## 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 1.0 \(\upmu\)m, which meets the requirements of automatic high-precision measurement.  
Conceptualization, Q.X. and S.X.; methodology, Y.Y., S.X. and M. W.; software, M.W. and Q.X.; formal analysis, S.X., M.W. and Y.Y.; investigation, Y.Y., S.X., M.W. and Q.X.; data curation, M.W.; writing--original draft preparation, Q.X., M.W. and Y.Y.; writing--review and editing, S.X. and Q.X.; supervision, Q.X. and S.X.; project administration, Q.X.; funding acquisition, Q.X. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No.** & **No.3 Point/mm** & **No.5 Point/mm** & **No.9 Point/mm** \\ \hline
1 & 10.329 & 10.201 & 9.990 \\
2 & 10.331 & 10.202 & 9.991 \\
3 & 10.330 & 10.202 & 9.992 \\
4 & 10.328 & 10.200 & 9.990 \\
5 & 10.330 & 10.199 & 9.991 \\ Averaged value & 10.330 & 10.201 & 9.991 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Repeated measurement results at the measuring point.  
Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Conflicts of Interest: Author [PERSON] was employed by the company Tonking New Energy Technology (Jiangshan) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.