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My latest project...MBot

Karl Lunt

karllunt@seanet.com

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I've included a couple of .jpg files showing MBot, a robot I brought to the January SRS meeting. MBot stands slightly over four inches high, with a footprint of about seven inches by five inches. It uses two 12 VDC gearhead motors for motion, a heavily modified BOTBoard+ for a computer board, and two H-bridge drivers built from TSC4427 dual MOSFET driver ICs.

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I designed MBot's frame so the electronics fits on the underside of the top, made of a piece of double-sided copper-clad PCB material. I used copper-clad because I can solder brass hinges to it, using a heavy-duty Weller soldering gun. As one of the photos shows, I can tilt the lid back to get full access to the electronics, yet run with the lid fastened in place, so the circuitry is protected.

Drive power comes from two 1.3 AHr 12 VDC gel-cell batteries, hook in parallel. This gives MBot plenty of running time, since the motors only draw a few tens of mA powered up.

I'm especially pleased with the wheels Dan Mauch and I designed. I've already sent an article on the design to Nuts & Volts magazine, so you should see full details in a month or so. Basically, the wheels are 3-inch plexiglass discs, 1/4-inch thick, bored to fit the 6mm shaft of the motor. I then fastened a 6mm split shaft collar, available from McMaster-Carr, to the front surface of the wheel. Now I can clamp the wheel/collar assembly onto the motor shaft by just tightening the collar's set screw. Simple, cheap, and elegant!

The electronics includes a BOTBoard+ with a psd813 chip wired in. If you haven't seen the full details on this hack at my web site ( http://www.seanet.com/~karllunt   ) , stop by and take a look. This one chip adds 128K of flash, 32K of EEPROM, 2K of static RAM, about 20 I/O lines, and 3000 gates of programmable logic. It is the perfect addition to a 68hc11, and a newer version of this device doubles the flash and brings the static RAM to 8K. Price is about $10 each from Arrow or Avnet, but you can get free samples from the factory rep, I-square in Bellevue (425) 822-9220.

The other boards you see in the photos include a National Semiconductor SimpleSwitcherŽ power supply demo board, a motor-driver board, and a wheel encoder board. The power supply board is an old one I had left over from when National was giving them away; now they cost $8 from the National web site; still a bargain, if you ask me. Check http://www.national.com  for details.

The motor driver board is WAY oversized, since it only has two TelComm TSC4427 MOSFET drivers on it. Each 8-pin device can supply up to 600 mA of current to a single motor when configured as an H-bridge; you can also use them as relay drivers. They are dirt-cheap at about $1.70 each from Digi-Key.

The wheel encoder uses a single 74hc14 Schmidt-trigger hex inverter to provide two encoder output channels, one for each wheel. I've added two LEDs so I have visual indication of encoder operation; this is far simpler than hooking up a serial cable and software. The actual encoders consist of IR LED/phototransistor pairs that I bought at Vetco years ago. Each pair is mounted so it faces a 90-segment encoder wheel that I printed on my laser printer, then fastened to the inside surface of the wheel.