Control System Fundamentals: Types and Components

Control Systems Defined

A control system is a set of interconnected elements that allows a change in a desired variable by manipulating another variable.

Open-Loop Control Systems

In open-loop control systems, the output signal does not affect the control action.

Consider a simple open-loop control system. Its operation is straightforward: when an input signal directs the control element to respond, an output is produced. The primary drawback of an open-loop control system is its inability to make automatic adjustments.

Closed-Loop Control Systems

In closed-loop control systems, the output information from the system affects the control action. The reference value is compared to the output signal, and based on any detected difference, the system takes appropriate action to compensate. Therefore, a closed-loop system is always equipped with a sensor, which monitors the output and compares it with the expected result.

Types of Control Systems by Mechanism

Mechanical Control Systems

These systems use mechanical elements (such as pulleys, gears, cogwheels, cams, and levers) to transmit movement and execute the control action. Examples include cisterns and non-electric watches.

Electromechanical Control Systems

These systems use electrical components such as switches, push-button switches, and relays to control the current flow that feeds a motor or a lightbulb.

Electronic Control Systems

These systems use electronic components such as diodes, transistors, and various resistors (e.g., LDR, NTC, PTC). Electronic systems are among the most developed control systems.

Key Control System Components

Input Signal

The input signal is the reference value, representing what we want the system to achieve or provide.

Comparator

A comparator is a device that compares the output signal (measured by the sensor) to the input signal. Based on this difference, the comparator produces an Error Signal and sends it to the controller.

Controller

A controller is a device that reads the error signal and acts to resolve it. If there is no difference between the two signals, the controller does not send an action. However, if there is a difference between the input and output values, the controller receives the error signal from the comparator and sends an instruction to the actuator to correct the difference.

Actuator

An actuator is a device that acts on a machine or process, modifying its operation following the controller's instructions. When an error is detected in the system's output signal, the actuator receives and executes instructions from the controller to make the system produce the desired output.

Process or System

This refers to all the elements that participate in the control action.

Output Signal

The output signal is the value that the system provides, representing the result of the controlled action.

Feedback Loop

A feedback loop determines that the output signal affects the control action, as it sends information to the comparator to be compared with the input signal.

Sensor

A sensor detects variations in the output signal and, thanks to the feedback loop, sends this information to the comparator.

Transducer

When the output signal is not the same variable as the reference value, a transducer is needed. This device translates one type of variable into another. For example, in a thermostat that controls the temperature of a room, the reference signal is the mechanical position of a dial with which we set the desired temperature.