The Analyst's Toolbox

Analysis of exotic evolved circuits is different to that undertaken as part of orthodox design. At an abstract level, the appropriate tools are sometimes more akin to neuroscience than to electronic engineering. It is especially important to recognise that an evolved system may not have a clear functional decomposition. A functional analysis decomposes the system into semi-independent subsystems with separate roles; the subsystems interact largely through their functions and independently of the details of the mechanisms that accomplish those functions [Simon 1996]. Systems designed by humans can usually be understood in this way, because of the `divide and conquer' approach universally adopted to tackle complex designs.

Although an evolved system may have particular functions localised in identifiable subsystems, this is not always so. Dynamic systems theory [Burton 1994] provides a mathematical framework in which systems can be characterised without a functional decomposition. Hence, what to many people is the essence of understanding -- being able to point at parts of the whole and say what function they perform -- is not always possible for evolved systems. In this case, more precisely formulated questions regarding the organisation of behaviour must replace fuzzy notions of `understanding' or `explanation' rooted in functional decomposition. In our case, these questions are centred around the suitability of an evolved circuit for engineering applications. Addressing these questions, such as those regarding long-term dynamics, is what we mean by `analysis.'

The successful action of a circuit can be considered as a property of the interface between its inner mechanisms and the external environment [Simon 1996]: the inner has been adapted so that the behaviour at the interface satisfies the specification. Observations at the interface (eg. at input and output connections) during normal circuit operation may reveal little about the inner mechanisms, but instead will largely reflect the demands of the specification. Analysis therefore requires internal probing, and/or observation under abnormal conditions, either internal or external.

There are surprisingly many tactics that can be used to piece-together an analysis:

• Probing and abnormal conditions. Abnormal conditions include: manipulation of the input signals, varying temperature or power-supply voltage, replacing parts of the circuit with alternative or non-functional pieces, and injecting externally generated signals at internal points. Monitoring an internal voltage always has some side-effect, often placing a mostly-reactive load at the probing point. This may have negligible consequences, but potentially perturbs the measurement, or even stops the circuit from working altogether. Probing internal signals of a circuit implemented on a VLSI (Very Large-Scale Integration) chip can require routing extra connections to reach the external pins of the device, with a danger of further disrupting the circuit under study.
• Mathematical techniques, including standard electronics theory, are preferable for their rigour and generality. If a whole unconventional evolved circuit is mathematically intractable, there may still be parts of it which yield.
• Simulation of a circuit allows rapid and extensive interactive exploration. As we shall see, circuits evolved not in simulation, but using real reconfigurable hardware, may rely on detailed hardware properties not easily modelled in a simulation. Attempts at simulation can at least help to clarify the extent of this dependence.
• Synthesis: a circuit can be implemented using alternative electronic devices. For example, a circuit evolved on a single VLSI reconfigurable chip, might then be constructed out of a number of hard-wired small-scale chips. This provides easy access for probing and manipulation of `internal' signals, and again can clarify what aspects of the hardware are important to the circuit's operation.
• Power Consumption for the most common VLSI technology (`CMOS'), is related to the rate of change of the internal voltages. After removing any power-supply smoothing capacitors, power consumption can be monitored with high temporal precision, while the circuit is exposed to test conditions.
• Electromagnetic emissions, resulting from rapidly changing electrical signals, can sometimes be detected using a tuned radio receiver. Circuit activity within a chip, which might be difficult to monitor directly, can thus be roughly characterised.
• Evolutionary history: The mechanism underlying a task-achieving behaviour may be more apparent soon after its evolutionary origin, rather than after evolution has refined it closely to match the specification. It may be possible to identify the innovation (perhaps caused by one or more mutations) giving rise to the behaviour's origin in an ancestor, and to relate this to the operation of the final circuit.
• Population diversity: Sometimes there can be several slightly different (but related) forms of high-fitness circuit in an evolutionary population, which can help to reveal the basic mechanisms used.

 

 

Although unconventional evolved circuits can seem dauntingly unfamiliar, the analyst is far from powerless.

1998-11-18