The paper presents a robot control system design method based on hierarchical Finite State Machines (FSMs). The system is decomposed into subsystems. Each one of them is specified in terms of a hierarchical FSM. Previously the description of subsystem activities had been presented as a single, complex FSM, where with each of its states a behaviour was associated. Each behaviour was described by a flowchart. In this proposal behaviours are defined in terms of individual FSMs, resulting in a unified description using only the concept of an FSM. By using FSM specification tools that provide automatic code generation, the robot control system implementation is significantly accelerated. This approach has been verified through the design of a table-tennis ball collecting robot.