pic: 2791's Chassis

[Teched3]

I've always been an advocate of using smaller diameter wheels for a little better grip on the carpet. You also obtain a little lower CG, which helps on Sat afternoon. Was there a reason that your standoffs appear to be reversed from one side to the other, or just an oversight. are you using belt or chains to drive your front/back wheels? Your rendering is awesome, and is motivating me to learn a 3D modeling program. any suggestions for an old-timer?

[Chris is me]

Originally Posted by JesseK

Looks like a very solid setup.

If you use the line following sensors, where would you put them?

If we opt to use line followers (I'm thinking an ultrasonic sensor and a gyro for dead-straight tracking is as good as we'll need), we'll probably cut a hole in our electronics board and mount them a little above the bottom of the front of the chassis.

Originally Posted by sdcantrell56

Id be real careful gearing at 12fps. We did that last year with 4cims and 2fp's and often times our battery was totally dead with around 30 seconds left in the match. We later dropped down to 9.5fps with the 6 motors and it was much better. Just be ready to swap out gearing after testing. Otherwise this looks super awesome!

One of the really cool things about this transmission is the belt reduction - we can adjust with a new belt and output pulley between 9 and 14 FPS without taking up as much space as a chain run would. It also lets us mount our motors in a very compact configuration - the two-motor model is literally 3 inches from the ground.

That being said this is a valid concern - does anyone know how I could predict battery discharge rates using math? I have no idea how to do it.

Originally Posted by Teched3

I've always been an advocate of using smaller diameter wheels for a little better grip on the carpet. You also obtain a little lower CG, which helps on Sat afternoon. Was there a reason that your standoffs appear to be reversed from one side to the other, or just an oversight. are you using belt or chains to drive your front/back wheels? Your rendering is awesome, and is motivating me to learn a 3D modeling program. any suggestions for an old-timer?

Our drivetrain's CG last year was about 5 inches from the ground.

This year, it's just under an inch and a half.

With the 9 foot tall arm we're working on, that could be pretty important.

[Matt H.]

Originally Posted by Chris is me

That being said this is a valid concern - does anyone know how I could predict battery discharge rates using math? I have no idea how to do it.

Looking at the spec. sheet for the batteries:
http://www.alliedelec.com/Images/Pro...A/610-0002.PDF

The last graph gives discharge characteristics. The C-rate or CA on the graph can be roughly calculated as Current Load/Rated Amp Hours so at your quoted 60 amps
CA~3.4
Visually extending the curve; you're right on the edge.

If you current usage rises to 80 amps with a greedy manipulator you'll almost certainly run of battery life.


Disclaimer: I'm not an expert on batteries and am only ~70% sure my interpretation of the C-Rate is correct.

Another method you could try is using Peukert's law.

Where the time to full discharge is approximated by:

t=H(C/(IH))^k
Here t is time to full discharge, C is the rated capacity, I is the current, H is the rated discharge time and k ranges between 1.1-1.5 for lead acid batteries (it is generally empirically determined, but we can use the discharge curves to estimate it)

Using the spec sheet again:
C=17.4 H=20 hours I=60 amps k~1.34

t~4.1 minutes

For 80 amps

t~2.8 minutes

So it all comes down to how much you want to trust the math. To me it looks like a very border line case especially if you have a high current (always on rollers, heavy lifting etc.) manipulator.

For more information:
http://en.wikipedia.org/wiki/Peukert's_law

[Chris is me]

That's pretty compelling, really. Though I doubt the drive will spend more than 30 seconds a match drawing maximum current, it is still something we should watch out for.

Our current plan as of today's meeting is to oder a set of pulleys to gear us for the more conservative 9.5 feet per second and to do extensive battery life testing. We've got a simple, probably low load manipulator planned - but we will still be careful. If during practice or competition we figure out we're going through batteries too fast - we'll swap those right out. Should take under 10 minutes.