# Review and Development Trend of Digital Hydraulic Technology
## 1 Introduction  
Although the foundation and development of hydraulic technology can be traced back to the middle of the 17 th century, the rapid development and application of it really began 100 years ago. The development of electro-hydraulic servo-control technology began in the 20 th century based on the continuous improvement of control theory and its engineering practice. After that, with the support of microelectronics technology, the hydraulic control unit became able to integrate with microprocessor, electronic power amplifier and sensor. So, the dynamic control accuracy, intelligence, reliability and robustness of the hydraulic control unit were improved [1; 2; 3]. When industry entered the 21 st century, the tremendous cost of labor and energy forced industry to develop towards energy conservation and intelligence. However, the high cost of hydraulic components does not correspond to the direction of industrial development. Achten P statistic shows that the cost of a hydraulic transmission component which is to 40-80 pounds/kg, is more than three times that of a mechanical transmission component [4]. Low energy efficiency is also a key factor limiting the further application and development of hydraulic technology. Generally, more than half of the output power of hydraulic pumps or motors is dissipated to throttle or overflow. Take the excavator hydraulic system as an example; up to 80% of energy dissipation occurs in such hydraulic systems [5]. In addition, after Germany put forward the definition of \"Industry 4.0\" in 2013, the industrial system put forward higher requirements for the intelligent hydraulic system and its application in the \"Intelligent factory.\" So, it can be said that if the hydraulic technology wants to survive from the intense market competition, high energy efficiency and low cost will be the inevitable direction of its development. And digital hydraulic technology is providing a feasible way to fulfil that purpose.  
After referring to the concept of \"digital\" in electronic technology, digital hydraulic technology gradually develops and takes shape; it accelerates the pace of intelligent hydraulic technology, andbecause of its huge advantages compared with traditional proportional servo control technology, it has attracted extensive attention from researchers. However, the definition of digital hydraulic technology has always been in dispute. There are two typical definitions of digital hydraulic technology. The one comes from [PERSON]; he says that \"digital fluid power refers to hydraulic and pneumatic systems that use discrete value component to actively control output of the systems,\" but from his further explanation, it can be found that his definition only includes the parallel technology and high-speed switching technology that are based on switching valve [6]. The other comes from [PERSON]; he presented a definition of digital hydraulic which included control signal discretization and fluid flow discretization [7]; there is also the content of digital signal indirect control (proportional control and servo control).  
The definition of digital hydraulic technology is not unified, which limits its development and application to some extent. With the intelligent and green development of global industry, only the technology meeting social needs can survive, and digital hydraulic technology is providing a feasible innovative development path for traditional hydraulic technology. Therefore, on the basis of a lot of research on digital hydraulic technology, this paper gives the exact definition of such a technology, and expounds its developmental course and trend, so as to make more researchers understand and promote the further development of it.