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Outdoor Robotics Power Plant

A hybrid power supply using a 4-stroke engine and an alternatorby Ken Gracey, Parallax Inc.kgracey@parallax.comRobot magazine is pleased to publish pioneering research by Ken Gracey on a hybrid robot powerplant for outdoor applications. Read the full article in the Fall 07 issue of Robot, available August 1st. Here is a sneak peek at Ken’s design philosophy and approach, with a key electrical schematic, video clip and photos of his new design.

Building an outdoor robot presents a whole new set of challenges to think about: navigation, traversing variable terrain, and more substantial power supplies. With this project I focused on the power supply. My goal was to build a supply that would provide plenty of power for an hour-long outdoor navigation excursion.

Converting a gasoline engine’s power to proportionally-controlled, bi-directional rotation is a challenging task for the hobbyist. The options include: (a) hydraulically, with a pump connected to the engine and two motors on the drive wheels; (b) a clutched direct connection between the engine and drive train, using brake steering for either side for forward-only motion; or (c) a hybrid, in which electric power is generated by the engine to drive electric motors. Transmissions are also an option, but they’re not readily available for gasoline engines this small.

I reasoned that a hybrid approach would provide the ease of electric motor control, fewer custom parts, and hopefully an abundance of power from a gasoline engine. The battery would only be necessary to provide the voltage the alternator needs to start generating current and to absorb large current surges by acting as a capacitor for peak power demands.

Automotive alternators generate at least 25 amps. After my first experiment it was apparent that I had an unlimited power budget. Rather than adding up my power needs and choosing a battery with capacity to run for an hour, I had the luxury of being able to allocate as much power as I wanted to an assortment of accessories! …

Choosing a Four-Stroke Engine and Alternator

Just a few years ago most of the smaller engines were two-strokes used on garden equipment. Putting such a two-stroke engine on your robot would have been loud and dirty. Furthermore, an alternator doesn’t require lots of low-end torque to start spinning. The new 4-stroke engines are quiet and much cleaner… my personal favorite is the Subaru Robin because it operates more smoothly and it has a built-in power switch…

Mechanical Connections

Every robot project seems to have a set of challenges worth at least several months of effort if pursued individually, but they’re all skill builders for future projects. One challenge in this project that I addressed fairly well was the mechanical connection between the engine and the alternator. There are two ways to go about this…


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Don’t miss the Fall 07 issue Robot, available August 1 on newsstands (and earlier to subscribers) for the details in this interesting, groundbreaking account.  Ken Gracey has opened up a world of “backyard robots” with  the power and longevity to do practical work.

–Tom Atwood,

See the Parallax hybrid robot video

Ken Gracey works for Parallax, Inc. in Rocklin, California,

His primary robotics interests include educational uses, small walking robots, machining and outdoor robotics.

Words by Ken Gracey