Deliberative vs. reactive  control 

NASREM - The standard deliberative control  architecture

NASREM = NASA standard reference model for telerobot control system architectures.

NASREM = conceptual and functional architecture for the control system architectures.

• Literature here.

The control system

The control system is a

• three-legged system of computing modules serviced by
• a global memory
• communication system,

The levels are

1. Servo: provides servo (motor) control for the actuators.
2. Primitive: Provides motion primitives to generate smooth trajectories.
3. Task: Decomposes actions on a single object into sequences of elemental moves.
4. Service bay: Converts actions on groups objects into sequences of tasks on single objects.
5. Service mission: Overall high mission plan converted into command for the service bay.

The modules are

1. Task decomposition: Plans and executes the decomposition of high level goals into low level actions. Decomposition is both

   - temporal -  into sequential actions along the time line and

   - spatial - into concurrent actions by different subsystems

2. World modeling: Consisting of

   Model = best estimate and evaluation of the history,  current, and possible future states of the world.

    Knowledge base: Contains state variables, maps, lists of objects and events and their attributes.

3. Sensory processing: Reads sensor values and provides the filtered and processed values to the world model. Moreover the controller is equipped with an

4. Operator interface: Provides a means by which human operators can observe and supervise the robot. Task commands into any level of the hierarchy can be either from the higher level task decomposition module, from the operator interface or both. Human interference is possible at  each level of the hierarchy. Human and autonomous control can be simultaneous. The operator interface is not present if the robot is working autonomously.

The timing of the tasks

Are each hierarchical level

• planner modules decompose task commands into strings of 2 - 10 planned subtasks for execution.
• strings of sensed events are summarized, integrated, and "chunked" (zusammenklumpen)  into single events at the next higher level.

Time horizon increases by an order of magnitude at each level of the hierarchy -> exponential increase in time.

Hierarchical planning

The principle of hierarchical planning: Higher levels in the hierarchies create  sub goals for the lower  levels

The picture shows the exponentially increase of time with the number of hierarchy steps.

General remarks on hierarchical control

Hierarchical control well suited for structured  and highly predictable environments (manufactures).

Additional material:

Shakey - one of the first mobile robots with planning (STRIPS planning system was developed for this robot).

Moved in an laboratory world. Could identify and manipulate (move) objects.

Reactive control systems

Reactive control:

• Direct  coupling of perception and action.
• Aim is timely robotic response in dynamic and unstructured worlds.
• Typically in the context of motor behaviors.

Reactive control architectures will be discussed on more detail with the behavior based control architectures.

The three major control paradigms

According to Arbib the architectures are characterized by the following scheme: