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BEAM Robots -- Feeding and Moving Behaviors

Over time, a community tends to develop its own dialect as what were once slang moves into acceptable usage. The BEAM robotics community is typical in this regard.

BEAM, with its emphasis on biology as a metaphor for robotic design, tends to borrow words from biology, neurology, and physiology. These then get mucked up with other robotic jargon. The upside is that this influx of words and ideas sparks creativity. The risk, however, is that the way we talk and think about our BEAM robots becomes less and less understandable to the outside community.

This article will connect some common BEAM terms back to their biological roots. Words, as much as anything else, will always be open to interpretation. If nothing more, this article documents my personal take on these terms and how they are used on this web site.

Are you eating that?

Photovore is probably the most common term used in BEAM designs. Vore comes from the Latin word for "swallow up." Some well known 'vores include carnivores, herbivores, and insectivores. These eat meat, plants, and insects. Tilden defines photovores in Living Machines as "light-eaters." That is pretty accurate and inline with biology terms.

To my knowledge, Omni Magazine was the first to misuse the term photovore in 1988. The "Omni Photovore" was a battery-powered car that tracked light. The problem was that the robot did not eat light. That is to say, it was not solar powered.

A 'vore is not a behavior but rather a power source. Chasing butterflies does not make you an insectivore. Nor does chasing light make a machine a photovore. The "Omni Photovore" should be appropriately thought of as an "Omni Phototaxis".

Animals take taxis while plants take turns.

There are three suffixes for movements in response to external stimulus: nastic, tropic, and taxis. Nastic implies a random response to stimulus, tropic means a turning toward the stimulus, and taxis implies moving towards the stimulus. There are a number of stimulus, including light (photo), sound (phono), and wind (aero). I have included a table of some common ones at the end of this article.

Bacteria and other simple animals display nastic and taxis behaviors. Meanwhile, plants are limited to nastic and tropic responses. Robots -- being neither animal, vegetable, nor mineral -- are not restricted to these categories.

In fact, there seems to be some confusion about which terms apply. Tilden, again in Living Machines, uses them both. When discussing Turbot 2, Tilden writes that it "exhibits aggressive phototropism to the point where it will systematically try every way to get over a significant obstacle to a brighter light environment. It has no focusing apparatus, yet in a general lighted environment it exhibits strong phototaxis."

Since tropic means "to turn", I limit my use of phototropic to 1-DOF heads. These literally turn towards the light, after all. I use phototaxis to describe robots that actively track light because taxis implies directed motion.

Positive about that?

Of course, the robots we build don't always run towards the light. We also set them up to run away from it. What word fits here? Sometimes, the BEAM community will speak of "photophobic" or "photophobia" (light-fear). While striking fear in the heart of the photovore might be amusing, it is not quite accurate.

Tropic and taxis motions can be described as positive or negative. A positive motion is one towards the stimulus, negative away. If positive or negative is not mentioned, the motion is assumed to be positive.

My light avoiding heads are displaying negative phototropism and are not afraid.

Why does it matter?

Using the right word for the behavior is like using the right tool for the job. We regularly review the tools we use and look for better ways to build. Likewise, I suggest we regularly review the words we use to look for better descriptions and deeper understandings. Each step of the way will only increase our understanding of BEAM principles.

J Wolfgang Goerlich
August 2004


Special thanks to Tom Gray for originally bringing these terminology oddities to my attention, for encouraging me to explore nastic motions, and for inspiring me to take a closer look at biology and etymology.

Stimuli and Motions Table

Stimulus Nastic Movement Tropic Movement Taxis Movement
Air currents n/a Aerotropism Aerotaxis
Chemical Chemonasty Chemotropism Chemotaxis
Light Photonasty Phototropism / heliotropism Phototaxis
Sound Phononasty Phonotropism Phonotaxis
Temperature Thermonasty Thermotropism Thermotaxis
Touch Thigmonasty Thigmotropism Thigmotaxis
Touch pressure n/a Haptotropism Haptotaxis
Water Hydronasty Hydrotropism Hydrotaxis

Reference Links

1. Living Machines (Hasslacher & Tilden; 1994)
http://www.solarbotics.net/library/ pdflib/pdf/living_machines.pdf

2. "The OMNI Photovore - How to build a robot that thinks like a roach"
Omni Magazine, October 1988, Pg 201-210,212

3. Tom Gray, "Photovore/Phototropism/Phototaxis", 28 Mar 2002