Industrial Automation and Control Systems

Process Control Education

Process control regulates a system's output by monitoring the amplitude of the output parameter and comparing it to the desired value. If there is any deviation, an error signal is generated to adjust the input variable and bring the output parameter to the desired value. For example, one can control the temperature of a room using a heater and a thermostat. The thermostat activates the heater when the room temperature goes below a certain value and turns off once the desired temperature is achieved (see Fig. 1). This process is known as on/off or deadband control. This is because the heater turns on and off to maintain the temperature. The temperature setting on the thermostat is called the set point. The deadband prevents the heater from frequently switching on and off, ensuring a stable room temperature.

Fig. 1 Block diagram of room temperature control using a heater and a thermostat.

Process control systems are essential for ensuring efficient and effective operation of industrial processes. The primary purpose of these systems is to achieve consistent production while minimizing energy waste. They are used in industrial settings to achieve two objectives: maintaining quality, yield, throughput, and energy efficiency and ensuring safe and profitable working conditions. By measuring, monitoring, and controlling manufacturing processes and activities, these systems can identify deviations or abnormalities from the specified values and correct them manually or automatically. Similar to the block diagram shown in Fig. 1, a sensor measures the process variable in industrial process control, which then serves as input to a controller. The controller responds to the process variable's value by taking appropriate action. Any industry has four typical process variables: level, flow, temperature, and pressure. Figure 2 depicts the measurements of these process variables in a mixing tank. In the figure, LT 101, TT 101 and FT 101 are the level, temperature and flow transmitters.

Fig. 2 Measurement of process variables in a mixing tank.

Fig. 3 The process control loop with PLC.

Programmable Logic Controller (PLC)

A PLC is an industrial computer built to last and can run programs continuously in a 24/7/365 setting. PLCs have several benefits and drawbacks and are frequently used in industrial automation systems. PLCs' high reliability and straightforward troubleshooting, which stem from their plug-in design and ease of maintenance and repair, are among their main advantages. They also require no standardization and have an easy installation process, which gives you more flexibility. PLCs are economical and energy-efficient due to their minimal power and space requirements. Moreover, they lack moving components and are easily scalable, facilitating prompt program modifications. PLCs do, however, have certain drawbacks. One is that working with different systems can be challenging for programmers due to the requirement for a standard programming language. Furthermore, because processing is done cyclically, they are slower than parallel logic systems, which can cause delays in the control system. However, because of their many benefits and adaptability, PLCs remain well-known as industrial control systems despite these drawbacks.

A process control loop that continuously monitors and modifies a process variable to keep output within predetermined bounds is shown in the block diagram in Fig. 3. A PLC controller, a flow control valve (FCV 101) as the last control element, and a flow transmitter (FT 101) as the measurement device comprise the control loop's three main parts. The flow rate is the process variable that the measurement device senses. When the measurement is obtained, it is compared to a target setpoint by the PLC controller, which determines the required corrective action. The last control element modifies the process to bring the output back to the intended setpoint.

PLC Ladder Diagram

A PLC program or algorithm must be defined to automate the above process, combining various functions. The IEC standard defines five popular PLC programming languages, of which Ladder Logic is the most popular due to its ease of implementation and troubleshooting. A ladder diagram visually represents the control logic used in PLC programming. It is similar to relay logic circuits and consists of rails and rungs that create a framework for the logic. Symbolic notation is used to depict the various logic operations. When implementing a PLC ladder diagram, it is essential to understand its critical parts, including rails, rungs, inputs, outputs and logic expressions (see Fig. 4). Understanding these components is crucial for creating effective and efficient ladder logic programs.

As shown in Fig. 4, ladder diagrams are a common way to represent logic circuits. The diagram consists of two vertical rails on the front end, with horizontal rungs connecting the rails to the logic expressions. The rungs are numbered sequentially. The inputs, like push buttons/limit switches, are represented by symbols such as normally open (NO) or normally closed (NC) contacts. The output devices, such as electric motors/solenoid valves, are represented by relay coil symbols. The logic expressions are combined with inputs and outputs to control operations.

Fig. 4 Basic PLC ladder diagram.

Labs: Virtual Laboratory for PLC-Based Process Control Education

Lab 1: Single Tank Level Control

Lab 2: Single Tank Temperature Control

Lab 3: Single Tank for Batch Mixing Process

Lab 4: Series Tanks Level Control

Lab 5: Parallel Tanks Level and Temperature Control

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