Application of acceleration sensor ADXL50 in missile march

0 Introduction
During the march of missile vehicles, the missiles on the vehicles will be affected by various vibration shocks. If this vibration shock is too large, it will cause damage or destruction to the missiles. This paper introduces an acceleration measurement method based on the ADXL50 monolithic integrated acceleration sensor, which is used to detect the vibration shocks that the missiles are subjected to during the march of the vehicle. The monolithic integrated acceleration sensor ADXL50 is small in size, good in directionality, high in precision, and small in time drift and temperature drift; it works under a single +5V power supply, is easy to use, and requires few peripheral components; it has high reliability and can withstand 500g acceleration shock when powered on, and can withstand 2000g acceleration shock when not powered on without damage. Therefore, it can be applied to missile transport vehicles.

1 Design scheme
The design scheme is shown in Figure 1: It includes three parts: sensor and peripheral circuit, signal processing device, display alarm circuit, etc. The sensor and peripheral circuit are encapsulated in a small shielded container and fixed on the missile carrier. The ADXL50 acceleration sensor collects vibration acceleration signals and outputs corresponding voltage signals; the signal processing part receives the analog voltage signal collected by the sensor, performs analog/digital conversion on it, displays the acceleration value on the LED, and then performs data judgment and processing. When it exceeds the safety range, an alarm is given to remind the driver to drive carefully.

2 Design of sensor and peripheral circuits
The scheme intends to use the ADXL50 accelerometer produced by the American AD company as the acceleration sensor. The device is integrated on a monolithic integrated circuit, uses differential capacitance as a sensitive element, and directly outputs a voltage signal, which is convenient for signal acquisition and processing.
2.1 Main technical indicators of ADXL50
The measurement sensitivity is 19mV/g; the full-scale measurement range is ±50g; the frequency response range is DC~1 kHz; self-test can be performed under the control of the switch signal; built-in buffer amplifier, which can be used for output sensitivity and zero acceleration output level adjustment.
2.2 Working principle and basic measurement circuit of ADXL50
ADXL50 is a complete acceleration measurement system, which combines a sensor component and the circuit related to acceleration measurement, including oscillator, demodulator, preamplifier, buffer amplifier, reference power supply and temperature compensation circuit. All are integrated on a silicon chip. The chip is packaged in a 10-pin T0100 tube shell, and the definition of these 10 pins is shown in Figure 2.

Figure 3 is the principle block diagram of the acceleration sensor. Three external capacitors and three external resistors are used. C1 is used to determine the bandwidth of the measurement system. The capacitance of C1 and C2 is generally selected from 0 to 0.22 μF, and C2 is generally 0.1 μF. Before ADXL50 leaves the factory, the manufacturer has adjusted the measurement sensitivity of its preamplifier to 19 mV/g, and its output voltage VPR at 0g is 1.8 V. Therefore, if the acceleration is ±50g, VPR=1.8V±0.95V, and if the acceleration is ±20g, VPR=1.8V±0.38V. Since the signal on VPR is not standard, it is not suitable for reading, displaying
or digital processing. In addition, the maximum injection current of the ADXL50 preamplifier can only be 25 μA, and the driving capacity is very small. Therefore, the signal on VPR needs to be further processed to meet the requirements. Generally, several external resistors and buffer amplifiers form an amplification link to adjust the 0g potential of the sensor's VPR and improve the sensitivity of the output signal.
After buffering and amplification, the acceleration signal should preferably have a variation range of 0.5 to 4.5 V, so that there is a certain margin from the high and low ends of the power supply. If the amplitude of the acceleration in the positive and negative directions is similar, the Og potential can be adjusted to about 2.5 V. In this way, the positive and negative acceleration signals have a variation range of ±2 V on the sensor output port Vout. Assuming that the acceleration range to be measured is ±50g, then VPR=1.8 V±0,95 V. To make Vout=2.5V±2.0V, the AC component in the signal needs to be amplified. The amplification factor of the buffer amplifier is -R3/R1, so if R1=50kΩ, it can be determined that R3=105 kΩ.
In Figure 3, one end of R2 is grounded. In order to make Vout=2.5V at 0g, R2 should meet the condition: R2=(1.8×R3)/(Vout-1.8)=270 kΩ.
2.3 Matters to be noted when using ADXL50
(1) ADXL50 has a sensitive axis, which points from the 5th pin to the 10th pin (i.e., to the positioning piece of the tube shell), as shown in Figure 2. The sensor measures the acceleration along this axis. Let this axis be the x-axis, and let the direction perpendicular to the x-axis in the plane at the bottom of the sensor tube shell be the y-axis. If the angle between the direction of the acceleration g and the xy plane at the bottom of the sensor is α, then the component of the acceleration g on the xy plane is gxy=gcosα. Let the angle between the acceleration component gxy and the sensitive axis of the sensor be β, then the component of gxy on the sensitive axis is gx=gxycosβ=gcosαcosβ. Therefore, when installing the acceleration sensor, try to make its sensitive axis consistent with the direction of the acceleration, otherwise it is necessary to make necessary modifications to the measurement results to avoid excessive errors.
(2) ADXL50 should be installed on a circuit board for use. When the resonance frequency of the circuit board is close to the frequency of the acceleration to be measured, if the circuit board is slightly loose, resonance will occur, making the measurement result biased and causing errors. Therefore, when installing the circuit board, choose a suitable position and fix it firmly. When testing the system, it is best to test it at different frequencies to find possible resonance problems.
(3) The role of the demodulator filter capacitor C1: determine the bandwidth of ADXL50; filter the demodulator signal.

3 Signal processing device
3.1 ADC0809
ADC0809 is a general-purpose programmable A/D converter that is powered by a single +5 V power supply and uses the principle of successive approximation conversion. It can perform time-sharing conversion on 8-way 0~+5 V input analog voltages. The working clock of the ADC0809 analog/digital converter has a wide applicable frequency range. The maximum effective working clock frequency applicable to this analog/digital converter is about 3MHz, and the minimum analog/digital conversion period can reach 24μs, so its sampling frequency can reach up to 41 kHz. The vibration frequency of the missile carrier during marching and erection is generally no more than 200 Hz. According to the sampling theorem, it can fully meet the measurement needs.
3.2 Microcontroller and its programming
The scheme intends to use the MCS51 series microcontroller, which can be directly connected to the ADC0809 analog/digital converter. Three acceleration sensors collect vibration signals in three directions (left and right, up and down, front and back), which are converted into digital signals through the three input terminals of the ADC0809 converter and then collected by the microcontroller. The main program of the single-chip computer first opens up three memory areas, each of which is a 10-byte unit, which is used to store the collected data of the three input terminals respectively. Then, the conversion results of the three input terminals (INO, IN1, IN2) of ADC0809 are read in turn and displayed on the LED display screen. At the same time, they are compared with the pre-set fixed data: if they are within the qualified range, the data of the next input terminal will continue to be read in turn, and the values ​​of the last 10 times of each input terminal will be stored; if the data of a certain input terminal read exceeds the qualified range, the data will be stored first, and then the data of the input terminal will continue to be read N times (N<9), and the data will be stored in time after each reading. After reading N times, the average value of the 10 data stored at the input terminal will be calculated. If it still exceeds the qualified range, an alarm will be quickly sounded, that is, the horn will beep and the warning light will flash through the driving circuit. Through this method of averaging data, some random interference such as singular values ​​can be filtered out.

4 Conclusion
Through the analysis of the missile transport process on the missile carrier, a solution is proposed to measure the vibration acceleration using the single-chip integrated acceleration sensor ADXL50 and the single-chip microcomputer circuit. It mainly introduces the composition of the ADXL50 single-chip integrated acceleration sensor and the single-chip microcomputer test system. And the smoothing filter method is used to filter out some random interference. The circuit of this solution is simple, and the design has high reliability and redundancy, which has a good reference role in testing the acceleration of the vehicle-mounted system.