Hardware circuit design of angle measuring instrument system using acceleration sensor

　　In modern control systems, angle measurement devices are critical components that require high precision, and their measurement accuracy directly affects the performance and accuracy of the entire system. Existing research on using acceleration sensors to achieve high-precision angle measurement mainly focuses on single-axis angle measurement. This article will focus on the hardware method of using the dual-axis acceleration sensor ADXL202 to achieve high-precision angle measurement.

　　This angle measuring instrument uses STM32F107 as the core chip for data processing. This is a low-power, high-speed 32-bit processor with a Cortex-M3 core. The angle measurement module uses a high-precision, low-power dual-axis acceleration sensor ADXL202, which can convert acceleration signals into digital square wave signal outputs and can be directly connected to STM32F107. The current tilt angle can be calculated through a certain algorithm. The display module uses a 12864ZW 128×64 dot matrix LCD display.

　　Angle measurement module

　　The angle measurement module uses the low-cost, low-power, high-precision dual-axis acceleration sensor ADXL202 produced by ADI, with a measurement range of -2g to +2g, which can measure both dynamic acceleration and static acceleration. Its operating voltage is 3.0 to 5.25 V, the operating current is less than 0.6 mA, and the maximum main frequency can reach 70 MHz. Therefore, considering power consumption, sensitivity and accuracy, ADXL202 is selected as the core chip of the angle measurement module.

　　Figure ADXL202 functional block diagram

　　Angle measurement module hardware circuit design

　　To ensure the high-precision and stable operation of ADXL202, it is necessary to configure the signal period and filter capacitor (determine the bandwidth of the signal) according to the chip technical documentation and actual usage. The output signal of ADXL202 is a pulse width duty cycle modulated signal, and the duty cycle T1/T2 is proportional to the measured acceleration. At 0g, its output is a 50% duty cycle, and the sensitivity is 12.5% ​​of the pulse width duty cycle change caused by each g. According to the technical documentation of the chip, the period of DCM can be set by the resistor RSET: ADXL202 sets the bandwidth of ADXL202 through XFILT and YFILT external capacitors CX and CY. This bandwidth determines its measurement accuracy. At the same time, capacitors CX and CY can remove aliasing and filtering. In order to minimize the error of the pulse width duty cycle, the analog bandwidth should be 1/10 lower than the frequency of the pulse width duty cycle. Table 2 provided in the technical document is analyzed, and T2 is set to 1 ms, the pulse duty cycle frequency is 1 kHz, and in order to meet the actual needs and the minimum DCM error requirement, a 0.05μF filter capacitor is selected, and the analog bandwidth is 100 Hz. According to the pin configuration diagram of the chip and the above configurations, the hardware circuit of the angle detection module can be designed. The circuit principle is shown in Figure 4, and its pulse output end is directly connected to the I/O port of the STM32F107.

　　Angle acquisition schematic diagram

　　As can be seen from the figure, ADXL202 is a complete dual-axis acceleration measurement system based on a monolithic integrated circuit. For the X and Y axes, the output loop converts the acceleration signal into a digital signal output with a pulse width duty cycle. These digital signals can be directly transmitted to the STM32F107 without the need for A/D conversion or other additional circuits.

　　This paper discusses the research and design method of a high-precision angle measuring instrument based on ADXL202, and introduces the angle measurement principle of ADXL202 and the input capture function of STM32F107 in detail. Through experimental tests, the measuring instrument can complete the angle measurement with high precision and good reliability, which has positive significance for the research and design of angle measurement.