Programmable Logic Controllers (PLC) An Overview (CH 1)
List six distinct advantages that PLCs offer over conventional relay-based control systems.
Increased reliability, flexibility, lower cost, communications capabilities, faster response time, easier to troubleshoot, easier to test field devices.
Describe how I/O modules connect to the processor in modular-type PLC configuration.
Modular I/O is connected to the processor through the backplane, or rack. When the modular I/O is slid into the rack, it makes an electrical connection with a series of contacts called the backplane, located at the rear of the rack.
What is the standard programming language and used with PLCs?
RLL (Relay Ladder Logic), which graphically represents rungs of contacts, coils, and special instruction blocks.
What are the two most common types of PLC programming devices?
The two most common types of PLC programming devices are handheld type or a personal computer running special application software.
Identify four tasks in addition to relay switching operations that PLCs are capable of performing.
timing, counting, calculating, comparing, and processing analog signals
What is a programmable logic controller (PLC)?
A PLC is an industrial grade computer that is capable of being programmed to perform control functions.
Explain the terms program and programming language as they apply to a PLC.
A program is a user-developed series of instructions that directs the PLC to execute actions. A programming language provides rules for combining the instructions so that they produce the desired actions.
The programmable controller operates in real time. What does this mean?
A real-time system gets its name because the outputs of the system controlled by the PLC depends on the input conditions.
Explain the differences between open and proprietary PLC architecture.
An open architecture design allows the system to be connected easily to devices and programs made by other manufacturers. A system with a closed architecture is one whose design is proprietary, making it more difficult to connect to other systems.
State two ways in which I/O is incorporated into the PLC.
Two ways in which I/O can be incorporated into a PLC are through Fixed and Modular I/O. Fixed I/O is usually associated with smaller PLCs, where the I/O is packaged with the processor. Modular I/O is separate from the processor and is connected to the processor through the backplane.
Explain the major function of each of the following major components of a PLC: a. Processor module (CPU) b. I/O modules c. Programming device d. Power supply module
a. A CPU is the "brain" of a PLC. The processor is used to implement logic and controlling communication amongst the modules. The CPU controls all PLC activity and is responsible for running the program. The CPU had repetitive scans, that begins with reading input statuses, then executes the logic, sets outputs, then runs internal diagnostic and communication tasks. b. The I/O system forms the interface by which field devices are connected to the controller. Two common types of I/O systems exist, Fixed I/O and Modular I/O. Fixed I/O is typical of small PLCs that come in one package, the processor and I/O are packaged together. Modular I/O is divided by compartments, which separate modules can be plugged. c. A programming device is used to interface with the PLC in order to develop and transfer logic programs, download or upload data, or supply diagnostic functions to troubleshoot PLC systems. d. The power supply supplies DC power through the backplane, to the processor and the other modules that plug into the rack.
Answer the following with reference to the process control relay ladder diagram of Figure 1-13 of this chapter: a. When do the pressure switch contacts close? b. When do the temperature switch contacts close? c. How are the pressure and temperature switches connected with respect to each other? d. Describe the two conditions under which the motor starter coil will become energized. e. What is the approximate value of the voltage drop across each of the following when their contacts are open? (1) Pressure switch (2) Temperature switch (3) Manual pushbutton
a. When the preset pressure value is reached. b. When the preset temperature value is reached. c. The pressure and temperature sensors are connected in series. d. The first condition is when both the pressure and temperature switches are active, the second condition is when the manual pushbutton is pressed. e. voltage drops across both the temperature and pressure switches are 0vac. The voltage across the manual pushbutton is 120vac.
