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- programmable logic controller (PLC)
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Each program file is created using one of the other types of programming languages. The SFC approach coordinates large, complicated programming tasks into smaller, more manageable tasks. There are many PLC systems on the market today. Other than cost, you must consider the following when deciding which one will best suit the needs of your application.
The following table shows a list of commonly used Acronyms that you see when researching or using your PLC. A small number of U. Company About Us History Partners. What is a PLC?
These controllers eliminated the need for rewiring and adding additional hardware for each new configuration of logic. The new system drastically increased the functionality of the controls while reducing the cabinet space that housed the logic. Execute the Control Instructions contained in the User's Programs. Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.
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Program Scan Executes the user created program logic 3. Housekeeping This step includes communications with programming terminals, internal diagnostics, etc These steps are continually processed in a loop. The company's products are listed below for your reference: This program dictates which output gets energized under which input conditions.
Programmable Logic Controllers (PLC) | Ladder Logic | Electronics Textbook
Although the program itself appears to be a ladder logic diagram, with switch and relay symbols, there are no actual switch contacts or relay coils operating inside the PLC to create the logical relationships between input and output. These are imaginary contacts and coils, if you will. When the pushbutton switch is unactuated unpressed , no power is sent to the X1 input of the PLC.
They do not exist as real electrical components. They exist as commands in a computer program—a piece of software only—that just happens to resemble a real relay schematic diagram. Once a program has been loaded to the PLC from the personal computer, the personal computer may be unplugged from the PLC, and the PLC will continue to follow the programmed commands. The true power and versatility of a PLC is revealed when we want to alter the behavior of a control system.
Since the PLC is a programmable device, we can alter its behavior by changing the commands we give it, without having to reconfigure the electrical components connected to it. For example, suppose we wanted to make this switch-and-lamp circuit function in an inverted fashion: In the following illustration, we have the altered system shown in the state where the pushbutton is unactuated not being pressed:.
Programmable logic controller
One of the advantages of implementing logical control in software rather than in hardware is that input signals can be re-used as many times in the program as is necessary. For example, take the following circuit and program, designed to energize the lamp if at least two of the three pushbutton switches are simultaneously actuated:. To build an equivalent circuit using electromechanical relays, three relays with two normally-open contacts each would have to be used, to provide two contacts per input switch. Take for instance this next system, a motor start-stop control circuit:.
An important point to make here is that fail-safe design is just as important in PLC-controlled systems as it is in electromechanical relay-controlled systems. One should always consider the effects of failed open wiring on the device or devices being controlled. In this motor control circuit example, we have a problem: So, we see there is no operational difference between this new design and the previous design.
The result, then, for a wiring failure on the X2 input is that the motor will immediately shut off. In this circuit, the lamp will remain lit so long as any of the pushbuttons remain unactuated unpressed. To make the lamp turn off, we will have to actuate press all three switches, like this:.
This section on programmable logic controllers illustrates just a small sample of their capabilities.
programmable logic controller (PLC)
As computers, PLCs can perform timing functions for the equivalent of time-delay relays , drum sequencing, and other advanced functions with far greater accuracy and reliability than what is possible using electromechanical logic devices. Most PLCs have the capacity for far more than six inputs and six outputs. Fit into a control cabinet, a PLC takes up little room, especially considering the equivalent space that would be needed by electromechanical relays to perform the same functions:.
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One advantage of PLCs that simply cannot be duplicated by electromechanical relays is remote monitoring and control via digital computer networks. Because a PLC is nothing more than a special-purpose digital computer, it has the ability to communicate with other computers rather easily. The actual pumping station is located miles away from the personal computer display:.