Hardware Reference
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Sensor
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Sensor
Real-Time
ENVIRONMENT
System
USER
Sensor
Figure 11.2
General structure of a monitoring system.
Monitoring systems do not modify the environment but only use sensors to perceive
its state, process sensory data, and display the results to the user. A block diagram
of this type of system is shown in Figure 11.2. Typical applications of these systems
include radar tracking, air traffic control, environmental pollution monitoring, surveil-
lance, and alarm systems. Many of these applications require periodic acquisitions
of multiple sensors, and each sensor may need a different sampling rate. Moreover,
if sensors are used to detect critical conditions, the sampling rate of each sensor has
to be constant in order to perform a correct reconstruction of the external signals. In
these cases, using a hard real-time kernel is a necessary condition for guaranteeing a
predictable behavior of the system. If sensory acquisition is carried out by a set of
concurrent periodic tasks (characterized by proper periods and deadlines), the task set
can be analyzed off-line to verify the feasibility of the schedule within the imposed
timing constraints.
Open-loop control systems are systems that interact with the environment. However,
the actions performed by the actuators do not strictly depend on the current state of
the environment. Sensors are used to plan actions, but there is no feedback between
sensors and actuators. This means that, once an action is planned, it can be executed
independently of new sensory data (see Figure 11.3).
As a typical example of an open-loop control system, consider a robot workstation
equipped with a vision subsystem, whose task is to take a picture of an object, iden-
tify its location, and send the coordinates to the robot for triggering a pick and place
operation. In this task, once the object location is identified and the arm trajectory
is computed based on visual data, the robot motion does not need to be modified on-
line; therefore, no real-time processing is required. Note that real-time computing is
not needed even though the pick and place operation has to be completed within a
deadline. In fact, the correct fulfillment of the robot operation does not depend on
the kernel but on other factors, such as the action planner, the processing speed of
visual data, and the robot speed. For this control problem, fast computing and smart
programming may suffice to meet the goal.
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