Abstract
The pneumatic pilot valve is one of pressure control valves and is used in pneumatic control instruments. The valve amplifies nozzle back pressure. This report deals with the so-called nonbleed type pilot valve with nonmetallic diaphragms. It is an advantage of this valve that it controls the large output flow rate consuming little control air. The valve contains two diaphrams for receiving the input and output pressures. There are three modes of actions with the valve; namely, (1) the steady mode when the both supply and exhaust valves are closed (2) the charge mode when the input signal increases and the supply valve opens, and (3) the discharge mode when the input signal decreases and the exhaust valve opens. The paper studies the characteristics of the valve under these three acting modes. A valve action of this type is constituted with these different modes, so there exists a strong non-linearity. In the charge and discharge modes, the pressure near the valve slit decreases steeply because the air speed is very high. The resultant force acts to close the valve. This force is called Bernoulli's force in fluid mechanics. Because it is difficult to estimate the magnitude of the force theoretically, we obtained it by experiment. The output flow rate is influenced by this force directly and decreased by about 20%.
The action of the pilot valve was generally described as a first order system neglecting influences from loads. But the output pressure influences the valve displacement and the output pressure is determined by the pressure-flow characteristics of the load. Hence it should be necessary to regard the output pressure as a feed-back signal to the valve. From this point of view we proposed a new graphic model of the pilot valve.
