Kitbot/cims/transmissions running too slow?
 

We had our kitbot running today and did a speed test.
Our configuration was using 6" wheels.
also used the 21 tooth for the counter shaft
and the 28 tooth for the wheels
we measured it's speed to be 5'/sec

If you figure those cims are running at 5300 free speed
reduced 12.75:1 (gear) and 1.333:1 (sprocket)
we should have been moving 8.15'/sec

There wasn't much if any efficiency loss in the way we had it assembled, and the speed test was fairly accurate +/- .5 sec

I'm thinking those cims are turning more like 3300 rpm

Has anyone else experienced this?

Originally I thought everyone using the kit chassis/trans. and 8" wheels would be up in the range of 10.5'/sec but after seeing this, it's looking like they'll be traveling more like 6.5'/sec

One thing I didn't check (as I'm thinking of it now) is that we may have not been applying full power to the speed controllers, as we just grabbed an old RC and ran it. But I doubt the code wouldn't be full power.

edit: reduction ratio (oops)

Re: Kitbot/cims/transmissions running too slow?
 

CIMs are 5342 rpm no load according to the spec sheet. No load means no load. Wheels, a gearbox, chains, etc, are a load.

Re: Kitbot/cims/transmissions running too slow?
 

Yah, of coarse there's going to be some loss, but over a 1/3?
and this test was giving it a good 10' of room before the start line with no load other than the chassis, and the gear reduction as you said. I could understand 1/10 loss but not 1/3.

Re: Kitbot/cims/transmissions running too slow?
 

Even under a normal driving load, those speeds wouldn't make any sense.
Assuming you don't have a MAJOR parasitic load somewhere (which you probably don't) this is totally crazy.

Dan, please check your voltage and get back to us.

My prediction, you're not giving those CIMs full power.

JV

Re: Kitbot/cims/transmissions running too slow?
 

If i'm not mistaken

5 ft per sec /(6"*pi/12)*60sec/min = rpm of wheels = 191

191 rpm * 2.63 * 12.75 = 6404 rpm at the CIM

I'm not sure what you mean by counter shaft, but assuming you mean the pinion sprocket (is that the proper term?) the sprocket ratio is 28/21, you have

191 rpm * 28/21 * 12.75 = 3247 rpm at the CIM

which is it?

Re: Kitbot/cims/transmissions running too slow?
 

I haven't put your numbers in, but I designed a spreadsheet based off of JVNs' motor calc sheets in the whitepapers, and from that I get an approximate speed of of 6.5 ft/sec for a full weight (120 lb) robot using the transimission as given. While the 8.15 ft/sec may be accurate with no load, you have to account for all of your weight as well.

Re: Kitbot/cims/transmissions running too slow?
 

The robot is actually closer to 134-136 lbs... you have to account for the battery in the physical weight even though you dont in weigh in :)

Re: Kitbot/cims/transmissions running too slow?
 

I assume your figuring on 6" wheels?

I think our assy weighs around 40lbs

I'd like to see what you're using from John's spreadsheet that adds the loss in weight.

Re: Kitbot/cims/transmissions running too slow?
 

there should be no (or almost no) loss in top speed because of weight. The only tiny bit of loss from weight would be the added resistance it puts on the bearings, but in every FIRST drivetrain i have seen, the bearings are well well below there ratings.

Re: Kitbot/cims/transmissions running too slow?
 

By weight I think he means the load that the motors have to push.

Re: Kitbot/cims/transmissions running too slow?
 

Dan,
Allow me to perform an analysis for "our viewers at home".
My calculations match your analysis:

The weight will have minimal affect on the top speed. The only difference will be an increased normal force on all wheel/wheelshaft bearing surfaces, which will increase friction slightly.

This increase in parasitic loss is negligible.
Therefore, we can simply eliminate weight from our analysis.

Please refer to the attached spreadsheet.
This sheet shows the speed of the robot at 8.47fps (assuming 100% efficiency). From here, we can increase the "speed loss constant" to 60% which then shows an output speed closer to what Dan is experiencing.

This means, (as Dan stated) the CIM motors are either spinning slower than expected, or there is a large parasitic loss in the gearbox.

If we assume no parasitic loss, we need to find out reasons for a slower spinning CIM.

Some reasons:
-Voltage provided to the motor is closer to 7.2V
-Motor is wound differently, and top speed is limited.

I would almost immediately rule out a motor flaw.

Dan, please recheck your voltage. If that fails, replace the Chip motors and see if the problem persists.

Thanks,
John

Re: Kitbot/cims/transmissions running too slow?
 

John,

Have you (or anyone you know) performed a speed test on this equipment and found it to perform as expected?

Re: Kitbot/cims/transmissions running too slow?
 

Yessir.
Paul Copioli performed testing on first articles from production as well as the original prototypes. FIRST Engineering also performed testing.

Everything functioned fine.

If it's not the motor voltage, I'll buy you a dew.

JV

Re: Kitbot/cims/transmissions running too slow?
 

For what its worth, I have compared free speed extrapolations to ground speed measurements for all the robots presently in our "garage." The trend is pretty obvious.

2001: two 3/8" drill motors
32% 6ft/s real, 18ft/sec projected

2003: two 1/2" drill motors in high
38% 10.5ft/s real, 27ft/sec projected
(resulted in intermittent breaker and controller resets!! )

2003: two 1/2" drill motors in low
68% 5.6ft/s real, 8.2ft/sec projected

2004: two 1/2" drill and two CIM
78% 7ft/sec real, 9ft/sec projected

You can see that our "speed efficiency" rose dramatically once my team started choosing reasonable gear ratios. If you were to take these speeds and work backwards to the assumed load on the motor to get that speed, and the resulting current draw... the currents are astronomical, obviously. I hate to think what was happening while we were turning. I've always wanted to use a clamp on meter to find out if all this empirical data is self-consistent.

Oh, and for all of these tests I also did a "wheels off the floor" rpm measurement, which without fail was very very close to the free speed expected. Take this measurement, and just see if robot weight on those axles doesn't somehow make a difference.

As far as our measurement methods, we timed the robots on the ground over a 50 foot window following a 30 foot accelleration zone. I imagine that was fairly accurate.

Given this history, my adjusted calculation to estimate the speed of our kit-built competitors was 6.3 ft/sec.

Re: Kitbot/cims/transmissions running too slow?
 

I tested the living daylights out of that gearbox. I tested the 4wd configuration with 8 inch wheels and it behaved as expected (around 9.5 ft/sec). I use a closed-form non-linear differential equation to solve for position, speed, and acceleration over time and it tracked exactly with my testing of the frame.

What type of six inch wheels are you using?

What type of bearings?

How much chain tension do you have?

Something is not quite right.

-Paul

Re: Kitbot/cims/transmissions running too slow?
 

For the past several years, Team 33 has been able to get our robots to reach over 90% of the theoretical free speed. If you are only getting 60%, something is wrong. Easiest way to find it: remove the chip motors and check for friction. Without the motors in the system, everything should spin freely when you turn the wheels by hand. If it doesn't, figure out why.

Re: Kitbot/cims/transmissions running too slow?
 

6" skyway wheel chair wheels (usual kit stuff). Chain had appropriate slack. Also the bot didn't drift to either side when full power - it went straight. Indicating the fact that if there were something wrong it would have had to happen in both gearboxes, or at least one motor in each box. So I'm almost certain there wasn't a mechanical problem. I also don't think we happened to receive (4) defective cims. So if everyone is positive these cims perform 5300 rpm free speed. Then it really only leaves one conclusion, that the RC wasn't providing full power. We also had the old victors (883's?). Anyhow, I'll measure the voltage tomorrow and let you know.

Dan

Re: Kitbot/cims/transmissions running too slow?
 

Well I couldn't get there today, but I had Amanda check the voltage on the output speed controllers, she told me they were reading 12v on the output at full power. I also had her spin the drive wheels - spun freely. Gearbox is in good shape. So I'm out of ideas... I prefer my dew cold please...

I'll have to make a little video to prove it all, since I doubt anyone will believe me now. I guess I'll try a different method of testing the speed, but the last was pretty good, and the chassis is visibly slow.

Re: Kitbot/cims/transmissions running too slow?
 

You say that you used the 6 inch kit wheels, but aren't they 8 inch. At least that is what our team got.

Re: Kitbot/cims/transmissions running too slow?
 

We've got 6" from years past kits. We also purchased a dozen from this past yard sale.

Re: Kitbot/cims/transmissions running too slow?
 

I'm stumped too.
Try switching the Chips... see what happens.

I'll bring you your dew in Atlanta.

JV

Re: Kitbot/cims/transmissions running too slow?
 

Dear god, you wanted 27 feet a second? I thought 11fps was absolutely insane.

Re: Kitbot/cims/transmissions running too slow?
 

What is the gear reduction ratio of the kit transmission for the chippewa motors? Are you running four chippewa motors for drive or just two? I don't have any numbers for it but our 2004 bot with two chippewa motor drive, 12 inch wheelchair wheels and about 29:1 final drive ratio would absolutely FLY!
I will agree with you, your problem has me stumped too. Since your driveline feels smooth and you have already ruled out voltage problems, I don't see why it would give you problems. As for having defective motors, it is highly unlikely that you would wind up with two lemons in the same kit. Good luck, Swampdude!

Re: Kitbot/cims/transmissions running too slow?
 

You might try taking off one CIM from each side. Since the interest is in top speed, a single motor should provide the answer. Also try the single in both positions, by trial and error (well somewhat scientific) you should be able to narrow it down to which motor(s) or gears are causing the problem. A burr or slight clash from the CIM gear can also reduce your speed.

I know you checked but any interferences?

Re: Kitbot/cims/transmissions running too slow?
 

Dan,
There are several threads discussing using a voltmeter on the output of the controllers. Don't believe them. The true indicator is the LED on the controller. If it isn't at full red or full green then you are not at full output. Both motors are connected and wired to run in the same direction? Are you sure of your wiring? Four Chalupas in trouble are going to draw a lot of current and therefore voltage drop in the wiring. Do the robot wheels turn faster when you lift the robot off the floor? Might be some misalignment in the wheel bearings. Try raising the robot and bypassing the speed controller, running breaker out to motor direct. Do they spin faster?

Re: Kitbot/cims/transmissions running too slow?
 

First let me preface this by saying it's now going 8.5'/s

OK last night I got some answers, but it's still not clear. The original setup had the 2005 motors in it, and transmissions assembled such that they made a little noise but not bad. They weren't greased yet. Just a student slapped together out of the box kit. But from a distance it all looked right. So we performed the tests and it ran slow - 5'/s.

Over the last few days we removed the 2005 cims. Milled and faced off some gears and Also added a gear to mate with the FP's ;) Then installed cims from 2003 and 2004. They now weigh 8.5 lbs with motors.

Last night I tested the output voltage to be 12v
then we put a zip tie on the wheel letting it whack a piece of plastic so it was loud and ran it for 20 seconds (up on blocks) with a stop watch. I counted in 4-4 timing marking on a sheet while keeping the beat (I gots rhythm). I counted 108 revolutions/20 seconds x 3 = 324 rpm at the wheel (and I got the same results twice). Which on a 6" wheel works out to be 8.5'/s

So what happened?
I did notice the students had the gearbox assembled incorrectly "After" the mods. So it's possible they we're incorrectly assembled prior to this and during the original testing. Also I saw that the cables were all hastily crimped loose and the connectors were not screwed tightly on the controllers.

Analysis:
1. Either our 2005 kit cims are bad.... Nah...
2. Our transmissions were assembled incorrectly.
3. Teenagers in a hurry wired it up.

Answer:
SLOW DOWN YOU CRAZY KIDS!!!!
Why are teens always in such a big hurry? Where's the fire? Why does everything always have to be done in a nanomicrosecond?
ok off my soapbox

After all this, we don't plan to use these on our competition bot. But it's turning out to be a great sparring/practice system.

Re: Kitbot/cims/transmissions running too slow?
 

Ghetto! :D

Re: Kitbot/cims/transmissions running too slow?
 

i'm not a pro, but this is what i'm going to say:
Temperature maybe is a small facter, but i think is the Voltage.
Did you change the pin inside the gearbox?

Re: Kitbot/cims/transmissions running too slow?
 

I assume you've check this, but make sure you're getting full power. The led's on the speed controls should be green in full "forward" and red in full "reverse." If they're not, recalibrate your speed controls.