Design and implementation of automatic zeroing servo tilt sensor

1. Introduction

Inclination sensors are devices that measure the inclination angle of a horizontal plane. They are widely used in engineering and technical fields such as civil engineering, hydrogeology, weapons, aerospace, and biomedicine. There are many types of inclination sensors. According to their working principles, they can be divided into three types: "solid pendulum", "liquid pendulum", and "gas pendulum". The research on solid pendulum inclination sensors has been relatively mature and widely used, but they are susceptible to external interference, such as mechanical vibration and shock; while liquid pendulum inclination sensors have the characteristics of high sensitivity, corrosion resistance, and moisture resistance, but their fatal disadvantage is that temperature changes will seriously affect their working characteristics, thereby limiting the development and application of liquid pendulum inclination sensors; gas pendulum inclination sensors have a simple structure and strong vibration and shock resistance, but they are greatly affected by ambient temperature and have low test accuracy. In short, the accuracy of existing inclination sensors requires a high cost to improve, and there are problems such as zero position deviation, time drift, and temperature drift.

In view of the above problems, this paper designs an automatic zeroing servo inclination sensor based on the automatic zeroing theory. Its basic idea comes from the old method used by carpenters and builders to rotate 180° for leveling with a bubble ruler, and is designed and implemented using stepper motor and single-chip microcomputer control technology. This automatic zeroing servo inclination sensor can well solve the problems of zero position deviation, time drift and temperature drift, so that the performance of the inclination sensor has been improved, and it has very important application value.

2. Theoretical basis

The automatic zeroing servo inclination sensor is designed to correct zero position deviation and drift from various sources. Its basic idea comes from the old method used by carpenters and builders to rotate 180º on the surface of the measured object for leveling. If the bubble shows the same result, it means that it is working properly, otherwise it indicates an error equal to half of the difference in the position of the bubble vertex. In the application of this paper, the servo inclination sensor is located on a rotating disk with an input axis IA parallel to its surface. When the offset correction operation is to be performed, it can be directly rotated 180º to the opposite position of the disk, as shown in Figure 1.

The zero position deviation of the servo inclinometer in Figure 1 is independent of the position of the sensor. Therefore, when the sensor attached to the horizontal rotating disk is rotated to two different positions, its output will not change.

The output of the inclination sensor at zero input (the base of the inclination sensor is on an absolutely horizontal surface) consists of two parts:

(1) The offset error VB is defined as the output that is independent of the position of the inclination sensor.
(2) The misalignment error angle e is mainly caused by the fact that the base of the inclination sensor is not absolutely parallel to the measuring axis. This causes an output voltage Ve that is proportional to the misalignment angle (for very small angles, sine≈e).

If the rotating base in Figure 1 is on an absolutely horizontal plane, it is obvious that the sum of the offset error output VB and the misalignment error output Ve of the inclination sensor at the two positions is:

Vo=VB+Ve. (1)

Assuming that the rotating base is tilted by an angle φ relative to the Y axis, the analysis process is as follows, as shown in Figure 2.

It is obvious that when the rotating sensor shown in Figure 2 is rotated, the angle φ is the same in the two positions. Obviously, the direction of the input axis is opposite, so the polarity of the output voltage Vφ is also opposite. The voltage outputs at position 1 and position 2 overlap:

V1 = Vo + Vφ = VB + Ve + Vφ (2)
V2 = Vo - Vφ = VB + Ve - Vφ (3)

Finally, subtracting the outputs at position 1 from those at position 2 (V1 - V2 = 2Vφ) gives the result:

Generally speaking, it can be seen from the equation that the offset error and misalignment error can be completely eliminated, so that the true angle can be obtained. In fact, this is the role of automatic zeroing. To apply these theories to practice, the following basic conditions must be met:

(1) The surface of the rotating disk at position 2 must be parallel to the surface at position 1.
(2) Because the inclination sensor is actually an accelerometer, the instrument must be stationary when performing the error correction operation, and vibration should be avoided as much as possible.
(3) The reading at the measurement position should be read when the output of the inclination sensor reaches a balanced state.

III. System working principle and structure

According to the theoretical basis of automatic zeroing proposed above, to achieve automatic zeroing of the inclination sensor, the inclination sensor needs to be accurately rotated 180º. After comparing various micromotors, a stepper motor that is easy to accurately control is selected to realize the rotation of the platform, and then the inclination sensor is accurately installed on this platform. The readings of the inclination sensor at two opposite positions need to be converted by an A/D converter with sufficient bit number, accuracy and response speed, and finally the final result is stored and calculated by a single-chip microcomputer. The digital output result is output through RS-232 or other forms, as shown in Figure 3.

IV. Test results

1. Temperature drift test

After studying the automatic zeroing principle of the inclination sensor, the automatic zeroing servo inclination sensor prototype was designed and implemented. In order to verify the automatic zeroing and drift elimination characteristics of the automatic zeroing servo inclination sensor, the inclination sensor was subjected to high and low temperature tests. The temperature test results are shown in Table 1, where V0 and V180 are directly read by the internal inclination sensor at two measurement positions during the automatic zeroing correction operation, and Vout is the calculated result after zeroing correction (the same below).

Then the results are corrected for data analysis, and the corrected results are shown in Table 2. The temperature change-output change characteristic diagram can be obtained, as shown in Figure 4. It can be seen that in the temperature range of low temperature -40℃ to high temperature 60℃, the output V0 of the internal inclination sensor has a maximum temperature sensitivity of 0.135V. After offset correction, the output Vout of the inclination sensor has a maximum temperature sensitivity of 0.01V. In other words, the zero temperature coefficient of this inclination sensor is less than 0.0001°/℃, and its zero adjustment accuracy reaches 0.01°, achieving the purpose of automatic zero adjustment and meeting the requirements of the test equipment.

2. Zero repeatability test

Zero repeatability refers to the change in inclination when the inclination sensor deviates from the zero position and then returns to the zero position. The data recording results are shown in Table 3. Analysis of the data records shows that the zero repeatability of the automatic zero servo inclination sensor is 0.001°.

3. Error analysis test

At room temperature 20℃, the inclination sensor was tested in the range of -10° to 10°, and V0, V180 and Vout were recorded respectively. Based on the measured data, the linearity diagram of this inclination sensor at 20℃ can be obtained as shown in Figure 5.

According to the above measured data, it can be calculated that the nonlinear error of the output Vout of this inclination sensor within the full range is 0.02°, which can meet the requirements of the test equipment.

4. Vibration test

Put the inclination sensor on the vibration table for vibration test. The technical conditions of the vibration test are shown in Table 4. Record the zero position output V0, V180 and Vout of the inclination sensor before and after the vibration test. The test data is shown in Table 5.

After analyzing the above data, the comparison chart of Vout before and after vibration is drawn as shown in Figure 6. It can be seen from the figure that the automatic zeroing servo inclination sensor has good anti-vibration and anti-impact performance, which can meet the requirements of the test equipment.

5. Continuous working test

The continuous working test refers to powering on the automatic zeroing servo inclination sensor to make it work continuously for 24 hours, and recording the values ​​of its data output V0, V180 and Vout every 1 hour. According to the continuous working test record, the output change chart of its continuous working can be drawn, as shown in Figure 7.

V. Test results

After testing the prototype of the automatic zeroing servo tilt sensor, its technical parameters are finally measured as follows:

VI. Conclusion

Zero deviation and drift are technical problems that all sensors need to solve. This paper studies the model and method of automatic zeroing and the algorithm of compensation, establishes a set of theories and methods for automatic zeroing of tilt sensors, and designs and implements the prototype of the automatic zeroing servo tilt sensor. The test proves that it has good automatic zeroing characteristics and can eliminate drift problems, which has very important application value.