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#16
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Re: Solution: Variable position of cylinder in compliance with <R74>
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#17
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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. |
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#18
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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. Quote:
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. |
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#19
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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.
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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. |
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#20
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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. |
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#21
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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. |
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#22
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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.
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#23
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Re: Solution: Variable position of cylinder in compliance with <R74>
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#24
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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. |
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#25
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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.
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#26
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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. |
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#27
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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 . |
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#28
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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. |
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#29
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Re: Solution: Variable position of cylinder in compliance with <R74>
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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. |
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#30
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Re: Solution: Variable position of cylinder in compliance with <R74>
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