Boe-Bot motors vs. servos

modemman

My Boe-Bot is getting bigger, heavier and clumsier. I have the Tamiya track set on (custom one bought from Parallax) which was fine for a while, but now the treads constantly fall off and the whole thing is kind of fragile, even for home only abuse (after all, it's really sitting on a bunch of 4-40 screws!). Again, love the Parallax kit, it's great for what it is (hey, I learned quite a bit about tracks), I just want to move on to some greater madness.

I'm looking at getting 2" track parts from Lynxmotion, their treads looks quite solid - and make my own design, with some aluminum (or plastic) boards, some standoffs and their sprockets, idlers and bushings. I love their Tri-Track Chassis Kit, but it's over 200 bucks and at this rate I'll have to start skipping lunches...

Questions:

1. Common sense tells me that the normal hobby servos (like the constant rotation ones that came with the Boe-Bot) won't make it. Or will they?

2. If the regular servos will choke (or heat up and explode in a big ball of fire), do I have to use 12Vdc motors, like Parallax does on their high end wheel system? I hate the idea of having to add a 12V battery.

3. If I really must use the 12Vdc motors, how do I interface them with a standard BS2 stamp on a Board of Education (both electrically and software)? How do you control these things with a Stamp?

Thanks for helping the new guy.

Mike Green

1) Hobby servos have relatively small motors. You can get them in all sorts of sizes from various vendors. You need to get ones that are matched for the mechanical load. DC motors come in bigger sizes.

2) There are DC motors that run off 3V, 4.5V, 6V, 12V, 24V, etc. The reason for the higher voltage is that, for larger motors, you can get more power with less current and more current requires larger wires which adds to the weight and size, etc. I'm sure you can find a 6V motor that will provide enough oomph for your robot.

3) Along with the 12V comes more current and the BS2 can't handle either directly. Look up "H-bridge" on the Wikipedia to see how DC motors are controlled. Go through Parallax's webstore and look at the motor controllers that they sell. That will give you some idea of how motors are controlled.

modemman

Thanks Mike.

I did read about the "H-Bridge" and understand now how to interface a normal DC motor to a microcontroller, such as the Stamp.

Words like "smoking" and "melting" made me decide to keep away from attempting to make those rather simple but high maintenance circuits.

Looked up ready made solutions and the most elegant (to me, currently) seems to be the HB-25 Motor Controller from Parallax. At 50 bucks a piece, plus the motors and a new battery pack, that's a whole new adventure. As tempting as it is to just purchase the Lynxmotion chassis and 2 Parallax HB-25 modules, I really want to try to stay low budget and try to make things work, figuring out clever ways to use what I already have - after all, that's what I loved about the Boe-Bot/Stamp concept initially, its simplicity and the fact that it makes you think and push limits.

I think I'll stick with trying to keep moving my bot with hobby servos for a while, and look for ways to improve the tracks mechanically, perhaps try to use 2 servos per side. Or maybe I'll just look for stronger servos (like the Hitec HS-625MG and similar) - after all, a servo is a box with a little motor, a pot, built-in H-Bridge and some gears, right?

Post Edited (modemman) : 3/23/2009 12:51:26 PM GMT

Ugha

Modemman: There are other, far cheaper H-bridges out there. They just can't handle as many amps or are a little harder to use.

A lot of people would recommend the L293 family of single-chip H-bridges... I wouldn't.
I think that the SN754410 is a far better H-Bridge as it contains flyback diodes (meaning you don't need anything external) and it can handle up to 1 amp per channel with two
channels per chip.

In other words, for a lot of small DC motors, all you need is your microcontroller and the SN754410 to control two motors.

You can control the direction of each motor very easily, and using PWM you can control the speed.

Best of all you can get it at www.sparkfun.com for $2.35 [noparse]:)[/noparse]

No, I'm not trying to sell the chip or anything... I just think it is a highly overlooked chip that has huge advantages over most other single-chip options.

PS: You can "piggy-back" SN754410s (sticking one on top of the other and soldering the pins together) to get around 2amps per channel if you need to. A large heat
sink will help as well.

roadrunner3g

So you are looking for the next big robot build, that's easy. you can go cheap or spend big $$$. the track thing your looking at big $$$. to part it out it will cost more then just getting the kit!!! I know I looked into getting some good tracks myself. I ended up getting some from DACTA.COM Lego dacta has some good parts, you can get hard to find Lego parts there. they are a little spendy, but if you have a lot of robot idea's and little $$$ , it's a good way to go. it's not cheap cheap but in the long run it will save some $$$. another good way to go is go to a thrift store and look for toys with motors. then all you have to do is add the electronics. so have, build on.

servo- good for robots but limited! continuous rotation servos are good for small robots and RC cars. or other small control movements that are needed.

ESC- electronic speed control. uses same pulsout commands as servos. good for med- large robots.





Post Edited (roadrunner3g) : 3/23/2009 10:44:53 PM GMT

Prettybird

I am very new at this and just built my first boe bot. I retrofitted some Matrack treads from a Radio Shack RC truck. They still may be available. Haven't tested it yet but it is not flimsy. I used a small set of erector set parts from ebay. My bot is getting heavy too and plan to use it outdoors. I may have to go to more expensive servos. Time will tell. Good luck.

iDave

Wow. I did a double take, thinking you bot bot looks like mine! ....And loe and behold it is mine! (the middle one) And yeah the erector parts were a quick & dirty way to get those suckers on.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"THE ONLY TRUE WISDOM IS IN KNOWING YOU KNOW NOTHING." - SOCRATES

iDave

Also increaseing the voltage feeding the servos from the vdd 5 volts to 7 volts(5 aa batteries) really helps get that weight moving

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"THE ONLY TRUE WISDOM IS IN KNOWING YOU KNOW NOTHING." - SOCRATES

Prettybird

·· Thanks. I have a· 5 cell ni-cad battery pack under the board. I am a laid of electrician / electronics tech. so the hardware is easy. It is the software I have to learn. lol· My bot can carry a bigger 7.2 pack and lots of extras but worry about the weight vs the motor size. I have only just begun. That erector set comtraption took me about 7 hrs. of trial and error. Thanks for the intrest.