Programmable Logic Controller: Manual and Auto Control

• Programmable Logic Controller is the topic.

Manual and Automatic Control

• Control is either manual or automatic.

Manual Control

• Manual control occurs when a user performs an action for the system to function.

Automatic Control

• Automatic control happens when an action is performed automatically in response to a set of conditions.

Control System

• A control system directs, commands, and regulates itself or another system.

Control System Classification

• Discrete control means the value to be controlled can be either ON or OFF for example turning a light ON and OFF.
• Continuous control mean the value to be controlled varies smoothly.

Programmable Logic Controller (PLC)

• A programmable logic controller (PLC) is a special form of microprocessor-based controller.
• It uses a programmable memory for storing instructions.
• It implements logic, sequencing, timing, counting, and arithmetic functions.
• Programmable Logic Controllers are used for the control and operation of manufacturing process equipment and machinery.

PLC Advantages

• Programmable Logic Controllers offer several advantages over a conventional relay type of controls, including:
  • Increased Reliability
  • More Flexibility
  • Lower Cost
  • Communication Capability
  • Faster Response Time
  • Easier to Troubleshoot

PLC Hardware

• An open architecture design enables easy connection to devices and programs made by other manufacturers.
• Open architecture design uses off-the-shelf components that conform to standards.
• A system with a closed architecture is proprietary, making it more difficult to connect the systems to other systems.

PLC Hardware Components

• Processor unit or Central Processing Unit (CPU) includes a microprocessor.
• The CPU interprets input signals, carries out control actions according to the stored program, and communicates decisions as action signals to the outputs.
• Power Supply Unit converts mains a.c. voltage to the low d.c. voltage (5 V) necessary for the processor.
• Programming Device - enters the program in the memory of the processor.
• Relay Ladder Logic - the most popular programming language used by all major manufacturers of PLCs.
• Ladder Logic Programming language uses graphic symbols that show the intended outcome, rather than words.
• Memory Unit - stores the program used for control actions.
• It also stores data from the input for processing and data for the output for outputting.
• Input and Output Sections - are where the processor receives information from external devices and communicates information to external devices.
• Communications Interface- transmits and receives data on networks from or to other remote PLCs.

Internal Architecture

• Internal architecture - of a PLC consists of central processing unit(CPU), a system microprocessor, memory, and input/output circuitry.

Central Processing Unit

• An arithmetic and logic unit (ALU) - manipulates data, carrying out arithmetic operations of addition/subtraction and logic operations of AND, OR, NOT and EXCLUSIVE-OR
• Memory - called registers, is located within the microprocessor and stores the information.
• A control unit - is used to control the timing of operations.

Buses

• Buses - are the paths used for communication within PLC through binary form, as a group of bits (0 and 1).
• WORD - is used for a group of bytes constituting some information.
• The DATA BUS - carries processing data by the CPU.
• The address bus - carries the addresses of memory locations
• The control bus - carries signals used by the CPU for control and provides timing signals.
• The system bus - communicates between input/output ports and the input/output unit.

Input/Output Unit

• Input/output units provide an interface between the system and the outside.
• It allows for connections to be made for input devices like sensors, and to output devices like motors and solenoids through input/output channels.

PLC Systems

• There are two common types of mechanical design for PLC systems
  • Single box or brick.
  • Modular Type

PLC System (Single Box)

• Single block PLC are used for small programmable controllers, supplied as a compact package with power supply, processor, memory, and input/output units.

PLC System (Modular)

• Modular types consist of separate modules for the power supply, processor, etc., which are mounted on rails in a metal cabinet.
• Rack type can be used for all sizes of programmable controllers.

Programming PLCs

• Hand-held - programming devices have enough memory to retain programs between locations.
• Desktop consoles - will likely have a visual display unit with a full keyboard and screen display.
• Personal Computers - are commonly configured program development work stations, though some require special communication cards to interface with PLC.

PLC vs Computers

• The Programmable Logic Controller is basically a computer designed for use in machine control, operating in an industrial environment.
• Unlike an office computer, a PLC is equipped with interfaces and a control programming language
• A common abbreviation used in the industry for these devices is PC.
• PC can be confusing because PC is also the abbreviation for "personal computers".
• Some manufacturers refer to their programmable logic controller as PLC.
• Unlike computers, PLCs are designed to operate in an industrial environment and are not affected by the electrical noise.
• The hardware and software of PLCs are designed for easy use by plant electricians and technicians.
• PLCs use relay ladder logic or easy-to-learn languages.
• Unlike computers, most PLCs execute one program in an orderly and sequential fashion from start to finish.
• PLCs are designed for installation and maintenance by plant electricians who are not required to be highly skilled computer technicians.
• Troubleshooting is simplified with fault indicators and written information displayed on the programmer screens of PLCs

PLC Size and Application

• Generally, there are five classes of PLCs: nano, micro, small, medium, and large.
• The criteria used in categorizing PLCs include functionality, number of inputs and outputs, cost and physical size.
• The I/O count is the most important factor.
• Nano PLCs can handle up to 16 I/O points.
• Micro PLCs can handle up to 32 I/O points.
• A nano or micro PLC can be used on elevators or carwashes.
• Small size PLCs, like the Allen-Bradley SLC 500 family, can handle up to 960 I/O points.
• Medium PLCs can handle up to 1024 IOs.
• Large size PLC can handle several thousand I/O points, having almost unlimited applications and controlling individual production processes or entire plants.
• There are three major types of PLC applications: single-ended, multitask, and control management.

Single Ended, Multitask and Control

• Single-Ended PLC Application:
  • Uses one PLC to control one process as a stand-alone unit that is small and does not communicate with computers or PLCs.
• Multitask PLC Application:
  • Calls for a medium-size PLC involving one PLC that controls several processes whose I/O capacity is a key factor.
• Control Management PLC Application:
  • Involves one PLC controlling several others using a large PLC processor to communicate with other PLCs and a computer, supervising by downloading programs.