Dual-axis acceleration sensor adxl202 and Its Application Design

Design of the dual-axis accelerometer ADXL202

Introduction

ADXL202 is a dual-axis acceleration measurement system developed by ADI. It simulates input and can measure dynamic and static acceleration. The measurement range is ± (2 ~ 10) g, the output is the cycle adjustable Pulse Width
Modulated signals can be directly connected to single-chip microcomputer or counter. The LPC2103 is a phillips family of microcontroller. It is mainly used for industrial control, medical systems, access control, POS machines, and communication
Communication gateway and other fields. In this paper, we use LPC2103 to collect and process the acceleration data of ADXL202.

1 adxl202 Acceleration Sensor
1.1 adxl202 pin definition and basic features

ADXL202 is a monolithic integrated circuit with a high degree of integration and simple structure. It contains a polysilicon surface micro-processing sensor and a signal control circuit to achieve an open-loop acceleration measurement structure. Similar to other Accelerometer
Compared with ADXL202, ADXL202 can greatly increase the working bandwidth, reduce the noise impact, and reduce the zero gravity deviation and temperature drift. Figure 1 shows the pin definition of the ADXL202 sensor.

ST: self-check, used to control the chip self-check function. When VDD is connected, the output waveform with a duty cycle of 10% indicates that the chip works normally.
COM: PIN 4, 7. During use, two COM terminals must be connected together and grounded.
T2: adjusts the output signal cycle by grounding the resistance RSET. Output signal cycle T2 = RSET/(125 MΩ S-1 ).
VDD: power supply. The operating voltage range is + 3. O ~ + 5.25 V, which can be connected to the power supply through 100 Ω decoupling resistance.
XFILT and YFILT: Used to change bandwidth, filter out noise, and suppress zero point drift.
Xout and Yout: outputs.
Figure 2 shows the internal structure of the ADXL202 sensor.

The ADXL202 sensor consists of an oscillator, an X-and Y-direction sensor, a phase detection circuit, and a duty cycle modulated device. It has a digital output interface and analog voltage signal output interface. There are two sensors in the X and Y directions.
Orthogonal acceleration sensors that work at the same time can measure the acceleration of dynamic changes and constant acceleration. The next-stage connected phase detector of the sensor is mainly used to correct the signal and determine the direction of the signal.
Disconnected. The signal output by the detector is driven by a 32 KB resistor, and the bandwidth is changed by the external capacitors CX and CY on the XFILT and YFILT pins.
1.2 calculation and processing of measurement data
(1) signal bandwidth calculation
Use CX and CY to set the bandwidth, connect the capacitor to the XFILT and YFILT pins, and use a low-pass filter to reduce noise. 3 dB bandwidth formula:
F = 5μf/C (x, y) (minimum capacitance: l 000 pF)
(2) Acceleration Calculation
Output signal cycle T2 = RSET/(125 MΩ s-1), as shown in 3.

After the signal passes through the low-pass filter, the duty cycle modulation converts the signal to a digital signal output. The cycle of T2 can be changed through the external resistance of T2 pin (O.5 ~ 10 MS), which is suitable for use in the case of different precision requirements. The output duty cycle signal can be calculated using the counter. The acceleration calculation can be obtained through the following formula:

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 (T1 T2) is, and the duty cycle of the output signal is 62.5%.
1.3 typical application of ADXL202
One of the most important applications of the ADXL202 sensor is the measurement of inclination. When measuring the inclination, the sensor's sensitive axis (X axis) must be perpendicular to the gravity direction. If it is parallel to the gravity direction, the effects of object skew on the acceleration data are negligible. Figure 4 shows the schematic diagram of the acceleration measurement.

When ADXL202 is perpendicular to the gravity vector, the output changes with the inclination of about 17.5 mg per degree. When the two are 45 °, the output variation value is only 12.2 mg per degree, and the resolution is reduced. Table 1 shows the relationship between tilt angle and acceleration.

2 Application Circuit Design
2.1 hardware interface design
LPC2103
Is a 32-bit ARM7TDMI-S CPU that supports real-time simulation and tracking, with 8 KB in-chip SRAM and 32
Embedded Flash memory in High-Speed chip in KB. The LPC2103 features a small LQFP48 package, extremely low power consumption, multiple 32-bit timers, 8-way 10-bit ADC, 2 external interrupts, and a maximum
Up to 32 gpios. Programmable in-chip PLL (possible input frequency range: 10 ~ 25 MHz) up to 70
MHz CPU clock frequency. The interface circuit 5 of ADXL202 sensor and LPC2103 is shown in.

After the T2 of the ADXL202 accelerometer is grounded by a 125 KB resistor, the signal output period is 1 ms. 13, 14 pin connections + 5
V power supply, XFILT and YFILT grounded by O.1 μF capacitor, used to set 50
Hz bandwidth. The two outputs are connected to the PO. O and PO.2 pins of LPC2103 respectively as data transmission lines. There are two methods for data transmission: Common GPIO port mode and timer capture interruption.
Method.
2.2 Common GPIO port Mode

Since the output of the sensor is a DCM signal, no matter how data is received, the timer/Counter is required to process the DCM signal. Therefore, the program must first initialize the timer
. Then, the high voltage and low level duration of the DCM signal are timing, the values t1 and t2 are obtained, and acceleration calculation is performed. Since gpio is a common I/0 method by default
You do not need to set the PIN-SEL register. The general gpio port method is as follows:

The general gpio port method is relatively simple. Although the execution of the program takes time, the frequency of the lpc2103 can reach 40.
MHz. It takes only a few microseconds to execute several commands, so the error is very small. However, when a general gpio program is executed, the CPU is waiting for the arrival of the rising or falling edge, greatly reducing the CPU usage.
Efficiency. You can use the xout and lpc2103 interfaces shown in Figure 5.
2.3 timer capture interrupt mode
As shown in figure 5, yout is connected to the po.2 pin of the lpc2103. With the reuse of Segment 2, the timer can capture the sensor data in interrupt mode. The main program section is as follows:

After the interrupt processing program is run, the signal cycle should be t2 = t1 + T2. Therefore, the acceleration is (fp32) T1/(fp32) T2 + (fp32) T1) a O.5) * 8. The use of the interrupt service program greatly improves the CPU usage efficiency, but the program is complicated and occupies an interrupt vector channel.

Knot

The application method of adxl202 sensor has been verified to be completely feasible and can achieve high measurement accuracy. Because of its high integration, the system consists of adxl202 and arm series microcontroller.
It is a security control system for vehicles, trains, and other transportation tools. Adxl202 is widely used in inertial navigation, tilt sensing, seismic monitoring, vehicle insurance, and other fields. It has high precision, high integration, and low power consumption.
It can completely replace the traditional acceleration sensor.