GB2420010A

GB2420010A - Piezoelectric motor with quadrant-type actuator

Info

Publication number: GB2420010A
Application number: GB0424484A
Authority: GB
Inventors: Kai Cheng; Amy Guo Wei Li; Feng Dong Chen; Yong Qiang Song
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2004-11-05
Filing date: 2004-11-05
Publication date: 2006-05-10
Also published as: GB0424484D0

Abstract

The piezoelectric motor comprises a rotor 12 having a recess (15, fig 2) defining an annular driving surface 16 and a stator housing 10 supporting an actuator 17. The actuator has a rectangular metal core 20 and a ceramic piezoelectric element 22 bonded to at least one side of the core. The or each piezoelectric element has a first face and a second face, the first face being divided into four quadrants 23, 24, 25, 26 covered by quadrant electrodes and the second face having a single common electrode in electrical contact with the core. The quadrant electrodes are connected so as to operate in pairs of diagonally opposite quadrants. A driving tip 21 extends from each of the two opposite ends of the core, the driving tips 21 and/or the annular driving surface 16 of the rotor being tapered in a direction parallel to the axis of rotation of the rotor. Spring means 28 urges the driving tips 21 into contact with the annular driving surface 16 of the rotor.

Description

A PIEZOELECTRIC MOTOR

Background of the Invention

The present invention relates to a piezoelectric motor and more particularly to a bi-directional piezoelectric motor.

Rotary piezoelectric motors produce either a travelling or progressive wave in the stator, or a standing wave. A travelling wave piezàelectric motor is more complicated and expensive to produce, both mechanically and electronically, than a standing wave piezoelectric motor.

Summary of the Invention

The present invention seeks to provide a bi-directional standing wave piezoelectric motor which is relatively simple to manufacture and cost efficient.

According to the present invention, there is provided a piezoelectric motor comprising a rotor having a recess defining an annular driving surface, a stator housing supporting an actuator, the actuator having a rectangular metal core and a ceramic piezoelectric element bonded to at least one side of the core, the or each piezoelectric element having a first face and a second face, the first face being divided into four quadrants covered by quadrant electrodes and the second face having a single common electrode in electrical contact with the core, the quadrant electrodes being connected so as to operate in pairs of diagonally opposite quadrants, at least one driving tip extending from each of two opposite ends of the core, the driving tips and/or the annular driving surface of the rotor being tapered in a direction parallel to the axis of rotation of the rotor, and spring means for urging the driving tips into contact with the annular driving surface of the rotor.

-- - Preferable and/or optional features of the invention are set forth in claims 2 to 8.

Brief Description of the Drawings

The invention will now be more particularly described with reference to the accompanying drawings, in which: Figure 1 is an exploded perspective view of one embodiment of a piezoelectric motor according to the present invention, Figure 2 is a sectional view taken through the assembled motor shown in Figure 1, Figure 3 is a perspective view of an actuator of the motor shown in Figure 1; Figure 4 is an exploded perspective view of another embodiment of a piezoelectric motor according to the present invention; Figure 5 is a sectional view taken through the assembled motor shown in Figure 3; Figure 6 is a perspective view of an alternative actuator as shown in the motor of Figure 4; and Figure 7 is a diagrammatic view showing the manner in which the actuator of Figure 5 rotates the rotor of the motor.

Detailed Description of the Preferred Embodiments

Referring firstly to Figures 1 and 2 of the drawings, the piezoelectric motor comprises a stator housing 10 which is closed by an end cap 11. A rotor 12 is mounted in the housing 10 by a bearing 13 supported by the end cap 11 for rotation about a rotational axis and an output shalt 14 of the rotor projects outwardly through the end cap 11.

The rotor 12 has a recess 15 defining an annular driving surface 16 which is of frusto conical shape and tapered inwardly towards the closed end of the recess 15 and the output shaft 14.

An actuator 17 is supported in the stator housing 10 within two elastomeric blocks 18 held in holders 19 and extends into the recess 15 of the rotor.

The actuator 17 is best shown in Figure 3 and comprises a rectangular metal core 20 of rectangular cross section with driving tips 21 extending from opposite ends of the core 20. The tips 21 can be formed by machining the core 20 and are preferably tapered so as to be complimentary with the annular driving surface 16 of the rotor 12. The core 20 of the actuator 17 acts as a vibrator as more particularly described hereinafter.

Each of two major sides of the core 20 is covered by a ceramic piezoelectric element 22. Each element 22 is glued or otherwise bonded to a respective side of the core 20.

The front face of each element 22 is divided into four quadrants, 23, 24, 25 and 26 and each quadrant is covered by a quadrant electrode. Both pairs of diagonally opposite quadrant electrodes 24, 26 and 23, 25 are electrically connected by a jumper lead (not shown).

The rear face of each element 22 is covered by a single ground electrode (not shown). The ground electrodes of the two ceramic elements 22 are electrically connected together by the core 20.

The driving tips 21 are urged into contact with the annular driving surface 16 of the rotor by a spring 28, preferably in the form of a spring washer mounted about a stator shaft 29 which extends through the actuator 17 and is supported by a bearing 30 in the output shaft 14 of the rotor.

An insulating washer 31 is provided between the spring 28 and the actuator 17 and the spring 28 is held in place on the stator shaft 29 by an E ring clip 32.

Referring now to Figures 4 and 5 of the drawings, another preferred piezoelectric motor comprises components similar to the motor of Figures 1 and 2.

The difference between this design and the aforesaid design is the preload applying mechanism. In the embodiment of Figures 4 and 5, the actuator 17 is held and positioned in the stator housing 10 within holders 19 and the stator shaft 29 is extruded from the center of the stator housing 10. The actuator 17 is fixed to the stator shaft 29 by an insulating E-ring clip 32. The preload between contact surfaces of the driving tips 21 and the annular driving surface 16 of the rotor 12 is provided by the spring 28 that is set against the rotor 12 and an inner ring of bearing 13 through a washer 31. The spring 28 may be a spring washer as used in the first embodiment or an axial compression spring as shown in Figure 4. To facilitate transfer of the preload to the actuator 17, the rotor 12 must be able to move with respect to the bearing 13.

This could be achieved by making the shaft 14 slidable within the bearing 13 but this may not be desirable for the load being driven by the motor. Accordingly, it is preferred, as shown that the rotor 12 is attached to the shaft 14 by axial splines 33 allowing the rotor to move axially on the shaft while being rotationally fixed therewith.

Compared with the first embodiment, the stator of the second embodiment is simpler and bearing 30 can be omitted. Also, in the latter design the stator housing is made from plastic, therefore the holders 19 are relatively flexible and the resilient support blocks 18 that are used in the previous design can be eliminated.

The actuator 17 of Figure 4 is shown in Figure 6. The actuator is similar to the actuator of Figure 3 except that it has a pair of driving tips 21 at each axial end of the core 20 of the actuator 17. Operationally, the actuators 17 are similar and either can be used in either embodiment. The pair of driving tips does give greater holding friction when not operating but only one tip of each pair drives the rotor when the motor is operating, giving reduced wear on the driving tips leading to longer life of the motor.

Operation of the actuator of Figure 3 will now be described with reference to Figure 7. When each ceramic piezoelectric element 22 is excited, it stretches. By exciting quadrants of each element, the quadrants are made to stretch. By selectively exciting diagonally opposite quadrants of each element, the element bends because of the unexcited adjacent quadrant. Bending of the two elements in the same direction causes the actuator 15 to bend and stretch as shown in Figure 7.

In the preferred embodiment, the diagonally opposite quadrants 24 and 26 are excited with a sinusoidal wave form to cause deformation as shown in Figure 4 where the voltage is high and positive and returns to normal or a relaxed state when the voltage is low. When the voltage is high and negative the excited quadrants 24 and 26 contract or shrink faster than the spring 28 can react and this thus causes the driving tips 21 to pull away from the annular driving surface 16 of the rotor 12 and the actuator to bend the other way (to the right as viewed). The effect of this is that the driving tips 21 undergo an elliptical motion as shown diagrammatically in Figure 7 and rotate the rotor in an anti-clockwise direction. If the quadrants 23 and 25 are excited, the rotor 12 will rotate in a clock-wise direction.

The actuator functions best when operated in resonance mode. That is when the frequency of the excitation wave form causes the actuator 17 to vibrate or stretch at near its natural frequency of resonance or a harmonic frequency thereof. This resultant vibration of the actuator 17 will provide nodes or areas of minimal lateral movement on the surface of the actuator 17 where the elastomeric mounting blocks 18 can resiliently hold and support the actuator 17 without preventing the actuator from vibrating and moving longitudinally.

The actuator of Figure 6 operates in a similar manner with only one driving tip 21 of each pair of driving tips at the ends of the actuator engaging the rotor during operation in one direction, as is readily apparent.

In the embodiments disclosed, the shape and dimensions of the actuator 17 are chosen so that the electrodes can be excited to cause first order longitudinal vibration and second order bending vibration of the actuator.

The embodiments described above are given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined by the appended claims. For example, it is only necessary for either the driving tips or the annular driving surface to be tapered.

Claims

1. A piezoelectric motor comprising a rotor (12) having a recess (15) defining an annular driving surface (16), a stator housing (10) supporting an actuator (17), the actuator having a rectangular metal core (20) and a ceramic piezoelectric element (22) bonded to at least one side of the core, the or each piezoelectric element having a first face and a second face, the first face being divided into four quadrants (23, 24,25, 26) covered by quadräi3t electrodes and the second face having a single common electrode in electrical contact with the core, the quadrant electrodes being connected so as to operate in pairs of diagonally opposite quadrants, at least one driving tip (21) extending from each of two opposite ends of the core, the driving tips (21) andlor the annular driving surface (16) of the rotor being tapered in a direction parallel to the axis of rotation of the rotor, and spring means (28) for urging the driving tips (21) into contact with the annular driving surface (16) of the rotor.

2. A piezoelectric motor as claimed in claim 1, wherein the driving tips (21) and the annular driving surface (16) are complementarily tapered.

3. A piezoelectric motor as claimed in claim 1 or claim 2, wherein the spring means (28) acts to urge the actuator (17) axially towards the rotor 12.

4. A piezoelectric motor as claimed in any one of the preceding claims, wherein the piezoelectric motor has a ceramic piezoelectric element (22) bonded to each of two opposite sides of the core (20).

5. A piezoelectric motor as claimed in any one of the preceding claims, wherein the actuator (17) is held within the stator housing (10) by resilient support elements (18, 19).

6. A piezoelectric motor as claimed in claim 5, wherein the support elements (18) are rubber blocks contacting the actuator (17) at surface node areas.

7. A piezoelectric motor as claimed in any one of the preceding claims, wherein the spring means (28) is mounted about a stator shaft (29) extending through the actuator (17).

8. A piezoelectric motor as claimed in claim 1 or 2, wherein the spring means (28) acts to urge the rotor 12 axially toward the actuator 17.

9. A piezoelectric motor as claimed in claim 8, wherein the rotor 12 is slidably fixed to a rotor shaft 14 for fixed rotation therewith.

10. A piezoelectric motor according to claim 9, wherein the stator housing 10 has an end cap 11, the motor shaft 14 is rotatably fixed to the end cap 11 by a ball bearing 13, and the spring means acts between the bearing 13 and the rotor 12 to urge the rotor 12 away from the end cap.

11. A piezoelectric motor as claimed in any one of the preceding claims, wherein the spring means is in the form of a spring washer or axial coil spring.