Cyber-Physical Control over Wireless Sensor - PowerPoint Presentation

Slide1

Cyber-Physical Control over Wireless Sensor and Actuator Networks with Packet Loss

Ahmed Alhafdhi

Feng

Xia*,

Xiangjie

Kong, and

Zhenzhen

Xu

School

of Software, Dalian University of Technology, Dalian 116620, ChinaSlide2

AgendaIntroduction to WSAN.Advantages of WSANs Over Hard

wire-lines.Challenges.Problems of Cyber-Physical Control Systems Over WSANs.

System

Architecture.

QoS

Challenges.

Wireless Channel

Characterization.

Packet Loss

Compensation.

Simulation.

Conclusion.Slide3

Introduction to WSANWSAN is a networked system

of geographically distributed sensor and actuator nodes equipped with: Some data processing.

Wireless

communication

capabilities.

Power supply.

Sensors gather and send

information

to actuators by sensing the

state of the physical world

.

Actuators

make

decision

about how to react

and to

change the behavior of the

physical environment.

Commonly, there is

a base station

which is responsible

for monitoring and managing the overall network

through communicating

with sensors and actuators.Slide4

Introduction to WSANSlide5

Introduction to WSANUnlike WSNs which primarily used to sense and monitor the state of the physical world, WSANs

respond to the sensed events/data by performing corresponding actions upon the physical system.

Turn

on the water sprinklers

on fire

handling

system.

Monitor

and manipulate the lighting in a smart

office.

Monitor and manipulate

speed

of a mobile

robot.

Today’s control systems are usually built upon hard

wire-lines.Slide6

Advantages of WSANs Over Hard wire-lines

Eliminating the installation and maintenance difficulties.Flexibility.

Expandability.

Less cost.

More

appropriate in

chemicals, severe vibrations and high

temperatures environments.

Satisfy

the requirements of

mobile systems.Slide7

ChallengesWireless channels have adverse properties such a:Path loss.

Multi-path fading.Adjacent channel interference.Doppler shifts.

Half-duplex operations

Transmitting radio signals

over wireless channels can be affected by many

factors:

Ambient noise.

Physical obstacles.

Node movement.

Transmission power.

Etc.Slide8

Problems of Cyber-Physical Control Systems Over WSANs

The network quality-of-service (QoS) cannot always be guaranteed.

Packet loss.

This cause

system

instability.Slide9

System Architecture

A controller, some sensors and actuators, used to:

Sense and compare

the operation of the physical

system.

Compute control commands and perform

actions onto the

system.

This feedback

architecture of a cyber-physical control system is also called

closed loop,

implying

that

the cyber space and the physical system are able to affect each other.Slide10

System ArchitectureSlide11

QoS Challengesmajor challenges in

QoS support in control systems over WSANs:Resource Constraints.

Platform

Heterogeneity.

Dynamic Network

Topology.

Mixed

Traffic.Slide12

Resource ConstraintsWireless sensor nodes are usually:

low-cost and low-power.Small devices with Limited data processing capability.

Limited

transmission rate, battery energy, and memory.

As result,

the available bandwidth and the radio range of the

wireless channel

are often

limited.

Actuator nodes have stronger

computation and communication capabilities and more energy

budget relative

to

sensors.

However,

resource constraints apply to

actuators as well.Slide13

Platform HeterogeneitySensors and actuators do not share the same level of resource constraints.

Possibly designed using different technologies and with different goals.

They are different in

many

aspects:

computing/communication capabilities, functionality

, and number

.

Different hardware and networking technologies used the

underlying

WSANs.

Platform heterogeneity makes

it

difficult

to make full use of the

resources.

Difficult to

achieve real-time and reliable

communication with the platform heterogeneity.Slide14

Dynamic Network TopologyThe actuators in WSANs

may be mobile.During runtime:New sensor/actuator nodes may be

added.

T

he

state of

a node

is possibly changed to or from sleeping

mode.

Some

nodes may even

die.

All of these factors

This requires

QoS

mechanisms to work

in dynamic

and even unpredictable environments.

QoS

adaptation

becomes necessary.

WSANs

must be adaptive and flexible at

runtime.

Slide15

Mixed TrafficIn many situations, diverse applications need to share the same WSAN, inducing both periodic

and aperiodic data. Some

sensors may be used to create the measurements of certain

physical variables

in a periodic manner for the purpose of monitoring and/or control.

For instance:

In

a smart

home:

Some sensors

are used to sense the temperature and

lighting.

Some

others are responsible

for reporting

events like the entering or leaving of a person.

It generates

traffic flows with different characteristics (e.g. message size and sampling rate).

QoS

management should take into account the

support of service

differentiation.Slide16

Wireless Channel CharacterizationThe problem of packet

loss:Packet loss degrades control performance and system instability.WSAN design should minimize the

packet losses.

Understand

the characteristics of

wireless channels

used by

WSANs:

Simulations was performed

based on a realistic

WSN link-layer

model developed by Zuniga and

Krishnamachari

.

Examine

the impact of two major

factors:

Communication distance.

Transmission power.Slide17

PLR & the Transmitter-Receiver Distance

The

whole area can be divided into three regions:

connected region, transitional region, and disconnected regionSlide18

PLR & the Transmitter-Receiver Distance

The

whole area can be divided into three regions:

connected region, transitional region, and disconnected regionSlide19

Packet Loss CompensationThey develop an approach to packet loss compensation

which conforms:Modify only the application layer

of the networks without exploiting any application-specific (lower layer)

network protocols.

Not

to use any statistic information about the distribution of packet loss rate

in any

specific

WSAN.

Not

to use the knowledge about the models of the

controlled physical

systems and the controller design of the applications.Slide20

Packet Loss CompensationA simple method on the actuator nodes to cope with packet loss occurring

in WSANs.When a sensor data packet is lost

The actuator produces

an estimate of the sensed value and compute the control

command

based

on it.

i.e.

y(k), is lostSlide21

Solution IllustrationInput: Sensed data

Output: Control command Begin If the sensed data

y(k) is lost then

Compute

yˆ(k) using (1)

Set

y(k) = yˆ(k)

End

if

Produce

control command with respect to

y(k) (through executing

control

algorithm)

Store

y(k) into memory

Discard

y(k–m) in the memory

Perform

actions corresponding to the control command

EndSlide22

Prediction AlgorithmsThree types of classic prediction algorithms:

1.

2.

3

. Slide23

SimulationSetup Overview:

Physical system that can be modeled in transfer function:Controller

uses the PID (proportional-integral-derivative) control

law.

The sampling period of the sensor is set to

10ms.

The integral of absolute error (IAE) is recorded

to measure

the performance. Larger

IAE values imply worse

performance.Slide24

Results and AnalysisSlide25

Results and AnalysisSlide26

ConclusionConstructing cyber-physical control systems

over WSANs that are unreliable.QoS challenges still an open problem.

It

remains open to

devise simple

yet efficient prediction algorithms for packet loss

compensation.Slide27

Questions?.Slide28

ReferencesCyber-Physical Control over Wireless Sensor and Actuator Networks with Packet Loss, Book Chapter, 2010