Introduction to PLC

History 

Before 1960, hardware control used relays, timers, and contractors to build logic circuits. Initially, an electrical circuit diagram was designed, and then the circuitry was completed by connecting the required components (relays, connectors) with cables. If a mistake was made in building the circuitry, it was difficult to identify the fault, and often a new circuit had to be built from scratch. Complex logical circuits required multiple relays and contactors, making cable connections cumbersome. Additionally, small electrical panels required large boards. To address these issues, the first Programmable Logic Controller (PLC) was introduced in 1968. Originally called Programmable Controllers, the term PLC was coined in 1971 by the Allen-Bradley Company. Richard Morley and Odo Josef Struger were the initial developers and evaluators of the PLC, and subsequent changes and modifications led to the development of modern PLCs.

What is PLC

PLC stands for Programmable Logic Controller, although some manufacturers refer to it as PMC (Programmable Machine Controller). PLCs are manufactured with standardized hardware, and machine manufacturers use them according to the requirements of their machines. PLCs are programmable and flexible compared to traditional relay logic. Changing the sequence of machine operations can be accomplished through simple program modifications without the need to change the PLC hardware. PLCs can be interfaced with various communication devices such as computers, display units, and data storage devices, making them highly versatile.

Commonly, a standalone PLC consists of a power supply unit, a central processing unit (CPU), and input/output (I/O) modules. In the case of PLCs used with CNC or NC machines, they are typically embedded with a controller unit. Additional modules can be added to a PLC as needed, and these units are given separate names by PLC manufacturers. A PLC always runs a software program called the PLC program, which is usually written on an external computer and transferred to the PLC's memory. PLCs can be programmed using different languages, including Ladder Logic or LAD, Statement List or STL, and Function Block Diagram or FBD. Ladder Logic or LAD is the most popular language and is supported by most PLC manufacturers.

 

Uses of PLC 

Currently, PLCs are used in various applications such as automated machine tools, CNC or NC machines with complex logical operations, elevators, process control systems, traffic light control, automatic door control, complex lighting control systems, movement of robotic arms, and small prototype automatic machines.

• Different CNC and NC machines.
• Production and assembly line.
• Automatic Door and Lift
• Automatic traffic signaling system.
• Different complex lighting control systems.
• Movement of a robotic arm.
• Small prototype automatic machines.

Advantages of PLC

Using a PLC in an automated control system offers several advantages:

• Low response time and high speed of operation.
• Easy modification of operational sequences.
• Reduced electrical wiring compared to conventional relay systems.
• Ease of developing complex logical operations.
• Ability to communicate with other devices such as PCs or printers.
• Easy identification and elimination of faults in the operation sequence.
• Cost-effectiveness for large and complex operations.
• Easy implementation of new systems through copy-paste options.

How PLC Works

A PLC has an independent power supply unit that provides electrical power to the CPU, I/O modules, and base unit. The input module of the PLC interfaces with various field devices such as sensors and switches. The input module receives electrical signals representing the state of a sensor (ON or OFF) and transfers this information to the input image table or register. The output modules provide control signals to different actuators such as solenoid valves, motors, and relays. The state of the input devices and the instructions or program written inside the controller determine the activation or deactivation of an actuator or output device.

To program a PLC, it is initially connected to an external programming device, such as a computer, through a communication port. The programming device uploads or downloads the program to be stored in the PLC's memory. The CPU of the PLC contains an EEPROM (electrically erasable programmable read-only memory) for storing the PLC program. Once the program is downloaded to the PLC, there is no need for an external programming device.

The PLC operates in a scan sequence, starting from the beginning of the program and proceeding to the end. The scan sequence repeats continuously as long as the PLC remains powered on. The time required to complete one cycle of the program is known as the scan time, which is typically very fast, taking only a fraction of a second. The scan time depends on the length of the PLC program. The scan cycle process of a PLC is illustrated in the accompanying picture.

Role of a Sensor with PLC

Sensors produce electrical signals based on the physical state of a machine part and interface with the PLC through an input module to provide information about the different states of the appliance (activated or deactivated). For example, a toggle switch can determine whether the switching status is open or closed. In this case, a 24V DC supply is passed through the switch contacts and connected to the terminal of the PLC's input module. If the switching contacts are open, there will be no 24V DC input present in the input module terminal, while in the closed switching contact, a 24V DC input will be present. The PLC monitors the presence or absence of the 24V DC voltage to determine the state or status of a sensor.

Sensors typically have two states: ON or OFF, which represent the activated and deactivated states of a sensor. PLCs consider the ON state of a sensor as Logic 1 or Logic high, and the OFF state as Logic 0 or Logic low. When a sensor is activated or in the Logic 1 state, a 24V DC voltage is present at a specific input module terminal of the PLC. Conversely, when the sensor is deactivated or in the Logic 0 state, no voltage is transferred from the input device to the PLC input module terminal. The picture below represents a switch with Logic 0 and Logic 1 states. 

 

How the Sensor connects to the PLC input module - The picture illustrates how switches, pushbuttons, and sensors interface with a PLC through an input module.

Specific input addresses are assigned to the respective input terminals of the PLC input module terminal strip. The PLC recognizes the status of the input address (0 or 1) to determine the state of the corresponding sensor. If the input address is 1, the PLC assumes that the sensor is activated, while a 0 status indicates that the sensor is deactivated. LEDs are often used with individual input addresses on a PLC input module to visualize the sensing status of a sensor from the outside. A glowing LED indicates that the corresponding sensor is activated. In the previous picture, the input address I0.3 LED is glowing, representing the activation of the corresponding push switch, while the remaining two inputs (I0.0 & I0.6) are deactivated. Input modules are available in different types based on factors such as the number of sensors to interface with, the current rating and operating voltage of the sensors, and the need for electrical isolation.

Role of Actuator with PLC 

An actuator changes the physical state of an appliance based on the electrical signal received from the PLC. Actuators can be directly connected to a PLC or interfaced through a relay. For example, a lamp can be connected directly to the output module of a PLC, while a motor may be interfaced through a relay. Based on the output signal received from the PLC, the actuators are activated or deactivated. Different actuators such as solenoid coils, connectors, and relays are employed in automated systems to change physical states or circumstances.

Actuators typically have two states: ON or OFF, representing the activated or deactivated statuses. PLCs consider the ON state of an actuator as Logic 1 or Logic high, and the OFF state as Logic 0 or Logic low. When an actuator is activated or in the Logic 1 state, a 24V DC voltage is supplied to a specific output module terminal of the PLC. Conversely, when the actuator is deactivated or in the Logic 0 state, no voltage is output from the output module terminal. Actuators can have different voltage ratings, such as 24V DC, 110V AC, and 220V AC. In the case of higher voltage rating actuators, 24V DC-operated relays are used to supply the required voltage. The following picture represents an actuator with Logic 0 and Logic 1 states. 

How an Actuator connects to a PLC output module - The picture represents different actuators and elements, such as a solenoid, lamp, and relay, interfacing with a PLC output module. 

Specific output addresses are assigned to the separate output terminals of a PLC output module terminal strip. When the status of an output address is 1, the PLC provides a 24V DC output to a specific output terminal of the output module. LED indicators are used with individual output addresses on the PLC output module to visualize the activation status of the corresponding actuator from the outside. A glowing LED indicates that the actuator or element is activated. In the previous picture, the output address Q0.3 LED is glowing, representing the activation of the corresponding element (in this case, a lamp), while the remaining two outputs (Q0.0 & Q0.6) are deactivated. For output address Q0.0, a 220V AC motor is connected through a relay and is activated by the 24V DC signal. The motor operates with 220V AC power. This is a simple example of controlling a higher voltage rating actuator with a PLC. The types of output modules available vary based on factors such as the capacity to interface with actuators, voltage, and current ratings.