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