Vibration wave driven motor

Abstract: This invention relates to a vibration type motor in which frequency signals differing in phase from each other are applied to an electro-mechanical energy conversion element disposed on a vibration member to thereby vibrate the vibration member and drive a movable member by the vibration force. This invention provides a motor of the described type in which a monitoring portion for detecting the vibrated state of the vibration member is provided at a location avoiding a location at which the rigidity of the vibration member exhibits rigidity differing from the rigidity of the entire vibration member so that the vibrated state can be accurately detected.

Description: This invention relates to a vibration type motor in which a vibration member and a member which is in contact with the vibration member are moved relative to each other by vibration generated in the vibration member. Related Background Art Figure 6 of the accompanying drawings shows the electrode arrangement of a piezo-electric element provided on the resilient member of a circular ring-shaped vibration wave driven motor according to EP-A-383 309 (CANON KABUSHIKI KAISHA) published 22.08.90. As shown, two groups of electrodes [A phase (A₁-A₆) and B phase (B₁-B₆) having a pitch of 1/2 of the wavelength of a vibration wave excited in a circular ring-shaped vibration member are spatially disposed with a pitch 1/2, i.e., with a phase deviation of 1/4 of said wavelength, and a sensor electrode S for detecting the vibrated state of the circular ring-shaped vibration member, i.e., an electrode such as a piezo-electric element generating an electromotive voltage by the vibration of the vibration member, and common electrodes C (C₁, C₂ and C₃) are further provided between these two groups of electrodes. The vibration member comprises a resilient member of a metal or like material and a piezo-electric element (e.g. PZT) as an electro-mechanical energy conversion element attached to the resilient member, and the associated electrode of the electro-mechanical energy conversion element which is adjacent to the resilient member is short-circuited through the metallic resilient member. The common electrodes C₁, C₂ and C₃ are rendered conductive with the resilient member by an electrically conductive paint or the like, and the potential difference from the resilient member is zero. When a potential of a certain magnitude relative to the potential of the common electrodes C is input to driving electrodes A₁-A₆ and B₁-B₆, a potential difference is produced between the front and back of the electro-mechanical energy conversion element and strain is created. Signs - and + shown on the electrodes indicate the directions of polarization of the piezo-electric elements in the electrode portions. By a voltage V=Vo sin ωt being applied to the group of A phase electrodes A₁-A₆ and a voltage V=Vo sin (ωt ± π 2 ) being applied to the group of B phase electrodes B₁-B₆, a travelling vibration wave is generated in the resilient member and a conventional movable member (not shown) such as a rotor which is in pressure contact with the resilient member is moved by friction. Also, by changing the signs (+) and (-) in the aforementioned equation, i.e., advancing or delaying the phase of one of the applied voltages with respect to that of the other, it is possible to change the direction of movement of the movable member, and in the case of (-), i.e., where a frequency voltage delayed in phase by π 2 (90°) with respect to the frequency voltage applied to the A phase is applied to the B phase, the movable member is rotated clockwise (CW), and in the case of (+), i.e., where a frequency voltage advanced in phase by π 2 (90°) with respect to the frequency voltage applied to the A phase is applied to the B phase, the movable member is rotated counter-clockwise (CCW). A signal detected from the sensor electrode S is a signal having a certain phase relation with the frequency voltage applied to the A phase when the movable member is in a resonant state, but it becomes a signal deviating from said certain phase relation with the frequency voltage applied to the A phase when the movable member is in a non-resonant state. So, by detecting the phase difference between the driving voltage and the detection signal, it is possible to know whether the vibration is in a reasonant state of great amplitude or how much the vibration deviates from a resonant state, and by determining from this information the frequency to be applied, the control of the rotational speed is possible. Figure 7A of the accompanying drawings is a fragmentary cross-sectional view of the resilient member of the vibration wave driven motor. A piezo-electric element 2 as an electro-mechanical energy conversion element is attached to the resilient member 1 of Figure 7A. Symbols below the element 2 represent the positions of the electrodes of Figure 6. A number of grooves each having a width t₁ and a depth h₁ are formed in the surface of contact of the resilient member with the rotor (not shown) over the entire circumference thereof. Some of these grooves are deep grooves of a depth h₂ to prevent the creation of noise. The spacing between the deep grooves of Figure 7A may be suitably selected, but if this spacing is 60°, the vibration of the 3rd-order mode can be suppressed. As described above, the arrangement of the deep grooves gives birth to the effect of suppressing a single or a number of modes of noise and preventing noise. At this time, the electrode pattern of the piezo-electric element which is an electro-mechanical energy conversion element and the pattern of said deep grooves are arbitrarily positioned. In the case of the combination of the sensor electrodes S and the deep groove pattern as shown in Figure 7A, the deep groove portion is near the sensor electrode S, and as shown in Figure 7B of the accompanying drawings, the rigidity of the portion of the sensor electrode S varies. At this time, there is obtained the relation of the frequency f vs. the phase difference ϑ A-S between the A phase and S phase as shown in Figure 8 of the accompanying drawings. When the frequency of the resilient member is at resonant frequency, the phase difference ϑ A-S is -135° for CW and -45° for CCW. However, since the rigidity of the portion near the sensor electrode S varies as shown in Figure 8, the amplitude and wavelength of the travelling vibration wave change delicately, and the CW rotation and the CCW rotation deviate from each other in opposite directions with respect to the ideal f vs. ϑ A-S curve (the portion indicated by dotted line in Figure 8). Consequently, there is the problem that the optimum resonant frequency is not detected even if the ϑ A-S phase difference is adjusted to -135° (for CCW, -45°). That is, in the example of the above-mentioned earlier apparatus described above, if the positional relation between the electrode pattern of the piezo-electric element which is an electro-mechanical energy conversion element and the deep groove pattern of the resilient member is arbitrary, the portion in which the rigidity of the deep groove portion varies is disposed near the sensor electrode S as shown in Figure 7B, and this adversely affects the signal of the sensor electrode S and thus, a difference occurs relative to the true f vs. ϑ A-S curve, as shown in Figure 8. That is, near the deep grooves, strain differs and the center of the electrode for a sensor becomes as if it deviated and thus, it follows that an optimum resonance frequency is not detected. An object of the application is to solve the above-noted problem peculiar to the earlier apparatus and to provide a vibration type motor in which a sensor electrode always enables the detection of an optimum resonant frequency. According to the invention there is provided a vibration type motor having a vibration member which has an electro-mechanical energy conversion element, said vibration member being adapted to vibrate on application to said conversion element of an alternating signal to form a driving force by said vibration; characterised in that said vibration member has a portion thereof of which the physical vibration characteristic differs from the characteristic of the remainder of the vibration member and a monitoring portion for detecting a vibration state, said monitoring portion being disposed in said vibration member at a region offset from said different physical characteristic portion. One aspect of the application is to provide a vibration type motor in which an electro-mechanical energy conversion element having at least a driving electrode and a vibration detecting electrode is disposed in a single layer a plurality of layers on a resilient member and frequency voltages differing in phase from each other are applied to the driving electrode to generate a travelling vibration wave, whereby said resilient member and a member which is in pressure contact with said resilient member are moved relative to each other, and wherein said vibration detecting electrode is disposed at the center of that portion of said resilient member in which the rigidity of said resilient member varies, or avoiding that portion in which the rigidity of said resilient member varies. Another aspect of the application is to provide a vibration type motor in which an electro-mechanical energy conversion element is disposed on a vibration member and a frequency signal is applied to said element to vibrate said vibration member and the vibration force of said vibration member is used as a drive force and wherein a sensor electrode for detecting the vibrated state of the motor is disposed in a portion wherein the variation in the rigidity of the vibration member is small or a portion wherein the variation in the rigidity of the vibration member occurs symmetrically with respect to the sensor electrode, whereby the vibration state of the motor is made accurately detectable. In the accompanying drawings : Figure 1 shows Embodiment 1 of a vibration wave driven motor according to the present invention, Figure 1A being a fragmentary cross-sectional view of a resilient member, and Figure 1B being a graph showing the relation between the position and rigidity of the resilient member. Figure 2 is a graph showing the f vs. ϑ A-S relation of Embodiment 1. Figure 3 shows Embodiment 2, Figure 3A being a fragmentary cross-sectional view of a resilient member, and Figure 3B being a graph showing the relation between the position and rigidity of the resilient member. Figure 4 shows Embodiment 3, Figure 4A being a fragmentary cross-sectional view of a resilient member, and Figure 4B being a graph showing the relation between the position and rigidity of the resilient member. Figure 5 is a plan view of a resilient member showing Embodiment 4. Figure 6 shows the electrode arrangement in a vibration wave driven motor according to the above-mentioned earlier apparatus. Figure 7 shows the vibration wave driven motor according to the earlier apparatus, Figure 7A being a fragmentary cross-sectional view of a resilient member, and Figure 7B being a graph showing the relation between the position and rigidity of the resilient member. Figure 8 is a graph showing the f vs. ϑ A-S relation in the example of the above-mentioned earlier apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Figure 1 shows Embodiment 1 of a vibration type motor according to the present invention, Figure 1A being a fragmentary cross-sectional view of a piezo-electric element 2 and a resilient member 1 near a sensor electrode S, and Figure 1B being a graph representing the rigidity at the same position. The disposition of the piezo-electric element and electrodes is similar to that shown in Figure 6. Referring to Figure 1A, the resilient member 1 is formed with first grooves 3 each having a width t and a depth h 1. Also, deep grooves 4 each having a depth h₂ are formed in some of these grooves 3. As shown in Figure 1B, the rigidity of the resilient member 1 varies near the deep grooves 4. However, the sensor electrode S is disposed avoiding the deep grooves 4 and therefore, the rigidity of the resilient member 1 is uniform. Here, the frequency (f) vs. phase difference ϑ A-S curve of a sensor signal obtained from the sensor electrode S when a voltage V=Vo sin wt is applied to driving A phase electrodes A₁-A₆ and a voltage V=Vo sin (ωt ± π 2 ) is applied to driving B phase electrodes B₁-B₇ is as shown in Figure 2. That is, it is seen from Figure 2 that the phase difference ϑ A-S at a regular resonance frequency fr has a difference of 90°, i.e., -135° in the case of CW rotation and -45° in the case of CCW rotation, but the frequency f vs. phase difference ϑ A-S curve coincides at all frequencies. By the sensor electrode S being disposed avoiding the portion in which the rigidity of the resilient member 1 varies as described above, the optimum resonant frequency can be accurately detected. Empirically, it is desirable that the amount by which the portion in which the rigidity varies is avoided be λ (wavelength)/4 or more from the portion in which the rigidity is lowest (or highest). Also, where the sensor electrode S is disposed at the center of the portion in which the rigidity varies, it is desirable that said amount be λ (wavelength)/8 or less. That is, if the sensor electrode is disposed at the center of the portion in which the rigidity varies, as indicated at a position S′ in Figure 1, the vibration state can be accurately detected because the variation in the rigidity occurs symmetrically with respect to the sensor electrode. Embodiment 2 Figure 3 shows Embodiment 2 of the present invention, Figure 3A being a fragmentary cross-sectional view of a piezo-electric element 2 and a resilient member 1 near a sensor electrode S, and Figure 3B being a graph representing the rigidity at the same position. Referring to Figure 3A, the resilient member 1 is formed with grooves 3 each having a width t and a depth h₁. Also, grooves 5 each having a depth h₁ and a width t′ and grooves 6 each having a depth h₂ and a width t′ are formed in some of these grooves 3. As can be seen from Figure 3B, the area in which the rigidity of the resilient member varies is decreased by providing the grooves 5 and 6 having the width t′ and the depths h₂ and h₁, and the sensor electrode S can readily avoid the portion in which the rigidity varies. Embodiment 3 Figure 4 shows Embodiment 3 of the present invention, Figure 4A being a fragmentary cross-sectional view of a piezo-electric element 2 and a resilient member 1 near a sensor electrode S, and Figure 4B being a graph representing the rigidity at the same position. Referring to Figure 4A, the resilient member 1 is formed with grooves 3 each having a width t and a depth h₁. Also, shallow grooves 7 each having a depth h₃ are formed in some of these grooves 3. In Embodiment 1, the rigidity is varied by deepening some of the grooves, whereas in the present embodiment 3, the rigidily is varied by making the grooves 7 shallow. The method of varying the rigidity of a portion of the resilient member 1 to thereby prevent the generation of noise can also be realized by attaching or removing a certain mass to or from a portion of the resilient member. Again in such cases, there exists a portion in which the rigidity varies and therefore, as in Embodiment 1, the sensor electrode S need be disposed avoiding that portion. Embodiment 4 Figure 5 shows Embodiment 4 of the present invention. In the present embodiment, grooves 8, 9 and 10 are at unequal pitches and the deep grooves 8 painted out in the figure are continuous at a unit of two. In Figure 5, the rigidity of the resilient member varies most in the portions wherein two deep grooves 8 are continuous, and the sensor electrode S avoids particularly those portions. Also, it is desirable that the sensor electrode S be disposed avoiding the portions in which the deep grooves 8 and 10 are provided at one location each. Each of the above-described embodiments is a circular ring-shaped resilient member, but the resilient member can also be made similar in a case where vibration detecting means is provided for a disk-shaped or linearly shaped resilient member. The number of the piezo-electric element which is an electro-mechanical energy conversion element is not limited to one, but piezo-electric elements may be provided in multiple layers. As described above, according to the present embodiment, the vibration detecting electrode is disposed avoiding the portion in which the rigidity of the resilient member varies, whereby the characteristic of the phase difference between the output of the vibration detecting electrode and the applied voltage can accurately detect the resonant frequency and can stably drive the motor. The embodiments are shown as the type in which a travelling vibration wave is formed, but the present invention can be applied to any type in which a vibration member is vibrated by a piezo-electric element and a movable member is driven by the vibration of the vibration member.

Claims: A vibration type motor having a vibration member (1) which has an electro-mechanical energy conversion element (2), said vibration member being adapted to vibrate on application to said conversion element of an alternating signal to form a driving force by said vibration; characterised in that said vibration member (1) has a portion thereof of which the physical vibration characteristic differs from the characteristic of the remainder of the vibration member and a monitoring portion (S) for detecting a vibration state, said monitoring portion being disposed in said vibration member at a region offset from said different physical characteristic portion. A vibration type motor as claimed in claim 1, characterised in that said monitoring portion (S) is comprised of a signal forming element provided on said vibration member (1) for forming an electrical signal proportional to the vibration of said vibration member, and a detecting electrode provided on said signal forming element. A vibration type motor as claimed in claim 2, characterised in that said signal forming element is a part of said energy conversion element (2). A vibration type motor as claimed in claim 1, 2 or 3, characterised in that said vibration member is formed with a plurality of slits (3) or projections, some of said slits (5, 7) or projections differing in shape from the other slits or projections to give said different characteristic, said monitoring portion being disposed in an area avoiding said slits or projections differing in shape. A vibration type motor as claimed in claim 4, characterised in that said some slits or projections each have a slit length or a projection length differing from that of the other slits or projections to form said portion of different characteristic. A vibration type motor as claimed in any one of claims 1 to 5, characterised in that said vibration member (1) is formed with a plurality of slits (3) or projections at predetermined intervals, some of said slits or projections being disposed at intervals differing from the intervals of the other slits or projections, said monitoring portion being disposed at a location avoiding the slits or projections disposed at said different intervals. A vibration type motor as claimed in any one of claims 1 to 6, characterised in that said electro-mechanical energy conversion element (2) is a piezoelectric element. A vibration type motor having a vibration member (1) which has an electro-mechanical energy conversion element (2), said vibration member being adapted to vibrate on application to said conversion element of an alternating signal to form a driving force by said vibration, characterised in that said vibration member (1) having a portion thereof of which the physical characteristic differs from the characteristic of the remainder of the vibration member, a monitoring portion (S) for detecting a vibration state, said monitoring portion being disposed in said vibration member and substantially the center of said monitoring portion is disposed in a region coincident with the center of said portion of different characteristic. A vibration type motor as claimed in claim 8, characterised in that said monitoring portion is comprised of a signal forming element provided on said vibration member (1) for forming an electrical signal proportional to the vibration of said vibration member, and a detecting electrode provided on said signal forming element. A vibration type motor as claimed in claim 9, characteristic said signal forming element portion is a part of said energy conversion element. A vibration type motor according to any one of claims 8 to 10, characterised in that said vibration member is formed with a plurality of slits or projections, some of said slits or projections different in shape from the other slits or projections, the center of said monitoring portion being disposed in a region substantially in coincidence with the center of said slits or projections differing in shape from the other slits or projections. A vibration type motor as claimed in any one of claims 1-11, characterised in that said portion of different physical vibration characteristic is formed by a different shape or weight or an added mass whereby the rigidity of said portion is different from that in the remainder of the vibration member.

Inventors: ATSUTA AKIO C O CANON KABUSHIK, ATSUTA, AKIO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H02N2
IPC codes: H02N2, H01L41