PLC Structure: The PLC Structure mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data as shown in Fig. 21.22. A PLC can be considered as a box full of hundreds of thousands of separate relays, counters, timers and data storage locations. (These counters, timers, etc. really do not exists physically but […]
PLC Definition(Programmable Logic Controller or Programmable Controller): PLC Definition – Programmable Logic Controller (PLC) or commonly simply called a Programmable Controller, is a solid state, digital, industrial computer. It is a device that was invented to replace the necessary sequential relay circuits for machine control. The PLC basically operates by looking at its inputs and
Cascade Control with Digital Controllers: A simplified system to control the temperature of a process employing a gas burner using two Cascade Control digital controllers is shown in Fig. 21.21. Each controller has a number of digital inputs and outputs, and also a number of and outputs, the actual inputs and outputs are limited to
Digital Process Control: A digital process control functionally needs to operate in much the same way as a conventional analog controller. That is, it must provide facilities to raise or lower the set point and to provide proportional, 2-term or 3-term control. It must be capable of operating in one of many modes including, manual,
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Direct Digital Control: Developments in digital electronics have led to many industrial processes being computer controlled. The first system used was Direct Digital Control (DDC), in which a computer measured each variable in the process, these signals being used to maintain the required set points in the process. As digital electronics developed, minicomputers become more
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Choice of Electronic Signal Transmission: The range of air pressure used for Signal Transmission in pneumatic system is limited by the dynamic operating range of the pneumatic devices used. But the choice of electrical transmission is much wider. Since either ac or dc current may be selected. In addition, a choice may be made between
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Temperature Controls using an Analog Electronic Controller: A Temperature Controls scheme using an Analog Electronic Controller of Fig. 21.14 is as shown in Fig. 21.16. The temperature setting, derived from a slide wire, is compared with the signal from a thermocouple situated in the oven being heated. The difference between these two voltages is amplified in an
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