introduction to programmable logic controller
Published on Sep08, 2021 | Category: basicShare this Page:
PLC Is Programmable Control System Which Reduced Complex Hardwired Relay Logic. Programmable Logic Controller (PLC) Store Instructions To Control Industrial Machines, Systems And Processes. Controlling Process Is Depends On Inputs, Outputs Condition Of System.
Basic Components of PLC Systems
Any PLC Systems will always consist of a processor (CPU), an input output system, a Memory unit, a programming device, a communication system, and a power supply.
- the central processing unit (CPU)
- input and output (I/O)
- the power supply
- programming device
PLC Power supply
The power supply supplies DC power to other modules that plug into the rack. For large PLC systems, this power supply does not normally supply power to the field devices. With larger systems, power to field devices is provided by external alternating current (AC) or direct current (DC) supplies. For some small micro PLC systems, the power supply may be used to power field devices.
the central processing unit (CPU)
The central processing unit (CPU) performs basic arithmetic and logical instruction and controlled input and output operation which specified by instruction. PLC have one or more standard or custom microprocessors as well as other integrated circuits that perform the logic, control, and memory functions. Memory requires for storing the results of the logical operations performed by the microprocessor.
- The processor reads the inputs, executes logic as determined by the application program, performs calculations, and controls the outputs accordingly.
Memory of PLC
Memory located in the same housing as the CPU. Memory is used to store the control program for the PLC system. The information stored in memory determines how the input and output data will be processed. size of memory is depends on the manufacturer of the programmable controller. Memory either volatile or nonvolatile. Volatile memory will lose its programmed contents if all operating power is lost or removed. Random access memory (RAM) is volatile memory of CPU, Nonvolatile memory will retain its data and program even if there is a complete loss of operating power. EEPROM is a nonvolatile memory.
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Memory are classified as application or user memory, system memory.
- system memory : directs and performs operational activities such as executing the control program and coordinating input scans and output updates. The user cannot access or change this process system memory, which is programmed by the PLC manufacturer.
- application or user memory : user can access two areas of memory in the PLC system which known as Program files and Data files.
- Program files : store the control application program, subroutine files, and the error file.
- Data files : store data associated with the control program, such as input/output status bits, counter and timer preset and Accumulated values, and other stored constants or variables.
input output system of plc
The Input and Output system provides the physical connection between the process instrument, field instruments, sensor, transmitter etc. and the microprocessor. process equipment switches, thermocouples, push button, level sensor, RTD, temperature controller, pressure transmitter, and position, solenoid valves, motors contactor, measure the physical condition of process and gives feedback to plc by Input and output system, so CPU Performs logical operation. The input devices measure or sense the process value depends on feedback of input devices signal controller execute logical operation and gives the output.
- Input Types These input signals come from such devices as push buttons, hand switches, thermocouples, strain gauges, and the like. These signals are connected to input modules where they are filtered and Conditioned for use by the processor.
- output types The outputs from the programmable logic controller energize or DE energize control devices to regulate processes or machines. These output signals are control voltages from the output circuits, and they are generally not high-power signals. For example, an output module sends a control signal that energizes the coil in a motor starter. The energized coil closes the power contacts of the starter. These contacts then close to start the motor. The output modules are usually not directly connected to the power circuit but rather to devices such as the motor starter and heater contactors that apply high-power (greater than 10 amps) signals to the final control devices.
- Digital module
- analog module
- network module
PLC DIGITAL MODULE This type of interface connects field input devices of the ON/OFF nature such as selector switches, pushbuttons, and limit switches. Likewise, output control is limited to devices such as lights, relays, solenoids, and motor starters that require simple ON/OFF switching. The classification of discrete I/O covers bit oriented inputs and outputs. In this type of input or output, each bit represents a complete information element in itself and provides the status of some external contact or advises of the presence or absence of power in a process circuit. Switches, pushbutton, limit switches, proximity switches, lamp, contactor, relay are all examples of discrete sensing devices.
PLC analog MODULE analog devices represent physical quantities that can have an infi nite number of values. Analog input modules normally have multiple input channels that allow 4, 8, or 16 devices to be interface to the PLC. The two basic types of analog input modules are voltage sensing and current sensing. all type of analog input module have Analog-to-Digital Converter (ADC) and analog output module have Digital-to-Analog Converter (DAC). ADC convert analog input to digital signal, and DAC Converts digital signal to analog signal.
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types of Analog Input and output module
- Voltage (0 to 10 volt, 0 to 5 volt)
- Current (0 to 20 mA, 4 to 20 mA)
- RTD (mV)
- Thermocouple (mV)
- Strain gauge (mV)
PLC Network Input output
Network module or communication module are used to communication between PLC to PLC, PLC TO PLC's, PLC to field devices, Modbus was digital protocols developed for PLC communication , originally for the Modicon brand of PLC. Modbus was adopted by other PLC and industrial device manufacturers. profibus is another example.
Programming Devices
The programming device such as personal computer (PC) is used to plc programming, downloading and uploading program to plc, monitor of the PLC. Programming devices also a dedicated portable unit. every manufacturer have different software for making a program or communicating plc to personal computer. PLCs have software available so that a PC can be used as the programming device. This software allows users to create, edit, document, store, and troubleshoot ladder logic programs
- Step7 : develop by siemens for s7-300 and s7 - 400 plc.
- TIA: total integrated automation developed by Siemens for s7 - 1200, 1500, 300, 400 etc plc
- rslogix 500 : develop by allen bradley for slc 500, micrologix 1100,1500 etc plc.
- rslogix 500 : develop by allen bradleyfor 5000 plc.
- FPWIN PRO : developed by Panasonic for Panasonic plc
- Fuji PLC : developed by fuji electrical.
operation of plc
The most basic function programmable controllers perform is to examine the status of inputs and, in response, control some process or machine through outputs. The logical combination of inputs to produce an output or outputs is called control logic. This control plan is stored in memory using a programming device. Once input in memory, the control plan is periodically scanned by the processor usually a high-speed microprocessor in a predetermined sequence.
- SCAN TIME - The period required to examine the inputs and outputs, perform the control logic, and execute the outputs is called the scan time.
- input scan - the time required for the controller to scan and read all input data. Typically accomplished within ms.
- program scan - the time required for the processor to execute the instructions in the program. The program scan time varies depending on the instructions used and each instructions status during the scan time.
- output scan - the time required for the controller to scan and write all output data; typically accomplished within ms.
The processor controls the operating cycle or processor scan for the PLC. This operating cycle consists of a series of operations that are performed sequentially and repeatedly. During the input scan the PLC examines the external input devices to detect whether a signal is present or absent, that is, an ON or OFF state. The status of these inputs is temporarily stored in an input image table or memory file. During the program scan, the processor scans the instructions in the control program, uses the input status from the input image file, and determines if an output will or will not be energized. The resulting status of the outputs is written to the output image table or memory file. Based on the data in the output image table, the PLC energizes or DE energizes its associated output circuits, thereby controlling external devices. This operating cycle typically takes 1 to 25 milliseconds (thousandths of a second). The input and output scans are normally very short compared to the time required for the program scan.
Book references
- PROGRAMMABLE LOGIC CONTROLLERS Fourth Edition by Frank D. Petruzella.
- Measurement and Control Basics, 3rd Edition by Thomas A. Hughes.