Yeah, looking into it, interfacing a Super Shifter in there with a 45:20 gear set would be feasible. I may have to buy an extended output shaft. The ratios wouldn’t be optimal though. By tweaking the way I drive the center wheel I could also use the Gen 1 shifters which offer slightly better ratios. But that would get pricey real quick.
What is “optimal” for this robot? IMHO 5-12 ft/s is is a very generous range. Any faster and an operator can’t control it too well, any slower and virtually anyone will be able to dodge you. Just a thought.
That would depend on the game, obviously if it were a game like last year I would say 8fps. For a game like 2008? 10fps seems more than fast enough for me. There are games that I would prefer more oomph (2007) and would settle for 6fps (For reference, in 2007 the team I was on never crossed past midfield) This is one of the major reasons I like AM products, they have a wide range of options that do not change the mounting/interface options. Heck, if I wanted to switch to 6" wheels I could do so relatively easily with a simple swap of gears in the transmission.
So if I’m understanding you right 5-12fps would cover you, with a reasonable margin, for any game within recent memory :rolleyes: I like where your design is going :]
Just a little thing I gambled on this year that paid off: keep your CG behind the bumper zone, not as low as possible (assuming the bumper zone isn’t insanely high, of course). 96.3% of the time robot-robot contact is bumper-to-bumper and having your CG/center of inertia in the bumper zone will help keep your robot from flipping. I’ll let you do the fizix.
Here’s how I would design a two-speed transmission. I always try to have specific goals for my low and high gearing to “optimize” the way that I’m using the motors’ mechanical power.
LOW GEAR ANALYSIS:
Goal: Be traction limited at 40A per motor
Rationale: This will let us push as hard as we can without ever popping breakers.
Assuming the load is something like 145 lbs (robot+battery+bumpers) and you are using roughtop with a static CoF ~= 1.3, the maximum amount of force that you can transfer to the ground is about 190lbs.
Assuming 4 drive motors (CIMs), and 4 inch wheels, each motor can contribute up to (190 lbs) / (4 motors) * (2 inch wheel radius) = 95 in-lb of torque before you slip your wheels.
Stall torque of a CIM is about 21.5 in-lb (at 133A). Torque at 40A is more like 6.2 in-lb. And then there’s gear-train inefficiency - let’s say you have a good spur gearbox for a total efficiency of 90%. Now your torque is 5.5 in-lb per motor.
So if you want to be able to push ~190lbs across the floor all the live-long day, a reduction like 95/5.5 = 17.2:1 would get the job done. When not pushing, you would expect to move at a clip of about 4.5-5 fps (I generally find that 80-85% of the “free speed” of the motor, divided by the gear ratio, gives a good top speed estimate. It is never exactly the free speed because of drag in the gear train).
HIGH GEAR ANALYSIS:
Goal: Be traction limited at 75% stall (100A per motor)
Rationale: We will be able to turn in the upper 25% of the motor power curve (since we need to skid our wheels in order to turn in a tank drive). At the same time, our max speed will be very high. We don’t worry about the 40A breakers, because they won’t trip unless there is a constant (> a couple seconds) heavy draw, and we will downshift if there is a need for that…
Assume all the same things as before…4 CIMs outputting 21.5 oz-in of torque and 190 lbs of total load. But now at 75% stall and 90% efficiency, we expect ~14 oz-in of torque instead of 5.5 as before. 95/14 ~= 6.8:1. We expect a top speed of about 11-12 fps.
Obviously, the goals at each design point will vary widely depending on what you are trying to do - these are only two possible criteria. For a single speed drive, you’d need to trade off between popping breakers/being traction limited and having a good degree of speed and agility.
Likewise, the missing “last step” that I would always go through would be to compare my desired ratios with those that I can easily achieve by utilizing COTS components and my teams’ manufacturing capabilities. For example, a SuperShifter (with the last stage removed) can achieve ratios of 10.67:1 and 4.17:1. Coupled with a chain reduction of about 1.6:1, I get 17:1 and 6.7:1, which are pretty close to my theoretical goals using 4 inch wheels.
Andrew, I don’t know a single employer who would take a napkin sketch and arbitrary numbers over a full CAD given a choice between the two. It’s a shame that you’re mocking everyone who requests feedback on various frame designs while also learning.
CAD is never a crutch; it’s simply one of the many pieces to the equation of a high quality product.
There are a good amount of people who post pretty renders of designs that aren’t even fully functional in my opinion, yet they spent the time to make a pretty render and CAD arbitrary details.
I agree with Andrew, you should be able to plan what actually is going to do the business (and all the details of it) in your head before you start making pretty CAD.
I don’t think Andrew intends to go into season with a napkin sketch and machine from there. A nice worked out concept will save much time when it comes to the detailed design in CAD.
973 works the same way; we CAD 100% and machine 100% to print, but almost every system is sketched out by hand, on a whiteboard, made from paper, etc. before we begin designing. We’ll often work out ideal ratios, sizes and stages of reduction before CAD even begins; it allows much more efficient design.
I am struggling with why having a high CoG would prevent you from flipping in the event of a collision.
If anything, lowering the traction of your wheels would seem to do the most good since it would prevent your entire robot from becoming a lever arm rooted in the ground.
I think the point is that if a designer/engineer has a very clear idea of what an elegant solution is, could be, they should be able to explain or convey the needed information with simple sketches, and then spend a minimum amount of time detailing the design in a CAD program. The N-CAD skill can be invaluable and it seems as if many students are missing it.
As an example: 2 months after I was hired I was responsible for designing a number of replacement parts for a high-g-load testing machine that was not working properly. This machine was too dangerous to run in our main engineering building, so a technician and I were over an hour away from any CAD-capable computer, but only 10 minutes from a machine shop when we were debugging it. My employer was VERY happy for me to make hand sketches of parts to give to the machine shop because saved time and money on the project as the deadline loomed nearer (not to mention many miles on my car!)
If you re-read Andrew’s post as if you were brand new to these forums, you may find that (specifically) the final paragraph is incredibly off-putting to someone trying to do a CAD by themselves for the first time, not knowing what they’re really supposed to know in order to do things correctly. Andrew has exemplified the common CD knowledge paradox. Sure, all of the recent CAD prototypes are ‘pretty’, and most of them are missing details, yet the negative implications of Andrew’s tone are as common as the missing details themselves.
Think about free body diagrams. The forces always go to the center of mass (in this case, gravity), of the object. A CG directly in line with the bumper zone will create a force applied that is parallel to the ground. This eliminates the levering issue, as well as flipping.
Am I looking for a job designing gearboxes? Do I work in that field? No. But I do know that most employers would rather me take the 30 seconds it takes to draw a rough sketch of an idea to make sure it would work than spend 2 hrs in CAD.
Would you call a lot of what has been posted recently full CAD or would you call it a bunch of cookie cutter components tossed together? I would say the latter. And yes, I am going to mock them. I am ok with them learning, heaven knows I still am, but this constant stream of “West Coast Drive” CADs that are only differentiable by their color is irritating. Sure, I could have put more time into my drawing but the important parts are there in my opinion. Wheel placement (having them overhung), chain placement, approximate weight, and a reasonable gearing based upon simple calculations. (Which I will be redoing thanks to Jared’s post)
CAD used as a tool to help further a design is a tool. CAD used to assemble parts that you downloaded off the internet and then making a pretty picture without any understanding of what you are doing is a crutch.
Yes, my last paragraph was meant to be that way. I, as a mentor whose job it is to inspire (not teach) students, would be floored if we were to have dozens of students asking how to properly do calculations for drive trains or arms. They have their college years to learn CAD (and will probably have to relearn a different piece of software for their jobs). Instead they are designing systems that, in all likelihood, will not work because they made some mistake that would have been solved had they first taken the time to do a quick sketch on a white board. So, if I turn off a couple people from jumping right into CAD I am fine with that as long as it also gets some people thinking about doing the math.
I know that I posted my designs to receive structural feedback, not feedback involving gear ratios and the math involved in the traction. I’m all for sketching by hand, but there is some structural feed back you just can’t get off of a napkin.
Which would be “doing the math” would it not be? I disagree that there is structural feedback you can’t get off a napkin since the lightening of 2337’s hanging system was a napkin* sketch but I will give you the benefit of the doubt. The ones that bother me are the ones that say, “We go X fps” and then when questioned about that number are unable to tell us their ratios. To me it screams that they pulled a number out of their behind.
*Actually, I think Spaz Dad used the back of another sheet of paper
I disagree there as well. A good, clean, efficient design should have a frame that can be sketched by hand and explained in 30 seconds or less.
A frame can still be strong if it doesn’t meet that requirement, but that means it’s probably pretty inefficient and complex.
I agree that things such as lightening patterns can be done easily on sheets of paper, but more complex mechanisms such as our kicker pull-back mechanism should be done primarily in CAD.
You can make a winch on paper…
You’d be surprised at how many things can be done on paper quickly, including “complex” mechanisms like kicker pull-backs.
Let’s go back in time a few years, to the airplane and tank designs of WWII. Did they have computers? No. At least, not that were capable of running CAD programs other than number-crunching to do analysis and things of that nature. So, how do you think they designed all their engines, landing gear, treads, guns, turret-turning devices, and all that sort of thing to create the given airplane or tank?
Yep, on paper, with enough accuracy to use them to make and assemble parts that worked, a few thousand times over per part.
How about the interrupter gear in WWI, which allowed a plane to shoot through its own propeller without damaging itself? Paper, and relatively complex. And I’d be willing to bet that Mr. Fokker, if placed in the modern world and told to sketch the interrupter gear, or even create a production drawing of it, could do it faster and just as accurately as a good CADder.
The point is, while CAD is nice because it can quickly turn a part model into a machining drawing (or, with assistance from other programs, into code for a CNC to make the part), pencil and paper is just as effective in the hands of someone who knows how to use it well.
“Don’t underestimate the ability of a highly skilled technician with simple tools…” --Dave Lavery, in response to someone saying something about not having good enough tools in http://www.chiefdelphi.com/forums/showthread.php?postid=119661#post119661
How long would it take you to change the spacing of your sprockets used in the pull back on paper?
How long would it take to create a piece you can water-jet with your paper sketch? About as long as it would take you to just CAD it originally.
I’m all for conceptual sketches being done on paper, but it seems unintelligent to not utilize all of your resources (CAD).
Didn’t see this when I hit reply…
Paper and pencil is to those times as CAD is to modern times. Sure, there are still uses for hand drafting in today’s environment, but CADding is becoming more and more important. I’m not suggesting you eliminate all drafting instruction, but that you consider what will be most useful in the future.