Networked Control Systems for Manufacturing: Parameterization, Differentiation, Evaluation, and Application Ling Wang ling.wang2@wayne.edu
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Introduction Applications in manufacturing Control Diagnostics safety Networks CAN-based Ethernet-based Wireless networks Etc Requirement Real time/ reliability/ rate/ determinism
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Bandwidth and Speed bandwidth CAN-based 1Mb/s Ethernet based 10Gb/s, currently mostly based on 100Mbps or 1Gbps switches Speed Time/bit Effective width Frame overhead (addressing and padding) Inter-frame time Retransmission/ACK
Delay and jitter Time delay Jitter variability in delay
Delay and jitter (contd.) Processing time may be long (95us transmit time)
Delay and jitter (contd.) Waiting time (access control/scheduling) Waiting time for devices with strobed message connection
Other QoS Metrics Reliability ACK message Retransmission Frequency hopping Security Traditional fieldbuses: physical isolation Authentication or encryption VPN Overhead
Network QoS vs. System Performance: Need control-networking co-design/joint-optimization Performance comparison of continuous control, Digital Control and networked control;
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Categorization of Networks Medium access control Time-Division Multiplexing ( master-slaving or token passing) Random Access with retransmission Random Access with prioritization for collision arbitration
Time-Division Multiplexing Representatives Modbus TCP / PROFIBUS / AS-I Advantages Collison free Waiting time is deterministic Disadvantages Clock synchronization Overhead
RA with Collision Arbitration Representatives CAN (CSMA/AMP) CAN Each message has a priority/identifier (11 bits) Send the identifier first bit-by-bit Listen to the network/ check the correctness of identifier Features Collison free Slow data rate Not suitable for large data size
Ethernet-Based Networks Advantages Low cost / Widespread availability High communication rate Disadvantages CSMA/ Not a deterministic protocol QoS cannot be guaranteed Representatives Hub-Based Ethernet (CSMA/CD) Switch-Based Ethernet (CSMA/CA) Industrial Ethernet
Hub-based Ethernet Feature Forwarded packets from any port to all other ports Carrier sensing before sending Collision detection Transmit 32 jam bits and wait a random length of time to retry Disadvantages Cannot eliminate collision
Switch-based Ethernet Features Forward the packet to destination port Cut through or store-and-forward Congestion control
Industrial Ethernet Representatives EtherNet/IP Modbus/TCP PROFINET Features Based on switched Ethernet Vary at all levels of OSI model Difference in devices rather than protocol Interoperability
Ethernet for High-Speed and Time Critical Applications Real time networks Guarantee the delivery of message before a preset deadline or within a time interval IEC 61784-2 specification RTE technologies: 16 communication profile families; 9 performance indicators: 1. Average message latency 2. Number of networked nodes 3. Basic network topology 4. Number of switches between end nodes 5. Throughput (for realtime traffic) 6. Throughput (for coexisting non-real-time traffic) 7. Clock synchronization accuracy 8. Jitter or variation in the period of cyclic messages 9. Redundancy or recovery time E.g., smart Switch, isochronous communication Time synchronization NTP (sub-ms accuracy) vs. PTP (sub-microsecond accuracy)
Wireless Networks Non-deterministic network
Wireless Ethernet (CSMA/CA)
Bluetooth Features IEEE 802.15.1 2.4G Hz frequency bands as 802.11b and g External interference Frequency hopping Forward error correction(fec)
Effects of wireless network performance Radio Propagation Loss Passive channel error (multipath fading) External interference Channel access errors: hidden terminals etc
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Evaluation of NCS Three Steps Theoretical performance Experimental perspective (overhead) Analysis (cost)
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Networks for control Lower level A high level of determinism Deterministic network (e.g., CAN) Switched Ethernet is becoming common Intermediate /high Level TP and Ethernet-based networks are commonly used Communicate larger amount of data and support network service Wireless networks are not commonly utilized for control yet Extensive and costly verification before deployment
Networks for Diagnostics Features Support large amounts of data Emphasis speed over determinism Wireless network is an ideal medium (flexibility)
Networks for Safety Features Have the strongest determinism and jitter requirement CAN-based and switched Ethernet-based networks are popular candidates The wireless network is possible (tradeoff between flexibility and reduced guaranteed response time)
Multilevel Factory Networking Example: Reconfigurable Factory Testbed
Outline Introduction Parameterization Differentiation Evaluation Application Future trends
Future Trends The move to wireless networks at all levels The domination of switched Ethernet The design of system to consider/leverage network attributes Conrol-networking joint optimization Wireless is not expected to replace wired in all applications Integration of traditionally separate industrial systems
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