normal use pose some other safety concern?? I can't imagine the latter, or there wouldn't be much market for closed-center solenoids...!

Based on what I have read, they want all pressure to be released with a single action, the opening of the plug valve. I don't think the relief valve is involved in that much. The desire to release all the air with this single action stems from concern about people working around potential energy, in this case in the form of air pressure differential.

A secondary concern might be teams using isolating air from the rest of the system and compressing it to dangerous levels (in excess of 120 (or even 240) psi, if one designs the system "properly") and releasing the potential energy in a highly energetic manner that could be destructive and dangerous, even if the system survives long enough for the release to occur on command, in the desired manner. Additionally, it could be possible to design similar, though not quite as extreme, systems accidentally, even if the team tries to stay within the rules. In most applications, the system would be designed with this in mind thus making the use of center blocking solenoids practical.

Originally Posted by Roboman01

That was the point. I was trying to describe a simpler system of controlling the cylinder.

I understand, and always have understood, that you were describing a different system. I just didn't want people to perceive either system as being more complex (or confusing) than it really was by trying to match the system I proposed to the system you provided a diagram of, since that is impossible due to the fact the systems are far from similar.

Also, 'simpler' is a matter of opinion, I consider mine to be simpler due to the fact that it doesn't require very much attention in the code, all you need to do is set the regulator and let it fine equilibrium with the spring on its own. Also, it isn't very complex physically either. I will draw a diagram and post it for clarity.

[cjwmam]

That's irrelevant, for the purposes of this discussion. All I'm saying is that with a 5/4 3-position valve with center pressure, you can only move the cylinder to a total of three positions - out, in, and a predetermined spot in between the two extremes


I am not sure why you say this will not work. but all I seen on our robot was I could stop and start cylinder any where in the 30" stroke. only used standard regulater on none shaft side of cylinder.

[Roboman01]

Originally Posted by cjwmam

That's irrelevant, for the purposes of this discussion. All I'm saying is that with a 5/4 3-position valve with center pressure, you can only move the cylinder to a total of three positions - out, in, and a predetermined spot in between the two extremes


I am not sure why you say this will not work. but all I seen on our robot was I could stop and start cylinder any where in the 30" stroke. only used standard regulater on none shaft side of cylinder.

Are you sure it's a center pressure valve? Also, when you were moving the cylinder, did you need to adjust the regulator to change the position of the rod, when it was not at either extreme?

[PAR_WIG1350]

Originally Posted by Roboman01

Are you sure it's a center pressure valve? Also, when you were moving the cylinder, did you need to adjust the regulator to change the position of the rod, when it was not at either extreme?

The requirement to hold the piston in place doesn't change as the piston moves along the cylinder, stopping it half way is the same as stopping it one third of the way out. The sum of the forces must be 0 to maintain constant motion or to stay at rest. If the regulator is set at a constant setting that balances the forces resulting from the pressure pushing against the 2 surfaces of the piston, which have different areas, then friction will quickly bring the cylinder to a stop and keep it there. The accuracy of such a system decreases with momentum.

You seem to be confusing volume or surface area of the inside of the cylinder with force exerted on the piston. The force exerted on the piston is what matters, the other two are irrelevant.

[Roboman01]

Originally Posted by PAR_WIG1350

The requirement to hold the piston in place doesn't change as the piston moves along the cylinder, stopping it half way is the same as stopping it one third of the way out. The sum of the forces must be 0 to maintain constant motion or to stay at rest. If the regulator is set at a constant setting that balances the forces resulting from the pressure pushing against the 2 surfaces of the piston, which have different areas, then friction will quickly bring the cylinder to a stop and keep it there. The accuracy of such a system decreases with momentum.

You seem to be confusing volume or surface area of the inside of the cylinder with force exerted on the piston. The force exerted on the piston is what matters, the other two are irrelevant.

According to the schematic symbol, it would appear that the two cylinder ports are tied together, and pressure is applied. Wouldn't this cause the cylinder to creep back to a position around the midpoint? In any case, it would appear that using a center-pressure valve for this application is not generally accepted in industry, according to the Norgren Valve Guide. From page 225, section 3: "3. 3-POSITION - Inlet Open to Cylinder Ports
a) Equal pressure supplied to both sides of the piston creates an
unequal force, due to rod area differential, which causes the
cylinders to extend. Many times a regulator will be installed in
the blind end cylinder line to drop the blind end pressure and
equalize the force (we all know this would be an improper
application of a regulator). The application of full-line pressure
to this regulator during normal cylinder cycling leads to
premature regulator failures.
b) Cannot be used to support vertical loads."

It would appear that is causes unnecessary and undesirable strain on the components.

[PAR_WIG1350]

Originally Posted by Roboman01

The application of full-line pressure
to this regulator during normal cylinder cycling leads to
premature regulator failures.

In FIRST, the lifespan of the components is not generally an issue, as long as it gets through the season (sometimes, even shorter lifespans , such as a single event or match) are also tolerated in FIRST.

It is not proper to:

• Use AC light or limit switches to switch DC current.
• Use the 90c to fill a tank with a volume exceeding one gallon.
• Do A LOT of other stuff that we do in FIRST.

[Roboman01]

Originally Posted by PAR_WIG1350

In FIRST, the lifespan of the components is not generally an issue, as long as it gets through the season (sometimes, even shorter lifespans , such as a single event or match) are also tolerated in FIRST.

It is not proper to:

• Use AC light or limit switches to switch DC current.
• Use the 90c to fill a tank with a volume exceeding one gallon.
• Do A LOT of other stuff that we do in FIRST.

Still, according to the manual, the proper way to jog a cylinder is to use a center-closed valve. Since there is a way to do this legally, as far as I can tell, there really is no reason why you shouldn't do it the proper way.

[wilsonmw04]

Originally Posted by Roboman01

Still, according to the manual, the proper way to jog a cylinder is to use a center-closed valve. Since there is a way to do this legally, as far as I can tell, there really is no reason why you shouldn't do it the proper way.

Center locking valves are not allowed this year, that's the whole purpose of this crazy thread.

[PAR_WIG1350]

*palm--> *

[Roboman01]

Originally Posted by wilsonmw04

Center locking valves are not allowed this year, that's the whole purpose of this crazy thread.

No, the valves themselves are, if you hook them up so that you can release all stored pressure.

*Sigh*, this is going around in circles. I'll have a video up next week demonstrating that there isn't any stored pressure with my system, and I'll talk to an inspector at St. Louis for further confirmation.

[PAR_WIG1350]

Originally Posted by Roboman01

No, the valves themselves are, if you hook them up so that you can release all stored pressure.

*Sigh*, this is going around in circles. I'll have a video up next week demonstrating that there isn't any stored pressure with my system, and I'll talk to an inspector at St. Louis for further confirmation.

that's what this is.