CHIPS

[Max Lobovsky]

Originally Posted by Matt Adams

I guess I'll just throw the comment out there that you simply can't be competitive with just one pair of motors and no transmission. You simply won't be able to move fast enough or be able to apply enough pushing force.

On another topic

Efficiency isn't really a concern for these motors. You don't worry about running out of juice in a match, you worry about moving faster and pushing harder than other machines. You want more power out of your motors. I'll take getting .5 HP out at 60% efficiency over .45 HP out at 99%. The batteries are rechargeable!!

Just some thoughts,

Matt

you are contradicting yourself, in a way. If running out of juice in a match is a factor, than efficiency is important. But i havent seena ny problems with running low on batteries so yes, i'd agree, max power at a reasonable current is more important.

[RogerR]

Originally Posted by Matt Adams

...it has to do with the internal motor windings...

sorry about the double post...

this is the second time that i've seen these 'windings' mentioned, and while i think i know what they are, i'm not sure, and i am curious about what is done to them that makes them cause this. does anyone have a (relatively) simple explanation they can give me?

[Ryan F.]

Originally Posted by RogerR

actually, this problem is not in the gearbox, but in the drill motor itself. the CIMs also have a slight bias, but it isn't nearly as noticable as in the drills. theres a couple threads on this subject out there...

The stock gearboxes are designed so that both drill motors will run the same way. On the other hand...if your using chips, I would recommend you get them running the same way. I listened to the good old "it's not a big difference"...but it became a very annoying problem on our robot this year. We would always drift right...making setting up autonomus to hit the tee ball a pain.

[roboteer49]

wow

ok, the reason for this post was because i want to keep the gearbox very simple adn achieve a speed of 9 ft/sec. also torque. This is the first time im designing a drive train and i want it to run smoothly. Im also scared of dual motors because theree is like a .5 mm shaft on the drill and i don't want it to crap out and its too complex for my first time. The reason i thought the chips would be better because they seem like they never break, and they have almost 5 time the rpm of a drill. From what im hearing is that the chip is only slightly less powerful, speed can be adjusted in gears but torque cannot, right? So, here's my question, if two robots were identical and one had chips and the other drills with gearbox(high and low) who would win in a pushing match?

thanks

[Cory]

Assuming that you designed each drivetrain to take advantage of the maximum power available, and efficiency and gear ratios were identical, the drills would win. Wheels would need to be the same too, as would weight.

If you gear just the Chippy's to 9 fps, you won't have any torque (Well, you will, but you wont be pushing much of anyone)

Cory

[RogerR]

Originally Posted by roboteer49

... speed can be adjusted in gears but torque cannot, right? So, here's my question, if two robots were identical and one had chips and the other drills with gearbox(high and low) who would win in a pushing match?

thanks

err, close...both speed and torque can be adjusted through gear ratios, but the output power of the motors stays constant.

Originally Posted by Matt Adams

Power output = torque x angular velocity.

with that being said, the winner of your hypothetical pushing match would depend on the gearing of both robots. but assuming that you geared each bot in such a way that they had identical coef. of friction, speed, and efficiency (in short, torque would be the only diff.) the bot powered by drills would come out on top since it is more powerful.

[Tristan Lall]

Originally Posted by roboteer49

wow ok, the reason for this post was because i want to keep the gearbox very simple adn achieve a speed of 9 ft/sec. also torque. This is the first time im designing a drive train and i want it to run smoothly. Im also scared of dual motors because theree is like a .5 mm shaft on the drill and i don't want it to crap out and its too complex for my first time. The reason i thought the chips would be better because they seem like they never break, and they have almost 5 time the rpm of a drill. From what im hearing is that the chip is only slightly less powerful, speed can be adjusted in gears but torque cannot, right? So, here's my question, if two robots were identical and one had chips and the other drills with gearbox(high and low) who would win in a pushing match? thanks

Hang on a second--speed and torque can be adjusted by gears; it's (mechanical) power that is constant. Once again, rotational speed × torque = power. Since power is constant, speed and torque are varied by the gear ratios.

Also, that little shaft on the drill has a steel pinion (i.e. small gear) pressed onto it. Some teams have managed to get it off, and replaced it with something more suitable to their needs, others elected to use the pinion as supplied. In any case, provided you're comfortable with assembling gears, the drills are only marginally harder to work with than the CIMs. (Finding a matching 0.7 module, 20° pressure angle gear is, however, quite annoying--try PIC Design, and order very early!)

(Or are you referring to the threaded shaft on the gearbox? That's another issue entirely.)

As for the CIMs never breaking, that's a little extravagant, I think! (We had a slightly defective one in the kit this year--it was replaced with a good one.) But you're absolutely right that they can stand much more abuse than the drills.

I've noticed that you seem to be thinking of the drills as the entire drill motor + transmission assembly. We've (or at the very least, I've) largely been thinking of the motor alone. If you don't feel like separating them, remember that they have high and low gears by default and therefore their torque and speed will differ in each gear. (In other words, you need to specify either high or low gear, or no gearbox, if you want to describe speed and torque for the drill motors.)

As for the actual question, I have to modify the wording a little:
Q. If the robots were identical, and they were geared to run at the same final drive speed when identically loaded, etc.
A. Theoretically, the drill-powered robot would win, since it is capable of outputting more torque (i.e. more power for a given speed). But that's not the whole story! Many drivetrains are traction-limited, so that the robot will actually spin its wheels at maximum torque, rather than doing any further useful work. If that's the case, and both robots are otherwise identical, it could well be that both robots spin their wheels and don't do anything productive. At this point, it comes down to which robot can sustain this condition the longest--and here's where it gets interesting. You'd need to look at the graphs and examine the electrical system (including efficiency), and determine which robot will run out of (electrical) power first. (I would tend to favour the drill, even under these circumstances, but that's an educated guess, since I haven't done this analysis of which I speak.)

9 ft/s is an average-to-high speed, depending on the year and the game. Without running the calculations through, I would tend to say that this sort of configuration would produce low-to-average torque--which isn't quite what you had in mind.

Edit: Beaten to it, not once, but twice--but they're saying the same sort of thing!

[Adam Y.]

From my understanding, this has nothing to do with the gearbox itself, it has to do with the internal motor windings. The gearbox functions the same backwards as forwards, so there shouldn't be any gearing differences that would cause the difference in speeds.

Hmm.... I thought it had to do with the position of the brushes. The way you place the brushes of the motor affects the timing of it. Placing them in differnt positions will affect the speed of the motor and the torque output but power will remain constant. Im 100% sure this has been discussed before.

[BillyGoats]

Simple Gearbox for one chip

http://www.valleytech.k12.ma.us/robo...2%20Gears1.jpg

Simple Gearbox for two chips

http://www.valleytech.k12.ma.us/robo...4_24wheels.JPG

or

http://www.valleytech.k12.ma.us/robo...ectronics2.JPG

better picture

http://www.valleytech.k12.ma.us/robo...4chipgear2.JPG

We did not have a drive train problem all year except for breaking 2 belts.

[Tristan Lall]

Originally Posted by BillyGoats

Simple Gearbox for one chip

http://www.valleytech.k12.ma.us/robo...2%20Gears1.jpg

Simple Gearbox for two chips

http://www.valleytech.k12.ma.us/robo...4_24wheels.JPG

or

http://www.valleytech.k12.ma.us/robo...ectronics2.JPG

better picture

http://www.valleytech.k12.ma.us/robo...4chipgear2.JPG

We did not have a drive train problem all year except for breaking 2 belts.

I see a bit of a problem here (regarding pictures 2 through 4), and I believe that this came up earlier (in this thread). You'll notice that the CIMs those pictures have (instead of an 8 mm keyed shaft) a sort of small gear built into the shaft (if memory serves, it's a 25° pressure angle piece--very non-standard). This version of the motor (the Atwood AP801-001 motor by CIM) was supplied in 2002 only. It was specifically legal in 2003, but not supplied in the kit (the FIRST version of the CIM motor, FR801-001 was introduced that year). For 2004, the FIRST-CIM was supplied, and the Atwood-CIM is no longer legal, per <R09> and <R70>. Electrically, they are very similar (but not quite identical, if the specs are to be believed--I'm looking at copies of both), but mechanically, they are different, due to the shaft configuration.

Now, there are pictures of a what appear to be a different version of the gearbox on their website--here, here, and here--was this modified version intended to use the correct CIMs? Or is this something else entirely? [Edit: Looking at other photos, those are something else entirely--they're for the winch.]

Also, note that per <R62>, to make the gearboxes kosher, they could have modified the output shafts on the FR801-001s to have the same profile as those on the old Atwood (this couldn't have been easy), but they couldn't replace it part-for-part (certain modifications are allowed, part substitutions are not).

(Note that they said that the problem of legality had been rectified by modifying the shafts, so I can't accuse them of impropriety--just of posting outdated photographs....)

And yes, this is slightly off-topic, and moot, given that the season is over, but it is of potential relevance to anyone hoping to build one of these systems in the future.

[BillyGoats]

yes its legal. we did nothing wrong. the good picture was taken with other ones while the new ones where being cut. we did nothing wrong. That gearbox is what we used this year and what we will be using in the offseason comp.

[Tristan Lall]

Please note that you're not being accused of wrongdoing--like I said, I'm taking you at your word that you replaced the outdated Atwood Mobile AP801-001 motors shown in those photos with FIRST-legal FR801-001 motors, and made whatever necessary and legal modifications that were required to make it work.

(No need to further derail the topic--for any teams wishing to adapt CIMs to drive systems, the existence of old motors is an issue. I thought you should know.)

[Paul Copioli]

First off, good post. We haven't had one of these in a while. Next, Matt Adams I have one small correction to your motor power calcs for the drill with and without gearboxes. The available mechanical power when you have a gearbox must be lower than the motor by itself due to the efficiency losses. I know you know this because you accounted for them (the reason the drill/drill high/drill low all have different power numbers at the 40 amp limit), but you show that the drill in low has more power than the drill in high and the drill by itself ... no way is that possible. I think you may have transposed the data, but the drill by itself has to have more available mechanical power than with a gearbox ... Efficiency losses.

Next, shifting and multiple motors (per side) are not a must. However, I think you must use one or the other. If you don't want multiple motor drives, then shift. If you don't want to shift, then use multiple motor drives. We shifted and had multiple motors in 2003, but we only used multiple motors in 2004 and we had no noticeable difference in time to the center of the field and pushing performance. With that said, 2002 was a different story. In 2002, the really competitive teams had to shift gears and use multiple motors (and yes, I consider switching drive trains the same as switching gears). My rule of thumb is that switching gears coupled with multiple motors is not necessary when transferring weight to your robot is not possible. When a significant amount of weight can be transferred to your robot, then I recommend doing both. Use multiple motors if your overall design allows for it, because it will help your drive train.

-Paul

[Brawler006]

Someone mentioned earlier that the Chips do not run the same speed forward as they do backward. This can be changed. Losen the bolts that hold the motor together (don't take them out, just losen them one or 2 turns) and hold the motor by the black part in one hand. Have another team member connect the motor to a battery. You will notice that if you turn the back peice of the motor in relation to the black body, the speed will change. With a tach, you can adjust the motors so that they run the same speed in forward and in reverse. This is nice because then you don't have to worry about which way the motors point. Doing this with the drill motors is MUCH more difficult, and i wouldn't reccomend it.

[Matt Adams]

Originally Posted by Paul Copioli

Next, Matt Adams I have one small correction to your motor power calcs for the drill with and without gearboxes. The available mechanical power when you have a gearbox must be lower than the motor by itself due to the efficiency losses. I know you know this because you accounted for them (the reason the drill/drill high/drill low all have different power numbers at the 40 amp limit), but you show that the drill in low has more power than the drill in high and the drill by itself ... no way is that possible. I think you may have transposed the data, but the drill by itself has to have more available mechanical power than with a gearbox ... Efficiency losses.

Paul-

I completely agree. I would like to explain how I came about the number for the horsepower.

I found in a post somewhere, that the ratio for low out of the planetary gearset was a ratio of 42.62 : 1.

My calcuation was as follows:

The stall torque of just the drill motor is 7.70 in-lbs, with a free speed of 19,670 RPM.

The expected speed with the gearbox with that gear ratio is 461.52 RPM. Hence, an efficiency of about 97.5%. This is honestly not realistic. However, I kept it uniform and assumed that this same loss would occur in the torque. So, to find the stall torque at 40 amps, I divided the stall torque of the motor times the ratio of the estimated free speed in low (450 RPM) to the ratio of the free speed of just the motor (19,670), then I multiplied that times the ratio of 40 amps / stall current (127 amps) and finally multiplied that by the efficiency of 97.5%.

The mistake of course, isn't TOO obvious, but here it is: the ratio is fixed and known, and I should have multiplied the stall torque by the true gear ratio, not the after-efficiency loss speed ratio. This would lower the overall output to somewhere around .493 HP in low, and .452 HP in high. This again, is not possible, but the benchmark for the actual motor without the gear box is based on one set of experimental data, the data I used in high gear is from another data set, and the low gear is purely theoretical.

However, I'll still say that I found these three pieces of data (though from independent sources) to be the most reliable pieces of information available to the general FIRST community at this time on this motor.

I hope this clears up some confusion.

Matt