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I used to oscillate, but now I've relaxed...
Storing energy for a rainy day

At the heart of most solar-powered robots is a circuit called the solar engine (variously called Solar Engines, solarengines, SEs; a.k.a,relaxation oscillators). The purpose of a solar engine is to act like a power "savings account" -- a small trickle of incoming energy is saved up until a useable amount is stored. This stored energy is then released in a burst, in order to drive some useful (if only sporadic and incremental) work.

The solar engine has a number of advantages:

  • A solar-powered robot can be made to work, even in relatively-low light levels.

  • Solar cell size is minimized
    • Saves money
    • Saves weight
    • Allows room for the solar cell to be ruggedized.

Four types of solar engines have been built to date:

  • Type 1 - voltage trigger. This is by far the predominant form of solar engine, since they are "efficient enough" for most uses, and pretty simple to build.

  • Type 2 - time trigger. These aren't terribly efficient, but are handy for 'bots that need activity at specific times.

  • Type 3 - charge curve differentiated (i.e., it triggers when the charge rate of the capacitors slow down). These are theoretically the most efficient, though type 3 designs are still in their infancy.

  • Nocturnal -- These solar engines charge up when it's light, and discharge (i.e., power a load) when it's dark.

Since solar engines are strictly circuits (and generally just a part of a BEAMbot's circuitry, at that), I've got a whole section of the BEAM Circuits Library devoted to more detailed information on them here.

For more information...

Ivar Thorson's writeup on solar engines (particularly handy since it includes efficiency comparisons for various designs) is here.

The excellent BEAM-robotics-Tek writeup is here.

Ian Bernstein of "BEAM-Online" fame has a writeup on 1381-based solar engines here, and a tutorial on how to free-form this kind of circuit here.

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Page author: Eric Seale
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