Need help selecting an ESC
 

I am acting as an advisor for my High School Science Olympiad team (I am a graduate), in their endeavors in building a competition robot. It's kind of a low budget thing so they are using an aircraft controller and receiver to control a tank style tracked building platform. I'd post a link but I haven't met the requirements to post links as a new member... It's a LynxMotion tri-track kit if you're curious.

If you can help me select an ESC that will correctly drive the motors it would help me a lot. I've only ever run nitro myself so I'm not very familiar with electric drivetrains, manufactures, part sources, etc. I am on the other hand a Computer Engineering major so I understand the material.

Here is a segment from the the spec sheet on the motor they are running...
__________________________________________________ _______________
12vdc 235rpm, Gear Head Motor / Part no.
GHM-12
Torque = 138.9 oz. in.
Weight = 7.15 oz.
Reduction = 30 :1
Length = 59.2mm (motor and gear)
Length = 22.6mm (shaft only)
Diameter = 36.8mm (motor and gear)
Diameter = 6mm (shaft) (0.157")
Current = 145mA (at 12v no load)
Current = 3.8A (at 12v locked shaft)
__________________________________________________ _______________


Specs are 3.8A stalled @ 12 volts. 12 volts is ideal but they can run at 10.8 or 9.6 although I would certainly prefer 12 volts since this is a "sumo" competition. Stalled motors defeats the point of treads ;). The part that I am having trouble with is finding one that has reverse. If they have "micro" ESCs that will work that would be great because weight class is looking pretty close for the whole thing. The real catch is that it's $0 budget so the cheaper the better.

Found this cheap model from r2hobbies: rcps61604 that fits all of the requirements (100A draw is supposed to be grossly overestimated) but at 83g it's a bit chunky considering that the receiver weighs just 8g.

A BEC ESC would be nice too. Competition rules are stupid, all cells on the bot can't total more than 14.4v, even if they are on different circuits. Otherwise I was just going to use a LM7085 voltage regulator to drop the voltage off of the 12v pack anyway.

Thinking about using Kan 2/3 AA 700maH NiMH cells to build the 12v battery packs. I would prefer to use some ELITE 2/3A cells (I run them in airsoft guns) but the weight is really cramping my style. Matches only last 2 minutes and with mixed loads I estimated 10+ minutes of runtime for a pack. Full load from 2 motors is 7.6A while these are capable of 10A draw so they seem like the best option. Are there any better batteries that are roughly the same weight (without going to lithium)?

Thanks for the help guys, I really appreciate it and I'm sure the team will too.

-Nick

You want one of these I believe. 2 channels, fits your voltage range, R/C versions, BEC, fairly light. Notice they have several models under the Motor Driver section. Also lots of other gizmos that might be helpful depending on what all you are trying to do.

http://dimensionengineering.com/Sabertooth2X12RC.htm

If weight is an issue you need to consider lipo batteries. Much lighter. Or li-fe or whatever they are called if you need/want more safety. Still better than ni-mh.

Good Luck!

Thanks for the help! It's a little over budget but I guess versus buying 2 ESC the additional $20 goes along way toward better quality and more options. Otherwise it should work awesome. Looks like 2x5 is a slightly better fit though since it's half the weight and is still above the rated current for the motors.

Can you take a look at this install guide and explain how the wiring works? It looks like this is designed for pistol grip type controllers. Not really a problem (if the coach hasn't shipped the ones he bought back yet) but it would seem more natural (to me anyway) if it was controlled by a joystick for each motor. Is that the feature of the "mixing mode" to turn on/off whether the turning is controller by one input for both or one for each?

add this after .com/ on dimension engineering's website since I can't link...

/datasheets/Sabertooth2X5RCQuickStart.pdf

Lithium packs would be awesome but I already have a speed charger for NiMH batteries so it helps offset the cost. Unless there is a way to set up 700maH ~12v lion packs for under $25 with a charger under $30 too.

Thanks,
-Nick

You’re on the right track. Independent mode will give you 2 separate forward/reverse channels. Typically on 2 joysticks, what I think you are after.

Mixed mode means one channel controls forward/reverse, the other steers. Probably best for a single joystick, forward/backward & left right.

Also depends on which channel you plug them into in the RC receiver, you could set it up all kinds of ways. Either a stick or pistol radio, the controller will work with either it appears. Pistol radio it would make sense to use mixed mode. Either mode would work with a stick radio, personal choice I think.

Either way you need to calibrate the controller to the radio, note the auto calibration feature.

There are others links on the site that should be helpful. Like the example wiring one.

For batteries and chargers, need to look at airplane stuff, they generally have a wider range of sizes than car stuff, including small & inexpensive. Do you have any local hobby shops that have a good plane selection? Strongly advised, to make sure you get everything that plugs together without messing around with different connectors and such if you can.

Backup plan, call one of the online/mail order shops and ask for help, most are fairly good. Sorry I don’t know of any good online/mailorder airplane sites to recommend, perhaps someone else will come along and help.

Give me a little time, let me do some looking. I see stuff that fits your budget I think, but I want to look a little more. Unless someone beats me to it!


Some battery basics: Lipos are 3.7v per cell nominal, 4.2V fully charged. So a 3s (3 cells in series) is 11.1V nominal, 12.6V full charge. Note it is very important to not run a lipo all the way down.

Li-on and Li-fe might be problematic. Li-on is typically low current, li-fe is generally a little more expensive to the best of my knowledge.

Nick, sorry, but I’m not finding enough information to give me confidence in recommendations, stuff that will work without possible connector issues. The biggest pain regarding lipos, the different connectors.

But some sites you can take a look at:

http://www.e-fliterc.com/Electronics_Accessories/

E-Flight is a common plane brand, several small lipos and inexpensive chargers. Be aware that many, but not all, small chargers are designed for 12V, like a car battery

If you don’t have a good local hobby shop, an online that has a good rep, and I have had good experiences with, call them and explain your deal:

http://www.amainhobbies.com/

Suggest getting a little bigger lipo than you think if you go that way, best not to use more than 80% of the capacity.

Good luck & Cheers

Well the prices on the lipo batteries seem to be about as expensive as the NiMH batteries packs that I was looking to build, possibly a little bit cheaper than the ones I was looking at. Plus they one holds an additional 450mAh with 20g less weight and probably about $5 cheaper by the time I paid for plugs.

Found this battery for $13 from amainhobbies (add link segment after the .com). 1500mAh and over twice the needed discharge rate but I have learned to not get too excited about stated values...

/product_info.php/cPath/1389_1393/products_id/149379/n/SuperTigre-15C-Li-Poly-Battery-Pack-111V-1500mAh

This one looks like a more proper price/capacity ratio and it is available from TowerHobbies where we are buying the radios so if Venom is a somewhat respectable company I like the looks of these batteries. There website says that they are based in the US and offer 1 year replacement on any malfunctioning battery. Although I have once again learned to be skeptical of such claims.

/product_info.php/cPath/1389_1393/products_id/143473/n/Venom-Power-3S-Li-Poly-20C-Battery-Pack-w-Traxxas-Connector-111V-1300mAh

And then a Team Losi pack
/product_info.php/cPath/1389_1393/products_id/24780/n/Team-Losi-3S-Li-Poly-20C-Car-Battery-Pack-111V-1000mAh

Multiple sources on Lipo chargers seem to show decentish ones for under $50 so I would respect Lipos as a decent option if they are going to invest in battery packs and charging system. 12v Charging may not be a problem because I have a 13.8v 3A power supply they could use, they could get one of the wall-warts that e-flite sells, or they may want to use an emergency car battery charger that they have.

Suggestions on a battery charger from TowerHobbies would be great since we are planning on buying a decent amount of stuff from them at one time. There's alot to choose from in the price range so I am not sure what is a decent charger. The capability to charge NiMH would be another great addition as well. If there is nothing good under $50 at Tower Hobbies I was looking at the "Dynam Supermate DC6 Double Power" since it charges all lipos as well as NIMH, and it has AC/DC source power. Makes my charger look like junk and I paid twice that much...

I'll send him an email suggesting he look into Lipos, possible call our local Hobby Haven and see what they'd be willing to do about an educational discount.

Thanks,
-Nick