Industrial Automation And Robotics Distributed Control Systems
Industrial automation and roboticsDistributed control systemsProf. Paolo Rocco (paolo.rocco@polimi.it)Politecnico di Milano, Dipartimento di Elettronica, Informazione e Bioingegneria
Distributed control systemsIn a control system composed by a control unit and afew sensors and actuators, communication can beperformed using analog or digital voltage or currentsignals. What happens, however, if we have manycontrol units connected to many sensors/actuators?Instead of using an analog point-to-pointconnection, we can choose a digital buscommunication system connecting all thesensors/actuators to one or more control units.Before discussing the communication systems inautomation, we need however to clarify what arethe real-time systems.Industrial automation and robotics – Distributed control systems – Paolo Rocco
Real-time systemsA discrete time control law periodically computes the control variable given a measure of thecontrolled variable and the reference signal. This computation is executed every sampling interval.To replicate the same behavior on a computer system we need an operating system that always takesthe same amount of time to complete the control task. We will call such a system a real-time system.How can we define a real-time system? a system in which the time at which output is produced is significant, as the input usually corresponds tosome event in the physical world, and the output has to relate to that same event; a system that must process information and produce a response within a specified time, else risk severeconsequences, including failure; a system whose correctness is based on both the correctness of the outputs and their timeliness; a system which has to respond to externally generated input stimuli within a finite and specified period.Industrial automation and robotics – Distributed control systems – Paolo Rocco
Real-time systemsReal-time does not mean fast! The deadline may be days or weeks. it means that it must produce theresponse by the required time.To measure the timeliness of a real-time system we introduce the notion of jitter. We define jitter theamount of error in the timing of a task over subsequent iterations of a program or loop. A good real-timesystem provides a low amount of jitter when programmed correctly.We can classify real-time systems into two groups: soft real-time system: systems that still function even if deadlines are sometimes not met (failure tomeet response time constraints leads only to a decrease of performance); hard real-time systems: systems where a failure to meet response time constraints leads to acatastrophic system failure.Industrial automation and robotics – Distributed control systems – Paolo Rocco
Real-time systemsValiditydeadlineIndustrial automation and robotics – Distributed control systems – Paolo RoccoResponsetime
Real-time systemsFrom a hardware point of view, a real-time system should have: one or more processors with suitable computational power; known and predictable instruction execution time (at least for the worst case) low and deterministic memory and I/O latency; automatic fault detection capabilities; hardware redundancy.From a software point of view, a real-time operating system should have: a task scheduler allowing different process priorities; a multitasking pre-emptive scheduler (a task can be temporarily interrupted and later resumed) allowingcontext switches; techniques to avoid deadlock conditions; mechanisms to support inter-process communication and process synchronization; mechanisms to handle interrupts.Industrial automation and robotics – Distributed control systems – Paolo Rocco
Real-time systemsExample of preemptive scheduling between three tasks (single core execution):Industrial automation and robotics – Distributed control systems – Paolo Rocco
Real-time systemsWriting software for a real-time system is far more complex than writing standard software. Rigoroustechniques and tools are required, for the design and for the testing phase as well.To simplify the development of real-time applications, supporting inter-process communication,synchronization, data exchange and distributed computing.Finally, we mention some real-time operating systems:QNX (commercial) https://blackberry.qnx.com/enVxWorks (commercial) E (commercial) https://en.wikipedia.org/wiki/Windows Embedded CompactRTAI (open), developed at POLIMI https://www.rtai.orgXenomai (open) https://www.xenomai.orgWind River Linux (open) l automation and robotics – Distributed control systems – Paolo Rocco
Point-to-point connectionsLet’s now consider the connectionbetween components of anautomation system.In the simplest case of a controlsystem hardware architecture, two“field” components, a transducer andan actuator, are connected to thecontroller, with analog connections(current or ndustrial automation and robotics – Distributed control systems – Paolo RoccoDriveHow are the connectionsorganized if there arenumerous field components?
Traditional centralized nsorIndustrial automation and robotics – Distributed control systems – Paolo RoccoOne possibility is a centralizedarchitecture with analogue pointto-point connections.However: High number of connections Cost of wiring High sensitivity to disturbances(need for shielding) Poor flexibility and scalability
Bus architectureA valid alternative is a bus architecturewith digital transmission of signalsPCPLCPLCDIGITAL erIndustrial automation and robotics – Distributed control systems – Paolo Rocco Savings on wiring costsEasy to add and remove devicesResource sharingFlexibilityFunctional decentralizationDistributed intelligence (localdiagnostic features)
Communication protocol000 .1For the transmission of complexinformation more sophisticatedcommunication mechanisms are required.To standardize the communication weneed to define communication protocols.Industrial automation and robotics – Distributed control systems – Paolo RoccoFor the transmission of simple digital data(e.g. bits) Simple digital I/O can be used:0-5 volts transistor-transistor (TTL) logic (used e.g. inArduino)0-3.3 volts for general purpose I/O (GPIO) logic (used e.g.by Raspberry Pi)0-10 or 0-24 volts (commonly used in PLC)
ISO-OSI communication protocolProtocols are organized in a hierarchical way in a protocol stack. In this way each protocol leverages theservices of the protocol below it.A conceptual model that standardizes the communication functions of a communication network,without regard to their underlying internal structure and technology, is the Open Systems Interconnectionmodel (OSI model). The model partitions a communication system into seven abstraction layers. A layerserves the layer above it and is served by the layer below it.The model is a product of the Open Systems Interconnection project at the International Organization forStandardization (ISO), maintained by the ISO/IEC 7498-1 standard.Industrial automation and robotics – Distributed control systems – Paolo Rocco
ISO-OSI communication protocolApplication: High-level APIs, including resource sharing, remote fileaccess, directory services and virtual terminals.Presentation: Translation of data between a networking service and an application;including character encoding, data compression and encryption/decryption.Session: Managing communication sessions, i.e. continuous exchange of information inthe form of multiple back-and- forth transmissions between two nodes.Transport: Reliable transmission of data segments between points on a network,including segmentation, acknowledgement and multiplexing.Network: Structuring and managing a multi-node network, includingaddressing, routing and traffic control.Data link: Reliable transmission of data frames between two nodesconnected by a physical layer.Physical: Transmission and reception of raw bit streams over aphysical medium.Not all the seven layers need to be implemented. In control systems the most used levels are:physical, data link and application.Industrial automation and robotics – Distributed control systems – Paolo Rocco
ISO-OSI and internetHow do the levels of theISO/OSI model map onthe internet?Industrial automation and robotics – Distributed control systems – Paolo Rocco
Physical layerThe physical layer of a transmission protocol defines the electrical and physical specifications of the data including:1. cable specifications (twisted, coaxial, optical fiber, )2. the layout of pins (number and mechanical specification of the connectors)3. voltages and line impedance,4. frequency (5 GHz or 2.4 GHz etc.) for wireless devices the encoding modality (NRZ, RZ, Manchester, ) the transmission mode (simplex, half duplex, full duplex) and thetype of the transmission (synchronous vs. asynchronous) the network topologyIndustrial automation and robotics – Distributed control systems – Paolo Rocco
Transmission mediatwisted pair wires– two conductors twisted together for the purpose of cancelingout electromagnetic interference– can be shielded (STP) or unshielded (UTP)– mid-high speed (up to 100 Mbps in local area networks)coaxial cables– an inner conductor surrounded by a tubular insulating layer,surrounded by a tubular conducting shield– mid-high speed (up to 100 Mbps in local area networks)optical fibers– transparent fiber made by drawing glass (silica) or plasticto a diameter slightly thicker than that of a human hair– permits transmission over longer distances and athigher bandwidths (up to 10 Gbps or more)Industrial automation and robotics – Distributed control systems – Paolo Rocco
Line codingBits are transmitted through the physical medium using a line code.Line coding consists of representing the digital signal to be transported, by a waveform that is optimallytuned for the specific properties of the physical channel. The common types of line encoding are:non-return-to-zero (NRZ), is a binary code in whichones are represented by a positive voltage, while zerosare represented by a negative voltagereturn-to-zero (RZ), the signal drops (returns) tozero between each bitManchester, the encoding of each data bit is eitherlow then high, or high then low, of equal time (the signalis self-clocking)Industrial automation and robotics – Distributed control systems – Paolo Rocco
Line codingBits can be alternatively transmitted through the physical medium using frequency modulationtechniques.For example the HART protocol uses the FSK method(Frequency Shift Keying) to encode bits.The two digital values “0” and “1” are assignedto the following frequencies:logical “0”, 1200 Hz sinusoidal signallogical “1”, 2200 Hz sinusoidal signalIndustrial automation and robotics – Distributed control systems – Paolo Rocco
Transmission modeAs for the transmission mode we might have: simplex, data can be sent only through one direction (unidirectional communication) half duplex, data can be sent in both directions but it is done one at a time (when the sender issending data, then at that time the receiver cannot send the sender a message) full duplex, data can be sent in both directions simultaneously (bidirectional communication)Almost all current equipment is, on a physical level, prepared for full duplex transmission.However, protocols can limit transmission to half duplex mode.Industrial automation and robotics – Distributed control systems – Paolo Rocco
SynchronizationFor the synchronization, we might have: asynchronous transmission, where the transmitter initiates the communication. Data are arranged incharacters (up to 8 bits) synchronous communication, that requires clocks of the transmitting and the receiving devices to besynchronized. Differently from asynchronous transmission, no start or stop bits are required. For thisreason synchronous communication allows more information to be passed over a circuit per unit timethan asynchronous communication.Industrial automation and robotics – Distributed control systems – Paolo Rocco
Data link layerThe data link layer: provides node-to-node data transfer, and detects and possibly corrects errors thatmay occur in the physical layer defines the protocol to establish and terminate a connection between two physicallyconnected devices defines the protocol for flow control between themStandard IEEE 802 further divides the data link layer into two sub-layers:– Medium Access Control (MAC) layer, responsible for controlling how devices in anetwork gain access to medium– Logical Link Control (LLC) layer, responsible for identifyingNetwork layer protocols, controlling for errors and frame synchronizationIndustrial automation and robotics – Distributed control systems – Paolo Rocco
Medium access controlThe MAC sublayer provides addressing and channel access control mechanisms that make it possible forseveral terminals or network nodes to communicate within a multiple access network that incorporates ashared medium.Examples of common multiple access protocols for wired networks are: CSMA/CD (defined by IEEE 802.3) Token bus (IEEE 802.4) Token ring (IEEE 802.5) Token passing (used in Fiber Distributed Data Interface, FDDI)Industrial automation and robotics – Distributed control systems – Paolo Rocco
Medium access controlCarrier Sense Multiple Access with Collision Detection (CSMA/CD) if the medium is idle any node can start a transmission at any time the transmitter monitors for collisions during transmission and, if a collision is detected,the frame is transmitted again not suitable for real-time applications as no bound on the transmission delay exists (dueto the retransmission mechanism)Token bus/token ring/token passing uses a special frame called a “token” that travels around a logical “ring” token passing is a channel access method providing fair access for all nodes, andeliminating the collision problem there can be master and slave nodes (slave nodes transmit only when asked by a masternode) access is deterministic hence suitable for real-time applicationsIndustrial automation and robotics – Distributed control systems – Paolo Rocco
The fieldbusIn modern distributed control systems, many standardized protocols are adopted, that areidentified as fieldbus. We will see some of them: CanBUS, DeviceNet, CanOPENProfibusEthernet, EtherCatProfiNetIndustrial automation and robotics – Distributed control systems – Paolo Rocco
CANbusCANbus (Controller Area Network bus) has been developed at Robert Bosch GmbH andreleased in 1986 and represents the standard communication protocol in automotive.Today, it is also adopted in non-automotiveapplication, though in a slightly modified manner(DeviceNet and CANopen).Industrial automation and robotics – Distributed control systems – Paolo Rocco
CANbusFeature (physical level)Adopted value in CANbusTransmission mediashielded twisted pairs (STP)Transmission speedfrom 125 kbps up to 1 Mbps (up to 40 m)Line encodingnon-return-to-zero (NRZ)Synchronization typeasynchronous (with clock synchronization to avoid bit slips)Feature (data link level)Adopted value in CANbusMAC typead hoc arbitration based on priority framesIndustrial automation and robotics – Distributed control systems – Paolo Rocco
CANbus: example of arbitration for the access to the busIndustrial automation and robotics – Distributed control systems – Paolo Rocco
ProfiBusProfiBus (Process Field Bus) is a standard for fieldbus communication in automation technology and wasfirst promoted in 1989 by BMBF (German department of education and research) and then used bySiemens.There are two versionsProfiBus DP (Decentralized Peripherals) is used to operate sensors and actuators via a centralizedcontroller in production (factory) automation applicationsProfiBus PA (Process Automation) is used to monitor measuring equipment via a process control systemin process automation applicationsIndustrial automation and robotics – Distributed control systems – Paolo Rocco
ProfiBusFeature (physical level)Adopted value in ProfiBus-DPTransmission mediashielded twisted (STP) pairs or fiber opticsTransmission speedup 12 Mbps (up to 1.2 km)Line encodingManchester (Manchester encoding Bus Powered, MBP)Synchronization typesynchronousFeature (data link level)Adopted value in ProfiBus-DPMAC typeToken-ring master and passive slave devicesIndustrial automation and robotics – Distributed control systems – Paolo Rocco
ProfiBus-DP: access to the busLogical network with token ring among master stations“active stations”, “master torSensor“passive stations”, “slave devices”Industrial automation and robotics – Distributed control systems – Paolo RoccoMTDriveTransmitter
EthernetEthernet was developed at Xerox PARC between 1973 and 1974 and is a family of computer networkingtechnologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and widearea networks (WAN).It has been first standardized in 1983 as IEEE 802.3 and has since been refined to support higher bit ratesand longer link distances.Over time, Ethernet has largely replaced competing wired LAN technologies. Thought Ethernet is notmeant to allow for real-time communication, other protocols have been developed on this standard.Industrial automation and robotics – Distributed control systems – Paolo Rocco
EthernetFeature (physical level)Adopted value in EthernetTransmission mediaunshielded twisted pairs (UTP), coaxial, or fiber opticsTransmission speedup 1 Gbps (up to 2 km)Line encodingManchesterFeature (data link level)Adopted value in EthernetMAC typeCSMA/CDIndustrial automation and robotics – Distributed control systems – Paolo Rocco
Industrial EthernetDifferent protocols exist that make Ethernet suitable for a real-time fieldbuscommunication system:PROFINET (Process Field Net) is an industry technical standard for datacommunication over Industrial Ethernet, with a particular strength in deliveringdata under hard real-time constraints ( 1 ms). The standard is maintained andsupported by Profibus & Profinet International.EtherCAT (Ethernet for Control Automation Technology) is an Ethernetbased fieldbus system, invented by Beckhoff Automation. The protocol issuitable for both hard and soft real-time computing requirementsIndustrial automation and robotics – Distributed control systems – Paolo Rocco
Industrial automation and robotics - Distributed control systems - Paolo Rocco. Distributed control systems. In a control system composed by a control unit and a few sensors and actuators, communication can be performed using analog or digital voltage or current signals. What happens, however, if we have many control units connected to
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