Design of wireless acquisition system for gait acceleration signal

In order to realize the wireless acquisition of gait acceleration signals, an effective method based on the embedded 8051 wireless transceiver chip CC1010 is proposed. Briefly introduce the working principle of the wireless acquisition system of gait acceleration signals, and explain the design and implementation of the system’s software and hardware in detail. The system uses routing and retransmission mechanisms to ensure reliable data transmission. Using this acquisition system, a 36-person gait acceleration database was successfully established, which can be analyzed by gait researchers in different fields.

In order to realize the wireless acquisition of gait acceleration signals, an effective method based on the embedded 8051 wireless transceiver chip CC1010 is proposed. Briefly introduce the working principle of the wireless acquisition system of gait acceleration signals, and explain the design and implementation of the system’s software and hardware in detail. The system uses routing and retransmission mechanisms to ensure reliable data transmission. Using this acquisition system, a 36-person gait acceleration database was successfully established, which can be analyzed by gait researchers in different fields.

Gait is the posture in which a person walks. As a biological feature, it has unique advantages such as being unaffected by distance, non-aggressive, difficult to disguise, and less affected by the environment, so it has attracted much attention in recent years.Many well-known universities and research institutions at home and abroad, such as the Massachusetts Institute of Technology, the Institute of Automation of the Chinese Academy of Sciences, etc., have extensively carried out research on gait recognition[1]. The uniqueness of gait provides effective clues for human identity recognition and authentication, and it can also play a significant auxiliary role in the prevention, diagnosis and rehabilitation of medical diseases such as asynchronous morbidity and hemiplegia. Moreover, in modern sports training, gait characteristics can also be used to monitor athletes’ physical consumption, movement accuracy, etc., and formulate scientific training programs. In addition, gait also plays an important role in scientific research such as robot walking and human behavior understanding.

At present, gait research at home and abroad has just started. Various gait research fields require a large amount of reliable raw gait data. The currently publicly available gait databases mainly include the SOTON gait database of the University of Southampton, the MIT gait database of the Massachusetts Institute of Technology, the CMU gait database of Carnegie Mellon University, and the NLPR gait provided by the Institute of Automation of the Chinese Academy of Sciences. database. The above databases are all based on images. However, images taken in a dynamic environment are affected by various factors such as changes in illumination and shadows of moving targets, which will bring greater difficulties to image-based gait feature extraction.Therefore, Heikki Ailisto et al.[2-3]A new method for acquiring gait data using acceleration sensors is proposed, which avoids the adverse effects of various factors in the dynamic environment on the captured images, reduces the difficulty of data processing, and opens up a new way to acquire gait data; but this method Using a laptop equipped with a DAQ1200 data acquisition card to collect data is not only costly, but also inconvenient for the test subject to carry.

The method of extracting gait acceleration signals has become a bottleneck in gait research; however, with the rapid development of various technologies, it has become possible to collect gait acceleration signals automatically and intelligently.

This article proposes an effective method for wireless acquisition of gait acceleration signals based on the wireless transceiver chip CC1010. This method uses the three-axis acceleration sensor MMA7260 to measure the acceleration signal of the gait, and uses Chipcon’s embedded 8051 wireless transceiver chip CC1010 as the core controller to control its built-in analog-to-digital converter to sample the acceleration signal and A/D conversion , And then realize the reliable wireless transmission of gait acceleration signal by means of routing between the wireless transmitting module and the receiving module. The collection system is powered by a 450 mAh lithium battery and can be used offline.

1 System design principle

First, a three-axis acceleration sensor is used to sense the three-dimensional acceleration signal generated when a person is walking, and then the analog signal is sampled by an A/D converter and converted into a digital signal, which is sent to the microprocessor for preprocessing. The processed gait acceleration data is sent to the router through the wireless transmitting circuit, and then forwarded to the wireless receiving device by the router. Finally, the gait acceleration data is sent to the computer through the serial port, which can be used by gait researchers in different fields. Figure 1 is a block diagram of the wireless acquisition system for gait acceleration signals. The wireless acquisition system of gait acceleration signal is composed of 4 parts: transmitting device, routing device, receiving device and PC. The transmitting device is composed of a three-axis acceleration sensor circuit, an A/D conversion circuit, a single-chip microcomputer and a wireless transmitting circuit. The routing device is composed of a wireless transceiver circuit. The receiving device is composed of a wireless receiving circuit, a single-chip microcomputer and a serial port circuit. The PC part is mainly composed of a PC and serial communication software.

Design of wireless acquisition system for gait acceleration signal
Figure 1 Block diagram of the wireless acquisition system for gait acceleration signals

2 hardware circuit design

The hardware circuit mainly includes the RF transceiver circuit between the CC1010 and the antenna, the interface circuit between the CC1010 and the acceleration sensor, the button control circuit, the LED indicator circuit and the alarm circuit.

The core part of the hardware circuit is the wireless acceleration transmission module. This module must not only meet the functional requirements of the transmitting and receiving device, but also be as small as possible for easy carrying. This makes the design of the hardware circuit very difficult. How to make high-quality PCB boards has become the focus of the entire design. PCB board design mainly includes schematic design, layout design and wiring design.

2.1 System schematic design

The quality of the schematic directly affects the difficulty of layout and wiring, and the performance of the final board. In order to clearly carry out the partition design when layout and wiring, the digital circuit, analog circuit and RF circuit should be separated when designing the schematic diagram; at the same time, distinguish the critical circuit from the non-critical circuit and which components have requirements for the location. In the design of wireless transmission module, CC1010 RF_IN (4 feet), RF_OUT (5 feet), L1 (10 feet), L2 (11 feet), etc. are RF circuits and also key circuits. Its ADC-related acceleration sensor circuit is an analog circuit. For the power supply terminal of analog devices, such as AVDD_ADC (pin 1), AGND_ADC (pin 64), AVDD_MIX_IF (pin 2), AGND_MIX_IF (pin 3), AVDD_LNA_PA (pin 6), AGND_LNA_PA (pin 7), AGND_PA (pin 8), etc. , The filtering performance should be carefully considered, and the noise interference of the digital circuit part should be avoided as much as possible. In addition, the crystal oscillator circuit is also a critical circuit, while the LED indicator and button circuits are non-critical circuits.

When designing a schematic diagram, in addition to the realization of the function and the correctness of the principle, the choice of the device must also be considered. First of all, the selected device should be easily available on the market; secondly, the package of the device must not only meet the requirements of the PCB board size, but also consider the difficulty of soldering. For PCB boards with RF, it is best to choose SMD packages to reduce the influence of unnecessary parasitic parameters.

(1) Design of RF transceiver circuit between CC1010 and antenna

This design uses the single-chip, multi-band, low-power, ultra-high frequency radio frequency chip CC1010 launched by Chipcon. The chip is made of 0.35 μm CMOS technology, embedded with a high-performance 8051 microcontroller, 32 KB of Flash program memory, 2 048+128 bytes of SRAM, 3-channel 10-bit ADC, 4 timers, 2 PWMs, 2 One UART, SPI and 26 general-purpose I/O, etc. CC1010 is suitable for many wireless applications such as home automation, security system, remote unlocking, remote sensing survey, remote control toys, etc. This design uses CC1010 to realize wireless acquisition of gait acceleration signals.

The circuit of the RF transceiver section is shown in Figure 2. C31 is the input matching capacitor, L32 is the input matching inductance, and L32 is also used to prevent the input of the DC bias signal; C41, C42 and L41 jointly realize the matching of the transmission output circuit. Through the transmit/receive switch circuit inside CC1010, the transceiver can transmit/receive through the same 50 Ω antenna. L1, C8, and C9 form a low-pass filter to filter out high frequency harmonics and increase frequency selectivity. Its impedance is 50 Ω. Component parameters can be in accordance with CC1010datasheet[2]The value given above can also be obtained using Chipcon’s SmartRF Studio software.

Design of wireless acquisition system for gait acceleration signal
Figure 2 Circuit principle of the wireless transceiver of the gait acquisition system

The voltage-controlled oscillator is embedded in the CC1010 chip, and only an external Inductor L101 is required when in use. The inductance is best to choose wire-wound inductance, and solder it according to the given reference value. Then use a spectrum analyzer to check its frequency, and adjust its parameters appropriately according to whether its center frequency meets the requirements. L101 should be as close as possible to CC1010 and arranged symmetrically with respect to the two pins 10 and 11. The package should be a small package of 0402 or 0603.

Generally, single whip antenna, helical antenna or loop antenna on PCB can be selected. The length of the single whip antenna is 1/4 of the wavelength, which can be calculated by the formula L=7125/f. Among them, L represents the length of the single whip antenna, and f is the transmit/receive frequency. The loop antenna is placed on the PCB and is very convenient to use; however, due to its poor radiation capability, the receiving/transmitting performance is also slightly worse. Helical antenna is a compromise between single whip antenna and loop antenna. Its size and receiving/transmitting capabilities are somewhere in between. You can choose a suitable antenna according to your needs. Generally speaking, helical antennas are more practical.

(2) The interface circuit design of CC1010 and acceleration sensor

This wireless acquisition system uses a low-cost, single-chip, three-axis acceleration sensor MMA7260 newly launched by Freescale. The miniature capacitive acceleration sensor integrates signal conditioning, single-pole low-pass filter and temperature compensation technology, and provides four acceleration measurement ranges, which are ±1.5 g, ±2 g, ±4 g, and ±6 g.

In the interface between CC1010 and MMA7260, the noise problem must be considered first. Because MMA7260 uses a switched capacitor filter internally, clock noise is generated, so it is necessary to connect RC filters to the XOUT, YOUT and ZOUT output terminals of MMA7260 respectively; secondly, the voltage matching problem should be considered, due to the X, Y, and Z axis directions The voltage output of CC1010 is 0.45~2.85, and the maximum input range of ADC of CC1010 is 0~VDD. Here VDD is 3.3 V, and its range is just within the ADC input range, so there is no need to consider additional voltage divider resistors. The interface circuit of CC1010 and MMA7260 is shown as in Fig. 3. R31/C31, R41/C41, R51/C51 are used to filter out the switching noise sampled inside MMA7260, and GS1, GS2 are used for range selection.

Design of wireless acquisition system for gait acceleration signal
Figure 3 Circuit principle of acceleration sensor

(3) Button control, LED indication and alarm circuit

This wireless acquisition system has 3 buttons S1, S2 and S3. Among them, S1 is the system reset button. S2 is the mode selection key, which can make the acquisition system in automatic working mode or manual working mode. If the collection system works in automatic mode, collection and wireless transmission are performed synchronously; if it works in manual mode, the acceleration data is stored in the memory first, and then the wireless transmission is started after the data transmission start button is pressed. S3 is the data collection start/stop key, which is used to control the start and stop of data collection and the data sending key after the collection is completed in manual working mode. The LED indication circuit includes a power indicator, a sending signal indicator, and a receiving signal indicator. The alarm circuit is composed of an amplifying circuit and a buzzer. When the data collection is completed, the buzzer will automatically alarm.

2.2 Layout design

A good layout is a prerequisite for successful wiring. During layout, components should be placed in the direction of the signal flow from the left input and right output, and the following aspects should be considered:
① First, draw the maximum size that can be supported on the Keepoutlayer layer in Protel, so that the layout can always be known, and the PCB board size will not be inappropriate;
② The placement position of this wireless acquisition system requires that the antenna must be located at the upper right corner of the PCB board. The shorter the RF signal path, the better, so the RF circuit should also be laid out in the upper right corner. The components of the RF circuit part should be as compact as possible. For example, L101 should be as close as possible to CC1010 and arranged symmetrically with respect to its 10 and 11 pins. For RF_IN and RF_OUT, the three principles of compactness, unobstructed flow, and uniform impedance should be observed.
③ It is better that there is no digital circuit in the analog circuit partition, otherwise the analog signal is very easy to be interfered by digital noise.
④ The crystal oscillator should be as close as possible to CC1010 and placed symmetrically with XOSC_Q1 (pin 18) and XOSC_Q2 (pin 19). The 15 pF capacitors at both ends of the crystal oscillator should be as close as possible to the crystal oscillator.

2.3 Wiring design

Wiring is the last step of PCB design. In order to ensure the good performance of the RF circuit part, all manual wiring is required, and the following rules must be followed: first, ensure that the wiring of key circuits such as RF and crystal oscillators are unblocked, preferably on the top layer; secondly, ensure that the wiring of the A/D part is unblocked; and finally Wire in a certain order. This design is wired from the upper right foot in a counterclockwise direction.

3 Software part design

Design of wireless acquisition system for gait acceleration signal
Figure 4 Acquisition system program flow

The software flow of the acquisition system is shown in Figure 4. The software part mainly includes initialization procedures, key processing procedures, A/D conversion, data storage, receiving and sending procedures. The initialization procedure includes one-chip computer port initialization, RF part initialization, ADC part initialization and T0 initialization. In the key processing program, Key2 is the automatic mode and manual mode selection key. If it is automatic mode, the data will be sent in real time; if it is manual mode, after the data collection is completed, press Key1 to send.

4 Test results and analysis

In order to test the feasibility of a wireless acquisition system for gait acceleration signals, the author did the following experiments. The test site is a corridor, and the transmitter of the acquisition system is carried by the test subject, all placed directly behind the belt, like carrying a mobile phone, except that it is located at the intersection of the spine and the waist belt, and at the same time, ensure that the MMA7260 is in the test standard position. The positive directions of the X-axis, Y-axis, and Z-axis respectively point to the direction in which the person is moving, the right side of the human body and the right above the human body. The router is placed at the entrance of the laboratory to ensure that it is visible to the launcher. The wireless receiving device is connected to the PC in the laboratory through a serial port.

A total of 36 subjects were tested, including 16 males and 20 females, aged between 24 and 30 years old. During the test, all test subjects are required to wear flat shoes and walk in a straight line at a normal pace as much as possible in the corridor. Each test object has to test 5 groups, and the result is a total of 1 800 groups of data, each group of data contains data in the three directions of the X-axis, Y-axis and Z-axis.

After the test of each test object is completed, the 5 groups of gait acceleration data continuously measured are then saved in the PC. Part of the data is shown in Figure 5. The first column of data in Figure 5 is the serial number of the data. In the experiment, each test object is required to collect 5,000 data per group; the second, fourth, and sixth columns represent the number of the analog-to-digital conversion channel occupied by the data; the third and fifth Column 7 is the acceleration data of the corresponding channel.

Design of wireless acquisition system for gait acceleration signal
Figure 5 Measured gait acceleration data

In the experiment, the data in the three directions of X-axis, Y-axis and Z-axis of all test objects are processed in the same way. After the computer receives these data, it first normalizes the data to make it all within the range of 0 to 1. In this way, using the PLOT() function in Matlab, you can visually see the waveform of the gait characteristic signal.

The following takes the data in the Z-axis direction of the No. 16 test object as an example. Part of the gait acceleration data in the Z-axis direction of test subject No. 18 is shown in the third column of data in Figure 5. In Matlab, the corresponding waveform can be obtained, as shown in Figure 6. It can be seen from the signal profile that the gait signal is a periodic signal. Because the “left” gait and the “right” gait are not necessarily completely symmetrical, the signal is divided into a gait and b gait.

Figure 7 shows the speed signals of No. 16 and No. 18 test objects in the Z-axis direction. It can be seen from Figure 7: Different test objects have obvious differences in the amplitude, period, and rate of change of the acceleration signal.

Design of wireless acquisition system for gait acceleration signal
Figure 6 #16 acceleration signal in the Z-axis direction
Design of wireless acquisition system for gait acceleration signal
Figure 7 #16 and #18 test object Z-axis acceleration signal

5 Conclusion

Through the above experiments, it is known that the embedded 8051 wireless transceiver chip CC1010 greatly simplifies the circuit design; at the same time, because the CC1010 uses a 3.3 V power supply and is in a sleep state when not working, it greatly reduces the power consumption of the acquisition system. In the experiment, the power consumption, signal stability and sensitivity of the gait acceleration wireless acquisition system have reached the expected results.

The realization method of the gait acceleration signal wireless acquisition system based on the wireless transceiver chip CC1010 proposed in this paper has the advantages of convenience, directness and effectiveness. The acquisition system is cost-effective, small in size, and easy to carry. It can be applied to fields such as human identification, medical technology, sports training, and sports fitness, and has practical application value.

The Links:   PD050VL1 6RI100G-160

Author: Yoyokuo