23-12-2010, 02:11 PM
Prepared by:Gurulingesh R.
[attachment=7571]
Roadmap
Introduction to RTS
Problem Definition / Motivation
Adaptive Cruise Control (ACC)
Driver Models
Functional Model & Task Model
Extensions to Functional Model
Conclusion & Future Work
References
Functional Design & Mapping
What is “real” about real-time?
computer world
e.g., PC
average response for user
Interactive
occasionally longer
reaction: user annoyed
computer controls speed of user
“computer time”
real world
Industrial system, airplane
environment has own speed
reaction too slow: deadline miss
reaction: damage, pot. loss of human life
computer must follow speed of environment
“real-time”
Real-Time Systems
A real-time system is a system that reacts to events in the environment by performing predefined actions
Real-Time Systems: Properties of Interest
Safety: Nothing bad will happen.
Liveness: Something good will happen.
Timeliness: Things will happen on time - by their deadlines, periodically, ...
In a Real-Time System…
correct value delivered too late is incorrect
e.g., traffic light: light must be green when crossing, not enough before
Real-time:
(Timely) reactions to events as they occur, at their pace: (real-time) system (internal) time same time scale as environment (external) time
Types of RT Systems
Dimensions along which real-time activities can be categorized:
how tight are the deadlines?
--deadlines are tight when
laxity (deadline -- computation time) is small.
how strict are the deadlines?
what is the value of executing an activity after its deadline?
what are the characteristics of environment? how static or dynamic must the system be?
Hard, soft, firm
Timing Constraints
Real-time means to be in time --- how do we know something is “in time”? how do we express that?
Timing constraints are used to specify temporal correctness e.g., “finish assignment by 2pm”, “be at station before train departs”.
A system is said to be (temporally) feasible, if it meets all specified timing constraints.
Timing constraints do not come out of thin air: design process identifies events, derives models, and finally specifies timing constraints
Overall Picture
Timing Properties
Periodic
activity occurs repeatedly
e.g., to monitor environment values, temperature, etc.
Aperiodic
can occur any time
no arrival pattern given
Sporadic
can occur any time, but
minimum time between arrivals
Who initiates (triggers) actions?
Example: Chemical process
controlled so that temperature stays below danger level
warning is triggered before danger point
…… so that cooling can still occur
Two possibilities:
action whenever temp raises above warn -- event triggered
look every int time intervals; action when temp if measures above warn -- time triggered
Other Issues to worry about
Meet requirements -- some activities may run only:
after others have completed - precedence constraints
while others are not running - mutual exclusion
within certain times - temporal constraints
Scheduling
planning of activities, such that required timing is kept
Allocation
where should a task execute?
Project Motivation
Motivation (Cont…)
Partitioning of system into TT and ET domains
Process Mapping
Optimization of parameters corresponding to communication protocol.
Sequence and Slots of TDMA (TTC)
Priorities of Messages (ETC)
Schedulability
Adaptive Cruise Control
Adaptive Cruise Control:
automatically adjusts vehicle speed to maintain a driver-selected safe distance from the vehicle ahead in the same lane.
It then returns to the set speed when traffic clears.
Requirements:
The speed should be kept close to the SET speed, if there is no vehicle ahead.
Timegap should be maintained at x sec.
Manual intervention, UI, etc…
Functions Identified
Computing Current speed of our vehicle
Leading Vehicle related Task
Controlling Speed of our Vehicle
Controlling the Throttle
Controlling the Brake
Detecting Manual Intervention
UI to the Driver
Periodicity of Tasks
Hard, Firm; Periodic, Aperiodic…
