## 1 Introduction  
Recently, pneumatic servo systems have been widely adopted for use in robotic systems and vibration-isolation devices. The use of air in such applications has several advantages, including the nonmagnetism of air, the achievement of high power-to-weight ratios, and reduced heat generation. Pneumatic servo valves are a key element in pneumatic servo systems. The characteristics of these valves are a critical factor in achieving satisfactory performance [1; 2; 3; 4; 5]. There are mainly two types of pneumatic servo valve: the nozzle-flapper type, and the spool type. Nozzle-flapper type servo valves can directly control pressure. The pressure-control systems associated with the valves are generally approximated to a first-order lag system assuming an isothermal state change for the air in the load chamber [6; 7]. These valves have both high responsiveness and high linearity. However, they require high exhaust flow rates in order to provide the precise control of pressure.  
Spool-type servo valves can directly control flow rate. Because their exhaust flow rates are smaller than those of nozzle-flapper type valves, spool-type servo valves are widely used in pneumatically driven robot systems [8; 9; 10]. However, spool-type valves still experience air leakage owing to the presence of a gap between the spool and the sleeve. Usually, leakage flow is approximately 3% to 5% of the maximum flow rate. With the commercialization of servo valves, it is highly desirable to decrease exhaust flow rates in order to reduce energy consumption and operating costs.  
To overcome this problem, poppet-type [11] and pinch-type valves [12; 13; 14] have been developed. Most of these valves only have two ports. The valve shown in Reference [14] has three ports, but the overlap condition should be improved. Three-port poppet-type servo valves had not yet been commercialized at the time of our investigation. A pneumatic cylinder can be controlled with a five-port servo valve, but two valves are required with a three-port valve. The use of three-port valves increases the cost. However, the use of two three-port valves becomes suitable for compliance control because each cylinder chamber can be individually controlled. A study [15] showed that the use of only one five-port servo valve may lead to very steep variation of the fluidic stiffness around zero velocity; independent pneumatic force control in each chamber, using two servo valves, surpasses this drawback. A zero-lap condition is desirable for precise control with three-port valves. Moreover, the dynamic response of servo valves is important for precise position control in a pneumatic servo system [16].  
The purpose of this study was to develop a three-port poppet-type servo valve with high dynamic response and nearly zero lap. The target leak flow was \(1\%\) or less than the maximum flow rate. Further, to allow the valves to be used in soft robots controlled at low flow rates, maximum flow rate was designed to be less than 30 L/min. The characteristics of the valve were measured, and the effectiveness of the valve is demonstrated through pressure and position-control experiments.  
The remainder of the paper is organized as follows: Section 2 describes the design of the proposed valve. The static and dynamic characteristics of the valve are discussed in Section 3, and Section 4 reports the results of the position-control experiments. Finally, the conclusions of this work are presented in Section 5.