1. Field of the Invention
The present invention relates to a piezoelectric valve opened or closed by means of displacement of a piezoelectric element, as well as an optical granular material-sorting apparatus that utilizes such piezoelectric valve and that sorts granular materials by blowing away defective materials, etc., with a blast.
2. Description of the Related Art
Optical granular material-sorting apparatuses have traditionally been known where grain kernels, resin pellets and other granular materials are sorted into good materials and defective materials by blowing away defective materials by a blast, or foreign objects, etc., that have mixed into granular materials are removed with a blast.
Granular material-sorting apparatuses of this type are designed to sort granular material as it drops from an end of a transfer channel along a specified path, by blowing away and removing defective materials, etc., based on defective material detection signals.
The aforementioned granular material-sorting apparatuses are intended to blow away defective materials, etc., from among a large amount of granular materials that drops continuously, by means of an air blast, and therefore require that a blast nozzle be provided with a responsive valve in order to accurately blow away only defective materials, etc., without allowing other granular materials to be blown away with the defective materials.
For this reason, the applicant of the present application for patent had earlier proposed a piezoelectric air valve that can be opened or closed at high speed by utilizing a piezoelectric element (refer to Patent Literature 1).
The piezoelectric air valve described in Patent Literature 1 utilizes the characteristics of a piezoelectric element offering excellent high-speed response, and is provided with a displacement amplification mechanism that amplifies a small displacement of the piezoelectric element based on the principle of leverage.
This piezoelectric air valve is designed in such a way that when voltage is applied to the piezoelectric element, a displacement of the piezoelectric element occurring in the stretching direction of the element is transmitted to a valve body via the aforementioned displacement amplification mechanism and consequently the valve body moves quickly and the valve opens.
This piezoelectric air valve is also designed in such a way that when the voltage applied to the piezoelectric element is removed, the piezoelectric element returns to its original state and during this process, the force generated by the element as it returns to its original state is transmitted to the valve body via the aforementioned displacement amplification mechanism and consequently the valve body quickly contacts a valve seat and the valve closes.
An optical granular material-sorting apparatus having the aforementioned piezoelectric air valve provides superior response of valve opening/closing compared to any conventional electromagnetic valve, and is therefore capable of blowing away defective materials, etc., accurately while keeping to a minimum the possibility of good materials before and after those defective materials getting blown away with the defective materials.
Note that with granular material-sorting apparatuses of this type, the air blasting time is often changed according to the dropping speed, size of granular materials, etc., in order to improve the sorting accuracy (refer to Patent Literatures 2 and 3).
However, the optical granular material-sorting apparatus having the aforementioned piezoelectric air valve is such that the piezoelectric air valve has a valve body that is moved via the displacement amplification mechanism, which means that if the air blasting time becomes longer, the valve body may vibrate and the amount of blast released from the nozzle may fluctuate, thereby preventing the sorting apparatus from demonstrating stable sorting action.    [Patent Literature 1] Japanese Patent Laid-open No. 2004-316835    [Patent Literature 2] Japanese Patent Laid-open No. Hei 11-179292    [Patent Literature 3] Japanese Utility Model Laid-open No. Hei 6-41876