Real-Time Systems
Chapter 1
Hermann Kopetz. Real-Time-Systems, Kluwer 1997.
Definition
• A real-time computer system is a computer
system in which its correctness depends on
– The logical results of the computation
– The physical time at which they are produced
Embedded Systems
• Embedded system: A RTCS that
controls a larger system (e.g., cars,
planes, washers).
• Properties:
– Not visible from outside the system.
– Limited resources.
Functional Requirements
• Data collection
– Variables or RT entities
•
•
•
•
In the sphere of control (SOC) of some subsystem
Cannot be changed outside its SOC
Only temporaly accurate; has accuracy interval
Collection can be time- or event-triggered
Functional Requirements
• Data collection
– Variables or RT entities
• …
– Signal conditioning
• Raw sensor data becomes measured data
• Then checked for plausibility: agreed data
– Alarm monitoring
• Abnormal behavior must be detected and generate
alarms (alarm showers)
• Logs must help identify a primary event
Functional Requirements
• Direct digital control
– From the data collected, the RTCS must
calculate the set points for the actuators
– The target is to reduce the difference
between the sensed values and the
reference values
– Control done periodically as a loop of data
collection, set points computing,and output
to the actuators
Functional Requirements
• Man-machine interaction
– A RTCS must inform the operator of the
state of the controlled object
– Bad interfaces can lead to accidents
– Contain
• Extensive data logging
• Data reporting
Temporal Requirements
• Mainly come from the control loop.
• Example:
– container with liquid
– target is selected temperature (set point)
– The RTCS only controls the steam valve
The Controlled Object
• Change in steam flow has two main temporal
parameters:
– dobject: initial inertia
– drise: time until new equilibrium
Controlling Computer System
• The system must
– Sample the temperature periodically
– Compare it with the set point
– Adjust the control valve accordingly
Temporal Parameters
Computer Delay Jitter
• Additional value error
• Must be small wrt the delay