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Unread 11-15-2005, 03:31 PM
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Direct Drive Four Motor System

I am looking at possible drive trains for next season. One thought I had was to run each CIM motor directly into each wheel (after running through a transmission). With a 12.75 reduction, you get about 9 ft/s with 6" wheels.

If you're having problems visualizing, think crab drive system without the wheel assemblies rotating and the motors running the same direction the wheels are turning (sorry I can't explain it more).

Does anybody have any experience with this type of drive? What were your problems?

Thanks!
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Unread 11-15-2005, 03:35 PM
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Re: Direct Drive Four Motor System

No experience Collin ... but thanks .... you've got me thinking ...

We did talk about doing something similar (CIM on each wheel - using DeWalt gearbox and short chain X 4) last year but we couldn't afford the weight hit of those 4 assemblies ...
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Unread 11-15-2005, 03:58 PM
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Re: Direct Drive Four Motor System

I would think the weight and size of four trannys might be a problem. We had four wheel drive robots in the past and turning was very hard. We have switched to six wheel drive with a lowered center wheel and turning is very easy. I know you must have your reasons, but can you tell us the advantage of having four trannys.
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Unread 11-15-2005, 04:33 PM
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Re: Direct Drive Four Motor System

Quote:
Originally Posted by Rod
I would think the weight and size of four trannys might be a problem. We had four wheel drive robots in the past and turning was very hard. We have switched to six wheel drive with a lowered center wheel and turning is very easy. I know you must have your reasons, but can you tell us the advantage of having four trannys.
The weight that you would gain using four tranny's could be lost from not having to use any chain.

I understand that turning is hard...but I feel that can be lessened by using wheels with lower traction.
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Unread 11-15-2005, 04:53 PM
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Re: Direct Drive Four Motor System

Quote:
Originally Posted by Collin Fultz
I understand that turning is hard...but I feel that can be lessened by using wheels with lower traction.
I understand that the four seperate transmission setup would give you a slight increase in pushing power, but using a wheel or tread with a lower coefficient of friction would just hurt you in pushing matches. You would be more likely to just spin your wheels without gaining any traction, therefore loosing any pushing power you gained through this setup. Based on the weight, pushing, and turning factors I would personally go with a 6 wheel drive setup. But this all really depends on what resources your team has to work with.

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Last edited by Jay Trzaskos : 11-15-2005 at 04:56 PM.
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Unread 11-15-2005, 05:13 PM
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Re: Direct Drive Four Motor System

Quote:
Originally Posted by Jay Trzaskos
I understand that the four seperate transmission setup would give you a slight increase in pushing power
not necessariyl. if one of those tires lose contact with the carpet, you only have a 3motor powered robot. 2 motor powered if 2 tires leave ground.. but if oyu have a chain connecting the wheels, even if one wheel leaves the ground, there are still 4 motors torquing the wheels still in contact with the groudn.
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Unread 11-15-2005, 05:38 PM
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Re: Direct Drive Four Motor System

1213 used this:


Each was a bit over built at 7-1/4 lbs. Should be able to take 1/2 lb. or so off.

Speed was 7-1/2 FPS with 4" wheels - that's with the Chippy driving the pins on the first stage of the DeWalt - I.E. planet gears replaced with adaptor keyed to the Chippy output shaft.

Mobility was outstanding with 95A durometer urethane wheels. Pushing was not much of an issue last year, but we managed to push when we needed to.

A chain added to connect front to back wheels would transfer power in case one end got lifted - but, I'd rather stay low a use a ramp than bother with chains.
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Unread 11-15-2005, 07:02 PM
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Re: Direct Drive Four Motor System

Jack -

Thanks for the info...that's exactly what we're looking at doing.
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Unread 11-15-2005, 09:46 PM
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Re: Direct Drive Four Motor System

Quote:
What were your problems?
Biggest issue is holding onto the naked DeWalt tranny. The leg between the motor and tranny is actually two pieces. One (1/4" Al) is the motor mount. The other is 3/8" and fully keyed (water cut) to fit the base of the tranny. I've got the DXF of the pattern if you go with something like it - saves weight - naked DeWalt is only 0.85 lbs.

We used 1/2" high-carbon keyed axles - machined 3/4" from one end to fit the 0.3 x 0.25 double-D tranny output. We found early on that unhardened shafts snapped. Heat treating solved that.
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Unread 11-16-2005, 06:54 AM
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Re: Direct Drive Four Motor System

I'd like to offer two strong opinions on this subject.

1.) Direct drive is good! We have been doing this up in Saginaw for a couple years (teams 902, 703, and 49). I love not have any more chain tensioning/jumping off issues! We just make the holes in the right places, but the thing together, and it works for the whole season. No adjustments!!

2.) I have always been a strong advocate of coupling the wheel rotations to each other. We do it with tank treads, but even with wheels you should make it so all of the motor power can go to both the front and/or the back. The problem comes with the inevitable pushing match. We all start with the same limits on power (same battery/fuses/motors). The trick is to put as much of it to the ground as possible. If you have a motor powering each wheel independently and you get a weight shift on your bot such as climbing a ramp or a pushing match, the wheels that get unloaded are no longer helping and you suddenly went from a 4 motor bot to a 2 motor bot.

(sorry, long winded)

Matt's $0.02

Last edited by MattB703 : 11-16-2005 at 11:33 AM.
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Unread 11-16-2005, 08:35 AM
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Re: Direct Drive Four Motor System

yes and no- if a wheel leaves the ground it will also spin freely,

which means it will draw far less current, your battery will be less loaded, and therefore there will be more voltage (power) available to the other three wheels

probabally not 25% more power, but it will be a significant amount.

Usually in a shoving match the wheels mnaking contact with the floor start spinning anyway - once a wheel starts to spin it doesnt matter how fast it spins, the force it applies is the same. Having twice the HP on a spinning wheel gets you no increase in pushing force.
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Unread 11-16-2005, 04:22 PM
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Re: Direct Drive Four Motor System

Quote:
Originally Posted by KenWittlief
yes and no- if a wheel leaves the ground it will also spin freely,

which means it will draw far less current, your battery will be less loaded, and therefore there will be more voltage (power) available to the other three wheels

probabally not 25% more power, but it will be a significant amount.

Usually in a shoving match the wheels mnaking contact with the floor start spinning anyway - once a wheel starts to spin it doesnt matter how fast it spins, the force it applies is the same. Having twice the HP on a spinning wheel gets you no increase in pushing force.
that would be driver. whenever my tires start slipping, i let off on the power a bit till i get my tires gripping again.
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Unread 11-16-2005, 04:38 PM
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Re: Direct Drive Four Motor System

//Only read if you care about off topic posting
OK, so I know that by posting what I am about to post I am not following the original intent of the thread BUT this topic was brought up and has enough relevancy for me to justify my single subject post.
//

I have been thinking about the pushing thing for a long time, and I believe it boils down to simple physics. If all robots have the same maximum weight to be pushing on the Normal force, the only other factor affecting a NON STALLING (from lack of torque required to turn wheel(s)) is friction. F=uN if I am not mistaken. To get the greatest force, you must have the greater amount of friction applied to the floor.
*This next part is a really long sentence with many side notes in parenthesis, it is hard to follow, but bear with me.**
I don't care how much horse-power you put into a drive system, as long as there is enough power to not stall the motors (or trip the breakers), which I find is EASILY accomplished with 2 CIMs per side (seeing as how my team doesn't think about stalling with 1 per side), you will win a pushing match with a highest amount of friction with the robot to carpet interface (wheel(s), tread, gum rubber roller ....etc.)

Pretty much
If you don't trip breakers and stall your motors, and you have a higher amount of total friction applied to the floor, you will have the great force in the desired direction, therefor win any pushing match.


Thats my long winded-off topic $.02.
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Unread 11-16-2005, 05:14 PM
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Re: Direct Drive Four Motor System

Henry's post got me thinking about this

the robot with the greatest force (most friction) will win the pushing match

but I have to admit now that is not the end of the story

OK, you are pushing another robot backwards, against their will. There is a good chance your tires are not slipping and his are

now the issue is, how fast are you pushing him backwards? if you have more HP available (more motors on your drivetrain) then you can get that torque at a higher wheel RPM, which means:

if you have superior traction to win the shoving match, the more motors you have, the faster you can push other bots around

and in a FIRST match, time (speed) is of the essence!


my 2 :^)
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Unread 11-16-2005, 07:19 PM
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Re: Direct Drive Four Motor System

Quote:
Originally Posted by Collin Fultz
I am looking at possible drive trains for next season. One thought I had was to run each CIM motor directly into each wheel (after running through a transmission). With a 12.75 reduction, you get about 9 ft/s with 6" wheels.


If you're having problems visualizing, think crab drive system without the wheel assemblies rotating and the motors running the same direction the wheels are turning (sorry I can't explain it more).

Does anybody have any experience with this type of drive? What were your problems?

Thanks!
Collin:

I have one pretty significant concern about using direct drive on each wheel - load sharing. In an ideal world of equal force and weight distribution, direct drive isn't a bad idea and it's not a bad idea in the 'real world' either, others will testify that it has worked for them!

However, take for example a robot that doesn't have weight evenly distributed, say it's 70% in the back and 30% in the front. You will need to set your ratios based on the largest amount of torque you'll see - in this case, the back wheels. Since you want to ensure the front and back wheels are spinning at the same rate (roughly, it's an open loop system)... you'll have to gear the front wheels to this same ratio. You will essentially be running slower than you would with a combined motor system.

To put this in a better frame of reference (numbers!)...

Let's say it takes 70 inch pounds of torque peak to turn the the back wheel and 30 inch pounds peak on the front wheel, and your peak current draw is at 10 inch pounds of torque from your motor. You will end up gearing both sets of motors at a 7 (plus some) to 1 ratio because you don't want your breakers to trip in a pushing match.

However, if you had these two motors combined on a single shaft that was linked via chain to two separate wheels, things change. Though each motor would still put out peak torque at 10 inch pounds, their combined torque would be 20 inch pounds at peak current. The max torque this pair of motors would see is 100 inch pounds. Thus, you'd gear the motors at 5 (plus some) to 1 ratio. This is a significant increase in speed (the difference between slow and average or average and fast!!)

Another case is assuming that your robot is lifted by another so it's only on it's back wheel- you now have two motors in the air doing no work, and you have the two rear motors seeing almost twice the load they were geared for - you'll be tripping breakers. That's no fun either!

For the two reasons above, I don't design direct drive robots. I also don't like the idea of creating 4 individual gearboxes, but it's mostly the load sharing benefit that keeps me sticking with motors in parallel. Others do direct drive and it works for them.

Fighting Words: It comes down to a mater of weight, and I think that if you're smart about your chain size (#25) and sprocket materials (aluminum) you'd be hard pressed to say the weight difference is enough to over come the disadvantages. I don't considering creating a good chain tensioning system upfront and spending a few minutes to tighten it every few matches at the first regional to be terribly inconvenient vs. the tolerances and assembly requirements of building 4 robust gearboxes.

Just some thoughts and fighting words,

Matt
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Last edited by Matt Adams : 11-16-2005 at 07:22 PM.
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