Application of ADXL202 in Combined Vehicle Navigation System

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Application of ADXL202 in Combined Vehicle Navigation System [Copy link]

introduction

With the rapid development and maturity of positioning and navigation technology, vehicle navigation systems have been more and more widely used in related fields. The function of the vehicle navigation system is to help users determine the vehicle position and provide correct operating instructions. Therefore, the accuracy of positioning has become an important indicator to measure the performance of a system. At present, the commonly used positioning methods include GPS satellite positioning, inertial navigation positioning and combined navigation positioning. The positioning method based on the acceleration sensor ADXL202 introduced in this article is one of the inertial navigation positioning. In the vehicle navigation system, the acceleration sensor completes the data acquisition task and obtains the instantaneous acceleration value of the vehicle. Then, the dead reckoning method is used to calculate the offset between the current position and the known reference position, thereby obtaining the absolute position of the vehicle. In a short period of time, the positioning accuracy obtained by this method is very high. For example, in the case of an average acceleration of 28.4mg, the positioning error obtained within 10s is about 0.5m, which is negligible compared with the average positioning error of 15m-20m of a single point of GPS; but it can be imagined that as time increases, the error accumulation effect will become larger and larger, seriously affecting the accuracy of navigation. Therefore, acceleration sensors are usually combined with GPS to form an integrated navigation system to improve positioning accuracy and enhance system performance.

Working Principle of ADXL202

ADXL202 is a dual-axis acceleration measurement system produced by Analogy Devices. It has analog input, can measure dynamic acceleration and static acceleration, and outputs a pulse width modulation signal with adjustable period, which can be directly connected to a microcontroller or a counter. It has the characteristics of high performance, high accuracy and low power consumption (input current is less than 0.6mA).

ADXL202 has a wide range of applications. It is mainly used in inertial navigation, tilt sensing, earthquake monitoring and automobile insurance. ADXL202 has high integration and simple structure. It is a monolithic integrated circuit, which contains a polysilicon surface microprocessor sensor and a signal control circuit to realize an open-loop acceleration measurement structure. Compared with other accelerometers, ADXL202 can greatly improve the working bandwidth, reduce the influence of noise, and the zero gravity deviation and temperature drift are relatively low. Its structural block diagram is shown in Figure 1:

The ADXL202 sensor is composed of an oscillator, X and Y direction sensors, a phase detection circuit and a duty cycle modulator (as shown in Figure 1). It is easy to use and has a digital output interface and an analog voltage signal output interface.

The X and Y direction sensors are two mutually orthogonal acceleration sensors. These two sensors work simultaneously and can measure dynamic acceleration and constant acceleration. ADXL can measure acceleration in both positive and negative directions, and has a wide measurement range, which can measure 2g/10g acceleration. Figure 2 shows that when the ADXL202 changes direction relative to the ground plane, the X and Y directions correspond to different outputs, so that the constant acceleration can be calculated.

After the sensor, a phase detector is cascaded, which is mainly used to correct the signal and determine the direction of the signal.

The signal output from the detector drives the duty cycle modulator through a 32K resistor. We can change the bandwidth by connecting capacitors CX and CY to the XFILT and YFILT pins. External capacitors are good for filtering noise and suppressing zero drift.

After the signal passes through the low-pass filter, the duty cycle modulator converts the signal into a digital signal for output. The period of T2 can be changed (0.5ms to 10ms) through the external resistor of T2 pin, which is very suitable for use in occasions with different accuracy requirements.

The duty cycle of the output duty cycle signal can be calculated by the counter. The acceleration can be calculated by formula (1).

A(g)=(T1/T2-0.5)/12.5%............(1)

For example, when the acceleration is 0g, the signal width T1 is the same as the idle width (T2-T1), and the duty cycle of the output signal is 50%; when the acceleration is 1g, the ratio of the signal width T1 to the idle width (T2-T1) is 5:3, and the duty cycle of the output signal is 62.5%.

ADXL202 can also directly output analog signals, and the acceleration value can be obtained through A/D conversion.

Design of combined vehicle navigation system based on ADXL202

The vehicle navigation system introduced in this article mainly uses acceleration sensor and GPS receiver as signal source for combined navigation. The embedded microprocessor EP7312 is used as the core processor of vehicle navigation, providing a working platform for the normal operation of the whole system, centrally processing the positioning signals from acceleration sensor ADXL202 and GPS receiver ITRAX02. The processed results are located on the electronic map through the map matching algorithm, and are displayed and updated regularly on the LCD screen. The structural framework of the whole navigation system is shown in Figure 2:

The acceleration sensor ADXL202 measures the acceleration value of the vehicle in the X and Y directions of the two-dimensional plane, and outputs it in the form of a pulse width modulation signal. The duty cycle value (i.e., T1/T2) is obtained through the CPLD, and an interrupt is triggered to send the ratio to the processor. After two integral operations, the relative displacement is obtained and cached in the memory; at the same time, the GPS receiver ITRAX02 receives the wireless satellite navigation carrier signal, and after demodulation, the signal frame is sent to the embedded microprocessor through the serial interface. The microprocessor extracts information according to the specified transmission protocol, thereby obtaining the two-dimensional plane position, altitude, ephemeris time, error analysis and other data, and caches them. After obtaining the relative displacement and GPS positioning data, the microprocessor EP7312 fuses the relative displacement data and GPS data through a distributed digital Kalman filter to obtain positioning information with relatively high accuracy, and uses a map matching algorithm to plot the current position on the electronic map. By refreshing the LCD screen, the positioning and navigation information is provided to the user in a timely manner. The system uses accelerometer and GPS as data sources for combined navigation, which can give full play to the advantages of accelerometer's high short-term positioning accuracy and autonomous navigation without external interference, and the advantages of GPS navigation's long-term stable positioning and simple use, reducing the impact of error accumulation and improving system performance.

CPLD Design

In the application of vehicle navigation system, it is inevitable to encounter thunderstorms, mountainous and low-lying paths, and such conditions will naturally cause serious interference to GPS signals. In order to reduce the inaccuracy of positioning, the acceleration sensor is used for further positioning while using the GPS receiver for positioning. CPLD mainly realizes the calculation of the duty cycle of the two-way signal.

The processing of CPLD is shown in Figure 3:

The counting controller is used to control the frequency divider so that the frequency of the output signal of the frequency divider is 1000 times the output frequency of the sensor. The signal output by the frequency divider samples the output signal of the sensor, counts the number of sampling values 1 in one cycle, and saves it in the cache.

When the counter is full, an interrupt is triggered to notify the CPU to read the data.

For low power consumption, the enable signal EN is used to control whether the entire CPLD is working. The EN signal can be controlled by the GPIO port of EP7312.

Application software design

We develop navigation programs on the Windows CE operating system platform. Windows CE is a small, ROM-based operating system with a Win32 subset API produced by Microsoft Windows. Compared with other embedded operating systems, it has stronger memory management, file management and GUI functions. In the car navigation system, there are very high requirements for displaying map scheduling and user interaction interface. Therefore, it is very suitable to develop applications based on Windows CE.

The vehicle navigation system application is a multi-tasking program. After startup, it initializes the processor serial port 1 connected to the GPS receiver ITRAX02, the processor serial port 2 connected to the ADXL202, and the LCD controller connected to the color LCD screen for displaying the vehicle's driving position. As the central processor, EP7312 monitors the interrupt signals of serial port 1 and serial port 2 in real time. When the pulse modulation signal of ADXL202 is converted into a duty cycle value by CPLD and sent to the processor, it will trigger the corresponding serial port interrupt.

When EP7312 detects an interrupt signal, the interrupt controller determines the entry address of the corresponding interrupt service program and calls the program for processing; when GPS receives a frame of satellite signal, it also triggers the corresponding serial port interrupt, and the interrupt controller determines the entry address of the corresponding interrupt service program and calls the program for processing. The interrupt service program for processing the duty cycle value first buffers the received data, filters it, integrates it twice, obtains the relative displacement within a cycle, records it, exits, and surrenders the right to use the CPU; the interrupt service program for processing GPS data first buffers the received data, extracts the direction and position information from it according to the pre-agreed format, records it, exits, and surrenders the right to use the CPU. The data processing task is mainly to fuse the relative displacement data with the GPS position data through the distributed digital Kalman filter method, to make up for the defect of low positioning accuracy of GPS data and the problem of accumulated errors of the acceleration sensor, so as to obtain positioning information with relatively high accuracy, exit, and surrender the right to use the CPU. After the map matching task is called, the map matching algorithm is used to project the positioning information onto the electronic map for plotting, and the positioning information is provided to the user in a timely manner by refreshing the LCD screen. The application flow is shown in Figure 4.

Conclusion

At present, the system has been designed and entered the final testing stage. After a period of testing, the hardware and software performance are good, which is a feasible design idea.