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What is a PLC?

A Programmable Logic Controller, commonly referred to as a PLC, is a ruggedized solid-state computing device used to control electro-mechanical equipment and processes, normally in an industrial facility. To do this, the PLC monitors the state of connected input devices, such as sensors, and makes logic-based decisions as programmed to control corresponding output devices such as drives, motor starters, valves, and solenoids.

Over the years, PLC controllers have become popular in automated manufacturing plants due to their ruggedness (they can withstand extreme industrial environments), reliability, flexibility, and adaptability to almost any control application along with their affordability and ability to perform multiple functions. They are also easy to program, install and maintain. In manufacturing facilities, PLCs are usually used to control conveyor systems, automobile assembly lines, food processing machinery, batch control, chemical processing, etc. They are also widely used in utility plants, mainly as part of SCADA (Supervisory Control and Data Acquisition) systems.

Hardware Components of a PLC

A PLC system is formed by combining several hardware components, which are interconnected and function to enable the controller to collect data from a variety of input devices, execute the programmed logic, and trigger appropriate outputs. These components include:

A) Processor Module

The processor module is the core component of a PLC unit, it includes the Central Processing Unit (CPU) and Memory. The CPU is a solid-state device responsible for data manipulation, arithmetic functions, and logic operations. A typical PLC processor includes two types of memory, ROM and RAM. ROM stores fixed data and programs normally used to define the capabilities of the PLC. On the other hand, RAM stores user-defined programs, I/O data, data variables, as well as timer and counter values.

Essentially, the processor unit is designed to perform the control functions of the PLC system including machine tool, process, and production control. Different PLC models are available with different types of processors. The type of processor or CPU used determines the different features of the PLC system such as available memory capacity, programming functions, size of available application logic, and processing speed.

B) Input Module

The input module transmits inputs signals from field input devices such as pressure sensors, operator inputs, various switches, START/STOP pushbuttons, etc., to the PLC processor. PLC input modules can be analog or digital, providing different types of inputs to the processor unit as per the programming of the PLC.

C) Output Module

The output module transmits executed data and control commands from the CPU to the connected field output devices such as solenoid valves, motors, relays, pumps, electric heaters, etc. PLC outputs are received by the field devices in different forms. Also, PLC output modules can be of digital or analog type depending on the type of corresponding input.

D) Power Supply Module

The power supply unit converts AC input source power (normally, 120 or 240 VAC) into a lower volts DC power (mainly, 24 VDC) required by the PLC internal circuitry. For example, the processor module operates on ± 5V DC power that’s supplied by the power supply unit. In some cases, this unit also provides isolated DC supply voltage for powering DC input circuits, LED indicators, switches, and other components of the PLC system. There are different types of power supplies available for use with PLCs ranging from 24V DC to 120/240V AC.

E) Programming Device

The programming unit enables users to develop PLC programs, which are then downloaded onto the processor memory. This device is also used to monitor the execution of the programmed logic and to modify the parameters stored in the program. Examples of PLC programming devices include laptops, personal computers, and Hand-Held Terminals (HHTs).

F) Communication Modules

Some PLCs have built-in communications capabilities with the processor unit, while others include separate communication modules. These modules allow for the digital exchange of data between the PLC unit and other systems or devices within the industrial facility. The modules are available for a variety of industry-standard networks, with the most common being Serial and Modbus communication cards.

In addition to the components, modern-day PLCs are equipped with HMI (Human-Machine Interface) modules that enable operators to interact with the PLC control system in real time (i.e., review the PLC’s operations and input operating parameters in real-time). These HMI interfaces can be big LCD touchscreen panels with advanced features or just simple displays including a keypad and a text readout.

How Does a PLC Operate?

A PLC controller starts operating once the user-defined program is downloaded from the programming terminal onto the processor unit and the appropriate I/O modules are connected. The CPU receives data collected from the connected input devices, processes the data, executes the program instructions, and delivers output commands based on the provided input information and programmed logic. Once the user has determined the necessary inputs and outputs, the CPU continually executes the above process in a repeating loop known as a Scan Cycle.

The PLC Scan Cycle involves the following steps:

  1. Input Scanning: The PLC processor sequentially reads and collects data on the state of all field input devices connected to the PLC system in a scanning manner. It then processes the data and stores it in the corresponding I/O memory area to be used in the next stage.
  2. Executing User Program: The CPU executes the written logic by sequentially scanning the user-defined program from top to bottom. Note that during program execution, the data and state of the inputs stored in the I/O memory area do not change. But the data and status of the outputs stored in the system RAM or I/O image area may change.
  3. Output Updating: Once the program execution is completed, the CPU refreshes the appropriate outputs according to the corresponding input variables and the results of executing the user-defined program, and then controls the corresponding peripherals through the output circuitry. This is the real PLC output used to control automated industrial machinery or processes.
  4. Housekeeping: During this step, the PLC processor continuously performs memory checks and internal hardware diagnostics, among other things, and it also constantly communicates with the programming devices to avoid any errors in the PLC program.

Note: The above four steps in PLC operation, take place sequentially in a repeating loop. Also, depending on the selected inputs and outputs, a PLC system can automatically start and stop industrial processes, monitor and record operational data such as operating temperature or machine productivity, generate malfunction/fault alarms, and more.

What is a PID?

The term PID stands for Proportional–Integral–Derivative control. It’s a type of feedback control loop mechanism largely used in science, industrial, and engineering applications that require continuously modulated control.

You’ll often come across PID controllers in high-precision industrial control systems used to regulate process variables such as speed, flow, temperature, pressure, etc. Most of these applications use several (2 or more) PID controllers in cascading networks to achieve the required control. The popularity of PID controllers in industrial control is partially attributed to their robust performance across an extensive span of operating conditions and their functional simplicity.

As the name suggests, a PID control algorithm includes three coefficients; Proportional, Integral, and Derivative, which are varied to give an optimal control strategy for process control. The PID controller evaluates an error value by continuously calculating the difference between a measured Process Variable (PV) and a required Setpoint (SP), and then automatically applying a responsive correction based on the Proportional-Integral-Derivative parameters. The three coefficients (P, I, and D) can be fine-tuned or weighted, to correctly adjust the process/function being controlled.

A practical example of PID control is cruise control in vehicles, where if a car is ascending uphill its speed tends to reduce if the applied engine power is constant. At this point, the PID algorithm operating the cruise control restores the measured speed of the car to the desired speed without any delays, by increasing the engine’s output power in a controlled manner.

How does a PID Operate?

To accurately control process variables, PID controllers operate in a closed-loop system with a feedback mechanism. The diagram below illustrates the structure of a typical PID process control consisting of a PID block with Proportional-Integral-Derivative coefficients and a Process/Plant block.  The PID block gives its output to the Plant block which consists of end-stage control devices such as control valves, actuators, and other devices controlling different industrial processes.

As stated previously, a PID controller functions in a closed-loop plant system with a feedback mechanism. So, the system evaluates the feedback variable from the Process block using a reference/response signal u(t) or a Setpoint (SP(t)) to generate an error value (e(t)). Essentially, the magnitude of the generated error is the variation between the Setpoint (SP(t)) and the Measured (MV(t)) value:

The PID block then takes the error signal e(t) as an input parameter and produces a controlled output signal u(t) whose magnitude is determined by the combined Proportional, Integral, and Derivative control calculations in the PID algorithm. This procedure continues until the error value gets to Zero, where the magnitude of the feedback signal is equivalent to the Setpoint value.

The functions of the three PID parameters (P, I, and D) in process control are as follows:

1 Proportional (P) Component: It compares the feedback signal or actual process variable to the SetPoint value. The resulting error value is then multiplied with a proportional constant (Kp), to get an output (up(t)) that’s proportional to the instantaneous error (e(t)), such that if the magnitude of the current error is zero, then the output of the Proportional component of the PID will be zero.

2. Integral (I) Component: This component is required to eliminate the steady-state error–an offset between the SetPoint value and the Feedback process variable– that exists with the Proportional component. It integrates the steady-state error over a duration of time till the magnitude of the error signal reaches zero. The integral runs from the start of the control loop to the present time.

3. Derivative (D) Component: Since the Integral component cannot predict the future characteristics of the error signal–instead it reacts whenever the Setpoint is changed–a Derivative parameter is used to overcome this problem. Its output (uD(t)) depends on the instantaneous rate of change of the system error in relation to time, which is then multiplied KD (the derivative constant).

By combining the outputs of the three components, the PID algorithm outputs a controlled output (u(t)) or a combined response that’s applied to the plant’s final control devices to alter the output of the system/process as desired.

Simply put, a PID controller operates with a control loop feedback strategy; reads a sensor, computes the desired output of the connected actuator (or any other control device) by calculating the Proportional (P), Integral (I), and Derivative (D) responses, and then sums up the outputs of the three components(P, I, and D) to compute the combined response.

What’s the Difference Between a PLC and PID?

From the discussion above, we can conclude that PLC and PID controllers are widely used in industrial applications to control automated machinery or processes. So, what sets a PID apart from a PLC? Let’s look at some of the features that differentiate the two.

Physical Differences: A Programmable Logic Controller (PLC) is a solid-state physical device consisting of a combination of several hardware and software components, whereas a Proportional-Integral-Derivative (PID) controller can be defined as a stand-alone control device or just a control algorithm–a mathematical construct. You can write a program to implement a given PID algorithm/control law and download it onto the PLC processor, for use in controlling a closed-loop automation system in your plant facility.

Functional Differences: A PID controller maps a physical input to the required output in a closed control loop via a PID algorithm–a specialty algorithm designed to control an industrial process with multiple or just one control loop–while a PLC can be programmed to carry out a variety of control functions including implementing a PID control law. Also, originally, PID controllers were developed as analog controllers, operating based on a generated error. But digital versions of PID controllers are currently available. In contrast, PLCs have always been configurable devices that use digital logic to control processes or machines.

In addition, PID control doesn’t need an in-depth understanding of the fundamental workings of the process to be regulated; the only requirement is that it should be possible to strongly influence a given measured process variable using a controlled variable. But a PLC controller must read all the connected inputs, execute a programmed logic, and trigger (energize or de-energize) certain outputs to control a given process; thus, an in-depth understanding of the basic workings of the automation process to be controlled is essential.

Application Differences: Most PID controllers are designed as dedicated instruments for controlling specific process variables such as speed, flow, temperature, pressure, etc. In contrast, PLCs are versatile in their applications, as they offer greater freedom and flexibility in programming to meet a wide range of application needs. That means while a PID can only control a single process output, a PLC can control an entire plant facility using its programmed logic.

Also, even though PID controllers are not optimum controllers they are largely used in science, industrial, and engineering applications due to their effectiveness and ease of implementation. In addition, PID control systems are less costly due to features such as auto-tune algorithms and flexible setpoint programming. Compared to PID controllers, PLCs are high-performance controllers but complex systems that are a bit costly due to additional costs that come with dedicated communication modules, other hardware components, and so on.

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