bzzt... wrong answer.

The differential idea (aka Planetary gearset) was the Thunderchicken's invention (and patented) long before Tytus even thought of it.

Such an idea would not work unless the stall torques of each motor were the same. The only way to overcome this fact is to put a anti-backdrive mechanism on the weaker motor, hence the worm gear on the 2002 CCT.


I've considered many similar designs to your clutch. Also, I would like to note that a simple search at McMaster Carr would yeld clutch bearings (aka ratchet bearings, one-way bearings). I would know because I've looked at them before for this purpose.

The bad side is: propulsion in reverse. You could only do this using the high-speed, low torque motor. Everybody knows that robots often get pinned. A high-torque reverse is a nice thing to have...

In any case, my point is that the differential design, whoever it is developed by, is different and is the subject of its own threads and discussion. Why it does or does not work isn't relevant to this thread. Thanks.

Quote:

I've considered many similar designs to your clutch. Also, I would like to note that a simple search at McMaster Carr would yeld clutch bearings (aka ratchet bearings, one-way bearings). I would know because I've looked at them before for this purpose.

I've looked at what's available through McMaster-Carr in the past and either ruled it out or flat out forgot about them when I began this design last week. I'll look again and see if those products are applicable to this concept. If they are, that's a great thing because it greatly reduces the machining complexity required and brings the design within reach of most teams.

Quote:

The bad side is: propulsion in reverse. You could only do this using the high-speed, low torque motor. Everybody knows that robots often get pinned. A high-torque reverse is a nice thing to have...

Agreed. But, remember, this is just a gearbox. There's nothing stopping anyone from attaching it to something fancy like a swerve drive or something. Additionally, if the products available through McMaster-Carr are viable (I'm concerned about the torque loads they're designed for), creating a similar system that works in forward and reverse becomes a bit less prohibitive. It's still possible in either case as I already mentioned, however. This is just the starting point and there are many improvements that could be made upon this concept.

Paul -- I am very interested in what you come up with and your suggestions.

I'll admit that my understanding of motors and gearboxes and things is a bit rudimentary. There could be large swaths of important information that I've failed to account for.

Also, forgive me for my bad comparison of your CCT to the Prius' arrangement. Where planetary gearsets are concerned, the moment someone talks about locking the ring gear in place, my mind shuts down. I've never been able to sufficiently imagine how that's accomplished. I'll go read through your whitepaper again. :)

Edit:
It seems like part #2489K14 from McMaster-Carr (www.mcmaster.com) would do the trick. The only mitigating factor might be how high above 40A the current can spike before the breaker trips -- if it's too high, above about 70A -- it'll exceed the specifications of the clutch bearing. This all assumes my drill gearing of 1:2 as well. Anything lower than that would also exceed the specifications of the clutch bearing.

Additionally, it seems like you can reverse the orientation of the clutch bearing on one side -- eliminating the usual need of having to run your drill motors in opposite directions and compensate for the difference in programming.

I'll work on a second iteration that includes this clutch bearing. The first problem I see is finding a way of mating the clutch bearing to a gear -- either by welding it (they seem to be plastic, partly) or by press fit. I really have no idea at all if a press fit is capable of transmitting 25-35Nm of torque.

Has anyone done some rough calculations on the power output of this transmission. I know from the past post that the main advantages were the lack of having to shift.

It would seem to me that you would have higher power available to you in both high and low at the output shaft using a shifter with the motors individually geared to match free speeds and all driving the output. From my first breeze over this I think everyone is saying that the Chip is overdriving the drill to go faster than the drill can when in high and the chip is essentially ideling in low. In the 45 shifter both motors are chipping in in both low and high. Shifting with the 45 design gives all of the power you could want in low (Can spin the treads!) and it is fast in high, and the gearbox is tiny by comparison.

I may be missing the whole point though??? Wouldn't be the first time.......

One Last Time....
 

I'll try to explain it one more time:

The Thunderchicken CCT is nothing like Tytus' differential idea. Please read the thread discussing the Tytus gearbox for the explanation. Tytus is using a differential with the motors connected where the wheels (on a car) would normally be. The output is where the input (on a car) would normally be. The Thunderchicken CCT uses a planetary (not a differential) and places one motor on the input and one on the ring gear.

JNadke,

The motors will not overrun each other on the CCT when both motors are powered. The reason we put the worm gear was to not overrun a motor with no power to it. During low speed mode, only one motor was running (much like what M. Krass is trying to do) and the ring gear motor was turned off. We decided to use a worm gear to HELP not backdrive the unpowered motor. In the end, we had to pulse the motor 'on' every once in a while in low speed mode in order to stop the occasional back driving. Both motors were turned on in high speed mode with the SAME torque available as high torque mode, but much more power consumed (aka battery life).

Matt R.,

I agree with your initial thoughts. I am doing some equations to show why combining motors at a compromised speed is still better than one motor optimized.

I hope this clears it up,

Paul

Like Swerve^2 :rolleyes:

Krass has a really neat design, but without doing any analysis to the design, I agree with Matt. I am looking forward to what Paul determines with regard to analysis.

Innovations like the Krass design and the Gerrish design make us all think, and that is a good thing. This is how concepting a new idea happens... we bat around ideas for a while until someone hits a winner.

Andy B.

Okay, I did some of my own very special, hackneyed "I think I know what I'm doing" math.

This is the conclusion I came to when comparing this design to 45's shift-on-the-fly transmission. I assumed different gearing for this comparison such that the output RPM in high and low matched those of the Technokat's design. The gearing I have now is far slower, but correspondingly more powerful.

In low, assuming the drill gearbox is in *high* rather than low (to achieve 375 RPM), with a 4:1 reduction, the output is 50.8 Nm. 9% more powerful.

In high, with the Chiaphua geared down to 1500 RPM via a 3.6:1 reduction, the output 9 Nm. 23% less powerful. The transition between motors would also overload the breaker on the Chiaphua motor.

In high, with the Chiaphua geared to 1000 RPM (67% of the Technokats top speed), the output is 13.5 Nm. While more powerful than 45's extreme end, it's still 23% less powerful at its own top speed.

All of these numbers are skewed because 45's whitepaper only gives the RPM under load conditions. My RPMs are assumed for no load. So, take the numbers with a grain of salt and assume that, for all speeds, 45's shift-on-the-fly transmission is more powerful by as much as 30%, I'd guess. Andy, can you give an idea of what the no load RPM of high and low is? I'm interested in seeing how far off I am.

So, that poses questions about the viability of the concept. Teams already have access to the Technokat's whitepaper for a more powerful gearbox and drivetrain.

My immediate conclusion is yes, it's still viable. Mostly, it's because I spent some time fleshing this out :). But, also, I think there are still two distinct advantages.

• It's still an automatic transmission. It will react to increased load on its own, it doesn't require pneumatics or additional motors, and, though less powerful, the moments saved could be crucial. It also seems to reduce the risk of popping breakers because it'll automatically distribute the load to the right motor.
  
• It's easier to manufacture. Given Jeremy's mention of the clutch bearings available at McMaster-Carr, I think that manufacturing this, after it's redesigned, will be within reach of most teams with access to a manual mill. There's really no complex machining work left once the spiral clutch is eliminated.

It has its advantages and disadvantages, I suppose. While not as powerful or fast as I'd hoped, it is a simple solution that seems like it could, at the very least, be competitive with the Technokat's and other's transmissions.

Again -- thank you everyone for offering your insight into this. Threads like this are the best kind here on CD because we can all learn from one another's ideas, successes and failures.

Actually, I like your idea of using a coupling as a clutch. It allows you to transmit more torque than a clutch bearing. I was merely trying to point out that there is more than one way of doing what you wanted to do.
Really, I've never ordered those, so I'm not sure if they could be adapted to work. You might be able to weld a gear around the clutch bearing, but it would probabaly damage the internal workings. I'm not really sure.

I understand that it's the first design, but you'd probabaly want to make the coupling smaller.

I'm sure it'd probabaly be the loudest robot on the field though with the constant clanking. :)

I thought about designing a belt-driven CVT, but the physics involved would be very complex, too much stuff that needs to be very precise. Basically, a belt-driven CVT works off a very similar principle as your clutch. You have a spring that applies a known force to half of a v-belt pulley. The other half is mounted permanently. Basically, as the robot goes faster, the half begins to separate (applies a centripetal force to the spring), thereby decreasing the radius. With that said, you can achieve anywhere between a 1:2 and 2:1 gear ratio.

I'm not sure if such an idea has yet been implemented on a FIRST robot, but I'd be willing to work through the math and find the correct parts if I had someone to draft it up for me :rolleyes:

From what I understand of the Prius, the CVT in the Prius is used to split the torque between the wheels and the generator. Their are three modes of the prius:
In the cruising mode, the CVT splits the engine torque between the drive shaft and the generator (generators induce a negative torque while they generate electricity). The generator, in turn, charges the batteries of the vehicle.
In high-acceleration mode, the CVT splits the engine toruqe between the drive shaft and the generator as before, but the electricity generated is sent back to the drive shaft where it powers a motor.
In low-speed and reverse, the vehicle is powered by the motor only.

I could be wrong, but I read a lot about it, and this is what most websites say. From a Physics point of view, this way would make sense.

I agree with the above posts. It is definitely a cool idea, and something that has never been done before. But, I also agree with what Matt said. I see no reason why building one of these is more beneficial than building a simple shifter. The Technokat style dog shifter is great, but somewhat difficult to make. But, anyone with a mill and a lathe can make a "pin style" shifter similar to what we made this year. *shrug*

It's certainly a cool idea though.
Keep 'em comin'!;) :D

John

M,

Your numbers are similar to what I would have thought. However, one of the biggest advantages to this design is be it's efficiency converting electrical power to mechanical power compared to the "averaging" design.

Do you have any way of figuring that out, without actually building one?

I don't know much of anything about anything when it comes to gearboxes, but I do know that when I was on RAGE last year we looked into constructing a "belt transmission" because it would allow us to achieve a _really_ high speed in a short period of time, with a near seemless journey there. So, my question.. shouldn't a belt transmission be geared towards robots that need to be really fast more than to robots needing to be "strong?" Throw something at me here, so I know what to think.

I don't think those are belts, the pulleys are drawn too narrow. Those are chains & sprockets.

Depending on the belt, chains and belts are nearly identical in the concept and function. The difference is that chains are typically more durable and can transfer more torque. If you use v-belts, they usually can transmit a certain amount of torque before they slip (slide without rotating the pulley), which would be a good thing if you have a very high-geared robot and you don't want the breakers tripping.

Other than being lighter and easier to assemble/dis-assemble, belts have no significant advantage over chains & sprockets. Of course there's the whole slip thing, which is good when you're dealing with combustion engines (stall = bad). On robots, however, we seem to like to stall our motors with all the pushing (since nobody has designed a true CVT, yet), so I guess this could be considered a bad thing.

Actually, 190 had a toroidal CVT in 2002, and was planning to use it again in 2003, but it had to be left out due to weight and time considerations. Also, I heard that 222 had a CVT designed and built this past year, but their robot design didn't need it. The hard part with a CVT is not the actual design and build, it's the programming. There is a fine balance between not reacting fast enough and occilating wildly that is very hard to overcome.

There is a good description of toroidal CVTs at http://www.barloworld-cvt.com/varibo...based_cvt.html

Interesting... didn't know a true CVT had been built before. I thought about a toroidial CVT, but then you have the problem that you brought up (control of gear ratios). Also, you have the issue of torque transfer between two non-interlocking metal surfaces. How did you guys solve this problem? I suppose you could use a high COF material and use acid-etching to make the metal surface porous. Or am I making this problem larger than it is?

If I remember correctly from the explanation given to me at 2002 Nationals, the toroids are polished metal, and it works flawlessly. Very high Mu, they told us :D