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Programmable Logic Controllers PLC’s

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1 Programmable Logic Controllers PLC’s
An Overview, Use in HVAC and Web Technologies By Sleepy Wombat

2 Overview What is a PLC ? History Overview of Technology
PLC Configuration and Selection Programming PLC’s PLC’s in HVAC Web Technologies

3 What is a PLC ? PLC (Programmable Logic Controller)
A PLC works by looking at its inputs and depending on their state, and the user entered program, turns on/off outputs. A PLC can be thought of as: Industrial Computers with specially designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input / output connections to the real world).

4 History 1/4 Early control systems consisted of huge control boards consisting of hundreds to thousands of electromechanical relays. An Engineer would design the system logic. Electricians would receive a schematic outline of logic then implement the logic with relays. The schematic was commonly called “Ladder Schematic” The Ladder displayed all switches, sensors, motors, valves, relays etc in the system. Problems: Long commissioning time, Mechanical Reliance, Any system logic design change required the power to the control board to be isolated stopping production.

5 History 2/4 General Motors was among the first to recognise a need to replace the systems “wired control board” Hydramatic Division of GM specified the design criteria for the programmable controller in 1968. Goal – Eliminate the high cost associated with inflexible, relay controlled systems.

6 History 3/4 New Controller Specifications: Solid State System
Computer Flexibility Operate in Industrial Environment (vibrations, heat, dust etc.) Capability of being reprogrammed Easily programmed and maintained by electricians and technicians.

7 History 4/4 In 1969 Gould Modicon developed the first PLC.
Strength – Programmed with Ladder Logic Initially called Programmable Controllers PC’s Now - PLC’s, Programmable Logic Controllers PLC’s have evolved from simple on/off control to being able to communicate with other control systems, provide production reports, schedule production, diagnose machine and process faults.

8 Overview of Technology

9 Basic PLC Schema CPU Power Supply Memory Input Blocks Output Blocks
Communications Expansion Connections

10 CPU Module The Central Processing Unit (CPU) Module is the brain of the PLC. Primary role to read inputs, execute the control program, update outputs. The CPU consists of the arithmetic logic unit (ALU), timing/control circuitry, accumulator, scratch pad memory, program counter, address stack and instruction register. A PLC works by continually scanning a program PLC Program SCAN 1

11 Memory The memory includes pre-programmed ROM memory containing the PLC’s operating system, driver programs and application programs and the RAM memory. PLC manufacturer offer various types of retentive memory to save user-programs and data while power is removed, so that the PLC can resume execution of the user-written control program as soon as power is restored. 1

12 Memory cont’d Many PLCs also offer removable memory modules, which are plugged into the CPU module. Memory can be classified into two basic categories: volatile and non-volatile. - Volatile memory is that which loses state (the stored information) when power is removed. - Nonvolatile memory, on the other hand, maintains the information in memory even if the power is interrupted.

13 Memory cont’d Some types of memory used in a PLC include:
ROM (Read-Only Memory) RAM (Random Access Memory) PROM (Programmable Read-Only Memory) EPROM (Erasable Programmable Read-Only Memory) EEPROM (Electronically Erasable Programmable Read-Only Memory) FLASH Memory Compact Flash – Can store complete program information, read & write text files

14 I/O Modules Input and output (I/O) modules connect the PLC to sensors and actuators. Provide isolation for the low-voltage, low-current signals that the PLC uses internally from the higher-power electrical circuits required by most sensors and actuators. Wide range of I/O modules available including: digital (logical) I/O modules and analog (continuous) I/O modules. 1

15 Inputs Modules Inputs come from sensors that translate physical or chemical phenomena into electrical signals. The simplest form of inputs are digital/discrete in AC/DC. In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8,16, 32, 64, 96 inputs of the same type on each card. 1

16 Inputs Modules The list below shows typical ranges for input voltages.
5 Vdc 12 Vdc 24 Vdc 48 Vdc 12 Vac 24 Vac 120 Vac 240 Vac 1

17 Example of Input Card 1

18 Outputs Modules Output modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. A common choice when purchasing output cards is relays, transistors or triacs. Relay are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower, cost more, and they will wear out after millions of cycles. 1

19 Relays The most important consideration when selecting relays, or relay outputs on a PLC, is the rated current and voltage. For transistor outputs or higher density output cards relay terminal blocks are available. Advantage of individual standard replaceable relays 1

20 Outputs Typical output voltages are listed below, 5 Vdc 12 Vdc 24 Vdc
24 Vac 120 Vac 240 Vac WARNING: Always check rated voltages and currents for PLCs and never exceed. 1

21 Example of Output Card 1

22 Analogue Cards Typical Analogue Input signals are:
Flow sensors Humidity sensors Load Cells Potentiometers Pressure sensors Temperature sensors Vibration Analogue Output signals control: Analogue Valves Actuators Chart Resorders Variable Speed Drives Analogue Meters Typical Analogue Signal Levels 4-20mA 1-5 Vdc 0-10 Vdc -10 – 10Vdc 1

23 Analogue Inputs/Outputs
Analogue input cards convert continuous signals via a A/D converter into discrete values for the PLC Analogue output cards convert digital values in then PLC to continuous signals via a D/A converter. Resolution can be important in choosing an applicable card Example, for a temperature input of 0 to 100 degrees C For 8 bit resolution the value in the PLC is 0 to 255 For 12 bit resolution the value in the PLC is 0 to 4095 For 12.5 bit resolution the value in the PLC is 0 to 6000 For 13 bit resolution the value in the PLC is 0 to 8192 For 16 bit resolution the value in the PLC is 0 to 32768

24 Special Modules RF ID Voice Gas Flow Calculation Weigh Cell
Hydraulic Servo ASCII Fuzzy Logic Temperature Sensor Temperature Control Heat/Cool Control Field Bus Cards DeviceNet, Profibus etc Lonworks, BACNet Fast Response (Interrupt) PID Loop Controller BASIC Cards RS232 Comm’s Modbus ASCII/RTU Ethernet Comm’s High Speed Counters Position Control Cards Per to Per Comm’s Controller Link DH+ Modbus Plus

25 Available Instructions
Sequence Input Output Control Logic Timer and Counters Comparison Range Comparison Data Movement Data Shift Step / Step Next Serial Communications Text String Processing File Manipulation Increment/Decrement Conversion ASCII Number Systems Math Floating Point Math Statistics Scaling PID PID with Auto tune Clock / Date Block Processing IF,THEN,ELSE,LOOP Table Processing LIFO, FIFO

26 Networks ASI Devicenet Interbus-S Profibus Interbus Fieldbus Ethernet I/P Smart Distributed System (SDS) Seriplex CANopen Lonworks BACNet Gateways enable communications between different network topologies


28 The Configuration of PLC
The configuration of PLC refers to the packaging of the components. Typical configurations are listed below from largest to smallest. Rack Type : A rack can often be as large as 18” by 30” by 10” Mini: These are similar in function to PLC racks, but about the half size. Dedicated Backplanes can be used to support the cards OR DIN rail mountable with incorporated I/O bus in module. Shoebox: A compact, all-in-one unit that has limited expansion capabilities. Lower cost and compactness make these ideal for small applications. DIN rail mountable. Micro: These units can be as small as a deck of cards. They tend to have fixed quantities of I/O and limited abilities, but costs will be lowest. DIN rail mountable. 1

29 Sizing of PLC Micro PLCs: I/O up to 32 points
Small PLC: I/O up to 128 points Medium PLC: I/O up to 1024 points Large PLC: I/O up to 4096 points Very Large: I/O up to 8192 points 1

30 Selecting a PLC Criteria Number of logical inputs and outputs Memory
Number of special I/O modules Expansion Capabilities Scan Time Communication Software Support Dollars 1

31 Selecting a PLC 1

32 Example of PLC Specifications

33 Example of PLC Specifications

34 Manufactures Major Brands OMRON Allen Bradley
Schneider (Modicon, Telemecanique, Square D) GE Fanuc Siemens Automation Direct (Koyo) Toshiba Mitsubishi Hitachi Keyence Festo Eberle Texas Instruments April

35 Programming PLC’s

36 Programming PLC’s Ladder Logic remains the most common technique for programming PLC’s


38 PLC Standardization Open Controller
IEC 61131 Based on IEC 1131 (1992) standard, developed to be a common and open framework for PLC architecture. IEC Overview IEC Requirements & Test Procedures IEC Data Types & Programming IEC User Guidelines IEC Communications IEC Fuzzy Control IEC Guidelines for the application and implementation of programming languages

39 IEC 61131-3 IL (Instruction List) – mnemonic programming
LD (Ladder Diagram) – Relay logic ST (Structured Text) – A BASIC like programming language FDB (Functional Block Diagram) – Graphical dataflow programming language SFC (Sequential Flow Chart) – Graphical method for structured programs

40 IEC 61131-3 Examples Ladder Instruction List Structured Text
Function Block Sequential Flow Chart

41 PLC’s in HVAC control

42 PLC’s in HVAC control Despite the versatility of today’s PLC’s, many controls professionals are skeptical of applying PLC’s to new applications such as HVAC control. Reasons included : Lack of knowledge of these systems Requirement for programming Lack of initial capability of technology Premium first up cost

43 Advantages PLC’s remain a predominate piece of equipment on the factory floor and will probably remain so for some time to come. The advantages they offer are: Cost effective for controlling complex systems Flexible and can be reapplied to control other systems quickly and easily Computational abilities allow more sophisticated control Trouble shooting aids make programming easier and reduce downtime Reliable components (high MTBF) ensure operation for years

44 Advantages continued….
Variety of I/O interfaces Small size Growing with technology, faster scan times, capability etc Quick I/O disconnects that aids in field servicing Modularity in hardware architecture Modularity in software design (programmer dependant) Software Timers/Counter, Relays Clean failure mode On-line programming Self inspecting for operation code during execution Availability of programmers/trouble shooters/integrators Availability of replacement parts

45 Benefits The benefits achieved with programmable controllers will grow with the individual using them: “The more you learn about PLC’s, the more you will be able to solve other control problems.”

46 Examples Southgate – Melbourne Millennium Dome – London
Woolworths – Britain ST Microelectronics – France Einstein III Building - Munich

47 Web Technologies

48 Web Enabled Automation...?
Is it just more Internet hype or is there some benefit behind the idea. Is it possible to connect all of these systems by their PLC’s to the Internet ? Is it practical ? Is it profitable ?

49 Short Answers…… Internet access can provide compelling competitive advantages linking customers, suppliers, subcontractors, front line management. Improve fault reporting and response time Enhance flexibility Optimise process control Proven affordable technologies are readily available to connect any PLC, Process Controller or I/O to the Internet and Intranet. Process to Internet access has the potential to become a standard utility.

50 What is Web Enabling Web enabling gives real time access to data and control, virtually anytime, anywhere. It uses communication with any manufactures PLC or I/O to send information via the Internet to anywhere in the world. Web enabled automation drives real-time accessibility “Vertically Down” to the control device level.

51 What can we do with it ? Check real time process data without waiting for end of shift reports Track equipment run time and initiate maintenance requests Automatically notify a technician via or GSM phone etc of faults and allow rectification Collect process data on a server PC and analyse with statistics and control (SPC) Use HTML help files including graphics Monitor and control remote operations reducing field staffing and unnecessary travel

52 What pieces and parts are required ?
Interface Web Server Data Service Browser

53 Interface An interface to the equipment to be monitored and controlled via the web (network) connection Ethernet Line Modem / Phone line ISDN Wireless Modem (eg b Ethernet) Type of connection is typically determined by the thin server and the existing network infrastructure – or lack thereof

54 Web Server Web Server (or “thin server” data service) to enable the remote browser to view the desired displays and web pages. Typical installation requires a connection to an existing PLC or proprietary controller. Most PLC’s support at least a serial connection using the vendor supplied protocol to communicate. Use an Open network controller (interface) Some PLC’s have Web Servers built into their Ethernet Units.

55 Data Service A data service or interface to handle exchanging data between the local equipment/ process (server) and the remote system (client) A common “language”, protocol is required.. XML (Extended Markup Language) Benefit of XML is its independence from senders and receivers hardware, OS, and application.

56 Browser Interface Use the thin servers assigned IP address to access the remote system. Simple browsing requires standard browser interface found on a desktop computer, PDA, WAP, or “Thin Client” For application information sharing (SPC, Enterprise Software), if the thin server and remote application software support a common interface such as OPC, setting up the data exchange can be accomplished in a couple of minutes

57 New term … OPC OLE for Process Control Object Linking and Embedding
One of industries most popular standards Maintained by OPC foundation OPC standard is non proprietary technical specification based on Microsoft's OLE/DCOM technology.




61 Web Servers Computer Based Displays
SCADA – Supervisory Control and Data Acquisition Citect, Wonderware, Fix, Iconics, Genius Ethernet, Serial, OPC (OLE for Process Control), Dedicated Computer Boards for Propriety Buses Dedicated Packages for BMS Citect SCADA Facilities Embedded Web Servers Open Network Controller Interface – Gateway Embedded Web Servers in PLC’s Ethernet cards New Dedicated HMI Screens – some have embedded Web Servers Example of Web Technology visit



64 Thank you…. Questions ?

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