04-09-2004 23:19

Arefin Bari

Re: pic: Iso view

looks nice... and simple... drivetrains that a lot of teams would go with... i dont see any pneumatics so i am guessing that gearbox cant be shifted into another speed... how fast you are expecting the robot to move and with how much torque... if possible, i would like to see a close view of the drive train... good luck...

04-09-2004 23:21

RogerR

Re: pic: Iso view

sweet drivetrain. about the weight estimates in CAD, is 60# the estimates for just the machined components, without the manufactured parts (i.e. motors, robot controller, battery) or did you already add those in? also, it looks like you might be using the cap screws in the front and back to move the wheel mounts back and forth, adjusting chain tension. is this the case? finally, how are the motors mounted? i can't tell from looking at the drawings, and its really messing with my head.

04-09-2004 23:34

Rod

Re: pic: Iso view

Single speed gearbox
Top speed is 8.1 FPS
Gear ratio chip to rear wheel= 8:1
Bosch to rear wheel=32:1
wheel Dia.= 6"
you do the math

04-09-2004 23:39

Rod

Re: pic: Iso view

60# total weight
Cap screws do adj. chains
Motor mounts are slotted and bolted from underneath for a cleaner look

04-09-2004 23:56

Arefin Bari

Re: pic: Iso view

I should have made another thread about the gearboxes... but okay...

I am just wondering if your motor mounts are strong enough keep the drill motor from spinning out because of torque in the gearbox... i am saying that since i have experienced it... we had something like a mexican hat which held the drill motor in place and then bolted that into the gearbox plate...

The design looks great so far... whoever drew it... put some quality time in it... good luck...

05-09-2004 00:24

Rod

Re: pic: Iso view

Thanks,
We have used this type of mount on the chip motors in the past with no problems. The mount is slotted on one side only and the hole is bored to fit tight on the motor. We'll see if it works, if not redesign

05-09-2004 00:37

Cory

Re: pic: Iso view

Looks really nice.

One comment.

It seems to me like the only thing holding the rear plate in alignment is the CIM motor on the left side. You might want to consider adding something in to link the two plates together

05-09-2004 09:05

Greg Perkins

Re: pic: Iso view

It looks awesome guys...

the one thing i see is that the bosch motors are being discontinued this coming year. i dont know what we are gettting but i wouldnt start anything until we find out. just a heads up...


Please dont take this post negatively... I know you must've spent numerous hours on it...and i dont want to sound like a buzzkill

05-09-2004 13:21

Rod

Re: pic: Iso view

It's just ink on paper now and will stay that way until after kickoff. If they do change the motors it will be a fairly easy edit in CAD. The gears in the gearboxes are Boston Gear change gears. Change gears can be purchased in one tooth increments from 20 to 96. The gears on the motors are Stem Pinion gears and come in 5,6,10 teeth. I chose these gears because they are all 3/8" face. They are all 5/8" bore. 1/8" key way. 20 DP 14.5 PA It makes it easy to mix and match to get the ratio you want. You can see the gears at bostongear.com. Go to Bos Spec 2 , open gearing, spur gears, change gears and stem pinion gears. You can download the CAD files, .STEP files seem to work best in AutoDesk Inventor. You will need to register to access Bos Spec 2, very easy. These gears seem to be a good option for teams like ours who do not want to try and make custom gears.
Gear List
GA20
GA40
NAR5

07-09-2004 11:10

Tytus Gerrish

Re: pic: Iso view

those gearboxes are HUGE!!! alot more material and complexity than is needed. the batery would be best placed in the front for some balance. it is a verry light tho i warn is a very flimsy chassis using the channel. swampthing is all channel and just a fwe hits tirned it into a crushed tin can. the atonomus had to be guessed everytime because after every match the wheeles would pointed in a diffrent direction.

07-09-2004 12:00

Billfred

Re: pic: Iso view

Perhaps it's time for FIRST robots to start using R/C car-type suspensions. You can adjust those things all day long...toe-in, camber, shock positioning, those were the days.

I have to agree--the channel (now that I realize it's channel) would probably be a little flimsy, given the robust trend lately. Perhaps some bracing would be in order? If that channel in the front or the back takes a few too many hits, then you're looking at no support to keep the sides from just ker-flumping inward. (I experienced the same phenomenon when one of my bed's rails at home warped. Ask my brother--we spent half an hour jumping on it to make it look half-decent...and even then, we had to prop a busted computer under the bed so that it'd hold my weight.) That wouldn't be too good in the middle of a match.

As for the weight distribution...I dunno. It would depend on your appendages du jour, but I think you might do well moving the battery back (more into the screen looking at that shot), and moving your RC to the other side. Or move the battery to the other side of the CIM motors and slide your RC inward some. That might help out, especially if the channel does give way. You don't want a good whack doing something to your RC.

Just my random blathering. With a few tweaks, that design should...what's the Chappelle's Show quote...SPIT HOT FI-YAH!

07-09-2004 15:16

Paul Copioli

Re: pic: Iso view

Rod,

I have done the math and something is not adding up.

CIM free speed = 5,500RPM
Gear Ratio to rear wheel = 8:1
Wheel Diameter = 6"

5500 / 8 = 687.5 RPM or 71.995 rad/sec
71.995 rad/sec * 3" (or .25 ft) = 17.99 ft/sec

Assuming some losses, you will be around 14 - 16 ft/sec, not 8!

-Paul

07-09-2004 16:23

Rod

Re: pic: Iso view

I figured the the top speed at 50% of the CIM free speed

07-09-2004 16:27

JVN

Re: pic: Iso view

Common mistake.
The motor will run muuuch faster than Max Power.

I usually use a "speed constant" of 85%.
Basically, gear the robot based on .85*free speed or in the Chips case: 4675RPM

Real world testing of my theoretical calculations shows this to be about right.

More accurate drive modelling can be done, but this simple method comes out pretty good. Then again, your milage may vary depending on your drivetrain.

Anyone else wanna confirm 85%?

JV

Edit:
See Andy's post below.
I guess I'm a little fast. Oops.
/Edit

07-09-2004 16:45

Andy Baker

Re: pic: Iso view

A few years ago, I was using 60% for what I called "working speed" (motor rpms at robot's top speed). The past few years, as our efficiencies have gotten better, this percentage of the free speed has gone up. Last year, we used 75%, I think.

I don't put too much faith in that number, though. It just gets us in the ballpark with regard to ratios, speed, and sizing. Our final number might have been 5% higher or lower, but it is about right.

John, you must have a very light robot or a very efficient drivetrain if you are running at 85% of your free speed.

Andy B.

07-09-2004 16:53

Max Lobovsky

Re: pic: Iso view

Andy, I assume you are talking about a dual motor (2 CIM and 2 Drill) drivetrain. The percentage is obviously significantly lower when you have half the maximum power. This question might also explain the discrepancy between John's and your numbers.

07-09-2004 17:07

JVN

Re: pic: Iso view

Or... maybe I'm just wrong.
Thanks for sharing, Andy.

You heard it here first, folks. 70% is where it's at.

John

07-09-2004 17:22

Paul Copioli

Re: pic: Iso view

Or you can measure the actual resistance force and use that to find your no load running torque to find the actual speed you are running at. John, you will find that you are probably closer to 80-85%.

07-09-2004 17:38

Andy Baker

Re: pic: Iso view

OK, I need to clarify my background. In '99, '01, '02, and '03, we designed treaded drive systems. These systems had much more drag and lower efficiency than wheeled systems. In '04, we did a wheeled system. I *think* that 75% was a good number for us to use, but I may be wrong. We did not do any after-the-fact testing to verify this number.

Also, Max has a good point. We used CIM motors and FP motors for our '04 wheeled drive base, so the CIM motor was doing the majority of the work on the system. Since we did not have two balanced motors on the drive system, this may be the reason that our "working speed" % number was lower.

If we used 2 equally powered motors, I can see this number jump from 75% to 80 or 85%.

Andy B.