|
|
|
![]() |
|
|||||||
|
||||||||
![]() |
| Thread Tools |
Rating:
|
Display Modes |
|
#16
|
||||
|
||||
|
Re: Mulit-postioning with pnuematics this year?
Quote:
It works. Believe me. Look at my sloppy diagram for more reference. My team wouldn't have dared built a lifting mechanism like that if we couldn't control where we stopped it. Manipulating the boxes with accuracy was a key last year. Any questions just ask. I really want to see the capabilities with a rotary actuator. Think 45 degrees. |
|
#17
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
I'm beginning to get an idea of what you're saying. Basically the system would be this..
Air tank -> Double Selenoid Valve Air tanks go into a Double Solenoid Valve. 1) Double Solenoid input/output hooked up normally to cylinder 2) 2nd input/outputs connect with a T connector to the input on the single solenoid. Single Solenoid: 1) One input/output is stopped with brass fittings. 2) Exhaust is outputted as normal 3) Stop in the other port (loop I don't really understand). So basically when you get the two valve's to work they limit the airflor of each other, correct or incorrect? |
|
#18
|
||||
|
||||
|
Re: Mulit-postioning with pnuematics this year?
You're getting a pretty good idea. Yeah it is airflow but air flow is due to difference in pressure so you're are equalizing pressure too.
Notes: I blocked both of the exhaust port of the single solenoid so I could switch the side of the output I wishes to use. My loop simply refers to a piece of tubing that uses a quick connect to the solenoid out but is connect to a quick connect tee with a piece of tubing connecting to the other part of the tee. It is lighter than brass fittings and easier to switch sides rather than a stop (but a stop would work too.) The exhaust isn't routed normally. Both are blocked. The open output is were the exhaust from the cyclinder goes through when you want the cylinder to move. Your basically correct. Ask question if you have any. A great way to perfect it is on a pneumatics board that way you can see how the system works. Seeing is Beleiving. My mentor hardly believed it when they saw my pneumatics board and it changed completely how our robot was built. Good luck. |
|
#19
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
I'm wondering if you have any close up pictures of the valves from last year. I have an idea of what they were like, but I don't fully remember the function of each port on the solenoids. That's why i'm having a problem grasping the concept actually, i so far have a firm understanding of what you're saying and it does make sense, but I'm the type of person that needs a visual to have it totally click. That, or maybe my mind just isn't thinking right because it's 3:30am. If you have aim, msg me at RFosgate15Subs, I'd really like to talk about this more.
|
|
#20
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
Quote:
|
|
#21
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
after staying up all night and suffering from lack of sleep, i finally figured out a way to make it work flawlessly, oh happy day!
|
|
#22
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
This is an excellent series of Posts.
I had never thought there was a way to do partial stops on these pneumatics. Leave it to a group of FIRST students to prove me wrong. I think I will have our pneumatics team build some circuits and play with it to see how we can use this new knowledge. Being able to clock the rotary pneumatic - that would be excellent! ![]() |
|
#23
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
the festo solenoid seems to have two extra contacts, is this because it inherently can close the exhaust?
|
|
#24
|
||||
|
||||
|
Re: Mulit-postioning with pnuematics this year?
Are you talking about the festos from 2003 or 2004? I'm more familiar with the 2003 but I'm setting my 2004 pneumatic board up tommorow. From the 2003 I know I had to block the exhaust of the Festo to keep it from leaking through when I would stop the one side. I not sure if they can inherently close the exhaust. If you are talking the 2004 Festos, disregard this post. I get my chance to work with them tommorow after taking a week to basically design a drivetrain/chassis.
|
|
#25
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
Quote:
![]() |
|
#26
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
Quote:
|
|
#27
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
This is still an excellent series of posts and the schematic described here will work. BUT, if the system is not properly controlled, part of the pneumatic system is operating above 60 psi if your system regulator is set at 60 psi.
Below I will try to explain some of the basics of the pneumatic system and why I say that. WHY 1? To hold a cylinder mid-position, the forces on either side of the actuator must be equal. This force is a function of the pneumatic pressure, but the pressures will be different. The pressures do not ‘equalize’, the forces on either side of the moveable inner plate equalize. WHY 2? One end of the cylinder has a larger surface area (working area) than the other. The rod end has a smaller surface area because of the rod, and this reduces the working area of that end. If you read the pneumatics table in the book, you will notice an “extended” and a “retracted” force. The working area is why they are different. WHY 3? Lets use a 2” cylinder as an example. On this cylinder, the major diameter is 2”. The rod diameter is 0.625”. Major end: 2” dia = 1” radius, the area is pi x (radius squared). 3.14 x 1 squared = 3.14 square inches. With 60 psi (pounds per square inch), the force of the cylinder is 3.14 x 60 = 188 pounds (as listed in the pneumatics manual). Minor end: The major area is the same – 3.14 square inches. However, the extension rod reduces the working area. The rod diameter is 0.625”, so the radius is .3125”. Using the same formula for area, the area of the rod is 0.307 square inches. To get the working area of the minor end, you have to subtract the rod area from the major area. So, 3.14 square inches – 0.307 square inches = 2.83 square inches. With 60 psi, the force of the cylinder on this end is 2.83 x 60 = 170 pounds (as listed in the pneumatics manual). This is why the cylinders we use will have more force in one direction than the other. And this is why part of the pneumatic system can be higher than 60 psi (the legal limit) when you use the solenoids to do partial stops. WHY 4? Assume the system regulator is set at 60 psi. and the cylinder is (somehow) - half way extended. On the major end, the force applied is 188 pounds (60 psi acting on 3.14 square inches). When using the cylinders normally, with the solenoid vents opened, this force will push the air from the minor end of the cylinder and the rod will fully extend. But in this situation, the vents are closed, so the air cannot escape. The rod will continue to move until the FORCES acting on the internal plate surface are equalized. This only happens when the air pressure on the minor end of the cylinder has increased to counteract the smaller working area. The movement of the major end of the cylinder will compress the air in the minor end – remember the minor end is a closed area because the solenoid vents are capped. The Major End will continue to extend until the pressure in the minor end reaches 66.5 psi. WHY 5? The minor end of the cylinder has a working area of 2.83 square inches. For this end to produce 188 pounds of force, it will require 66.5 psi (188 divided by 2.83 = 66.5 psi). This pressure is generated by the compressing action of the major end of the cylinder moving and reducing the area in the minor side of the cylinder. By closing the vents, you have created a system that will operate above the maximum allowable pressure (60 psi) by 10%. SOLUTION: You can still utilize the setup described, but you must reduce the main operating pressure in the system to do so. If you want to be able to stage the extension of the 2” cylinder mid-extension, you must limit the system pressure to 54.1 psi. WHY 6? The pressure on the minor end becomes your limiting factor – you need to keep it at or below 60 psi. Using the 2” cylinder At 60 psi, the minor end of the cylinder can produce 170 pounds of force at a 60 psi working pressure (2.83 square inches x 60 psi). To be able to hold the cylinder mid-extension, you need to limit the force of the major end to 170 pounds – remember that the FORCES must equal, not the pressures. So, to limit to 170 pounds with a 3.14 square inch area, the pressure must be limited to 54.1 psi (170 divided by 3.14). For the other two cylinders in the kit: 0.75” Cylinder with 0.250” dia rod – 26.5 pounds Major, 23.5 pounds Minor. The working area of the minor (rod) end is 0.392 square inches x 60 psi = 23.5 pounds. The working area of the major end is 0.441 square inches. 23.5 pounds divided by 0.441 square inches = 53.3 psi. 53.3 psi is the maximum allowed system pressure to allow you to hold the cylinder mid-extension and keep below the 60 psi maximum allowable in the closed end of the pneumatics. 1.5” Cylinder with 0.437” dia rod – 106 pounds Major, 97 pounds Minor. The working area of the minor (rod) end is 1.61 square inches x 60 psi = 97 pounds. The working area of the major end is 1.76 square inches. 97 pounds divided by 1.76 square inches = 55 psi. 55 psi is the maximum allowed system pressure to allow you to hold the cylider mid-extension and keep below the 60 psi maximum allowable in the closed end of the pneumatics. It has been awhile since I sat in a fluids class so if I made a wrong statement here please advise and I will correct it. |
|
#28
|
||||
|
||||
|
Re: Mulit-postioning with pnuematics this year?
I'm not sure about the numbers, I'm not in fluid mechanics until next year, but I understand the concept. I still would like to re-read some of the pneumatic rules and I am still not sure on my personal thoughts as to it breaking FIRST rules, i.e. safety, or the legality of the rules. I will for sure look into this more in-depth. I know the 60 psi has been broken before i.e. robot with pneumatic arm running into wall or other robot, but I'm not sure how FIRST would take this. I see them as taking it on only if it poses a safety hazard and I know my system did not. If I was unsure of it's merits safetywise I would make changes to my current robot and not consider using it in its full capacity this coming up year. I take your comments with great seriousness because as last year rules guy I know there importance. I have on questions for you though. If the system ran in full capacity as it is now, with this possible extra compression taking place, what would be your thoughts about its safety, FIRST legality and intent, and any other you think I should consider?
The only reference I see to this says: "Working" air pressure on the robot must be no greater than 60 psi. I do not see the exhaust air that is being compressed aka greater than 60psi as "working" but as being "worked on". By this definition I see my system as being FIRST legal and will take it that way until I get a chance to ask them the question. I would still like your thoughts though. Last edited by ngreen : 18-01-2004 at 23:44. |
|
#29
|
|||||
|
|||||
|
Re: Mulit-postioning with pnuematics this year?
In my opinion, the system probably safely operate above 60 psi. The presssure between the tank and the first regulator valve is 120 psi, and we use similar components (especially the tubing) through-out. The 60 psi limit is probably both to add a safety factor in the system (a blown hose or other component could seriously injure a student) as well as put a limit on the power of the pneumatics. Remember the commetns from the kick-off - don't turn into a lawyer. Your goal should be to keep the system under 60 psi and it can be done just by reducing your regulated pressure.
Operating through the exhaust like you are is probably not significant, but I thought it should be mentioned. One difference in this design circuit vs. running into a wall is that running into the wall is unintentional, but building the pneumatic circuit is intentional. I had three reasons for the post - one is that I laid out the schematic and it looks like it would work and I think it is great that studetns have found a way to do this, another was to clarify / correct a few statements about pressures and forces to be sure others walked away with the right concepts, and the third was just to be sure students were aware that part of the system had to be over 60 psi to make this work (if the system regulator was set at 60). Please post any info you get from FIRST. And good luck in fluids. Last edited by Chris Fultz : 19-01-2004 at 00:01. Reason: added statement |
|
#30
|
||||
|
||||
|
Re: Mulit-postioning with pnuematics this year?
Thanks. I agree with this post completely but my only problem is I had planned to use this system with a 2" cyclinder to possibly hang. The plan was to use the retracted force, 170 pounds and the 60psi. This will cause the systems pressure to drop by 12psi and will put the force on the border of 130lbs, enough to lift the robot. Reducing this pressure to much would probably limit this ability and I would have to change to extended force to have enough power. I had reasons why I want use this system but I imagine if FIRST rules against this I can re-engineer a workable plan. Thanks for your insight into this. I had honestly never put any thought into this. I knew the compression stops the pistons (equalizing of forces not pressures) but I had not thought that it would be compression past 60 psi and possibly be ineligible. I'll try to post if I hear anything from FIRST. Thanks.
Last edited by ngreen : 19-01-2004 at 00:20. |
![]() |
| Thread Tools | |
| Display Modes | Rate This Thread |
|
|
Similar Threads
|
||||
| Thread | Thread Starter | Forum | Replies | Last Post |
| Championship Qualification - How you would've done it | Ken Leung | Championship Event | 6 | 26-10-2003 14:00 |
| Problems with this year... and a solution? | archiver | 2001 | 2 | 24-06-2002 03:23 |
| Wow! what a year | archiver | 2000 | 8 | 23-06-2002 22:43 |
| why why why...? | archiver | 2001 | 34 | 23-06-2002 22:13 |
| Making heads or tails of the new announcement... | Jessica Boucher | General Forum | 66 | 26-09-2001 11:13 |