ENGG4420: LECTURE 3 --CHARACTERISTICS OF REAL-TIME …

ENGG4420: LECTURE 3 -- CHARACTERISTICS OF

REAL-TIME SYSTEMS

TIME CONSTRAINTS: every real-time task is

associated with some time constraints: a)

Deadline associated with tasks -- common

constraint; b) Delay; c) Duration

A.

It is the responsibility of the Real-Time Operating

System (RTOS) to ensure that all tasks meet their

respective time constraints.

A logically correct result produced after the

deadline would be considered incorrect.

a.

CORRECTNESS CRITERION: correctness in real-

time system implies both logical correctness of

results, and correctness of time at which the

results are produced.

B.

Safe system is one that does not cause any

damage even when it fails

a.

Reliable system is one that can operate for

long durations of time without any failure

b.

SAFETY-CRITICALITY: In many real-time systems

the issues related to safety and reliability are

bounded together to generate safety-critical

systems -- safety-critical system is required to be

highly reliable.

C.

CHAPTER 1 By Radu Muresan University of Guelph Page 1

Sensor data may be sensed periodically,

aperiodically -- therefore real-time systems must

process data from all the sensors concurrently,

otherwise signals may be lost and the system may

malfunction

a.

These systems can be considered non-

deterministic, since the behaviour of the system

depends on the exact timing of its inputs

b.

A non-deterministic computation is one in which

two runs using the same set of input data can

produce two distinct sets of output data.

c.

CONCURRENCY: a real-time system usually needs to

respond to several independent events within very

short and strict time bounds

D.

STABILITY -- under overload conditions, real-time

systems need to continue to meet the deadlines of

the most critical tasks, though the deadlines of non-

critical tasks may not be met

E.


The different events of interest arise at

geographically separated locations

○

There is need to handle locally the events and

responses produced to them to prevent

overloading of the communication network

○

Therefore, the sensors and the actuators may be

located at places where the events are

generated -- example is a petroleum refinery plant.

○

Many distributed as well as centralized real-time

systems have a feedback structure as shown below

○

DISTRIBUTED AND FEEBACK STRUCTURE -- in many

real-time systems, the different components of the

system are naturally distributed across widely

geographic area.

F.


Criticality of a task is determined by examining

how critical are the results produced by the task

to the proper functionality of the system.

○

The higher the criticality of a task the more reliable it

should be made

�

Criticality of tasks needs to be taken in

consideration when we design for fault-tolerance

○

If a highly critical task fails, immediate failure

detection and recovery are important

○

Note that task priority is a different concept then

task criticality and the priority of a task should

not be determined solely based on its criticality

○

TASK CRITICALITY: is a measure of the cost of failure

of a task

G.

Ex: in a chemical plant that is setup to work non-

stop taking corrective actions on a failure

becomes difficult in the absents of operators

○

Even if corrective actions cannot be immediately

taken it is important that a failure does not result

in a catastrophic situation.

○

A failure needs to be detected and the system

should continue to function in a graceful

degraded mode, rather then shut down abruptly.

○

EXCEPTION HANDLING -- many real-time systems

work round-the-clock.

H.


REACTIVE: a reactive system is one in which an

ongoing interaction between the computer and

the environment is maintained.

I.

Traditional systems compute the output data as

some function of the input data (without

controlling the environment)

○

In each interaction step, the results computed are used

to carry out some actions on the environment.

�

The reaction of the environment is sampled and is fed

back to the system -- the computations in real-time

system can be considered non-terminating.

�

In contrast real-time systems do not produce any

output data but enter into an ongoing interaction

with their environment

○

CUSTOM HARDWARE: a real-time system is often

implemented on custom hardware that is

specifically designed and developed for the

purpose.

J.

Ex: cell phones use various custom components,

the MPFI use specialized embedded processor.

○


REAL-TIME EMBEDDED COMPUTERS -- are

computers that implement one or more

functional elements of a real-time system and is

not a stand-alone computing machine

K.

Connection to the environment through a wide

range of interface devices, and

�

Receive and send variety of stimuli�

Example of an embedded system is the Multi-Point

Fuel Injection (MPFI) system.

�

Common features of real-time systems and

embedded computers are:

○

Complex due to its environment interaction○

Difficult to test during the design process○

Need to use plant models and environment

models in order to develop a functional design

○

LabView is a program that is very useful in

simulating environment, modeling plants, testing,

and implementing support software.

○

Majority of the 41x designs that I supervise have

used LabView to demonstrate proof of concept

for their embedded applications (real-time or

not).

○

DESIGN OF REAL-TIME EMBEDDED SYSTEMS


TEST ARCHITECTURES


CLASSIFICATIONS OF REAL-TIME SYSTEMS BASED

ON SYNCHRONIZATION

Tasks are internal actions carried by the computer○

External processes have their own time scale○

Tasks are connected by physical devices to external

processes. We need to consider that:

•

Computer tasks operate in real-time if their actions

are related to the time-scales of the external

processes.

•

Operations are carried out according to a time schedule�

Clock based -- based on passage of time or actual

time of the day

○

Event based -- the events determine the

synchronization -- Ex. Closure of a switch ...

○

Ex. a set of operations should be completed within

a predetermined time.

�

Interactive -- relationship between actions in the

computer and processes is loosely defined.

○

Synchronization types between external processes

and the tasks (internal actions) are:

•

IMPORTANCE OF PLANT CHARACTERISTIC

The time constant of a plant is a measure of the

time taken by the plant to respond to a change in

input or load and is called the plant characteristic

•

For feedback control the required sampling rate is

dependent on the time constant of the process.

•


Real-time requirements -- computer used for control

must be able to carry out all the required operations

(i.e., measurement, control, actuation) within each

sampling interval.

•

Synchronization is obtained by adding a real-time

clock to the computer system and using interrupts

from this clock to carry out operations.

•

The task is to run once per period T, or it is to run

at exactly T unit intervals

○

CLOCK BASED TASKS -- are referred to as cyclic or

periodic

Ex: turning off a pump, closing a valve, or switching

a motor off in response to the closure of a position

microswitch.

�

Event based systems are also used extensively to

indicate alarm conditions and initiate alarm actions

in industrial processes.

�

Actions are performed in response to some event ○

The general real-time requirement for event-

based systems is that the system responds within

a given maximum time to an event.

○

EVENT BASED TASKS -- events occur at non-

deterministic intervals -- also called aperiodic tasks


Interrupts -- to inform the computer that action

is required

○

Polling -- the computer periodically asks (polls)

the various sensors to see if action is required.

○

Implementation of event based systems•

Example: a task may be required to start within 0.5

seconds or produce an output within 0.5 seconds

of an event occurring

�

Such tasks may have deadlines expressed in

terms of start or finish times (or even both).

○

Events based tasks are called aperiodic tasks --

events occur at non-deterministic intervals

•

The real-time requirement for these system is

usually expressed in terms such as: the average

response time should not exceed 20 seconds

○

INTERACTIVE SYSTEMS -- reservation system for

airlines, hotels, etc. Or a bank teller system.

Interactive versus event based -- interactive systems

respond at a time determined by the internal state

of the computer and without reference to the

environment.

•

Interactive versus clock based -- clock based are

tightly synchronized to an external process through

a clock while interactive are not.

•


TYPES OF REAL-TIME SYSTEMS AND TASKS

Using the deadline constraint we can have two major

categories of real-time tasks: hard real-time and soft

real-time.

•

Hard, soft, or firm real-time task○

Some authors divide the real-time tasks based on

the consequences of a task missing a deadline in:

•

Within a real-time system it is not necessary to have

all task belonging to the same category.

•

The system is considered to have failed whenever

any of its hard real-time tasks does not produce

its required results before their deadlines.

○

Robot system -- a robot cyclically carries out various

tasks such as communication with the host, logging

all completed activities, sensing the environment to

detect any obstacle, tracking the object of interest,

path planning, effecting next move.

�

Consider that a robot suddenly encounters an

obstacle -- the robot needs to detect it and try to

escape colliding with it quickly -- if concerned tasks

are not completed in time then collision takes place

and the robot would be considered to have failed.

�

Examples:○

HARD REAL-TIME TASKS -- it is a task that is

constrained to produce its results within certain

predefined time bounds


APPLICATIONS that have hard real-time tasks

are normally safety-critical.

•

Any failure in a hard real-time task would

result in a severe consequence for the

application.

•

As a result, the hard real-time tasks are

extremely critical

•

Extremely critical to not so critical○

Criticality of a task can range from:•

Task criticality is a different dimension than

hard or soft characterization of a task.

•

Criticality of a task is a measure of the cost of

a failure.

•

Q: Give an example of a hard real-time

system that is not safety-critical??


OTHER examples of hard real-time systems is the

anti-missile system, etc.

•

HOMEWORK: 1) describe why the anti-missile

system is a hard real-time system; 2) think of other

examples that are real-time hard system and why?

In practical system the time bounds for hard real-

time tasks usually range from several micro-seconds

to a few milliseconds.

•

There is no reward in completing it ahead of its

deadline

○

It should be noted that a hard real-time task does

not need to be completed within the shortest

possible time but within the specified time bound

•

FIRM REAL-TIME TASKS -- the results of a firm real-

time task is associated with a predefined deadline,

but unlike a hard real-time task if a firm task does

not complete before its deadline the system does

not fail (the late results are discarded)


Firm real-time tasks are typically encountered in

multimedia applications

•

The system takes pictures of an enemy territory

and beams it to a ground station computer frame

by frame

�

The ground computer proceeds to find positional

difference of various objects of interest with

respect to previous frames

�

When the computer is overloaded a new image

could be received before an old one has been

processed -- in this case the old one can be

discarded

�

Satellite-Based Tracking of Enemy Movements○

Video Conferencing -- HOMEWORK -- explain why this

application is a firm real-time system.

○

Examples:•

The associated time bounds for firm real-time tasks

range typically from a few milliseconds to several

hundreds of milliseconds.

•

SOFT REAL-TIME TASKS: these tasks also have time

bounds associated with them but the timing constraints

of soft real-time tasks are not expressed as absolute

value -- instead, the constraints are expressed in terms

of the average response times required.


Once a request for reservation is made, the

response should occur within 20 seconds on an

average

�

Response result may be in the form of a printed

ticket or an apology on an unavailability ticket

�

Alternatively, we might state the constraint on the

ticketing task as: at least in case of 95% of

reservation requests, the ticket should be processed

and printed in less than 20 sec.

�

Railway seat reservation system○

Using the above example -- if the ticket is printed in

about 20 sec, we feel that the system is working

fine and have a feel of obtaining instant results

�

Missed deadlines of soft real-time tasks do not

result in system failure. However, the utility of the

result produced by a soft real-time task falls

continually with time after the deadline expired

�

IMPACT of the deadline failure in a soft real time task○

Examples of Soft Real-Time Systems•


In practical applications, the time bounds for typical

soft real-time tasks usually range form a fraction of

a second to a few seconds

•

NON-REAL-TIME TASKS: a non real-time task is not

associated with any time bounds

Most interactive computations we perform

nowadays are handled by soft-real time tasks.

•

A few examples of non-real time tasks could be

batch processing jobs, e-mail, background tasks, etc.

•

But, we could argue that even these tasks could be

considered soft real-time to some degree.

•

WHAT IS THEN THE DIFFERENCE BETWEEN a non-

real time tasks and a soft real-time tasks??

For non-real-time tasks the associated time bounds

are typically of the order of a few minutes, hours, or

even days.

•

In contrast for the soft real-time tasks the time

bounds are the most of the order of a few seconds.

•


SUMMARY OF TASK CLASSIFICATION BASED ON

TIME BOUNDS

Hard real-time task -- time bounds associated

to the task deadlines are from several µs to a

few ms.

1.

Firm real-time tasks -- time bounds associated

to task deadlines range from a few ms to

several hundreds of ms.

2.

Soft real-time tasks -- time bounds usually

range from a fraction of a second to a few

seconds.

3.

Non-real-time tasks -- the associated time

bounds are typically in the order of a few

minutes , hours or even days.

4.


ENGG4420. CHAPTER 1: Real-Time Computer Control. Developed by Radu Muresan, Univesity of Guelph 17

Hard Soft

Periodic(Cyclic)

A periodic(Event)

Periodic(Cyclic)

A periodic(Event)

ee Tit ≤)(atit sc ±=)( ∑=

±=n

i

asc tit1

)(n

1

stTn /=

∑=

≤n

i

ae Tit1

)(n

1

stTn /=

tc(i) the interval between the i and i – 1 cycles,te(i) the response time to the ith occurrence of event e,ts the desired periodic (cyclic) interval,Te the maximum permitted response time to event e,Ta the average permitted response time to event e measured over

some time interval T,n the number of occurrences of event e within the time interval T,

or the number of cyclic repetitions during the time interval T,a a small timing tolerance.

FORMAL CLASSIFICATION OF REAL-TIME TASKS


CLASSIFICATION OF PROGRAMS

Real-time tasks and non-real-time tasks.○

Subdivide the real-time tasks into hard and soft○

It is important to separate the computer control

activities into:

•

The division of software into small, coherent

modules is an important design technique.

•

Verification of the applications ...�

Sequential -- actions are strictly ordered as a time

sequence. The behaviour of the program depends

only on the effects on the individual actions and

their order.

○

Processes or tasks are partly sequential but are executed

concurrently -- communicate through shared variables

and synchronization signals.

�

Verification of the applications ...�

Multi-tasking -- the actions required to perform

are not necessarily disjoint in time (concurrency).

Such programs can be built from a number of

parts called processes or tasks.

○

Verification -- need models of environment.�

Real-Time -- in addition to its action not

necessarily being disjoint in time, the sequence of

some of its action is determined by the

environment

○

Types of programming•


DESIGN OF REAL-TIME SYSTEMS, GENERAL

INTRODUCTION

Design Example•

Single-Program Approach•

Foreground/Background System•

Multi-Tasking Approach•


DESIGN EXAMPLE -- REAL-TIME COMPUTER

CONTROL FOR HOT-AIR BLOWER SYSTEM

The approach to the design of real-time control

systems is no different in outline from that

required for any computer-based system.

○

The planning phase -- is concerned with interpreting

user requirements to produce a detailed specification of

the system to be developed and an outline plan of

resources, people, time, equipment, costs-required to

carry out the development. At this stage preliminary

decisions regarding the division of functions between

hardware and software will be made -- The outcome of

this stage will be a specification or requirement

document.

�

The development phase -- is composed of various

stages such as: preliminary stage, design stage, and

testing stage. During these stages there is need for

extensive cooperation between hardware and software

designers.

�

The work can be divided into two main sections:○


HOT AIR BLOWER PLANT -- MAIN PLANT OF THE

SYSTEM EXAMPLE

BridgeCircuit

& Amplifier

FullyOpen

ThyristorUnit

TubeAir Flow

PowerInput

A

Computer

B

TemperatureMeasurement

(0-10V)

Auto Man

Air Inlet Heater

ManualFullyClosed

Air Inlet Position

On/OffClockwise/

Anticlockwise

Blower

MotorControl

Operator

VariableAir Inlet

centrifugal fan (blower),○

thermistor,○

heating element,○

reversible motor,○

potentiometer wiper for inlet air,○

microswitches,○

slider potentiometer for the reference temperature,○

operator panel.○

This plant example (a hot-air blower) is presented in order

to illustrate the various operations of a computer control

system. As seen from the diagram, the components of the

hot-air blower plant are:


COMPONENTS DESCRIPTION

Centrifugal fan -- blows air over a heating element and into a

tube.

•

The thermistor bead -- placed at the outlet end of the tube

forms one arm of a bridge circuit (a common configuration

used to measure small resistor variations). The amplifier output

of the bridge circuit is available at B and provides a voltage, in

the range 0 to 10 volts, proportional to temperature.

•

The current supplied to the heating element can be varied by

supplying a DC voltage in the range 0 to 10 volts to point A.

•

The position of the air-inlet cover to the fan is adjusted by

means of a reversible motor (can be adjusted by the operator

or computer). The motor operates at constant speed and is

turned on or off by a logic signal applied to its controller; a

second logic signal determines the direction of rotation.

•

A potentiometer wiper is attached to the air-inlet cover and

the output voltage is proportional to the position of the cover.

•

Microswitches are used to detect when the cover is fully open

and fully closed.

•

In manual mode the heat output and fan cover position can

be adjusted using potentiometers. Switches are provided to

operate the fan and heater.

•

Panel lights indicate fan on, heater on, cover fully open,

cover fully closed, and auto/manual status.

•

The operator panel: automatic and manual control•

The desired output temperature is known as the set point and

is set by the operator using a slider potentiometer. The

computer can read this setting.

•

The question is: how do we implement a system like this?•


FEATURES OF THE PLANT

Panel lights: fan on; heater on; cover fully open;

cover fully closed; and auto/manual status

•

Set point for the control system -- the desired

output temperature is set by the operator and it can

be read by the computer.

•

In manual mode the heat output and fan cover

position can be adjusted by the operator using

potentiometers.

•

Switches are provided to operate the fan and the

heater.

•

COMPUTER CONTROL FEATURES

Monitoring of the plant,○

Control of the plant,○

Actuation of the plant,○

And communication to the operator.○

The operation of the plant requires that software be

provided to support:

•


COMPUTER CONTROL OF A HOT-AIR BLOWER

Digital Input

ADC

Computer

DAC Digital Output

BridgeCircuit

Air InletPosition

Heater Circuit

Motor Control

OperatorPanel

AirFlow

Thermistor

AirInlet

Close

AirInlet Open

Blower

HeaterElement

DirectionAuto/Manual

ToOperator

Panel

On/Off

AirInlet

Air Inlet Motor

A general schematic of the system is shown above. The

computer interacts with the digital input and output interfaces

and with the ADC and DAC. The interfaces are directly

connected to the plant. Complex micro-processors incorporate

many I/O interfaces on the same chip with the ALU.

Many of these interfaces are part of the embedded processor.


MONITORING FUNCTION

Monitoring involves obtaining information about

the current state of the plant.

•

Analog signals: air temperature, fan-inlet cover

position, temperature reference.

○

Digital (logic) signals: fan-inlet cover position

(i.e., fully open, fully closed); status signals (i.e.,

auto/manual, fan motor on, heater on).

○

In our example, the information is available from

the plant instruments in the following two forms:

•

ACTUATION REQUIREMENTS

The provision of a voltage proportional to the

demanded heat output to drive the heater control;

•

Logic signals indicating on/off and the direction in

which the fan-inlet cover is to be moved;

•

Logic signals for the operator display.•


CONTROL -- involves parallel logic operations,

time-sequential control and timing of operation.

Specifically we need to implement:

The temperature, and ○

The position of the fan-inlet cover.○

The digital equivalent of continuous feedback

control (direct digital control DDC) for control of:

•

For example, the heater should not be on if

the fan is not running.

○

Sequence and interlock control operations are

also required

•

The change over needs to be done without

disturbing the temperature of the air at the

output of the tube -- bumpless transfer.

○

The computer needs also to handle automatic

change-over from simply tracking (monitoring)

the manual control operations to controlling the

system when the operator requests a change

from manual to automatic control:

•

BUMPLESS TRANSFER -- a change from manual

to auto mode such that no disturbance in the

plant output is caused.


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ENGG4420: LECTURE 3 --CHARACTERISTICS OF REAL-TIME …