An online detection system for inner diameter, outer diameter and wall thickness of pipes

An online detection system for inner diameter, outer diameter and wall thickness of pipes

Abstract: This paper introduces an online detection system for the inner diameter, outer diameter and wall thickness of pipes, and describes in detail the structure, measurement principle, hardware circuit and software design of the detection system.
Keywords: inner diameter, outer diameter, wall thickness, online detection, displacement sensor

1 Introduction
In industries such as boilers and pressure vessels, pipes are commonly used raw materials. At present, the detection of their inner diameter, outer diameter and wall thickness mainly uses measuring instruments such as calipers. The main disadvantages of this method are that the human factor makes its measurement error large, the labor intensity is high, and the efficiency is low? This paper introduces an online detection system for the inner diameter, outer diameter and wall thickness of pipes. The system realizes the automation of detection and improves the detection accuracy.
2 Structure composition
The structure of the detection device is shown in Figure 1. On a fixed bracket, four identical inductive displacement sensors A, B, C, and D are installed in the same vertical axis direction, and are connected to a control box through the round hole on the bracket with wires. The control box is equipped with an amplifier, A/D converter, single-chip system, micro printer, display, keyboard, etc. During the detection, the measured pipe is sent into the bracket slot. At this time, the output signal of the inductive displacement sensor is amplified by the amplifier and converted by the A/D converter and sent to the single-chip system. After the single-chip system collects the signal, it is processed, displayed and printed.

3 Detection principle
The size of the bracket of the detection device is determined according to the inner and outer diameters of the pipe to be detected. The detection device needs to be calibrated before the detection. The method is to process a precision pipe as the standard pipe of this detection device according to the inner diameter, outer diameter and wall size of the pipe to be detected, and send the standard pipe into the bracket. After the detection device detects this, there is a standard parameter value on the display or printer, and this parameter value is stored in the single-chip computer as its standard value. When actually detecting the pipe to be tested, the detection system also detects its parameter value, and compares the value of the pipe to be tested with the standard value pipe to obtain the inner diameter, outer diameter and wall value of the pipe to be tested.
Assume that the inner diameter of the standard pipe is d0, the outer diameter is D0, the inner cylindrical diameter of the bracket groove is m0, the inner diameter of the bracket groove is M0, the inner diameter of the measured pipe is d, and the outer diameter is D. When the standard pipe is sent into the bracket groove for testing, as shown in Figures 2 and 3, the inner diameter of the standard pipe is:

where rB, rC are the displacements of inductive displacement sensors B and C in the same diameter direction of the standard pipe.

Then the outer diameter of the standard pipe is:

Where rA, rD are the displacements of the inductive displacement sensors A and D in the same diameter direction in the standard pipe.
Similarly, when the pipe to be tested is sent into the bracket slot for testing, as shown in Figures 4 and 5, the inner diameter of the pipe to be tested is:

Then the outer diameter of the pipe to be tested is:

(8)
In summary, from formulas (6) and (8), it can be seen that as long as the displacement of the inner diameter and outer diameter of the pipe to be tested relative to the inner diameter and outer diameter of the standard pipe is detected, the inner diameter and outer diameter of the pipe to be tested can be obtained by adding the inner diameter and outer diameter of the standard pipe.

Then formula (6) can be obtained as:

Here C is the conversion coefficient between the output signal of the inductive displacement sensor and the digital quantity.
It can be seen from equations (9) and (10) that as long as the displacement of the inductive displacement sensor in the standard pipe ΔU1, ΔU3, the displacement of the inductive displacement sensor in the measured pipe ΔU2, ΔU4 and the inner diameter and outer diameter of the standard pipe are taken into account, the inner diameter and outer diameter of the measured pipe can be calculated from equations (9) and (10). The wall thickness of the measured pipe is obtained from equations (10)-(9):

It can be seen from equation (11) that when only the inner diameter and outer diameter of the measured pipe are known, the wall thickness of the measured pipe can be directly calculated or measured.
4 Hardware Circuit
4.1 Single-Chip Microcomputer System
Single-chip microcomputer is an important branch of microcomputers. It integrates CPU, I/O interface, timer, counter and memory on one chip. With the rapid development of microelectronics technology, its price has become very cheap and there are many types. Here, a single-chip microcomputer ATMEL 89C51 with many applications is selected as the detection control core.
89C51 has 4 I/O interfaces, 2 timer counters, 5 interrupt levels, 128 bytes of data storage, 4K program storage (electrically erasable), and is compatible with the MCS-51 system microcontroller.
4.2 Micro printer
Micro printers are commonly used external devices in microcontroller application systems. Commonly used are TPMP40A/16A, GPl6, PP40, etc. This type of micro printer can meet the printing requirements of this detection system, but the first thing to consider when choosing a micro printer is its service life. This detection system uses the PP40 plotter printer, which can not only print numbers, letters, characters, etc., but also print graphics, and its service life is long.
4.3 A/D converter
Since this detection system has a total of 4 inductive displacement sensors, the selection of the A/D converter requires an analog electronic switch in the A/D converter. Here, the ADC0809 8-way successive approximation A/D converter is selected, and the conversion speed and accuracy can meet the requirements of this detection system.
4.4 Amplifier
The amplifier converts the millivolt voltage signal output by the inductive displacement sensor into a voltage signal of several volts. The amplifier is required to have low drift, low offset, and meet the accuracy requirements. Here, the 7650 operational amplifier is selected. Its circuit is very simple and can be found in many materials.
5 Software Design
The flowchart of the detection system is shown in Figure 6. The entire detection control is controlled by the keyboard of the detection system. When the system is powered on, it is first initialized, and then the inner diameter d0 and outer diameter D0 values ​​of the standard pipe are input to the detection system. Then the standard pipe is tested to find the ΔU1 and ΔU3 values. Finally, the tested pipe is tested to find the ΔU2 and ΔU4 values. In order to improve the accuracy of the detection system, it is necessary to measure ΔU1, ΔU3, ΔU2, and ΔU4 of the standard pipe and the pipe to be tested 10 times in succession, and take the average value as the accurate value to substitute into (9), (10), and (11) to calculate the inner diameter d, outer diameter D, and wall thickness δ of the pipe to be tested, and display and print this value on the display.

6 Conclusion
This detection system is small in size, simple in structure and high in precision. It can be made into a portable on-site online detection of the inner diameter, outer diameter and wall thickness of pipes. It is especially suitable for large-scale detection of pipes. In addition, since the method of comparing standard pipes with the pipes to be tested and the computer to process the detection data, as long as the parameters of the sensors, amplifiers, A/D converters, brackets, etc. that affect the measurement results are stable, the requirements for the parameters of components, machined parts, etc. are greatly reduced, and the cost of this detection system is minimized. Therefore, this detection system has a high performance-price ratio and fully meets the detection needs of various pipes.

References:

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