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  #16   Spotlight this post!  
Unread 25-06-2003, 23:34
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OK take all that stuff And Turn it around . Look! this thing didnt come off of a truck! It Will never have to do what a diffrental on an automobile does, IN FACT it does the exact oppsite. the Cg model works, the Lego model works, the popcicle stick model works, AND THE PRODUCTION MODEL WILL WORK!
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Unread 25-06-2003, 23:40
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Quote:
Originally posted by Tytus Gerrish
OK take all that stuff And Turn it around . Look! this thing didnt come off of a truck! It Will never have to do what a diffrental on an automobile does, IN FACT it does the exact oppsite. the Cg model works, the Lego model works, the popcicle stick model works, AND THE PRODUCTION MODEL WILL WORK!
If you'll read the explanation I provided at the end of my post, you'll see that, assuming I've done this correctly, the math shows that what you're doing is actually less powerful than using a single motor.

It will work, yes, but it will be completely unnecessary and act as a burden! Something working on a computer is meaningless if you're not using some hefty software to run your analysis. It looks like you're using 3D Studio MAX. That's not in the same ballpark by any stretch.

The Lego version will work, but again, you're actually outputting less torque than you're inputting on a single side. The second motor/finger/whichever is dragging on the system.

There's no need to scream. I'm sorry I wasted my time trying to explain what's actually happening here. I'll remember that for the next time.
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Last edited by Madison : 25-06-2003 at 23:43.
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Unread 25-06-2003, 23:47
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what made you come up with the idea
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Unread 25-06-2003, 23:55
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Re: Re: Sorry

Quote:
Originally posted by M. Krass
If I've made some grossly wrong assumption here, please correct me. Thanks.
You're on the right track, but you've made a few wrong assumptions. I've done some work (and a lot of math) thinking of a way to use a differential in reverse as a gearbox.

In the end, I came up with several conclusions:
1. A differential is merely a planetary gearbox, designed differently.
2. A differential is less efficient due to the bevel gears needed.

With a differential, the housing is the input, with the two sides being the outputs. A differential allows either the two sides to have the same speed and torque. It also allows torque and speed to shift to one output for the sake of turning. One of the outputs could have twice the input speed, while the other has none.

Unfortunately, with such a design, as Paul Copioli said, the power of the second ("control") motor is basically lost.

In using a differential as a gearbox, you basically put yourself back at the Thunderchicken's problems, because it behaves like a planetary transmission. You cannot have two motors with dissimilar torque and speed characteristics hooked up without experiencing motor fighting. The motor with higher torque will stall the other motor and will win every time. The result is that your robot won't go anywhere. The only way to make such a design work is using worm gears. I then determined that the Thunderchicken's way of using the worm gears would be more efficient.


If you have two motors with similar torque and speed characteristics (or geared to match), you can, essentially, create a continuously variable gearbox similar to the Thunderchicken's CCT. Using one motor to spin the housing, and the other as a control on one of the side outputs, you can simulate a load on that output. Thus, it will behave normally as a differential, achieving a variable gear ratio anywhere from 1:1 to 1:2. The maximum torque is that of a single motor, while the maximum achievable speed is twice that of a single motor, without all the quirks.
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Last edited by Jnadke : 26-06-2003 at 00:14.
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Unread 26-06-2003, 00:07
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So, for my reference, which of my assumptions were incorrect?

I've said exactly what you have, but without drawing the parallel to a planetary gearbox.
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Unread 26-06-2003, 00:13
Jnadke Jnadke is offline
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Mostly the part about the power output being less than a single motor. Although, if working incorrectly, this can be the scenario, it can produce a power output that is the sum of the two motors if working correctly.


The design is hard to make out so until I can get a more detailed view, I will refrain from mentioning whether or not it will work.
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Last edited by Jnadke : 26-06-2003 at 00:17.
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Unread 26-06-2003, 00:16
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YES! THE RPM AND THE FPT GET AVREGED bUT tHE HORSEPOWER IS ADDED AND AFTER YOU take into account the fricton from the bevls wich BTY dont rotate too much Anyways I still have the most powerful robot ever!
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Unread 26-06-2003, 00:22
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Quote:
Originally posted by Tytus Gerrish
YES! THE RPM AND THE FPT GET AVREGED bUT tHE HORSEPOWER IS ADDED AND AFTER YOU take into account the fricton from the bevls wich BTY dont rotate too much Anyways I still have the most powerful robot ever!
Currently, from what I see, your design connects both motors to the output shafts of the differential (which is connected to the bevel gears), without any attempt at matching them.

Overall, your maximum speed will be an average between the two motors while your maximum achievable torque would be twice that of the weakest motor. Once it stalls, and the breaker trips, all torque will go to the spot with least resistance, which is there.


When designing things, remember that for every action there is an equal and opposite reaction. I see too many people look at the actions and not the consequences. If something can rotate freely, it will, and the device will fail.
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Last edited by Jnadke : 26-06-2003 at 00:27.
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Unread 26-06-2003, 00:35
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Quote:
Originally posted by Jnadke
Mostly the part about the power output being less than a single motor. Although, if working incorrectly, this can be the scenario, it can produce a power output that is the sum of the two motors if working correctly.
...I think what I've discerned, and probably didn't explain to well, is that such an arrangement is capable of outputting more torque at a higher RPM than a single motor would, but not more torque overall.

Also, as best as I can tell, this new output isn't the sum of the two motors, but some average.

I don't know right now. This kid has me so confused I'm practically guessing. I'll have to think about it more using real motor data and draw some better conclusions. It seems that without gearing the motors to one another at some level, while there may be a potential benefit at some point, there'd be a greater overall loss in performance across the entire operating range.
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Unread 26-06-2003, 00:43
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This seems like the sort of thing that is just easier to build and test out rather than make guesses or calculations about it's performance.

/me takes out his old lego sets
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Unread 26-06-2003, 01:25
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Tytus,

You could be on to something here. Your basic idea of coupling two motors through a differential is good and could be used in a compact manner on a FIRST robot.

However, in my opinion, you have some more work to do in order to turn this concept into a working model. I'll try to give some suggestions and clarify what Maddie and Jeremy are saying above:

1. Your 1:1 bevel gear mating of the CIM and drill motor is very inefficient. This pair of motors will run at an rpm that is higher than the 5,500 free speed of the CIM, but you need to realize that once the CIM motor gets near or over 5,500, it becomes very inefficient. Electric motors, in general, run most efficiently at about 1/2 of their rated free speed.

Since the free speed of the drill is about 20k, then you should incorporate a ratio between 3.5 and 4:1 in order to couple these two motors together.

2. Realize that bevel gears are less efficient than spur gears. I don't know the exact numbers, but I am guessing that spur gears are around 97% and bevel gears are around 90-92%.

3. It appears that you are actually speeding up your rpm's through a smaller bevel gear on your output shaft. I would suggest slowing your rpm's down. Most FIRST robots operate their wheels around 150 - 500 rpm, so you are gonna want to definitely reduce the speed of your output shaft and then do another reduction in order to get a manageable rpm.

Keep designing and good luck.

Andy B.
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Unread 26-06-2003, 01:45
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Okay, I am back after a bit of playing with the Legos. I had two identical motors hooked up to the same power source. One motor was directly driving one side of the differential. The other motor was geared down 24:17 (or 17:24 depending on how you look at it). First, I ran one side at a time with the other side motor disconected and axle locked in place. I felt with my fingers the torque of the system and listened to the pitch of the sound to indicate speed. Then, I ran the system with both motors together. The speed was more than that of the geared down motor alone but less than the direct drive motor alone. The torque of the system was more than either motor alone.

So, my guess is that this design does work. The speed is averaged and the torque is added.

As for Andy Baker's post above, perhaps use a planetary differential instead like this one

EDIT:
The way that one in the pic works is the top left and bottom right planetary gears are in contact with the sun gear which is attatched to the output on one side. All the planetary gears are in contact with each other. There is another sun gear that is not pictured that comes into contact with the bottom left and top right gears in the pic. That second sun gear is on the output shaft on the other side. I hope that helps explain it. I have the unit opened up here on my desk if you have any questions.
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Last edited by sanddrag : 26-06-2003 at 01:57.
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Unread 26-06-2003, 02:29
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Quote:
Originally posted by Andy Baker
You could be on to something here. Your basic idea of coupling two motors through a differential is good and could be used in a compact manner on a FIRST robot.

Keep designing and good luck.

Andy B.
Really, the design isn't a whole lot different from the functionality of a planetary system. The output shafts are akin to the sun and ring gears. The housing would be the planet carrier. These similarities are in terms of functionality. If you rotate both the sun and ring gear in the same direction, the carrier will rotate at an average of both speeds. The torque will be combined.

You can't overlook these similarities, because then you will easier understand what will work and what won't. Although the torque with such a system is combined, you have to see that the maximum torque you can ever exert (for an extended period of time) is twice that of the weakest motor (stall torque). If you exert more than this, the breakers will trip due to excessive current draw. All torque will be spent on spinning the (now) dead motor, instead of accelerating your robot.

I wish I had my laptop out. A few months back I was talking with a teammate about prototyping a drivetrain with a similar idea of using a differential as a gearbox. Though, I realized these limitations, which is why I went with the idea above (using two similar motors on the housing and side input).

In all practical applications, lego "prototypes" have no usefulness for a FIRST robot beyond design concept. We have various restrictions, both electrical and mechanical, that cannot be modelled accurately. The Thunderchicken's found this out last year - the hard way. They blew motors and tripped breakers like crazy, all using a similar design. You are only doomed to repeat their failures if you fail to heed my warnings.

I do believe such a design would work, and well, but not without substantial thought and time in creating a simple and efficient design.
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Unread 26-06-2003, 07:58
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Dr Joe speaks...

Wow, a lot of good discussion.

Here are some of my thoughts.

Jnadke is 100% correct in saying that the a differential is mathmatically the same as planetary geartrain.

Those of you who say, "the torques add and the speeds average" are essentially correct (assuming you account for ratios -- and don't forget the SIGN of the ratio too, sometimes you are adding in a negative number... ...also known as subtracting!).

People need to keep in mind that motors have a speed-torque curve for every voltage. For a given voltage and torque, each motor will run at a given speed, period. Don't forget that the speed-torque line extends into negative torques and negative speeds. By this I mean that if you can run a motor faster than its free speed by applying a negative toque. Similarly, if you apply a high enough torque, a motor will run backwards!

This second idea (that a motor can be made to run backwards even if it is trying to run forwards due to the voltage being applied), is why Paul C. has said you can only use a worm gear to drive your ring gear. Worm gears with low lead angles have the property that they are like mechanical diodes for torque -- by this I mean that torque can drive from worm to worm gear but not the other way around (it is not an exact analogy -- to be honest, I just thought of it and I have not noodled through whether I like it or not). Essentially, a worm/worm gear combo prevent the weaker motor from being driven in reverse.

IN ANY CASE... ...I will state again that there is no real magic to 2 motor systems. They act like one motor with a different speed torque curve. This is not to say that there are not advantages -- there are -- but that they do not allow you to make bricks without straw.

Here are my benefits in order as I would rank them:

1) Higher current limit (due to multiple circuit breakers)
2) Higher power (more torque at the same speeds or more speed at the same torque)
3) Cooler motors (for a given amount of mechanical power, you have more surface area to dissipate the electrical power lost -- HEAT-- AND you have the potential to run the motors nearer to their peak efficiency point which means that you have less heat generated per unit of mechanical power)
4) The ability to shift the speed-torque operating point between the two motors (this can be done in software by giving different voltages to the different motors OR in hardware by mechanically have load sharing as in the case of a differential or planetary geartrain OR by both software and hardware) -- note that this does not make a "continuously variable gearbox" but it does give you some properties of a CVT (provided that you don't look too closely at it).

I really need to write a white paper on this subject.

Joe J.
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Unread 26-06-2003, 12:14
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Thanks andy and joe

I like hearing this and id like to add ThAt this Is a Demonstration Not a production The Final 2:1 bevel output is to show where the power goes In the end (now that i know bevel gears are less efficvent) they will probaly be chabges to somthing else Like a spur , Or if i could mill it a helical geer , Im sorry i use caps so mych with these guys ,there useing math to prove their point But I think they left Somthing out I take what they have said And it looks like they say I am destroying Energy Wich I Obviously cant do The idea of this device is to have the motors work in harmony at any powre arc and while being slowed down wile pushing Where normel 2-motor gearboxes Fight themselves
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