A sensor is a detection device, a tool for acquiring information, or a machine for transmitting sensation. In its broadest definition, a sensor is a device, module, or subsystem commonly used in automatic control and measurement systems. A sensor can be explained as a device or apparatus that can convert a measured physical quantity (such as velocity, temperature, sound, light, etc.) into another physical quantity (usually an electrical quantity) that is easier to transmit and process. Sensors are commonly known as probes, and are sometimes also called transducers, converters, transmitters, or detectors. The national standard GB7665-87 defines a sensor as: a device or apparatus that can sense a specified measurand and convert it into a usable output signal according to a certain law, usually composed of a sensing element and a conversion element.
Sensors generally consist of a sensing element, a conversion element, a signal conditioning circuit, and auxiliary circuits. However, not all sensors must include both a sensing element and a conversion element. If the sensing element directly outputs an electrical quantity, it also functions as a conversion element. Sensing element (pre-converter): This refers to the part of the sensor that directly senses or responds to the measured quantity (non-electrical quantity) and outputs another quantity (non-electrical quantity) that has a definite relationship with it. Conversion element: This refers to the part of the sensor that converts the measured quantity sensed or responded to by the sensing element into a usable output signal (usually an electrical signal) suitable for transmission or measurement. Signal conditioning circuit: This is the circuit that converts the electrical signal output by the conversion element into a useful electrical signal that is easy to display, record, process, and control. Auxiliary circuit: This usually refers to the power supply, i.e., the AC or DC power supply system.
Sensors play a crucial role in the Internet of Things (IoT). They create an ecosystem for collecting and processing data about specific environments, making them easier, more efficient, and manageable. IoT sensors are used in homes, the field, automobiles, aircraft, industrial environments, and others. Sensors bridge the gap between the physical and logical worlds, acting as the eyes and ears of computing infrastructure, analyzing the data collected from sensors and processing that data.
Sensors can be classified in several ways. One common approach is to categorize them as active or passive. An active sensor is one that requires an external power source to respond to environmental inputs and generate an output. For example, sensors used in weather satellites typically require some power to provide meteorological data about the Earth's atmosphere.
On the other hand, passive sensors do not require an external power source to detect environmental input. They rely on the energy of the environment itself, using energy sources such as light or heat. A good example is a mercury glass thermometer. Mercury expands and contracts with temperature fluctuations, causing the liquid level in the glass tube to rise or fall. External markings provide a human-readable instrument for viewing the temperature.
Some types of sensors, such as earthquake and infrared sensors, come in both active and passive forms. The environment in which the sensor is deployed typically determines which type is best suited for the application.
Another way to classify sensors is based on the type of output they produce: whether it's analog or digital. Analog sensors convert environmental input into a continuously changing analog output signal. Thermocouples used in gas water heaters are a good example of analog sensors. An indicator light on the water heater continuously heats the thermocouple. If the indicator light goes out, the thermocouple cools down and sends a different analog signal, indicating that the gas should be shut off.
Unlike analog sensors, digital sensors convert environmental inputs into discrete digital signals transmitted in binary format (1s and 0s). Digital sensors have become quite prevalent in all industries, replacing analog sensors in many cases. For example, digital sensors are now used to measure humidity, temperature, atmospheric pressure, air quality, and many other types of environmental phenomena.
Similar to active and passive sensors, some types of sensors (such as thermal or pressure sensors) come in both analog and digital forms. In this case, the environment in which the sensor operates usually determines which is the optimal choice.
Sensors are characterized by miniaturization, digitization, intelligence, multifunctionality, systematization, and networking. They are the primary link in achieving automatic detection and control. The existence and development of sensors have given objects senses such as touch, taste, and smell, gradually bringing them to life. They are typically classified into ten major categories based on their basic sensing functions: thermal sensors, photosensors, gas sensors, force sensors, magnetic sensors, humidity sensors, acoustic sensors, radiation sensors, color sensors, and taste sensors.
