Realtime systems chapter 1

Prepared by:
Binay Ghimire
REALTIME SYSTEMS
Binay Ghimire, course Instructor, RTS 1

Books:
 Text Book(RTS Bible):
RealTime Systems - Jane W S Liu
 Reference Materials:
-The Concise Handbook of Real-Time
Systems :Timesys Corporation
- Lecture notes/slides
- Internet resources

INTRODUCTION
Any system in which a timely response by the
computer to the external stimuli is vital
RT System must not only produce right
answer but also meet time constraints
System that must satisfy explicit or bounded
response time constraint or risk severe
consequences including system failure
A failed system is one which cannot satisfy
one or more of the requirements laid out in
the formal system specification

INTRODUCTION
RTS
 Schedulability
-Performance must be schedulable
 Responsiveness:
-Must respond in worst case by each task
Non RTS
 Throughput
 Average case response time
 Fairness is more important than deadline

Facts about RTS
 Predictable computing rather than fast computing
 RT Programming is not hand coded
- A number of automated tools can be used to generate
efficient codes for Real-time programs
 RT system performance engineering emphasizes more on
timeliness than the raw performance
 RT systems not necessarily work in static environment
- Rather there are a number of situations where RTS are
deployed in environment were the operating condition
changes dynamically

EXAMPLES OF RTS
Cell phones, Digital camera , Microwave
oven
Avionics, Radar Control System, Industrial
Process Control
Command and Control
Multimedia system
ElectronicsWarhead Control System
MissileTracking System

EXAMPLE
 MissileTracking System: System
responsiveness is faulty but calculations are
correct so target is missed

Logical / Temporal Correctness
 If a system misses its deadline the resultant action
could be abort, continue, abandon depending upon
system option
 Real-time System is a system whose specification
includes logical / temporal correctness
 Logical correctness:
- Indicates that system must produce correct
output (must be verifiable)
Temporal correctness
- Implies that system must produce output at
correct or right time

Characteristics of Real-time
Systems:
i. Event driven, reactive in nature (continual
interaction with the environment)
ii. Cost of failure is high
iii. Require concurrency or multiprogramming
iv. Standalone / Continuous operations
v. Reliability and fault tolerant requirements
vi. Predictable behavior

EXAMPLE: Driving a Car
 Mission: To reach destination safely
 Controlled system: car
 Operation environment: road, traffic, other
cars
 Real-time controller : driver
 Controls: accelerator, steering wheel, brake
paddle
 Actuators: wheels, engine , brake
 CriticalTasks: braking system, steering
 Non CriticalTask: radio, cassette player

EXAMPLE:Thermo Nuclear Power
Plant
 Two events signaled by interrupts:
i. Event is triggered by any of several signals at
various security points that will indicate a
security breach.
-The system must respond within one second
ii. Event indicates that nuclear core has
reached an over temperature
-The signal must be dealt within 1 millisecond
-Danger of meltdown if deadline is missed

Types of Real-time Systems
 Depending upon temporal behavior of
Real-time system they can be
characterized as
hard,
soft and
firm Real-time system

Hard Real-time System
 A system crashes if the deadline is not met by the
tasks
 A hard deadline is imposed on a job as a late result
produced by the job after the deadline may have
disastrous consequences

Hard Real-time System
 e.g. an Onboard computer of a moving
aircraft:
- Measures the velocity , altitude, position ,
acceleration wind pressure etc
- Collects data with sensor within fixed
amount of time say 20 ms
- Compares these data against the stored
values
 There’s a stiff deadline for all these
computations

Firm Real-time System
 System doesn’t fail but throws the
results (ignores) in case of failure

Firm Real-time System
 Every 1/30th of sec 1 frame has to arrive
 Destination will not wait for frame to arrive but
switch over to next frame if 6th frame does not
arrive
 Hard RTS are not periodic
 Most periodic hard Real-time systems are firm
RT systems
Reset clock
Wait 20 m/s
Capture frame
If capture buffer ok (picture captured)
store frame
else repeat

Soft Real-time System
 Here utility value becomes less/drops with time
 e.g. exam hall: Delay by 30 minutes allowed. More
delay, you will not be allowed to write the exam.
 e.g.Word processors,Airline reservation system,
games, simulations

Digital control
 Real-time systems are usually control systems
embedded in sensors and actuators and
function as digital controllers

Digital control
 The plant is a controlled system e.g. engine, brake,
an aircraft or a patient
 The state of the plant is monitored by sensors and
can be changed by actuators
 The Real-time (computing) system estimates from
the sensor reading the state of the plant
 Computes a control output based on the
difference between the current state and the
desired state (reference input)
 This computation is called the control law
computation of the controller
 The output thus generated activates the actuator,
which brings the plant closer to the desired state

Sampled Data Systems
 A sampled data system typically reads and digitizes
the analog sensor readings periodically and carries
out its control law computation every period
 The sequence of digital outputs thus produced is
then converted back to an analog form, needed to
activate the actuators
 Analog sensor reading y(t) gives the measured state
of the plant at time “t‟
 If e(t) = r(t) – y(t) denote the difference between
the desired state r(t) and the measured state y(t) at
time “t‟ then:
ek = rk - yk is the kth sample value of e(t)

Multi-Rate Systems
 A plant typically has more than one degree
of freedom
 Its state is defined by multiple state variables
(e.g. rotation, speed , temperature etc)
 These are monitored by multiple sensors
and controlled by multiple actuators

High Level Controls
 Controllers in a complex monitor and control
system are typically organized hierarchically.
 One or more digital controllers at the lowest level
directly control the physical plant.
 Each output of a higher-level controller is a
reference input of one or more lower-level
controllers.
 Examples: Flight Management System(FMS),Air
Traffic Control(ATC), assembly robots in a factory,
patient monitoring system in an ICU.

Hierarchical Control System

Reference of Hierarchical control
system
 Controllers in complex monitors and control systems are typically
organized hierarchically
 One or more digital controllers at the lowest level directly controls
the physical plant while the controller at higher level provide reference
inputs to the controller at lower level
 Controllers at high level may have interface with the operator
 Example: Patient Care System
◦ LLCs are micro-processor based controllers to monitor and control
patients blood pressure, respiratory glucose and so on
◦ HLCs are expert system with operator (nurse/doctor) and provides
reference value of the health indications
 Computation done by LLC are simple accurate and deterministic while
computations done by HLC are more complex and variable.
 Periods of control law computations for LLC may range from
milliseconds to seconds
 Period of control law computations for HLC may range from minutes
to hours

Guidance and Control
 In Flight Management System(FMS), the control activity is
handled by low level digital controller while a second level
controller performs the guidance activity such as path
planning, finding the most desirable trajectory out of the
trajectories that meet the system constraints
 The guidance function takes the constraints into
consideration such as air craft characteristics max/min,
cruise speed, descent/ascent rates etc) and environmental
characteristics (ground track, altitude, profile weather
condition)
 It may also impose various cost functions such as fuel
consumption, constrained fixed time of arrival

Real-time Command and Control
 In control hierarchy, controllers at highest level
is a command and control system
 Air Traffic Control (ATC) system is a command
and control system
ATC system monitors the air-crafts in its coverage
area and weather condition and provide necessary
information required by the ATC system operators
Output of ATC system goes to the on board Flight
Management System. so ATC system indirectly
controls the lowest level embedded controllers

ATC Example

ATC Example
 ATC collects information about aircrafts through radar
 Radar asks aircraft for state info and aircrafts send
data for state variables (identifier, position, altitude,
(track record) and current trajectory (track)
 ATC processes these data and stores in DB which is
picked up by display processor and surveillance
processor to inform the operator about the potential
hazards if any (collision)
 Timing requirements of command control system are
less stringent
 Low level controllers run on one or few computers
with the small network of dedicated links
 Command and control system may be a large
distributed system with hundreds of heterogeneous
computers

Signal Processing System
 Most signal processing applications
have Real-time requirements
 Response time few millisecond to few
seconds
 e.g. Digital filtering (audio and video),
Voice and video compression and
decompression

Radar signal processing

Subsystems
 I/O Subsystem: Samples and digitizes the
radar echo signal and places the sampled values
in a shared memory buffer called bin
 DSP Subsystem: An array of DSP processors
processes these sample values
 Data Processing Subsystems: Processes and
analyses the DSP output to produce output to
interface with the display system commands to
control (move) the radar, and signal processing
parameters for the next cycle

Working of a Radar System:
For object search , radar points antenna in
one direction at a time and sends a short RF
pulse, then it collects and analyzes the echo
signal
The echo signal having background noise
only indicates no object hit
If the signal hits any object at distance “x‟
and echo arrives after 2x/c
where c = 3 x 108 ie. Speed of light

Tracking
 An application that examines all the track records in order to
sort out false returns (wrong data of presence of object) from
real ones and update the trajectories of detected objects is
called a tracker
 The tracker assigns each measured value(position and velocity)to
a trajectory.
If the trajectory is an existing one, the measured value assigned to it
gives the current position and velocity of the object moving along the
trajectory.
If the trajectory is new, the measured value gives the position and
velocity of a possible new object.
 Tracking is carried out in two steps:
i. Gating
ii. Data Association Binay Ghimire, course Instructor, RTS 34

Gating and Data Association
 Gating is the process of putting each measured value into
one of two categories established trajectories or new
one.
The gating process assigns a measured value to an established
trajectory if it is within a threshold distance G away from the
predicted current position and velocity of the object moving along
the trajectory.
 Under adverse conditions, the assignment produced by gating
may be ambiguous, that is, some measured value is assigned to
more than one trajectory or a trajectory is assigned more
than one measured value.
 The Data Association step is then carried out to complete
the assignments and resolve ambiguities. E.g. the nearest
neighbor algorithm

Further Reading
 Refer Textbook for some more examples
on RealTime systems and their
applications

THANKYOU

Realtime systems chapter 1

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