introduction to sensor and actuator networks · introduction to sensor and actuator networks wcng...
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Introduction1. Classification of WSAN2. WSAN Challenges3. Control systems aspect
1. Existing Protocols for WSAN2. Stability of Wired Sensor and Actuator Networks
4. Research directions
Definition and characteristics
l Wireless Sensor and Actuator Networks (WSAN): network of sensor nodes that can measure stimuli in environment + network of actuators capable of modifying this environment
l Actuators tend to have more energy, very heterogeneous networks
l Oftentimes, actuators are mobile, in smaller numbersl Packets containing measurement are obsolete (dropped)
by the time of new measurement è time delay is particularly stringent
Taxonomy of Actuators
Mobile Not Mobile
Fixed
Self-Improving
Not Fixed
DeliverySystems
Camera with swivel
Robomote
Sprinklers
Medication delivery nodes
Agriculture Management nodes
Applications:
• Advanced target tracking
• Disaster response
• Precision agriculture
• Dr Sensor
• Ultra-precise chemical production
Taxonomy of WSA Networks
In networkprocessing
Out of networkprocessing
Localizedprocessing
Remoteprocessing
SingleActuator
MultipleActuators
In network processing[1]
Event area Sink / BS
Local ProcessingRemote / Out of network Processing
[1]: BWN Lab, GaTech
Single / Multiple Actuators[1]
Event area Single ActuatorsMultiple Actuators
• Choose the best actuator for the task
• Requires coordination of sensors in the sensing area.
• Coordination between actuators may be needed to cover the whole area
• Need for clusters of nodes associated with one actuator
Actuator – actuator coordination[1]
l Communication between actuators similar to nodes in networks
l Actuator-actuator comm. needed if actuator cannot act (low energy, small action range…)
l Need to find the best (set of) actuator(s) for the task at hand: exact number of actuators, capabilities vs. task (Distributed Decision)
Some suggestions[1]
l Power management plane:– to decide whether to act, route or not based on remaining
energy [1]
– if mobile, go where battery recharge[1]
l Mobility management plane:– replenish batteries[1]
– improve network connectivity[1][2]
– improve coverage[3]
l Coordination plane:– what to do with sensed data[1]
– distributed decision
[2]: Robomotes, Berkeley
[3]: Autonomous Helicopter Project, Carnegie Mellon
Protocol
Protocol
Protocol
Protocol
Pow
er Managem
ent
Mobility
Coordination
Constraints on protocols[1]
l Transport layer:– get accurate reports from sensors: exact type, location, intensity,
etc. Act according to reliability– aggregation and dissemination of sensed information reliably– reliable communications between actuators
l Routing layer:– route to sink, or coordinator, or actuator(s)– Ad-hoc routing can be used for actuator-actuator routing
l MAC layer:– Use mobility for connectivity between sensors and actuators
For all layers: deliver in timely manner!
WSAN and control systems[4]
l When event detected or command issued, information from sensors read
l Then activation of actuatorsl Environment changes, sensor readings change (loop)
Control/ Network
Sensors
ActuatorsEnvironment
Environment
User
Σ+
-
[4]: V. Hölttä , Wireless Sensor And Actuator Networks
Challenges[4]
l Robustness: network should reliably deliver packets so that loop not interrupted (sensor ßà control and control ßà actuator)
l Small delay introduced or may have over shooting and instabilityl So far: stability of networked system ? stability of SAN.
t
What introduces delay?[4]
Control ActuatorsΣ+
-
τa
SensorsτsN
etw
ork
τca
τsc
τc
Ref. Output
Sampling interval
Depends on actor
Depends on net. (cong.,
etc.)
Packet loss: infinite delay
Packet reorderingSmall throughput
In network decision[6]
l Stringent delay constraints in event detectionl Aggregation within one cell of grid and 2-level
aggregationl Consider anycast: if event received by any actuator =
successl Pick next-hop that guarantees timely delivery (calculates
how far actuator is)l Other protocols[7] propose building tree rooted at event
(in fact all sensor nodes maintain table of closest actuators)
l Dynamically assign actuators to region with more events
[6]: Ngai, Zhou, Lyu, and Liu, “Reliable Reporting of Delay-Sensitive Events in WSAN”
[7]: Hu, Bulusu, and Jha, “A Communication Paradigm for Hybrid Sensor/Actuator Networks”
In network decisions[8]
l Goal is to have in network decision like reflexes found in biology:
l Select sensor-actuator pairs form matrix linking actuator intensity and sensor measurement
l Total Response: Tx = Σdlal: al is actuator influence, dl its weightl dl evaluated as inner products of action with wanted optimal
response and depends on all sensor readings
[8]: C. J. Rozell and D. H. Johnson, Evaluating local contributions to global performance in wireless sensor and actuator networks, DCOSS’06.
Brain
Local neurons
1
2
In network decisions[8]
l è Too many connections required!l Sensors quantify impact of removing sensor i reading on
resulting action Tx using frame theory (inner product of reconstruction basis and actuator influence).
l è maps expected response to (fixed) event to connection diagrams between sensors and actuators
l Non-adaptive, a-priori choose sensor-actuator communication links.
l Sensors and actuators form two basis (measurement and action), not ortho-normal: frames. Sensors have action frame, actuators reconstruction frame.
Stability of SA networks
ActuatorsΣ+
-τa
Sensorsτs
Net
wor
k / S
ink
τnOutputApplication
Control Actuators τa
Sensorsτs
Net
wor
kτca
τsc
τc
Output
Iterative method[5]
l An actuator decision is made on current sensor readings (level of CO2 Lt and soil moisture Wt), target value (weed-density Yt).
l Paper claims also depends on history of actuation St, related to residual level of pesticides (not measured)
l Actuation strategy depends on Wt, Lt and St. Yields new Yt
Strategyg(Wt, Lt, St)
g*(Lt, St)
Yt
Lt
Wt St
l What if other strategy used and that lets us turn off sensors? è save lifetimel Slow control, very simplistic view of control
[5]: M. Coates, “Evaluating Causal Relationships In WSAN”
System block diagram
SystemState xk
Estimator
Controller
Network
Network
disturbance wt
measurement yttx̂
Controller design and analysisNoisy and missing measurements
l Estimator may use a Kalman filter: recursive filter that estimates state of dynamic system from incomplete and noisy measurements
l Kalman filter yields minimum mean square error estimate based on received sensor measurements
l Controller calculates control command from estimate (usually in linear systems only need closed-loop gain matrix)
Controller design and analysisNetwork-induced delays
l [9] studies the stability of networked CS in presence of network delay and packet drop out
l Establishes the stability regions as function of sampling interval and net. delay
l When packets lost, estimate state of system and evaluate stability
[9]: W. Zhang, M. S. Branicky, and S. M. Phillips, “Stability of Networked Control Systems”, IEEE Control Sys. Mag. 2001
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