A Programable logic conroller (PLC) refers to a coomputer designed specifically for use in the industry.The PLC controls specifically a different process which is programmed with the instructions of the operational requirements of that process.Differenit industries use sequenntial processes respeective to their nature of operations.In relation to the sequential process used by industries depnds on the relays,timers,steping drum as well as controls which are necessary in reprogramming due to the changes in the nature of production in the industries.Most of the times the system has to be redesgned and the old one is scrapped off.The PLC control system was introdced to to curb this problem relating to sequential processes.
Programing Logic Controller refers to a microprocessor that relies on the programming technique in in performance of the industrial processes. It operates in a similar manner like the ordinary controller with respect to the industrial process but the Programming Logic Controller use a software in its operation. The change of a PLC only entails the change of the software controlling the process and therefore there is no need to change the whole process.
The Programming Logic Controller takes the instructions of input activities in the form of computer software or the ladder diagram instructions. These input instructions are decoded in the CPU where the CPU send the signals to operate as well as controlling the devices of the system. The PLLC is often affected when the position of these devices are changed leading to a change of the controlled variable in the system. The ladder network of the PLC helps in performance of the requirements of each system.
A typical diagram of the Programing Logic Control unit above includes the Central processing unit(CPU),programming device as well as the input and output model. The Central Processing Unit refers to the heart of the Programming Logic Control Unit. The CPU is therefore based on the microprocessor which replaces the counters, central relays, sequences and timers. The processors tend to appear one once in the Programming Logic Control unit in form of a word processor or one-bit. One bit processors are used to adequately deal with the logic operations. The Programing Logic Units with the word processors are very useful in processing of numerical data as well as texts, gauging, calculations, recording and controlling as well as the simple processing of signals in form of binary codes. The Central Processing Unit reads and accept the input data from the sensing devices then executes the user program from the memory. The appropriate commands are then send to the appropriate output commands in controlling the devices. In the CPU, a direct power current (DC) source is necessary in production of low-level voltage used by the I/O modules and the processor. The power source can be mounted separately or may be housed on the CPU unit depending on the manufacturer of the CPU.
Input and Output Modules of PLC
The input-output modules are mainly used in creating an interface between the input devices (sensors, limited switch and selector switch) and the microprocessor which is essential in operations of the industrial process. The output devices are also interfaced with the microprocessor with the use of these input-output modules. The input information is transferred to the processor through the input module while the output information is also is taken to the processor with the help of output models.
The input/output module is therefore useful in provision of isolation between the processor and the input devices. This module also provides an isolation between the output devices and the processor.
The input devices including limited switches, push buttons and selector switches are used in are wired to the terminal on input modules. The output devices (motor starters, small motor, and solenoid valve and light indicators) are also hardwired to the output modules. The input modules accepts the signals from machine processors and devices where they are converted into the form that can be used by the controller and processor output. These signals are further converted into external signals used in controlling the machine or the process. The main task performed by the circuit and block diagram include sensing when signal is received at the input of the machine. The input voltage is then converted to correct the voltage level for the PLC. The PLC is separated from the fluctuation of the input signal current or voltage. The signal is finally send to the PLC.
PLC Output Module
The output module is used in interfacing between the processor and output load. The function of the output module is similar to that of input modules but in the reverse order.
The keyboard and monitor are mainly used for programming a Processor Logic Unit. The programmer can therefore communicate with the PLC processor with the help of programming devices. The programming unit passes information to the processor through parallel or serial data transmission link. The personal computer can also be used for the programming of PLC if it contains the required software.
The PLC may be based on microprocessor control system. The information signal is transferred from the sensor in analog form and transferring it into digital form which can be easily read and processed by the microprocessor. The key board allows the user to set point values and reading the current values of the process variables.
Kaftan, J. (2011). PLC Basic Course with SIMATIC S7. Würzburg: Vogel Buchverlag.
Kaftan, J. (2011). PLC basic course with SIMATIC S7: Structure and function of programmable logic controllers, programming with the SIMATIC S7. Würzburg: Vogel.