1. Introduction
In marine engineering, floating raft systems are widely used in kinds of ships, warships, submarines and other related structures, equipment, and facilities [
1]. Usually, vibration isolators are installed to collect the raft system and base. On the one hand, they could protect the machines and devices installed on the rafts from sudden external shocks under certain circumstances. On the other hand, and more importantly, the floating raft is a key technique for vibration and noise control.
The vibration and noise control are vital for submarines. Normally, a submarine has a circle hull and a relatively long body and the raft system is installed inside compactly.
Figure 1 gives a schematic diagram of a floating raft system installed in a submarine [
1]. Many vibration isolators are needed between the floating raft structures and supporting base in all directions, along with the hull circle. There are kinds of vibration isolator types, like airbags, also known as gasbags, which are commonly employed due to their large bearing capacity and good vibration isolation performance [
2]. As technology advances, especially at present when anti-submarine technology keeps improving, the underwater quieting performance of submarines has been unprecedentedly challenging [
3]. Therefore, any aspects negatively affecting the concealment ability should be considered and avoided.
Many factors show a great impact on the vibration and noise control performance, such as the isolator type and its installed station. The good installation status of floating raft isolation systems is very important in reducing vibrations and noises which would affect, both directly and prominently, the overall performance.
Figure 2 gives an installation schematic diagram of a vibration isolator in the horizontal direction. In the installation area, two metal plates, i.e., the upper backing plate and the lower backing plate in the schematic, are fine machined. This helps get a relatively correct distance between the two plates. For the installing process of big vibration isolators, ideally, it is best to make the vibration isolator height exactly equal to the distance between the base and the surface to be supported. However, the isolators usually could not match the spacing between the base and the distance. When there is a difference between the isolator height and the distance, it is necessary to install adjusting shims [
4], which means the sizes of the shims need be obtained. The common way to ascertain the thickness of the shim for the isolator is to manually measure the spacing between the mounting surfaces and then subtract the isolator height. Since the isolator height is usually known, the mounting space needs to be measured.
The traditional technique of measuring the installation space is to use tools like micrometers and vernier calipers, which have many drawbacks. When measuring, operators usually use a vernier caliper, setting it vertical to the mounting plates and ensuring the two measuring jaws are in contact with the two surfaces. The displacement could then be read and recorded. And the basic dimensions of the shims could be obtained. Afterwards, an initial shim would be manufactured for preinstallation. And the shim will be adjusted several times until a satisfying shim shape is reached. Many problems may show up in the manual measuring processes. Firstly, the measurement accuracy cannot be guaranteed, so that the adjustment shims may be made with large errors, which affects the installation of the rafts and equipment. Secondly, since there are only a few sampling points available during the measurement for adjustment shims, the reconstruction of shim mounting surface is only usually determined by three measured points. In this case, the mounting surfaces of the shims and the rafts may not be in contact effectively. As a result, vibrations and noises could occur and the wear of the shim surfaces may speed up. On the other hand, due to limited operation and installation room, the labor intensity of workers is high and the working efficiency is low. Practice shows it may take minutes for just one single measured point. Furthermore, human mistakes would happen easily. In summary, many problems exist in the present traditional measuring and installing technique. In order to improve the quality and efficiency of vibration isolation installation, the research and development of a fast and accurate measurement system is extremely urgent and of great importance.
As an important indicator for evaluating the planeness quality of the parts [
5,
6], surface flatness is also significant, in this work, in the vibration isolation performance. In practice, it is necessary to judge whether the manufactured quality of the backing plates is satisfied (see
Figure 2). Through multi-point displacement measurements, it is feasible, in some way, to characterize the flatness of a surface. Up until now, the main planeness measuring instruments have included autocollimators [
7,
8], white light interferometers [
9] and the clearance method [
10]. CMMs (coordinate measuring machines) are often used to measure large movable planes [
11]. Another flatness measuring method is to use a CCD (charge coupled device) camera as a data acquisition instrument and to process the acquired image data to obtain the flatness. The data processing process is relatively complex [
12]. Furthermore, an instrument developed based on the laser ranging method [
13] is also an option. It generates three-dimensional coordinate information by acquiring a large number of point cloud data [
14], and then the error value could be obtained according to a reasonable flatness calculation algorithm [
15]. The accuracy of this method is high and the measured data are reliable, whereas the cost is high [
16]. These existing measuring instruments and techniques are mainly employed to measure the planeness of a single surface at one time.
Based on the backgrounds mentioned above, in this work, a measuring system was developed for a certain isolator on the basis of high-precision eddy current displacement sensors in order to efficiently obtain the distances at multiple points simultaneously between two metal plates. A matched LABVIEW (Laboratory Virtual Instrumentation Engineering Workbench, National Instruments, NI) program was also exploited for data processing, showing, and analysis. The measured spacing results could be used for calculating the flatness of the metal plates through related data processing. And the system is proven to be valid and it could work efficiently.
The rest of this paper is structured as follows.
Section 2 gives the related theories including the measurement principle.
Section 3 reports the developed system. The measured results are obtained through experimental tests and discussed in
Section 4, followed by the conclusions in
Section 5.
5. Conclusions
The traditional way of measuring the installation space of the vibration isolation for raft structures has many problems, like low accuracy, large measurement error, and tedious multi-point measurement steps. This work tries to achieve the fast and accurate measurement of the installation space. The spacing measurement between two parallel metal surfaces was concentrated. A measuring system composed of multi eddy current sensors, a controlling component, as well as a calibration platform has been designed and developed. The experimental tests show the developed system works well and can obtain the digital spacing results once in several seconds and with relatively high precision, which has many more advantages than the traditional ways. The surface flatness of the measured metal plates could also be evaluated. The measuring system could be employed in other related applications. And the corresponding scheme and method could be employed as a guide or inspiration under other probable circumstances. Specifically, several conclusions can be drawn as follows.
(1) A scheme that can simultaneously measure the multi-point distance between two metal surfaces has been developed, in which the surface flatness could also been analyzed through modeling.
(2) The hardware designs, based on an eddy current displacement sensor, were introduced as well as the software program based on LabVIEW. The purpose of the measurement scheme, mechanical design, software development, algorithm implementation, and test measurement results and analysis were detailed and elaborated.
(3) The experimental tests demonstrated that the proposed scheme can realize fast displacement measurement and flatness analysis. The tests show that each measurement only takes several seconds to obtain results. And the uncertainty of the measurement error could reach less than m, which meets the requirements of automatic high-precision measurement.