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Packet Priority Assignment for Wireless Control Systems of Multiple Physical Systems
Wenchen Wang, Daniel Mosse, and Alessandro V. Papadopoulos
May 8th, 2019 @ISORC, Valencia, Spain
Interconnected World Made of Computing Systems
!2
IoT Connections
4.1 Billion Connections
in 2024!!!
Photo-illustration: iStockphoto
404 CAR NOT
FOUND
Photo-illustration: iStockphoto
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment
Not easy to deploy and
maintain
Wired Control System
Plant
Sensors
Controller ActuatorsTargets
& Objectives
Environment At most 4 robots can be connected to 1 cabinet
[Salman et al., Fog-IoT 2019]
Not easy to deploy and
maintain
Wireless Control System (WCS)
Plant(s)
Sensors
Controller(s)
Actuators
Targets &
ObjectivesEnvironment
Network
Network
Delay and Message Loss
0 0.2 0.4 0.6 0.8 1time
01020
PS1
105
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS2
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS3
Major Challenges of WCSInstability
When the physical system is unstable, the plant or the device can be damaged and leads to serious safety issues and financial loss.
Performance Degradation Induced additional error
Network-induced error
0 0.2 0.4 0.6 0.8 1time
01020
PS1
105
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS2
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS3
Major Challenges of WCSInstability
When the physical system is unstable, the plant or the device can be damaged and leads to serious safety issues and financial loss.
Performance Degradation Induced additional error
Network-induced error
Unstable
0 0.2 0.4 0.6 0.8 1time
01020
PS1
105
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS2
0 0.2 0.4 0.6 0.8 1time
-1
0
1
PS3
Major Challenges of WCSInstability
When the physical system is unstable, the plant or the device can be damaged and leads to serious safety issues and financial loss.
Performance Degradation Induced additional error
Network-induced error
Unstable
Degraded Performance
Problem Formulation
Shared multi-hop networkDifferent paths p_1, p_2, …, p_m Each path with delay D_jTDMA fixed topologyTime-varying delivery ratio dr_j
Remote Controller
…
Multi-hop Network
N Physical Systems (PSs)Dynamic network reconfiguration in [Wang, RTNS 2018]
Different Requirements, Shared Network
time
time
time
Demand
Demand
Demand
LO-Critical HI-Frequency
HI-Critical HI-Frequency
HI-Critical LO-Frequency
Different Requirements, Shared Network
time
time
time
Demand
Demand
Demand
LO-Critical HI-Frequency
HI-Critical HI-Frequency
HI-Critical LO-Frequency
Different Requirements, Shared Network
time
time
time
Demand
Demand
Demand
LO-Critical HI-Frequency
HI-Critical HI-Frequency
HI-Critical LO-Frequency
Different Requirements, Shared Network
time
time
time
Demand
Demand
Demand
LO-Critical HI-Frequency
HI-Critical HI-Frequency
HI-Critical LO-Frequency
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
RCARequested
Change Amount
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
RCARequested
Change Amount
RCDRequested
Change Duration
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
RMSEi = 1Ttrans
Ttrans
∑t= 0
∥yWi (t) − yWL
i (t)∥2
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
RMSEi = 1Ttrans
Ttrans
∑t= 0
∥yWi (t) − yWL
i (t)∥2
Wired
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
RMSEi = 1Ttrans
Ttrans
∑t= 0
∥yWi (t) − yWL
i (t)∥2
Wired Wireless
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
RMSEi = 1Ttrans
Ttrans
∑t= 0
∥yWi (t) − yWL
i (t)∥2
Wired Wireless
Transient
Problem FormulationThe control is more or less difficult based on
Setpoint (or reference) tracking Nonlinearity of the controlled systemReliability of the communication path
Objective: Minimize Control performance degradation Induced by the wireless realizationWithout redesigning the control system
RCARequested
Change Amount
RCDRequested
Change Duration
RMSEi = 1Ttrans
Ttrans
∑t= 0
∥yWi (t) − yWL
i (t)∥2
Wired Wireless
Transient
∀i ∈ Physical Systems
Solution: Dynamic Packet Priority Assignment
Priority Assignment Static heuristic (baseline)Dynamic heuristicPID Dynamic heuristic
Path Selection Network Path Quality Determination
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
Static Heuristic - BaselineOffline analysis
Assuming no packet lossFor different requested changes in demand
For all PS i compute
Assign the priorities that minimise the averageDo not change the priorities online
rRMSEi(Tsim) = 1Tsim
Tsim
∑j= 0
∥ri( j) − yi( j)∥2
rRMSEi(Tsim)
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
Dynamic HeuristicAt every time t, for all PS i compute
Sort the PS i by
Assign the highest priority to the PS with highest value of
rRMSEi(t) = 1t
t
∑j= 0
∥ri( j) − yi( j)∥2
rRMSEi(t)
rRMSEi(t)
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
PID Dynamic HeuristicWe define the tracking error as
The priority for every PS i is computed as
strange formula, isn’t it?It is a PID controller!
ei(t) = |ri(t) − yi(t) |
πi(t) = KP (ei(t) + λt
t
∑i= 1
ei(t)) + KD (ei(t) − ei(t − 1))
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
PID Dynamic HeuristicWe define the tracking error as
The priority for every PS i is computed as
strange formula, isn’t it?It is a PID controller!
ei(t) = |ri(t) − yi(t) |
πi(t) = KP (ei(t) + λt
t
∑i= 1
ei(t)) + KD (ei(t) − ei(t − 1))
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
The Path Quality Model: PQModelAfter we determine the priority of the measurement packetsIncludes
Network delayNetwork reliability
We compute the path quality for all the paths as
PQ = Dnet + α nloss Δcsp
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
The Path Quality Model: PQModelAfter we determine the priority of the measurement packetsIncludes
Network delayNetwork reliability
We compute the path quality for all the paths as
PQ = Dnet + α nloss Δcsp
End-to-end delay
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
The Path Quality Model: PQModelAfter we determine the priority of the measurement packetsIncludes
Network delayNetwork reliability
We compute the path quality for all the paths as
PQ = Dnet + α nloss Δcsp
End-to-end delay Control Sampling Period
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
The Path Quality Model: PQModelAfter we determine the priority of the measurement packetsIncludes
Network delayNetwork reliability
We compute the path quality for all the paths as
PQ = Dnet + α nloss Δcsp
End-to-end delay Control Sampling Period
Consecutive Packet losses
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
The Path Quality Model: PQModelAfter we determine the priority of the measurement packetsIncludes
Network delayNetwork reliability
We compute the path quality for all the paths as
PQ = Dnet + α nloss Δcsp
End-to-end delay Control Sampling Period
Consecutive Packet lossesDesign parameter
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
Case studies
Parameters Inverted Pendula (IP) Nuclear Power Plant (NPP)
Sampling period Ts 0.01s 0.1s
Simulation time Tsim 100s 300s
RCA (6 + 4l) meters (2 + 2l) MWatts
RCD 5j seconds 15j seconds
ST change [0s, Tsim-RCD] [0s, Tsim-RCD]
l=0,…,4
j=1,…,8
x3 x3
PID Tuning
-4-2
2.6
0
2.8
3
10-3
RM
SE
2
3.2
0
KD
3.4
5 410 6
-4-2
IP
3
0
KP
4
5
10-3
20
6
KD
7
5 410 615
-4-2
3
0
KP
4
-4
5
10-3
2-2
6
0
7
42 4 66
-4-2
0.08
0
0.1
0
RM
SE
2
0.12
KD
0.14
10 420 6
-4-2
NPP
0.1
0
KP
0.15
0 2
0.2
KD
0.25
10 420 6
-4-2
0.1
0
KP
-4
0.15
2-2
0.2
0
0.25
42 4 66
! Tuning
The IP is more sensitive to large delays
" decreases with high network interference
Comparison of the Different Heuristics
PID provides better and
more stable performance
Improvement With PQModel
The PQ model outperforms end-to-end approaches
ConclusionWe explored the interaction between dynamic packet scheduling and the control system performance in WCS
Highly nonlinear systems are not heavily explored in the literature
Three heuristics for packet priority assignmentPID: most promising
Path quality modelTradeoff between delay and reliability
Questions?
Alessandro V. Papadopoulos [email protected]
Remote Controller
…
Multi-hop Network
Priority Assignment
Path Selection
