by Tom Jenner
Robotic nervous systems attempt to simulate the
movement patterns of animals. With the exception of a
few lower invertebrates, animals have a nervous system that
utilizes central pattern generators to coordinate and
synchronize their movements. The central pattern generator
has a pacemaker neuron.
The pacemaker neuron, when combined with a
phase-shifting network or interacting pacemaker neurons,
causes the generation of an oscillating signal that is received
at the muscle tissue through inter-neurons and motor
neurons. These neurons communicate with voltage spikes
and so this type of processing is called “spike based
computing” (also known as “integrate and fire”).
A spike based computing form of communication can be
effective and robust, especially in a noisy environment where
signal attenuation may occur over a long distance; e.g., from
the spinal cord to the hand. Spike computing is also efficient
at processing information in the time domain.
A substantial amount of research has taken place in the
Circuit 1. Basic motor neuron.
18 SERVO 07.2004
©2004 New School Technologies, LLC
field of spike based computing with respect to robotics and
artificial life. This research tends to be not only complicated,
but expensive. Very complex circuits using custom VLSI (Very
Large Scale Integrated) analog silicon, digital signal processors,
or a combination of both have been created to simulate how a
biological central pattern generator and nervous system work.
Others have attempted to create simple nervous systems
for robots. Most notably, Dr. Mark Tilden — the founder of
BEAM robots — uses an adaptive, oscillating ring network to
pattern the movement of robotic legs, each of which is
independent. It uses a pulse delay circuit wired up to another
pulse delay circuit that functions as an artificial neuron.
Some of the disadvantages of this approach are that the
motors have no idea where they are in their phase space (unless
potentiometers are used or circuits that are set up as “edge
cells,” such as limit switches) and that they are not taking
advantage of the computing power of variable analog circuitry.
It should be understood, however, that Dr. Tilden was trying to
use the simplest circuits possible to create a nervous system
using the “integrate and fire” approach of biological neurons
and was a pioneer in the field of robotics in the process.
Nature Doesn’t Have the Benefit
of Silicon ...
This nervous system for locomotion differs from most
(all?) others in that it can synthesize the action of biological
nervous systems by using a wide variety of oscillators that
can operate from around 1/2 to 3 Hertz. Also, there are