Article
Optimized Design of the SGA-WZ Strapdown Airborne Gravimeter Temperature Control System Juliang Cao, Minghao Wang *, Shaokun Cai, Kaidong Zhang, Danni Cong and Meiping Wu Received: 10 November 2015; Accepted: 24 November 2015; Published: 1 December 2015 Academic Editor: Vittorio M. N. Passaro College of Mechatronics and Automation, National University of Defense Technology, Changsha 410072, China;
[email protected] (J.C.);
[email protected] (S.C.);
[email protected] (K.Z.);
[email protected] (D.C.);
[email protected] (M.W.) * Correspondence:
[email protected]; Tel.: +86-132-7240-7961; Fax: +86-731-8457-6305 (ext. 8209)
Abstract: The temperature control system is one of the most important subsystems of the strapdown airborne gravimeter. Because the quartz flexible accelerometer based on springy support technology is the core sensor in the strapdown airborne gravimeter and the magnet steel in the electromagnetic force equilibrium circuits of the quartz flexible accelerometer is greatly affected by temperature, in order to guarantee the temperature control precision and minimize the effect of temperature on the gravimeter, the SGA-WZ temperature control system adopts a three-level control method. Based on the design experience of the SGA-WZ-01, the SGA-WZ-02 temperature control system came out with a further optimized design. In 1st level temperature control, thermoelectric cooler is used to conquer temperature change caused by hot weather. The experiments show that the optimized stability of 1st level temperature control is about 0.1 ˝ C and the max cool down capability is about 10 ˝ C. The temperature field is analyzed in the 2nd and 3rd level temperature control using the finite element analysis software ANSYS. The 2nd and 3rd level temperature control optimization scheme is based on the foundation of heat analysis. The experimental results show that static accuracy of SGA-WZ-02 reaches 0.21 mGal/24 h, with internal accuracy being 0.743 mGal/4.8 km and external accuracy being 0.37 mGal/4.8 km compared with the result of the GT-2A, whose internal precision is superior to 1 mGal/4.8 km and all of them are better than those in SGA-WZ-01. Keywords: airborne gravimetry; SGA-WZ-02; optimized design; temperature control; heat analysis
1. Introduction The information about the Earth’s gravity field is becoming more and more important in geophysics, geodesy, geodynamics, etc. Airborne gravimetry is an effective method to collect highly precise information about large areas of the gravity field. Different principles are used in different airborne gravimetry systems; the three-axis inertial platform navigation system is used in GT-1A, GT-2A and AIRGrav, the 2-axis stable platform technique is used in LCR Air/Sea Gravity System, and Strapdown Inertial Navigation System (SINS) is used in SISG, GT-X and SGA-WZ [1]. The strapdown airborne gravimeter has the advantages of being structurally simple, small sized, light weight, low cost and low power consumption. The strapdown gravimeter is also able to implement vector gravimetry, and gravity vector information is significant in many subjects, like geophysics [2,3]. Therefore, airborne gravimeters based on SINS have been a major development trend in airborne gravimetry. The core sensor in the strapdown airborne gravimeter (e.g., SGA-WZ) is the quartz flexible accelerometer. The advantages of the quartz flexible accelerometer are good stability and simple structure, but the output of the quartz flexible accelerometer has an obvious drift due to temperature
Sensors 2015, 15, 29984–29996; doi:10.3390/s151229781
www.mdpi.com/journal/sensors
Sensors 2015, 15, 29984–29996
variation because the quartz flexible accelerometer is based on springy support technology and the magnet steel in electromagnetic force equilibrium circuits of quartz flexible accelerometer is greatly affected by temperature. Under general inertial navigation conditions, the problem of temperature drift can be solved through the method of temperature compensation [4]. However, at the mGal (10´5 m/s2 ) level of high precision airborne gravimetry applications, temperature compensation cannot meet the demands of airborne gravimetry. In most mature airborne gravimetry systems, temperature control measurements are adopted. The temperature control precision in the GT-2A system is better than 0.1 ˝ C, and its day drift is smaller than 0.1 mGal. The temperature control precision in the AIRGrav system is better than 0.05 ˝ C. Some airborne gravimeter systems cannot provide practical results, because their thermal control accuracy cannot meet the requirements. Addressing the problem of airborne gravimeter temperature control systems, some research, according to the structure and material properties of the quartz flexible accelerometer, have built the temperature field distribution of the quartz flexible accelerometer under the condition of only considering heat transfer [5]. Others, mainly focusing on control algorithm, have ignored the importance of the thermal structure design [6–8]. What’s more, research about airborne gravimeter temperature control systems is rare. Based on the design experience of the SGA-WZ-01, the first strapdown airborne gravimeter developed in China, the temperature control system and the thermal structure are optimized redesigned in the SGA-WZ-02. In the following section, the temperature control system in SGA-WZ-01 is presented and its temperature control system shortcomings are pointed out. In the Section 3, the optimized design of the SGA-WZ-02 temperature control system is described. The experimental results are given in Section 4. Finally, a discussion and some conclusions are given in Section 5. 2. Description of the Temperature Control System in SGA-WZ-01 2.1. The Three-Level Temperature Control System in SGA-WZ-01 The temperature drift of the quartz flexible accelerometer used in the SGA-WZ is about 10 mGal/˝ C and the max permitted error caused by the quartz flexible accelerometer in the SGA-WZ is 0.5 mGal. To meet the precision requirements of the SGA-WZ airborne gravimeter, the temperature stability in the SGA-WZ should be superior to 0.05 ˝ C [9]. To achieve this precision, the temperature control system in the SGA-WZ should be carefully designed. The alternative temperature control systems are a single-level temperature control system or a multi-level temperature control system. In practice, the highest precision of a single-level temperature control system is about 0.1 ˝ C, which can’t meet the system requirements. To ensure the temperature control precision and minimize the effect of temperature on the gravimeter, the SGA-WZ temperature control system therefore adopts a three-level modular control method. A sketch of the SGA-WZ temperature control system is shown in Figure 1. Every level of the temperature control unit has its own thermal insulation layer, heating element, temperature sensors, drive module, etc. The first level temperature control aims at keeping the temperature of the sensor box as homogeneous and stable as possible, providing a good working condition to lase gyros and a preliminary isolation of the effect of temperature on the quartz flexible accelerometer. The second level temperature control tries to manage the air temperature of the accelerometer component and further isolates the effect of temperature on the quartz flexible accelerometer. On the basis of the former two level control units, the third level temperature control directly controls the temperature of the quartz flexible accelerometer.
29985
Sensors 2015, 15, 29984–29996 Sensors 2015, 16, page–page Sensors 2015, 16, page–page
Figure Figure 1. 1. The The sketch sketch of of the the SGA-WZ SGA-WZ temperature temperature control control system. system. Figure 1. The sketch of the SGA-WZ temperature control system.
2.2. Control System System 2.2. The The Performance Performance of of SGA-WZ-01 SGA-WZ-01 Temperature Temperature Control Temperature Control System The The SGA-WZ-01 SGA-WZ-01 has has already already undergone undergone 2400 2400 h h of of static static tests tests and and about about 2000 2000 km km surveying surveying operation, with the precision of the SGA-WZ reaching 1.5 mGal/4.8 km. The tests and surveying The tests and surveying operation, with the precision precision of of the the SGA-WZ SGA-WZ reaching reaching 1.5 1.5mGal/4.8 mGal/4.8 km. km. The results show that the temperature control system in SGA-WZ can essential satisfy the requirements. results show that the temperature control system in SGA-WZ can essential satisfy the requirements. Figure Figure 22 is is aa photo photo of of the the SGA-WZ-01, SGA-WZ-01, where where the the electric electric box box is is in in the the front front of of the the picture picture and and the the sensors box is behind the electric box (the white one). sensors box is behind the electric box (the white one).
Figure 2. The photo of SGA-WZ-01. Figure 2. The Figure 2. The photo photo of of SGA-WZ-01. SGA-WZ-01.
Figure 3 shows the temperature change and quartz flexible accelerometer output in the static Figure 3 shows the temperature change and quartz flexible accelerometer output in the static FigureFigure 3 shows the temperature changechange and quartz accelerometer output in the static test, while 4 shows the temperature in theflexible surveying and mapping operation and test, while Figure 4 shows the temperature change in the surveying and mapping operation and test, while Figure 4 shows the temperature change in the surveying and mapping operation and represents that the temperature control system meets the requirement and the effects of temperature represents that the temperature control system meets the requirement and the effects of temperature represents that the temperature meets theand requirement the effects of SGA-WZ-01 temperature on the SGA-WZ are below the control index insystem the surveying mappingand operations. The on the SGA-WZ are below the index in the surveying and mapping operations. The SGA-WZ-01 on the SGA-WZ are system below the index the surveying operations. TheisSGA-WZ-01 temperature control only usesinheating to keep and the mapping temperature stable, that to say, the temperature control system only uses heating to keep the temperature stable, that is to say, the temperature control system only to keep the situation, temperature thatof isthe to 1st say,level the natural heat dissipation is the onlyuses wayheating of cooling. In this the stable, set value natural heat dissipation is the only way of cooling. In this situation, the set value of the 1st level natural heat control dissipation is the only way ofthan cooling. In this situation, the set value of the 1st temperature system must be higher the environmental temperature by about 10 °C. temperature control system must be higher than the environmental temperature by about 10 °C. level large temperature system must bethe higher than the environmental temperature by about With numberscontrol of flight experiments, control method using only heating has the following With large numbers of flight experiments, the control method using only heating has the following 10 ˝ C. With large numbers of flight experiments, the control method using only heating has the shortcomings: shortcomings: following shortcomings: (1) The aircraft cabin easily exceeds the set value of the 1st level temperature control unit in (1) The aircraft cabin easily exceeds the set value of the 1st level temperature control unit in hot aircraft weather. As easily a result of a the lack of heat of dissipation, the temperature can’tunit meet (1) The cabin exceeds the 1st level temperature control in the hot hot weather. As a result of a lacksetofvalue heat dissipation, the temperature can’t meet the stability requirements and further impacts the quartz flexible accelerometer’s precision. weather. As a result of a lack of heat dissipation, the temperature can’t meet the stability stability requirements and further impacts the quartz flexible accelerometer’s precision. Figure 5 shows thefurther performance the quartz temperature control system in hot weather.Figure 5 requirements and impactsof flexible accelerometer’s precision. Figure 5 shows the performance ofthe the temperature control system in hot weather. (2) shows Laser the gyro’s best working temperature is at about 30 °C, but the set value of the 1st level performance of the temperature control system in hot weather. (2) Laser gyro’s best working temperature is at about 30 °C, but the set value of the 1st level temperature control unit is set at 40 °C. Higher temperatures affect the service life of the temperature control unit is set at 40 °C. Higher temperatures affect the service life of the laser gyro and reduce the reliability of the system. laser gyro and reduce the reliability of the system. 29986 3 3
Sensors 2015, 15, 29984–29996
Sensors 2015, 16, page–page (2) Laser gyro’s best working temperature is at about 30 ˝ C, but the set value of the 1st level (3) Figures 1 and 2 show that the sensors’ box is so big that it increases Sensors 2015, 16, page–page temperature control unit is set at 40 ˝ C. Higher temperatures affect the service life of the consumption of the temperature control laser gyro and reduce the reliability of thesystem system. by some degrees.
the power
1 and 22 show showis that the sensors’ is itsoincreases bigSGA-WA-01 that it increases the power (3) Figures 1 and that the sensors’ box is sobox big make that the power consumption (4) (3) TheFigures SGA-WA-01 design too conservative to the relatively light, for of the temperature control systemcontrol by somesystem degrees.by some degrees. consumption of the temperature there have not been any previous experiments. (4)SGA-WA-01 The SGA-WA-01 designis is too to make the SGA-WA-01 relatively light, for therelight, for (4) The design tooconservative conservative to make the SGA-WA-01 relatively anyany previous experiments. there have havenot notbeen been previous experiments. Environment temperature (std:0.28℃) 18
℃
17.5
17.5
0
℃
16.5
5
17
42.7
℃
Environment temperature (std:0.28℃)
18
17
16.5
42.65
10
0
5
25
10
15
hour
20
25
℃
42.65
0
5
10
42.6 0
59.11
℃
20
Sensor box temperature (std:0.015℃)
42.7
42.6
15
hour
Sensor box temperature (std:0.015℃)
5
℃
20
25 25
hour
Z-axis Q-flex accelerometer temperature (std:0.0016℃)
59.105 59.11 59.1
15
hour
Z-axis Q-flex accelerometer temperature (std:0.0016℃)20 10 15
59.105
0 59.1
5 0
10
15
hour
20
Z-axis Q-flex accelerometer output 15(std:0.29mGal) 10 hour
5
9.79131
25
20
25
Z-axis Q-flex accelerometer output (std:0.29mGal)
9.7913
m/s
2
9.79129 9.79129
0
5 0
10 5
15
hour
10
20
15
hour
25
20
25
Figure 3.Figure Temperature change and accelerometer output static tests 3. Temperature change andZ-axis Z-axis accelerometer output of static tests. Figure 3. Temperature change and Z-axis accelerometer output ofof static tests Z-axis Q-flex accelerometer ℃, ,peak peaktotopeak peak value:0.04 Z-axis Q-flex accelerometertemperature temperature (std:0.009 (std:0.009℃ value:0.04 ℃)℃) 58.0458.04 58.0358.03 58.0258.02 58.0158.01
1
1
22
33 hour hour
4
5
4
6
5
6
Sensor box temperature(std:0.048 (std:0.048℃ ℃℃ ) ) Sensor box temperature ℃,peak ,peaktotopeak peakvalue:0.25 value:0.25 37.95
℃
℃
37.95 37.9 37.937.85 37.85 37.8 37.837.75 37.75 0 0
1
2
1
3 hour 3
2
4
5
4
6
5
6
hour
Environment temperature (std:0.328℃,peak to peak value:1.94℃)
Environment temperature (std:0.328℃,peak to peak value:1.94℃)
℃
℃
28.5 28 28.5 27.5 28 27 27.5 26.5
27 26.5
0
0
1
2
1
3 hour
2
4
3
5
4
6
5
6
hour Figure 4. Temperature change in surveying and mapping operation.
Figure 4. Temperature change in surveying and mapping operation.
Figure 4. Temperature change in surveying and mapping operation. Z-axis Q-flex accelerometer temperature
Z-axis Q-flex accelerometer temperature
58.2
℃
58.15
58.2 58.1
58.1558.05 58.1 58.05
58 10
12
14
16
18
20
22
24
hour 29987
58 10
12 41
℃
℃
m/s 2
9.79131
9.7913
40
14
Sensor16box temperature 18 hour
Sensor box temperature
20
22
24
℃
28.5 28 27.5 27 26.5 0
1
Sensors 2015, 15, 29984–29996 Figure 4.
2
3 hour
4
5
6
Temperature change in surveying and mapping operation. Z-axis Q-flex accelerometer temperature
58.2
℃
58.15 58.1 58.05 58 10
12
14
16
18
20
22
24
hour
Sensor box temperature
℃
41 40 39 38 10
12
14
16
18
20
22
20
22
hour
Environment temperature 34
℃
32 30 28 26 10
12
hour
14
16
18
24
Sensors 2015, 16, page–page Figure 5. The performanceofoftemperature temperature control system in hotinweather. Figure 5. The performance control system hot weather.
4 3. SGA-WZ-02 Design 3. SGA-WZ-02Temperature Temperature Control Control System System Optimized Optimized Design The SGA-WZ-02 SGA-WZ-02 is is the the new new airborne airborne gravimeter gravimeter developed developed by by the the Laboratory Laboratory of of Inertial The Inertial Technology of of the the National Technology National University University of of Defense Defense Technology. Technology. The The airborne airborne experiments experiments show show that that the precision of SGA-WZ-02 is better than 1 mGal/4.8 km. Figure 6 shows a photo of the the precision of SGA-WZ-02 is better than 1 mGal/4.8 km. Figure 6 shows a photo of the SGA-WZ-02, SGA-WZ-02, where the sensor box is on the bottom of the electric box. where the sensor box is on the bottom of the electric box.
Figure 6. 6. Photo Figure Photo of of the the SGA-WZ-02. SGA-WZ-02.
3.1. The Optimized 1st Level Temperature Control Design Based on a Thermoelectric Cooler 3.1. The Optimized 1st Level Temperature Control Design Based on a Thermoelectric Cooler In order to conquer the problem caused by hot weather in the SGA-WZ-01 Temperature In order to conquer the problem caused by hot weather in the SGA-WZ-01 Temperature Control Control System, a thermoelectric cooler (TEC) is adopted in the 1st level temperature control. TEC System, a thermoelectric cooler (TEC) is adopted in the 1st level temperature control. TEC is based on is based on the Peltier Effect, which is the presence of heating or cooling at an electrified junction of the Peltier Effect, which is the presence of heating or cooling at an electrified junction of two different two different conductors. Therefore, the TEC works like a solid-state active heat pump which conductors. Therefore, the TEC works like a solid-state active heat pump which transfers heat from transfers heat from one side of the device to the other, with some consumption of electrical energy. one side of the device to the other, with some consumption of electrical energy. The temperature control system of the SGA-WZ-02 adopts a one-style design with compact The temperature control system of the SGA-WZ-02 adopts a one-style design with compact and and reasonable structure, prominently reducing the volume and the power consumption. The reasonable structure, prominently reducing the volume and the power consumption. The structure structure of the 1st level temperature control is shown in Figure 7 and a sketch of the optimized 1st of the 1st level temperature control is shown in Figure 7 and a sketch of the optimized 1st level level temperature control design is shown in Figure 8. temperature control design is shown in Figure 8. In order to estimate the performance of the new design, stability experiments and max cool down capability experiments are carried out. Figure 9 shows the result of the stability experiments. 29988 In the stability experiments, the set temperature is 30 °C. Figure 9 indicates that a temperature gradient exists in the temperature control system, but the temperature gradient is unavoidable in every single input temperature control system. The decisive element in the temperature control
is based on the Peltier Effect, which is the presence of heating or cooling at an electrified junction of two different conductors. Therefore, the TEC works like a solid-state active heat pump which transfers heat from one side of the device to the other, with some consumption of electrical energy. The temperature control system of the SGA-WZ-02 adopts a one-style design with compact and structure, prominently reducing the volume and the power consumption. The Sensorsreasonable 2015, 15, 29984–29996 structure of the 1st level temperature control is shown in Figure 7 and a sketch of the optimized 1st level temperature control design is shown in Figure 8. In performance of the newnew design, stability experiments and max In order ordertotoestimate estimatethe the performance of the design, stability experiments andcool maxdown cool capability experiments are carried out. Figure 9 shows the result of the stability experiments. In the down capability experiments are carried out. Figure 9 shows the result of the stability experiments. stability experiments, the set temperature is 30 ˝ C.isFigure indicates that a temperature gradient In the stability experiments, the set temperature 30 °C.9Figure 9 indicates that a temperature exists in the temperature control system, but the temperature gradient is unavoidable in every single gradient exists in the temperature control system, but the temperature gradient is unavoidable in input temperature control system. The decisive element in the temperature control system of the every single input temperature control system. The decisive element in the temperature control airborne is its stability, and Figure 9 shows the 9temperature meets thestability system system ofgravimeter the airborne gravimeter is its stability, and that Figure shows that stability the temperature ˝ C). requirements (superior to 0.5 meets the system requirements (superior to 0.5 °C).
Figure 7. The the 1st 1st level level temperature temperature control control in in SGA-WZ-02. SGA-WZ-02. Figure 7. The structure structure of of the Sensors Sensors 2015, 2015, 16, 16, page–page page–page
5
Figure Sketch of of the the 1st Figure 8. 8. Sketch 1st level level temperature temperature control control in in SGA-WZ-02. SGA-WZ-02. Temperature Temperature of of the the sensor sensor box boxbottom bottom (std:0.07) (std:0.07)
30 30
℃℃
29.9 29.9 29.8 29.8 29.7 29.7 29.6 29.60 0
50 50
100 100
150 150
200 200 min min
250 250
300 300
350 350
400 400
450 450
Temperature Temperature of of the the sensor sensor box box right right part(std:0.10) part(std:0.10)
℃℃
28 28 27.8 27.8 27.6 27.6 0 0
50 50
100 100
150 150
200 200 min min
250 250
300 300
350 350
400 400
Figure 9. The stability experiment results. Figure 9. The stability experiment results.
Figure Figure 10 10 shows shows the the results results of of the the max max cool cool down down capability capability experiments. experiments. Room Room temperature temperature is is 29989 about 25 °C in the experiments. To evaluate the cool down capability, the set temperature about 25 °C in the experiments. To evaluate the cool down capability, the set temperature is is 00 °C. °C. The The temperature temperature difference difference between between the the outer outer radiator radiator and and the the air air outlet outlet is is about about 30 30 °C, °C, so so that that the the temperature difference essentially reaches the max temperature difference of the selected TEC temperature difference essentially reaches the max temperature difference of the selected TEC and and proves the 1st level temperature control unit has a good heat sink capability. From the experimental
℃
27.8 27.6
0
Sensors 2015, 15, 29984–29996
50
100
150
200 min
250
300
350
400
Figure 9. The stability experiment results.
Figure 10 10 shows shows the the results results of of the the max max cool cool down down capability capability experiments. experiments. Room Room temperature temperature is is Figure ˝ ˝ about 25 25 °C in the the experiments. experiments. To cool down down capability, capability, the the set set temperature temperature is is 00 °C. about C in To evaluate evaluate the the cool C. ˝ The temperature difference between the outer radiator and the air outlet is about 30 °C, so that the The temperature difference between the outer radiator and the air outlet is about 30 C, so that the temperature difference difference essentially essentially reaches reaches the the max max temperature temperature difference difference of of the the selected selected TEC TEC and and temperature proves the the 1st 1st level level temperature temperature control control unit unit has has aa good good heat heat sink sink capability. capability. From From the the experimental experimental proves ˝ results, the max cool down capability is about 15 °C compared with the air outlet temperature and results, the max cool down capability is about 15 C compared with the air outlet temperature and ˝ about 11 11 °C compared with with the the right right part part of of sensor sensor box. box. Figure shows that that the the max max temperature temperature about C compared Figure 55 shows ˝ rise in the aircraft cabin is less than 10 °C, so the cool down capability of 1st level temperature rise in the aircraft cabin is less than 10 C, so the cool down capability of 1st level temperature control controlthe meets the requirements foruse. actual use. meets requirements for actual Temperature of the air outlet
11.5 ℃
11 10.5
0
100
200
300
400
500
600
min
Temperature of the sensor box right part
℃
15 14 13
0
100
200
300
400
500
600
500
600
min
Temperature of the outer radiator
40 ℃
39 38 37
0
100
200
300
400 min
Figure Figure 10. 10. The max cool down capability capability experiment experiment results. results.
6 3.2. The 2nd and 3rd Level Temperature Control Unit Heat Analysis As a result of the equipment being a newly optimized design, heat analysis is needed in the 2nd and 3rd level temperature control units to meet the temperature control precision requirement. In the 2nd and 3rd level temperature control units, three basic ways of heat transfer (radiation, conduction and convection) exist to different degrees. The main forms of heat exchange in temperature control units can be any of the following three: (1) Heat conduction of the component itself; (2) The radiation from the unit surfaces to the environment; (3) The convection between the internal and external surface of unit heat transfer. Besides the ways of heat exchange, the material’s thermal physical properties, boundary conditions, related thermal parameters and the heat transfer coefficient used in the calculations are necessarily clarified in the heat analysis. The thermal physical properties are listed in Table 1. Table 1. Material physical properties. Material
Density pkg¨ m3 q
Specific Heat pJ{pkg¨ kqq
Heat Conduct Coefficient pW{pm2 ¨ kqq
Steel Heat insulation material
7840 30
450 1400
49.8 0.03
The quartz flexible accelerometer unit, which contains 2nd and 3rd level temperature control units, is shown in Figure 11. The quartz flexible accelerometer unit is assembled into the IMU component from the bottom side. The 2nd level heating is on the outer wall of the unit shell. The heat insulation measures use foam in the outer of 2nd level heating sheet. The 3rd level heating is on the bottom of the accelerometer base and directly controls the temperature of the accelerometer. 29990
Thequartz quartzflexible flexible accelerometer accelerometer unit, unit, which contains The contains 2nd 2nd and and 3rd 3rd level leveltemperature temperaturecontrol control units,isisshown showninin Figure Figure 11. 11. The The quartz quartz flexible accelerometer units, accelerometer unit unit isis assembled assembledinto intothe theIMU IMU component from the bottom side. The 2nd level heating is on the outer wall of the unit shell. The component from the bottom side. The 2nd level heating is on the outer wall of the unit shell. The heat insulation measuresuse usefoam foam in in the the outer outer of of 2nd Sensors 2015, 15, 29984–29996 heat insulation measures 2nd level level heating heatingsheet. sheet.The The3rd 3rdlevel levelheating heatingisisonon the bottom of the accelerometer base and directly controls the temperature of the accelerometer. the bottom of the accelerometer base and directly controls the temperature of the accelerometer.
Figure 11. Structure diagram of the accelerometer with temperature temperature control system. system. Figure11. 11.Structure Structurediagram diagramof ofthe the accelerometer accelerometer installation installation Figure installation with with temperaturecontrol control system.
Using the finite element analysis software ANSYS [10] to analyze the temperature field of Using [10] to to analyze analyze the the temperature temperaturefield fieldofof Usingthe thefinite finite element element analysis analysis software software ANSYS [10] accelerometer unit, we can make an appropriate simplification. We ignore the accelerometer’s heat accelerometerunit, unit,we wecan canmake make appropriate simplification. ignore accelerometer’s accelerometer anan appropriate simplification. We We ignore the the accelerometer’s heat generation, because accelerometer’s heat generation is much less than that of the heat sheet. We also heat generation, because accelerometer’s heat generation is less much lessthat than of the heatWe sheet. generation, because accelerometer’s heat generation is much than of that the heat sheet. also need to refine finite element mesh division of the structure joint, like heat transfer of the We also need to refine finite element mesh division ofstructure the structure joint, like heat transfer of need to refine finite element mesh division of the joint, like heat transfer of the accelerometer fixed screw in the internal system [11]. Finally, as the finite element model of the accelerometer fixed screw in the internal system [11]. Finally, as the finite element model of accelerometer screw in 12 theshows, internal [11]. Finally, the finite element model accelerometer fixed unit in Figure the system nodes and units of theasdivision of finite elements areof accelerometerunit unit in in Figure Figure 12 12 shows, shows, the nodes and units of the division of finite elements are accelerometer units of the division of finite elements are 258,755 and 111,539, respectively. Needless to say, intensive meshing is helpful to improve the 258,755 and 111,539, respectively. Needless to say, intensive meshing is helpful to improve the 258,755 and 111,539, respectively. Needless to say, intensive meshing is helpful to improve the accuracy of the simulation [12]. accuracyofofthe thesimulation simulation[12]. [12]. accuracy
Sensors 2015, 16, page–page
Figure 12. The finite element model of the accelerometer unit. Figure of the the accelerometer accelerometerunit. unit. Figure12. 12.The Thefinite finite element element model model of 7
To decide the temperature measurement point 7 in the 2nd and 3rd level temperature control units, To ANSYS was to analyze the temperature temperature field simulation control result is decide theused temperature measurement pointfield. in theThe 2nd and 3rd level temperature shown Figurewas 13. used In thetoanalysis, level heating is set at 40 simulation °C and thatresult of 3rd units, in ANSYS analyze the the 2nd temperature field.temperature The temperature field ˝ level is 45 °C. From the simulation results, it is obvious that the temperature of accelerometer is shown in Figure 13. In the analysis, the 2nd level heating temperature is set at 40 C and that unit of 3rd level isbed’s 45 ˝ C.top From the simulation it is obvious the temperature accelerometer foundation surface and the results, temperature of the that accelerometer base’softop surface are unit foundation bed’sThus, top surface the 2nd temperature of the accelerometer top temperature surface are relatively uniform. settingandthe level temperature control base’s system relatively uniform. the 2nd level control system temperature measurement measurement point Thus, on thesetting accelerometer unit temperature foundation bed’s top surface and setting the 3rd level point on thecontrol accelerometer unit foundation bed’s top surface and setting the 3rd level temperature temperature system temperature measurement point on the accelerometer base’s top surface control system temperature measurement point on the accelerometer base’s top surface is better than is better than setting it on other places, because the single point represents the temperature of the setting it on other places, because themakes singlethe point represents the temperature whole surface on whole surface on those surfaces and control system’s input reflect of thethe temperature change surfaces and makes the control system’s input reflect the temperature change of the whole unit. ofthose the whole unit.
Figure field of ofthe theaccelerometer accelerometerunit. unit. Figure13. 13.The Thesteady-state steady-state temperature temperature field
Hiding all other structures and with heater open, the steady-state temperature field of 29991 accelerometer’s base and three accelerometers is shown in Figure 14. From Figure 14, we can get the information that the temperature gradient of the two lateral accelerometers is much larger than that of the plumb accelerometer, and the max temperature difference of the two lateral accelerometers
Sensors 2015, 15, 29984–29996
Figure 13. The steady-state temperature field of the accelerometer unit.
Hiding heater open, open, the the steady-state steady-statetemperature temperaturefield fieldof of Hidingallallother other structures structures and and with with heater accelerometer’sbase baseand andthree threeaccelerometers accelerometersisisshown shownininFigure Figure14. 14.From FromFigure Figure14, 14,we wecan canget getthe accelerometer’s the information thattemperature the temperature gradient of two the two lateral accelerometers is much larger than information that the gradient of the lateral accelerometers is much larger than that the plumb accelerometer, max temperaturedifference difference of of the of that the of plumb accelerometer, andand thethe max temperature the two twolateral lateralaccelerometers accelerometers ˝ ˝ head 0.15 whilethe themax maxtemperature temperature difference difference of head is 0.05 head is is 0.15 °C,C,while ofthe theplumb plumbaccelerometer accelerometer head is 0.05C.°C.
Figure fieldof ofthe theaccelerometer accelerometerbase. base. Figure14. 14.The Thesteady-state steady-state temperature temperature field
The temperature field of the three accelerometers is shown in Figure 15. It is evident that the The temperature field of the three accelerometers is shown in Figure 15. It is evident that the plumb accelerometer is much much more moreconsistent consistentthan thanthat thatofofthe the two lateral plumb accelerometerbody’s body’stemperature temperature field field is two lateral accelerometers, of which the max temperature difference is 0.44 °C, while the plumb accelerometer accelerometers, of which the max temperature difference is 0.44 ˝ C, while the plumb accelerometer body’s max temperature body’s max temperaturedifference difference is is 0.18 0.18 ˝°C. C.
Sensors 2015, 16, page–page
8
Figure of the the three threeaccelerometers. accelerometers. Figure15. 15.The Thetemperature temperature field field of
The analysis results show that the temperature control methods significantly improve the The analysis results show that the temperature control methods significantly improve the uniformity of the temperature field of the accelerometers compared with the accelerometer uniformity of the temperature field of the accelerometers compared with the accelerometer temperature data benefitsthe thestability stabilityofofthe theaccelerometer’s accelerometer’s output. temperature datainingeneral generalconditions conditions [9], [9], which which benefits output. 3.3.3.3. TheThe Optimized Design Control Optimized Designofofthe the2nd 2ndand and3rd 3rd Level Level Temperature Temperature Control ordertotominimize minimizethe thetemperature temperature gradient, gradient, we should In In order should optimize optimizethe thelayout layoutofofthe theheating heating sheet. The original3rd 3rdlevel levelheating heatingsheet sheet is is deployed deployed on base, as as sheet. The original on the the bottom bottomofofthe theaccelerometer accelerometer base, showninin Figure Figure 16a. optimized deployment of the of second-level heat sheetheat is shown 16b. in shown 16a.The The optimized deployment the second-level sheetin Figure is shown The temperature field comparison of the accelerometer unit using ANSYS, is shown in Figure 17. in Figure 16b. The temperature field comparison of the accelerometer unit using ANSYS, is shown Comparing the temperature field in the original layout with the optimized one, the newly deployed Figure 17. Comparing the temperature field in the original layout with the optimized one, the newly scheme can significantly lessen the temperature difference in the two lateral accelerometers, that deployed scheme can significantly lessen the temperature difference in the twoso lateral the temperature more homogeneous. datum of temperature differences in of accelerometers, sofield thatcan thebetemperature field canThe be detailed more homogeneous. The detailed datum the two deployment scheme is shown in Table 2. temperature differences in the two deployment scheme is shown in Table 2.
29992
sheet. The original 3rd level heating sheet is deployed on the bottom of the accelerometer base, as shown in Figure 16a. The optimized deployment of the second-level heat sheet is shown in shown in Figure 16a. The optimized deployment of the second-level heat sheet is shown in Figure 16b. The temperature field comparison of the accelerometer unit using ANSYS, is shown in Figure 16b. The temperature field comparison of the accelerometer unit using ANSYS, is shown in Figure 17. Comparing the temperature field in the original layout with the optimized one, the newly Figure 17. Comparing the temperature field in the original layout with the optimized one, the newly deployed scheme can significantly lessen the temperature difference in the two lateral deployed scheme can significantly lessen the temperature difference in the two lateral accelerometers, so that the temperature field can be more homogeneous. The detailed datum of Sensors 2015, 15, 29984–29996 accelerometers, so that the temperature field can be more homogeneous. The detailed datum of temperature differences in the two deployment scheme is shown in Table 2. temperature differences in the two deployment scheme is shown in Table 2.
Figure 16. (a) (a) Theoriginal original deployment of 3rd level heating sheet layout (high lightpart); green part); Figure deployment of 3rd levellevel heating sheetsheet layoutlayout (high light The Figure 16. 16. (a)The The original deployment of 3rd heating (highgreen light green(b)part); (b) The optimized deployment of 3rd level heating sheet layout (high light red part). optimized deployment of 3rd level heating sheet layout (high light red part). (b) The optimized deployment of 3rd level heating sheet layout (high light red part).
Figure 17. (a) The original temperature field; (b) The optimized temperature field. Figure (b) The The optimized optimized temperature temperature field. field. Figure 17. 17. (a) (a) The The original original temperature temperature field; field; (b) Sensors 2015, 16, page–page
Table 2. Temperature differences in the two layout method. Table 2. Temperature differences in the two layout method.
9 Two Lateral Accelerometers Max 9 Layout MethodTwo Lateral Accelerometers Layout Method Max Temperature Difference The original The original The optimized
The optimized
Temperature Difference 0.44 ˝ C 0.44 °C 0.21 ˝ C 0.21 °C
Plumb Accelerometers Plumb Accelerometers Max Max Temperature Difference Temperature Difference 0.18 ˝ C 0.18 °C 0.17 ˝ C 0.17 °C
Besides using using aa new new heating heating sheet sheet layout layout to to reduce reduce the the temperature temperature gradient gradient of of accelerometer accelerometer Besides unit, intensifying intensifying the The heat heat insulating insulating layer layer currently currently unit, the heat heat insulation insulation is is another another good good method. method. The used in the accelerometer unit is shown in Figure 18a. used in the accelerometer unit is shown in Figure 18a.
Figure Figure 18. 18. (a) (a) The The original original heat heat insulating insulating layer; layer; (b) (b) The The optimized optimized heat heat insulating insulating layer. layer.
As seen in Figure 18, there is a large amount of air in the whole unit and the air’s heat transfer As seen in Figure 18, there is a large amount of air in the whole unit and the air’s heat transfer conditions are relatively complex, which can influence the temperature stability. The optimized conditions are relatively complex, which can influence the temperature stability. The optimized heating layout is shown in Figure 18b and the material used in the two structures is the same (thermal conductivity ≤ 0.03 W/m·k). The mold with an optimized heat insulating structure and low 29993 coefficient of thermal coefficient coats the accelerometer unit. The new insulating structure reduces the amount of air in the unit and increases the thickness of insulating layer, so it is conducive to the temperature stability of the accelerometers.
Figure 18. (a) The original heat insulating layer; (b) The optimized heat insulating layer. Sensors 2015, 15, 29984–29996
As seen in Figure 18, there is a large amount of air in the whole unit and the air’s heat transfer conditions are relatively complex, which can influence the temperature stability. The optimized heating layout layout is is shown shown in in Figure Figure 18b 18b and and the the material material used used in in the the two two structures structures is is the the same same heating (thermal conductivity ≤ 0.03 W/m·k). The mold with an optimized heat insulating structure and low (thermal conductivity ď 0.03 W/m¨ k). The mold with an optimized heat insulating structure and low coefficient of of thermal thermal coefficient coefficient coats coats the the accelerometer accelerometer unit. unit. The The new new insulating insulating structure structure reduces reduces coefficient the amount of air in the unit and increases the thickness of insulating layer, so it is conducive to the the the amount of air in the unit and increases the thickness of insulating layer, so it is conducive to temperature stability stability of of the the accelerometers. accelerometers. temperature 4. The 4. ThePerformance Performanceof ofSGA-WZ-02 SGA-WZ-02 Airborne Airborne Gravimeter Gravimeter order to to check the performance of SGA-WZ-02 with the optimized temperature control In order system design, both aa static output of system static experiment experiment and andan anairborne airborneexperiment experimentare arecarried carriedout. out.The The output SGA-WZ-02 Z-axis quartz flexible and of SGA-WZ-02 Z-axis quartz flexibleaccelerometer accelerometerininthe thestatic staticexperiment experimentisisshown shown in in Figure 19 and compared with withthat that in SGA-WZ-01. Compared with 3,Figure 3, quartz the z-axis flexible compared in SGA-WZ-01. Compared with Figure the z-axis flexiblequartz accelerometer accelerometer output in the SGZ-WZ-02 better than the accelerometer output inand thethe SGZ-WZ-01 output in the SGZ-WZ-02 is better than theisaccelerometer output in the SGZ-WZ-01 accuracy and the accuracy the needs of airborne gravimetry. satisfies the needssatisfies of airborne gravimetry. The comparison of SGA-WZ-01 and SGA-WZ-02 Z-axis Q-flex accelerometer output in the static experiment SGA-WZ-01 std:0.29mGal SGA-WZ-02 std:0.21mGal
9.7913
m/s2
9.79129
9.79131
9.7913
Sensors 2015, 16, page–page 9.79129
The SGA-WZ-02 airborne experiment was carried out in western China with GT-2A in the same aircraft, and Figure 20 shows the aircraft cabin, in which both SGA-WZ-02 and GT-2A are hour shown. The result of repeated surveying is shown in Figure 21 and the result comparison with Figure 19. The comparison of SGA-WZ-01 SGA-WZ-01 and SGA-WZ-02 flexible accelerometer Figure 19. The comparison of SGA-WZ-02 accelerometer GT-2A is shown in Figure 22, with the internaland accuracy beingZ-axis 0.742 quartz mGal/4.8 km in the experiment output in the static experiment. output in the static experiment. and the external accuracy compared with the result of GT-2A being 0.37 mGal. The flight track is shown in Figure 23. The result is better than the internal accuracy in SGA-WZ-01, which is about 10 The SGA-WZ-02 airborne experiment was carried out in western China with GT-2A in the same 1.5 mGal/4.8 km. aircraft, andexperiment, Figure 20 shows the aircraft in which both SGA-WZ-02 are shown. In the the problem of cabin, temperature control system seenand in GT-2A SGA-WZ-01 does The not result ofinrepeated surveying is shown in Figure 21 and with GT-2A is shown appear SGA-WZ-02. The reason is that the TEC usedthe in result the 1stcomparison level temperature control works in Figure 22, to with the internal accuracy being 0.742caused mGal/4.8 km inheat the generation experimentand andhot theweather, external well enough neutralize the temperature change by inner accuracy compared design with the of GT-2A being mGal. Thecontrol flight track is shown Figure 23. and the optimized ofresult the 2nd and 3rd level0.37 temperature guarantees theinstability of Thequartz result is better accelerometer than the internal accuracy in SGA-WZ-01, which is about 1.5 mGal/4.8 km. the flexible output. 0
5
10
15
Figure 20. Cabin Figure 20. Cabin of of the the aircraft. aircraft. -70 -80
G al)
-90 -100
Repeat surveying line result (internal accuracy:0.742mGal/4.8Km)
29994
20
25
Sensors 2015, 15, 29984–29996
In the experiment, the problem of temperature control system seen in SGA-WZ-01 does not appear in SGA-WZ-02. The reason is that the TEC used in the 1st level temperature control works well enough to neutralize the temperature change caused by inner heat generation and hot weather, and the optimized design of the 2nd and 3rd level temperature control guarantees the stability of the quartz flexible accelerometer output.Figure Figure 20. 20. Cabin Cabin of of the the aircraft. aircraft. Repeat surveying surveying line line result result (internal (internal accuracy:0.742mGal/4.8Km) accuracy:0.742mGal/4.8Km) Repeat
-70 -70 -80 -80
dG dG(m (mGGal) al)
-90 -90 -100 -100 -110 -110 -120 -120 -130 -130 -140 -140
79.2 79.2
79.4 79.4
79.6 79.6
79.8 79.8 longitude(deg) longitude(deg)
80 80
80.2 80.2
80.4 80.4
Figure 21. The repeatsurveying surveyingline lineresult result(internal (internal accuracy: accuracy: 0.742 0.742 mGal/4.8 km). Figure The repeat surveying line result (internal km). Figure 21.21. The repeat accuracy: 0.742mGal/4.8 mGal/4.8 km).
Sensors 2015, 16, page–page
Figure22. 22.The Thesurveying surveying result result compared compared with GT-2A. Figure 22. The surveying Figure result comparedwith withGT-2A. GT-2A.
11flight track Repeat 11 40.35 40.3
latitude(deg)
40.25 40.2 40.15 40.1 40.05 40 79.2
79.4
79.6
79.8 longitude(deg)
80
80.2
80.4
Figure23. 23.The The flight flight track track of Figure of repeat repeat line lineexperiment. experiment.
5. Conclusions 5. Conclusions The optimized design of the temperature control system in the SGA-WZ-02 based on the The optimized design of the temperature control system in the SGA-WZ-02 based on the design experience of the SGA-WZ-01 system was presented. The SGA-WZ-01 cannot overcome the design experience of the SGA-WZ-01 system was presented. The SGA-WZ-01 cannot overcome the temperature changes caused by hot weather and the set value of the 1st level temperature control temperature changes caused by hot weather and the set value of the 1st level temperature control was was too high to ensure the service life of laser gyros, so TEC is added to the temperature control toosystem high toofensure the serviceThe life experiments of laser gyros, so TECthat is added to the accuracy temperature control system the SGA-WZ-02. indicate the stability of the new 1st level of thetemperature SGA-WZ-02. The experiments indicate that the stability accuracy of the new 1st level temperature control system is superior to 0.1 °C, and the max cool down capability is about 15 °C ˝ C, and the max cool down capability is about 15 ˝ C and 11 ˝ C control system is superior and 11 °C compared withto the0.1 outlet air temperature and the right part of sensor box, respectively. compared with the air temperature of sensor box, respectively. Using ANSYS, Using ANSYS, a outlet finite element model ofand the the 2ndright and part 3rd level temperature control components is built to analyze the temperature field. Through heat analysis, besides the new temperature measurement points of the 2nd and 3rd level 29995 temperature control system selected, the optimized heating sheet layout and the new heat insulating layer are given. The max temperature difference of the accelerometer is 0.44 °C before the optimized design and 0.21 °C after optimizing the heating sheet layout. A new heat insulating layer is used to guarantee the temperature stability of the
Sensors 2015, 15, 29984–29996
a finite element model of the 2nd and 3rd level temperature control components is built to analyze the temperature field. Through heat analysis, besides the new temperature measurement points of the 2nd and 3rd level temperature control system selected, the optimized heating sheet layout and the new heat insulating layer are given. The max temperature difference of the accelerometer is 0.44 ˝ C before the optimized design and 0.21 ˝ C after optimizing the heating sheet layout. A new heat insulating layer is used to guarantee the temperature stability of the accelerometer. Both the static experiments (0.21 mGal/24 h) and airborne experiments (0.742 mGal/4.8 km) of SGA-WZ-02 show better results compared with those of SGA-WZ-01 (0.28 mGal/24 h in static experiments and 1.5 mGal/4.8 km in airborne experiments). The conclusion of the optimized design can provide meaningful ideas for the construction of the strapdown airborne gravimeter temperature control system, and can also offer references for the temperature compensation [13] and heat optimization of strapdown inertial navigation systems. Acknowledgments: This study was supported by the National High-Tech Research & Development Program of China (Grant No. 2013AA063902). Author Contributions: In this study, Juliang Cao designed the temperature control system in SGA-WZ-01 and SGA-WZ-02, maintained the hardware of SGA-WZ02 system and contributed other assistant tools. Minghao Wang performed the experiments, analyzed the temperature field and wrote the paper. Shaokun Cai analyzed the test data, Meiping Wu and Kaidong Zhang conceived and designed the experiments. And Danni Cong modified English errors. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Cai, S.K. The Research about Airborne Vector Gravimeter and Methods of Errors Separation. Ph.D. Thesis, National University of Defense Technology, Changsha, China, 2014. Zhang, K.D. Research on the Methods of Airborne Gravimetry Based on SINS/DGPS. Ph.D. Thesis, National University of Defense Technology, Changsha, China, 2007. Cai, S.K. Long-term stability of the SGA-WZ strapdown airborne gravimetry. Sensors 2012, 12, 11091–11099. [CrossRef] [PubMed] Zhang, P.F.; Long, X. Research on temperature compensation model of inertial sensor in mechanically dithered RLG’S SINS. J. Astronaut. 2006, 27, 522–526. Wang, Y.; Shang, S. The temperature field analysis of quartz-flex accelerometer. J. Transducer Technol. 1996, 3, 8–14. Yu, Y.; Zhong, Y. High precision robust temperature control for an accelerometer unit with multi-operating conditions. J. Syst. Sci. Math. Sci. 2007, 27, 378–386. Yu, Y.; Zhong, Y. High precision two-stage robust temperature control for accelerometer Unit. Acta Aeronaut. Astronaut. Sin. 2009, 30, 1103–1104. Yu, X.; Zhang, P.; Tang, J. Finite element analysis and experiments of temperature fields of mechanically dithered ring laser gyroscopes. Opt. Precis. Eng. 2010, 18, 913–920. Li, J.L. Research on Thermal Control Technology of Airborne Gravimetry System. Ph.D. Thesis, National University of Defense Technology, Changsha, China, 2011. ANSYS Co. ANSYS-Simulation Driven Product Development. Available online: www.ansys.com (accessed on 27 November 2015). Incropera, F.P. Fundamentals of Heat and Mass Transfer, 6th ed.; Chemical Industry Press: Beijing, China, 2007. Li, J. Airborne Gravimetry System Thermal Control Technique Research. Ph.D. Thesis, National University of Defense Technology, Changsha, China, 2011. Becker, D. Drift reduction in strapdown airborne gravimetry using a simple thermal correction. Geodesy 2015. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons by Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
29996
