Three-tier architecture

In hybrid architectures, reactions are combined with algorithmic calculations. One type of hybrid architecture, far superior to others in its popularity, is a three-tier architecture , which consists of reactive, executive, and algorithmic levels.

The reactive level provides low-level control of the robot. A distinctive feature of this level is the presence of a rigid “perception-action” cycle. Decision time at this level is often only a few milliseconds.

The executive level (or ordering level) serves as an intermediary between the reactive and algorithmic levels. It takes directives from the algorithmic level and arranges them for transmission to the reactive level. For example, at the executive level, processing of a set of intermediate points formed by an algorithmic path planner can be performed, and then decisions are made about what reactive behavior should be caused. Decision time at the executive level is usually about one second. The executive level is also responsible for the integration of sensory information in the form of an internal state representation. For example, at this level, the robot localization and operational mapping procedures can be implemented.

At the algorithmic level , global solutions to complex problems are developed using planning methods. Due to the computational complexity associated with making such decisions, the decision time at this level is of the order of a few minutes. At the algorithmic level (or planning level), models are used to make decisions. These models can be prepared in advance or formed by training based on available data, and they usually use state information collected at the executive level.

In most modern systems of robotic software, certain variants of the three-tier architecture are used. But the criteria for decomposition into three levels can not be called very strict. In addition, some robotic software systems have additional levels, such as user interface levels, that control user interaction, or levels that are responsible for coordinating the actions of the robot with those of other robots operating in the same environment.