|
|
|
![]() |
|
|||||||
|
||||||||
![]() |
| Thread Tools | Rate Thread | Display Modes |
|
#1
|
||||
|
||||
|
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:
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. Last edited by PAR_WIG1350 : 23-02-2011 at 23:41. |
|
#2
|
|||
|
|||
|
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. |
|
#3
|
||||
|
||||
|
Re: Solution: Variable position of cylinder in compliance with <R74>
Quote:
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. |
|
#4
|
|||
|
|||
|
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.) Last edited by Roboman01 : 24-02-2011 at 02:36. |
|
#5
|
||||
|
||||
|
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.
|
|
#6
|
|||
|
|||
|
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?
|
|
#7
|
|||||
|
|||||
|
Re: Solution: Variable position of cylinder in compliance with <R74>
Quote:
How about a mechanical stop for that 'middle' position? A tiny servo moving a locking pin/bar could manage that. |
|
#8
|
|||
|
|||
|
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.
|
|
#9
|
|||
|
|||
|
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. |
|
#10
|
|||||
|
|||||
|
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. |
|
#11
|
||||
|
||||
|
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.
|
|
#12
|
|||
|
|||
|
Re: Solution: Variable position of cylinder in compliance with <R74>
Quote:
|
|
#13
|
|||||
|
|||||
|
Re: Solution: Variable position of cylinder in compliance with <R74>
Quote:
|
|
#14
|
|||
|
|||
|
Re: Solution: Variable position of cylinder in compliance with <R74>
Quote:
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. Last edited by Roboman01 : 26-02-2011 at 10:33. |
|
#15
|
||||
|
||||
|
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...! |
![]() |
| Thread Tools | |
| Display Modes | Rate This Thread |
|
|