Piezoelectric pump

The present application is a continuation of International Application No. PCT/JP2007/073555, filed Dec. 6, 2007, which claims priority to Japanese Patent Application No. JP2006-332692, filed Dec. 9, 2006, the entire contents of each of these applications being incorporated herein by reference in their entirety.

The present invention relates to piezoelectric pumps, and more particularly, to a piezoelectric pump using a diaphragm that is bent by a piezoelectric element.

Piezoelectric pumps are used as cooling pumps in small electronic devices, such as notebook personal computers, and fuel transportation pumps in fuel cells. A piezoelectric pump is a pump using a diaphragm that is bent by the application of voltage to a piezoelectric element, and has advantages of a simple structure, a low profile structure, and low power consumption. In a piezoelectric pump using a piezoelectric element as a driving source, check valves are respectively provided at an inlet and an outlet. However, reliability of the check valves is reduced with use for a long period, and the fluid is not sufficiently transported because of adhesion of foreign substances, such as dust, to the check valves. Further, when the piezoelectric element is driven at a high frequency, the check valves, to which foreign substances, such as dust, adhere, do not follow the driving, and transportation of the fluid is impossible.

Patent Documents 1 and 2 propose a piezoelectric pump in which a diaphragm is mounted in contact with a pump body having an inlet and an outlet and in which a plurality of piezoelectric elements are mounted on the diaphragm so as to be arranged from the inlet to the outlet. In this pump, the piezoelectric elements are sequentially driven from a piezoelectric element close to the inlet to a piezoelectric element close to the outlet, so that the diaphragm can be sequentially bent from the inlet toward the outlet so as to push out the fluid from the inlet toward the outlet. When the application of voltage to the piezoelectric elements is stopped, a channel between the inlet and the outlet is closed by restoration of the diaphragm. Therefore, it is possible to omit check valves from the inlet and the outlet.

Unfortunately, since a plurality of piezoelectric elements need to be arranged in a plane in the piezoelectric pump having this structure, the piezoelectric pump has a large size and a complicated structure. Moreover, a driving circuit for sequentially driving the piezoelectric elements is complicated, and this increases the cost.

Patent Document 3 discloses a fluid pump having no check valve. In a fluid pump disclosed particularly in FIG. 10 of Patent Document 3, a pump chamber is formed between a pump body and a diaphragm, a first opening is provided in a center portion of the pump body, a second opening is provided in a peripheral portion of the pump body, an elastic buffer is provided in the diaphragm, and the diaphragm is bent by causing the center portion of the diaphragm by another driving means to reciprocate. When the diaphragm opens the first opening, fluid is drawn into the pump chamber from the first opening. When the diaphragm closes the first opening, the buffer portion corresponding to the second openings is bent, and the fluid is discharged from the second openings by the elastic restoring force of the buffer portion.

In Patent Document 3, since the diaphragm is merely driven in a reciprocating manner by the single driving source, the structure is simple. However, since only the portion of the diaphragm opposing the first opening, that is, only the center portion of the diaphragm is displaced and the peripheral portion (buffer portion) of the diaphragm is bent after the displacement, the diaphragm needs to be formed of a soft material. This makes it impossible to increase the discharging pressure. For example, when the fluid is compressible fluid such as air, a very soft material, such as rubber or resin, needs to be used in order to elastically deform the buffer portion of the diaphragm, and the discharging pressure is decreased. As a result, the fluid sometimes cannot be reliably discharged under a condition that the pressure outside the pump chamber is high.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2-149778

Patent Document 2: Japanese Unexamined Patent Application Publication No. 4-86388

Patent Document 3: Japanese Unexamined Patent Application Publication (Translation of PCT application) No. 10-511165

Accordingly, an object of a preferred embodiment of the present invention is to provide a piezoelectric pump that has a simple structure and that can increase the discharging pressure.

In order to achieve the above object, the present invention provides a piezoelectric pump including a pump body; a diaphragm fixed at an outer peripheral portion to the pump body; a piezoelectric element bonded to a center portion of the diaphragm; a first opening provided in a portion of the pump body opposing the substantial center portion of the diaphragm; and a second opening provided in an intermediate region between the center portion and the outer peripheral portion of the diaphragm or in a portion of the pump body opposing the intermediate region. The diaphragm is formed by a metal plate, and the piezoelectric element has a size such as to cover the first opening and such as not to reach the second opening. A portion of the diaphragm opposing the first opening and a portion of the diaphragm opposing the second opening are bent in opposite directions by applying a voltage having a predetermined frequency to the piezoelectric element so that fluid is drawn in from one of the first opening and the second opening and is discharged from the other opening.

Unlike Patent Document 3 in which fluid is pushed out by using an elastic restoring force of the diaphragm itself, according to the present invention, a metal plate having a high Young\'s modulus is used as the diaphragm and the fluid is discharged by forcibly bending the diaphragm by the piezoelectric element. In particular, since the piezoelectric element has a size such as to cover the first opening and such as not to reach the second opening, the portion of the diaphragm opposing the first opening and the portion of the diaphragm opposing the second opening can be efficiently bent in opposite directions. For this reason, the discharging pressure can be increased, and the fluid can be reliably discharged even under a condition where the pressure on the discharging side is high. In particular, since the diaphragm is formed by the metal plate having a high Young\'s modulus, it can properly follow the piezoelectric element, and this allows operation at a high frequency.

While the frequency of the voltage applied to the piezoelectric element can be arbitrarily selected, it is preferable that the piezoelectric element be driven at a frequency near the resonance frequency of a displacement member defined by the diaphragm and the piezoelectric element, since the displacement volume of the diaphragm is quite large, and a high flow rate can be obtained. When driving is performed in a primary resonance mode (first resonance frequency), fluid can be drawn in from the first opening, and can be discharged from the second opening. When a tertiary resonance mode (tertiary resonance frequency) is used, fluid can be drawn in from the second opening, and can be discharged from the first opening. While driving can be performed at a high frequency in both the primary resonance mode and the tertiary resonance mode, in particular, when the tertiary resonance mode is used, operation can be performed at a quite high frequency that is about three times of that in the primary resonance mode. Since this allows driving at a frequency above an audible region, noise can be avoided. As for this, for example, when a soft material is used as the diaphragm, as in Patent Document 3, there is a time lag between displacement of the center portion of the diaphragm and displacement of the peripheral portion of the diaphragm. Therefore, the fluid pump in Patent Document 3 cannot be driven at a frequency higher than or equal to a frequency corresponding to the time lag. In contrast, since the piezoelectric pump of the present invention uses a metal plate having a high Young\'s modulus as the diaphragm, it can be driven at a high resonance frequency of the first resonance mode and the tertiary resonance mode. In particular, when driving is performed in a tertiary resonance mode beyond the human audible region, noise is not produced, and a high flow rate can be obtained. Further, since the displacement is small, stress generated in a fixed portion between the pump body and the diaphragm is reduced, and reliability is thereby improved. It is preferable that the Young\'s modulus of the diaphragm be 100 GPa or more. When the Young\'s modulus is 100 Gpa or more, a high follow-up ability is obtained when driving is performed in any of the primary resonance mode and the tertiary resonance mode. Moreover, since the loss during driving is small, the amount of generated heat is small, and the power efficiency is high.

The piezoelectric pump of the present invention is suited to transport compressible fluid such as air. When a piezoelectric pump discharges imcompressible fluid such as liquid, in general, check valves formed of a soft material, such as rubber or resin, are respectively provided at the inlet and the outlet, and a piezoelectric element is driven at a low frequency of about several tens of hertz. When such a piezoelectric pump is used as a pump for discharging compressible fluid such as air, the displacement amount of the piezoelectric element is quite small, and little fluid can be discharged. When the piezoelectric element is driven near the resonance frequency (primary resonance frequency or tertiary resonance frequency) of the displacement member defined by the diaphragm and the piezoelectric element, the maximum displacement can be obtained. However, since the resonance frequency is a high frequency of the order of kilohertz, the check valves cannot perform a follow-up operation. Since a check valve is not provided in the present invention, even when the piezoelectric element is driven at the frequency near the resonance frequency, imcompressible fluid can be efficiently transported without being restricted by the check valve. Further, there is no fear that operation failure will be caused by adhesion of dust or the like to the check valve, and a highly reliable piezoelectric pump can be provided.

It is preferable that the second opening be provided at a position where the diaphragm is maximally displaced in a tertiary resonance mode or outside the position. While the position where the diaphragm is maximally displaced in the tertiary resonance mode differs in accordance with the area ratio of the piezoelectric element and the diaphragm or the Young\'s modulus of the diaphragm, when the second opening (inlet) is provided at the position of maximum displacement or outside the position, a sufficient sealing ability of the second opening (inlet) can be obtained when discharging the fluid from the first opening (discharging port) in an operation cycle of the piezoelectric pump, and backflow of the fluid to be discharged can be prevented. This increases not only the discharging pressure, but also the discharging flow rate.

A plurality of the second openings may be provided on the same circumference centered on the first opening. When driving is performed in a tertiary resonance mode, the second opening serves as an inlet. If one second opening is provided, fluid does sometimes not rapidly flow into an annular pocket space formed between the pump body and the peripheral portion of the diaphragm, and a sufficient amount of fluid is not discharged. In contrast, when a plurality of second openings are provided on the same circumference, the fluid can rapidly flow into the annular pocket space, and the amount of discharged fluid can be increased.