What are the common sensors and how do they work?

What is a sensor?

Miniaturization, digitalization, intelligence, multi-functionalization, systematization, and networking. It is the first link to realize automatic detection and automatic control. The existence and development of sensors allow objects to have senses such as touch, taste, and smell, allowing objects to slowly become alive. It is usually divided into ten categories: thermal elements, photosensitive elements, gas-sensitive elements, force-sensitive elements, magnetic sensitive elements, moisture-sensitive elements, acoustic components, radiation-sensitive elements, color-sensitive elements and taste-sensitive elements according to their basic sensing functions.

What are the common sensors and how do they work?

Temperature sensor

A temperature sensor is a sensor that senses the temperature and converts it into a usable output signal. Temperature sensors are the heart of temperature measuring instruments and come in a wide variety of varieties. According to the measurement method, it can be divided into two categories: contact and non-contact, and according to the characteristics of sensor materials and electronic components, it can be divided into two categories: thermal resistance and thermocouple.

Contact

The detection part of the contact temperature sensor has good contact with the measured object, also known as the thermometer. The thermometer achieves thermal equilibrium by conduction or convection, so that the thermometer's indication directly represents the temperature of the object being measured.

In general, the measurement accuracy is high. The carburized glass thermal resistance made by carburizing and sintering porous high silica glass is a kind of temperature sensing element of the cryogenic thermometer, which can be used to measure the temperature in the range of 1.6-300K.

Contactless

Its sensitive elements are not in contact with the measured object, also known as non-contact temperature measuring instrument. This instrument can be used to measure the surface temperature of moving objects, small targets, and objects with small heat capacity or rapid temperature changes (transients), as well as to measure the temperature distribution of the temperature field. The most commonly used non-contact temperature measuring instruments are based on the basic law of blackbody radiation, known as radiometric thermometers. Radiometric thermometry includes luminance (see Optical pyrometers), radiometry (see Radiation pyrometers), and colorimetric methods (see Colorimetric thermometers).

Advantages of non-contact temperature measurement

The upper limit of measurement is not limited by the temperature resistance of the temperature sensor, so there is no limit to the maximum measurable temperature in principle. For high temperatures above 1800°C, non-contact temperature measurement methods are mainly used. With the development of infrared technology, radiation temperature measurement has gradually expanded from visible light to infrared, and has been used below 700 °C until room temperature, and the resolution is very high.

How it works

Resistance sensing

As the temperature of a metal changes, so does its resistance value.

For different metals, the resistance value changes differently for each degree of temperature change, and the resistance value can be used directly as an output signal.

There are two types of variations in resistors

Positive temperature coefficient

An increase in temperature = an increase in resistance

Lower temperature = less resistance

Negative temperature coefficient

Increased temperature = decreased resistance

Decrease in temperature = increase in resistance

Thermocouple sensing

A thermocouple consists of two metal wires of different materials that are welded together at the ends. By measuring the ambient temperature of the unheated part, the temperature of the heating point can be accurately known. Since it must have two conductors of different materials, it is called a thermocouple. Thermocouples made of different materials are used in different temperature ranges, and their sensitivity is also different. The sensitivity of a thermocouple is the amount by which the output potential difference changes when the temperature of the heating point changes by 1°C. For most metal-supported thermocouples, this is between 5 and 40 μV/°C.

Selection method

thermocouple

Thermocouples are the most commonly used temperature sensors for temperature measurements. Its main benefits are a wide temperature range and adaptability to a wide range of atmospheric environments, as well as being robust, inexpensive, without the need for power supply, and the cheapest. A thermocouple consists of two different metal wires (metal A and metal B) connected at one end, and when one end of the thermocouple is heated, there is a potential difference in the thermocouple circuit. The temperature can be calculated using the measured potential difference.

Thermistor

Thermistors are made of semiconductor materials, and most of them have a negative temperature coefficient, that is, the resistance value decreases with the increase of temperature. Temperature changes can cause large resistance changes, so it is the most sensitive temperature sensor. However, the linearity of thermistors is extremely poor and has a lot to do with the production process. Manufacturers can't give a standardized thermistor curve.

Accelerometers

An accelerometer is a type of sensor that is capable of measuring acceleration. It is usually composed of mass, damper, elastic element, sensitive element and adaptation circuit. During the acceleration process, the sensor uses Newton's second law to obtain the acceleration value by measuring the inertial force on the mass. Depending on the sensor sensing element, common accelerometers include capacitive, inductive, strain, piezoresistive, piezoelectric, etc.

classify

Piezoelectric

Piezoelectric accelerometers are also known as piezoelectric accelerometers. It is also an inertial sensor. The principle of piezoelectric accelerometers is to use the piezoelectric effect of piezoelectric ceramics or quartz crystals, and when the accelerometer is vibrate, the force of the mass on the piezoelectric element also changes. When the measured vibration frequency is much lower than the natural frequency of the accelerometer, the change in force is proportional to the measured acceleration.

Piezoresistive

Based on the world's leading MEMS silicon micromachining technology, piezoresistive accelerometers have the characteristics of small size and low power consumption, and are easy to integrate in various analog and digital circuits, and are widely used in automotive crash experiments, test instruments, equipment vibration monitoring and other fields.

Capacitive

Capacitive accelerometers are capacitive sensors with varying pole distance based on the capacitance principle. Capacitive accelerometers/capacitive accelerometers are more versatile accelerometers. There are no substitutes in some areas, such as airbags, mobile phones, mobile devices, etc. Capacitive accelerometers/capacitive accelerometers use a microelectromechanical systems (MEMS) process that is economical when produced in large quantities, thus guaranteeing low costs.

Servo-type

The servo accelerometer is a closed-loop test system with good dynamics, large dynamic range and good linearity. Its working principle, the vibration system of the sensor is composed of "m-k" system, the same as the general accelerometer, but the mass m is also followed by an electromagnetic coil, when there is acceleration input on the base, the mass deviates from the equilibrium position, the displacement magnitude is detected by the displacement sensor, and is converted into current output after amplification by servo amplifier, and this current flows through the electromagnetic coil, produces electromagnetic restoring force in the magnetic field of the permanent magnet, and tries to keep the mass in the original equilibrium position in the instrument housing, So the servo accelerometer works in a closed-loop state.

Fiber optic sensors

The basic working principle of the optical fiber sensor is to send the optical signal from the light source to the modulator through the optical fiber, so that the parameters to be measured interact with the light entering the modulation area, resulting in the optical properties of the light (such as the intensity, wavelength, frequency, phase, polarization state, etc.) of the light change, becoming the modulated signal source, and then sent to the optical detector through the optical fiber, after demodulation, the measured parameters are obtained.

principle

The basic working principle of the optical fiber sensor is to send the light from the light source to the modulator through the optical fiber, so that the parameters to be measured interact with the light entering the modulation area, resulting in a change in the optical properties of the light (such as the intensity, wavelength, frequency, phase, polarization state, etc.) of the light, which is called the modulated signal light, and then the influence of the measured on the transmission characteristics of the light is used to complete the measurement.

How fiber optic sensors work

(1) Functional - made by using a certain sensitive feature or function of the optical fiber itself

(2) Light transmission type - the optical fiber only plays the role of transmitting light, and it is equipped with other sensitive components on the end face or middle of the optical fiber to feel the changes to be measured.

peculiarity

High sensitivity;

The geometry has multifaceted adaptability, and can be made into arbitrarily shaped fiber optic sensors;

It can manufacture devices that can sense various different physical information (sound, magnetism, temperature, rotation, etc.);

It can be used in high voltage, electrical noise, high temperature, corrosion, or other harsh environments;

It is also intrinsically compatible with fiber optic telemetry.

What are the common sensors and how do they work?

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