## 2 Characterization and Methodology  
### Description of VCM-DHPV  
Figure 1 illustrates the schematic diagram of VCM-DHPV, which consists of two parts: the pneumatic on/off valve and the VCM actuator. The on/off valve is regarded as an electro-pneumatic converter, which has the functions of the power amplified element and the energy conversion [20]. The VCM actuator proposed is shown in Figure 2. The main part of the VCM is composed of an iron yoke and a cylindrical permanent magnet, which can make a radially-oriented magnetic field in the air gap of the VCM. The moving part of the VCM is composed of the plastic bobbin and the wire winding,  
which can provide an Ampere force in the vertical direction perpendicular to the current flow and the magnetic flux when the coil is energized [21]. The spool connected directly with the slider is driven linearly by the VCM actuator. The permanent magnet of actuator is fixed with the valve body as the stator. To eliminate the excess pneumatic force acting on the spool, the lock nut is designed to balance the area difference of the spool, and then only the flow force and seal friction should be overcome by VCM. Therefore, the electromagnetic force of VCM is decreased and the dynamic characteristic of VCM-DHPV is improved. A high-accuracy position sensor mounted on the VCM is used to measure the displacement of the valve spool, and the position signal is fed back to the system controller, which generates the coil current. The poppet valve face-seal type is used in the valve port to reduce the leakage of VCM-DHPV when working under high-pressure condition. In addition, the face-seal in the valve port is made of graphite sleeve dipped with the phenolic resin, so the graphite valve port is softer than the valve cone made of ceramic, which is sprayed at the surface of titanium alloy spool. This kind of soft-hard contact sealing can archive zero leakage of the valve port. As a control component, the VCM-DHPV is designed to control gas flows through the pipelines and the chambers in the pneumatic actuator. Therefore, based on the position control system, the displacement of the valve spool is regulated accurately, so that it can control the gas quickly through the valve port from inlet to outlet.  
When there is no exciting voltage signal, the spool is compressed tightly on the graphite sleeve by the spring, and then the VCM-DHPV is closed. As positive exciting voltage signal is given, the spool moves upward under the action of the electromagnetic force by VCM, and then the valve is opened. As a result, the inlet port and outlet port are interlinked. The high-pressure air flows from the inlet port into atmosphere through the outlet port. As negative exciting voltage signal is given, the spool moves downward under the action of the electromagnetic force by VCM, and then the VCM-DHPV is closed, and the inlet port and outlet port are disconnected.  
The dynamic characteristic of VCM-DHPV is one of the main research focuses. According to ISO 5598-2008, the dynamic response time of fluid power components can be described as the elapsed time between the action initiation and the resulting reaction, which is tested under stated conditions. In this research, the opening response time is considered as the elapsed time between the initiation point when the exciting voltage is given and the point when the on/off valve is fully opened.  
Figure 1: Schematic diagram of the developed high-speed pneumatic on/off valve (VCM-DHPV).