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Introduction to PLC and its role in automation

Discuss about the Control Logic Error Monitoring and Prediction.

A programmable logic controller (PLC) plays a vital role in the field of microprocessor. It’s very simple and easy for the users to handle since it could consist of hardware and software components which could handle all the industrial components, and the work of the engineer is to program the plc using ladder logic which could do certain type of automation and control in the industrial equipments. The greatest advantage of using PLC is that they can be reprogrammed depending on the user who handles it. They have an astounding effect on the automation since these PLC components are highly flexible and reliable at their implementation stage. Since the functionalities of the microcontrollers has been increased as well as their reduced cost had increased their scope and made it to be used in the variety of fields. PLC had been used in the field of hydraulics to control fluid flow in the place of electromechanical relays. In recent days, programmable logic controllers are established into an erudite and largely flexible component in the control system that is able to perform complicated mathematical calculations as well as work in a faster speed. Some leading PLC[1] manufacturers are ABB, Allen Bradley, Honeywell, Siemens, Mitsubishi, Modicon, etc.

In the Electro Pneumatic system the output is said to be the movement of solenoid. This could be done either by using relays or with the help of PLC’s[2]. The work of the relay is to get the input signal from the various sensors[3] and perform the function of normally opened or normally closed operation. With the movement the solenoid is operated. This could also be done with the help of programmable logic controllers. The required logic diagram is made with the certain timers and sequential operation. At last the signal is provided to the solenoid to the final control element to carry out the operation at various cylinders.

Control systems acts as the constituent division in this industrial world. The necessity of the control system is to automate the process without the need of the human intervention. This could avoid some human error during the process. This automation process could be greatly supported with the PLC’s[4] which could be used in the process of designing, testing and optimizing the process. The basic advantage of automating a system: increase in the production, maximize the accuracy, promotes safety to the environment, more flexible etc., There are four main features which make the automation process to be in the lead of the control system engineering:

  • Transfer of information from the analog system into the digital format which could be supported by the computer based systems.
  • The control methods for the process of automation are carried out with the help of the language based variable with the exception of the heavy and tedious mathematical calculations.
  • Amendment of communication is done by the human intervention with the less effort[5] for processing the information.
  • Utilization of multifaceted automation processed system in the recent days had increased the productivity. Especially PLC is used to operate the water tanks[6].

Electro-pneumatic controllers using PLC

There are three main steps involved in the electro-pneumatic controllers: 1) Input devices, which could consists of various sensors and the signals obtained from them is transferred to the relays and contactors. 2) Signal processing devices, that consists of set of relay switches or PLC’s and finally 3) Output devices, that activates the solenoid to make a mechanical movement and these output devices can be an alarm or some indicators. The electrical devices used in the controllers could be push buttons, limit switches, timers, relays, solenoids, temperature and pressure switches. A sensor such as proximity sensors which is used in the indication of the certain thing and various electrical counters is also used. This controller combines the electrical and pneumatic technology which is largely used in much application. Either AC or DC source signal is applied here. Compressed air is used in this process as a medium of working. The operating voltage is around 12-220 Volts.

Programming a PLC is not a complicated task when the sufficient programming technique is adopted. Many concepts can be used to enter a program in PLC[7]: 1) Draw a ladder logic diagram. 2) Functional blocks 3) Various Boolean expressions based on the low level 4) High level programming language. But the most commonly used method is the ladder logic diagram which is an easy and efficient approach. This ladder logic could consist of programming logic in a way identical to the switching circuit. With the help of certain conventions the ladder logic is converted into PLC ladder diagram. With the help of the cathode ray tube and the keyboard we can integrate certain components into a logical ladder program. The input switches, relays are indicated in the form of a contact switches and the solenoids, relays, counters and timers are denoted as coils in the PLC’s.  Each and every step of a ladder program is denoted as rung. The program[8] has to place the necessary contacts and coils depending on the process. This could be verified rung by rung and the result could be displayed in the CRT screen. The PLC ladder logic has two vertical lines which are also termed as rungs which are supplied by a positive voltage in the left side and zero voltage at the right side. The horizontal lines (rungs) are in the middle of these vertical rungs which are the process of the automation and the process could be carried out depending on the horizontal rung positions. Between these two sides are the horizontal rungs for the assumed power flow. Various operations are carried out in a PLC ladder diagram and they are Arithmetic operations, Timer operations, PLC[9] Bit logic operations, Comparison operation and timer operation.

Advantages of using PLC in Control Systems Engineering

NO Contact of PLC: The representation of NO contact is to scan the input signal ON (1) in the specific bit address. When the signal is ON then the switch tends to close and performs the operation. Shortly we could say that NO switch is opened normally and when the power flows it will act as a closed switch.

NC Contact of PLC: This is exactly the opposite of the NO switch, which will scan the input signal OFF (0) in the specific bit address. When there is no signal then the switch tends to remain in the closed position which gets opened when the signal is turned ON.


A push button plays a major role in starting and stopping an operation. This could be indicated by an open and closed switch in a PLC[10] diagram. There is also manually operated push button which could be used for some emergency circumstances. The functionality of the push button is to move the position of the actuator into housing. This is supported by the spring mechanism which could open or close the contact. There are two types of push button namely momentary push button and Maintained contact push button.  Momentary push buttons[11] will come to its actual position once the button is manually released and the maintained push button which is mechanically latched has a certain latching mechanism to hold its position. The push buttons can be categorized as Normally opened (NO), Normally closed (NC) and Change over (CO) type. At the position of NO the switch is opened and during the actuator action[12] the contacts are closed which could permit the flow of air. At NC position the air is forbidden in this position and during the actuator position the contacts are closed. Below figure1,2,3 represents the position of the push buttons.

Limit switches are similar to the push button and the contradictory performance is that the push button is manually operated and the limit switches are said to be mechanically actuated. The switch performs with the position of the fluid which could be a piston rod or a motor shaft. The limit switch provides an electric signal that provides an applicable system response. The limit switch is classified into two types based on the actuation of the contacts: 1) Lever Actuated[13] Contacts, in which the contacts operate at a very slow rate and 2) spring loaded contacts that make the contacts to perform expeditiously.

Steps involved in electro-pneumatic controllers

Pneumatic-electric signal conversion is done with the help of the pressure switch. The work of the pressure switch is to detect the transformation of the pressure change. With this information the electrical switch will be opened and closed when a prearranged pressure point is attained. Diaphragm or bellows will act as a pressure sensor. The change in the pressure is detected by the diaphragm which id in the form a plate that contracts and expands. In this similar way, the bellows also react to Pressure change. The pressure that comes through the inlet is noticed and when this pressure reaches a limit then the diaphragm or the bellows will expand which will make a spring loaded plunger to break/make the contact.

In the Electro-pneumatic[14] controllers, the control valve which is electrically operated to actuate is the part which forms as an interaction layer. The important activity of the directional control valves is to regulate the supply of air (ie, it should either switch to ON or OFF position), compression and rarefaction of the cylinder drives. This switching operation could be made possible with the help of the solenoid. The mode of operation[15] is of two types:  1) Spring valves which return to its position until the power supply is provided to the solenoid. 2) Double solenoid valves cling to the finally operated state although when the solenoid is not supplied with the voltage.  During the first step, every solenoid will not be energized and hence the directional control valve does not operate which make them inactive. There is no beginning position for the double valve since it will not return the spring to its position. The operating voltages of every solenoid are 12 V DC, 12V AC, 12 V 50/60 Hz, 24V 50/60 Hz, 110/120V 50/60 Hz, 220/230V 50/60 Hz. The numerous ways in which the solenoid operates are of the following: 1) 3/2 Way single solenoid valve, spring return. 2) 5/2 Way single solenoid valve, spring return. 3) 5/2 Way single double solenoid valve.


This is a very simple device which could withstand any harsh environment and these relays are said to be electromagnetically operated. These relay switches[16] acts as a signal processing device. The relays are designed in such a manner to withstand the heavy power surges which could cause damage to the circuits. The system consists of the coil core through which the voltage is applied. This coil converts the electrical power into the electro-magnetic energy which will attract the armature towards the winding. The armature will actuate[17] the relay making it to be in the open or closed position. Depending upon this operation the system provides the output. There is a spring attached in order to return the armature to the initial position. There is an interlocking capacity supported by the relays. This interlocking[18] capacity will avoid the instantaneous switching of relays to ON and OFF. The relays are indicated by K1, K2, and K3 etc. The relay contact with their operation is indicated in the figure 3 given below.

Programming a PLC - methods and techniques

Timers play a major role in the PLC[19]. In the pneumatic process when one side of the cylinder expands then the other side should contract and this process is done successfully with the help of timers which could provide certain delay in the time of expansion and compression. Every control task which is automated needs a time to perform perfectly. These time delays find their own memory spaces in the system. The representation of the timer circuit purely depends on the manufacturer. The timer implemented in the PLC’s is in the form of the software module and it could provide the digital way of representing the timing. The typical PLC’s[20] could have timers with the bit address of 64, 128, 256, 512 and many more. The timers in the ladder program could be indicated as T1, T2 etc. To explicitly reset timer, the logic of 1 should be applied to the reset port. The timers are of two types namely on delay timer and off delay timer.

When the signal is received in the start input signal the timer will be in the ON state. The output signal changes from 1 to 0, till the preset timing is attained (there will be a delay in the timing operation). When the signal reaches a preset timing, then the output will be changed from 0 to 1. The function of On-delay timer is described below.

This is exactly the opposite of the on delay timer. When the start input is provided with some signal then the timer starts to operate and gives the output 1. When the result logic changes from 1 to 0 then the timer will not stop at a sudden. There is a delay in their off time till a preset time. When this time is reached, the output will change to 0. The function of off-delay timer is described in the diagram given below.      

The count of the events and the particles kept to the process is indicated with the help of the counters. The controllers should be operated with the help of the counters. For instance, let’s see the bottle filling mechanism. The counter counts the number of bottles which is to be filling with the particular solution. When the conveyor belt moves the counter counts the bottles kept in the conveyor belt. This could be used in the place of a sorting device. There are two types of counters: 1) Up counter-In this counter the accumulator value will be increased to 1 how long the input is set to true. 2) Down counter- This works by decrementing the accumulator value until a preset value is reached. The circuit diagram of up and down counter is shown below.

Working of NO and NC contacts in PLC

The transitional values are placed in the memory elements. They are represented as flags. The operation is performed by set and reset coil in the ladder diagram[21]. The diagram below shows the set and the reset coil. Let us denote the output as D. When the switch A gets closed then the latch sets itself to logic 1 and the output will be 1. This output remains until the switch B performs its operation. When the switch is opened then also latching operation is continued. When the switch B gets opened then unlatching process takes place which will set the output D as 0.


Relays are said to be an electro-magnetic switch and here we’ll get into a small example of wiring a relay circuit into a ladder diagram. This is very necessary since the PLC[22] does not understand code. Many PLC’s convert the ladder diagram into coding. The following are the steps to be carried out.

Step 1: The PLC doesn’t care about the external equipments and devices. These devices should be indicated by ladder symbol. For instance, the input devices are not cared by the PLC. Either it analyze whether the given input is ON or OFF and the process it does is considered.

Step 2: The input voltage is very necessary. The AC supply is not considered. Rather the voltage for the process to get in operation is considered with is indicated in the form of two vertical bars (rungs) for the positive voltage and the ground.

Step 3: This could be indicated in the form of a circular symbol. This could be a bell or an alarm or a motor.

Step 4: We have to indicate whether the PLC is loaded (which means addressing a device). Then the process is done rung by rung in a horizontal manner. These could be seen in the modicon equipments. The program does its operation and we can set the switch to automatic or manually operated depending on the user who handles the equipment. This is purely based on the engineer who handles it. He should have the entire knowledge regarding the PLC.

Step 5: Other than this we have temperature switch which could sense the temperature. These switches can be wired either normally opened or normally closed depending on the temperature variations. Other than these, sensor plays a major role in the field of PLC[23].

Conclusion:

The pneumatic controller with the implementation of the PLC[24] is quite easy since it reduces the size, very easy to maintain, consumes very less voltage, less cost and highly accurate, easily programmable etc. Although we have some difficulties in finding the error and wiring becomes a challenging task, this is very rugged to certain temperature and vibrations.

Push buttons and limit switches in PLC

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