+ MSI Wind L1300 netbookCompUSA; $285
+ Motor mount and wheel kitParallax; $280
+ HB-25 motor driversParallax; $50 (x 2)
+ Ping ultrasonic sensorParallax; $30
+ SLA-12V18 lead-acid batteryBattery Mart; $35
+ ACC-1206S charger for lead-acid battery Battery Mart; $33
+ In-car adapter to power netbookeBay; $18
+ Arduino Duemilanove microcontrollerSparkfun Electronics; $30
+ Logitech C905 webcamStaples; $80
+ SPT-50 pan-and-tilt mechanismServocity.com; $20
Note: This list reflects prices at the time the project was built.
In last month's issue, my fellow IEEE Spectrum editor Erico Guizzo investigated the up-and-coming universe of telepresence robots—electromechanical proxies that allow you to be there without actually being there. As a telecommuter, I was intrigued by the possibility of being able to "walk" over to a colleague's office, just as if I were working down the hall. The problem is that commercial telepresence robots are pricey. The one Erico tested, made by the company Anybots, will sell for close to US $15 000. I'm too sheepish to ask the boss to shell out for something like that.
But I'm not too sheepish to try to build one.
So with Erico's help I cobbled together a simple telepresence robot for about $1000 in parts. Initially, we thought about copying Sparky Jr., a telepresence robot built on an iRobot Create base. Construction is detailed at http://sparkyjr.ning.com, a Web site "dedicated to DIY, open-source mobile telepresence." But this robot's cat-size stature wouldn't be adequate for office life; I'd need something taller to gossip with others around the water cooler and Xerox machine. And the iRobot Create is too small and light to support much of a superstructure. So instead I rolled a robotic base of my own design.
To propel it, I used the motor mount and wheel kit ($280) from Parallax, of Rocklin, Calif. The kit includes a pair of 12-volt DC motors with worm-screw reduction gearing that were surely designed for automobile electric windows. The gearing has more play than I would have liked, but Parallax integrated these motors with nice aluminum mounts, axles, and wheels. Each kit also includes an optical encoder and a position-controller board, providing closed-loop control of wheel movements. But the kit doesn't contain any motor-driver circuitry, so I purchased separate 25-amp units ($50 each), also from Parallax.
Many mobile robots, including the iRobot Create, have four wheels spaced around a circle at 90-degree intervals. Only the two wheels at either side are driven, the two at front and back merely being swiveling casters to keep things level. The nice thing about such an arrangement is that the robot can rotate in place.
I was set to arrange mine the same way, but a friend with considerable experience advised me otherwise. The problem with four wheels, he explained, was that an irregularity in the floor could cause one of the drive wheels to lift, sending the robot off in an unintended direction.
With three wheels, that can't happen. So I mounted the two driven wheels forward on each side and put just one swiveling caster in the rear. In truth, I incorporated five wheels in all, if you include the two small casters I added to the very front. These don't touch the floor: I added them only to prevent the robot from tipping over if it stopped suddenly or was bumped from behind. To be sure, with a heavy 18-ampere-hour lead-acid battery ($35) mounted on the base, there's really not much chance that this robot could topple anywhere.