# Review and Development Trend of Digital Hydraulic Technology
## 5 Developments and the Current Situation of Digital Hydraulic Technology
### High-Speed Switching Digital Hydraulic Technology
#### 5.2.1 High-Speed Switching Digital Hydraulic Valve  
The research of high-speed switching hydraulic technology focuses on the R&D of high-speed switching valves, which has two aspects. The one is the research and development of new high-speed switching valves, and the other is the control method of high-speed switching valves. The development of the high-speed switching valve can be traced back to the end of the 1970s. The company Lucas in Britain developed the high-speed solenoid switching valve using two special shape electromagnets: spiral tube type and taper type, which were called the \"Helenoid valve\" and \"Celenoid valve.\" They overcame the problem that the acceleration of armature is inversely proportional to electromagnetic force. But due to their complicated structures and difficulty in processing and manufacturing, they are not widely used [56; 57]. Since then, many companies and research institutions launched various types of high-speed switching valves. BKM company launched a three-way spherical, cartridge type, high-speed electromagnetic switching valve with a response time of 2-3 ms, and its working pressure is 10 MPa [58]. But this valve can only be used for direct digital control of electronic unit injectors because of its small flow rate. [PERSON] developed a two-way high-speed switching valve and a three-way high-speed switching valve in the 1980s; their response times are all around 3 ms [59]. In the late 1980s, [PERSON] developed an ultra-high-pressure high-speed switching valve with working pressure of 120 MPa and response time of 0.4 ms [60; 61; 62]. However, because these valves do not overcome the small flow rate, their applications are limited to the field of fuel injection. Bosch company also successfully developed a high-speed electromagnetic switching valve suitable for an ultra-high-pressure environment, and its response time is between 0.3 and 0.65 ms [63]. Linz center developed a high frequency switching ball valve which is shown in Figure 26a, and the spool is controlled by current feedback. The spool position of this valve is only 5 mm, and its frequency response is up to 1 kHz; its flow rate can reach 14 L/min [64]. But for now, the exemplary application for such a valve could be the actuation of automotive wet clutches; in particular, those of dual clutch systems. Tampere optimized the surface material and the heat-treatment process of the cone valve, and they obtained a high-speed switching water hydraulic valve with high reliability and long service life (which is shown in Figure 26b) [65]. Minnesota university developed a rotary high-speed switching valve with a special structure pilot spool which is shown in Figure 26c; this valve's maximum flow rate is 40 L/min, and its frequency response is 100 Hz [66]. Zhejiang University designed a PZT piezoceramic high-speed switching valve with temperature compensation, which is shown in Figure 26d. This valve's working pressure can reach 20 MPa, and its frequency response and flow rate are, respectively, 200 Hz and 10 L/min [67]. Guizhou Honglin machinery factory cooperated with BKM company developed a thread cartridge HSV high-speed electromagnetic switching valve, whose opening time is 3 ms, and closing time is 2 ms. This valve's highest working pressure is 20 MPa, and its flow rate can reach 2-9 L/min [68]. Zhejiang University of Technology developed a high-speed switching valve with a high frequency and large flow rate. This valve's working pressure is 21 MPa, and its flow rate is 450 L/min. The spool position of the valve reaches 6 mm, but the response time is only about 8 ms [69]. The control of high-speed switching valve has a crucial effect on its performance, but few commercial controllers can be used for high-speed switching valve at present [70; 71]. To solve such a problem, Linjama developed a set of controllers with response time of 8-12 ms that can directly control the cartridge valve [59]. Zhejiang University put forward an intelligent voltage control method based on current feedback.