Working of a PLC

WORKING OF PLC 

Most of the Engineering projects use PLC as heart of the project for control. PLC stands for Programmable Logic Controller. This article gives a overview of PLC and its working.

 

Basis of a PLC function is continual scanning of a program, under scanning we mean running through all conditions within a guaranteed period. Scanning process has three basic steps: Step 1: Testing input status. First, a PLC checks each of the inputs with intention to see which one of them has status ON or OFF. In other words, it checks whether a sensor, or a switch etc. connected with an input is activated or not. Information that processor thus obtains through this step is stored in memory in order to be used in the following step. Step2: Program execution. Here a PLC executes a program, instructed by instruction. Based on a program and based on a status of that input ad obtained in the preceding step, an appropriate action is taken. This reaction can be defined as activation of a certain output, or results can be put off and stored in memory to be retrieved later in the following step. Step 3: Checkup and correction of output status. Finally, a PLC checks up output status and adjust as it is needed. Change is performed based on the input status that had been read during the first step, and based on the results of program execution in step 2. Following the execution of step 3 PLC returns to the beginning of this cycle and continually repeats these steps. Scanning time is defined by the time needed to perform these three steps, and sometimes it is an important program feature. 

LADDER DIAGRAM Programmable controllers are generally programmed in ladder diagram (or “relay diagram”) which is nothing but a symbolic representation of electric circuits. Symbols were selected that actually looked similar to schematic symbols of electric devices, and this has made it much easier for electricians to switch to programming PLC controllers. Electrician who has never seen a PLC can understand a ladder diagram. 

NORMALLY OPEN AND NORMALLY CLOSED CONTACTS Since we frequently meet with concepts “normally open” and “normally closed” in industrial environment, it is important to know them. Both terms apply to words such as contacts, Inputs, outputs etc., (all combinations have the same meaning whether we are talking about input, output, contact or something else). Principle is quite simple, normally open switch won’t conduct electricity until it is pressed sown, and normally closed switch will conduct electricity will conduct electricity until it is pressed. Good examples for both situations are the doorbell and a house alarm. If a normally closed switch is selected, bell will work continually until someone pushed the switch. By pushing a switch, contacts are open and the flow of electricity towards the bell is interrupted. Of course, system so designed would not in any case suit the owner of the house. A better choice would certainly be a normally open switch. This way bell wouldn’t work until someone pushed the switch button and thus informed of his or her presence at the entrance. Home alarm system is an example of an application of a normally closed witch. Let’s suppose that the alarm system is intended for surveillance of the front door to the house. One of the ways to “wire” the house would be to install a normally open switch from each door to the alarm itself (precisely as with a bell switch). Then, if the door was opened, this would close the switch, and an alarm would be activated. This system could work, but there would be some problems with this too. Let’s suppose that switch is not working, that a wire is somehow disconnected or a switch is broken etc., (there are many ways in which this system could be dysfunctional). The real trouble is that a home owner would not know that a system was out of order. A burglar could open the door, a switch would not work, and the alarm would not be activated. Obviously, this isn’t a good way to set up this system. System should be set up in such a way so the alarm is activated by a burglar, but also by its own dysfunction, of if any of the components stopped working. (A house owner would certainly want to know if a system was dysfunctional). Having these things in mind, it is far better to use a switch with normally closed contacts which will detect an unauthorized entrance (opened door interrupts the flow of electricity and this signal is used to activate a sound signal), or failure on the system such as disconnected wire. These considerations are even more important in industrial environment where a failure could cause injury at work. One such example where outputs with normally closed contacts are used is a safety wall with trimming machines. If the wall doors open, switch affects the output with normally closed contacts and interrupts a supply circuit. This stops the normally closed contacts and interrupts a supply circuit. This stops the machine and prevents an injury. Concepts normally open and normally closed can apply to sensors as well. Sensors are used to sense the presence of physical objects, measure some dimension of some amount. For instance, one type of sensors can used to detect presence of box on an industry transfer belt. Other types can be used to measure physical dimensions such as heat etc., still most sensors are of a switch type. Their output is in status ON or OFF depending on what the sensor “feels”. Let’s take for instance a sensor made feel metal when a metal object passes by the sensor. For this purpose, a sensor with a normally open or a normally closed contact at the output could be used. If it were necessary to inform a PLC each time an object passed by the sensor, a sensor with a normally open should be selected. Sensor output would set off only if a metal object were placed right before the sensor. A sensor would turn off after the object has passed. PLC could then calculate how many times a normally open contact was set off at the sensor output, and would thus know how may metal objects passed by the sensor. Concepts normally open and normally closed contact ought to be clarified and explained in detail in the example of a PLC controller input and output. The easiest way to explain them is in the example of a relay. 

PROGRAMMING THE PLC PLC controller can be reprogrammed through a computer (usual way), but also through manual programmers (consoles). This practically means that each PLC controller can be programmed through a computer if you have the software needed for programming. Today’s transmission computers are ideal for reprogramming a PLC in factory itself. This is of great importance to the industry. Once the system is corrected, it is also important to tread the right program in to a PLC again. It is also good to check from time to time whether program in a PLC has not changed. This helps to avoid hazardous situations in factory room (some automakers have established communication networks which regularly check programs in PLC to ensure execution only of good programs).Almost every program for programming a PLC possess various useful options such as: forced switching on and off the system inputs/ outputs (I/O lines), program follow up in real time as well as define failures and malfunctions. Programmer can add remarks, names of input or output devices and comments that can be useful when finding errors, or with system maintenance. Adding comments and remarks enables any technician to quote precisely part numbers if replacements would be needed. This would speed up a repair of any problems that can come up due to bad parts. The old way such that a person who developed a system had protection on the program, so that nobody aside from this person could understand how it was done. Correctly documented ladder diagram allows any technician understand thoroughly how the system functions.