In autosystem, the PLC is commonly regarede as the heart of the control system. With a control application program ( store within the PLC mem.) in execution, the PLC consutaly monitors the state of the system through the field input devices feedback signal. It will the base on the program logic to determine the course of action to be carried out at the field output devices.
The PLC may be use to control a simple and repetitive task, or a few of then may be interconnected together with other host controllers or host controllers of host computers through a sort of communication network, in order to intergrate the control of a complete process.
Input devices
Interlligence of an automated system is greatly depending on the ability of a PLC to read in the sign from varius type of automatic sensing and manual input fiel devices
Keypad, push button and togge switches, witch form the basic ma-machine unterface, are types of manual in put device. On the other hand, for detection of workpice, monitoring of moving mechanism, checking on pressure and or liquid level and many others, the PLC will have to tap the signal from the specific automatic sensing devices like proximity switch, limit switch, level sensor, photo sensor and …Type of input signal to the PLC would be of ON/OFF logic or analogue. These input signals are interfaced to PLC through various types of PLC input module.
Output device
An automatic system is incomplete and the PLC system is virtually paralysed without means of interface to the field output devices Some of the most commonly controlled devices are motor, solenolds, relay indicator, buzzers and etc. Through activation of motors and solenoids the PLC can control from a simple pick and place system to a much complex servo positioning system. These type of output devices are the mechanism of an automated system and so its direct effect on the system performance.
However some output devices is only alarm and notting for the man, which is pilot lamp,buzzes. Like input signal interface, signal from output devices are interfaces to the PLC through the wide range of the PLC output module.
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