Automation Systems Discrete Event Control Systems and Networked Automation Systems

Transcription

1 Automation Systems Discrete Event Control Systems and Networked Automation Systems 10 th Lecture Signals and Communication in Automation Systems

2 Outline Signals analogue vs. digital cyclic vs. event-driven Analogue vs. Digital Transmission and Control Communication on different layers sensor/actuator-bus, field bus, plant bus Medium Access 264

3 Information in the Automation Pyramid Communication Networks in general are used to transmit information from one point to another Type of information is important for the design of communication system Networks in Automation transport different types of information depending on their placement in the Automation Pyramid Higher Levels: refined data (status reports) low frequency long response time (s, min, h) Lower Levels: raw data (measured sensor values) high frequency short response time (ns or ms) real-time 265

4 Data on Lowest Level: Physical Values Physical data to be measured for automation purposes Continuous in value and time Examples: temperature, velocity, Physical data to be influenced for automation purposes Continuous in value and time Examples: same as above 266

5 Sensors and Actuators Measurement in automation: Sensors (Sensoren, Messglieder) Convert physical values from different domains (mechanics, thermodynamics, chemistry, ) into electrical ones Measurement may be continuous or discrete Measurement of temperature using thermo-couple Measurement of position using limit-switch Influence on the process: Actuators (Aktuatoren, Stellglieder) Convert electrical values to other phyiscal values Actuation may be continuous or discrete Acceleration using motor with variable speed switching of a motor In most cases sensors already include Transducers (Messwertumformer) to produce electrical signals conforming to some standardized range (4..20mA, 0..10V, 0..24V, etc.) Today the output of a Transducer is often already in digital form 267

6 Example: Analogue speed measurement: tachometer A tachometer is a rotating permanent magnet that induces a voltage into a stator winding. This voltage is converted into an analogue voltage or current, conforming to some standard range. Alternatively, the output is directly converted to a digital signal; Transducer contains an A/D-Converter (A/D-Wandler). angular speed U ~ transducer analogue: ma digital: Source: Prof. Kirrmann 268

7 Vorlesungstitel 4-20 ma loop standard Sensor Transducer Instrument 1 Instrument 2 Instrument 3 voltage source V R1 R2 R3 measurand angular speed i = f(v) 0, ma Transducer acts as a current source delivering a current between 4 and 20 ma, proportional to the measurand (Messgrösse) 0 ma signals an error (wire disconnection) Number of loads connected in series is limited by the operating voltage Covered distance is limited due to induced noise Simple devices are powered directly by the residual current (4mA) allowing to transmit signal and power through a single pair of wires Source: Prof. Kirrmann 269

8 Vorlesungstitel Example of analog transducer High voltage Field house Transducer Current Transformer Protection 0..1A rms R = Load ma PLC Emergency panel Control Room Source: Prof. Kirrmann 270

9 Pros and Cons of Analogue Transmission Pros Very simple/cheap sensors (no intelligence needed) Simple installation (only electrical setup, no programming) Cons Interference-prone Only short distances (high energy loss rate on long distances) Practically no error detection and correction possible detection: limited to detection of complete breakdown correction: limited to filtering No intelligence at sensor level possible No diagnosis One-way communication No parameterization Changes require re-wiring Large amount of wiring Today data is processed in digital form anyway (first thing at an analogue input of a PLC is A/D-conversion) Aside on Analog vs. Digital Computation 271

10 Vorlesungstitel Control (Computation) using analogue values AT 3 Example endim 2000 Modular Analogue Computer Based on electron tube technology Calculation-Voltage: 100V Producer: Rechenelektronik Glashütte (Sachsen) Built: , (20 Pieces) Example Meda 42 Modular Analogue Computer Based on transistor technology Calculation-Voltage: 10V Producer: Aritma AT, Prague Built: from 1968, until late 70s 272

11 Vorlesungstitel Control (Computation) using analogue values AT 3 Programming Example Meda 42 Modular Analogue Computer Based on transistor technology Calculation-Voltage: 10V Producer: Aritma AT, Prague Built: from 1968, until late 70s 273

12 Handling of Analogue and Binary Data in PLCs y(t) y(t) y(k) u(k) u(t) time analogue variable (e.g mA) filtering & scaling sampling analogdigital converter input memory digitalanalog converter amplifier analogue variable e.g. -10V..10V binary variable (e.g V) filtering & scaling sampling processing output memory transistor or relay binary variable (e.g V) y(t) y(t) u time 274 time

13 Analogue to Digital Conversion Digital Signals are quantized in values sampled in time encoded and serialized quantized & sampled quantized sampled encoded & serialized t preamble start payload end delimiter 275

14 Cyclic (equidistant) versus Event-Driven sampling / communication hysteresis x% cyclic: sample/send value strictly every n milliseconds sample period n ms misses the (fast) peak always the same, why transmit? time event-driven: sample/send when value change by more than x% of range how much hysteresis? - coarse (bad accuracy) - fine (high frequency) misses values (limited update frequency) nevertheless transmit: - every m ms as I m alive sign - when data is internally updated - upon quality change (failure) 276

15 Pros and Cons of Digital Transmission Pros Less interference-prone Longer distances are possible Error detection and correction possible (depends on encoding) Additional information can be added to the bit stream diagnosis Two-way communication possible parameterization Easier change of an installation Sharing of resources is possible Networking Cons Protocols are needed for communication Less simple/cheap sensors (intelligence needed, A/D-converter, ) Less simple installation (additional know-how needed) 277

16 Digital Communication using direct connections Advantages of digital communication can be gained (and have been) without networking: A/D-conversion directly at sensor level Grouping of sensors/actuators possible: several (dumb) components connected to an intelligent access point point-to-point communication using serial or parallel interfaces parallel: + fast, - much more wiring, - possible crosstalk serial: - slower, + less wiring, + less crosstalk Example: RS232 Completely different and very successful approach: HART Protocol HART uses the well-known analogue approach (4-20 ma) Digital Information is added on the analogue connection Measurement of the process variable Configuration or re-configuration data Diagnostic information Values of additional measured variables (multi-variable devices) Device Status 278

17 HART: Concept HART ("Highway Addressable Remote Transducer") popular digital communication protocol for industrial process measurement applications uses low-level modulation superimposed on the standard 4-20 ma current loop widely used for such measurements average value of the HART signal is zero (FSK) without significant effect on analogue current signal analogue signal can still be used compatibility with existing systems digital advantage: device configuration, status checking, diagnostics 279

18 HART: Applications Communication to field devices with additional information (bidirectional) Closing of control-loops in the actuator-sensor-level Multi-drop communication with up to 15 devices (analog value is fixed to 4 ma and communication is only based on digital signal, polling) Final step to digital communication 280

19 Digital Communication using direct connections Remaining problem: Amount of wiring (HART in multi-drop has limited number of devices) Response-Time (HART: 2 updates per second, 4 in burst-mode) Idea: Sharing of the communication medium (wires) bus possible since digital signals are only transmitted at intervals between two samplings the communication medium can by used by others also digital transmission allows to add additional information (source, ) 281

20 Vorlesungstitel The original idea: save wiring field bus COM I/O switching cabinet dumb devices PLC PLC field bus Note: the number of end-points remains the same! energy must be supplied to smart devices 282

21 What is a field bus? Communication network, in an automation system, characterized by: transmission of numerous small data items (process variables) with bound delay (1ms..1s) harsh environment (temperature, vibrations, EM-disturbances, water, salt, ) robust and easy installation by skilled people high integrity (no undetected errors) high availability (redundant layout) clock synchronization (milliseconds down to a few microseconds) continuous supervision and diagnostics low attachment costs moderate data rates (50 kbit/s 5 Mbit/s) but large distance range (10m.. 4 km) non-real-time traffic for commissioning (e.g. download) and diagnostics Source: Prof. Kirrmann 283

22 Main Problem of Networking Medium Access How to make sure that on a shared communication line only one connected participant sends at a time? Side-conditions for this problem keep waiting times low use the available medium bandwidth provide deterministic response time (depends on application) Medium Access deterministic (controlled) non-deterministic (carrier sense multiple access) centralized de-centralized collision detection collision avoidance master/slave token bus CSMA/CD CSMA/CA token ring 284

23 Master/Slave General Idea: There is one node in the bus that handles the communication control: MTER The master assigns the right to write on the bus to the slaves according to some internal algorithm (e.g. fixed sequence) Advantage: dumb slave nodes address Bus Master devices (slaves) plant Example: Cyclic activation of slaves time [ms] 285

24 Token Problem of Master/Slave: Single point-of-failure (MTER) General Idea: Each node at the bus can handle the communication control The right is handed over using a token Two possibilities: token bus: physically a line (bus), however logically a ring token ring: physically as well as logically a ring Drawback: More intelligence needed in all nodes 286

25 Token Bus Token Bus A node receives the free-token Now he has the medium or a pre-defined maximum time After finishing its transmission the token is sent to the next (by address) node The node with the highest address sends it to the node with the lowest ( logical ring) 287

26 Token Ring Token ring A node receives the free-token If the node wants to submit data it converts it in a busy-token, appends the data and sends it Next station receives data (if it is the receiver, data is copied into internal memory) Data is sent on to the next station until it comes back to the sender (check) After finished sending, the token is sent to the next station 288

27 Drawbacks of Deterministic Approaches Access strategies have to be defined off-line Extension of the system requires careful choices in setting new timeslots priorities, Idea of non-deterministic access: Just connect nodes to the network Let the nodes check for themselves whether they can send Requires some intelligence 289

28 Main Problem of Networking Medium Access How to make sure that on a shared communication line only one connected participant sends at a time? Side-conditions for this problem keep waiting times low use the available medium bandwidth provide deterministic response time (depends on application) Medium Access deterministic (controlled) non-deterministic (carrier sense multiple access) centralized de-centralized collision detection collision avoidance master/slave token bus CSMA/CD CSMA/CA token ring 290

29 Drawbacks of Deterministic Approaches Deterministic Ideas: Master/Slave, Token-Bus, and Token-Ring Access strategies have to be defined off-line Extension of the system requires careful choices in setting new timeslots priorities, Use of medium not optimal Long waiting for medium even if others do not want to communicate Problem of Single-Point-Of-Failure in Master/Slave Similar problems in Token Approaches Broken ring in Token-Ring Lost token if active device fails Idea of non-deterministic access: Just connect nodes to the network Let the nodes check for themselves whether they can send Requires some intelligence 291

30 Carrier Sense Multiple Access (CSMA) Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium, such as an electrical bus, or a band of electromagnetic spectrum. "Carrier Sense" describes the fact that a transmitter listens for carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the node waits for the transmission in progress to finish before initiating its own transmission. "Multiple Access" describes the fact that multiple nodes send and receive on the medium. Transmissions by one node are generally received by all other nodes using the medium. In pure CSMA, only the carrier sense is used to avoid collisions. If two nodes try to send a frame at nearly the same time, neither detects a carrier so both begin transmitting at the same time. The transmitters do not detect collisions, so transmit the entire frame. Receivers cannot distinguish between collisions and other sources of frame errors, so collision recovery relies on the ability of the communicating nodes to detect frame errors and invoke an error recovery procedure. 292

31 CSMA Extensions In Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA), each node must inform other nodes of an intent to transmit. When the other nodes have been notified, the information is transmitted. This arrangement prevents collision because all nodes are aware of a transmission before it occurs. However, collisions are still possible, and are not detected so have the same consequences as in pure CSMA. In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), sending nodes are able to detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in much more efficient use of the media since the bandwidth of transmitting the entire frame is not wasted. In Carrier Sense Multiple Access with Bitwise Arbitration (CSMA/BA), all of the nodes on the bus are assigned an identification number or priority code. When a collision occurs, one of the nodes that are attempting to send at the same time will be given priority to transmit according to its identification number or priority code (as opposed to waiting a random amount of time and then retransmitting, as in CSMA/CD). 293

32 Summary of Chapter 10 This lecture gave an introduction to Signals and Communication in Automation Systems considering the following aspects: Types of Data in Automation Systems Analogue vs. Digital Data and D/A-Conversion Analogue vs. Digital Communication including HART Extension of Digital Communication to Networks Bus Systems (Linear Architecture in Networks) Methods for Medium Access in Networks 294