Control of Hexapod Walking - A Decentralized Solution Based on Biological Data

In order to investigate hexapod walking, experimental studies on animals have been performed in parallel with software and hardware simulations of the control structures and the body to be controlled. In this paper, we will describe the basic behavioral properties of hexapod walking, as they are known from stick insects, and a simple neural network called Walknet, which exemplifies these properties and also shows some interesting emergent properties. The network contains internal recurrent connections, but important recurrent connections utilize the loop through the environment. This approach leads to a subnet for controlling the three joints of a leg during its swing which is arguably the simplest possible solution. The task for the stance subnet appears more difficult because the movements of a larger and varying number of joints (9-18: three for each leg in stance) have to be controlled such that each leg contributes efficiently to support and propulsion and legs do not work at cross purposes. Already inherently non-linear, this task is further complicated by four factors: 1) the combination of legs in stance varies continuously, 2) during curve walking, legs must move at different speeds, 3) on compliant substrates, the speed of the individual leg may vary unpredictably, and 4) the geometry of the system may vary through growth and injury or due to non-rigid suspension of the joints. This task appears to require some kind of “motor intelligence”. We show that an extremely decentralized, simple controller, based on a combination of negative and positive feedback at the joint level, copes with all these problems.This is possible because we exploit the physical properties of the system to simplify explicit calculations. Walknet is able to control walking at different speeds with an emerging tripod or tetrapod gait pattern, curve walking without any explicit calculation of leg trajectories, walking uphill and downhill, climbing over obstacles, and walking with partly amputated legs.