The present invention relates to a proportional control valve capable of controlling a flow rate of a fluid to be at a desired level.
As a control valve for controlling a flow rate of a chemical liquid or an exceptionally high-purity liquid, it is preferable to use a valve made of Teflon™ so as to prevent entry of metal ions into the liquid. Further, to enable a flow rate of a chemical liquid to be precisely controlled, a diaphragm type valve is preferably used in which a degree of valve opening can be controlled as desired. A diaphragm made of Teflon™ is directly subject to a fluid pressure, and it is necessary to impart by way of an actuator a force that is sufficiently high to counteract this fluid pressure. Generally, as an actuator in this field of application, a linear actuator comprising a precision gear and a stepping motor is used (reference is made to, for example, U.S. Pat. No. 6,145,538).
However, use of a linear actuator of this type gives rise to various problems, such as high cost, difficulty in making fine adjustments due to gear backlash, low responsiveness, complexity of a control circuit, and so on.