1.  Field of the Invention
The present invention relates to a vibration actuator for moving a vibration member and a contact member relative to each other by vibrations. 
2.  Related Background Art
A vibration motor in a prior art will be described with reference to FIGS.  5A to 7C.
FIGS.  5A to 5D are views showing the structure of a vibration motor proposed by the present inventor. 
Referring to FIG.  5A, a vibration member 1 serves as a power generation source of the vibration motor.  The vibration member 1 has a shape shown in FIG.  5B and is fastened and fixed to a motor end plate 3 by screws 4 at a central boss portion 1c.  The vibration member 1 is made of stainless steel.  A large number of radial slits 1b are formed in the peripheral edge of one end face of the vibration member 1 to define a large number of tooth-like projections 1a, as shown in FIG.  5B. An annular piezoelectric element 2 is adhered and fixed to the other end face of the vibration member 1 so as to correspond to the annular portion on which the tooth-like projections 1a are formed.  A flexible printed board (not shown) for applying a drive voltage to the piezoelectric element is adhered to the piezoelectric element 2. 
A bearing 5 is fixed at the center of the motor end plate 3, and a rotating shaft 6 is rotatably supported in the bearing 5.  A motor case 12 having the other end plate portion is fixed to the motor end plate 3 by screws 13, and the rotating shaft 6 is also rotatably supported in a bearing 14 fixed to the end plate portion of the motor case 12.  A flange 6a mounted on the rotating shaft 6 by shrink-fit or the like is disposed between the bearings of the rotating shaft 6.  An intermediate member 8 serving as a support member shown in FIG.  5B is fixed to the flange 6a by screws 7, so that the intermediate member 8 rotates together with the rotating shaft 6.  The intermediate portion 8 has a peripheral flange 8a facing the tooth-like projections 1a of the vibration 1, as shown in FIG.  5B. An annular aluminum alloy support member 10 for supporting a slide member is attached to the end face (i.e., an end face facing the tooth-like projections 1a of the vibration member 1) of the peripheral flange portion 8a through an annular, elastic rubber sheet member 9. 
As shown in FIG.  5C, the support member 10 comprises a base portion 10a fixed to the flange portion 8a of the intermediate member 8, a stepped portion 10b circumferentially formed along the peripheral edge of the base portion 10a, and an annular flange portion 10c extending outward from the flange portion 8a.  An annular slide member 11 made of a composite resin is adhered to the surface of the flange portion 10c, which faces the vibration member 1.  The slide member 11 is pressed against the tooth-like projections 1a of the vibration member 1 and moved relative to the tooth-like projections 1a by the friction therebetween in accordance with circumferential traveling wave vibrations produced in the tooth-like projections 1a.  As a result, the intermediate member 8 is rotated through the support member 10. 
Note that a member obtained by coupling the slide member 11 and the support member 10 is called a moving member herein. 
A conical compression spring 15 brings the slide member 11 into tight contact with the tooth-like projections 1a of the vibration member 1.  The conical compression spring 15 has a planar shape, as shown in FIG.  5D. The conical compression spring 15 is disposed between the bearing 14 and the intermediate member 8 to bias the intermediate member 8 toward the vibration member 1. 
The slits 1b and the tooth-like projections 1a of the vibration member 1 and the electrode arrangement of the piezoelectric element 2 will be described with reference to FIGS.  6 and 7A to 7C.
The annular piezoelectric element 2 is adhered to the lower surface (i.e., the surface without the tooth-like projections 1a) of the vibration member 1 in correspondence with the arrangement position of the tooth-like projections 1a.  Eight regions having the same length and polarized alternately in opposite directions are formed along one half of the circumference of the piezoelectric element 2, and eight regions polarized alternately in opposite directions are also formed along the other half of the circumference of the piezoelectric element 2.  Electrodes A1 to A8 and electrodes B1 to B8 are formed on the surfaces of the respective regions.  The distance between the centers of the adjacent ones of the electrodes A1 to A8 and B1 to B8, i.e., the pitch between the centers of the adjacent polarized regions is 1/2 the wavelength .lambda.(.lambda./2) of the circumferential standing wave vibration produced in the tooth-like projections 1a.  The center of each region, i.e., the center of each electrode coincides with the position of one slit 1b.  The vibration member 1 and the piezoelectric element 2 are adhered to each other such that four tooth-like projections are located between the center of each electrode and the center of its adjacent electrode.  When a first AC voltage A is applied to the electrodes A1 to A8 arranged along the first half of the circumference, a first standing wave vibration A' is generated along the tooth-like projections 1a of the vibration member 1, as shown in FIGS.  7A to 7C. When an AC voltage B phase-shifted by 90.degree.  from the first AC voltage is applied to the electrodes B1 to B8 arranged along the other half of the circumference, a second standing wave vibration B' is generated along the tooth-like projections 1a. 
The electrode A1 is phase-shifted from the electrode B1 by 3.lambda./4.  In this region corresponding to 3.lambda./4, two polarized regions each having a circumferential length of .lambda./4 and two polarized regions each having a circumferential length of .lambda./8 are formed on the piezoelectric element 2.  These four regions are negatively polarized on the surface of the piezoelectric element 2.  Ground electrodes G are formed in the two regions which are respectively adjacent to the electrodes A1 and B1 and each of which has a circumferential length of .lambda./8.  A first detection electrode S.sub. A for detecting the first vibration occurring in the vibration member 1 by the first AC voltage and a second detection electrode S.sub. B for detecting the second vibration occurring in the vibration member 1 by the second AC voltage are formed in the two regions .lambda./4 long in the circumferential direction between the two regions in which the ground electrodes G are formed.  The centers of the two detection electrodes S.sub. A and S.sub. B i.e., the centers of the polarized regions in which the detection electrodes are formed are arranged to match the corresponding slits 1b of the vibration member 2, respectively.  The central position of the detection electrode S.sub. A matches the antinode position of the first standing wave vibration A', and the central position of the electrode S.sub. B matches the antinode position of the second standing wave vibration B'. 
On the other hand, a region having a circumferential length of .lambda./4 is present between the electrodes A8 and B8.  This region is polarized positively on the surface of the piezoelectric element 2, and at the same time, a ground electrode G is formed on the surface of this region. 
The annular portion of a flexible printed board (not shown) is adhered to the surface portions of the piezoelectric element which are polarized as described above and have the electrodes thereon.  The respective electrodes of the piezoelectric element are pressed against the corresponding electrodes formed on the board, so that an external control circuit and a power supply are electrically connected to the piezoelectric element 2 through the board. 
The axial thickness of the vibration member 1 is represented by H in FIG.  7A, and the depth of each slit 1b is represented by h. The width of each slit 1b is t, and the overall thickness (i.e., the thickness including the electrode film) of the piezoelectric element 2 is b.
The characteristics required of the vibration motor are shown in Table 1 below. 
TABLE 1 ______________________________________ (Required Characterestics) ______________________________________ Rated Value 8 kg .multidot.  cm or more at 22.5 rpm Accuracy of 0.03% or less at 33.3 rpm and a Rotation torque of 1 kg .multidot.  cm, using a laser rotary encoder (81,000 PPR) Service Life 2,000 hours ______________________________________
TABLE 2 ______________________________________ (Main Design Specifications) (Unit: mm) ______________________________________ Piezoelec- Material Tokin N-61 tric Ele- Size 73 (outer diameter) .times.  57 ment (thickness) (inner diameter) .times.  .05 Electrode Wave number: 9, vibration detection electrode count: 2 Vibration Material Martensite-based stainless Member steel, SUS420J2 Size 73 (outer diameter) .times.  57 (inner diameter) .times.  6.5 (thickness) Slit Number: 72, width: 1, depth: 3.5 Hardening Ni-P-PTFE (2.5 wt %) alloy film*.sup.1 product heat-treated at 300.degree.  C., H.sub.v = 800 Member for Material Aluminum alloy 5056 Supporting Size Thickness of flange por- Slide Mem- tion: 1.5, length: 3 ber Slide Mem- Material Composite resin H.sub. R M = 80 to ber 110 Size 68 (outer diameter) .times.  65 (inner diameter) .times.  1 (thickness) Axial Load 15 kgf ______________________________________ *.sup.1 Fluoroplastic eutectoid electroless nickel: Nippon Kanizen, Kanifuron B
Table 2 shows the main design specifications of the vibration motor.  An evaluation test of the vibration motor of the prior art was conducted in accordance with the required characteristics, but the following unsatisfactory results were obtained.  When the T-N characteristics (torque-rotational speed characteristics) of the motor were measured at a large vibration amplitude at which "sound noise" did not occur, the following results were obtained. 
(1) Although the rating of the required characteristics was satisfied, a sufficient margin was not assured, and the torque did not increase sufficiently. 
(2) Disturbances occurred in the T-N characteristics at high amplitudes. 
(3) Ripples often occurred in a high-torque range.