Robotik & Elektronik

Power requirements in mobile robots.

The motors in most small robots aren't very picky about their power source

slap a hurking battery on the frame and your motors will do the rest. But your microcontroller (MCU) needs a much better power supply if you want decent performance. Here are some tips to keep your 'bot and its brain happy.

First, pick the right battery. For larger machines, where the battery is usually set deep inside the robot's guts, choose a lead-acid gel-cell battery. They are cheap on the surplus market, take plenty of recharge cycles, and provide a fairly good power to weight ratio.

The design of a smaller machine, however, may let you use a removable battery pack. Here, you can opt for NiCds or Renewal alkaline batteries. NiCds can supply high currents for short stretchs, but self-discharge if left idle for more than a few weeks. Renewal alkalines have a shelf life of 5+ years, but don't have the current density of NiCds.

I've used Renewal batteries for two or three years, taken good care of them, and only had to replace four or so out of more than forty in all that time. The secret to getting long life from Renewals: DON'T RUN THEM DOWN! Unlike NiCd batteries, Renewals suffer (or die) if you drain them completely. Watch your 'bot's performance, and when the motors seem sluggish or the torque starts to drop, pop in a fresh set of Renewals and put the tired ones in the charger.

Second, keep your MCU voltage clean. Wire your robot so the MCU gets its power on a separate voltage line from that used by the motors. Use plenty of bypass capacitors on your MCU board. Bypass both motor leads to the motor's case, using 0.1 uf capacitors, then add a 0.1 uf capacitor across the motor leads themselves. Make sure all robot electrical components use 18 AWG or higher wire in their main ground lead, and tie all ground leads to a single ground connection near the battery.

Third, use an efficient power supply. Don't just throw a 7805 regulator onto a 12 VDC battery; you'll waste nearly 60% of your battery's available power as heat. Instead, build a small supply using a power IC such as National Semiconductor's Simple Switcher (tm). The LM2595 comes in a 5-pin power TO-220 package and supplies 5 VDC at 1 AMP from an input voltage of up to 40 VDC. You can get 85% efficiency from this chip, using only 5 components. The data sheets include a printed circuit board (PCB) layout that measures only 1 square inch! Check National's web site at http://webdirect.natsemi.com for details and data sheets. If that site is busy, try www.national.com.

So make your next 'bot a lean, mean, charging machine. Drop in the right battery and an efficient supply; your plastic pal will thank you for it!

by Karl Lunt

Published on: 2002-12-16 (4453 reads)

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