APPLICATION DESIGN ISSUES - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 408

When the frequencies of hierarchical nested servo loops differ for about an order
of magnitude, the analysis of the stability and the design of the control laws are
significantly simplified.

For instance, if at the lowest level a joint position servo is carried out with a period of
1 ms , a force control loop closed at the middle level can be performed with a period
of 10 ms , while a vision process running at the higher control level can be executed
with a period of 100 ms .

11.3 HIERARCHICAL DESIGN

In this section, we present a hierarchical design approach that can be used to develop
sophisticated control applications requiring sensory integration and multiple feedback
loops. Such a design approach has been actually adopted and experimented on sev-
eral robot control applications built on top of a hard real-time kernel [But91, BAF94,
But96].

output to
the user

high level goal

F3

SU CU

3 3

S3 C3

F2

SU CU

2 2

S2 C2

F1

SU CU

1 1

S1 C1

SU = Sensing Unit

ROBOT

CU = Control Unit

Figure 11.9 Example of a hierarchical control system.