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

I believe I have found a solution to most issues arising from the illegality of exhaust restricting solenoids that has been a hindrance to all plans involving stopping a cylinder mid stroke.
Unfortunately, due to the apparent desire of the GDC to prevent the trapping of air within the pneumatics system,
http://forums.usfirst.org/showthread.php?t=16642
the solution is slightly more complex than it otherwise would be, but that can be worked around.

The solution, in compliance with this years rules is as follows:

1. Connect one secondary regulator per cylinder to be controlled to the 60psi side of the pneumatics system.
   .
2. Attach a servo to the knob that controls the pressure setting of the secondary regulator.
   .
3. Connect one port of the pneumatic cylinder to the secondary regulator output (technically, a solenoid is unnecessary, but this seems to make them mandatory, use your best judgement in deciding placement, powering it and programming it are unnecessary.)
   .
4. Attach an adjustable flow control valve to the second port. Leave it fully open at first and reduce the flow if the spring induces significant oscillations.
   .
5. Place a variable force device (i.e. a spring) such that it applies force to the piston opposite the force resulting from the pressurized air in the cylinder.
   .
6. Remember, the restoring force of a spring is kX. where k is the spring constant and X is displacement; and the force of the piston is (Pi*[bore/2]^2)*pressure if the cylinder is extending and (Pi*[{bore/2}^2-{rod diameter/2}^2])*pressure if it is retracting.
   .
   Set the spring force equation equal to the appropriate pneumatic actuator force equation substituting in the correct values for bore, k, and, if necessary, rod diameter and the desired value for X. For unbalanced mechanisms, account for the force applied by the mechanism in calculations by adding this force to the appropriate side of the equation.
   .
7. Solve for pressure.
   .
8. Program the servo to adjust the regulator to the pressure value found using this method.
   .

Note:constants are red, independent variables (inputs) are blue, and dependent variables (ouputs) are green.

I am not an official source of information, I cannot verify the legality of this method of cylinder control.

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

Did you actually try this? I can think of a few immediate issues, like valves need a minimum pressure to operate, and a servo has 170 degrees of throw, where a regulator has multiple turns.

I haven't tried it, but I also think servo will not have enough power to turn a KOP regulator.

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

1) there are no valves that require actuation so that issue is avoided entirely. The only valve is a single solenoid that is open by default and never needs to be switched.

2) Not all servos have such limits. Sail winch servos can travel 3 full rotations, I believe, and some servos can rotate indefinitely, but have no position feedback (see point 3)

3)It doesn't have to be a servo, a motor (or continuous rotation servo) with a potentiometer or encoder would also work with some extra programming. Gearing could also increase available torque for weaker motors/ servos.

No, I have not tried it physically, but the physics works out, at any given pressure, a certain pneumatic cylinder has a set force. Additionally, at any given displacement, a certain spring has a calculable restoring force, if these forces, along with forces applied by the weight of the mechanism (assuming an unbalanced mechanism) balance out to zero, the mechanism will be stationary (technically, it might exhibit oscillations, but by reducing the flow rate out the open port on the cylinder, they should be dampened to the point at which the amplitude is nominal).

However, I do have 2 corrections to make and they shall be made momentarily.

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

Well, it's not much of a first post, but I just had to put in my $0.02 on this subject.



Now, why wouldn't something like that work? This is just for the control of a single cylinder; the rest of the circuit continues past the black arrow. Each check valve prevents backflow from the accumulator into the cylinder, while simultaneously allowing venting of all pressurized areas from a single valve. (Note: I was intending to use a solenoid flow-control valve, but I seem to have lost the link.)

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

Based on what I have read about pneumatic schematics, this looks like a fancy way of operating a cylinder in 2 positions, out or in, since when one side is pressurized, the other would be vented to the atmosphere. I considered a similar approach myself, but abandoned it when I realized there was no way to make it work properly without complex programing or trapping pressurized air in one side of the cylinder. Also, I'm not sure if solenoid flow control valves are legal, but in this configuration they certainly are since it is a second solenoid and according to <R74> only one solenoid can be used to control every commanded action of a pneumatic actuator.

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

I started reading this, and it seemed so complicated I just gave up. I don't typically give up at understanding things, but this just seems like so complex of a system, that perhaps the design should be reconsidered if it requires all this. Motors anyone?

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

I'm normally a guy who understands stuff, but I'm also lost here. I'd need to see it I guess.

How about a mechanical stop for that 'middle' position? A tiny servo moving a locking pin/bar could manage that.

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

After reading this a second time, I totally get it now. In theory, it's actually a rather nifty idea. Not nearly as complicated as I thought. In practice, I think it would leave a lot to be desired.

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

The valve in the schematic above is a 5-port, 4-way, 3-position valve with the center position blocking both pressure and exhaust ports. This allows the pneumatic cylinder to be controlled similarly to a hydraulic ram, since one can shut off and trap the air inside the cylinder, preventing significant movement.

Of course, <R73> specifically states that this is not allowed, so one must be able to vent the trapped pressure at the same time as the rest of the system, from a single valve. The two check valves allow this. Each check valve allows air to flow (mostly) unobstructed out of the supply to each cylinder, but does not allow the high-pressure air to reach the cylinder supply lines. Since the pressure in the cylinder is lower than that of the accumulator, little air will be lost, due to the natural tendency for the higher pressure air to seal off the check valve.

During normal operation, this will cause the cylinder to act as if it were connected via a normal piece of tubing. However, in the event that we need to vent all of our stored pressure rapidly, the check valve will allow the stored pressure in the cylinder to flow straight out the dump valve.

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

Oh, I get it - the check valves allow the whole system to vent to atmosphere when high pressure is released, thus making the 3-way valve (center position blocking) legal.

I'd be prepared to explain carefully to the inspector why the high pressure side seems to be connected to a cylinder.

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

Just to clarify, the system explained in the first post and the schematic in the second are actually 2 very different methods of controlling cylinders. I imagine it would be very difficult to try to match the schematic to the description in the first post, since they don't actually go together.

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

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

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

The bolded words are why it won't work (at least not legally). <R65> and <R66> combine to heavily limit the types of pneumatic devices allowed on the robot. Check valves are not one of those allowed devices.

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

Oh dear, that could prove problematic.

EDIT: A unidirectional inline flow control valve, when it's closed entirely, will act like a check valve. If each check valve in my above schematic was replaced with one of these, it would perform the same function, and remain legal.

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

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...!

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

That's why you couldn't have multiple release valves. Unless you could find something that can simultaneously vent multiple outputs while keeping each output isolated, you cannot use your idea.

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

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.

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

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.

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.

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

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.

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.

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

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.

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

Al,
I value your opinion because you tend to relay facts in a cool and level headed manner. That being said, I have a question for you: What makes pneumatics different than a motorized mechanism? If air is locked inside a piston, after the relief valve is opened, it will stay in that position. What makes that different from an arm that cannot be back driven? Will it not stay in the position it was last in? What makes one "safe" and another "unsafe"?

I am not arguing with the rules. I'm just trying to understand the logic/rationale behind them.

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

If parts move with or without power, it is bad regardless of it being caused by electrical or pneumatic action. The vent plug valve, like that main circuit breaker, attempts to insure that no action on a participant's part will cause unexpected robot movement. We all know that the more wear a part gets, the more it is likely to leak air. In your example, should a actuator be locked due to an external valve and either the valve fails or the tubing goes open, movement will occur. This could be a violent action depending on the failure. We have all witnessed team members working on pneumatics, pull a tube while pressurized or cut into a tube while trying to use wire cutters to trim a wire tie or wire.

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

Now I see where they are coming from. Thanks for the explanation. But wouldn't window motor driven arms be subject to the same mechanical failures?

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

(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.

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

If you are referring to a total window motor gearbox failing and letting the arm fall, the answer is yes. That possibility exists with the robot powered or unpowered and can be extended to mounting hardware or mounting holes failing as well. Teams should exercise caution whenever working under or in front of a moving appendage that is not at rest. They should also plan to move the appendage to it's most safe position before transport.

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

1: Good point, I hadn't thought of that.

2: That makes sense, but the system will not be as authoritative in its movements, if you're varying the pressure, and therefore, the output force.

3: If you're entirely set of using the adjustable regulator, why not go with an electronic one? That would cut down on your overall weight immensely. Also, my team is not measuring the extent of the cylinder directly; rather, we're actuating an arm. The arm has an encoder at the joint, which obviously measures the position of the arm itself. For our purposes, my system will work far more efficiently, since we won't have a variable output force. Also, my system will most likely weigh far less than yours, as we don't need a motor, gearbox, or secondary regulator. This is important, at least for my team, since we are within a few pounds of the limit.

4: Actually, my system could possibly be easier to program for, at least, in my team's application. For us, we either need the valve in one extreme or the other, or simply off. Also, we can gain higher positioning accuracy without any extra electronics, other than the single encoder on the joint of the arm. In order to make sure that you are positioned correctly, you would need both an encoder/potentiometer on the regulator, as well as an encoder/potentiometer on the arm, assuming you're using this for an arm.

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

Why would you need 2 encoders/pots? One for the regulator would be sufficient. Also, the additional motor is offset by the deletion of the solenoid (at least from th electrical and software systems). I don't know if it would be legal to get rid of it completely or not.

Our team also has only one position sensing device on the arm, but then again, we aren't attempting variable positioning with pneumatic actuators anywhere on our robot.

Additionally, I may prefer my method over alternatives for variable positioning of pneumatic actuators, but I also prefer motors over pneumatics:p .

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

You would need two encoders/pots because the load on the arm varies, based on what kind of tube you're picking up, as well as the location of the end effector in relation to the pivot point. Unless you have some method of reliably measuring the load on the cylinder, your method would not be the most precise option.

We also only have one positioning sensor on the main segment of our arm, but we're switching the motor out for a pneumatic cylinder, since the BaneBots gearbox seized up, causing the motor to melt.

I happen to prefer pneumatics over motors, and I've been trying to influence my team in that direction since ever since I joined.

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

Make no mistake, I love pneumatics and prefer them over motors for some applications. However, for variable positioning applications, I prefer motors.

Also, while it is possible to use only one encoder while using my system, I must admit that it is more prudent to use two. That being said I have already admitted that for this application you solution would be a better fit. However, for applications with a more predictable load, my system becomes more competitive.

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

Okay, that makes sense. I admit, your design is a rather clever workaround to the problem presented in <R74>. I can see how, with the correct setup (possibly an electronic regulator), your system could be superior to mine, in terms of weight savings.

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

question to GCD 03/04/2011

2011FRC2221
Pneumatic Solenoid
R73> The pressure vent plug valve must be connected to the pneumatic circuit such that, when manually operated, it will vent to the atmosphere to relieve all stored pressure. The valve must be placed on the ROBOT so that it is visible and easily accessible. If the compressor is not used on the ROBOT, then an additional vent valve must be obtained and connected to the high-pressure portion of the pneumatic circuit off board the ROBOT with the compressor (see Rule <R69>).
First forums GDC reply 01-31-2011, 02:05 PM “Trapping the air under pressure as described would be a violation of R68/R73. The rule clearly states that when the required relief valve is opened, it will vent to the atmosphere to relieve all stored pressure.”
<R66> In addition to the items included in the KOP, pneumatic system items specifically permitted on 2011 FRC ROBOTS include the following items.
Pneumatic pressure vent plug valves functionally equivalent to those provided in the KOP,
<R66> allows multiple pressure plug valves.
Question: Can you plumb pressure plug valves in the pneumatic lines to the cylinder using a 5 port 3 position center port closed solenoid valve and position these valves adjacent to the original pressure vent plug valve and still be in compliance with <R73> .
Thank you
Team 2221
2011FRC2221

Reply from GCD
03-15-2011, 01:35 PM
The purpose of this forum is to answer specific rule questions, not to perform design reviews for legality. That decision will be left up to the inspectors at each event.
GDC

Look like you can proceed at your own risk at each regional

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

I think this would be illegal. From what I have read/heard/understood is this: your system must be void of all stored and working pressure when any one relief valve is opened. If there is pressure anywhere in the system after the valve is opened, the system is illegal.

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

Why not just use multiple cylinders, mounted end to end?

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

When you open the purge valves on the inlet and outlet of the cylinder you will release all stored energy. Let me draw up a sketch and I will post it shortly.

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

multiple purge valves (which are legal) sketch. You do not need check valves (which are illegal)

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

The fact remains that if you open up one of the valves (only one) ALL of the pressurized air must be released. Also, some other components, such as some flow control valves (which are legal), can act as check valves so the check valves in the schematic don't have to be actual check valves. If it was as simple as adding more plug valves, we would never have designed the other two, more complex, systems featured in this thread.

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

We had to change our design also. The rules do not limit the number of purge valves. The GCD failed to set a quantity and they do not state opening only one purge valve. They have shifted the responsibility to the inspectors. It is kind of a mute point for us this year because our competition is over. The GDC did not answer our question till two days before we had to compete.

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

ouch! I did notice a large gap in Q&A responses, what was up with that? IDK how much luck you had this year, but I hope you have better luck next year:D

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

We ended up keeping the system as a "Plan C," since the BaneBots motor on the shoulder of our arm burnt up the night before shipping, and we weren't sure if we could get the new gearbox to work at the San Diego regional. However, in testing, we did find that we needed "purge valves" on each line to the cylinder, since we would be trapping atmospheric pressure in the cylinder, preventing movement. Since it wasn't technically pressurized, it was still a legal setup. We haven't actually implemented the system in a competition. Our motor/gearbox setup has worked through two competitions, so far (San Diego and LA), so we haven't seen a need to change it out, despite the fact that the pneumatic system would be lighter and easier to control. However, this was an interesting learning experience, and I hope to implement something to this effect in next year's competition.

EDIT: I created a PDF documenting the process of designing our system. I thought you guys might find it interesting.

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

If it is trapped, any change in volume can lead to pressurization (that's why we aren't allowed to block the ports on cylinders to make gas shocks). I don't know if this affects you, but if it does, it could be important to know.

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

Thanks for the heads-up, but it doesn't really affect us. The push-button valves are just in case we need to move the arm, since it would generate a vacuum in one side of the cylinder. The check valves will allow any built up pressure to equalize with the atmosphere, but they will not allow air in. I tested the arm with a pressure gauge in each cylinder supply line, and the vent valve dumped all pressure when opened. Under load, the vacuum usually isn't enough to hold the arm up on its own, but it does make manual repositioning slightly more difficult.

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

I am just wondering how you read...

R68
D. An easily visible and accessible pressure vent plug valve to manually relieve the stored pressure (see Rule <R73>).

to mean more than one vent valve is legal? Everything in that sentence looks to be singular in my mind.

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

As per my understanding, you can have more than one vent valve, as long as there is a singular valve that will relieve the entire system. The two extra valves are only there to allow us to move the arm after we depressurize the system.

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

That sentence would make the system illegal. Any one valve would have to remove ALL pressure from the system. We had two on our robot last year (because we forgot to take one off while testing part of it) and the inspector made sure that we didn't have any pressure behind it when the main valve was released.

Again, any air trapped when any one valve is opened will cause your robot to fail inspection. Ignore our multiple warnings at your own risk.

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

Trapped air that is not pressurized beyond atmospheric pressure is legal, to my knowledge. The only thing that is making the arm difficult to move is the vacuum generated inside the cylinder, since the check valves prevent air from entering the system. The extra push buttons let air into the cylinder. Vacuums are not subject to the rules governing pneumatic systems, according to <R66>, part G. Once the vent plug valve is open, any positive pressure created by the movement of the arm is exhausted as it is created, through the check valves, and the only pressure that exists is negative (vacuum).

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

I think that is a rather liberal way of reading the rule:
and
Good luck to you when you play. I just hope you have a "plan B" ready before you pack up for your regional, however.

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

Actually, we didn't end up using this system at either the San Diego or the Los Angeles regional. We brought the system along with us, as a "Plan C," in case neither of our new gearboxes would lift the arm. Come to think of it, I should have had an inspector look at the system to determine its legality. I'll have to see about doing that at St. Louis.

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

Yes, vacuum systems are exempt but you are talking about a hybrid pneumatic/vacuum system. Therefore, the system would almost certainly be considered pneumatic by an inspector.

Also, check valves are not legal this year.

JMHO,

Mike

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

Still, it's not generating any positive pressure, which is generally what "pressure" is considered. If I were to hook up gauges, such as the ones from the KOP, to each cylinder supply line, both would read 0 psi, since they only measures on a positive scale. The mere fact that a pneumatic system is a "hybrid" in a certain configuration does not necessarily mean that it is illegal. It would be considered a vacuum-generating device when pressure is released, and a pneumatic device when pressurized. In a week, after my spring break is over, I'll create a video to better demonstrate my point.

Also, since the "check valves" that I'm using are technically flow-control valves, they are legal, as I stated previously.

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

I know what you have stated and I'm not going to argue with you as it seems that you have made up your mind.

However, for the sake of other readers, the GDC has stated that if a component contains a check valve, it is illegal.

I do not need to like their ruling and I will not defend it... However, if it walks like a duck and sounds like a duck...

JMHO,

Mike

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

The post you are linking to is from 2010. Also, flow control valves with built-in check valves are included in the KOP.

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

My apologies on the 2010 link... I hate when I do that...

I don't know of any flow control valves in the KOP this year but I'll not press it...

Good Luck...

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

<R73> The pressure vent plug valve must be connected to the pneumatic circuit such that, when manually operated, it will vent to the atmosphere to relieve all stored pressure.

Doesn't indicate positive or negative pressure, simply all stored pressure.

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

True, but negative pressure is a vacuum, and vacuums are not subject to the rules.

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

If you use 5 port 4 way 3 position valve with presure center it will hold cylinder in any position and drain all air when you open your dump valve you may have to use regulator on non shaft side of cylinder to make up for area differance of shaft to avoid drift

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

If you use a valve in which the center position applies pressure to both ports, the cylinder rod will center itself. This may be desirable in certain cases, but it will not easily allow variable positioning.

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

Originally Posted by cjwmam
If you use 5 port 4 way 3 position valve with presure center it will hold cylinder in any position and drain all air when you open your dump valve you may have to use regulator on non shaft side of cylinder to make up for area differance of shaft to avoid drift

If you use a valve in which the center position applies pressure to both ports, the cylinder rod will center itself. This may be desirable in certain cases, but it will not easily allow variable positioning.
__________________
BE A NERD!

We have a 30" cylinder on the center lift of our robot we can stop & start any where we want going up or down works great with the above valve when you open drain valve air exits both sides cylnder

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

Are you sure? That seems highly illogical, to me.

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

It would most certainly not center itself!

No matter where in the stroke it is, if the pressure on the 2 sides is equal, then the side with the larger surface area would be the side that "wins". The cylinder would extend. If a regulator were used on the other hand, then the valve would stop wherever it was.

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

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

Okay, I see what you're saying. Still, a valve that applies pressure to both ports in the middle position should only allow for three positions, unless you use an electronic regulator on one side. If you use an electronic regulator, you might as well just use the system described in the initial post - it will save weight and reduce complexity.

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

There are three states, not three positions: extending, retracting, and stopped. [there are 5 states if you include full extension and complete retraction]

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

If it's a 3-position valve that is center pressure, then you can only have three possible positions of the cylinder, without adjusting the regulator. If you use a center-closed valve, like I show in the schematic I posted, theoretically, you can have an infinite number of cylinder positions.

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

what's the difference between half and and say 3/4?

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

I think you are missing the true meaning of this...

G. For the purposes of the FRC, a device that creates a vacuum is not considered to be a pneumatic device and are not subject to the pneumatic rules (although they must still satisfy all other appropriate rules). These include, but are not limited to, venturi-type vacuum generators and off-the-shelf vacuum devices (as long as they are powered by provided or permitted motors).

The storage is what is under discussion and storage is most definitely under the rules.

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

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.

Even though my team probably won't decide to use this system, I'll bring it along with me to St. Louis, to have it looked over by an inspector.

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

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.

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

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?

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

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.

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

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.

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

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?

Yes it is center pressure valve, I started with closed center but could not dump all air from system so went to presure center. once set you do not need to adjust regulator. you can stop & start at any point. up or down the stroke with only powering silonoid on one side or the other

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

I see. Still, that is an improper application of a center-pressure valve, as my previous post stated. You could easily blow out your regulator with that setup. According the the Norgren Pneumatics Manual, the only way to properly jog a cylinder is to use a closed-center valve.

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

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.

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

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.

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

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

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

*palm--> :] *

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

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.

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

that's what this is.

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

Precisely. After all, that was the entire point of the schematic...

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

:o