real-time systems chapter 1 hermann kopetz. real-time-systems, kluwer 1997
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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
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