SEISMIC DATA RECORDER UNIT USING FLASH MEMORY SD CARD

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1 SEISMIC DATA RECORDER UNIT USING FLASH MEMORY SD CARD Santiago Cruz Lauro 1, Hernández Arias Humberto 1, Mijares Arellano Horacio 2 Instituto de Ingeniería, 1 Coordinación de Instrumentación, 2 Coordinación de Ingeniería Sismológica Universidad Nacional Autónoma de México lsc@pumas.iingen.unam.mx ABSTRACT This work describes a seismic data recorder unit, developed for a digitizer unit named SR04. The paper shows the way to solve the currently problem faced by researches who want to use the SR04 unit as a stand-alone system. The SR04 unit is a seismic data digitizer. When it is used for data collecting and processing information, it must be connected on line to a personal computer (PC). If we want to use this unit in a stand-alone process that it is not possible, in the cases we want to let the equipment in the field registering long sample periods of time that will not be possible without a PC. This paper presents a solution to the described problem. It shows the hardware and software items of a seismic data recorder unit using flash memory cards. The developed interface unit can operate as a stand- alone system. With this unit we can collect, display and record the data coming from the SR04 unit. Using this unit we can register data for long periods of sample time. Developed around a microcontroller, it contains serial USB and SR232 communications interfaces, a display, selection buttons, a flash memory SD (Secure Digital) Card (1 to 8 Gigabits capacity) and digital control electronic circuits. The data is recorded using the FAT (File Allocation Table) protocol for 16 and 32 bits file systems, so we can read the collected data in any PC and process it with any software for processing seismic data or to use proprietary software developed for such purpose. It concludes, after several months of test, that the researchers can confidence use the developed unit for seismic data collection and to solve the problem of a stand-alone system for long periods of work. We have been working with the unit at 50, 100, 200 samples per second and registered data for many months. Key words: FAT, Flash SD, microcontroller, File System. 1. INTRODUCTION In the Coordinación de Ingeniería Sismológica del Instituto de Ingeniería (II) de la Universidad Nacional Autónoma de México (UNAM), we have seismic data acquisition equipments. These equipments are taken to the field to make data registration using portable computers. One of the most used equipment is the SR04 seismic unit. This unit is a data digitizer, used to convert a seismic signal to a digital one. One of the problems that we have with the unit SR04 is that for saving, collecting and processing information, it always has to be connected to a portable computer. When we want to use it as a stand-alone device for long periods of sampling time, this wouldn t be possible without a computer. In order to solve the mentioned problem, we proposed to build an interface for collecting and saving the data, also that we can display it; with this interface, the SR04 unit could work without the need of a computer. The data registration is made in flash SD Card memory, which is of an actual technology and with the feature of to be portable. The gathered information could be read directly from the memory card with any personal computer that used the FAT protocol. 1

2 With the interface, in a Liquid Crystal Display (LCD), we can see the information sent by the three seismic sensors of the SR04 unit. Also with the LCD and four buttons we can configure the operation of the SR04 unit. 2. FLASH SD Card MEMORY The flash memories are the dominant technology on applications that demands a lot of quantity of storage information in a solid state memory with an affordable cost. This is the technology used in the memory cards. Now a days the memory flash cards are popular on portable applications, this is because they have a low consume of energy, reduced size, durability, portability and lower cost. One of the formats of memory flash card, more popular and most used, is the SD, made by Panasonic Corporation, SanDisk Corporation and Toshiba Corporation. The principal features of the SD Card are: big capacity of storage, the range of the feed voltage (2.7 to 3.6 V) and the read/write speed. The interconnection of the SD card and a microcontroller could be done using the communication protocol SPI (Serial Peripheral Interface). 3. FILE SYSTEM The memory flash cards, as the hard disk, compact disc and others storage devices, need to organize the data that is going to be saved or to be accessed. This action is done with the file system. A file system could be explained like a data base of specific purpose, which manipulates, organize and gives a structure to the data that is going to be saved. The purpose of these tasks is that the registered data could be read later with other equipment that works with the same protocol. The most used operative system around the world is Windows, and it works with the file system named File Allocation Table (FAT). There are three types of these file system: FAT12, FAT16 and FAT 32. The most used are FET16 and FAT32, the basic difference that exists between them is the number of bits they used in their work. A file system consists of 4 basic regions, figure 1: 0. Reserved region 1. FAT region. 2. Directory root region (doesn t exist for the FAT 32). 3. File, directories and data region. Figure 1. Common Structure of a FAT format. 2

3 4. SEISMIC UNIT SR04 The SR04 unit contains 3 seismic sensors and 2 electronics cards, as is show on the figure 2. The sensors are geophones, used to measure the 3 components of movement (x, y, and z). One of the cards is the SADC20, this card makes the analog to digital conversion, using 24 bits, while the other one is the GPSDCF, containing a Global Positioning System (GPS) receptor used to establish the reference time for the data. Figure 2. SR04 Seismic Unit. The data coming from the SR04 unit are 5 bytes for the data information of each one of the channels and 9 bytes for the data time. Sending some commands to the SADC20 we can get the version of the firmware and the information of the EEPROM that it has. Also, we can configure the parameters operation of the unit, like the time, the date and clock compensation, this commands consists of 6 bytes. 5. DEVELOPMENT Hardware The figure 4 shows the principal elements that contain the interface unit. The nucleus of the system is a microcontroller of 8 bits. The features that this device have satisfies the requirements of the interface, this means the interconnection with the SD memory, the use of the graphic display with the keyboard and the acquisition of the data coming from the seismic unit. Figure 3. Block diagram of the proposed system. 3

4 The developed system allows the use of the standard memory SD and the SD cards of high capacity (SDHC). The communication between the SD card and the microcontroller is made thru the Serial Peripheral Interface (SPI) protocol. The SD card works with a voltage between 2.7 and 3.3 V, in fact all the system will be feed with 3.3 V. To communicate the microcontroller with the SR04 unit we use the RS232 serial protocol. Software The program to operate the developed system was built using C language and compiled using the C18 compiler of Microchip. When we start the developed program it shows a menu in the display, figure 4, where we can select the following task: Configuration, Test mode and Register mode. The selection is made using a four push buttons keyboard. Figure 4. The developed system. In the Configuracion option we can choose between the 3 options of sampling frequency that the SR04 uses; 50, 100 or 200 samples per second. The communication rate is made at bauds, using 8 bits of data, 1 bit of stop and without parity. If the chosen option is Modo de prueba, the display will show another menu, in which we can choose between the 3 channels information and the package time information that the GPS card will give. When we choose one of the 3 channels, we can see the graphic of the information of that sensor; if we choose the option of time, we will see the date and time GMT in that moment. At the Registro option, it could be set if we want a register based on hours (maximum 255) or if we prefer on days (max. 30). In this last option the interface will start to collect and to register the data with time previously selected. 6. TEST RESULTS In order to evaluate the correct operation of the interface, we tested it to comply the defined requirements. For this work we used a sinusoidal signal coming out from a function generator as an input to the digitizer. The signal was of 0.5V in amplitude and with a frequency of 1 Hz. In the Modo de Configuración we define different options of sample frequency, with the purpose of obtaining different graphics at different rates of register of the input signal. In figure 5 we show the gathered signal for the three channels at 100 samples per second (sps). The acquired signals were of the same amplitude and frequency as the input. 4

5 Figure 5. Three channels display at 100 sps. Also, the test option allows us to verify if the GPS of the GPSDCF card is synchronized. When selected the time package the interface links with the seismic unit and it started to take data, waiting for the identification byte of the time package. The program of the interface reads the information of time and displays it as we can see on the figure 6. Figure 6. Decoded time package. To verify that the collected information of the time package is the right one, we compared it with the date that we got from internet, at the web page that gives the GMT exactly time. In figure 7 it is shown the screen with the GMT time. Figure 7. GMT time. 5

6 Another test was to program the interface to operate for 30 days, collecting information at 100 mps. At the end of this test and after we took out the SD card from the interface, we got the information gathered in it. Using a PC we read the files registered in the SD card. As we can see on the figure 8, it is confirmed the creation of 15 files of the maximum requirement size, that contain the information created by the seismic unit SR04 during the 30 days of operation. Figure 8. The registered files, for 30 days at 100 mps. We made data register tests for the 3 different options of sampling frequency; 50, 100 and 200 samples per second. In the figure 9 we can see that the generated graphics for the 3 channels are almost the same, also we verified that for this test we obtain a period of the signal in 50 samples. For the amplitude, doing a zoom for any of the graphics, we can see that the registered signal has 4.69x10 6 accounts. Considering that each account is equal to 119nV, the amplitude of the register signal has a value of 0.55V, that is congruent with the value of the input signal. Also we read from the SD Card the data of the time package, which this information we generated a table as it is shown in figure 10. We can see that the collected data match with the date and time of the test and that there are not information loses. 6

7 Figure 9. Graphic generated with the information in the SD memory. Figure 10. The registered data of time. Finally we realized a Register test of data, for evaluating the system operation on his final stage, using real information coming from the sensors connected to the digitizer, card SADC20, during a period of 15 days, with 200 samplings per second. In the figure 11 we see the features of the 15 generated files; on figure 12 we show the data time gathered in the 17AGO10A.TXT and 17AGO10O.TXT files. In the figure 13 we show the graphics that we got with the storage information in the 17AGO10E.TXT file. 7

8 Figure 11. The registered files, for 15 days at 200 mps. Figure 12. Time marks extracted from the 17AGO10A.TXT and 17AGO10O.TXT files. 8

9 Figure 13. Graphics from the 17AGO10E.TXT file. We did many tests for different periods of time and we got confidence about the operation of the interface. 7. RESULTS AND CONCLUSIONS The most important results from the development of the register interface are: - The developed interface built around a microcontroller has digital electronic circuits, a graphic display, a keyboard and slots for the SD Card and RS232 and USB serial communications ports. The interface allow us to selected different functions like: Sampling frequency configuration To test the behavior of the connected sensors for each channel of the unit SR04 To show the Greenwich Mean Time (GMT), date and time. To program the operation time in which the interface will collect and register data -Using the Configuración option we can establish the sampling frequency for the work of the seismic unit, it can be chosen between 3 different frequency of samplings value: 50, 100 y 200 mps -Using the Prueba option we can select one of the 3 channels of the SR04 unit, so the interface can communicate with the unit, to get the generated information by this channel and to plot it. Also it is show the time and date current give by the GPS. -By the Registro option it can be chose the amount of time in which the interface will be registering the incoming data of the seismic unit. -The register data it is made by files, these files can be read by any computer equipment that has compatibility with FAT files. With the help of a basic prosecution we get the information that the seismology experts need. -The system is able to register to 30 days of information at 100 mps, need for this a 4 GM memory card. 9

10 The interface will display ERROR if we use a non compatible SD Card or if it has a different capacity from 1, 2, 4, and 8 GB. The final development prototype is shown on the figure 14. It allows to integrate the functions mentioned to the SR04, making easy the task of the specialist on seismologic engineering, making this without a computer and doing the register of data for big periods of time and not for configured the sampling frequency, and also that allows to verify the correct operation of the seismic unit SR04 and the sensors connect to the computer. Figure 14. The developed Interface. After several months of test, we conclude that the researchers can confidence use the developed unit for seismic data collection and to solve the problem of a stand-alone system for long periods of work. 8. REFERENCES [1] SD Specifications, Part 1 Physical Layer Simplification Version 2.00 September 25, 2006, SD Group [2] 8-bit PIC Microcontroller (PIC18F452) datasheet. [3] Microsoft Extensible Firmware Initiative FAT 32 File System Specification FAT: General Overview of On-Disk Format. Version 1.03, December 6, 2000 Microsoft Corporation [4] SADC10/18/20/30-Communication protocol. Document revision 23 rd March 2004 SARA di Mariotti Gabriele and C snc Perugia [5] Understanding the FAT32 File system [6] Very low drop voltage regulators with inhibit (LP33CV) datasheet. [7] 3.0V to 5.5V, Low-Power, up to 1 Mbps, True RS-232 Transceivers Using Four 0.1µF external Capacitors (MAX3222) datasheet. [8] Kernigham Brian W., Ritchie Dennis M., El lenguaje de programación C, segunda edición, Pearson Prentice Hall, México [9] Lázaro Antoni Manuel, del Rio Joaquín, LabVIEW 7.1 Programación Gráfica para el control de instrumentación, Thomson, [10] Palacios Enrique, Remiro Fernando, López Lucas, Micro controlador PIC16F84 Desarrollo de proyectos, primera edición, Alfaomega, 2004 [11] MPLAB C18 C Compiler Libraries Microchip Technology Inc,