**7. Conclusions**

This paper explains the mainstream definition of digital hydraulics, and it gives a more accurate definition of digital hydraulic technology. Meanwhile, this paper presents a review of developmental works on digital hydraulic components and digital hydraulic technology. The main outcomes of this review work are as follows:


and other problems caused by the frequent opening and closing of high-speed switching valve, and the lives of high-speed switch components themselves. At the same time, the problem of high-speed switching valves' insu fficient service lives also needs to be solved.


The definition of Industry 4.0 promotes the progress of the whole society and industrial system. Only the technology that meets the needs of society can survive and develop. And digital hydraulic technology provides a feasible way for the traditional hydraulic industry to develop in the direction of intelligence and greenness. At the same time, as the innovation of and successor to traditional hydraulic technology, digital hydraulic technology will certainly make fluid power technology developing in line with "Industry 4.0."

With continuous in-depth research, digital hydraulic technology will continue to innovate. More mature digital hydraulic components will also have more extensive engineering application prospects.

**Author Contributions:** Q.Z. is responsible for article writing and research on digital hydraulic technology. X.K. is responsible for the general idea of this article, and he gives the defination of digital hydraulic technology. B.Y. investigates the digital hydraulic components. K.B. investigates the features and advantages of digital hydraulic technology. Z.J. investigates the developments and the current situation of digital hydraulic technology. Y.K. investigates the developing trend of digital hydraulic technology. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by NATIONAL KEY RESEARCH AND DEVELOPMENT PROGRAM, gran<sup>t</sup> number 2018YFB2000701, and the NATIONAL NATUAL SCIENCE FOUNDATION OF CHINA, gran<sup>t</sup> number 51975506.

**Conflicts of Interest:** The authors declare no conflict of interest.
