File Name: plc ladder logic questions and answers .zip
Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions.
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Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions. As an acronym, it meant Mod ular Di gital Con troller, and later became the name of a company division devoted to the design, manufacture, and sale of these special-purpose control computers.
Other engineering firms developed their own versions of this device, and it eventually came to be known in non-proprietary terms as a PLC , or P rogrammable L ogic C ontroller.
The purpose of a PLC was to directly replace electromechanical relays as logic elements, substituting instead a solid-state digital computer with a stored program, able to emulate the interconnection of many relays to perform certain logical tasks. In an effort to make PLCs easy to program, their programming language was designed to resemble ladder logic diagrams.
Thus, an industrial electrician or electrical engineer accustomed to reading ladder logic schematics would feel comfortable programming a PLC to perform the same control functions. PLCs are industrial computers, and as such their input and output signals are typically volts AC, just like the electromechanical control relays they were designed to replace.
Although some PLCs have the ability to input and output low-level DC voltage signals of the magnitude used in logic gate circuits, this is the exception and not the rule. The following illustration shows a simple PLC, as it might appear from a front view. In this way, the PLC is able to interface with real-world devices such as switches and solenoids.
The actual logic of the control system is established inside the PLC by means of a computer program. This program dictates which output gets energized under which input conditions.
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. Consider the following circuit and PLC program:. 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: push the button to make the lamp turn off , and release it to make it turn on. In the following illustration, we have the altered system shown in the state where the pushbutton is unactuated not being pressed :. 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.
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. 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:.
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. Module 7: 3 Real-world application example. Module 8: A brief touch on troubleshooting using PLC.
Input and Output sink, N. C wiring connection. Sensor Light-On, Dark-On. Don't have an AAC account? Create one now. Forgot your password? Click here. Latest Projects Education. Home Textbook Vol. Control System Behavior The true power and versatility of a PLC is revealed when we want to alter the behavior of a control system. The actual pumping station is located miles away from the personal computer display:.
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This is an era of technology and everyone relies on technology. Technology has changed mankind a lot. Technology has the immense effect on mankind. It has basically given birth to new mankind in which men just have to design certain tools or devices for their work and then command them to work accordingly. One such device, which has the immense effect and is based on logical technology in the field of different industries, is PLC. It is a type of computer. But it is not an ordinary computer; it is a digital computer which has been designed especially for industries which deal with the manufacture of assembly lines for video games, automobile devices etc.
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I am stealing ladder logic examples for inspiration and solutions to my PLC programs. By looking at examples of ladder logic programs, you will be able to find a solution to your problem very fast. You may even find a smarter solution in the ladder logic examples than your own solution. There are several reasons to use examples of PLC ladder logic.
Programmable Logic Controllers are industrially hardened micro computers, designed to replace electromechanical relays, used to control machines and processes. PLCs are highly reliable, easily programmable, Small and Inexpensive, PLCs can be designed with the communication capabilities so they can converse with the local or remote computer, They can sustain in robust environment less maintenance. What is the programmable language used in PLC?