Presentation on

Real Time Systems andAdaptive Cruise Control

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

HW1 HW2 HW3 HW4Hardware Interface

RTOS/Drivers

Thr

eadArchitectural

Design

F1F2

F3

F4

F5Functional

Design

(F3) (F4)

(F5)

(F2)

Source:Source:Ian Phillips, ARMIan Phillips, ARM

VSIA 2001

Source:Source:Ian Phillips, ARMIan Phillips, ARM

VSIA 2001

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”

A real-time system is a system that reacts to events in the environment by performing predefined actions

I/O - data

I/O - data

Real-Time Systems

Real-timecomputing system

event

action

within specified time intervals.

time

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, ...

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?

deadline (dl)

+

Hard, soft, firm

• Hard -- result useless or dangerousif deadline exceeded

Ex: Aircraft, Chemical Plant value

time

-

hardsoft

• Soft -- result of some - lower value if deadline exceeded

Ex: Multimedia, Interactive video games

• Firm -- If value drops to zero at deadline

Timing ConstraintsReal-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 correctnesse.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

Physical properties of environment

Model-design

Timing constraints

Analysis, Testing

Run-time dispatching

(In field use)

Functional

Temporal

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

mint

time

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…

Human Driver Model

Structure of Human Driver in Car-Following

• Stimulus-Reaction Model

Car Following Models

• Linear Follow-the-Leader Model– Stimulus: Velocity Difference b/w Leader and Follower– Reaction: Acceleration command to vehicle

• Look-Ahead-Model– Driver observes the behavior of three vehicles ahead of

him.– Stimulus: Majority direction of Acceleration– Reaction: Acceleration command using switching logic

• Others…

Simple Car Following Model

vl Velocity of Leader

vf Velocity of Follower

rl Retardation of Leader

rf Retardation of Follower

tr Short Reaction Time

Acceleration profile of vehicle

Dmin = Di – Di-1

Di = D1i + D2i + D3i

Src: Prof. Shashikant's Control System Lec-1 in DEP Mode

ACC System Design

(desired vehicle speed)(desired vehicle speed)

Control I/PControl I/PPhysical Physical ProcessProcess

SensorsSensors

ActuatorsActuators

AdaptiveAdaptive

Cruise Cont.Cruise Cont.

Reference InputReference Input

Actual outputActual output

Sensor NoiseSensor Noise

Actuator NoiseActuator Noise

Sensed O/PSensed O/P

DesiredDesired

Control I/PControl I/P

DisturbancesDisturbances(accelerator pedal (throttle) position, brake pedal position)(accelerator pedal (throttle) position, brake pedal position)

(wheel speed sensor)(wheel speed sensor)

(air drag, grade,(air drag, grade,

friction etc)friction etc)

(vehicle speed)(vehicle speed)

Src: Prof. Shashikant's Control System Lec-1 in DEP Mode

Process Model

Physical Physical ProcessProcess

Actual OutputActual Output

Control I/PControl I/P

DisturbancesDisturbances

EE 1/M1/MGG 1/R1/Rww

Pictorial View

Friction Estimator

Speed Sensor

Radar System

Roadside Signals

Control Algorithm

Throttle System

ABS

ActuatorsSensors

Schematic Picture of Control Algorithm and its Environment

SPEED Module

DISTANCE Module

Min-value

Control Signal to the Actuators

The structure of Control Algorithm

as

ad

Block Diagram

Flow Chart

Flow Chart

(cont…)

State Diagram

g

a

b

Wheel S

IR S

f

Speed Set

Throttle S

Brake S

c

d

e

Throttle A

Brake A

Curr_Thr Pos

Curr_Br Pos

Precedence Graph showing communication relation

Task Graph

Extensions to Functional Model under consideration

• Adaptive to

– Driver Reaction Time– Roadside Signals– Friction b/w road and tyre (ABS)– Relative positioning in the lane

Future Work

• Partitioning tasks as TT and/or ET and as Soft, Hard or Firm.

• Writing Algorithm

• Allocation of Tasks

• Schedulability

• One or two similar application if time permits