|A hybrid power supply using a 4-stroke engine and an alternatorby Ken Gracey, Parallax Inc.email@example.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…
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…
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, firstname.lastname@example.org
Ken Gracey works for Parallax, Inc. in Rocklin, California, www.parallax.com.
His primary robotics interests include educational uses, small walking robots, machining and outdoor robotics.
Words by Ken Gracey