Design of Digital Inclinometer

Digital inclinometer is a measuring tool for measuring small angles. It can be used to measure the inclination of the horizontal position, the parallelism and verticality of two parts, the straightness of machine tools and instrument guide rails, the flatness of the workbench, and the flatness of the plate, etc. It has a very broad application prospect. The  

inclinometer in the design is an inclinometer based on thermal convection. The inclinometer has a small mass and generates a small inertial force under large impact or high overload. Therefore, it has a strong ability to resist vibration or impact. It is one of the few inclinometers that can have the advantages of simple structure, high reliability, and universal sensor integrated circuits. This design measures the inclination of the platform through the inclinometer, outputs a voltage signal, and after amplification by the operational amplifier, outputs it to the single-chip microcomputer circuit for data processing. Finally, the size of the inclination is displayed through the digital tube, and it can also be used as a control system to output a control signal to adjust the inclination of the platform.  

1 Principle of digital inclinometer  

The overall hardware design principle of the inclinometer is shown in Figure 1. It consists of a sensor, a preamplifier circuit, an A/D conversion circuit, and a single-chip microcomputer system. The sensor collects acceleration information and outputs voltage values ​​after internal conversion. The voltage values ​​are divided into two paths, one for measuring the acceleration in the X-axis direction; the other for measuring the acceleration in the Y-axis direction. After low-pass filtering and amplification by the operational amplifier, the two voltage signals are output to the A/D conversion module. The A/D conversion module outputs 8-bit digital quantities to the single-chip microcomputer for processing. The single-chip microcomputer finally outputs serial data and outputs the tilt angle of the platform through the digital tube display circuit.  


  
2 Working principle of sensor  

The sensor used in the design is MXA2050A, which is a complete acceleration measurement system located on a single-chip integrated circuit CMOS IC. The sensor is based on natural convection for heat conduction. In MXA2050A, the gas is the only moving body in the sealed cavity. It has a small mass and generates small inertial force in large shocks or high overloads. Therefore, it has strong resistance to vibration or shock. It is an extremely low-cost dual-axis accelerometer. Because it has no moving parts, the device is particularly reliable. Moreover, it can be manufactured based on a sub-micron CMOS method. This method can produce small-sized chips, so this device is very cheap and versatile. The MXA2050A acceleration measurement range is: ±10gn, and the output is two analog voltage signals. It allows resolution of less than 10 mg in a 1 Hz bandwidth. The frequency response can be extended to 40Hz. With a simple external circuit, the frequency response is extended to 160Hz. The  

thermal convection accelerometer contains a closed cavity filled with fluid. There is a heating element that heats the fluid around the heating element in the cavity. The heated fluid expands and the density decreases. It rises under the action of gravity, and the surrounding relatively cold fluid replenishes the empty position. In this way, repeated cycles cause heat convection conduction. In the static state, the temperature curve is symmetrical. In the case of acceleration, due to natural convection heat conduction, acceleration in any direction will disturb the temperature and cause temperature asymmetry. Therefore, the temperature and output voltage of the four thermopiles in the accelerometer are different. The output voltage of different thermopiles is directly proportional to the acceleration. The accelerometer has two identical acceleration signal channels. One is to measure the acceleration in the X-axis direction; the other is to measure the acceleration in the Y-axis direction. The peripheral circuit is shown in Figure 2.  


  
3 Operational amplifier, A/D conversion and display circuit  

In the application of sensors, the high-impedance preamplifier has two main functions: one is to amplify the weak signal of the sensor; the other is to convert the high-impedance output of the sensor into a low-impedance output. The preamplifier in the design is mainly composed of the AD623 amplifier circuit, and the circuit diagram is shown in Figure 3. The  


  
operational amplifier AD623 can realize the functions of subtraction, addition, and amplification at the same time. The signal VX output by the sensor is first subtracted from its zero voltage in the horizontal direction, and then amplified by G times, and then added with the reference voltage VREF. The final output is the  


  
Amplification factor can be adjusted by adjusting the size of RG1. The reference voltage is output by the sliding rheostat. The voltage signal generated by the sensor is first output to the A/D conversion circuit after passing through the amplifier. The A/D conversion uses TLC5510, which can realize the speed of conversion controlled by the single-chip microcomputer. The output digital quantity is 8 bits, which can achieve the accuracy requirements of the design. [page]

The display circuit is mainly composed of two parts: shift register and digital tube display. The data processed by the single-chip microcomputer is converted into serial format data. Under the action of the clock signal, the shift display is realized through the shift register. The display circuit uses a total of 8 shift registers and 8 digital tubes to realize the 4-bit angle display in the x and y directions respectively, with an accuracy of 0.01°. The shift display circuit is shown in Figure 4.  


  
4 Experimental results and data processing  

In the process of experimental data processing, multiple groups of data are measured and their arithmetic mean is calculated as the final measured result. Therefore, it is necessary to study the uncertainty determination criteria of the arithmetic mean. If multiple groups of repeated series of measurements are made on the same value under the same conditions, each series of measurements has an arithmetic mean. Due to the existence of random errors, the arithmetic mean of each measurement column is also different. They have a certain dispersion around the true value of the measured value. This dispersion explains the uncertainty of the arithmetic mean, and the standard deviation σ of the arithmetic mean is a parameter that characterizes the dispersion of the arithmetic mean of each independent measurement column of the same measured value, which can be used as the determination standard of the uncertainty of the arithmetic mean. It is known that the arithmetic mean x is  


  
From this, it can be seen that in the equal-precision measurement series of n measurements, the standard deviation of the arithmetic mean is the standard deviation of a single measurement. When the number of measurements n is larger, the arithmetic mean is closer to the true value of the measurement, and the measurement accuracy is higher. Increasing the number of measurements can improve the measurement accuracy. However, it can be seen from the above formula that the measurement accuracy is inversely proportional to the square root of the number of measurements. Therefore, to significantly improve the measurement accuracy, a lot of work must be done. Practice also shows that after n>10, σ-x has decreased very slowly. In addition, since the larger the number of measurements, the more difficult it is to ensure the constant measurement conditions, which may introduce new errors. Generally, n=10 is taken. Therefore, in the experiment, the arithmetic mean of the measurement results of 10 times is taken as the final measurement result. The voltage inclination relationship curves drawn after measurement are shown in Figures 5 and 6.  





  
5 Conclusion  

In order to meet the market demand of machinery manufacturing, equipment installation, road and bridge, construction engineering and other industries and exports, a digital display inclinometer based on acceleration sensor is designed, with a resolution of 0.01° and a comprehensive accuracy of 0.03°, reaching the level of similar products at home and abroad; it has high cost performance. It is widely used in surveying and mapping instruments, construction machinery, antenna positioning, robotics, tank and ship artillery platform control, aircraft attitude, automotive electronic control, oil exploration, offshore platform monitoring, etc.