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The global goal (the robot's mission objectives) is successively broken down into sub-goals: the tasks and actions, each handled by control functions on the appropriate layer, until the elementary "control outputs" can be issued to the robotic devices (e.g., currents to the joint motor drives).
The "vertical" branches of the FRM are based on the concept of feedback. The center branch is called forward control (FC). FC functions are responsible for activity decomposition, execution planning and control by taking the most appropriate a priori information known to each layer into account. The left branch consists of nominal feedback (NF) functions for the refinement and update of a priori knowledge ("world models"), based on the actual, but essentially expected, evolution of the process, and consequently the formulation of controlled adjustments of the FC. By FC and NF, "cascaded" control loops are closed on action, task, and mission layers and are equipped with everything necessary for the "nominal" course of events. Besides FC and NF, the FRM foresees a third branch of "non- nominal feedback" (NNF) functions responsible for the monitoring of discrepancies between actual and allowable states in both the FC and NF functions, diagnosis of their origins, and generation of directives (including recovery strategies and constraints) for FC.
A much more refined definition of the FRM is given in (Putz and Mau, 1992). It should also be acknowledged that the FRM concept and structure have been heavily and beneficially influenced by the NASREM architecture developed at the US NIST (Albus, et al., 1989).
2.5 Application reference model
The FRM is by its nature still very generic, and it applies to general automation control systems. To be more specific for the frequent application to "classical" robot control, the CDM has elaborated a more detailed reference model for this application class, called the application reference model (ARM).
The control concepts relevant to an industrial robot are completely located in the action layer of the FRM. In fact, the robot controller has to implement the control functions relevant to action planning, execution and control, while mission- and task-level functions are in the charge of the robot and workcell engineer, who has to program a mission execution as a suitable sequence of tasks, and tasks as sequences of actions. In the same way, it is a human operator that has to intervene to "rescue" unexpected, non- nominal situations. Therefore the ARM is a refinement of the FC and NF functionalities in the action layer.