Design and implementation of regional integrated environmental measurement for China space station

The monitoring of cabin air pressure, temperature, humidity, microgravity and other environmental parameters is very important for the long-term and reliable operation of large manned spacecraft. Based on the characteristics of 1553B bus, a design method of regional integrated environmental measurement for the space station was proposed, which realized the high precision and reliable measurement of environmental parameters such as pressure, temperature, humidity and microgravity of china space station. The measurement data can accurately reflect the environmental changes of astronauts’ in-orbit living, working and extravehicular activities, which verifies the effectiveness of the design. It provides the necessary new principle application verification for higher precision pressure measurement in lunar landing missions.


Introduction
Manned space technology is one of the hot spots of space development in the world.It can not only reflect the development level of a country's space technology, but also become an important symbol to measure a country's economic, technological, military and comprehensive national strength.After 11 years of unremitting efforts, the Tianhe Core module was successfully launched into orbit on April 29, 2021 by a Long March-5B carrier rocket.China has completed the assembly and construction of the space station and turned to the formal operation stage through 11 launches and in-orbit missions [1].In order to ensure the life safety and physical and mental health of astronauts and provide a comfortable living and working environment, we must conduct independent, comprehensive and long-term reliable monitoring of environmental parameters such as air pressure, temperature, humidity and microgravity in the cabin.
Air pressure sensors are installed on the ISS node module to monitor cabin air pressure in the range of 0kPa to 104.8kPa(0psia to 15.2psia) [2][3].The small logistics pressure chamber (MPLM) is equipped with a pressure sensor to monitor the pressure internal the module.The measurement range is 1hPa ~ 1100hPa, with an accuracy of ±16hPa.The space shuttle adopts two kinds of pressure sensors, metal cup strain type and silicon piezoresistive type, to monitor the internal pressure of space shuttle.The measuring ranges are 0psia-15psia and 15psia ~ 20psia respectively.Microgravity survey heads used Allied Signal's quartz flexible accelerometers on spacecraft, the latest generation of the SAMS-II uses high-performance A/D converters in a distributed layout that scales up to five points.The above environmental measurement is characterized by a wide variety of parameters, scattered measurement locations and complex data processing [4].The measurement system is based on the overall framework of multi-point distribution sensor and acquisition control and processing device, which has disadvantages such as complex installation and wiring, easy interference and inconvenient data interaction [5][6][7][8].
Compared with distribution sensor and acquisition control and processing device method of the International Space Station, it overcomes the drawbacks of complicated sensor layout and wiring, high risk of connection failure, easy interference of analog signals, limited sensor principle, difficult data sharing and so on.This paper presents a simple and high precision environmental measurement system for manned spacecraft.Based on the characteristics of china space station system, a design method for the integrated environment measurement of the space station is proposed in this paper.Environmental parameters such as pressure, temperature, humidity and microgravity in the cabin of the China space station have been measured.It has the characteristics of simple wiring, convenient expansion, independent management and data interworking.It fits the design idea of China space station as a multi-functional space test platform in long-term orbit operation.

Architecture design of environmental measurement system
According to the layout characteristics of the China Space Station, the environmental measurement equipment adopts the regional integrated design idea, and uses the advantages of 1553B bus to connect multiple digital integrated measurement units, forming a comprehensive, multi-parameter and intelligent environmental measurement network.Taking Tianhe Core cabin as an example, the environmental measuring equipment of the space station includes integrated measuring equipment, pressure and temperature measurement equipment 1, pressure and temperature measurement equipment 2 and microgravity measurement equipment.The topology structure of the environmental measuring equipment is shown in Figure 1

Microgravity measuring equipment
Figure 1.The topology structure of the environmental measuring equipment The integrated measuring unit acts as the bus BC, the pressure temperature and humidity measuring equipment 1/2, and the microgravity measuring equipment to transmit measurement data and telemetry data.Meanwhile, as the bus RT, the measurement data of various environmental parameters are framed and sent to the integrated measuring unit.The pressure, temperature and humidity measuring equipment and microgravity measuring equipment can respectively measure the pressure, temperature, humidity and microgravity environment at different positions in the china space station.

Pressure, temperature and humidity measurement
There are two pressure and temperature measurement devices in the Tianhe core cabin, which are respectively arranged in the junction cabin of the core cabin and the front cone of quadrant I of the large column section(sealed cabin), as shown in Figure 2 below.Each pressure and humidity measuring device consists of one 10kPa capacitive pressure sensor, three 150kPa silicon resonance pressure sensors, one temperature and humidity sensor, one signal acquisition and processing communication module.The silicon resonant pressure sensor is used to measure the total pressure in the cabin.The silicon resonant pressure sensor has the characteristics of high precision, high stability, no lag, corrosion resistance, etc.Its measuring accuracy and long-term stability are one order of magnitude higher than the conventional principle pressure sensor.The actual accuracy of the whole machine is better than 110Pa, and the range covers 0.5kPa ~ 150kPa.The principle of capacitive pressure sensor is to use the external load of thin film bending, resulting in the principle of capacitor pole distance change, belongs to the pole distance change type capacitive sensor.The tensioned metal diaphragm is used as a common electrode, and the ceramic matrix is designed as a double electrode to form a coaxial ring dual capacitance sensor.The center is the measuring capacitor Cp, and the edge ring is the reference capacitor Cr.The subsequent signal conditioning circuit deals with the difference between the two capacitors.In order to realize the floating installation of the sensitive element, the base and the sensitive element are electrically isolated through the polyimide gasket.The operating temperature of the polyimide gasket is -250℃~260℃.Silicone rubber O-ring in the internal and external polyimide formation groove to achieve a vacuum seal, silicone rubber O-ring use temperature of -70℃~225℃.Radial positioning of the inner ring polyimide is achieved through the groove of the base, and radial positioning of the outer ring polyimide is achieved through the contact between the edge and the sensor.The capacitance structure and electrical isolation design of the sensor are shown in Figure 3. 150kPa pressure sensor design.The 150kPa pressure sensor uses a silicon resonant pressure sensor sensing principle that is more accurate and stable than piezoresistive and many other pressure measurement techniques.The main characteristic is that the silicon is completely elastic before its breaking point.The mechanical properties give the sensor a high quality resonance (high Q value factor), and its frequency stability is not affected by the electronic properties within the product.The silicon resonant structure is driven by an electrostatic field.When pressure is applied to the diaphragm, the silicon resonator is stretched like a guitar string, changing its frequency, which is directly related to the pressure applied.The main material of the product is stainless steel material, and the surface of the stainless steel material is machine state.The sensitive component is connected to the base by laser welding, and the sensitive component is made into the sensitive component after electric assembly.The sensitive component is welded with the support, and the positioning requirements are met here.The positioning is carried out by the marks on the support and the alignment marks on the circular amplified circuit board.After electrical assembly and assembly, the shell is welded.The main dimension of the product is Φ40mm×97mm, and the maximum dimension of the product is Φ40mm ×109mm, as shown in Figure 4.

Temperature and humidity sensor design
The temperature and humidity sensor mainly consists of temperature sensitive element, humidity sensitive element, signal conditioning circuit board, pillar, protective cap, connector and shell.The humidity is measured by a humidity sensitive capacitor element.The J555 oscillation circuit converts the capacitance signal output by the humidity sensitive element into a pulse signal with adjustable width.The pulse width is proportional to the capacitance value, and the frequency signal is output.The temperature sensitive element uses MF51 thermistor manufactured at 715 factory to output voltage values through a 5KΩ partial voltage resistor.
The thermistor and the humidity sensitive capacitor are welded on the signal conditioning circuit board, the circuit board is loaded into the shell and fixed, and then the pillar is screwed into the top of the shell, and the gap between the pillar and the circuit board is sealed and fixed by glue.The top of the heat insulation pillar is equipped with a protective cover, and finally, the plug-in, cover plate and heat insulation seat are installed.The sensor structure diagram is shown in Figure 5.

Microgravity measurement
Microgravity measurement adopts the microgravity measuring instrument and microgravity sensor, the sensor is miniaturized and compact design, the weight is only 530g, the measurement bandwidth is up to 300Hz, the sampling rate is up to 2KHz, the microgravity sensor can be installed at different points to be measured according to the demand, and the microgravity measurement can be quickly expanded.Through the patented static calibration and software solution method, the cross-coupling Angle correction is realized.The Angle error correction and the repeatability of installation are realized through the cubic mirror precision measurement and the mounting face.The measuring range is ± 30mg0, and the uncertainty is better than 2mg0 in the bandwidth of 0.1-120Hz, which realizes the high-precision triaxial microgravity measurement.The microgravity measuring instrument is composed of a power module, a signal acquisition and processing communication module, and a control and storage module.The power module implements the interface with the primary power supply and the conversion and filtering of the secondary power supply, and supplies power to the microgravity sensor, signal acquisition and processing communication module and control and storage module.The functional composition of microgravity measuring instrument is shown in Figure 6. .Microgravity sensor equipment composition The microgravity sensor uses a small quartz flexible servo accelerometer for its sensitive head, which is characterized by high measurement accuracy, good temperature characteristics, good longterm stability and high reliability.In the part of signal conversion, conditioning and filtering, the current signal output by the accelerometer is firstly converted into a voltage signal proportional to the input acceleration through the sampling resistance, and the voltage signal is properly amplified and filtered, and the final voltage signal is sent to the signal acquisition and processing communication module of the microgravity measuring instrument for data processing, and the measurement results of microgravity are obtained.The output results include the maximum value, minimum value, average value and effective value of X axis, Y axis and Z axis.The microgravity sensor and microgravity measuring instrument can measure the vibration and transient acceleration of measuring points inside the space station cabin, as shown in Figure 7 below.

Experimental verification in orbit
On April 29, 2021, the Tianhe Core Module was successfully launched into orbit .On June 17, the Shenzhou-12 manned spacecraft completed forward docking with the Tianhe Core module.On July 4, the Shenzhou-12 astronauts completed the first undocking of the Chinese space station.Figure 8 below shows the pressure and temperature and humidity measurements of the Shenzhou 12 manned spacecraft crew during the first exit.The figure 9 below shows the X-direction, y-direction, and Zdirection microgravity measurements of the space station during its orbit on July 4.
It can be seen that at about 5:00 a.m. on July 4, the total pressure of the joint cabin of the sky and core cabin began to drop from 93.7kPa.After 3 hours, the joint cabin pressure dropped to close to vacuum (< 100Pa) and stabilized, indicating that the joint cabin door had been completely opened at this time, and the total pressure ladder of the sealed cabin rose to 101.2kPa.At about 15:00, the pressure of the node cabin began to recover from the vacuum, and at about 15:40 minutes later, the total pressure of the node cabin recovered to about 93.4kPa, completing the re-pressurization of the node cabin.At the same time, the total pressure of the sealed cabin decreased from 101kPa to 9.34kPa.The node cabin and the sealed cabin were connected, and the total pressure was balanced.
During this period, the temperature in the sealed cabin changed little and was stable on the whole.The temperature fluctuation range was 26.3℃ ~ 28.7℃, while the temperature fluctuation range of the node cabin was 24.5℃ ~ 27.6℃.The two maximum values appeared before and after the exit and entry moments, reflecting the influence of astronaut activities on the temperature of the node cabin.
The fluctuation range of relative humidity of the sealed cabin is 28.1%RH ~ 38.4%RH.At 7:40 am, the humidity of the sealed cabin starts to decrease significantly, reflecting that the humidity of the working cabin starts to decrease compared with the condition of three persons because there is only one person staying in the working cabin.The humidity of the working cabin continues to decrease after exiting the cabin, and the minimum humidity occurs at the moment before entering the cabin, which is about 28.1%RH.The relative humidity of the node cabin fluctuates from 0%RH to 45.0%RH on the same day.About 5:00, the humidity of the node cabin began to rise, indicating that the astronauts began to enter the node cabin, and then with the pressure of the node cabin decreased, humidity also began to decline, until about 8:10 pressure into a vacuum, humidity will also be 0%RH.At about 15:00, the pressure of the node cabin began to recover from the vacuum.40 minutes later, at about 15:40, the total pressure of the node cabin recovered to about 93.4kPa, and the corresponding humidity of the node cabin rose and gradually recovered to be basically the same as that of the sealed cabin.In-orbit microgravity measurements of the core module Based on the above data and analysis, the design method of regional integrated environmental measurement for the space station proposed in this paper has realized the high precision and reliable measurement of environmental parameters such as pressure, temperature, humidity and microgravity in the cabin of the space Station, which can accurately reflect the status and activities of the space station and astronauts.

Conclusion
Sensor technology is one of the important basic technologies supporting the development of space technology.Combining with the system characteristics of China's space station, this paper puts forward the design method of regional integrated environment measurement for the space station, and realizes the measurement of environmental parameters such as pressure, temperature, humidity and microgravity in the cabin of China's space station.The design is of great significance to ensure the safety of astronauts and the space station.It realizes real-time monitoring and evaluation of the overall micro-vibration environment of the space station during flight, orbit transformation, docking, astronaut activities and robotic arm movements, etc., and provides important basic data for the platform safety and application system to carry out microgravity tests.At present, the design only carries out high precision measurement of temperature, humidity and microgravity of manned spacecraft.In the future, the measurement parameters can be further expanded according to the demand, such as carbon dioxide concentration and harmful gases.

Figure 2 .
Figure 2. Pressure, Temperature, Humidity Measuring Equipment 2.1.1.10kPa pressure sensor design.The 10kPa pressure sensor is used to accurately measure the micropressure below 10kPa.It has the characteristics of high measurement accuracy and low noise[9-10].The sensor includes a capacitive pressure sensor and a demodulation circuit, which outputs digital signals.The sensor includes a signal conditioning circuit board, which converts the output capacitance of the sensor into digital signals.The sensor is composed of a connector, a shell, a conditioning circuit printed board, a sensitive element and a base.The principle of capacitive pressure sensor is to use the external load of thin film bending, resulting in the principle of capacitor pole distance change, belongs to the pole distance change type capacitive sensor.The tensioned metal diaphragm is used as a common electrode, and the ceramic matrix is designed as a double electrode to form a coaxial ring dual capacitance sensor.The center is the measuring capacitor Cp, and the edge ring is the reference capacitor Cr.The subsequent signal conditioning circuit deals with the difference between the two capacitors.In order to realize the floating installation of the sensitive element, the base and the sensitive element are electrically isolated through the polyimide gasket.The operating temperature of the polyimide gasket is -250℃~260℃.Silicone rubber O-ring in the internal and external polyimide formation groove to achieve a vacuum seal, silicone rubber O-ring use temperature of -70℃~225℃.Radial positioning of the inner ring polyimide is achieved through the groove of the base, and radial positioning of the outer ring polyimide is achieved through the contact between the edge and the sensor.The capacitance structure and electrical isolation design of the sensor are shown in Figure3.

Figure 3 .
Figure 3. Double Capacitance Structure and Electrical Isolation Design of Sensitive Component 2.1.2.150kPa pressure sensor design.The 150kPa pressure sensor uses a silicon resonant pressure sensor sensing principle that is more accurate and stable than piezoresistive and many other pressure

Figure 4 .
Figure 4. Composition and outline of silicon resonant pressure sensor

Figure 5 .
Figure 5.General View of Temperature & Humidity Sensor

Figure 6
Figure 6.Microgravity sensor equipment compositionThe microgravity sensor uses a small quartz flexible servo accelerometer for its sensitive head, which is characterized by high measurement accuracy, good temperature characteristics, good longterm stability and high reliability.In the part of signal conversion, conditioning and filtering, the current signal output by the accelerometer is firstly converted into a voltage signal proportional to the input acceleration through the sampling resistance, and the voltage signal is properly amplified and filtered, and the final voltage signal is sent to the signal acquisition and processing communication module of the microgravity measuring instrument for data processing, and the measurement results of microgravity are obtained.The output results include the maximum value, minimum value, average value and effective value of X axis, Y axis and Z axis.The microgravity sensor and microgravity measuring instrument can measure the vibration and transient acceleration of measuring points inside the space station cabin, as shown in Figure7below.

Figure 7 .
Figure 7. Distribution of microgravity measurement points

Figure 8 .Figure 9 .
Figure 8.The pressure, temperature and humidity measurements of the Shenzhou-12 astronaut crew during the first extravasation