Understanding Automatic Control Systems: The Difference Between Open Loop and Closed Loop

 1. Introduction to Control Systems: Open Loop vs. Closed Loop

Automatic control systems are an essential part of various engineering fields, particularly electrical, mechanical, and industrial manufacturing. These systems enable a process or machine to run automatically as desired without the need for continuous human intervention.

What Is a Control System?

A control system is a circuit or mechanism that regulates or controls the behavior of a system to achieve a desired output. This system works by observing system conditions, comparing them to target values (setpoints), and taking corrective action if there are any differences.

Open Loop

An open-loop control system works without using information from the output to change the input. In other words, this system issues commands directly without observing the results or responses.

Example of an Open-Loop Control System:

An electric oven with a timer: The oven will run for a specified time, regardless of the actual temperature. If the temperature is too low or too high, the system will not correct because there is no feedback.

Advantages of an Open Loop System:

• Simple and inexpensive
• No feedback sensors required

Disadvantages:

• Inaccurate due to lack of correction
• Sensitive to disturbances or environmental changes

Closed-Loop Control System

A closed-loop control system uses feedback from the output to regulate the input. This means the system monitors the results and automatically adjusts to meet the desired output.

Example of a Closed-Loop Control System:

An air conditioner that regulates room temperature: A temperature sensor monitors room conditions and turns the air conditioner on or off to maintain a stable temperature at the setpoint.

Advantages of a Closed-Loop System:

• High accuracy and adaptability to changes
• Can correct errors and disturbances

Disadvantages:

• More complex and expensive system
• Requires sophisticated sensors and controls

2. Control System Components: Sensors, Actuators, and Controllers

Automatic control systems consist of several main components that work together to control processes.

2.1 Sensors

Sensors function as measuring devices that detect physical variables in the system, such as temperature, pressure, position, speed, and others. Sensors convert these physical quantities into electrical signals that can be processed by the controller.

Types of sensors:

• Temperature sensors (thermocouples, RTDs)
• Pressure sensors
• Position sensors (encoders, potentiometers)
• Speed sensors (tachometers)
• Light, humidity sensors, and others
• Sensors act as the "eyes" of the control system, providing real-time information about the state of the process.

2.2 Actuators

Actuators are devices that perform physical actions in the system based on commands from the controller. Actuators convert electrical signals into mechanical movement or other physical changes.

Examples of actuators:

• Electric motors (servo, stepper, DC motors)
• Solenoid valves
• Pumps
• Heaters or heating elements
• Pneumatic and hydraulic actuators
• Actuators are the "arms" of a control system that execute commands.

2.3 Controller

The controller is the brain of the control system. It receives signals from sensors, compares them to the setpoint, and calculates corrective actions to regulate the actuators.

Types of controllers:

• On-Off Controller (simple switching)
• Proportional (P) Controller
• Proportional-Integral (PI) Controller
• Proportional-Integral-Derivative (PID) Controller
• The controller's job is to maintain a stable system output that meets the target.

3. Introduction to PID Controllers: Concept and Implementation

PID controllers are the most widely used type of controller in industry due to their ability to regulate systems stably and responsively.

What is a PID Controller?

PID stands for three control actions that work simultaneously:

• Proportional (P): Produces an output proportional to the current error. If the error is large, the control output is also large.
• Integral (I): Calculates the accumulated error over time, thereby eliminating offsets or steady-state errors.
• Derivative (D): Anticipates error changes by taking into account the rate of change of error, thereby increasing stability and reducing overshoot.

PID Controller Formula

The control output  u(t) is usually formulated as:

Where:

• e(t) = error (difference between setpoint and actual value)
• Kp = proportional constant
• 𝐾𝑖 = integral constant
• 𝐾𝑑 = derivative constant

Functions of Each PID Component

• Proportional: Reduces error directly, but does not always eliminate it completely.
• Integral: Eliminates residual error by taking into account the sum of errors over time.
• Derivative: Reduces overshoot and speeds up response by taking into account the trend of error changes.

PID Controller Implementation

PID controllers can be implemented in:

• Analog: Using electronic components such as op-amps and resistors.
• Digital: Using a microcontroller or PLC with special programming.

PID Tuning

The process of adjusting the parameters 

Kp​, ​Ki​, and ​Kd is called tuning. Tuning aims to achieve the best system performance: fast response, minimal overshoot, and stability.

Popular tuning methods:

• Ziegler-Nichols Method
• Cohen-Coon Method
• Trial and error with simulation

4. Examples of Automatic Control Applications in Industry

Automatic control systems are widely applied in various industrial sectors to improve production efficiency, safety, and quality.

4.1 Temperature Control in Industrial Processes

In industries such as food, chemical, and metal manufacturing, temperature control is crucial. PID systems control heaters or coolers to maintain the target process temperature.

Example: Industrial ovens producing electronic components must maintain a stable temperature to prevent component damage.

4.2 Liquid Level Control

In the oil, chemical, or water treatment industries, maintaining liquid levels in tanks is crucial to prevent spills or shortages.

The system uses level sensors and valve actuators to automatically fill or discharge liquids.

4.3 Motor Speed Control

In conveyors, cranes, or industrial robots, the speed of electric motors is controlled to ensure smooth process operation.

Variable Frequency Drives (VFDs) and PID controllers are often used to regulate motor speed and torque as needed.

4.4 Robotic Control and Factory Automation

Industrial robots use complex, closed-circuit control systems with sensors and actuators to precisely perform tasks such as welding, painting, and assembly.

Automated control systems are also used in automated production lines to increase speed and product consistency.

4.5 Chemical Process Control

Control systems regulate critical variables such as temperature, pressure, pH, and material flow in chemical reactors to ensure optimal and safe reactions.

Conclusion

Automatic control systems are a critical technology that enables industrial processes and machine tools to run effectively and efficiently. The main difference between open-loop and closed-loop systems is the presence of feedback in a closed system that allows for automatic correction. Control systems consist of sensors, actuators, and controllers working together.

PID controllers are the most common control method due to their ability to accurately regulate systems by utilizing three control actions: proportional, integral, and derivative. They are widely used in various industries, from temperature control to robotics.