Direct Drive Four Motor System

[Jason Kixmiller]

Actions speak louder than words (said or typed) and a winning season speaks for itself. Whether you custom built every piece or assembled from "the shelf," having a successful season allows your performance to speak for you. Many teams have great features/machines, but arguing about the best is in many ways missing the point.

Back to the true topic at hand, I have a question:

Is it feasible for the 4 independent motors to be electronically controlled/programmed to have the same output speed?

[Ken Leung]

I fail to see the point of bragging and insisting who is better than who. Are they supposed to make people feel better about themselves or gain respects from others?

There are plenty of opportunities to prove yourself in this program, the least of which are the myriad technical awards, the chance to win regionals and Championship, not including the more prestigious Chairman’s and Engineering Inspiration awards, and the ultimate price of FIRST: the inspiration and motivation of students into Science and Engineers, as well as just being decent human beings. I would assume those are the places to compete and strive to be the best, not the brief moments you spend so much around here.

In a community that boast inspiration above all else, the evidence of which is shown here: http://www.chiefdelphi.com/forums/sh...threadid=40485 , I am very disappointed that people are spending so much time on such trivial issues. I suggest you reexamine your reason for participation in this program if you intend to continue these kinds of discussions.

Inspiration is a way of life, not a show in front of others. Quality and excellence come from within, from what you do, from who you are, not from what you look like.

Now please, go have some meaningful discussions that will inspire us all.

[Madison]

Y'know, I'd fully intended to split this thread in two -- one that focuses on the initial question at hand about direct-drive and its merits and a second that focuses on... well, I'm not sure. I couldn't think of a thread title that was less derogatory than, "Fighting over when, how, and why you should mention your past work, who decides if it's worth mentioning at all, and discussing why we feel compelled to tell people it's annoying when overdone -- repeatedly"

The point's been made, I think, and I'm certain that your criticisms of Sanddrag's behavior have been duly noted. Do I need to make y'all play dumb team-building games that make you say things like, "I liked... I didn't like... I wish you would have..."? I'll do it.

[wilshire]

any drive train that can allow your robot to move around the field in a reasonable manner, be very reliable, and allow you to complete the game objectives is a good drive system in my book. Just my 2 cents
like Mr. Bill Beaty says: "Drive train, Drive train, Drive train, Drive train"

[Matt Adams]

Originally Posted by Jason Kixmiller

Actions speak louder than words (said or typed) and a winning season speaks for itself. Whether you custom built every piece or assembled from "the shelf," having a successful season allows your performance to speak for you. Many teams have great features/machines, but arguing about the best is in many ways missing the point.

Back to the true topic at hand, I have a question:

Is it feasible for the 4 independent motors to be electronically controlled/programmed to have the same output speed?

Absolutely, but it requires feedback, such as an encoder, resolver, tachometer, and a good controls algorithm, especially to control the speed of four different axis. However, I'll ask another question for you:

It is worthwhile to electronically control the speed of each individual tire in a closed loop fashion on FIRST robots?

This is an interesting question, and there are a few distinct and divided groups on these forums about drive wheel feedback / controls in general. There are some very adamant people who swear up and down that closed loop control (position and/or velocity feedback on the wheel) is critical, others who deem the resources / benefit are too small, and still others who ask, "Closed loop? What's that?" I consider myself to be in the 2nd group.

My thoughts are merely, "Can't a good driver with some practice on the robot before it is shipped 'aim' an open loop drive system 'straight enough'?" The advantage of a closed loop system is pretty nice if you have two pairs of motors controlling two wheels on each side - you can essentially ensure that when you push both joysticks full forward, that the robot will move straight and not veer off by say up to 10 degrees or so as it might in an open loop system. Driving is more intuitive, this is a very good thing.

Correct me if I'm wrong, but this thread has sort of now technically spun into a suggestion about a 4 wheel robot with independently driven wheels, each with an independent gear box, as well as a closed loop feedback setup on each axis to ensure that the wheels all spin at the same rate.

Others may disagree, but my opinion about this can be said in a single word: Overkill.

Matt

[Veselin Kolev]

Back to the main topic,

Both the times I have done direct drive to four wheels with 4 motors, I used a very similar setup.

I had a gear bolted to the wheel and the wheels rolled freely on bearings and shafts. The gearing was as follows: small gear on the CIM output mates with a larger gear, which is on the same shaft as a smaller gear. The smaller gear then mates with the larger gear that is bolted to the wheel. EX: CIM speed of 5342 x two 12:40 reductions = 480 rpm = about 12 feet per second on 6" wheels.

The reason for the two reductions being the same was so the first gear (on the CIM, 12 tooth) and the last gear (on the wheel, 40 tooth) could be on the same rotational axis. This proved for a very small drive train, and if you mess around with the shafts and gears, you can pull it off with very few parts, meaning less fabrication.

My reason for using a gear on the wheel itself is so it can act as the final reduction, instead of having extra bearings and shafts to have that reduction inside the transmission, only to run a shaft out to the wheel.

I really like the 4 wheel 4 motor independent direct drive system, it doesnt clutter your robot with drive shafts or chains, it makes everything modular. Also, fabrication is a blast, you're making 4 of the same part for the drive trains, and the gear reductions arent that complex or anything. No CNC required (unless you have interesting cheeseholes) and I would say this is a really reliable system, very low maintainance if done right.

The only way you can go wrong is if you use a face width that is too small for the gears. Then you start snapping teeth. I used .375 on all the gears, it worked quite fine. Also, it is a must to have your wheel on stable bearings and shafts, because if the gear on the wheel and the gear on the transmission misalign, the teeth get worn down or they snap. And the last rule of thumb, use loctite! When direct driving like this, there tends to be more vibrations through the drive train (it is usually dulled by the chain in chained transmissions). These vibrations make set screws and bolts back out and your drive train to misalign.. etc. Keep your drive trains screwed together!

[Jack Jones]

Is it worthwhile to electronically control the speed of each individual tire in a closed loop fashion on FIRST robots?

In my opinion, answer is: Maybe! But before you go piling on a whole bunch of complexity, you should try to design something simple that’ll go straight all on it’s own – build it – test it – if it works, you’re done.

That’s what we did with the above example of our experience, as Collin was asking, with four-wheel direct drive. It was a low risk experiment for us. The CIMs were known to be unbiased, and they gave us four. The parts were water-cut from scrap in about two hours – including the welding jig, which made for 30 minutes to weld. The DeWalts cost $21 each. The wheels cost $17. So, with a $200 investment, the kit frame, and some assembly, we had it on the ground in two days. It went straight.

OTOH, suppose that this year’s game has an autonomous that is worth doing and requires precise positioning… Who knows! Except that one thing we’re sure is we’ll try to keep it as simple as we can.

[Alan Anderson]

Originally Posted by Matt Adams

My thoughts are merely, "Can't a good driver with some practice on the robot before it is shipped 'aim' an open loop drive system 'straight enough'?" The advantage of a closed loop system is pretty nice if you have two pairs of motors controlling two wheels on each side - you can essentially ensure that when you push both joysticks full forward, that the robot will move straight and not veer off by say up to 10 degrees or so as it might in an open loop system. Driving is more intuitive, this is a very good thing.

You're missing something relatively important in your thinking here. Yes, a good driver can effectively be the element that closes the loop, but only when there is a driver at the controls. However, a reliable autonomous routine pretty much requires closed loop software control of motor speed.

[Billfred]

Originally Posted by Alan Anderson

You're missing something relatively important in your thinking here. Yes, a good driver can effectively be the element that closes the loop, but only when there is a driver at the controls. However, a reliable autonomous routine pretty much requires closed loop software control of motor speed.

I'd qualify that a bit with "an autonomous routine that has a decent chance of screwing up."

If you're just knocking off the hanging tetra, you can go open-loop (as the only thing you have to do is drive forward a smidge and raise an arm). There's very little chance of this screwing up, as you're not going far, and the hanging tetra is a big target.

Now consider the 10-point ball from FIRST Frenzy. Tiny target, really far away. Break out the encoders, folks.

[Jack Jones]

Originally Posted by Alan Anderson

... However, a reliable autonomous routine pretty much requires closed loop software control of motor speed.

Please don't tell our robot. We took the encoders off the drive wheels and mounted them on two boggie wheels about 6 inches each side of dead center - got better distance measurement that way. Direction was not a problem. Ask the folks at Rah Cha Cha or Novi - it would cap the center side goal dead solid perfect, or, with the gyro telling it when to stop turning, it could snatch a tetra from the autoloader and hold it over the center goal. The motor control was binary - on, then off.

[Gary Dillard]

Collin:
Sorry I didn't see your post until just now.

S.P.A.M used 4 CIM's, 4 NBD gearboxes, and 4 "half tank treads" last year, and we loved it. We'll probably go with a similar design this year if it fits the game. It's the fastest we've ever assembled our chassis and drive system. The biggest drawback was the cost of 4 Breckoflex belts; they don't get much cheaper even though they are half the length.

side view

top view

Sorry, these are the best pictures I could find on our website. We drove the center sprocket on each tread with the output of the Dewalt gearboxes so we theoretically had 4 wheel control although we just ran them in pairs like a full tank tread. The treads are slightly angled down to the center to give the same "boogie wheel" effect that allows turning without dragging the treads.

We have always build our own custom gearboxes and other than the omniwheel drive system we built in '04 we have always used tank drive, but with 4 identical motors this was the first year we split them up that way. As has been posted, this system seemed more powerful than Fluffy. Credit of course goes to Gary and James Jones (James was a featured presenter on drive trains at the '04 FIRST conferences).

[CJO]

On a similar topic, our six wheel "direct" drive originally started out as a 4 motor/ 4 wheel design, which we then expanded to six wheels, and used driveline to connect all of the wheels. Upside, no chin/belts/tensioning . . . Downside? Weight, it was fairly heavy, but it worked well, and we were able to add encodes to get a good idea of where we were. Not quite as modular as a moter per wheel design, but we did it with just the 4 cims, and it did work quite well.

I would strongly encourage teams to try out more designs which use direct/line drives instead of chain/belt.

P.S. here is a picture of the front 4 wheels of our system: http://www.chiefdelphi.com/forums/sh...highlight=1097

[GMKlenklen]

Ok, there is all this talk of "tripping breakers" and such... So I ask what does it take to trip the stock breakers?
Also, because last year was my rookie year... I only have experience with one drive train... but I don't remember all these sorts of problems to do with it.
We had 2 stock trannys, with 2 cims per tranny. 4-wheel drive with the stock skyway wheels, i.e. your standard 4-cim 4-wheel drive slip steering set up.
We didn't ever trip anything... I pushed people around quite a bit, i flew down the field, and I always drove strait. Oh, and we never broke a chain.

Somehow we where amazingly lucky and evaded all of the aforementioned problems... if anyone knows how, I'd like to know!!!