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ОглавлениеIntroduction to Programmable Logic Controllers
Objectives: Define the term PLC.
Understand the history of PLC.
Identify four major components of PLC.
Explain the operation principle of a PLC system.
Describe the benefits of using PLCs.
Identify typical PLC applications.
Know PLC categories.
Overview
PLCs are digital electronic apparatus with a programmable memory for storing instructions to implement logic, sequencing, timing, counting, and arithmetic functions. Since its inception in 1969, PLC technology has staged through six generations of development. Memory size and added functions have been increased from one generation to the next.
There are four major components in a PLC: processor unit, input modules, output modules, and programming device. A processor unit is the brain of the PLC that consists of three elements: central process unit, memory, and power supply. Input modules provide the physical connection between the processor unit and input field devices such as limit switches. Output modules provide the physical connection between the processor unit and output field devices such as motor starters. A programming device allows creating PLC programs and entering the programs to PLCs.
The basic operation principle of a PLC is program scanning. A PLC processor scans a program in a cyclic manner, starting from left to right at the top rung, then proceeding to the next rung. During scanning, the processor simultaneously updates the status of input and output instructions in both the program and the memory, as well as interacting with input and output modules.
The benefits of using PLCs are very obvious. They are flexible and programmable, reliable, and cost saving. PLCs provide ease of installation and implementation, as well as ease of maintenance and troubleshooting. There are five categories of PLCs according to their size and configuration, including micro PLCs, small PLCs, medium PLCs, large PLCs, and very large PLCs.
1.1 Definition of PLC
The term PLC is the abbreviation for Programmable Logic Controller. PLCs were initially called Programmable Controllers (PCs). The use of this term, PCs, caused some confusion when personal computers (PCs) became popular. To avoid this confusion, PCs are referred to as personal computers and PLCs programmable logic controllers. The National Electrical Manufacturing Association (NEMA) defines a programmable controller as follows:
“A programmable controller is a digital electronic apparatus with a programmable memory for storing instructions to implement specific functions, such as logic, sequencing, timing, counting, and arithmetic to control machines and processes.”
More specifically, a PLC can be considered as an industrial computer that is especially designed for use in industrial, rugged environments. It performs the following functions:
•Receives and interprets the signals from various input switches and sensors
•Implements the control logics designed in the form of programs
•Outputs the control signals to activate the power devices such as motor starters, solenoids, contactors, etc.
1.2 History of PLCs
The inception of PLCs resulted from the necessity and the emergence of computer technology. Electromechanical relays that provide logic controls for industrial systems had been successfully implemented for many generations. The main problems with those hard-wired, relay-based systems include:
•Lack of flexibility of reprogramming
•Limit to relay types of applications
•Susceptive to mechanical failure due to physical contact and wear
•Prone to wiring errors
•Difficulty of trouble shooting
•Limited to small-to-medium size of control systems
•Costly to implement when the size of control systems increase
GM’s Hydromatic Division realized the need for using a solid-state system with computer flexibility to replace hard-wired relay control panels, which were huge, costly, and inflexible. In 1968, Hydromatic defined the design specifications for the first programmable logic controller. Some of the initial specifications are outlined below:
•The new control system had to be a solid-state device with the flexibility of a computer.
•The system had to sustain an industrial environment (vibration, heat, dirt, etc.).
•The system had to be re-programmable and reusable for other tasks.
•The input and output interfaces had to be easily replaceable.
•The system had to be easily programmed and maintained by plant electricians and technicians.
•The system had to be cost competitive with the use of hard-wired relay systems.
GM solicited interested companies to develop a system that met the above design specifications. Richard E. Morley, founder of the Modicon Corporation, built the first practical PLC in 1969. Since then, PLC technology has steadily advanced, in both hardware and software, with the computer technology. We can grossly classify the PLC development into six generations. The year, added functions, advancement in hardware and software, and the applications of each generation are summarized in Table 1.1.
1.3 PLC Components
A typical PLC system consists of four major components: processor unit, input modules, output modules, and programming device (Figure 1.1).
1.3.1Processor Unit
The processor unit is the brain of the PLC. It consists of three parts: central process unit (CPU), memory, and power supply (Figure 1.2). They are briefly described below:
Table 1.1: PLC Development
Figure 1.1: PLC components
Figure 1.2: Processor unit
CPU: executes operating system, manages memory, receives inputs, implements ladder logic instructions, outputs control signals to field devices, and handles communications with other devices.
Memory: has two types — system memory and user memory. The operating system is stored in a Read-Only Memory (ROM) type of system memory for converting the ladder diagram to instructions that the CPU can recognize. User memory is normally of Random Access Memory (RAM) type. It is used to (1) store status of inputs and outputs, (2) store contents of variables for timers and counters, and (3) reserve spaces for the processor work area.
Power Supply: produces low voltage DC power used by the logic circuits of the processor and input and output modules.
1.3.2Input Modules
Input modules provide the physical connection between the processor unit and the input field devices. They detect and transmit the input signals from input field devices to the processor (Figure 1.3). Using input modules, the PLC processor can sense status conditions such as Open or Closed, and actuated or not-actuated switches, as well as measure process quantities such as pressure, temperature, flow, and position. Using the input data gathered from input modules, the PLC processor processes the data according to the instructions in the PLC program, then issues control commands to the proper slot in output modules, which in turn control various output field devices such as solenoids, motor starters, etc.
Figure 1.3: Input modules
1.3.3Output Modules
Output modules provide the physical connection between the processor unit and output field devices to facilitate the PLC processor sending the control signals to output devices (Figure 1.4). Output modules perform the following functions:
•Provide connection terminals for output field devices
•Receive control data from the PLC processor and pass on to control output field devices
•Give isolation between the PLC system and field voltages
Figure 1.4: Output modules
Figure 1.5: Hand-held programming device
Figure 1.6: Programming terminal
1.3.4Programming Devices
The main purpose of a programming device is to enter the control programs to PLCs. There are several ways you can perform this task. The available programming devices you can use for a particular PLC vary from manufacturer to manufacturer. There are three basic types of programming devices: hand-held programmers, programming terminals, and personal computers.
Hand-held programmers are an inexpensive and portable way to program small PLCs (Figure 1.5). They have a display and keyboard with numeric keys and programming instruction keys for input. Programming terminals are used in larger PLCs. They are proprietary products from PLC manufacturers and are more costly. They look like a combination of computer monitor and keyboard (Figure 1.6). One programming terminal can serve multiple PLCs.
Personal computers can work together with PLC programming software to create PLC programs for any PLC types. Each brand name PLC has more than one type of programming software for you to prepare programs; the software allows you to download programs to PLCs and monitor the status of the systems.
1.4 PLC Operation Principle
The successful implementation of a PLC project depends on integrating field devices, the PLC program, and the PLC system. The keys to integrate and coordinate these three elements are interface wiring and address assignment (Figure 1.7).
Figure 1.7: PLC operation principle
1.4.1Input Wiring
There are two types of interface wiring: input wiring and output wiring. Input wiring connects input devices to the input modules. There are a fixed number of connection terminals in each input module. Each connection terminal has its assigned address in the PLC memory (Figure 1.8). The actuation of an input device generates a digital ON (1) or OFF (0) signal to appear at its connected terminal of the input module (Figure 1.9). The PLC processor picks up the signal while it reads the input module and stores the signal in the assigned memory address. In this way, any status change in input devices can be immediately reflected on the PLC memory.
Figure 1.8: PLC memory address for input module
Figure 1.9: Signals produced from input module
1.4.2Output Wiring
Similarly, output wiring connects output devices to the output modules. Each connection terminal has its assigned address in the PLC memory. The PLC processor sends the digital signal to connection terminals based on the content in their corresponding memory address (Figure 1.10). An ON (1) signal to a particular output terminal causes the wired output field device to be energized or turned ON. An OFF (0) signal to the output terminal de-energizes the output field device (Figure 1.11).
Figure 1.10: PLC memory address for output module
Figure 1.11: Signals from PLC to output module
1.4.3PLC Program
The PLC program is the road map of the operation. In most PLC systems, PLC programs are written in ladder diagram format. A ladder diagram consists of two rails and several rungs (Figure 1.12). Two rails, arranged vertically, represent the power lines. Program instructions are arranged along the horizontal rungs. The number of rungs increases as the program becomes larger and complicated. Each input and output instruction is assigned a memory address. A PLC program is implemented in cycles. Each cycle involves three steps:
Figure 1.12: A PLC program
1.Reading the status of input field devices in input modules and writing these signal data to their memory address
2.Scanning the program to update the status of input instructions and placing the output results to their address
3.Sending the control signals to the output modules
1.4.4PLC Scanning
The PLC processor scans a program in cyclic manner. The scanning cycle starts from left to right at the top rung and proceeds to the second rung until reaching the bottom rung to complete a cycle. It then returns back to the top rung to continue the next cycle (Figure 1.13). During scanning, the processor simultaneously updates the status of input and output instructions, in both the program and the memory, as well as interacting with the input and output modules.
Figure 1.13: PLC scanning cycle
1.5 PLC Applications
1.5.1PLCs versus Hard-Wired Relay Systems
PLCs initially were intended to replace hard-wired relay systems. They offer many advantages over their hard-wired counterparts. Table 1.2 compares these two types of control systems.
Table 1.2: Comparison between PLC control and relay control
| Feature | Hard-Wired Relays | PLCs |
|---|---|---|
| Functions | Limited to relay types of control. | Extended and advanced control functions available. |
| Feasibility | Complex systems require many relays to control complicated tasks. | Capable of controlling any complex systems. |
| Flexibility | Need to change wiring when applications change. | Programmable. Simply replace a program for a new application. |
| Reliability | Reliable, but susceptive to poor contact and limited life. | Highly reliable due to semi-conductor devices. |
| Expandability | Hard to expand. | Can be expanded to sizable memory for control program. |
| Maintainability | Requires regular maintenance and inspection. Need to replace components in their due life. | Easy to maintain. Replace single module if needed. |
| Required technical knowledge | Many people know relay logic. | Do not need to understand the hardware. |
| Space requirement | Relatively large. | Relatively small. |
| Required time for design and implementation | Takes time to prepare drawings, installation and testing. | Relatively short. |
| Cost | Cost-effective if the number of relays used in the system is less than ten. | Cost-effective if the number of relays used in the system is more than ten. |
| Data collection | Cannot store the data gathered in the control system for further analysis. | Data gathered from the system can be stored in the memory for further use and analysis. |
1.5.2Benefits of Using PLCs
The architecture of PLCs is modular and flexible in nature, which permits hardware and software elements to be integrated in any combination. They can be uniquely tailored by adding or removing some elements to meet a specific application. The benefits of using PLCs can be summarized in the following five items:
•Flexible and programmable
•Ease of installation and implementation
•Reliable
•Ease of maintenance and troubleshooting
•Cost saving
Flexible and Programmable
PLCs allow the control systems to be modularly configured to meet specific needs whether they are big or small, simple or complicated, long-term or short-term use. They are programmable so that changes in a control program results in a different application. It is also easy to make any change to the control program without involving much effort in programming and hard-wiring.
Ease of Installation and Implementation
PLCs are relatively small size compared to their hard-wired relay counterparts. It takes less than half the space required by its equivalent relay control panel. The amount of wiring is significantly reduced due to the elimination of hard-wired relays, counters, timers, etc. Any changeover can be made readily by connecting the input and output devices to the terminal strips.
Reliable
PLC systems are highly reliable because they use solid-state elements that have no mechanical wear, low component failure, and low space and power consumption. They use standard devices and standardized wiring diagrams that eliminate customized interfaces. All of these contribute to them being more reliable systems than their relay hard-wired counterparts.
Ease of Maintenance and Troubleshooting
Most system components are solid-state type. Problems with mechanical wear, short-circuiting, and unexpected accidents from wiring and operation mistakes are significantly reduced. Because most system components are solid-state and modularized, maintenance is essentially reduced to replacing plug-in components if needed.
PLC components normally come with fault detection circuits and LED indicators. They detect any malfunction of the components and give prompt identification of component failures. Modern PLC systems are loaded with diagnosis programs and online monitor systems to show the actual status of each control element. All of these facilitate the troubleshooting of the system when they go wrong.
Cost Saving
Generally speaking, when the number of relays used in the system is more than 10, the use of a PLC becomes cost-effective. Today there are many micro PLCs that cost less than two hundred dollars. It is cost-effective using these low cost PLCs to implement those small control systems.
1.5.3PLC Applications
Since its inception, the functionality of PLCs has gone beyond simple relay replacement. With their added advanced functions, PLCs have been widely used in almost every sector of industry. Typical applications include:
•Discrete logic controls
•Monitoring
•Continuous control
•Analog measurement and control
•Diagnostic information gathering
•Data logging
•Production reports generation
•Communication network
Table1.3 tabulates some applications in various industries.
Table 1.3: PLC applications by industry
| Industry | Applications |
|---|---|
| Chemical | Batch processing, blending, off shore drilling, material and product handling, pipeline control, etc. |
| Food and Beverage | Baking, mixing, blending, brewing, distilling, filling, material and product handling, sorting conveyor control, warehouse storage and retrieval, palletizing and wrapping, etc. |
| Glass and Film | Cullet weighting, finishing, forming, material handling, packaging, palletizing, etc. |
| Lumber, Pulp, and Paper | Bark burning, batch digesters, chip handling, coating, cutting, pulp batch blending, wrapping and stamping, etc. |
| Manufacturing | Conveyor systems, assembly machines, plastic injection molds, test machines, machine tool control, work cell control, etc. |
| Power | Burner control, coal handling, fuel control, sorting, process control, etc. |
| Snack Foods | Oven control, batching systems, material handling, cooker systems, extrusion and cutting systems, slurry mixing and distribution, etc. |
| Fiberglass | Furnace control, batching systems, material handling, forming systems, cutting and slitting systems, binder mixing and distribution, etc. |
| Material Conveying | Material handling, batching systems, variable speed drives and systems, motor control center systems, slurry mixing and distribution, valve sequencing control systems, and plugged line countermeasure controls, etc. |
| Extrusion Systems | Forming barrel control systems, variable speed drives and systems, cutting control systems, and batching systems, etc. |
| Pharmaceuticals | Autoclave control systems, batching operations, vial capper machines, validation reports, R&D operations, vial washing, vial labeling, etc. |
| Electronic Board Manufacturing | Material handling, material positioning, inventory control system, variable speed drive systems, product conveying systems, machine control systems, etc. |
| Steel Industry | Material positioning, inventory control system, product conveying systems, variable speed drive systems, sequencer control, pump control systems, valve control systems, temperature control, pressure control, etc. |
1.6 PLC Categories
PLCs are made in various sizes and configurations. They can be classified in six categories according to I/O count, memory size, processor type, function, and application. The various sizes are nano PLCs, micro PLCs, small PLCs, medium PLCs, large PLCs, and very large PLCs. Table 1.4 summarizes these sizes and their PLC categories.
1.6.1Rockwell Automation Allen-Bradley PLCs
There are more than 60 PLC manufacturers. Some top PLC makers include Rockwell Automation Allen-Bradley, Siemens, Mitsubishi, GE Fanuc, Omron, and Schneider Modicon. Some Rockwell Automation Allen-Bradley PLCs are shown in Table 1.5.
Table 1.4: PLC categories
Table 1.5: Rockwell Automation Allen-Bradley PLCs
| PLC Model | Features | |
|---|---|---|
| Pico PLC | Small, simple, and flexible. Ideal for relay replacement applications. Low cost. | |
| Micro 800 PLC | Offers a wide range of small controllers with 10 to 48 points. Designed for standalone machines. Optional plug-in modules. | |
| MicroLogix | Provides 24 or 28 built-in basic I/O. Expandable I/O capability. | |
| CompactLogix | Designed for small and mid-size applications. Machine-level control is its typical applications. I/O modules range from 4 to 32 points per module. | |
| SLC 500 | A modular PLC system at minimum consists of a processor module and I/O modules in a single chassis with a power supply. It can be configured with up to 3 local chassis, for a total of 30 local I/O or communication modules maximum. | |
| ControlLogix | Provides discrete, drives, motion, process, and safety control together with communication and state-of-the-art I/O in a small, cost-competitive package. The systems are modular. | |
| PLC 5 | PLC-5 processors are high-speed, single-slot processors used for control and information processing. They are designed for larger sequential and regulatory control applications with specialized I/O requirements. |
Table 1.6: Siemens PLCs
| Siemens PLC Model | Feature |
|---|---|
| Logic module | Is perfectly suited for small-scale automation projects and makes life much easier by replacing many time switches and relays, counters, and protective relays. |
| Simatic S7 | Simatic S7 controllers are modular controllers that can be expanded flexibly at any time via pluggable I/O, functional, and communications modules. |
| Embedded controller | Embedded bundles are ideal for control or any other applications that need to be implemented on a sturdy, open platform. They combine a variety of tasks such as open-loop control, operator control and monitoring, data processing, as well as safety and communications applications on a shared hardware platform. |
| PC-based controller | They are industrial PCs with typical applications ranging from human machine interfacing to data processing. |
1.6.2Siemens PLCs
Four groups of Siemens PLCs are shown in Table 1.6. They are Logic module, Simatic S7, Embedded controller, and PC-based controller.
1.6.3Mitsubishi PLCs
Three categories of Mitsubishi PLCs are shown in Table 1.7, including Alpha2 Sequence controller, FX compact controller, and L series controller.
1.6.4GE Fanuc PLCs
Four categories of GE-Fanuc PLCs are summarized in Table 1.8. They are Durus controller, VersaMax Micro controller, Series 90-30 controller, and PAC8000 controller.
1.6.5Omron PLCs
Table 1.9 summarizes four categories of Omron PLCs including Compact CP1 series, Modular CJ1 series, CJ2 series, and Rack CS1 series.
Table 1.7: Mitsubishi PLCs
| PLC Model | Features |
|---|---|
| Alpha2 Sequence controller | The Alpha2 is a controller designed to address simple control applications at the lower end of the industrial and commercial control markets. With simple analog processing integrated, a straightforward programming style, and a built-in display, the Alpha 2 is a highly affordable control solution. |
| FX series | They are expandable compact controllers. They are the ideal choice regardless of needing a simple control system requiring up to 34 I/O, or a more complex system with up to 384 I/Os. |
| L series | The L Series is Mitsubishi’s newest controller line, donning an innovative and easy-to-expand rack-free design. The single-CPU architecture includes built-in Ethernet and Mini-USB interfaces, a SD/SDHC memory card slot for program storage and data logging, and 24 I/O for positioning and high-speed counter functions. L Series ideal for both stand-alone machines as well as networked stations in larger applications. |
Table 1.8: GE Fanuc PLCs
| PLC Model | Features |
|---|---|
| Durus controller | Durus controllers integrate I/O, operator interface, and control in one small package. They are ideal for applications requiring less than 44 inputs and outputs such as vending machines, packaging lines, security, and assembly where it could result in cost reduction by replacing timers and counters. |
| VersaMax Micro | When you need amazing functionality in a compact package, VersaMax Micro PLCs are ideal for fast cycle times, a robust instruction set, and extensive memory that multiplies your programming options. |
| Series 90 – 30 | Series 90*–30 PLCs are a family of controllers, I/O systems, and specialty modules designed to meet the demand for versatile industrial solutions. They are ideal for applications such as high-speed packaging, material handling, complex motion control, water treatment, continuous emissions monitoring, mining, food processing, elevator control, injection molding, and many more. |
| PAC8000 Controllers | The PAC8000 Controllers consist of a Process Controller, Logic Controller, and HybridController, which combines the capabilities of both. These controllers provide a tight control loop response, generating a control output in response to input data within 100 ms. |
Table 1.9: Omron PLCs
| PLC Model | Feature |
|---|---|
| Compact CP1 series | Designed for compact machines, the CP1is the advanced high-speed all-in-one compact PLC. Four high-speed counters and four pulse outputs are ideal for multi-axis positioning control. The CP1H can be expanded with different I/O. |
| Modular CJ1 | The smallest members of the CJ1 family are fully upward-compatible to the CJ1G/H and CS1 series regarding instruction set, communication commands, and memory organization. Even small machines can be built in a consistently modular way, reducing the cost to enhance, expand or customize machines without a complete redesign of the control system. |
| CJ2 series | The CJ2H series is ideal for advanced machine automation needs, such as those required in image processing inspection of electrical components and high speed sorting on conveyors. It has special instructions that provide direct data access to high-speed analog I/O units and serial communication units. Position control units can be synchronized for coordinated control of up to 20 axes. |
| Rack CS1 | CS1 is Omron’s most extensive PLC family, with a maximum capacity of 5120 local digital I/Os. Up to 7 expansion racks can be connected to a single CPU rack, bringing the maximum number of I/O units to 80. Any combination from over 200 models of digital I/Os, analog I/Os, control units, and communication units can be mounted in any order. |
Review Questions
1. What is a PLC?
2. What functions does a PLC perform?
3. Explain the main problems with those hardwired relay-based systems.
4. Describe the initial specifications of PLCs proposed by GM’s Hydromatic Division.
5. List the five generations of PLC development and briefly describe each of their contributions.
6. What are the four major components of a PLC system? Briefly describe each.
7. What are the three parts of a processor unit?
8. Describe the functions of a CPU.
9. What are the two types of memory used in PLCs? Describe each type.
10. Explain the purpose of input modules?
11. Use a drawing to help describe the functions of an input module in a PLC project.
12. Use a drawing to help explain the functions of an output module in a PLC project.
13. What are the three types of PLC programming devices? Briefly describe each.
14. List the two types of interface wiring.
15. Use a drawing to help describe the input wiring.
16. Use a drawing to help describe the output wiring.
17. Explain a ladder diagram.
18. Describe the three steps of a PLC implementation cycle.
19. Use a drawing to help describe the PLC scanning process.
20. Compare PLCs and hard-wired relay control systems.
21. List at least five benefits of using PLCs.
22. List five typical applications of PLCs.
23. List the six PLC categories.
