Kitbot/cims/transmissions running too slow?

[Swampdude]

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)

[Max Lobovsky]

Originally Posted by Swampdude

If you figure those cims are running at 5300 free speed

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

[Swampdude]

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.

[JVN]

Originally Posted by Max Lobovsky

CIMs are 5500 rpm no load according to the spec sheet. No load means no load. Wheels, a gearbox, chains, etc, are a load.

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

[Max Lobovsky]

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?

[Hieb]

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.

[Cory]

Originally Posted by Hieb

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.

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

[Swampdude]

Originally Posted by Hieb

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.

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.

[Max Lobovsky]

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.

[russell]

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

[JVN]

Originally Posted by Swampdude

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.

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

[Swampdude]

John,

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

[JVN]

Originally Posted by Swampdude

John,

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

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

[jimfortytwo]

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.

[Paul Copioli]

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