Relay Control System PDF

TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL RELAYS AND PROGRAMMABLE LOGIC CONTROLLER (PLC) LEARNING OBJECTIVES After completing this lesson, you will be able to: ❑ Define PLC and relays and its function ❑ Explain the...

TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL RELAYS AND PROGRAMMABLE LOGIC CONTROLLER (PLC) LEARNING OBJECTIVES After completing this lesson, you will be able to: ❑ Define PLC and relays and its function ❑ Explain the difference between hard wired control and PLC control ❑ Explain the different section of PLC ❑ List the advantages of PLC over electromagnetic relays ❑ Explain the functions of major components of PLC ❑ Explain various programming approaches used in PLC ❑ Describe the functions of memory functions, timers and counters ❑ Convert the logic functions into ladder diagram ❑ Design PLC circuits for single and multi actuator INTRODUCTION The term “discrete” refers to individual or distinct elements. In engineering, a “discrete variable” or measurement denotes a true- or-false condition. Therefore, a discrete control system is designed to operate on Boolean signals—specifically, "on" or "off" signals—supplied by discrete sensors, such as process switches. A fundamental aspect of discrete control, commonly covered in introductory electronics courses, involves the use of circuits known as logic gates. These circuits take one or more Boolean inputs and produce a Boolean output based on predefined rules, such as those governing "AND" or "OR" operations. This framework allows for the implementation of binary decision-making in various control applications. Industrial control systems rarely use logic gates directly for discrete control systems, although the fundamental concepts of “AND,” “OR,” and other gate types are universally applied. Instead, control functions are typically implemented using electromechanical relays and/or programmable digital devices, such as Programmable Logic Controllers (PLCs). 57 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL An “AND” function is equivalent to series-connected normally open contacts in a relay control circuit, as the lamp will energize only if both switch A and switch B are actuated. An “OR” function is equivalent to parallel-connected normally open contacts in a relay control circuit, as the lamp will energize if either switch A or switch B is actuated. The “NOT” function is equivalent to a single normally closed contact in a relay control circuit, as the lamp will energize only if the switch is not actuated. CONTROL RELAYS An electromechanical relay is an electrical switch that is actuated by an electromagnet coil. As switching devices, relays exhibit a straightforward “ON” and “OFF” behavior, with no intermediate states. They are highly valuable in various applications, as they enable a single discrete electrical signal to control much higher levels of electrical power, as well as multiple power or control signals that may be isolated from one another. For example, a relay can be activated by a low-voltage, low-current signal that passes through a delicate switch, such as a limit switch, proximity switch, or optical sensor. Once activated, the switching contacts of the relay can control a much higher-voltage, higher- current circuit, or even multiple circuits, depending on the number of switching contacts available. This functionality makes relays essential for safely interfacing low-power control systems with high-power electrical loads. 58 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Types of Relays based on Poles and Thow The electronic schematic symbol for a simple single-pole, single-throw (SPST) relay. A coil of wire wrapped around a laminated ferrous core provides the magnetic field necessary to actuate the switch mechanism. This electromagnet coil’s actuating influence on the relay’s contact(s) is represented by the dashed line. This particular relay is equipped with normally open (NO) switch contacts, which means the switch will be in the open (off) state when the relay coil is de-energized. A single-pole, single-throw relay with a normally-closed (NC) switch contact. In the electrical control world, the labels “Form-A” and “Form-B” are synonymous with “normally open” and “normally closed” contacts, respectively. Thus, we could have labeled the SPST relay contacts as “Form-A” and “Form-B” respectively. An extension of this theme is the single-pole, double-throw (SPDT) relay contact, otherwise known as a “Form- C” contact. This design of switch provides both a normally-open and normally-closed contact set in one unit, actuated by the electromagnet coil. “Form D” relay is also an SPDT relay and has the same principle as Form C relay but it is “make-before-break” contact relay. A double pole, single throw (DPST). The double pole means it can control two completely isolated individual circuits. The single throw means that each pole has one position in which it can conduct. A further extension on this theme is the double-pole, double-throw (DPDT) relay contact. This design of switch provides two sets of Form-C contacts in one unit, simultaneously actuated by the electromagnet coil. DPDT relays are some of the most common found in industry, due to their versatility. Each Form-C contact set offers a choice of either normally-open or normally-closed contacts, and the two sets (two “poles”) are electrically isolated from each other so they may be used in different circuits. A common package for industrial relays is known as the “ice cube relay,” named for its clear plastic case that allows for easy inspection of the internal components. These relays plug into multi-pin base sockets, facilitating straightforward removal and replacement in the event of failure. A DPDT (double pole, double throw) ice cube relay is depicted in the following photographs: one showing the relay ready to be plugged into its base (left), and another with the plastic cover removed, exposing both sets of Form-C contacts (right). These relays connect to the socket using eight pins: three pins for each of the two Form-C contact sets, and two additional pins for the coil connections. Due to the total pin count of eight, this style of relay base is often referred to as an octal base. Industrial control relays typically feature connection diagrams printed on their outer shells, indicating which pins correspond to the internal elements of the relay. The style of these diagrams may vary, even among relays that serve identical functions. For instance, the diagrams shown here illustrate three different brands of DPDT relays, each with its unique representation of the pin connections. This variation highlights the importance of consulting the specific diagram for accurate installation and troubleshooting. 59 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Types of Relays based on Operation Principles These following types of relays are classified based on their different operation principles: ❑ Electromechanical Relay (EMR) This type of relay, known as an electromagnetic relay (EMR), consists of an electromagnetic coil and a mechanically movable contact. When the coil is energized, it produces a magnetic field that attracts the armature (movable contact). When the coil is de-energized, the coil loses its magnetic field, and a spring retracts the armature to its normal position. This relay can be designed for either AC or DC sources, depending on the application. The construction of AC and DC EMR relays differs slightly in their coil design. The DC coil includes a freewheeling diode to protect against back EMF when the coil is de- energized. While the polarity of the source in an EMR relay does not matter, allowing it to energize the coil regardless of the direction, the presence of a back EMF diode necessitates consideration of polarity. The main disadvantage of the EMR relay is that its contacts can produce an arc during switching, which leads to increased resistance over time and reduces the relay's lifespan. ❑ Solid State Relay (SSR) This relay composed of semiconductors rather than mechanical parts, and it functions by isolating low-voltage circuits from high-voltage circuits using an optocoupler. When a control input is applied to the SSR, an LED lights up, producing infrared light. This light is detected by a photosensitive semiconductor device, which converts the light signal into an electrical signal to switch the circuit. SSR relays operate at relatively high speeds and have very low power consumption compared to Electromechanical Relays (EMRs). They also have a longer lifespan because there are no physical contacts that can burn out. However, the main disadvantage of SSRs is the nominal voltage drop across the semiconductor, which results in power being wasted in the form of heat. ❑ Reed Relay This relay consists of a reed switch and an electromagnetic coil, equipped with a diode for back EMF protection. A reed switch is composed of two metal blades made of ferromagnetic material, hermetically sealed in a glass tube that also supports the metal blades. The glass tube is filled with inert gas. When the coil is energized, the ferromagnetic blades attract each other, forming a closed circuit. Because there is no moving armature, there is no issue of contact wear. Additionally, the inert gas within the glass tube helps prolong the relay's lifespan. ❑ Electrothermal Relay (Thermal Relay) This relay consists of a bimetallic strip made from two metals with different thermal expansion coefficients. When current flows through the conductor, it generates heat, causing the temperature of the bimetallic strip to rise and expand. The metal with the higher thermal expansion coefficient expands more than the other metal, resulting in the strip bending and closing the contacts, which typically activates the trip circuitry. Thermal relays are commonly used for electric motor protection. ❑ Polarized and Non-polarized Relay The polarized relay utilizes a permanent magnet in conjunction with an electromagnet. The permanent magnet provides a fixed position for the armature, while the electromagnetic coil alters the position of the armature around a fixed pivot. The position of the armature depends on the polarity of the control input. The non-polarized relay does not use permanent magnets, allowing its coil to be energized in either direction without affecting its operation. However, some relays equipped with back EMF diodes do have polarity, as the diode will bypass the coil if the connection is reversed. Relay Circuits Electromechanical relays can be interconnected to perform logic and control functions, effectively acting as logic elements similar to digital gates (e.g., AND, OR). A widely used form of schematic diagram illustrating the interconnection of relays for these purposes is known as a ladder diagram. In a ladder diagram, the two poles of the power source are represented as the vertical rails of a ladder, while the horizontal “rungs” display the switch contacts, relay contacts, relay coils, and final control elements (such as lamps, solenoid coils, and motors) situated between the power rails. Ladder diagrams differ from standard schematic diagrams commonly used by electronics technicians primarily in the strict orientation of the wiring: vertical power “rails” and horizontal control “rungs.” Additionally, the symbols used in ladder diagrams vary slightly from those in conventional electronics notation; for instance, relay coils are depicted as circles, and relay contacts are represented in a manner resembling capacitors. 60 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Unlike schematic diagrams, where the association between relay coils and relay contacts is represented by dashed lines, ladder diagrams associate coils and contacts by labels. In some cases, relay contacts are labeled identically to their corresponding coil (e.g., a coil labeled CR5 will have all contacts for that relay also labeled CR5). In other instances, suffix numbers are used to differentiate individual contacts within each relay (e.g., a coil labeled CR5 may have its three contacts labeled CR5-1, CR5-2, and CR5-3). Another notable convention in relay circuits and their ladder diagrams is that each wire in the circuit is labeled with a number corresponding to common connection points. This means that wires connected together share the same number, designating a condition of electrical commonality (all points bearing the same number are equipotential). Wire numbers change only when the connection passes through a switch or another device capable of dropping voltage. Example of Ladder Diagram To illustrate this concept, let us examine a relay control circuit in which a pressure switch activates an alarm light. This image shows a simple electrical control circuit diagram, typically used in industrial systems. Here’s a breakdown of the components: L1 and L2: These represent the electrical power lines that supply voltage to the circuit. L1 is the live or positive side, and L2 is the neutral or return path. Pressure Switch (Trip = 50 PSI): This is a normally open switch that closes when the pressure reaches or exceeds 50 PSI. The pressure switch will trip (close) when the pressure threshold is met, allowing current to flow through the circuit. CR1 (Control Relay 1): CR1 represents a relay coil that will energize when the pressure switch is closed, allowing the circuit to operate other components. When the pressure reaches 50 PSI, the pressure switch closes, energizing this relay. CR1-1 (Normally Open Contact): This is a normally open contact associated with the relay CR1. When CR1 is energized, this contact closes, allowing current to flow to the next part of the circuit. Alarm Lamp: When CR1-1 closes, the alarm lamp turns on, indicating that the pressure has reached the threshold of 50 PSI. How it works: Initially, the pressure switch is open, and CR1 is not energized. No current flows through the circuit. When the pressure reaches 50 PSI, the pressure switch closes, energizing CR1. The CR1 relay closing causes the CR1-1 contact to close, completing the circuit to the alarm lamp. The alarm lamp turns on, indicating that the pressure has exceeded 50 PSI. This is a typical alarm or control system in pressure monitoring applications, often used for safety or process monitoring. 61 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Electriomechanical Relays as Interposing Relays In addition to directly performing logic functions, electromechanical relays can also serve as interposing devices between mismatched sensors, controllers, and control devices. When devices have incompatible electrical characteristics—such as low- power switches and high-power loads—relays act as a bridge, allowing the safe and effective control of high-power equipment using low-power input signals. A simple example of this is shown in the following circuit diagram, where a delicate toggle switch, which cannot handle high current, is used to control a bank of high-power lights on an off-road vehicle. In this setup: Toggle Switch: A small, low-current toggle switch is used by the operator to control the lighting system. Since this switch is not rated to directly control the high-power lights, it cannot be connected to them directly. Relay Coil: When the toggle switch is closed, it energizes the relay coil. The coil draws only a small current, making it safe to be controlled by the low-power toggle switch. Relay Contacts: Upon energizing the relay, the relay contacts close. These contacts are designed to handle the higher current required by the lights. High-Power Lights: The lights are powered by a separate high-current circuit. When the relay contacts close, current flows through the lights, turning them on. How It Works: When the toggle switch is flipped, it energizes the relay coil with a low current, suitable for the switch’s rating. The relay coil activates, closing the high-power contacts in a separate circuit. This allows high current to flow through the lights, turning them on, without subjecting the delicate toggle switch to dangerous levels of current. This example demonstrates how a relay can act as an intermediary, ensuring that mismatched components (in this case, a low-power switch and high-power lights) work together safely and effectively. ❖ In industrial automation, electromechanical relays are often used as interposing devices between mismatched components, ensuring compatibility between different electrical systems. A common example involves integrating a DC output sensor with a Programmable Logic Controller (PLC) input channel that operates on 120 volts AC. Without an interposing device, the DC proximity switch would be unable to directly interface with the AC-rated PLC input, as their electrical characteristics are incompatible. Review of fundamental principles ❑ Amplification: The control of a relatively large signal by a relatively small signal. This concept is relevant to the role of relays as interposing devices. ❑ Interposing: The use of a relay as an intermediary between electrically incompatible devices. ❑ “Normal” Switch Status: The “normal” status of a switch contact, as defined by the manufacturer, refers to its resting condition (minimum stimulus). ❑ “Seal-in” Circuit: A condition in which an electrical relay uses one of its own switch contacts to maintain its coil energization after the initial triggering event has passed. This concept is relevant to various relay control circuits. PROGRAMMABLE LOGIC CONTROLLER (PLC) A Programmable Logic Controller (PLC) is a specialized computer engineered to function reliably in harsh industrial environments. It withstands extreme temperatures and adverse conditions such as wet, dry, or dusty surroundings. PLCs are designed to automate industrial operations like assembly lines in manufacturing plants, ore processing facilities, or even wastewater treatment systems. 62 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL While PLCs share many characteristics with personal computers, including a power supply, CPU (Central Processing Unit), inputs and outputs (I/O), memory, and operating software, the operating software of a PLC is personalized specifically for industrial control and automation. This allows it to execute real-time tasks efficiently and withstand demanding operational conditions. Programmable Logic Controllers (PLCs) have developed industrial logic control by replacing the complex wiring of electromechanical relays with a microprocessor. This not only simplifies the control system but also introduces advanced functionalities, such as counters, timers, sequencers, mathematical operations, and communication capabilities. One of the key advantages of PLCs is the ease of modifying control logic through programming, rather than physically rewiring relays. Ladder-logic programming, a commonly used method for PLCs, offers a significant advantage by translating the technician’s understanding of traditional relay control circuits into a virtual environment. In this virtual form, contacts and coils interact to perform real-world control functions. However, an essential concept to hold is the association of real-life conditions to the switch status, which depends on the “normal” representation of switch contacts—whether they are physical (relays) or virtual (PLCs). Once this concept is understood, both hard-wired relay control circuits and PLC programs become much easier to comprehend and apply. PLCs were invented by Dick Morley in 1964 and have since revolutionized the industrial and manufacturing sectors. PLCs offer a wide range of functions, including timing, counting, calculating, comparing, and processing various analog signals. One of the main advantages of a PLC over a traditional "hard-wired" control system is its flexibility. Once programmed, a PLC can be easily reconfigured or updated at minimal cost—essentially just the time required by the programmer. In contrast, modifying a hard-wired control system requires physically removing and re-routing wires, which is not only more expensive but also time-consuming. This adaptability makes PLCs a more efficient and cost-effective solution for industrial automation. What is Programmable Logic Controller (PLC)? A Programmable Logic Controller (PLC) can be defined as a digital electronic device that uses programmable memory to store instructions and implement functions such as logic, sequencing, counting, timing, and arithmetic, all aimed at controlling machines, processes, and instrumentation. PLCs are user-friendly digital computers designed for making logic decisions and providing output. They consist of solid-state digital elements and serve as replacements for hard- wired electromechanical relays in controlling pneumatic systems. The term “Programmable Logic Controller” is defined by the IEC 1131 standard. “A digitally operating electronic system designed for use in an industrial environment. It utilizes programmable memory for the internal storage of user-oriented instructions, enabling it to implement specific functions such as logic, sequencing, timing, counting, and arithmetic. This functionality allows it to control various types of machines or processes through digital or analog inputs and outputs. Both the PLC and its associated peripherals are designed for easy integration into an industrial control system and can be used efficiently for their intended functions.” A Programmable Logic Controller (PLC) is quite similar to digital computers, but it also possesses certain features that are specific to logic controllers: 1. Rugged Design: PLCs are built to withstand harsh industrial environments, including vibrations, temperature fluctuations, humidity, and noise. 2. Integrated Interfacing: The interfacing for input and output is incorporated within the controller, simplifying connections and configurations. 3. Ease of Programming: PLCs are easily programmable and primarily utilize logic and switching functions to control processes and machinery. Hard-wired Control Systems In hard-wired control systems, relays are employed for control functions. For example, in electrical control, the wiring of control elements such as sensors, solenoids, and counters is managed through relay controls. Such relay-controlled systems are referred to as hard-wired control systems because any modification in the control program necessitates rewiring of the circuit. Consequently, hard-wired controls can be cumbersome and challenging to modify when production requirements change regularly. Maintenance is also difficult in hard-wired control systems, as any minor design issue can lead to significant problems related to tracing and rewiring. Hard-wired control systems consist of three main divisions: 1. Input Section: This section comprises push buttons, switches, and sensors, which transfer signals to the processing section. 2. Processing Section: This section consists of relay coils and contacts, determining the relationship between the received inputs and the required outputs. 3. Output Section: This section includes solenoids, lamps, and contactor coils, where the processed signals are transferred to operate the output devices. 63 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Programmable Logic Controller Systems PLC systems offer numerous advantages over hard-wired electromechanical control systems. Unlike electromechanical relays, PLCs are not hard-wired to perform specific functions. Therefore, when system operation requirements change, a software program can be readily modified instead of having to physically rewire the relays. Additionally, PLCs are more reliable, faster in operation, smaller in size, and can be easily expanded. PLC systems consist of three main divisions: 1. Input Section: This section includes push buttons, switches, and sensors that are connected to specific input addresses in the program, transferring address information to the processing section. 2. Processing Section: In this section, the microprocessor receives input signals from the input section, executes the instructions in the software program, and sends the processed signals to the output section. 3. Output Section: This section receives signals from the processing section and modifies these signals from the processor to operate output devices connected to specific output addresses. Advantages of PLCs over Electromechanical relays Programmable Logic Controllers (PLCs) are increasingly replacing electromechanical relays due to several key advantages: ❑ Reliability and Speed: PLCs are more reliable and offer faster operation compared to traditional relays. ❑ Compact Design and Expandability: PLCs are compact in size and can be easily expanded to accommodate additional functions. ❑ Lower Power Requirements: PLCs consume less electrical power, making them more energy efficient. ❑ Cost-Effectiveness: When compared to hard-wired systems with the same number of control functions, PLCs are generally less expensive. ❑ Flexibility: Unlike hard-wired electromechanical relays, which lack flexibility, PLCs can be easily reprogrammed to adapt to changing system operation requirements without the need for rewiring. ❑ Fewer Hardware Failures: PLCs experience significantly fewer hardware failures compared to electromechanical relays, leading to increased system reliability. ❑ Ease of Implementing Special Functions: PLCs can easily perform special functions, such as time-delay actions and counting operations, enhancing their versatility in control applications. ❑ Comparison between Relay and PLC How does a PLC work? The operation of a Programmable Logic Controller (PLC) can be understood through a cyclic scanning method known as the scan cycle. The PLC scan process involves the following steps: 1. Initialization: The operating system begins its cycle, monitoring time and system status. 2. Input Reading: The Central Processing Unit (CPU) reads data from the input module, checking the status of all inputs. 3. Program Execution: The CPU executes the user or application program, which is typically written in relay-ladder logic or another PLC programming language. 4. Internal Diagnostics: The CPU performs internal diagnostics and communication tasks to ensure proper system operation. 5. Output Update: Based on the results of the executed program, the CPU writes data to the output module, updating all outputs accordingly. 6. Continuous Operation: This process repeats continuously as long as the PLC remains in run mode. 64 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Major Components of PLC As discussed earlier, a PLC is essentially a microcomputer consisting of hardware and software. The major components: 1. Power Supply module 2. Input module 3. Central processing unit (CPU) 4. Output modules 5. Software 1. Power Supply Module Typically, input and output modules in a Programmable Logic Controller (PLC) require a 24V DC power supply, while the processor operates at 5V DC. The power supply unit is usually an integral component of the PLC system. These power supply units convert standard 120V or 230V AC line voltage into the required 24V DC or 5V DC using standard rectifier circuits. 2. Input Module Input devices include push buttons, sensors, potentiometers, pressure switches, and more. The primary function of the input module is to convert high voltage signals from these input devices into low-level logic voltages that the CPU can utilize for processing. The input module can handle both analog and digital inputs, although digital inputs are more commonly preferred in industrial applications. ❑ Analog Input Module: This module converts analog signals from devices such as temperature sensors and pressure sensors into digital signals using an Analog-to-Digital Converter (ADC). The analog signal typically varies within a voltage range of 0-12 V or a current range of 4-20 mA. These voltage or current values are converted into an integer value (e.g., a 16-bit word). ❑ Digital Input Module: This module is responsible for converting digital input signals into 5 V digital signals that the CPU uses internally to execute user programs. The input module of a PLC has four main functions: 1. Signal Reception: The input module receives signals from process devices operating at 220 volts AC. 2. Signal Conversion: It converts the received input signals into 5 volts DC, which can be utilized by the PLC. 3. Isolation: An isolator block is employed to protect the PLC from voltage fluctuations and potential damage. 4. Signal Transmission: Finally, the processed signal is sent to the output end, i.e., the PLC. 3. Central Processing Unit (CPU) The CPU serves as the brain of the PLC, controlling and processing all operations within the system. It is responsible for executing various arithmetic and data manipulation functions involving both locally and remotely located input/output sections. Additionally, the CPU performs essential communication functions, enabling it to interface with personal computers, remote input/output devices, other PLCs, and various peripheral devices. The CPU performs several critical functions: 1. It receives input from various sensing devices and switches. 2. It executes the user-defined program. 3. It makes decisions to control the operation of equipment or processes. 4. It performs various arithmetic and data manipulation functions. 5. It delivers corresponding output signals to load control devices, such as relay coils and solenoids. The CPU module of a Programmable Logic Controller (PLC) comprises a central processor, Read-Only Memory (ROM), and Random Access Memory (RAM). ROM stores the operating system, drivers, and application programs, while RAM is utilized for storing programs and data. The CPU serves as the brain of the PLC and typically features an octal or hexadecimal microprocessor. 65 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL As a microprocessor-based unit, the CPU replaces traditional components such as timers, relays, and counters. It reads input data from sensors, processes this information, and subsequently sends commands to controlling devices. A DC power source, as previously discussed, is required to provide the necessary voltage signals. Additionally, the CPU includes various electrical components that facilitate the connection of cables to other units. 4. Output module The output module of a PLC operates similarly to the input module but in reverse. Its primary function is to interface the output load with the processor. The output module consists of two main sections: ❑ Logic Section: This section processes the control logic generated by the CPU based on the input signals and programmed instructions. It determines which output devices should be activated. ❑ Power Section: Following the logic section, the power section manages the actual power delivered to the output devices, such as motors, relays, or actuators. It converts the control signals from the logic section into appropriate voltage and current levels to drive the connected loads. Together, these sections ensure that the PLC can effectively control various output devices in response to the processed input signals, facilitating efficient automation of industrial processes. 5. Software A Programmable Logic Controller (PLC) consists of two main components: the operating system and the user program. The PLC operating system provides comprehensive support for various tasks, from creating the project structure to developing user programs. It is accessed through a graphical user interface (GUI), often referred to as the main window. This main window encompasses all the functions required to set up a project, configure hardware, and write and test programs. User programs can be developed in any standard PLC programming language, such as ladder diagram or statement list. During the execution of a PLC program, the CPU operates in a cyclic manner, scanning and executing the main program. Each program scan cycle comprises sequential operations, which include the input scan, program scan, and output scan. In the input scan phase, the CPU updates the process image input table to reflect the current status of input devices. During the output scan, the CPU updates the process image output table to control the output devices based on the program logic. Upon completing each scan cycle, the CPU returns to the start and initiates the next cycle, continually repeating this process. The duration required to complete a single scan cycle is referred to as the scan cycle time. Other Components ❑ Rack or Chassis In all PLC systems, the PLC rack or chassis is the most critical module, serving as the backbone of the system. PLCs come in various shapes and sizes to accommodate different applications. For more complex control systems, larger PLC racks are required to support additional components and functionalities. Small-sized PLCs typically feature a fixed I/O pin configuration, which limits their flexibility. To address this limitation, modular rack PLCs have been developed. These modular PLCs accept different types of I/O modules that can be easily slid in and fitted into the rack. This design allows for greater customization and scalability, as all I/O modules reside within the rack or chassis, enabling the system to adapt to specific operational needs. ❑ Communication Interface Module Intelligent I/O modules play a crucial role in transferring information between the CPU and communication networks in industrial automation systems. These specialized communication modules facilitate connections between Programmable Logic Controllers (PLCs) and remote computers or other PLCs, enabling efficient data exchange across various locations. By incorporating advanced processing capabilities, intelligent I/O modules enhance the overall communication process, allowing for better integration, monitoring, and control of automated systems. This technology is essential for optimizing operations and ensuring seamless communication in distributed industrial environments. Types of PLC: There are 4 different types of PLC: ❑ Fixed integrated PLC: have a single unit that houses both the controller and the I/O. ❑ Distributed PLC: A network connects the controller and input/output devices. ❑ Soft PLC: Instead of using a specific controller, a general-purpose computer runs the PLC as a program. ❑ Modular PLC: contains separate modules for the controller and IO that are inserted into a chassis. Based on hardware setup Most commonly used PLC are: Fixed PLC and Modular PLC. 66 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL ❑ Fixed integrated PLC The Fixed I/O PLC is the most popular name for this kind of PLC. Actually, “Fixed I/O” stands for Fixed “Input/Output.” When purchasing Compact PLCs, users will observe that the microcontroller itself houses both the PLC’s input and output parts. Additionally, with this kind of PLC, the number of inputs and outputs may not be increased. Small-scale applications are controlled or carried out using these PLC types. A small package houses the processor, power supply, memory, inputs, and outputs. These PLCs typically have set numbers of inputs and outputs, such as 8, 16, 24, 32, and 40 inputs and 4, 8, 16, 32, and 40 outputs. The amount of inputs, outputs, and memory configurations vary according on the manufacturer and model. Inputs include analogue and digital inputs, while outputs include digital and analogue outputs. The primary benefit of fixed type PLCs is their reduced price. The lack of flexibility with fixed type PLCs is one of their drawbacks. Additionally, certain versions require the replacement of the complete system if any component malfunctions. Advantages: ▪ Fixed PLCs can only store a certain amount of data. This is due to the fact that every component is contained within a single entity. ▪ Fixed PLCs may be substantially less expensive. Disadvantages: ▪ Less memory is frequently built into fixed PLC than into their modular equivalents. Fixed PLC have a fixed number of inputs and outputs since they are manufactured with I/O components already installed. ▪ Because the system is put together as a whole, it is difficult to fix if one component breaks. Longer downtimes may result from this for repairs. ▪ Fixed PLC cannot do more complex tasks as a result. The pre-assembled feature of fixed PLC has the additional drawback that repairs could be more difficult. ❑ Soft PLC A programmable logic controller, or PLC, often known as a fully complete embedded computer, can be created using a software technology called a soft PLC. It combines high performance computer networking, data management, and computational capabilities with discrete, PID, and analogue I/O control offered by PLCs. As a result, soft PLCs provide reliable operation, incredibly quick and deterministic program scan durations, a great set of instructions, clear data table memory, an unlimited number of user programs, and above all, an open architecture platform that enables users to connect to a variety of I/O systems, networks, and other devices. A soft PLC needs to be implemented under the proper system requirements in order to function properly. An Ethernet port, USB port, parallel port, or user-specified port, 32 MB RAM, 386 or higher compatible CPU, 64 MB disc, I/O ports or interface cards, as well as other communication interfaces like the COM ports, are among the minimum software and hardware requirements. 67 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Soft PLCs combine the following operations: ▪ PLC’s ▪ Data monitors ▪ Information Gateways ▪ Embedded computing Advantages: ▪ Soft PLCs can assist in lowering production costs. They foster efficiency, which leads to more output and, ultimately, higher profits. ▪ With benefits including simpler configuration and maintenance, Soft PLCs support a variety of programming languages. They are therefore the ideal choice in high-volume industrial settings where frequent updates are necessary. ▪ In keeping with current industry requirements, a Soft PLC device offers sophisticated security solutions. To prevent unwanted access, for instance, enables you to modify access restrictions to production data. Disadvantages: ▪ There is no other software included beside Soft PLC. ▪ Not every application requires for Soft PLC, and vice versa, due to the potential addition of needless programming costs. ❑ Distributed PLC A network connects the system’s distributed controllers together. The multiple instruction, multiple data (MIMD) structure is used to implement a multiple core controller architecture that uses independent bit and word units. By using a semaphored memory system in its basic form, the calculation process is effectively synchronized. The suggested compiler automatically divides up the program into several processing units. A graph-based representation of the standard program is utilized for distribution and scheduling purposes. Selected standard languages can be combined into an independent graph-based form thanks to a developed conversion model. Utilizing Field Programmable Gate Array (FPGA) devices, the distributed controller architecture has been prototyped, allowing for precise implementation and performance assessment. Token passing-based deterministic protocol was used to implement the data exchange. Advantage: ▪ Compared to other systems, the Distributed PLC system requires less and simpler wiring. ▪ The distributed PLC is capable of remote control. ▪ The connected system receives a quick response from the distributed PLC. Disadvantages: ▪ The high-speed implementation of distributed PLC does not work. ▪ The distributed PLC costs more when used infrequently. ❑ Modular PLC The term “modular” refers to a form of PLC that enables various expansions of the PLC system through the use of modules. Due to the independence of each component, modules usually make it simpler to use the programmable logic controller and provide more functionality like more I/O units. To build a PLC control system, you must manually connect the power supply, communications module, and input/output module since they are all independent of the microcontroller itself. The rack-mounted or rack mount PLC is a type of modular PLC. All connections are centralized in a rack mount PLC because the communications module of the PLC is housed in the rack itself. Separate modules can be put into the compartments that make up this sort of PLC. A rack, a power supply unit, a CPU module, an input, and an output make up the modular basic control. It has an operator interface for running programs and keeping track of things. The rack has the modules hooked in. These PLCs are employed in industrial settings. Depending on the PLC brand and type, the input and output racks can be upgraded as needed and have more storage capabilities. This form of PLC has the benefit of allowing for industry scaling without the need for downtime, but it also costs more than compact PLCs. The modular PLC is simpler to maintain than the compact PLC. 68 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Advantage: ▪ The memory and data storage capacity of modular PLCs are significantly greater. ▪ The modular PLC is more useful since it can handle more complicated procedures. ▪ In order to develop and adjust processes for smooth expansion, modular PLCs were created. ▪ Modular PLCs can quickly identify problems and rectify them while continuing to run some processes. ▪ Modular PLC has higher long-term economic security. Disadvantages: ▪ Reduced space efficiency caused by modular PLC. ▪ When modules fail, modular PLC might increase troubleshooting cost. ▪ Configuration complexity increases as a result of modular PLC. ▪ Because modular PLC have low IP ratings, enclosures are required. ▪ Maintaining more spare inventory on-site for modular PLC ▪ Modular PLC I/O racks promote centralized rather than distributed I/O, which is certainly not standard way in the modern world. Types of PLC Based on Physical Size Programmable Logic Controllers (PLCs) can be classified into three categories based on their physical size: ❑ Mini PLCs They are small, low-cost controllers that are ideal for simple control applications. They typically have fewer input/output (I/O) points than larger controllers and can be programmed using ladder logic or other programming languages. Mini PLCs offer fast installation due to their small size and often come with built-in I/O capabilities such as digital inputs, analog outputs, and pulse outputs. ❑ Micro PLCs They offer an intermediate level of complexity between mini models and modular designs; they are usually compact devices capable of controlling multiple processes simultaneously without requiring additional hardware components like expansion cards or rack units found in some modular models. Microcontrollers can also feature integrated communication functions such as Ethernet networking protocols for easy integration into a distributed automation system architecture. ❑ Nano PLCs They represent the latest generation of programmable logic controllers – these ultra-compact devices use advanced microcontrollers combined with specialized programming software tools to reduce costs while providing high levels of processing speed and accuracy. Nano PLCs are even used in highly complex applications involving multiple axes movement or sophisticated machine vision operations like object recognition algorithms & pattern matching techniques. Types of PLC Based on Output ❑ Analog Output To control equipment that needs varying degrees of control, analog outputs are used that produce continuous voltage or current signals. Analog output offers precise control by altering the voltage or current levels they produce. ❑ Relay Output The electrical separation between the PLC and the external devices is provided by relay outputs in PLCs. They are used to manage machinery such as huge motors, contactors, or power relays that need larger current or voltage levels. ❑ Triac Output Triacs are solid-state devices that can turn on or off during the AC power waveform, giving the linked devices power to be precisely controlled. Devices like lamps, heaters, and small motors that require AC voltage control are controlled by triac outputs. ❑ Transistor Output Transistor outputs are an output module in PLCs that use transistors as switching devices. Transistor outputs can handle higher currents and voltages compared to digital outputs, making them suitable for controlling devices that require more power. 69 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Some of the manufacturers of PLCs include: ❑ Allen Bradley ❑ General Electric (GE) PLC ❑ ABB ❑ Honeywell PLC ❑ Siemens ❑ Panasonic ❑ Mitsubishi PLC ❑ Schneider ❑ Hitachi PLC ❑ Fuji Electric ❑ Delta PLC ❑ Omron Industrial Applications of PLC In industrial automation, PLC performs a wide variety of manufacturing production, monitoring machine tool or equipment, building the system, and process control functions. Here are some of the example where PLC has been used. PLCs are used in, ❑ Transportation System likes Conveyor Belt System. ❑ Packing and Labeling System in Food & Beverage. ❑ Automatic Bottle or Liquid Filling System. ❑ Packaging and Labelling System in Pharma Industries. ❑ Transportation System like Escalator and Elevator. ❑ Industrial Crane Control System for Operation of Overhead Traveling Crane. ❑ Glass Industries for glass production and recording data. ❑ Paper Industries for the production of Pages, Books or Newspapers, etc. ❑ Cement Industries for manufacturing or mixing the right quality and quantities of raw materials, and accuracy of data regarding. ❑ Automatic Drainage Water Pump Monitoring and Controlling System. ❑ Time and Count-based Control System for an Industrial Machine. ❑ Temperature Controller or Humidity by using the Sensors Input to the PLC system. ❑ Fault Detection and Protection of Industrial Machines like an Induction Motor. ❑ Wind Turbine System for Maximum Efficiency, Recording Data, and Safety Purposes. ❑ Conveyor Belt System controls the Sequence of Conveyors and Interlocking procedure. ❑ Energy Management System like Boiler, Ball Milling, Coal Kiln, Shaft Kiln, etc. ❑ Oil and Gas Industries for controlling the Purging Procedure. Power Station Applications of PLC For the electrical power system analysis, PLC plays operation for maintenance and other main roles in the power plants and the smart grid system. ❑ PLC uses for the Smart Grid System to Monitor and Detect fault conditions. ❑ It is used in the Power Generation, Transmission, and Distribution System. ❑ In the Power Substation, PLC can use the Auto Assembly Line System. ❑ Some Electrical Equipment (like Circuit Breaker Tripping, Capacitor Switching) can be automatically operated with PLC. ❑ A Single-Phase or Three-Phase Sequence Detect by using the PLC. ❑ In Oil, and Gas an Automation Power Plant, PLC needs for Valve Switching for Changeover of Fuels, Pilot Light ON or OFF, Flame Safety Checking, Oil Filtering procedure, and more things. ❑ Real-time PLC uses in Underground Coal Mine or Water Level Sensing and Data Survey. Commercial Applications of PLC We can see the growth of PLC in commercial control applications. With the use of PLC, applications can easily operate without or with very minimal manpower or physical hard work. Here are some basic commercial application uses PLC: 70 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL ❑ Smart Traffic Control Signal System. ❑ Smart Elevator Control System. ❑ Fire Detection and Alarm System. ❑ Automatic Machine Handling System. ❑ Automatic Vehicle Washer System ❑ Automated Guided Vehicle System. ❑ In the Roller Coasters Machine. ❑ Automation System for Well Drainage System. ❑ Luggage Handling System. For example, at the Airport. ❑ Pressure Controller in Multi-Motor Pump Applications. ❑ Sequence or Numerical Counting and Packing System. ❑ Mining Equipment Line Detection and Remote Control System. ❑ For Wind Turbine Operation, PLCs convert signals from the Wind Speed and Direction Sensors to better control the Wind Turbines. Domestic Applications of PLC For the domestic purpose, PLC act as a remote operating device or automatic sensing device. We can automate some day-to-day activities with PLC. Here are some useful domestic applications we can automate with PLC: ❑ Water Tank Level Control System ❑ Car Washing and Parking System. ❑ Flashing Light Controlling System. ❑ Automatic Door Opening/Closing System. ❑ Remote Monitoring Application like Air compressor (AC), Fan. ❑ ON/OFF Switching Application like Light, Motor, and More daily life applications of PLC. Education Applications of PLC Engineering students mostly prefer the automation system for doing their academic or research projects. It is a big trend. As part of the automation projects, you can automate any commercial or domestic applications using PLC. The project should be designed to automate a specific task. It should work under real-time and with superior reliability and best performance. What are the major areas of Application of PLC? PLC works in an industrial automation environment where PLC replaces the relay system. Now, we will see the top automation companies list where PLC is needed. Automation Industries are: ❑ Steel Industry ❑ Automobile industry ❑ Glass Industry ❑ Food Processing System ❑ Paper industry ❑ Oil and Gas Power Plant ❑ Textile industry ❑ Wind Turbine System ❑ Cement Industry ❑ Robotic Automation System ❑ Chemical industry ❑ Underground Coal Mine and many more industries In the above automation industrial area, PLC helps to monitor input and output and makes the logic-based decision, automatic sequential count, time-based control system for the automated process. PLC Programming When using a PLC, it’s important to design and implement concepts depending on your particular use case. To do this we first need to know more about the specifics of PLC programming. A PLC program consists of a set of instructions either in textual or graphical form, which represents the logic that governs the process the PLC is controlling. There are two main classifications of PLC programming languages, which are further divided into many sub-classified types. 71 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL 1. Textual Language ❑ Instruction List (SL) ❑ Structured Text (ST or STX) 2. Graphical Form ❑ Ladder Diagrams (LD) (i.e. Ladder Logic) ❑ Function Block Diagram (FBD) ❑ Sequential Function Chart (SFC) Textual Language ❑ Instruction List (IL) This is one of the PLC programming languages which is like an assembly programming language. You will find the series of instruction lists in this language. The mnemonic codes like LD, AND, OR, A, etc. are used in this PLC programming language. Sometimes it is easy to remember the code while using this programming language. Advantages: ▪ High Execution Speed. ▪ Less Memory Consumption as compared to other PLC programming languages. ❑ Structured Text (ST) The ST or STX is the short abbreviation of Structured Text, one of the PLC programming languages. It is a high-level programming language is like a ‘C’ or ‘Pascal’. The ST consists of various statements with complex statements and instructions like IF, WHILE, CASE, RETURN, FOR, REPEAT, etc. It is a very powerful language that can easily execute complex mathematical logic. Advantages: ▪ Very good with complex algorithms and mathematical logic. ▪ Easy to modify programming due to standard coding format. Graphical Form ❑ Ladder Diagram (LD) A ladder diagram is usually called a “Ladder Logic”, this represents a program by a graphical diagram. This looks like relay racks, each device in the relay rack would be represented by a symbol on the ladder diagram. The name ladder diagram is based on the programming language pattern similar to a ladder, with two vertical rails that show electrical connection among a series of horizontal rungs between them. Advantages: ▪ Easy to learn, understand and follow ▪ More reliable than electronic circuit controller ▪ A most convenient way to represent the discrete logic ▪ Easy to fault diagnose 72 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL ❑ Functional Block Diagram (FBD) This is another PLC programming language that is a popular one and looks like a graphical type like a Ladder Diagram (LD). In Function Block Diagram inputs and outputs are connected in blocks by connection lines. Function blocks are mainly used to do repetitive tasks like starters, closed-loop control, PID loops, etc. Advantages: ▪ Easier because of a graphical representation method ▪ One block consists of several lines of logic which is like a repetitive task ❑ Sequential Function Chart (SFC) Sequential Function Chart (SFC) is also a graphical programming language that is similar to a flow chart like logic. In this PLC programming language, the program is divided into steps that act as a major role. Steps consist of an action that occurs when a programmer wants them to happen. Steps can be active or inactive. Transitions are the instructions that are used to move the program from one step to another. Advantages: ▪ Easy to understand overall program control. ▪ Easy to design and modify logic due to repeated instructions. PLC PROGRAMMING EXAMPLES Example No. 1: Test lamp In this example, a signal lamp is required to be switched on under two conditions: ❑ When the pump is running and the pressure is satisfactory. ❑ When the lamp test switch is closed. To achieve this, both AND and OR logic gates are used in the control logic. AND Logic: The lamp should turn on only if both the pump is running and the pressure sensor indicates satisfactory pressure. This requires an AND gate because both conditions (pump running and satisfactory pressure) must be true to produce an output that will switch on the lamp. OR Logic: The test switch allows the lamp to be turned on independently. When the test switch is closed, the lamp should turn on regardless of whether the pump is running or the pressure is satisfactory. This requires an OR gate, as only one condition (either the AND logic or the test switch being closed) needs to be true to turn the lamp on. In a ladder diagram, these conditions would be represented by two branches: The first branch would have the AND logic (pump running and pressure satisfactory) to control the lamp. The second branch would have the OR logic with the test switch, which bypasses the AND conditions to turn the lamp on when closed. Finally, in the ladder diagram, instructions like END or RET are used to indicate that the PLC has reached the end of the program. This tells the PLC that the logic scan for that cycle is complete, and the system can reset to the start of the program for the next scan. This ensures that the control logic continues to be monitored and executed in real-time, based on the inputs. 73 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Ladder Diagram and Functional Block Diagram Solution Example No. 2: Operate valve As another example, consider a valve that is to be operated to lift a load when a pump is running and either the lift switch is operated or a switch operated indicating that the load has not already been lifted and is at the bottom of its lift channel. OR logic is used for two switches and an AND logic is used with two switches and the pump. Valve will be operated only if the pump is ON and two switches are operated. Ladder Diagram and Functional Block Diagram Solution Explanation: The contacts X400 (Lift Switch) and X402 (Not Lifted Switch) are in parallel, meaning if either condition is true, the logic will continue. The series connection with X401 (Pump) ensures that the Valve will only activate if the pump is also running. Example No. 3 74 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL The image represents a ladder logic diagram, commonly used to show industrial control systems. The circuit consists of relays, pushbuttons, sensors, and indicator lights that work together to monitor and control certain parameters like level, pressure, temperature, and flow. Let’s break it down: Power Supply: L1 and L2: These represent the main power supply lines of the circuit. The voltage is applied between these two lines. Circuit Components: 1. Pushbutton A (Normally Open): When pressed, this pushbutton allows current to flow and energizes Relay CR1, provided other conditions (such as sensors being triggered) are met. 2. Pushbutton B (Normally Open): Similar to Pushbutton A but is used to trigger specific lights (Blue) depending on the state of other components in the circuit. 3. CR1 (Control Relay): This is a relay that controls various parts of the circuit. When CR1 is energized, its contacts close, allowing current to flow in other parts of the circuit. o CR1 Contact: The normally close contact of the relay CR1 opens when the relay is energized. o CR1 Contact: The normally open contact of the relay CR1 closes when the relay is energized. 4. Sensors (with Trip Points): o Trip = 4 ft: This sensor closes its contact when the level reaches 4 feet (e.g., a tank level sensor). o Trip = 30 PSI: A pressure sensor that closes when pressure reaches 30 PSI. o Trip = 178°F: A temperature sensor that closes when the temperature reaches 178°F. o Trip = 5 GPM: A flow sensor that closes when the flow rate is 5 gallons per minute. 5. Indicator Lights: o Blue Light: Indicates a certain condition has been met, such as when Pushbutton B is pressed and the circuit involving CR1 and the pressure sensor (30 PSI) is closed. o Red Light: This light turns on when the flow sensor detects 5 GPM. o Amber Light: Lights up when Pushbutton B is pressed, and certain conditions are met (e.g., CR1 being energized). How the Circuit Works: 1. Top Circuit (Relay CR1 Activation): o The Trip = 4 ft sensor closes when a level of 4 feet is reached. If Pushbutton A is pressed, it energizes Relay CR1, closing the contacts associated with CR1 in other parts of the circuit. 2. Middle Circuit (Pressure Sensor and Blue Light): o When CR1 is closed, and the pressure sensor reaches 30 PSI (Trip = 30 PSI), pressing Pushbutton B will turn on the Blue Light, indicating the pressure condition has been met. 3. Bottom Circuit (Flow and Temperature Sensors, Red and Amber Lights): o When CR1 is energized, the circuit conditions depending on the Trip = 178°F temperature sensor and Trip = 5 GPM flow sensor are met. If these conditions are satisfied, the Red Light will turn on to signal flow rate detection. o The Amber Light will illuminate when Pushbutton B is pressed, and the CR1 relay is activated. Summary: This diagram is used to control a system that monitors level, pressure, temperature, and flow rate. It provides visual indicators (lights) depending on the status of these parameters and allows manual intervention through pushbuttons. The system will operate based on the conditions set by the sensors and the activation of the control relay CR1. ASSIGNMENT ❑ From Functional Block Diagram (FBD), do a Ladder Diagram in the specific situation. 75 | P a g e  TUP MANILA COLLEGE OF ENGINEERING ELECTRICAL DEPARTMENT INSTRUMENTATION AND CONTROL Problem Definition: Drinking Machine Consider a drinks machine that allows the selection of tea or coffee, milk or no milk, sugar or no sugar, and will supply the required hot drink on the insertion of a coin. From the below-shown figure, it is seen that either tea or coffee is selected using the first OR logic gate. Consider a drinks machine that allows the selection of tea or coffee, with the options of adding milk or sugar. The machine dispenses the chosen drink upon the insertion of a coin. Drink Selection: Either tea or coffee can be selected using an OR gate. Coin Insertion and Drink Selection: The first AND gate produces an output when either tea or coffee is selected, and a coin is inserted into the machine. Hot Water Application: The output from the first AND gate is passed to a second AND gate, which operates when hot water is combined with the selected drink (tea or coffee). Optional Additions: Milk and sugar are optional additions, which can be selected after a coin has been inserted. The machine ensures that the drink is dispensed only when the proper selections are made and the payment is completed. The first AND gate give an output when either Tea or coffee is selected and a coin is inserted into the machine. The output from this AND gate is given to the second AND gate. The second AND gate operate only when hot water combines with tea. Milk and sugar are optional additions that can occur after a coin has been inserted. Functional Block Diagram Solution REFERENCES https://www.studocu.com/in/document/delhi-technological-university/basic-civil-engineering/lecture-42/47760880 https://www.researchgate.net/publication/328578763_Electric_Relays_Principles_and_Applications https://www.allaboutcircuits.com/projects/use-relays-to-control-high-voltage-circuitswwith-an-arduino/ https://www.electrical4u.com/programmable-logic-controllers/?fbclid=IwAR2GW-0vsaY4F1A4- KiJ7oeiKDjcKJX575TjrV3NwT_x4nRRoPx5XfXd59Q#google_vignette https://automationforum.co/what-are-the-types-of-plc/ https://trainings.internshala.com/blog/different-types-of-plc/ https://dipslab.com/plc-applications/ https://instrumentationblog.com/types-of-plc-programming-languages/ https://www.youtube.com/watch?v=Qf32qtHfowQ 76 | P a g e