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Temperature sensors are widely used and numerous, ranking first among all kinds of sensors. The development of temperature sensors has roughly gone through the following three stages:
1. Traditional discrete temperature sensors (including sensitive components) mainly convert between non-electric quantity and electric quantity. 2. Analog integrated temperature sensor/controller.
3. Intelligent temperature sensor. At present, the new temperature sensor in the world is developing from analog to digital and integrated to intelligent and networked.
Classification of temperature sensors
According to the contact mode between the sensor and the measured medium, temperature sensors can be divided into two categories: one is contact temperature sensor, and the other is non-contact temperature sensor.
The temperature measuring element of the contact temperature sensor should have good thermal contact with the measured object, and the thermal balance is achieved through the principles of heat conduction and convection, which is the indicated value of the measured object. This temperature measurement method has high accuracy and can measure the temperature distribution inside the object. But for moving objects with small heat capacity and corrosive effect on temperature sensing elements, this method will produce great errors.
The temperature measuring element of non-contact temperature measurement is not in contact with the measured object. The principle of radiant heat exchange is usually used. The main feature of this stability measurement method is that it can measure small moving targets and objects with small heat capacity or rapid change, and it can also measure the temperature distribution of temperature field, but it is greatly affected by the environment.
Development of temperature sensor
1. Traditional discrete temperature sensor-thermocouple sensor
Thermocouple sensor is the most widely used temperature sensor in industrial measurement, which is in direct contact with the measured object and is not affected by the intermediate medium, and has high accuracy. Wide measuring range, continuous measurement from -50~ 1600℃. Special thermocouples, such as gold-iron-nickel-chromium, can measure as low as -269℃ and tungsten-rare earth can measure as high as 2800℃.
2. Analog integrated temperature sensor
Integrated sensor is made of silicon semiconductor integrated process, so it is also called silicon sensor or monolithic integrated temperature sensor. Analog integrated temperature sensor came out in 1980s. It integrates the temperature sensor on one chip, which can complete the functions of temperature measurement and analog signal output.
The main characteristics of analog integrated temperature sensor are single function (only measuring temperature), small temperature measurement error, low price, fast response, long transmission distance, small volume and low power consumption. Non-linear calibration is not needed, the peripheral circuit is simple, and it is suitable for long-distance temperature measurement.
2. 1 optical fiber sensor
Principle of optical fiber temperature measurement
Optical fiber temperature measurement technology can be divided into two categories: one is based on the principle of radiation measurement, which uses optical fiber as a conductor to conduct luminous flux and uses photosensitive elements to form a structural sensor; Second, the optical fiber itself is a temperature sensing element and a functional sensor for transmitting luminous flux. Optical fiber has good flexibility, wide transmission spectrum, low transmission loss, convenient local use or remote transmission, small fiber diameter, simple structural arrangement and small volume, and can be used in single beam, bundle, Y shape or array mode. Therefore, as a thermometer, it can be applied to almost all kinds of detection objects, and can be used in special occasions where other thermometers are difficult to apply, such as sealing, high voltage, strong magnetic field, nuclear radiation, strict explosion-proof, waterproof, anti-corrosion, extra-small space or extra-small workpiece. At present, optical fiber temperature measurement technology mainly includes total radiation temperature measurement, single radiation temperature measurement, dual-wavelength temperature measurement and multi-wavelength temperature measurement.
2. 1. 1 total radiation thermometry
Total radiation thermometry is to measure the radiation energy of all bands, which is determined by Planck's law:
In the measurement, the radiation of the surrounding background, the test distance, the absorption, emission and transmittance of the medium will seriously affect the accuracy. At the same time, the emissivity is difficult to predict. However, because of its simple structure, convenient operation, automatic measurement and wide temperature measuring range, pyrometer is generally used as a fixed target temperature monitoring device in industry. The measuring range of this kind of optical fiber thermometer is generally 600 ~ 3000℃, and the maximum error is 65438 06℃.
2. 1.2 single radiation thermometry
According to the law of blackbody radiation, the monochromatic radiation of an object at a certain temperature is a single-valued function of temperature, and the growth rate of monochromatic radiation is much faster than that of temperature rise, so the temperature information can be obtained by measuring the brightness of a single radiation. In the ordinary temperature and wavelength range, the monochromatic radiation brightness is expressed by Wien formula:
2. 1.3 dual-wavelength temperature measurement method
Dual-wavelength thermometry is to determine the temperature of an object by using the single-value relationship between the ratio of signals with two different working wavelengths and the temperature. The ratio of the two signals is given by the following formula:
In international application, after R(T) is measured, the temperature T can be obtained by looking up the table. At the same time, if λ 1 and λ2 are properly selected, so that ε(λ 1, t) and ε(λ2, t) of the measured object are approximately equal in these two specific bands, the real target temperature independent of emissivity can be obtained. This method has fast response speed, is not affected by electromagnetic induction and has strong anti-interference ability. Especially in the harsh environment such as dust and smoke, it has obvious advantages to measure the temperature of moving or vibrating objects where the target does not fill the field of view. However, it is limited in practical application because it assumes that the emissivity of the two bands is equal, which can only be satisfied by the gray body. The temperature range of this kind of instrument is generally 600 ~ 3000℃, and the accuracy can reach 2℃.
2. 1.4 Multi-wavelength radiation thermometry
Multi-wavelength radiation thermometry is to use the multi-spectral radiation measurement information of the target to process the data and obtain the real temperature and spectral emissivity of the material. Considering that the multi-wavelength pyrometer has n channels, the output signal Si of the ith channel can be expressed as:
By combining any of formulas (9) ~ (13) with formula (8), the temperature t and spectral emissivity can be obtained by fitting or solving this equation. In 1988, Coates discussed the data fitting method and accuracy of multi-wavelength pyrometer under the assumptions of equations (9) and (10). 199 1 year Mansoor[ 10] summarizes the data fitting method and accuracy of multi-wavelength pyrometer. This method has high accuracy. At present, Hiernaut and others in the European Union and the United States have developed a submillimeter 6-wavelength pyrometer (Figure 4) to measure the true temperature of 2000-5000K[ 1 1]. Harbin Institute of Technology has developed a 35-wavelength pyrometer with splitting prism to measure the real temperature of ablative materials. Multi-wavelength pyrometer shows great potential in radiation true temperature measurement. Multi-wavelength pyrometer is a promising instrument in measuring the real temperature of high temperature, ultra-high temperature, especially transient high temperature objects. This instrument has a wide temperature range and can be used to measure the true temperature in the temperature range of 600 ~ 5000℃, with an accuracy of 65438 0%.
2. 1.5 conclusion
The development of optical fiber technology provides very favorable conditions for the application of non-contact temperature measurement in production. Optical fiber temperature measurement technology solves many problems that thermocouple and conventional infrared thermometer can't solve. In the field of high temperature, optical fiber temperature measurement technology is showing more and more powerful vitality. The total radiation thermometry is to obtain the temperature by measuring the radiation energy of all bands. The radiation of surrounding background, the change of medium absorption rate and the prediction of emissivity εT will all bring difficulties to the measurement, and it is difficult to achieve high accuracy. The narrower the band, the better. However, if the bandwidth is too narrow, the energy received by the detector will become too small, which will affect its measurement accuracy. Multi-wavelength radiation thermometry is a very accurate method, but it is difficult to popularize and apply because of its complicated process and high cost. Dual-wavelength temperature measurement adopts narrow-band wavelength comparison technology, which overcomes many shortcomings of the above methods. Under very harsh conditions, such as smoke, dust, steam and particles, and the emissivity of the target surface changes, high accuracy can still be obtained.
2.2 Semiconductor absorption optical fiber temperature sensor is a light-transmitting optical fiber temperature sensor. The so-called optical fiber temperature sensor means that in the optical fiber sensing system, only optical fiber is used as the transmission path of light waves, and other sensitive elements such as optics or machinery are used to feel the change of the measured temperature. This type mainly uses step multimode fiber with large numerical aperture and core diameter. Because it uses optical fiber to transmit signals, it also has the advantages of electrical insulation, anti-electromagnetic interference and safety and explosion protection of optical fiber sensors, and is suitable for measurement in places where traditional sensors are not competent. Among these sensors, semiconductor absorption optical fiber temperature sensor is one of the more in-depth research.
Semiconductor absorption optical fiber temperature sensor is composed of semiconductor absorber, optical fiber, optical transmitter and signal processing system including optical detector. It has the advantages of small volume, high sensitivity, reliable operation, easy manufacture and no stray light loss. Therefore, it has high application value in some special occasions, such as temperature measurement of high-voltage power devices.
Temperature measurement principle of B semiconductor absorption optical fiber temperature sensor
Semiconductor absorption optical fiber temperature sensor is realized by using the characteristic that the absorption spectrum of semiconductor materials changes with temperature. According to the research, in the temperature range of 20~972K, the band gap energy Eg of semiconductor is related to the energy of semiconductor.
The relationship between temperature t is
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3. Intelligent temperature sensor
Intelligent temperature sensor (also known as digital temperature sensor) came out in the mid-1990s. It is the crystallization of microelectronics technology, computer technology and automatic test technology. At present, a variety of intelligent temperature sensor series products have been developed internationally. Intelligent temperature sensor includes temperature sensor, A/D sensor, signal processor, memory (or register) and interface circuit. Some products also include multiplexer, central controller (CPU), random access memory (RAM) and read-only memory (ROM).
Intelligent temperature sensor can output temperature data and related temperature control variables, and adapt to various microcontrollers (MCU). The testing function is realized by software, that is, intelligence depends on the development level of software.
3. 1 digital temperature sensor.
With the continuous progress and development of science and technology, there are more and more kinds of temperature sensors. Digital temperature sensors are widely used in industrial control, electronic thermometers, medical instruments and other temperature control systems, because they are suitable for automatic temperature control systems composed of various microprocessor interfaces, and can overcome the shortcomings of signal conditioning circuits and A/D converters when analog sensors interface with microprocessors. The representative digital temperature sensors include DS 1820, MAX6575, DS 1722, MAX6635, etc.
I. working principle of DS 1722
Main features of 1 and DS 1722
DS 1722 is a three-bus digital temperature sensor with low cost and low power consumption. See table 1 for its main features.
2. Internal structure of DS1722
Digital temperature sensor DS 1722 is available in 8-pin m-SOP package and 8-pin SOIC package, and its pin arrangement is shown in figure 1. It consists of four main parts: precision temperature sensor, analog-to-digital converter, SPI/ three-wire interface electronic device and data register, and its internal structure is shown in Figure 2.
At the beginning of power supply, DS 1722 is in a power-off state. After power-on, the user changes the resolution of the register to make it in continuous switching temperature mode or single switching mode. In the continuous conversion mode, DS 1722 continuously converts the temperature and stores the result in the temperature register. Reading the contents of the temperature register will not affect its temperature conversion. In single conversion mode, DS 1722 performs temperature conversion, and the result is stored in the temperature register, and then returns to off mode. This conversion mode is suitable for temperature-sensitive applications. In application, the user can set the resolution register through the program to realize different temperature resolutions. There are five resolutions: 8-bit, 9-bit,1bit,1bit or 12-bit, and the corresponding temperature resolutions are 1.0℃ and 0.5 respectively. DS 1722 has two communication interfaces: Motorola serial interface and standard three-wire interface. The user can select the communication standard through the SERMODE pin.
3, DS 1722 temperature operation method
Sensor DS 1722 converts the temperature into a digital quantity and stores it in the temperature register in two's complement format. Through SPI or three-wire interface, the data at addresses 0 1H and 02H in the temperature register can be read out. The address of output data is shown in Table 2, and the exact relationship between binary form and hexadecimal form of output data is shown in Table 3. In Table 3, it is assumed that DS 1722 is configured with 12 bit resolution. The data is transmitted continuously through the digital interface, the MSB (most significant bit) is first transmitted through SPI, and the LSB (least significant bit) is first transmitted through three wires.
4. Working procedure of DS1722
By choosing the appropriate status register address, all the working programs of DS 1722 are completed by SPI interface or three-bus communication interface. Table 4 is a register address table, showing the addresses of two registers (status and temperature) of DS 1722.
1SHOT is the single-step temperature conversion position, and SD is the closed circuit breaker position. If the SD bit is "1", continuous temperature conversion will not be performed. When 1SHOT bit is written into "1", DS 1722 performs temperature conversion, and the result is stored in address bits 0 1h(LSB) and 02h(MSB) of the temperature register. After the temperature conversion is completed, 1722 performs temperature conversion. If the SD bit is "0", it will enter the continuous conversion mode, and DS 1722 will continuously perform temperature conversion, and all the results will be stored in the temperature register. Although the data written to the 1SHOT bit is ignored, the user can still read/write the bit. If SD is changed to "1", the ongoing conversion will continue until the conversion is completed and the results are stored, and then the device will enter the low-power shutdown mode.
When the sensor is powered on, the default 1 trigger bit is "0". R0, R 1 and R2 are temperature resolution bits, as shown in Table 5 (x= arbitrary value). By default, users can read and write R2, R 1 and R0, and R2 = "0", r1= "0" and r0 = "1"(9-bit conversion). At this time, the communication port remains valid, and the user has read/write permission to the SD bit, and its default value is "1" (off mode).
Second, the principle and application of intelligent temperature sensor DS 18B20.
DS 18B20 is an improved intelligent temperature sensor newly developed by Dallas Semiconductor Company. Compared with the traditional thermistor, it can directly read the measured temperature, and realize the 9 ~ 12 digital value reading mode through simple programming according to the actual requirements. The digital quantities of 9 bits and 12 bits can be completed in 93.75 ms and 750 ms, respectively. The information read or written from DS 18B20 only needs one port line (single-wire interface) to read and write, and the temperature conversion power supply comes from the data bus, which can also supply power to the connected DS 18B20 without need. Therefore, using DS 18B20 can make the system structure simpler and more reliable. Compared with DS 1820, it has greatly improved the temperature measurement accuracy, conversion time, transmission distance and resolution, and brought more convenient use and satisfactory results to users.
Internal structure of 2DS 18B20
DS 18B20 adopts 3-pin PR35 package or 8-pin SOIC package, and its internal structure block diagram is shown in Figure 1.
(1) 64 b flash ROM has the following structure:?
The first 8 digits are the serial number of the product type, followed by the unique serial number of each device. * * * has 48 bits, and the last 8 bits are the CRC check codes of the first 56 bits, which is why multiple DS 18B20 can communicate on one line.
(2) The nonvolatile temperature alarm triggers TH and TL can be written into the upper and lower limits of user alarm through software.
(3) high-speed temporary storage
The internal memory of DS 18B20 temperature sensor includes a temporary RAM and a nonvolatile electrically erasable E? 2 a.m. The latter is used to store TH and TL values. Data is written into RAM first, and then sent to E? At 2 am, the configuration register is the fifth byte in the high-speed register, and its content is used to determine the digital conversion resolution of the temperature value. When DS 18B20 works, the temperature is converted into a value with corresponding precision according to the resolution in this register. Byte bits are defined as follows:
The lower 5 bits are always 1, and TM is the test mode bit, which is used to set whether DS 18B20 is in working mode or test mode. When DS 18B20 leaves the factory, this bit is set to 0, so the user cannot change it. R 1 and R0 determine the accuracy of temperature conversion, that is, set the resolution, as shown in table 1 (DS 18B20 is set to 12 bits when it leaves the factory). ?
As can be seen from the table 1, the higher the set resolution, the longer the required temperature data conversion time. Therefore, resolution and conversion time should be considered in practical application.
In addition to the configuration register, the temporary memory contains another 8 bytes, which are allocated as follows. In which the temperature information (byte 1, 2), the 3rd, 4th, 6th-8th bytes of th and TL values are not used, showing full logic1; The CRC code of all 8 bytes before the 9th byte reading can be used to ensure correct communication. ?
When DS 18B20 receives the temperature conversion command, it starts the conversion. After conversion, the temperature value is stored in 1 and 2 bytes of the temporary memory in the form of 16-bit signed extended twos complement. Single-chip microcomputer can read data through single-wire interface. When reading, the low bit comes first, the high bit comes last, and the data format is 0? 062 5 ℃/LSB form. The format of the temperature value is as follows:?
Corresponding temperature calculation: when the sign bit S=0, the binary bit is directly converted into decimal; When S= 1, the complement is converted into the original code first, and then the decimal value is calculated. Table 2 shows some corresponding temperature values. ?
After the temperature conversion of DS 18B20 is completed, the measured temperature is compared with th and TL. If T >；; TH or t < TL, set the alarm flag in the device and respond to the alarm search command sent by the host. Therefore, multiple DS 18B20 can be used to simultaneously measure the temperature and conduct alarm search.
(generation of CRC)
Cyclic redundancy check code (CRC) is stored in the most significant byte of 64 b ROM. The host computer calculates the CRC value according to the first 56 bits of the ROM, and compares it with the CRC value stored in DS 18B20 to judge whether the ROM data received by the host computer is correct. ?
Temperature measurement principle of 3DS 18B20
The temperature measurement principle of DS 18B20 is shown in Figure 2. In the figure, the oscillation frequency of the crystal oscillator with low temperature coefficient is little affected by temperature [1], which is used to generate a pulse signal with a fixed frequency and send it to the subtraction counter 1. The oscillation frequency of high temperature coefficient crystal oscillator changes obviously with temperature, and the generated signal is used as the pulse input of subtraction counter 2. The figure also implies a counting gate. When the counting gate is open, the opening time of the DS counting gate is determined by the oscillator with high temperature coefficient. Before each measurement, put the cardinality corresponding to -55℃ into the subtraction counter 1 and the temperature register respectively, and the subtraction counter 1 and the temperature register are preset in? -55 ℃? The corresponding base value. The subtraction counter 1 subtracts the pulse signal generated by the crystal oscillator with low temperature coefficient. When the preset value of the subtraction counter 1 is reduced to 0, the value of the temperature register will be increased by 1, and the preset value of the subtraction counter 1 will be reloaded. The subtraction counter 1 starts counting the pulse signals generated by the crystal oscillator with low temperature coefficient again, and so on, and the temperature will not stop until the subtraction counter 2 counts to 0. The slope accumulator in Figure 2 is used to compensate and correct the nonlinearity in the temperature measurement process, and its output is used to correct the preset value of the subtraction counter. As long as the counting door is still open, the above process is repeated until the temperature register value reaches the measured temperature value, which is the temperature measurement principle of DS 18B20.
In addition, because the single-wire communication function of DS 18B20 is time-sharing and has a strict concept of time slot, the reading and writing timing is very important. All operations of the system on DS 18B20 must be performed according to the protocol. The operating protocol is: initializing DS 18B20 (sending reset pulse) → sending ROM function command → sending memory operation command → processing data. The timing diagram of various operations is the same as DS 1820. Please refer to [2]. ?
Typical Interface Design between 4DS 18B20 and Single Chip Microcomputer
Taking MCS5 1 single chip microcomputer as an example, Figure 3 adopts parasitic power power supply mode, P 1? The 1 port is connected to a single-wire bus. In order to ensure that sufficient current is provided in the effective DS 18B20 clock cycle, a MOSFET and a P 1 of 89C5 1 can be used. 0 to complete the pull-up of the bus [2]. DS 18B20 must have strong pull-up on the bus during memory write operation and temperature A/D conversion operation, and the maximum pull-up opening time is 10μ s ... parasitic power power supply mode is that both VDD and GND terminals are grounded. Because a single-wire system has only one wire, the sending and receiving ports must be tri-state. The upper computer controls DS 18B20 to complete the temperature conversion, which must go through three steps: initialization, ROM operation instruction and memory operation instruction. Assuming that the crystal oscillator frequency used by the single chip microcomputer system is 12 MHz, according to the initialization time sequence, writing time sequence and reading time sequence of DS 18B20, three subprograms are written respectively: init is the initialization subprogram, WRITE is the writing (command or data) subprogram, and read is the reading data subprogram. All data reading and writing starts from the lowest bit. In fact, this method is not used in the experiment, just add a pull-up resistor 4 to the data line. ?
Exact delay problem of 5DS 18B20
Although DS 18B20 has many advantages, it is not easy to use. Due to the single bus data transmission mode, the data I/O of DS 18B20 is completed by the same line. Therefore, the operation timing of reading and writing is strict. In order to ensure the strict I/O timing of DS 18B20, more accurate delay is needed. In the operation of DS 18B20, the time delays used are 15 μs, 90 μs, 270 μs, 540 μs, etc. Because these delays are integer multiples of 15 μs, we can write a DELAY 15(n) function with the following source code:
As long as this function is used to delay about 15 μs×N n n, with more accurate delay guarantee, we can read and write, convert the temperature and display DS 18B20.
3.2 The new trend of intelligent temperature sensor development
(1) Improve the accuracy and resolution of temperature measurement.
The intelligent temperature sensor adopts 8-bit A/D converter, which has low temperature measurement accuracy and the resolution can only reach 1℃. At present, many kinds of high-speed and high-resolution intelligent temperature sensors have been introduced abroad, and the resolution can generally reach 0.5 ~ 0.0625℃ by using 9 ~ 12-bit A/D converter. DS 1624 high-resolution intelligent temperature sensor newly developed by Dallas Semiconductor Company in the United States can output 13 binary data with a resolution of 0.03 125℃ and a temperature measurement accuracy of 0.2℃. In order to improve the conversion rate of multi-channel intelligent temperature sensor, some chips adopt high-speed successive approximation A/D converter. Take the AD78 17 5-channel intelligent temperature sensor as an example, its conversion time for local sensor and each remote sensor is only 27 microseconds and 9 microseconds respectively.
(2) Increase the test function
The test function of temperature sensor is also increasing. For example, DS 1629 single-wire intelligent temperature sensor adds a real-time calendar clock (RTC) to make its function more perfect. DS 1624 also adds a storage function, which can store users' short messages by using the 256-byte E*EPROM memory inside the chip. In addition, the intelligent temperature sensor is developing from single channel to multi-channel, which creates good conditions for the research and development of multi-channel temperature measurement and control system.
The sensor has a variety of working modes to choose from, mainly including single conversion mode, continuous conversion mode, standby mode, and some of them also add low temperature limit expansion mode, which is very simple to operate. For some intelligent temperature sensors, the host (external microprocessor or single chip microcomputer) can also set its A/D conversion rate, resolution and maximum conversion time through corresponding registers.
Can I go to /s? wd = % CE % C2 % B6 % C8 % B4 % AB % B8 % D0 % C6 % F7 % D4 % AD % C0 % ED & amp； lm = 0 & ampsi = & amprn= 10。 ie = GB 23 12 & amp； ct = 0 & ampcl = 3 & ampf= 1。 Rsp=8。 Look up. Many choices. good luck