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Sep 9 2019

Classification of Temperature Sensor and Application


Temperature, as one of the seven basic quantities of the international unit system, is a variety of sensors for measuring temperature. The temperature sensor is the core part of the temperature measuring instrument. In a word, the temperature sensor is a sensor that can sense temperature and convert to an available output signal. With the support of semiconductor technology, semiconductor thermocouple sensors, PN junction temperature sensors and integrated temperature sensors have been developed in this century. With the support of material technology, ceramics, organic, nano and other new materials can be used in temperature sensors to make the measurement and control of temperature more scientific and accurate. Due to the reasonable cooperation of the software and the hardware of the intelligent temperature sensor, the function of the sensor can be greatly enhanced, the accuracy of the sensor can be improved, and the structure of the temperature sensor can be more simple and compact, and it is more convenient to use. Therefore, intelligent temperature sensors are a research hotspot today. The introduction of the microprocessor makes the integration of temperature signal acquisition, memory, storage, synthesis, processing and control, and the temperature sensor develops toward the intelligent direction.

Article Core



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I The Overview of Temperature Sensor

1.1 Temperature

1.2 Terminology

II Classification of Temperature Sensor

2.1 Classification with Measurement Mode

2.2 Classification with Characteristics of Sensor Materials and Electronic Components

2.3 Classification with Signal Output Mode

III Development of Temperature Sensor

3.1 Analog integrated Temperature Sensor

3.2 Intelligent Temperature Sensor

IV Applications of temperature Sensor

4.1 Application in Automobile

4.2 Application in Life

I The Overview of Temperature Sensor

  1.1 Temperature

Temperature is a physical quantity used to characterize the degree of heat and cold of an object. The temperature should be quantified by numbers, and the temperature scale is the numerical representation of temperature. The commonly used temperature scales are centigrade temperature scale and thermodynamic temperature scale. The Celsius temperature scale is set to 100 degrees Celsius at standard atmospheric pressure, the temperature of the ice-water mixture is set to 0 degrees Celsius, and 100 equal parts between 100 degrees Celsius and 0 degrees Celsius,each of which is 1 degree Celsius. The thermodynamic temperature scale was put forward by William Tom. Based on the second law of thermodynamics, the thermodynamic temperature scale, which is only related to the amount of heat and independent of matter, is established. Because it is summed up by Calvin, it is also known as the Calvin temperature scale.

  1.2 Terminology

The temperature sensor converts the temperature into the available output signal by using the law that the physical properties of the substance change with the temperature. Like all sensors, the temperature sensor converts the change in the physical vehicle to a readable measure indicative of a change. For example, in a mercury thermometer, the change in external heat causes the liquid mercury in the glass body to expand or contract, thereby rising or falling in a thin tube marked with a temperature scale, wherein the temperature scale changes linearly with the ambient heat energy. The sphere containing mercury is the temperature sensor of the thermometer, and the scale along the length of the glass tube is readable. Temperature sensors play a critical role in the wide range of domestic and commercial products. In household appliances, its ensure that ovens, refrigerators and central air-conditioning thermostats function correctly, keeping the temperature in a certain range, starting refrigeration or heating components whenever the temperature exceeds the set range, and adjusting the ambient temperature back to a specific steady-state level. In industrial applications, for example, in chemical engineering, its need sufficient sensitivity to detect subtle changes in temperature to properly control the chemical reaction. All temperature sensors respond to thermodynamic changes: with the increase of heat energy, the more intense the molecular movement, the expansion of the system or medium and the increase of temperature.

II Classification of Temperature Sensors

According to the measurement mode, it can be divided into contact and non-contact, and can be divided into thermal resistance and thermocouple according to the characteristics of sensor materials and electronic components. It can be divided into thermocouples, thermistors, resistance temperature detectors (RTDs) and IC temperature sensors. IC temperature sensors include analog output and digital output. The development of the temperature sensor has experienced the following three stages: the original conventional discrete temperature sensor (including the sensitive element), mainly capable of switching between non-electric quantity and electric quantity. Analog integrated temperature sensors / controllers appeared in the 1980 s. And then to the hot research of intelligent temperature sensors today. At present, the new type of temperature sensor in the world is developing from analog to digital, integrated to intelligent and networked.

  2.1 Classification with Measurement Mode

    2.1.1Contact Temperature Sensor

The temperature measuring element of the contact temperature sensor is in good thermal contact with the object to be tested, also known as a thermometer. The heat balance is achieved by the principle of heat conduction and convection, and the indication value is the temperature of the measured object. This method has high accuracy and can measure the temperature distribution inside the object. However, for the objects with small thermal capacity and corrosion to the temperature sensing elements, the method will produce great errors. The main features of this type of temperature measurement method are small targets that can measure motion state and objects with small or rapid heat capacity can measure the temperature distribution of the temperature field, but it is greatly affected by the environment.

The commonly used contact thermometer is bimetallic thermometer, glass liquid thermometer, pressure thermometer, resistance thermometer, thermistor and thermocouple. They are widely used in the fields of industry, agriculture, commerce and so on, and people often used in daily life. With the application of low-temperature technology in the fields of national defense engineering, space technology, metallurgy, electronics, food, medicine and petrochemical industry, the low-temperature thermometer with the temperature below 120K has been developed, such as low-temperature gas thermometer and vapor pressure thermometer. An acoustic thermometer, a paramagnetic salt thermometer, a quantum thermometer, a low-temperature thermal resistor, a low-temperature thermocouple, and the like. Low temperature thermometer requires small volume, high accuracy, good reproducibility and stability of temperature sensing elements.

  Solid Map of Contact Temperature Sensor

Figure 1 Solid Map of Contact Temperature Sensor

    2.1.2 Non-contact Temperature Sensor

The sensitive element of the contactless temperature sensor is not in contact with the measured object, also known as the contactless thermometer. This instrument can be used to measure the surface temperature of moving objects, small targets and objects with small heat capacity or rapid (transient) temperature change, and can also be used to measure the temperature distribution of temperature fields. The most commonly used non-contact thermometer is based on the basic law of blackbody radiation, so it is called radiation thermometer. The upper limit of the measurement of the non-contact temperature sensor is not limited by the temperature resistance of the temperature sensing element, so that the maximum measurable temperature is not limited in principle. For the high temperature above 1800 ℃, the non-contact temperature measurement method is mainly used. With the progress of infrared technology, radiation temperature measurement gradually extends from visible light to infrared ray, which has been used below 700 ℃ to room temperature, and the resolution is very high.

Radiation thermometer includes luminance method (optical pyrometer), radiation method (radiation pyrometer) and colorimetric method (colorimetric thermometer). All kinds of radiation temperature measurement methods can only measure the corresponding photometric temperature, radiation temperature or colorimetric temperature. Only the temperature measured on blackbodies (objects that absorb all radiation and do not reflect light) is the real temperature. If you want to determine the true temperature of the object, you must correct the surface emissivity of the material. However, the surface emissivity of a material depends not only on temperature and wavelength, but also on surface state, film and microstructure, and thus is difficult to measure accurately. In automatic production, it is often necessary to use radiation temperature measurement to measure or control the surface temperature of certain objects, such as steel strip rolling temperature in metallurgy, roll temperature, forging temperature and the temperature of various molten metals in smelting furnaces or crucibles. In these case, that measurement of the surface emissivity of the object is quite difficult. For automatic measurement and control of solid surface temperature, additional mirrors can be used to form blackbody cavity together with the measured surface. The effect of additional radiation can improve the effective radiation and effective emission coefficient of the measured surface. And the effective emission coefficient is used for correcting the measured temperature through the instrument, and the real temperature of the surface to be measured can be finally obtained.

Non-contact Temperature Sensor 

Figure 2 Non-contact Temperature Sensor

  2.2 Classification with Characteristics of Sensor Materials and Electronic Components

    2.2.1Thermal Resistance Sensor

The thermal resistance measures the temperature by using the characteristic that the resistance of the substance changes with the change of temperature. The heating part of the thermal resistance (temperature sensing element) is a skeleton made of fine wire evenly wrapped around the insulating material. When the temperature in the measured medium changes, the measured temperature is the average temperature in the medium in which the temperature is sensitive. If the resistance value rises as the temperature rises, it is called the positive resistivity. If the temperature rises, the resistance value decreases, which is called the negative resistivity. Most resistive temperature sensors are made of metal, so for different metals, the change in resistance varies for every degree of temperature change. At present, platinum and copper are the most used, and the thermal resistance is now started with materials such as nickel, manganese and iron. The thermal resistance usually requires the transfer of the resistance signal through the lead to the computer control or other disposable instrument. Compared with other sensors, the thermal resistance has the following characteristics:

A. High accuracy up to 1 mk

B. Strong output signal and high sensitivity;

C. Wide temperature range and good stability. In a small and suitable environment, it can maintain stability below 0.1 degrees Celsius for a long time;

D. The temperature value can be directly obtained by the measured resistance value without a reference point;

E. With good output linearity, the linear output can be obtained only by simple auxiliary circuit, and the display instrument can be evenly scaled.

Structure of Resistance Temperature Sensor 

Figure 3 Structure of Resistance Temperature Sensor

    2.2.2 Thermocouple

It is a traditional separation type sensor, which is one of the most widely used temperature sensors in industrial measurement. The working principle is: according to the Seebeck effect in physics, that is, in the circuit formed by two metal conductors, if their contacts maintain different temperature, the electromotive force corresponding to the temperature difference is generated in the circuit. The thermocouple has the advantages of simple structure, wide temperature range, fast response, accurate measurement and good reproducibility. In addition, the temperature in the microzone can also be measured by using fine coupling wire, and there is no need for power supply.

Thermocouple temperature sensor 

Figure 4 Thermocouple temperature sensor

The thermocouple consists of two metal wires of different materials that are welded together at the end. The ambient temperature of the unheated part can be measured again, and the temperature of the hot spot can be accurately get. Because it must have two conductors of different materials, it is called a thermocouple. In general, thermocouples made of different materials are used in different temperature ranges, and their sensitivity is different. The sensitivity of thermocouple refers to the change of output potential difference when the hot spot temperature is changed by 1 ℃. For thermocouples supported by most metal materials, this value is about 5 × 40 microvolts / ℃. Since the sensitivity of the thermocouple is independent of the thickness of the material, the temperature sensor can be made with very fine material. Also because of the good ductility of the metal materials used to make thermocouples, this subtle temperature measuring element has a very high response speed and can measure the rapidly changing process.

The Structure of Thermocouple 

Figure 5 The Structure of Thermocouple

   2.3 Classification with Signal Output Mode

    2.3.1Analog Temperature Sensors

The analog temperature sensor has a variety of output forms (absolute temperature, Celsius temperature and Fahrenheit temperature) and voltage offset. The latter allows the component to monitor the negative temperature value with a single power supply. The output of the analog temperature sensor can also be sent to the comparator to generate the overtemperature indication signal, or directly to the the analog-to-digital converter, which can be used to display real-time temperature data. So analog temperature sensors are suitable for applications that require low cost, small volume and low power consumption.

    2.3.2Digital temperature sensors

The demand for closer control, higher precision and greater resolution has led to the development of digital temperature sensors. The measured temperature signal is converted from the non-electrical quantity received by the sensitive component to the digital signal that can be processed by the microprocessor, and there are many links, and the analog signal is subjected to more interference and large error in the process of long-distance transmission. Therefore, from non-electric conversion to digital signal, the processing process is generally integrated into the internal part of a single IC device, thus a powerful and accurate digital sensor was formed. Compared with analog sensor, digital sensor has obvious advantages in reliability, anti-interference ability and device miniaturization due to its high integration design and digital processing. However, due to the limitation of semiconductor device itself, the digital sensor still has the following unsatisfactory places.

III Development of temperature Sensor

In recent ten years, in order to meet the requirements of high precision, extreme conditions, miniaturization and high integration on chip, analog integrated temperature sensor and intelligent temperature sensor have been researched and developed.

  3.1 Analog Integrated Temperature Sensor

The integrated sensor is made using a silicon semiconductor integrated process, so it is also called silicon sensor or single chip integrated temperature sensor. Analog integrated temperature sensor came out in 1980s. It integrates temperature sensor on a chip and can complete the functions of temperature measurement and analog signal output. The main characteristics of analog integrated temperature sensor are single function (measuring temperature only), small temperature measurement error, low price, fast response speed, long transmission distance, small volume, micro power consumption and so on. It is suitable for long distance temperature measurement and does not need to be non- Linear calibration. The integrated temperature sensor has a distinct advantage over conventional discrete temperature sensors. Most of the traditional temperature sensors are based on discrete temperature sensors, such as pt100, which require certain peripheral circuits to measure. But for today's highly integrated electronic products, this is obviously not practical and convenient. The integrated temperature sensor, of course, also has the inherent disadvantages to the discrete sensors: the measurement accuracy is limited by process fluctuations. Although the literature of CMOS integrated temperature sensor with high precision has been published in academic circles (±0.1 °C) [5], with the high cost of TRIM, it can not be fully applied to the actual large-scale mass production. Considering the need for practical applications, not all applications need to achieve such accuracy,

  3.2 Intelligent temperature sensor

Intelligent temperature sensor (also known as digital temperature sensor) is combined with various microprocessors to connect to the network, and the intelligent theory(artificial intelligence technology, neural network technology, fuzzy technology, etc.) is used to process the sampled data for forming an intelligent temperature sensor with signal processing, temperature control, logic function and other functions.

Intelligent temperature sensors were introduced in the mid-1990s. It is the crystallization of microelectronic technology, computer technology and automatic test technology(ATE). At present, a variety of intelligent temperature sensor products have been developed internationally. The intelligent temperature sensor internally includes temperature sensor, A / D converter, signal processor, memory (or register) and interface circuit.  And some products also come with multi-channel selector, central controller (CPU), random access memory (RAM) and read-only memory (ROM). The intelligent temperature sensor can output temperature data and related temperature control quantity, adapt to various microcontrollers (MCU), and can realize the test function through software. Its intelligence depends on the development level of the software. Here are some new trends in development of intelligent temperature sensors:

A. Improving the accuracy and resolution of temperature measurement

B. Adding test function

C. Standardization and Standardization of bus Technology

D. Reliability and safety design

E. Virtual temperature Sensor and Network temperature Sensor

IV Application of Temperature Sensor

  4.1 Application in Automobile

The function of temperature sensor is to measure the temperature of engine intake, cooling water, fuel, etc., and to convert the measured results into electrical signals for transmission to ECU. For all gasoline engine electronic control systems, intake air temperature and coolant temperature are the two temperature parameters necessary for the ECU to control, while the other temperature parameters vary with the type and control needs of the electronic control system.

The intake air temperature sensor is usually installed in the air flow meter or the air inlet or air flow meter from the air cleaner to the throttle body, and The water temperature sensor is arranged in an appropriate position above the engine, cylinder head, or block. The sensors that can be used to measure temperature are winding resistance type, diffusion resistance type, semiconductor transistor type, metal core type, thermocouple type and semiconductor thermistor type. at present, the most widely used in the measurement of intake temperature and coolant temperature are thermistor temperature sensors.

Intake Air Temperature Sensor 

Figure 6 Structure of Intake Air Temperature Sensor

Figure 7 The Relationship Between Resistance and Temperature

  4.2 Application in Life

Using temperature Sensor to adjust the temperature of toilet. The temperature sensor can also adjust the temperature in the bathroom, especially when taking a bath, it is necessary to automatically adjust the temperature in the bathroom. The temperature and humidity sensor and the gas sensor can control the environment in the bathroom well so that we can have a comfortable life. Nowadays, most hotels and some public places have realized automatic adjustment, while the toilets of ordinary households are still manually operated. The main reason for not achieving automatic adjustment is that the average customer does not know that they can use sensors to achieve automation. With the further understanding of people in the future, the bathroom of the ordinary family can also be automatically adjusted.

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