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  #61   Spotlight this post!  
Unread 21-11-2003, 15:24
KenWittlief KenWittlief is offline
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Another easy solution to the SW control issue.

The driver will know when he is in a shoving match - you could have a button or use the trigger on the joystick to powerdown the high speed motor (give it some power, but not alot)

this then becomes an automatic transmission most of the time - when the bot is not pushing an obstical, you can apply full power to both motors from a standstill, and the bot will acellerate out of the low gear range quickly, and transisition into 'high gear' smoothly

and then when the driver wants to really load up the high torque motor, he can squeeze the trigger or button, to keep the high speed motor from over heating

kinda like pulling your auto tranny into low when you are going down a hill, or plowing through deep snow.
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Unread 21-11-2003, 15:31
KenWittlief KenWittlief is offline
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Another possible way to implement this

if you want 4 wheel drive - put the drill motors, geared way down, with the one way clutch, on one set of wheels (lets say the back wheels)

and the high speed motors on the other set

when you are moving around quickly you dont really need 4 wheel drive

and when you are pushing someone, most of your weight is shifted to the back wheels

might save on some of the complexity of the mechanics.
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Unread 21-11-2003, 15:40
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Yeah, I'm glad I found this tread, it has a lot of good information, but I have worked on a design and have finished it today! But there are a few problems, I haven't been able to decide a way to make the planet gears to stop when I need them to, but I have a pretty good idea of how it should work, and it also relies a good amount on making the ring gear rotate by friction, because at some points you must let the planets rotate freely and such. The biggest problem is that one motor must overcome the resistance put forth from it's opposing sun gear, this could cause problems in the electrical system. But the two sun gears are where the power comes from, and then transfered to the planet gears, and then to the ring gear. The ring gear is the gear that will be attached to the rest of the drive system. I'll post the link as so as I get it uploaded.
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Unread 21-11-2003, 17:08
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Think Power...

There is a ton of good thinking in this message thread...

...but... there is another ton of sloppy thoughts in this message thread.



Too much to deal with on a case by case basis, so I will leave it as an exercise for the student to sort it all out.

but I do have a thought I two I would like to share.

#1 No one discussed the fact that the drill transmissions themselves have DOUBLE ACTING one way clutches in them.

The motor can drive the output, but the output cannot drive the input. Using such devices on two inputs to a planetary gearset (or its equivalent -- any of a family of devices with equations of the form w1 + (R2*w2) + (R3*w3) =0 where R2 & R3 are allowed to be negative), you can avoid the some of the problems that Paul has had with his Thunder Chicken Tranny -- you don't need to use a worm gear drive for example.

#2 People who design robots really should think in terms of power. The speed and torque of a motor are more or less just an accident of birth. Given the right amount of power, you can always use a ratio to trade one for the other.

Thinking in terms of power you can see the advantages of using 2 motors to drive a differential even though the input torques to the two motors have to balance -- the output speed doubles at any given torque -- this is another way of saying that the POWER doubles -- if you pick your ratios right, you can do twice as much work per second. This is often a good thing.

#3 To my mind, the main reason for multiple motor drives is to increase power. If you want to a very different speed/torque curve (more speed now, more torque later) I think you will almost always be better off with a shifting transmissionrather than motors that engage and disengage, especially if you want to use the drill transmission as your shifter.

Relatively robust systems can be made tolerably easliy if you are willing to live without shift on the fly. Using multiple motors makes using the standard drill tranmission a bit more tricky but not as difficult as rolling your own shifter (imho). CVT's and other more complicated systems quickly reach the point of diminishing returns.

I have more to say, but that will have to do for now.

Joe J.
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Unread 21-11-2003, 21:37
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Hey Joe, I really want to hear more about your thoughts on drive trains. The members who were a driving force in design last year graduated, and now we have people that are creative and innovative, but we need some of the skills, our team sponsor/teacher is a genius when it comes to this stuff, but Chainsaw is only one man. By the way here is the link to my design, planetary gearset.
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Unread 22-11-2003, 18:21
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Complicated issue...

Ivey,

I am really sorry that I have more or less give up monitoring forums, mainly watching from a distance and only once in a while writing a reponse.

I love these forums and would like to be more involved but between my family, my church, my work and FIRST, something had to give in my life.

Now to your point. I can't really go into the detail that is needed, but I will say that robust drivetrains are the most important part of a robot. They are really worth all the discussion this thread and others have put into them.

That said, I have really become a believer is simple simple simple drive trains (and this from a former 4 wheel drive, 4 wheel INDEPENENTLY driven swerver advocate -- to be honest, I have a 4 wheel swerver, 4 wheel drive SHIFT ON THE FLY design sketched up that I had been noodling with as a future Chief Delphi drive train). The K-3 rule and the push-a-thons that FIRST has been giving us lately have really forced me back to basics.

For my money, a simple geartrain with a chain drive is a real winner.

As to chain, if you are careful, you can usually use #25 chain, but, I have become a big believer in #35 chain of late (based on our robot last year and based on 3 years of "OCCCRA" robots). It is SO easy to work with and it lets you get away with SO many mistakes and SO much abuse, that I wouldn't even consider #25 chain for my drive system.

As to the number of wheels and orientation of them, this is very game dependent. All other things being equal, I like all my surfaces that touch the ground to be powered. But, I will say right now, I am not a big fan of treads, though people of good will can disagree on this point (for me, I think there is almost always a wheeled solution that is simpler and more robust -- but I have already admitted my bias on this point). Back on driven wheels vs. casters, depending on the game (and especially the floor), casters can be an excellent solution (espcially if there is not a ramp).

As to 8 wheel drive (4 on a side) or 6 wheel drive (3 per side) or 4 wheel drive. I confess to be totally in love with 6 wheel drive with the middle wheels lowered by 1/4 to 1/2 inch (the machine rocks). This system is a good compromise in my view between 4WD with the wheels in the corner (which can lead to a very difficult machine to turn -- especially if you have grippy tires -- one year we had a 4WD machine that would not turn at all unless one wheel was powered in reverse, full power to one side with the other side off STILL drove the machine in a straight line) and 8WD or NWD (N>6). The ratio of distance between the sides and the distance between the 4 wheels that are touching the ground is very important for determining how much scrubbing occurs when your machine turns (this can be managed with omni-wheels but that is another topic). Bigger is better. This is why I advocate lowering the middle set of wheels on a 6WD system -- it effectively doubles this ratio over a similar 4WD machine.

As to types of wheels, I am a fan of pneumatic wheels now that they are legal. 8 inch "Mountain board" wheels worked pretty well for us and a number of other teams. They typically come with a 2 piece hub that has a bolt pattern that makes mounting a sprocket pretty much a walk in the park (the hub itself is your hole template -- how hard is that?)

As to transmissions, I am pretty much agnostic on the whole roll your own vs. use the drill tranmission controversy. Good arguments can be made on both sides. For me it comes down to putting your resources where they have the biggest impact. If you simply cannot live with the package space taken up by the drill transmissions OR you are convinced shift on the fly is your ticket to Atlanta this year, by all means roll your own transmission. BUT AGAIN, only make it as complex as you need it to be.

I don't think I speak too braggingly to say that I have designed some pretty sweet FIRST gearboxes in my day and I have never had to use anything other than a 2 or at most 3 flat plates to hold the bearings for my shafts. I pretty much use straight spur gear transmission. Most of the complexity of my gearboxes comes from acknowledging that I am not smart enough to always get my ratios right (sometimes, I the game is just not what I thought it was so I need more torque or more speed or both -- if I need both I need a shifter OR more likely I need another motor). The complexity this drives is that I have to think hard and have BERG, Small Parts, Stock Drive Products & Mcmaster catalogs open so that I can make sure that I can do 2 things: buy my gears off the shelf (thus the catalogs) AND change ratio without having to change my gearbox plates (this is the hard thinking part). If I can, I try to put a relatively large set of gears (not wide, but large) no more than one stage after the motor. I attach these gears the shafts using trantorques. I know this will shock a lot of folks, but I love them for 2 reasons #1 The only modifications I have to do to off the shelf gears is to put in the right size hole for the trantorque and #2 I can change my gear ration in a few minutes.

If you can use the drill transmissions, I think that it would be hard to justify not using them, especially since they went to the 1/2 inch drill output.

One final point is that before you decide you just HAVE to have that multi motor drive system, make sure you are not using the cross axis helical gear system FIRST provides or that you are using some other inefficient stage in your gearbox (if you have bevel gears or worm gears or -- to a lesser extent -- a home brew planetary gearbox or differential in your drive system THIS MEANS YOU). The best you are going to get out a cross axis drive like the ones in the kit is something like 70% effeciency. If you can get that stage out of you drive train it is almost like a 50% increase in output power to the wheels. This often makes the difference between willing a pushing match and constantly popping breakers.

Well... ...this message has definitely gotten out of hand.

I will end it now but again, not because I don't have more to say but because time is short.


Joe J

Last edited by Joe Johnson : 22-11-2003 at 18:34.
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Unread 23-11-2003, 09:27
KenWittlief KenWittlief is offline
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I thougth of one limitation of the transmission idea presented in the first post.

When you take a 10 speed bike and roll it backwards, the one way clutch engaged, and drags the pedals around backwards.

so what this means is, not only would the low rpm motor not be able to apply a force in reverse direction, it will also limit that max reverse speed

because the low rpm motor will have to spin backwards (powered) to allow the clutch to rotate backwards

so if your low speed motor is geared down to have a max speed of 1mph, then that is the fastest you will be able to go in reverse - even though it is only the high speed motor that can move the robot backwards - the low speed motor will have to be spun backwards to 'get out of the way'

this could easily be fixed by holding the clutch open mechanically or pnuematically - but then you have implemented a shifter

and if you start down that path, you have to do a tradeoff study against a normal (shifted) two speed transmission.
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Unread 23-11-2003, 22:56
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Re: Complicated issue...

Quote:
Originally posted by Joe Johnson
As to transmissions, [...] I pretty much use straight spur gear transmission. [...]

If I can, I try to put a relatively large set of gears (not wide, but large) no more than one stage after the motor. I attach these gears the shafts using trantorques. I know this will shock a lot of folks, but I love them for 2 reasons #1 The only modifications I have to do to off the shelf gears is to put in the right size hole for the trantorque and #2 I can change my gear ration in a few minutes. [...] Joe J
OK Joe, NOW you got my attention... I love trantorques, too. I used them at Huron in the CDI two years ago. The idea of using them in a gearbox for ratio changes is intriguing.

But I'm missing something here. HOW do you change a gear ratio in a two plate gearbox without having to drill new holes in the plate??? Changing a gear to a different tooth count changes it's diameter, which affects the spacing between the shafts and changes everything. Can you please describe (or show an image of) your "floating gear" mounting to allow diameter changes to still engage its neighboring gears properly without a new plate set?

BTW, when you make a gearbox, do you make different versions for the two robot sides so that clockwise on a motor always drives the robot in the same direction instead of having to reverse power to one side? We've been noticing "arcs" in our robots from gearbox 180 degree rotations because forward and reverse motor behaviors are different for the same power level. If you ARE making different versions per side, how are you implementing it: via a jackshaft, an extra gear to reverse one side, a double sided output shaft, or what?

Thanks!

- Keith
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  #69   Spotlight this post!  
Unread 23-11-2003, 23:04
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I appreciate the continuing discussion in this thread.

As for changing ratios using a trantorque -- wouldn't that simply mean that you can change to any ratio that maintains the same center-to-center distance of your initial gearset? Maybe there's a better way, but that's what I interpreted that to mean.
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Unread 24-11-2003, 08:45
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Trantorques and Biased motors...

As to keeping the center distance, M. Krass is right, all you have to do is keep the gear set such that the gears add up to the same number of teeth.

I often refer to it as "flipping a tooth" from on gear to another. For example, I may use 13:47 and 14:46 on the same center distance.

The problem with the above scenario is it is not much of a ratio shift in this case and more importantly, they are gears that are almost certainly not available from PIC Design, W.M. Berg, McMaster, Stock Drive Products, or Small Parts Inc. (In addition it is a bad example because unless the module is huge -- meaning the D.P. is small -- a 13 Tooth gear is not likely to fit on a Trantorque).

While I am thinking about it, I always add a bit (5-10 thousandths of an inch) to the theoretical center distance. Backlash will almost never hurt you. Gear tooth interference from out of round gears is a killer.

Back to the changeable gear ratio gearbox, it is not as simple as making sure that Trantorques fit in the center of the gear and that off the shelf gears are available. You have to really layout the gearbox to make sure all the gears actually package in your gearbox. Sometimes I have to move boltholes between the plates so that they miss the gears in extreme cases. It is easy to put the holes in place while I am making the plates. The key is to plan them from the start not to have to add them after the fact. You also sometimes have to reduce shaft diameter or move a motor or whatever in order to make sure all your gear combinations can be accommodated.

Now for this whole, motor bias issue. Essentially this is when a motor performs differently in one direction vs. the other. When you have a robot drive system that has the right and left sides mirror images of each other, the motors drive in opposite directions when the robot goes forward or backward. This leads to a robot that wants to turn (especially at the start).

Yes I agree that motor bias is real, particularly on drill motors. Where I disagree with folks is where the fix should be. Many people feel very strongly that it should be fixed by either putting another gear stage in one of the motors to flip it's direction or doing something else to make sure the motors turn in the same direction.

My take on this is that if this is easy, be my guest, but if it is driving ANY complexity at all, it is not worth the bother. Given a competent driver and drive time, it is very easy for a driver to learn how to compensate for this problem. Alternatively, in 2 lines of code, you can have the computer compensate somewhat for this effect. Many folks even go so far as to use the Yaw Rate Sensor to allow the computer to fix this problem.

Last year, for whatever reason, we had one side that was a bit less efficient than the other side; this made the machine want to go in an arc. WE LEFT IT ALONE. Our driver was comfortable making the corrections himself and we were comfortable working on other, more pressing problems.

I am sure that I will get a flood of zealots on the other side of this issue calling me a heretic, but... ...so be it. I have to call 'em how I see 'em.

Joe J.

Last edited by Joe Johnson : 24-11-2003 at 08:48.
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Unread 24-11-2003, 09:03
KenWittlief KenWittlief is offline
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we used the yaw rate sensor last year to close the loop on steering - it worked so well I cant believe we never tried it before

and it only takes two lines of code to implement.

You take the difference between the joysitck X input, and the yaw rate sensor output (the difference between what the driver is telling the bot to do and what it is actually doing)

mulitiply the differnce slightly if you want tighter response (we used 2X)

then use this difference signal in place of the joystick X axis command - it works SO nice - the robot responds like a servo.
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