Positioning System Performance Based on Different Pressure Sensors

Transcription

1 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp. -6 Sensors & Transducers 4 by IFSA Publishing, S. L. Positioning System Performance Based on Different Pressure Sensors Zhengqun Hu National Astronomical Observatories, Chinese Academy of Sciences, A Datun Road, Chaoyang District,, Beijing, China Graduate University of Chinese Academy of Sciences, 8 Zhongguancun East Road, Haidian District, 86, Beijing, China Tel.: huzq@nao.cas.cn Received: March 4 /Accepted: 3 May 4 /Published: 3 June 4 Abstract: The atmospheric pressure sensor is used to be auxiliary method in positioning system, which is used as an elevation constraint to solve positioning equation, the pattern in the height measurement accuracy is better than m. Pressure sensors in different sensitivities are with different performance, two classic atmospheric pressure sensors MS5534C and BP567 were analyzed in the performance and depth analysis of the sensitivity in the pressure sensors is done, due to the measurement sensitivity, resolution is different, the performance in positioning system may be different. The experiments show that the two sensors both have a good highprecision in measurement. However the stability of BP567 is better than the MS5534C at fixed pot measurement. The impact of temperature on MS5534 is greater than BP567 which is further analyzed. BP567 applied in the positioning system has a good performance than MS5534C in Differential Barometric Altimeter (DBA), especially in elevation auxiliary positioning system, which will have a great application prospect in the relevant field. Copyright 4 IFSA Publishing, S. L. Keywords: DBA, Sensor, MS5534C, BP567, MEMS, Sensitivity, Positioning system.. Introduction Sensor technology has been widely applied in our daily life. High-sensitivity, high-precision is the direction of sensor development in the industry. If sensing information is used as a secondary condition in positioning system, which can effectively improve the performance of positioning system. Efficient sensor technology is used to assist positioning system to complete the combination navigation, and then obtain precise location information. This method in today's growing demand for location information services will have a good prospect for development []. Generally, the measurement accuracy of the elevation is not ideal in current navigation positioning system, especially in the positioning system based on base station [-3], the mobile user and the base station in elevation almost appear a coplanar status, the user can t form a good measurement constraint reference in the direction of elevation, so the measurement accuracy of elevation is lower, usually some auxiliary methods are used to improve the measurement accuracy of elevation [4-6]. This article will introduce MS5534C and BP567 into the system of Differential Barometric Altimeter (DBA) based on base station, research and analysis performance of differential barometric altimeter

2 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp. -6 system based on base station by the sensor MS5534C and BP567, the different resolution and sensitivity brings into different impact on the DBA system performance.. Acquisition Data in MS5534C and BP567 Micro-electro mechanical system (MEMS) is developed based on the microelectronics technology into a cutting-edge research of multi-disciplinary field, which has become one of the major scientific and technological fields of the world's attention. The sensors MS5534C and BP567 are both based on MEMS technology, which both can measure air pressure and temperature. After resetting the interface system initialization, CPU reads MS5534C register to obtain word word4 through the serial port. It uses logical operation and shift operation to calculate the compensation parameter C C6 and reads pressure data D and temperature data D in 6 bits through three lines of synchronous serial interface. Pressure and temperature calculated by compensation algorithm can be obtained with D, D and C C6, which will output after software filtering. Software is designed by modular method, which includes system initialization (relevant register initialization) [7], sensor reset, read word word4, conversion between pressure and temperature, C C6 coefficients calculation, temperature and pressure calculated by compensation algorithm, temperature compensation, filtering, etc. Software flow chart of MS5534C is shown in Fig.. Fig.. Software flow chart of MS5534C. A SPI interface is used between microprocessor and sensor BP567 to be interactive communication, so we can obtain pressure and temperature data. There are five basic commands in sensor BP567: reset - read PROM digital (C ~ C6 coefficient) - D conversion - D conversion - read ADC and output data [8]. Through these five basic instructions we can obtain pressure and temperature acquisition data in BP567, the workflow is shown in Fig.. Fig.. Software flow chart of BP Sensitivity Analysis of Pressure Sensor Based on MEMS Sensors based on MEMS technology have the advantages such as can improve the signal-to-noise ratio, adjust the output signal to improve sensor performance, realize multifunction sensing, can be ignored influence of attached SMD chip tantalum capacitors plus quality to the system. Generally, sensitivity is an important physical quantity of the measuring instruments, to improve the sensitivity can achieve higher measurement accuracy, but the higher of the sensitivity, the measuring range is often narrowed, the stability is often worse. Therefore, the sensitivity of design range is also an important technical indicator. MS5534C and BP567 sensor both belongs to piezoresistive silicon micro-mechanical sensor [7-8], ADC conversion will transform the uncompensated analog voltage of pressure value and temperature value form the piezoresistive sensor into digital signal (for MS5534C is 6 bits, for the BP567 is 4 bits), this analog voltage signal is prototype-data of the final output atmospheric pressure value. Resistance silicon strain sensitive factor is of 5~ times higher than the metal strain gauge, the sensitivity of this type sensor is very high, which has an output full-scale about mv in general. For silicon piezoresistive sensor when the piezoresistive change, the output voltage signal change as follows. R Vinπ Vout Vin δ - δ () t R In the case of constant pressure, the pressure sensitivity of sensor is defined as the amount of relative change of output signal caused by per unit pressure change.

3 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp. -6 S Vout R π δ δ t, () V P R P P hours and calculates the height of user by the Eq. (4). The calculated height is shown in Fig. 4. where ( - t ) is the difference of orthogonal stress, ΔP as the pressure change amount, π is the constant, therefore, enhance the difference between orthogonal stress can increase the sensitivity of such sensors and then improve the measurement accuracy. Under normal circumstances, we always hope that the sensitivity is a constant [9], which is conducive to the corresponding relationship between the input value and the output quantity, for easy reading, but in fact the sensitivity mostly only maintains a constant in a measurement range. So we must determine the linear range before we use it. Therefore, in the application of the atmospheric pressure sensor, the sensor must be located as far as possible to meet the linear range to work, such as to meet the temperature and other conditions. For a measurement system which is made of multiple links, the total sensitivity can be obtained by product of sensitivity in each link. If all sensitivities of each link were S, S, S 3,, S n, the systemic sensitivity can be expressed as S S S (3) Sensitivity describes the absolute relationship between input and output, which reflects the degree of sensitivity for the input quantity in the measurement system []. The sensitivity is higher, the range may be smaller. It may greatly affect the scope of application for the measurement system. Therefore, the sensor system needs to balance the relationship among the sensitivity, resolution and accuracy in application. S n 4. Experimental Analysis The Eq. (4) shows mathematical model of Differential Barometric Altimetry []. t H H 73.5 m 84 lg P, (4) P where t m is the average centigrade degree between the two barometric surfaces of P and P, P is the barometric value of the reference point, P is the barometric value of receiver in user, H is the exact height of the reference point. 4.. Experiment of Single Fixed Point Stability The diagram of difference barometric altimetry is shown in Fig. 3. Barometric altimeter module by MS5534C and BP567 in user is put on the base station location, the height of base station is assumed to be m. The module collects pressure and temperature values in Fig. 3. Diagram of difference barometric altimetry. - - : 3: 6: 9: : 5: 8: : Beijing time(hh:mm) (a) Calculated height with time in hours by MS5534C : 3: 6: 9: : 5: 8: : Beijing time(hh:mm) (b) Calculated height with time in hours by BP567 Fig. 4. Calculated height of single fixed point by MS5534C and BP567 in hours. The statistics of calculated height by different sensors in user including minimum, maximum and standard deviation are shown in Table. Table. Statistics of single fixed point calculated height within hours. Sensors Min/m Max/m Standard deviation/m MS5534C BP From the Fig. 4 and Table, we can obtain that BP567 have higher pressure measurement sensitivity, the elevation measurement accuracy is more high compare with MS5534C in differential barometric altimeter. The advantage of highsensitivity, high-resolution pressure sensor on the elevation measurement accuracy is well represented. 4.. Analysis to Positioning System Affected by Different Sensors From the Section 3., we can know that elevation measurement accuracy used sensor MS5534C can achieve m ( 3 δ ), elevation measurement accuracy of used sensor BP567 can reach. m ( 3 δ ) in DBA system. 3

4 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp. -6 Effect of the error in DBA to the threedimensional accuracy is always independent, which is an approximately linear relationship with the threedimensional error. The linear coefficient can be approximated to. Assuming that the threedimensional positioning error with elevation constraint solving equations is 35.4 m, the relationship factor between elevation error in DBA and three-dimensional positioning error is.3. Fig. 5 shows effect curves between the two different sensors and the three-dimensional error in the elevation auxiliary positioning system. Three-dimensional error (m) Three-dimensional error (m) Error in height by DBA (m) (a) Relationship between three-dimensional error and height error by MS5534C in DBA Error in height by DBA (m) (b) Relationship between three-dimensional error and height error by BP567 in DBA Fig. 5. Relationship between three-dimensional error and height error by different sensors in DBA. Therefore during the height constraint solving orientation equations, the height error of DBA approximately equals to the three-dimensional positioning error introduced by elevation constraint conditions Experiment for Distinguishing Different Heights The differential station height is defined as. We respectively use MS5534C and BP567 sensor altimeter module to distinguish different heights. The MS5534C altimeter terminal distinguished to.5 m and m. The BP567 altimetry terminal respectively resolved to. m and m. The curves of calculated height are shown in Fig. 6 for MS5534C and BP567. From Fig. 6, we can know that BP567 elevation resolution exceeds one order of magnitude higher than the MS5534C. Theoretically BP567 has. mbar pressure resolution which can realize elevation resolution to achieve better than. m, but due to the interference impact of hardware design factors which will have reservations in the practical applications. Even so, as shown in Fig. 6, for the height of. m, BP567 still can realize a good distinguishing to the height, but the MS5534C almost presents a fuzzy state in distinguishing sub-meter (a) Distinguish to.5m by MS5534C (b) Distinguish to m by MS5534C (c) Distinguish to.m by BP (d) Distinguish to m by BP567 Note: The solution curve is obtained by the sensor moving up in different height and returning to the original place. Fig. 6. Solution curve of different calculated heights by MS5534C and BP567. 4

5 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp Analysis to the Sensors Affected by Temperature Due to the effects of temperature on the sensor, the same external pressure will produce a different stress distribution under different temperature conditions. Under relatively high temperature conditions, the flat membrane and the beam diaphragm of MEMS sensor chip may also have a relatively large change which is not conducive to the linear characteristic of sensor. Therefore, it is necessary to research and analyze the differential performance between of these two sensors in differential barometric altimeter system by the temperature factor. Many temperature fields are established, atmospheric pressure is relatively stable within a short period of time. Therefore, by changing the external temperature conditions, the amount of change in measured air pressure equivalent to the pressure measurement error affected by temperature. Fig. 7 shows the curve of pressure error with temperature under different temperatures between MS5534 and BP567, Fig. 8 shows the curve of calculated height with temperature in Differential Barometric Altimeter by MS5534 and BP567 sensors. In the Differential Barometric Altimeter system, the influence of temperature on the performance of the sensor MS5534 is greater than BP567. The temperature is higher, the bend degree of the internal sensor diaphragm is greater and the linearity is lower. 5. Conclusions Efficient means of sensor technology as positioning information auxiliary measures have been in-depth development. Practice has proved that DBA based on base station in positioning system has a wide range of applications. This paper has done a research and analysis of the application performance by the two sensors MS5534C and BP567 in the DBA system. ) Sensitivity and stability for BP567 is better than MS5534C in DBA system. ) We can use the accurate elevation information to constraint solving plane positioning solution in order to further improve the accuracy of the positioning system and then can enhance the role of auxiliary to the positioning system. Affect to three-dimensional error by height error via BP567 is less than MS5534C in DBA system. 3) Due to the sensitivity (. mbar) of the BP567 is a high sensitivity pressure sensor, its variation in the height is more sensitive, the distinction will be higher. 4) We analyzed the sensors affected by temperature. The influence of temperature on the performance of the sensor MS5534 is greater than BP567 in DBA system. Pressure error (mbar) Pressure error (mbar) Calculated height (m) (a) Pressure error with Temperature in MS (b) Pressure error with Temperature in BP567 Calculated height (m) Fig. 7. Curve of pressure error with temperature (a) Calculated height with temperature in differential barometric altimetry by MS (b) Calculated height with temperature in differential barometric altimetry by BP567 Fig. 8. Curve of calculated height with temperature in differential barometric altimeter. The method based on digital pressure sensor for improving measurement accuracy in elevation which can make up for deficiencies of poor elevation measurement accuracy in positioning system. Different type of sensor in positioning system presents different performance. We need to choose the suitable sensor from different type of sensor for using in practice. Typically, BP567 sensor is more suitable for DBA system compared to MS5534C. All of these researches will have a certain reference value in relevant technical field. Acknowledgments This work was supported by the Key Research Program of the Chinese Academy of Sciences (KGFZD ); the National High Technology Research and Development Program of China (863 Program) (AA8); the Pilot Program for the New and Interdisciplinary Subjects of Chinese Academy of Sciences (KJCX-EW-J); the Knowledge Innovation Program of Chinese Academy of Sciences (KGCX-EW-47) and the 5

6 Sensors & Transducers, Vol. 7, Issue 6, June 4, pp. -6 Young Researcher Grant of National Astronomical Observatories 3, Chinese Academy of Sciences (O8353). [5]. References []. Zhengqun Hu, Lirong Zhang, Junxia Cui, et al., A Novel Study of the Pedestrian Integrated Navigation Monitoring System, Sensors & Transducers, Vol. 6, No., December 3, pp. -8. []. Zhengqun Hu, Lirong Zhang, Barometric altimeter in wireless communication network indoor positioning system, Journal of Beijing Institute of Technology, Vol., No. 3, 3, pp [3]. Cheng Liu, Lirong Zhang, Xiaohui Du, Terrestrial Mobile Network Positioning Method Assisted by Barometric Altimetry, Technology, Journal of Guilin University of Technology, Vol. 3, No.,, pp [4]. Lirong Zhang, Lihua Ma, Haifu Ji, et al., Positioning Accuracy of CAPS Subject to Earth Height [6]. [7]. [8]. [9]. []. Constraint, in Proceedings of the nd China Satellite Navigation Conference,, pp Xiaohui Du, Jun Pei, Lirong Zhang, et al., Differential barometric altimetry method based on mobile phone base stations, Journal of Beijing University of Aeronautics and Astronautics, Vol. 39, No., 3, pp Zhengqun Hu, Lirong Zhang, Application of differential barometric altimetry in indoor positioning system based on MS5534C, Transducer and Microsystem Technologies, Vol. 3, No.,, pp MEAS.MS5534C barometer module datasheet [EB/OL]. (6-7-7). MEAS. BP567 barometer module datasheet [EB/OL]. (--7). Weiwei Zheng, Xueguan Liu, Guangxia Zhao, Parameter Design and Analysis of Sensitivity MicroPressure Sensor, Instrument Technique and Sensor, 7,, pp Guangyu Liu, Ming Chen, New sensor technologies and applications, Beijing University of Aeronautics and Astronautics Press, Copyright, International Frequency Sensor Association (IFSA) Publishing, S. L. All rights reserved. ( 6