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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.

There are four types of solar engines to date:

  • Type 1 - voltage controlled 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 controlled 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.

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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