Hydraulic cylinder inner wall parameter measurement data acquisition and processing system

Hydraulic transmission is widely used in vehicles, lifting and transportation machinery, engineering machinery, mining equipment and other machinery. As an indispensable component of hydraulic transmission, the accuracy of the parameters of the inner diameter of the oil cylinder will directly affect the performance of the transmission system.
The aperture measurement method similar to the inner wall of the oil cylinder is generally divided into direct measurement method and indirect measurement method. The direct measurement method is a method of directly measuring the aperture by positioning at two or three points. Its commonly used measuring tools include internal diameter dry ruler, universal length gauge, horizontal length gauge, etc., general length measuring tools and special aperture measuring tools such as internal diameter micrometer and flexible coordinate measuring machine, but this kind of measurement method is inefficient and has large errors, and is not easy to be used for online measurement of deep aperture and high-precision products. The indirect measurement method uses the principle of three points to determine a circle to measure the coordinate values ​​of any three points on the circumference of the measured hole, and then calculates the coordinates of the center of the circle according to the relevant algorithm, and then converts the aperture size and other parameters. This method has high efficiency and small error, and can be used for online detection. This detection system adopts the indirect measurement method.

1 Overview of the measurement system
The design concept of the measurement system is to install four high-precision displacement sensors on the designed sensor fixture, and put it into the measured cylinder. Under the traction of the air-floating guide rail, it passes through the measured cylinder smoothly. The obtained signal change of the displacement sensor is transmitted to the host computer. The principle of the indirect measurement method is applied to convert the measured data into coordinate change. Through the least squares fitting data, according to the relevant data processing operations, multiple parameters required for the inner wall of the cylinder are obtained.

This detection system is mainly composed of two parts: hardware and software. Among them, the hardware part is mainly composed of 4 high-precision displacement sensors, 4-channel inductance box, data acquisition card, air-floating guide rail and host computer. The overall structure block diagram of the system is shown in Figure 1. One end of the inductance displacement sensor is connected to the 4-channel inductance box, and then the output end of its 4-way voltage signal is connected to the analog input channel port of the data acquisition card. The A/D function module inside the data acquisition card performs analog-to-digital conversion on the signal. The converted digital signal is transmitted to the host computer through the PCI bus interface for related data processing. At the same time, the host computer transmits various control signals of the air-floating guide rail to the data acquisition card through the PCI bus, and then controls the movement of the air-floating guide rail. In this way, the host computer can collect the signals of the 4 displacement sensors on site and transmit the control signals of the air-floating guide rail. The software part is mainly based on the development and design of the host computer data acquisition and processing software of VB 6.0.

2 Hardware Introduction
The main purpose of the detection system is to detect the parameters such as the cylindricality, straightness and diameter of the inner wall of the oil cylinder. Since the measurement accuracy of the detection system is required to be (-15μm, +15μm) and it is a dynamic measurement, the selected sensor must be a displacement sensor with high accuracy and fast response speed. According to the above requirements, the system uses the inductive displacement sensor of Swiss TESA, also known as the inductive sensor head. It is a half-bridge sensor that converts displacement changes into electrical signals. The measurement range is ±0.5mm, the axial stroke is 1.25mm, the sensitivity is 73.75±0.5mV (V/mm), the linear error is <0.2%, and the repeatability is <0.2μm.
Since the power supply of the selected sensor is a 13 kHz 5 V AC voltage source, in order to ensure the high stability of the sensor output signal and the convenience of transmission, it is equipped with a channel inductance box.
During the detection process, a large number of sensor signals need to be transmitted to the host computer for real-time processing and analysis, which requires a high-speed and large-capacity data acquisition device. The system uses Advantech's PCI-1711 high-speed data acquisition card, which provides users with the required measurement and control functions. It can provide 16 channels of single-ended A/D input, 12 bit A/D conversion, and a sampling rate of up to 100 kHz. The gain of each input channel can be programmed separately. Users can select different gain coefficients according to the different input voltage types of each channel and set the corresponding input range. The card has a 1 kB sampling FIFO buffer, programmable counter/timer, and automatic channel/gain scanning.

3 Software Design
3.1 Data Acquisition
The programming methods for data acquisition by PCI-1711 can be divided into three types: software trigger method, interrupt method, and DMA method. The software method is that the software command triggers the data conversion. This method is relatively simple to program, but the data acquisition speed is slow and is mostly used for low-speed data acquisition. The interrupt transmission method has a higher sampling speed than the software transmission method. There are two types of analog input interrupt transmission methods: one method generates an interrupt for each conversion; the other method is to save the conversion data in the FIFO. Depending on the hardware, an interrupt is generated when the FIFO is half full or full. After receiving the interrupt, the device driver will send different events to inform the user of the current sampling status; the DMA method is the fastest data transmission among the three. The data is directly transferred between the device and the memory without CPU intervention. The device driver will detect the data conversion status and send appropriate events to notify the user. The system calls the dynamic link library in the software program and uses the FIFO function under the DMA method to transmit signal data. The data acquisition of this system uses the third method - DMA method, combined with the Visual Basic6.0 development platform, and the data acquisition process is shown in Figure 2.

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3.2 Data Processing
The fundamental principle of using four inductive displacement sensors to measure the parameters of the inner wall of the oil cylinder is to convert the displacement changes measured by the four sensors of the measuring section into coordinate values ​​in the coordinate plane where they are located, and then use the least squares fitting to fit the actual center position coordinates of the measured section of the oil cylinder and the cylinder diameter through these four coordinate values. The key point of this measurement process is to convert the changes measured by the inductive displacement sensor into corresponding changes in coordinate values. Under ideal conditions, the axes of the sensors installed on the fixture are orthogonal, and the intersection of the axes coincides with the center of the standard ring gauge. A rectangular coordinate system is established with the intersecting axes as the coordinate axes and the intersection as the origin. The converted coordinates measured by the four displacement sensors are (x1, y1), (x2, y2), (x3, y3), and (x4, y4) respectively.
3.2.1 Principle of least squares fitting
Assuming that the measured section is XY, the position coordinates measured by the four displacement sensors after transformation are P1(x1, y1), P2(x2, y2), P3(x3, y3), and P4(x4, y4). These four points are all on the fitted circle, and the ideal circle equation is

Only when the center coordinates (x0, y0) are small enough, can a linear transformation be performed. If (x0, y0) is not small enough, it will cause linear errors. At this time, the obtained center coordinates (x0, y0) are used as the origin of the new coordinates, and the coordinates of the measured data points are translated and then the least squares fitting is performed until the coordinates (x0, y0) are small enough. The fitting center coordinates (x0, y0) of this measurement system are small enough, so the measured coordinates do not need to be transformed.
3.2.2 Error Processing
The measurement errors of the system mainly include the deformation error of the measuring device caused by temperature changes, the sensor accuracy error, the sensor installation error, etc. Since the measurement environment can be controlled by constant temperature and the selected sensor is a high-precision sensor, the first two errors can be ignored, but the sensor installation error should be eliminated. The sensor installation error can be divided into three aspects:
(1) The eccentricity error of the origin of the coordinate system, that is, there is a slight deviation between the intersection of the orthogonal axes of the displacement sensor and the center of the measured object.
(2) The coordinate position error caused by the non-orthogonality of the displacement sensor
installation axis. (3) The error caused by the non-orthogonality of the displacement sensor installation axis and the eccentricity error of the origin of the ideal coordinate system during measurement.
By the above fitting method, the straightness, cylindricity and radius of the cylinder can be quickly obtained through real-time data processing by the software written. Figure 3 is the software interface of the measurement system.

4 Conclusion
The data acquisition and processing system for hydraulic cylinder inner diameter parameter detection introduced in this paper has high measurement accuracy and efficiency. It can be used in an online detection environment. By measuring the diameter, straightness, cylindricity and other parameters of the inner wall of the hydraulic cylinder, it can quickly check whether the product meets the factory accuracy requirements. It has strong practicality. The system can be promoted in the field of deep hole parameter detection of products.