Differences Between Ladder Logic Programs And Relay Logic Diagrams
- Differences Between Ladder Logic Programs And Relay Logic Diagrams For Beginners
- Hardwired Relay Logic
Table of Contents.Description:PLC Ladder Logic Programming examples- The PLC ladder logic programming examples that I am about to share with you guys will really help you in developing complex ladder logic diagrams. The PLC ladder logic programming is really easy as compared to the Arduino or any other microcontroller programming. I will start with the very basic ladder logic diagrams so that you can easily understand how to write complex ladder logic programs. These ladder logic diagrams are developed for the Fatek PLC FBS-14.If you are new to the PLC programming, then I highly recommend first read my previous two articles in which I have explained the very basics like for example. Best PLC for beginners.
How to write your first PLC program. How to run a PLC simulation. Basic PLC gates etcIn this article I am not going to explain how to run a simulation or how to add a contact and how to add comments because I have already explained these things in my previous articles, you can find links in the related projects section.
Before the advent of solid-state logic circuits, logical control systems were designed and built exclusively around electromechanical relays. Relays are far from obsolete in modern design, but have been replaced in many of their former roles as logic-level control devices, relegated most often to those applications demanding high current and/or high voltage switching.Systems and processes requiring “on/off” control abound in modern commerce and industry, but such control systems are rarely built from either electromechanical relays or discrete logic gates. Instead, digital computers fill the need, which may be programmed to do a variety of logical functions.The History of Programmable Logic ControllersIn the late 1960’s an American company named Bedford Associates released a computing device they called the MODICON. As an acronym, it meant Modular Digital Controller, 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 Programmable Logic Controller. 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.Ladder Logic and Programming PLCsA PLC has many “input” terminals, through which it interprets “high” and “low” logical states from sensors and switches. It also has many output terminals, through which it outputs “high” and “low” signals to power lights, solenoids, contactors, small motors, and other devices lending themselves to on/off control.
In an effort to make PLCs easy to program, their programming language was designed to resemble. 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 120 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.Signal connection and programming standards vary somewhat between different models of PLC, but they are similar enough to allow a “generic” introduction to PLC programming here.
Differences Between Ladder Logic Programs And Relay Logic Diagrams For Beginners
The following illustration shows a simple PLC, as it might appear from a front view. Two screw terminals provide connection to 120 volts AC for powering the PLC’s internal circuitry, labeled L1 and L2. Six screw terminals on the left-hand side provide connection to input devices, each terminal representing a different input “channel” with its own “X” label.
The lower-left screw terminal is a “Common” connection, which is generally connected to L2 (neutral) of the 120 VAC power source.Inside the PLC housing, connected between each input terminal and the Common terminal, is an opto-isolator device (Light-Emitting Diode) that provides an electrically isolated “high” logic signal to the computer’s circuitry (a photo-transistor interprets the LED’s light) when there is 120 VAC power applied between the respective input terminal and the Common terminal. An indicating LED on the front panel of the PLC gives visual indication of an “energized” input:Output signals are generated by the PLC’s computer circuitry activating a switching device (transistor, TRIAC, or even an electromechanical relay), connecting the “Source” terminal to any of the “Y-” labeled output terminals.
The “Source” terminal, correspondingly, is usually connected to the L1 side of the 120 VAC power source. As with each input, an indicating LED on the front panel of the PLC gives visual indication of an “energized” output: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. The program is entered and viewed via a personal computer connected to the PLC’s programming port.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. Following the program, which shows a normally-open X1 contact in series with a Y1 coil, no “power” will be sent to the Y1 coil. Thus, the PLC’s Y1 output remains de-energized, and the indicator lamp connected to it remains dark.If the pushbutton switch is pressed, however, power will be sent to the PLC’s X1 input.
Hardwired Relay Logic
Any and all X1 contacts appearing in the program will assume the actuated (non-normal) state, as though they were relay contacts actuated by the energizing of a relay coil named “X1”. In this case, energizing the X1 input will cause the normally-open X1 contact will “close,” sending “power” to the Y1 coil. When the Y1 coil of the program “energizes,” the real Y1 output will become energized, lighting up the lamp connected to it:It must be understood that the X1 contact, Y1 coil, connecting wires, and “power” appearing in the personal computer’s display are all virtual. 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.Equally important to understand is that the personal computer used to display and edit the PLC’s program is not necessary for the PLC’s continued operation. 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.
I include the personal computer display in these illustrations for your sake only, in aiding to understand the relationship between real-life conditions (switch closure and lamp status) and the program’s status (“power” through virtual contacts and virtual coils).Control System BehaviorThe 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.
The “hardware” solution would require that a normally-closed pushbutton switch be substituted for the normally-open switch currently in place. The “software” solution is much easier: just alter the program so that contact X1 is normally-closed rather than normally-open.In the following illustration, we have the altered system shown in the state where the pushbutton is unactuated ( not being pressed):In this next illustration, the switch is shown actuated (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. Using a PLC, however, we can program as many contacts as we wish for each “X” input without adding additional hardware, since each input and each output is nothing more than a single bit in the PLC’s digital memory (either 0 or 1), and can be recalled as many times as necessary.Furthermore, since each output in the PLC is nothing more than a bit in its memory as well, we can assign contacts in a PLC program “actuated” by an output (Y) status. Take for instance this next system, a motor start-stop control circuit:The pushbutton switch connected to input X1 serves as the “Start” switch, while the switch connected to input X2 serves as the “Stop.” Another contact in the program, named Y1, uses the output coil status as a seal-in contact, directly, so that the motor contactor will continue to be energized after the “Start” pushbutton switch is released. You can see the normally-closed contact X2 appear in a colored block, showing that it is in a closed (“electrically conducting”) state.If we were to press the “Start” button, input X1 would energize, thus “closing” the X1 contact in the program, sending “power” to the Y1 “coil,” energizing the Y1 output and applying 120 volt AC power to the real motor contactor coil.