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Adrenalynn
06-25-2008, 02:46 PM
Maybe the question's more about the preferred motor controllers...

The Sabertooth and the Banebots both start getting flakey at anything less than 6v. The GM* motors list voltage at 3-6v. So a 6v battery will yield flakey motor controllers after just a few minutes of use (As voltage drops to 5.x, and as current load dips the battery below 6v)

So, can the Solarbotics motors be run safely at 7.2v, or what motor controllers should I be looking at for 3-5v operation?

TIA!

lnxfergy
06-25-2008, 03:00 PM
I've been running my GM8s on 7.2V without a problem (actually, I wasn't aware they were rated 3-6V until you said so....). I would guess the one set has around 20-30 hours of runtime on it (of course, not at full speed all the time).

-Fergs

Adrenalynn
06-25-2008, 03:03 PM
Cool, thanks Fergs! What ESCs are you using?

lnxfergy
06-25-2008, 03:39 PM
Cool, thanks Fergs! What ESCs are you using?

I don't use an ESC, just an on-board TI SN754410 quad-half H-bridge chip. Motor control is then via the HW timers on the AVR.

-Fergs

Adrenalynn
06-25-2008, 04:02 PM
4.5-36DC @ 1A. Gotcha, thanks!

lnxfergy
06-25-2008, 04:25 PM
A few more observations as I thought about it:

1) I run the motors off a 7.2V mini-T pack. So fresh battery is about 7.8-8.4V, which means that with the transistor drop the motors get up to 7.2-7.8V for a little while.

2) You are talking about not dropping below 6V to keep your ESC happy, right? In this case, sure you can run 7.2V, but I'm not sure that speed difference is there between 6 and 7.2V, in which case you will pretty much no speed variability for control.

3) On the other hand, Solarbotics shows specs for 9 and 12V for this motor. I'm not so sure 6V is an upper limit. http://solarbotics.com/assets/datasheets/motor_data_summary-gm8.pdf

4) I can actually get the SN754410s to go down to about 2V on the motor terminals. Additionally, they allow a peak of 2Amps - and while I can't say it will work for everyone, I have definitely thrown a small heatsink on the chip and had it run 2 motors each at ~1.2A continuous in 100degree heat for several hours. It got warm, but it held up.... and hey, it's only like $1.65 at Digikey...

-Fergs

Adrenalynn
06-25-2008, 04:43 PM
That spec sheet was especially helpful, thank you!

Looks like the RM3 motor is the key there. The RM2 is "not recommended" at 9.6 and 12v.

I'll need to measure it, but certainly the RPM should go up linearily betwixt 6 and 7.2v, assuming the pack can deliver the current. I have an photoRPM meter here, so I'll measure it at some point.

@Alex: Do you know which motor the GM3 and GM9 come with per-chance? I tore one apart and don't see any designation...

I agree that you an overdrive those transistors. It's all just a function of heat. If you refridgerate them with a peltier and run them at -30C, you can really push 'em. Not that I'd know anything about cryogenically cooling driver transistors. ;) ;) ;)

JonHylands
06-25-2008, 04:52 PM
If you're still looking for a low-voltage motor driver, here's the one I've seen:

http://www.pololu.com/catalog/product/120

Adrenalynn
06-25-2008, 05:06 PM
Interesting, thanks! It's teeny! Very cool. Perfect deskpet accessory.

lnxfergy
06-25-2008, 05:23 PM
That spec sheet was especially helpful, thank you!

Looks like the RM3 motor is the key there. The RM2 is "not recommended" at 9.6 and 12v.

I'll need to measure it, but certainly the RPM should go up linearily betwixt 6 and 7.2v, assuming the pack can deliver the current. I have an photoRPM meter here, so I'll measure it at some point.

@Alex: Do you know which motor the GM3 and GM9 come with per-chance? I tore one apart and don't see any designation...

I agree that you an overdrive those transistors. It's all just a function of heat. If you refridgerate them with a peltier and run them at -30C, you can really push 'em. Not that I'd know anything about cryogenically cooling driver transistors. ;) ;) ;)

The stock motor in all of those gear motors is the RM3. RM2 is an aftermarket upgrade you have to buy separately.

-Fergs

Adrenalynn
06-25-2008, 05:40 PM
Awesomeness, thanks!!!

A-Bot
06-27-2008, 10:54 AM
I have four of the GM14a. I'm using the Sabertooth 2x5 controller with a 1600mah 6V 2/3A pack powering the bot.

Adrenalynn
06-27-2008, 11:36 AM
My 2x10's drop under acceptable voltage and get wonky pretty quick at 6v. I don't think a GM9 could even carry a 16A/h pack. :)

lnxfergy
06-27-2008, 01:41 PM
My 2x10's drop under acceptable voltage and get wonky pretty quick at 6v. I don't think a GM9 could even carry a 16A/h pack. :)

I'm thinking he means 1.6Ah. 2/3A packs are the smaller size for mini-rc cars. Typical cells have 1100-1800mAh. So this would have to be some crazy pack (6v = 5 cells, times 10 in parallel, thats like a 50 cell brick, you'd be better off with D size batts).

If it is a 6V 2/3A 1.6Ah pack, then it's similar to my packs on that little green GM8-powered bot (Xr-A4 uses a 7.2V 1100mAh pack)

-Fergs

Adrenalynn
06-27-2008, 01:44 PM
I figured something similar, that's why I added the smiley face. I'm running mine right now on 7.2v/4500mAh (Sub-C) NiMH - thanks for the clarification!

lnxfergy
06-27-2008, 02:07 PM
I figured something similar, that's why I added the smiley face. I'm running mine right now on 7.2v/4500mAh (Sub-C) NiMH - thanks for the clarification!

So the robot can drive around for what, about 5 days, on a charge??? I get a couple of hours out of a fairly crappy 1050mAh pack, 4500 is gotta be like 2 days of continuous runtime..... (although it's also probably a lot heavier).

-Fergs

Adrenalynn
06-27-2008, 02:21 PM
I'm not sure of runtime yet. I'll probably go to 9.6 x 5400 at some point. The point to this little guy is to just roam around and learn his environment. The longer he can explore the better.

I love Tenergy's high capacity "competition" cells. They make a 10Ah D-Cell NiMH now!

A 4500mAh 7.2v pack with Tamaya-type connector and wires is 380g...

A-Bot
06-27-2008, 02:27 PM
Sorry, I forgot the decimal.

Adrenalynn
06-27-2008, 02:34 PM
I was all excited thinking you might have found some ubercool but reasonably priced new battery. :)

A-Bot
06-27-2008, 02:58 PM
I was all excited thinking you might have found some ubercool but reasonably priced new battery. :)

No, but did you hear that McCain wants to offer a $300 million prize for the next great leap in battery tech?

The most advanced electric cars (like Tesla Motors) are still using Lithium.

Adrenalynn
06-27-2008, 03:02 PM
Yeah, I worth with LiIon and LiPoly professionally, but I just don't like 'em for 'bots. Too many downsides to the technology for the hobbiest in my humble opinion.