Wednesday, April 18, 2012

Student Robotics projects-how cheap can you get?

I am done with robotics projects for a while now, other than an afternoon gig I will be doing with some Boy Scouts later in the month.  Everything gets packed up for the summer months.

Every year when I wind up my basic robotics and my advanced robotics classes I get asked: "How can I do this on my own?"  Which makes me recall how I accumulated the pile of surplus stuff that allows me to create a robotics program with virtually zero ongoing expenses.

Lets take it by steps.

STEP ONE.  SMALL, CHEAP AND ALMOST LEGAL

The basic combat robotics class I do just uses servos and junk.  So really all you would need would be a transmitter, a receiver, two servos and some batteries.  And the ability to charge the batteries.  There is a ton of this stuff out there, probably available for free or near free.  The problem is that most of it is on air frequencies, 72 mhz.  Technically this is illegal to use for ground based projects.  Now, some of our early combat robots did spend time flying though the air after being hit, but we did not use that as a technicality, and always tried to stay legal. 

Not that I would ever encourage even harmless flaunting of FCC rules (almost nobody is using 72 mhz anymore, that's why there is so much of it around).  But here are a few recent ebay offerings:
This gets you transmitter, receiver, crystals for both and a charger.  Usually the charger has ports for both the transmitter and for the receiver battery (not included).  So really all you would have to add would be a 6 volt RC battery and some servos.  Asking price when I looked this one up was $7.50 plus postage.  Most do not sell even at that price.  See, nobody wants to operate expensive airplanes with twitchy controllers.  Stuff crashes.

How 'bout some servos?

Here is a pair of older Futaba servos on ebay for asking price 99 cents and postage.  Make sure you get this kind of connectors, some of the really old servos have an odd three pronged connection that will not fit receivers....unless you found some really, really old receivers too!

I have to say, my local RC shop seems to have stuff like this from time to time.  The owner knows about my student projects and saves things for me.  You could ask about any vintage gear that might be around as RC guys upgrade to newer control systems.  For the record, 27, 49 and 75 mhz are ground legal.  72 mhz is not. 

Oh, regards servos, you can hack Futabas but the Hitec servos are equally abundant and a bit easier.  I have not mentioned 6 volt RC receiver batteries.  You can buy used as above.  Or if handy with a soldering iron make your own.  I have found some cheap surplus cell phone batteries at Axman that can be put together into 6 volt packs.

STEP TWO.  SMALL, CHEAP, LEGAL AND RELIABLE

Lets assume you have at least a minimal budget to play with.  You want to buy some stuff that will not give you an education-the best kind btw-by having to figure things out and fix them.  Alright then.



This is a Vex system that was bidding at $149 with one day to go.  These are used for a lot of student robotics programs and are excellent.  You have to keep your eyes open for bargains, as the prices seem to be all over the map.  The above gets you transmitter, receiver, charger, batteries and a pile of mechanical stuff.  Servos too.  The transmitters in particular are nice, I use them preferentially in my classes.  The grey square object in the lower left is a Vex microprocessor.  If you purchase some additional software you can use this to do actual programming and make autonomous robots that can be programmed to carry out tasks. 

Vex systems seem to come on the market in waves, spring and summer are good times to look for them as school based programs are finishing up and extra stuff hits ebay.

If the whole package is a bit pricey just shop for the Vex transmitter.  It comes with a battery system that uses AA's, usually comes with a crystal and it works with some futaba 75mhz receivers such as these:
The Vex transmitter is basically a re-labled Futaba FM unit, so it works with certain Futaba receivers.  The above Futaba R153F and R133F three channel receivers work just fine.  You will need crystals that match, often the Vex transmitter you bought comes with a set.  And be advised, the Vex transmitters use an odd proprietory crystal type:

If you have a mind to buy servos new I can only suggest my source http://www.servocity.com/ .  Good prices, good folks to work with.  Hitec HS 311, 322  and 325 are all cheap and easy to hack.

STEP THREE.  BIG, FAIRLY CHEAP, RELIABLE AND LEGAL.

Most people start out small.  But if you have a mind to build the bigger stuff, most of the equipment above can be used.  You just need to add speed controllers and bigger batteries.

For speed controllers there are few more adaptable and durable than the IFI Victor 884.

This image is from the Vex Robotics site and gives useful specs and add ons.  You will need to interface between your radio receiver and your speed controllers.  Futaba receivers need a PWM signal booster, which you can get on the Vex site.  Or, if you happened to pick up a Vex microprocessor earlier you can route the PWM connectors through that without the need for a booster.  But you will need to make connectors that link the microprocessor to the speed controller, so order a four pack of the 12 inch 3 wire cable extenders and splice them into two wires with the appropriate connectors.  Yes, I admit this is a little complicated, that's why you should start with the simpler stuff.

On the Vex site the 12 volt Victor 884s list for 89 bucks.  Not bad, but if you keep an eye out on ebay you can occasionally pick them up for about half that.

You need one Victor for each side of your robot, they can drive either one or two motors per side depending on how much current you are putting through them.

Regards motors and such either canniblize some derelict Barbie Jeeps, or keep an eye out for worn out electric wheel chairs.  For batteries anything that sources 12 volts would work, but be advised that car batteries spill acid when tipped, and are in general a less than good choice.  Sealed batteries are more commonly found in motorcycle and lawnmower versions.  Gel cells of various types can be found on the cheap, I find them at Axman Surplus, but they are used and do not have the vigor of new units.  Batteries Plus has fresh units in the 12 to 13 amp-hour size at a somewhat reasonable price.

Clear enough? 

No?  Fine then, just email me.

2 comments:

Paul, Dammit! said...

I have a half-dozen partially-wrecked 80's RC cars and a dozen futaba speed controllers and servos- what's the best place to unload them?

Tacitus2 said...

Paul
I guess ebay is the most realistic option. I have occasionally posted on some of the RC car forums and found little interest in the older stuff.
Futaba FM 75mhz receivers would interest me, if you have such email me at dagmarsuarez@gmail.com That email is mostly a spamtrap but if you post here and tell me to look there I sure would.
T2