Design of seismic detector based on acceleration sensor

introduction

A seismic geophone is a special sensor used in geological exploration and engineering measurement. It is a sensor that converts ground vibrations into electrical signals. It can convert ground vibrations caused by seismic waves into electrical signals, which are converted into binary data through an analog/digital converter for data organization, storage, and calculation processing.

An accelerometer is an electronic device that can measure acceleration force. It is typically used in mobile phones, laptops, pedometers, and motion detection. This design uses Freescale's MMA7455L to implement the design of a seismic detector tester. It has signal conditioning, temperature compensation, self-test, and can be configured to detect 0g or pulse detection of rapid motion. The product has the characteristics of low power consumption, easy to carry, high accuracy, and high speed.

1 Hardware Circuit Design

The core hardware of the seismic detector is DSP controller, acceleration sensor, Flash memory, keyboard, LCD display and serial port external interface, etc. Some circuit diagrams are shown in Figures 1 to 3.

The relevant pin signal descriptions of MMA7455L are listed in Table 1.

MMA7455L provides I2C and SPI digital interfaces, but MMA7455L should be used as a slave device. When CS is pulled high, it is an I2C interface, and when CS is used as a slave selector, it is an SPI interface. The I2C interface is used in this device, and its slave address is 0x1D, which supports multi-byte read and write. AVDD is in the range of 2.4 to 3.6 V, with a typical value of 2.8 V; DCC 10 is between 1.71 V and AVDD, with a typical value of 1.8 V. Considering the circuit and external interface conditions, the DVDD_IO voltage is selected according to the I2C interface level of the CPU. If a switching power supply is used, it should be noted that the switching frequency must be greater than 250 Hz to prevent interference with the internal ASIC of the chip. [page]

The maximum sampling rate of MMA7455L is 250 Hz, and its data rate can be greater than 2.5 kbps; there is no relevant calculation formula for the I2C pull-up resistor in the obtained information, so the empirical value of 4.7 kHz can be used. If it needs to pass through an analog switch or the wiring is long, it can be appropriately reduced.

MMA7455L provides two interrupt output pins, and INT1 is shared with DRDY. In actual use, the output of INT1 and INT2 should be determined according to the application requirements. In the above typical circuit diagram, only INT1 is used.

The memory part uses AMD's 32 Mb Flash memory AM29LV033C.

The system control processor uses TMS320DM642, which is suitable for occasions with large amounts of digital signal processing and high real-time requirements. Its powerful computing power can meet the system's real-time data computing and graphical interface display. TMS320DM642 is a TI series DSP, and 24WC256 is a 256 Kb I2C serial CMOS EEPROM.

2 Control Software Design

The data acquisition and processing program flow is shown in Figure 4. The initialization includes system power-on, I/O port initialization, and human-machine interface initialization; the accelerometer requires zero point calibration and self-test to meet the accuracy requirements; after the system calls the detection task, the external interrupt is turned on, waiting for the interrupt report of the accelerometer and the sampling data collection.

When designing the MMAT455L driver software, it should be noted that when IADDR0 is connected to GND, its slave address is Ox1D, and when it is connected to DVDD_10, its slave address is Ox1E; MMA7455L has three working modes (measurement mode, level detection mode, and pulse detection mode). In the measurement mode, the DRDY pin is shared with INT1, and data reading can clear DRDY, but in the other two modes, both INT1 and INT2 need to be cleared by writing registers.

The following is the accelerator detection part of the program:

Conclusion

By using embedded technology and modern digital signal processing technology, this system can achieve all the functions of the previous front-end detectors, and has obvious advantages in size, weight and power consumption. After further optimization and improvement, the system will become an effective tool for seismic detection.