this is exactly what i would call it too just my two random cents
I told you guys we needed to come up with a catchier name than 6-bar :/. If we had made a big deal of calling it “the AURA linkage” when we first revealed our robots, we probably would have got away with it.
Guys,
A word to the wise. Do some reading before making too many bold statements about multi-bar linkages and their names. They existed, were named, and were studied thoroughly, long before VRC gave you and me a chance have fun with them.
A 4-bar and other linkages are quite clearly defined by the engineers and other folks who have developed the terms currently used in the real world.
You will want to use those terms correctly so that you don’t embarrass yourself, especially in front of someone you might want to ask to reccommend you to a college or to a potential employer.
There have been 1 or 2 potentially embarrassing assertions in the recent posts in this thread… :eek:
So, keep having fun with the mechanisms, and learn their proper names so that you will truly be ahead of the folks who aren’t getting the same head start you are. 
Blake
then what would be the proper name for the so called “extended 4 bar”?
I think we all get this. I have done a bit of checking, and as far as I’ve been able to find out this thing has no other name than a four-bar that just happens to be sitting on top of another four-bar. That doesn’t surprise me, since outside of a few special situations like VEX its applications are very limited.
The thing is that this mechanism does have some significance in the context of VEX and the 30" problem. As such it’s to be expected (from a linguistic rather than an engineering point of view) that it might well have a VEX-specific name regardless of whether or not it had a properly defined name in the engineering community at large. The fact that it appears not to have a ‘proper’ name is all the more reason why it would be expected to have a VEX-specific one.
Some of us - and I may only speak for myself here - do VEX for fun and to learn new skills. I guess you could say that out of VEX I would prioritise getting employability over employment. I do hope that having done VEX might look good to future employers, but I don’t let that get in the way of the experience.
No one is claiming to be the first to have invented this thing, if that’s what you’re thinking. Yes, we invented it independently (not that there was much to invent, but it was something) and yes, as far as i know we introduced it to VEX. We don’t claim to be the first people to have ever built one.
I’m sure if anyone managed to find a reference to a name for this mechanism in the literature, people would start using it. So far, as far as I know, no one has and so we’re just going to have to try extra hard to keep ahead of those folks in other ways to compensate ;).
Guys,
I didn’t say that everyone was making a mistake.
There are many posts in this thread - I chose not to point out the specific mistake(s), and I’m going to stick to that plan.
If I were asked I would say that the mechanism is simply two 4-bars that share two links. If there is a more specific name for it I don’t know it.
In a separate, but related vein, something interesting to notice (see jgraber posts in this thread) is that it is the same sort of connected links used in scissor lifts and in cartoon boxing glove extenders. The stationary/anchored parts are different than in the lifts and boxing gloves, but the concept of connected parallelograms is the same.
About keeping feet out of mouths in the future… The point isn’t whether any given participant is using VRC explicitly to improve college or career prospects (I certainly hope that fun is the central reason and that the other benefits are a bonus).
The point is that we should all strive to form good habits early in life so that they can serve us well thereafter. Bad habits are hard to break. I know this from personal experience. I have a few I wish that I had never developed, that now cling to me like barnacles.
Blake
wow! this is a really nice thread and its loaded with info! thanks super sonic sparks for making this!
WARNING: Many large images in this post.
So, as it has been quite a long time since I have posted an update, I figured I would do so now:
As of a couple weeks ago, we have started our regular weekly meetings up again! So that means many more updates, and much more frequently, with luck. Our robot is coming along, and we’ve already finished a couple major parts of our newly designed robot!
Mainly we started work on our drivetrain, and here is what we hoped each half would look like:
http://bit.ly/ruKrqH
Also, a view of what the chain run would look like:
http://bit.ly/qDbpIs
The idea was to have two 3-Wire Motors and one 2-Wire 393 Motor powering each side. The motors would all be geared together using a continuous chain run, using 12- and 6-tooth High Strength Sprockets to gear the two Omni-Directional Wheels up 1:2 for a great increase in speed. We felt this motor configuration could still allow our 7-bar linkage lifting mechanism to preform alright, and would add some additional strength to our drive.
We used just 4 Omni-Directional Wheels because we felt the greater ease in turning and maneuverability would prove to be useful, and that being shoved from the side probably wouldn’t come into play enough to be a large issue. We will see how it plays out in our next tournament in late October.
As of now we do not have any plans to add a 5th drive-wheel to the center for strafing, but there is a possibility of being able to remove a motor off of our roller-intake for that purpose. However, the strafing wheel would have to be geared down, most likely, because moving an entire ~12/13 lb robot with a single 3-Wire Motor going 1:1 with a 4" wheel would probably give us trouble.
Now onto our 7-bar linkage. We redesigned it, as you may have seen in an earlier post, and we have since made very minor changes, but here is what we hope both of them side-by-side will look like:
http://bit.ly/pF6yHT (Too many images – please click.)
Single 7-bar linkage render:
http://bit.ly/naZssv (Again, please click.)
Finally, the photos of what we actually built:
Halfway finished pieces of the drive:
http://bit.ly/n1VnMc
Complete drive halves temporarily mounted together for PID/Gyroscopic Sensor code testing, along with what we have gotten done on our 7-bar linkages thus far:
[
Enjoy, and feel free to comment. 
~Jordanhttp://bit.ly/oBah2x](“http://bit.ly/oBah2x”)
The chain runs on the middle sprockets look a tiny bit too low. Try and squeeze more coverage out of those if you can.
Didn’t you decide after your first competition that a fast drive train was not desirable? What made you stick with the 2:1 ratio?
Fantastic pictures; Don’t forget elastic on the arm.
2:1 on 4" wheels =2.9 nominal fps, which seems fast,
but Aura claims 3:1 on 2.7" wheels (3.6fps nom).
Such small HS sprockets (12:6) is going to be a little choppy.
24:12 (higher count sprockets) would be smoother, because the variation in diameter is smaller. But its likely too late now.
You might try cad’ing the layout with larger sprockets, and reverse the chain wrap on the driving sprockets. Does that let the unused top of the outside sprockets be used as idlers, so you can eliminate your existing idlers?
You mean because the chain at the bottom may touch the ground? I’m not seeing this as an issue, as we have >1/2" of clearance between the floor and the bottom of the C-Channel.
Or do you instead mean because the chain may slip off of the sprockets? So I should try raising the outer two 12-tooth sprockets? I considered that, but if I’m remembering correctly it seemed that the chain would start hitting places, such as the idler gears at the top and the standoffs, which I did not exactly want.
Nice catch! I did say this, however we had a discussion, and ended up deciding that most likely what we needed was more driving practice, and that once we could drive more efficiently we would want that extra speed.
Thank you!
Of course – I am figuring it will be close, but that we can pull off the two-393 motor lift with just the right amount of elastic.
I was thinking of trying this. Might be good to change out before we put everything together, but I tried driving around our robot and nothing happened unless the robot made undesirable movement, such as ramming into a wall or going full speed forward, then full speed reverse. Then only the 3-Wire Motor on each side which is in the back when traveling forwards, front when traveling backwards, would skip a little. I’ll be making sure this doesn’t happen in programming, most likely. By that point the robot probably would have tipped over, if the arm were to be raised. 
Still, I may end up CAD’ing what you suggested!
Thanks so much to everyone who commented!
~Jordan
wow, nice render!
Can’t wait to see it in competition!
this = what?
Its possible to program avoiding full-forward/full-backward driving;
however, doing that makes it slightly more likely to ram into a wall.
Its harder to program avoiding ramming into a wall.
How much slop is there between the chain and the wheels?
(Hold the robot still and pull the chain back and forth,
or count on the motors to be still, and see how far the robot will rock back and forth on its wheels without moving the motors)
This can only get worse over time, as axle holes round out with use.
Using larger sprockets on the wheels gives you the ability to bolt the sprocket to the wheel, to avoid passing torque through the wheel axle.
Thank you! I’ll be sure to post video after our next tournament.
Hoping that our drivers would be able to refrain from driving the robot into the wall at full speed for an extended period of time (maybe a second or more), I was referring to keeping the robot from being able to go from -127 to 127 nearly instantaneously.
The chain is about as tight as we would ever want it. I’d be scared to tighten it further, if anything maybe loosen it, which we may have to do if it proves to be causing an issue. Or are you talking about the wheels shifting on the axles?
~Jordan
Both sprockets and wheels can shift on their axles, during hard torque reversals.
Bolting the sprockets (or gears) to the wheels removes this completely.
Now that “any threaded fastener < 2 inch long” is allowed, I’m tempted to convert to 3/16" thick chromed polished round steel dead axles for wheels.
If the chain is so tight, how can the drive sprockets skip on the chain during reversals? More chain wrap (or low-pass filter programming) can reduce that skip.
Or just ignore it if it is not a serious problem.
The chain slips because of the lessened amount of chain wrapped around those certain sprockets. A little more wrapping would probably keep it from happening almost completely, as it already only happens in one direction, but I’m thinking that it isn’t worth adding another sprocket.
Still need to finish all the programming for this (I actually just took a quick break from doing so now to post this), and we also need to finish building the robot to test and tweak the code, but we’re hoping we’ll be finished with the main portions of the robot by early October, so we should have a couple weeks to get our code working, get our drivers practice, and maybe a little autonomous code worked out… . 
~Jordan
Another update on 24C!
Our robot is really coming along. We’ve finished the drive, mounted both 7-bars onto it, and have now added our first version of our intake! The intake design was originally based off of the one on 2921A Free Range Robotics, in Clean Sweep. (http://bit.ly/ocRNiV) Mainly just because they were the first (and only, that I’m remembering) ones I had seen use the 6" Wheel Leg parts as an intake, and it inspired me to think about doing a similar thing with our robot. Then, after watching the video of 2438A (http://bit.ly/r61pNu) at the McKinley VEXhibition (http://bit.ly/np6a7s) we decided to try adding rubber bands running between them, and once we built the intake, it seems to work quite well!
By the way, here’s the first video of our new design, as we test out the intake: [24C VEX Gateway - Intake Test 2011/09/27 - YouTube
Enjoy, and as always, feel free to comment/critique or ask questions!
~Jordan](24C VEX Gateway - Intake Test 2011/09/27 - YouTube)
Nice work, Sparks!
Good job guys!