Design of wireless gait data acquisition system based on accelerometer

Introduction
Gait, as one of the biometric features, has important research significance in identity recognition and motion analysis. Many scholars at home and abroad have invested in this hot research. Gait research needs to be based on a large amount of reliable original gait data. Currently, the public gait database is based on gait images. However, images captured in dynamic environments are easily affected by various factors such as lighting changes and the shadow of moving targets, which brings great difficulties to the extraction of gait features. Ailisto H. et al. proposed a new method to obtain gait data using an accelerometer, which avoids the adverse effects of various factors in dynamic environments on capturing images, reduces the difficulty of data processing, and thus opens up a new way to obtain gait data. However, this method uses a laptop equipped with a DAQl200 data acquisition card to collect data, which is not only costly, but also inconvenient for the test subject to carry.
In recent years, with the development of sensor technology and the continuous improvement of manufacturing technology, MEMS (Micro-Electro-Mechanical-System) accelerometers with the advantages of small size, high precision, and low power consumption have entered the application field, making gait research based on motion sensors more convenient. The gait acceleration signal wireless acquisition device introduced in this article is designed using the main components such as MEMS three-axis accelerometer LIS3LV02DQ, wireless transceiver chip nRF2401, and 8-bit microcontroller μPD78F0547.

1 Hardware circuit design
The gait acceleration signal wireless acquisition device is mainly composed of two parts: data acquisition and wireless transmission module, wireless receiving and data transmission module. The main interface circuits include the CSIA0 serial interface circuit between the microcontroller and the accelerometer, the CSIll serial interface circuit between the microcontroller and the wireless transceiver chip, and the UART to USB bridge circuit. Its principle is shown in Figure 1 and Figure 2. The data acquisition and wireless transmission module fixed on the back of the human waist acquires and wirelessly transmits the three-dimensional acceleration data of the human body during movement; the wireless receiving and transmission module on the computer side transmits the received data to the computer through the USB serial interface as the data source for subsequent data processing.

2 Control software design
The data acquisition and wireless transmission program flow is shown in Figure 3. The initialization includes the initialization of the microcontroller's I/O port, serial interface initialization, interrupt initialization, and initialization configuration of the accelerometer and wireless transceiver chip. After the initialization is completed, wait for the key command. The first key press enters the data collection state, and the second key press stops the data collection. The key is controlled by the person being tested.

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The main program code for the accelerometer is as follows:

Note:
① When CSIA0 works in 1-byte communication, if only receiving operation is performed, communication can only be started by writing virtual data to the serial I/O shift register SIOA0. After the communication is completed, an interrupt request signal (INTACSI) is generated. At this time, reading SIOA0 is the desired value.
② After each set of acceleration values ​​is collected, the statement:
write_comm (WRITE_STATUS_REG, Ox00) must be executed. By clearing the status register STATUS_REG, the Data-Ready signal output by the accelerometer can be cleared, thereby generating a new set of acceleration values.
The wireless receiving and data transmission program flow is shown in Figure 4. After completing the initialization, it enters the loop listening state. When receiving data, the wireless transceiver chip nRF2401 sends a data ready signal to the MCU. After reading the data, the MCU communicates with the computer through serial communication and saves the data in the form of a file to the computer.

3 Experiments
The wireless transmitting and receiving antennas in this design use the circular wires on the PCB board, which have weak transmission capabilities. Therefore, the experiment was completed under the condition that the transmitting and receiving ends were within the visual range and there were no obstacles blocking them. The test subject fixed the data acquisition and wireless transmission modules of the device on the back of the waist, and made the positive directions of the three axes of the accelerometer point to the side, vertical and forward directions of the human body as required. The experimental results show that when the accelerometer works at any sampling rate within its sampling frequency range, the device can meet the requirements of collection and transmission. Figure 5 is a time-acceleration diagram received and saved in the computer when the accelerometer works at 160Hz and the test person walks on flat ground.

Conclusion
The wireless gait data acquisition device designed in this paper has made an effective attempt to further complete the portable acquisition device by completing the acquisition and storage of gait acceleration data in a specific environment. Through further improvement and perfection, it will become an effective tool for establishing the original gait acceleration database, tracking and detecting, and recording the movement status of people.