Using 6 motors in a drive system?

[FIRST JerseyKid]

If you were to put 6 motors in a robot just to drive it around for the 2 minutes and 15 seconds would you have enough battery for 1) The whole round and 2) For other things 3) A compressor ???

[Steve W]

We did last year. You must have an efficient drive system and it also depends on how much you use other motors and how much your compressor runs.

[phrontist]

Originally Posted by FIRST JerseyKid

If you were to put 6 motors in a robot just to drive it around for the 2 minutes and 15 seconds would you have enough battery for 1) The whole round and 2) For other things 3) A compressor ???

Uh, depends on a lot of factors. Here is a tip: Battery capacity is measured in Amp-Hours. So if a battery is rated at 10 Amp-hours (that figure it entirely ficticious) this means you could draw 10 amps for an hour, or one amp for 10 hours (although this is not a perfect relationship, because batteries have internal resistances). You'll need to lookup the datasheets on everything you intend to put on the bot and make reasonable estimates for how much they will draw in the application you intend to use them in.

[CJO]

I think that the exide batteries are 18 amp-hours

[Joe Ross]

It is important to note that when a battery is specified at 18 AH, that is for the current draw over a long period of time. For our battery, it was for 20 hours. When you draw more current, the battery has a lower capacity.

It's defintely worth your while to look at the battery's data sheet: http://www2.usfirst.org/2005comp/Specs/batex.pdf

For our battery it is 18AH over 20hrs. Over 1 hour, it is only 10.8 hr. Looking at the discharge characteristics, At approximately 50 amps, it will last between 5 and 8 minutes. At 100-120 amps, you are probably getting close to the 2 minute 15 second match length.

Also note that the 120 amp circuit breaker doesn't trip as soon as it reaches 120 amps. The data sheet is here: http://www2.usfirst.org/2004comp/Spe...it_Breaker.pdf

The circuit breaker will actually allow 200+ amps for 10-20 seconds. It will also allow at least 140 amps for the full 2 minute match.

With those two pieces of information, I have no doubt that it is possible to completely drain the battery in 2 minutes and 15 seconds without tripping the main 120 amp circuit breaker.

[Tristan Lall]

Originally Posted by Joe Ross

With those two pieces of information, I have no doubt that it is possible to completely drain the battery in 2 minutes and 15 seconds without tripping the main 120 amp circuit breaker.

I've personally verified this through calculations and empirical observation; Woburn Robotics has used variations of 6-motor drivetrains for the last three years, with occasionally stunning results.

In 2002, the 60 A circuit breaker was a major liability; we removed the two drill motors at our second event (and used the CIMs and Johnson F-Ps), after several breaker-throwing episodes. (Incidentally, we needed a 2-speed, badly, that year.)

In 2003 with a single-speed transmission, all but the most hard-fought matches were handled easily, with no breaker issues (on the more rugged 120 A main breaker); however, matches with more than a minute of consistent pushing would reduce the robot to 6 or 7 V under load, and disable the robot controller. We once managed to push one reasonably powerful opponent up and over the ramp, from our end zone to theirs, before running out of juice, and ourselves getting pushed most of the way back. (Consider that my only driving mistake.... ) The breaker never tripped. For high-speed driving (and the robot was geared rather high), it was very driveable, and extremely responsive.

In 2004, the robot was given a 2-speed gearbox, and though (in hindsight), the gear ratios were too far apart, it demonstrated superb speed and pushing power, while never encountering a power consumption issue. (Batteries were, however, religiously changed after every match; there was no desire to find ourselves short on power.)

The bottom line: it's never been easier to do it, because of the four CIMs, and, if you don't want to put those motors elsewhere, it's a nice place to make use of them. Remember, if you have trouble, you can always remove the offending motors!

[FIRST JerseyKid]

Originally Posted by phrontist

Uh, depends on a lot of factors. Here is a tip: Battery capacity is measured in Amp-Hours. So if a battery is rated at 10 Amp-hours (that figure it entirely ficticious) this means you could draw 10 amps for an hour, or one amp for 10 hours (although this is not a perfect relationship, because batteries have internal resistances). You'll need to lookup the datasheets on everything you intend to put on the bot and make reasonable estimates for how much they will draw in the application you intend to use them in.

I would do this if I had an extra 8 hours and all the information put into 1 packet but maybe after build season I will. For rite now I think we will just do 4 motor drive system.

[Mike Betts]

Originally Posted by FIRST JerseyKid

If you were to put 6 motors in a robot just to drive it around for the 2 minutes and 15 seconds would you have enough battery for 1) The whole round and 2) For other things 3) A compressor ???

You have an 18 AH battery. That is 60*18=1080 amp*minutes. For a 2.25 minute match, you can draw 1080/2.25=480 amps. You have a circuit breaker which will limit you to less than that.

Answer: Yes.