The term ``action'' has been used widely in the ethology, psychology and robotics literature, sometimes with quite different meanings. A popular definition of action, in the context of action selection in animals, is given by Tyrrell Tyrrell92a: an action refers to one of the mutually exclusive entities at the level of the behavioral final common path [McFarland and Sibly1975]. The level of the behavioral final common path is the lowest level of control in an animal or agent's control system, whereby all behaviors are expressed. However meaningful and accurate from an ethologist's point of view, this definition leads to a certain degree of confusion from a designer's point of view. More precisely, it gives the impression that an action is ``a movement'', hence the action selection mechanism in an anthropomorphic robot, for example, is responsible for determining the detailed movements of every muscle. As a result, one cannot help wondering how the action selection mechanism is any different from the whole control system of an agent. An alternative is therefore to define an action as a motor skill, such as ``move forward'' or ``turn to the right''. Depending on the mechanical model of an agent, each of its actions may require a simple single movement of a single actuator or muscle, or a complex series of coordinated movements of a number of actuators.
Defining an action as a motor skill allows us to deal with the problem of action selection at a higher level by differentiating mechanisms for motor control from those for action selection. More specifically, given this definition, motor control mechanisms are responsible for controlling and coordinating actuators in an agent so as to form useful motor skills, i.e. actions, while action selection mechanisms are only responsible for choosing an action without knowing how it is implemented. This concept underlies the design of the artificial fish. We first build a motor control system to implement a set of basic motor skills, including swim forward and backward, turn right and left, glide, yaw, pitch, roll, and brake. The behavior control system is then built to control the selection of these motor skills in order to produce realistic behaviors. Fig. illustrates how action selection differs from motor control in a general design scheme. Note that exclusive actions, i.e., actions that use the same actuators/muscles, cannot be selected simultaneously. For example, one can not walk while sitting. However, non-exclusive actions can be selected simultaneously. For example, one can walk while eating.
Figure: Differentiating action selection from motor control in design.
It should be realized that, currently, motor control in complex animated creatures, such as articulated figures, still remains an unsolved problem. This limits the present feasibility of implementing the above described behavior control scheme in such creatures. However, this should not indicate the lack of generality of the control scheme for we believe that the separation of action selection from motor control is necessary in achieving high level behaviors in most creatures, especially complex ones.
|Xiaoyuan Tu||January 1996|