Drive Circuit for a Vibration Wave Motor

1. Field of the Invention

The present invention relates to a drive circuit for a vibration wave motor which frictionally drives a movable member by a travelling vibration wave, and more particularly to a drive circuit for causing the wave to vibrate in a stable resonance state.

2. Description of the Prior Art

FIG. 1 shows a schematic view of a vibration wave motor driven by a travelling vibration wave, which motor has recently been put into practice. Numerals 1a and 1b denote electrostrictive devices which may be made of PZT (zirconium lead titonate) and numeral 2 denotes a vibration member which is made of an elastic material on which the electrostrictive devices 1a and 1b are bonded. The vibration member 2 as well as the electrostrictive devices 1a and 1b are held on a stator (not shown). Numeral 3 denotes a movable member which is press-contacted to the vibration member 2 to form a rotor. The electrostrictive devices 1a and 1b are bonded as shown in FIG. 1B and one group of electrostrictive devices 1a are displaced from the other group of electrostrictive devices 1b by one quarter of a wavelength .lambda. of the vibration wave. The electrostrictive devices 1a in the one group are arranged at a pitch of one half of the wavelength with adjacent ones being oppositely polarized. The electrostrictive devices 1b of the other group are also arranged at the pitch of one half of the wavelength with adjacent ones being oppositely polarized. Electrode films (not shown) are formed on front and back sides of the electrostrictive devices 1a and 1b so that A.C. voltages are applied to the electrostrictive devices 1a and 1b.

In the vibration wave motor thus constructed, an A.C. voltage of Vo Sin .omega.T is applied to the one group of electrostrictive devices 1a and an A.C. voltage of Vo Cos .omega.T is applied to the other group of electrostrictive devices 1b. Accordingly, the A.C. voltages having a phase difference of 90.degree. are applied to the electrostrictive devices of the respective groups with adjacent ones being oppositely polarized. Thus, the electrostrictive element expands and shrinks. Since this vibration is propagated to the vibration member 2, it makes a bending vibration at a pitch of the arrangement of the electrostrictive devices 1a and 1b. When the vibration member 2 projects at alternate electrostrictive device positions, it recedes at other alternate electrostrictive device positions. Since the electrostrictive devices 1a are displaced from the electroctrictive devices 1b by one quarter of the wavelength, the bending vibration travels. While the A.C. voltages are applied, the vibrations are sequentially excited and propagated through the vibration member 2 as a travelling bending vibration wave.

The travel of the wave is shown in FIGS. 2A, 2B, 2C and 2D. Assuming that the travelling bending vibration wave travels in a direction of an arrow X.sub.1 and 0 denotes a center plane of the vibration member in a quiscent state, a vibration state is represented by a chain line. On a neutral plane 6, bending stresses are balanced. On a sectional plane 7.sub.1 which is normal to the neutral plane 6, no stress is applied to a crossing line 5.sub.1 of those two planes and it merely vibrates vertically. The sectional plane 7.sub.1 makes a lateral pendulum motion around the crossing line 5.sub.1. Similarly, sectional planes 7.sub.2 and 7.sub.3 make lateral pendulum motions around crossing lines 5.sub.2 and 5.sub.3, respectively.

In FIG. 2A, a point P.sub.1 on a crossing line of the sectional plane 7.sub.1 and the surface of the vibration member 2 facing the movable member 3 is a right dead center of the lateral vibration and it makes only the vertical motion. When the crossing line 5.sub.1, 5.sub.2 or 5.sub.3 is on a positive side of the wave (above the center plane 0), a leftward (opposite to the travel direction X.sub.1 of the wave) stress is applied, and when it is on a negative side of the wave (below the center plane 0), a rightward stress is applied. In FIG. 2A, the crossing line 5.sub.2 and the sectional plane 7.sub.2 indicate the former state and the stress in the direction shown by the arrow is applied to the point P.sub.2. The crossing line 5.sub.3 and the sectional plane 7.sub.3 show the latter state and the stress in the direction shown by the arrow is applied to the point P.sub.3. As the wave travels and the crossing line 5.sub.1 comes to the positive side of the wave as shown in FIG. 2B, the point P.sub.1 makes the leftward motion and the vertical motion. In FIG. 2C, the point P.sub.1 makes only the leftward motion at the top dead center of the vertical motion. In FIG. 2D, the point P.sub.1 makes the leftward motion and the rightward motion. As the wave further travels, it makes the rightward and downward motion, the rightward and upward motion and returns to the state of FIG. 2A. Through those series of motions, the point P.sub.1 makes a rotating elliptic motion. On the other hand, the movable member 3 is press-contacted to the vibration member 2, and the rotating elliptic motion of the point P.sub.1 on the vibration member 2 frictionally drives the movable member 3 in the direction X.sub.2. The points P.sub.2 and P.sub.3, and all other points on the vibration member 2 similarly frictionally drives the movable member 3.

The vibration wave motor is efficiently driven when the vibration is in a resonance state. The resonance frequency is determined by dimensions of the electrostrictive device and vibration member, the temperature and the contact pressure of the movable member.

When the electrostrictive resonator is driven, the resonance frequency of the resonator varies with a load. Accordingly, if the drive frequency is fixed, the resonator may not be vibrated in the resonance state. Therefore, in the prior art, a voltage proportional to a strength of the mechanical vibration of the resonator is fed back to an input terminal of an amplifier of the oscillator to change the oscillation frequency to follow the resonance frequency of the resonator.

However, it has not been known to adopt the above method in the vibration wave motor which drives the movable member by generating the travelling vibration wave. There are various problems in feeding the mechanical vibration of the resonator to the amplifier, such as the need for a complex circuit.

It is an object of the present invention to provide a drive circuit for a travelling vibration wave motor which drives an electrostrictive resonator.

It is another object of the present invention to provide a drive circuit for a vibration wave motor having an oscillation circuit which is simple in configuration, follows a change of an impedance of the resonator and has a high Q.

It is yet another object of the present invention to provide a suitable mount space of electrostrictive device used to pick up a voltage proportional to the strength of the mechanical vibration of the vibration wave motor.

It is a further object of the present invention to improve a drive effeciency by cutting a high frequency component of the drive circuit for the vibration wave motor by providing a filter in the drive circuit.

FIG. 1 is a schematic view of major portions of a vibration wave motor,

FIG. 2 illustrates a principle of drive of the vibration wave motor,

FIG. 3 is a circuit diagram of one embodiment of a drive circuit of the present invention,

FIG. 4 shows a relation between a drive frequency of the vibration wave motor and an impedance of an electrostrictive device,

FIG. 5 shows a modification of the drive circuit shown in FIG. 4,

FIGS. 6A, 6B and 6C show circuits to be inserted into the circuit blocks shown in FIG. 5,

FIG. 7 is a schematic view of major portions of a vibration wave motor to which a second embodiment of the drive circuit of the present invention is applied,

FIG. 8A is a plan view showing an arrangement of electrostrictive devices of the vibration wave motor shown in FIG. 7,

FIG. 8B is a plan view showing another arrangement of electrostrictive devices,

FIG. 9 is a block diagram of the second embodiment of the drive circuit of the present invention,

FIG. 10 shows a relation between an impedance of the vibration wave motor and a frequency,

FIG. 11 is a sectional view of a vibration wave motor to which other embodiment of the drive circuit of the present invention is applied, and

FIG. 12 shows a series circuit of a strain gauge R.sub.32 and a resistor R.sub.31 of the vibration wave motor shown in FIG. 11, which is used in place of a resistor R.sub.3 shown in FIG. 9.

FIG. 3 shows a drive circuit in accordance with the present invention. Numeral 10 denotes a voltage source, numeral 11 denotes an oscillator, numeral 12 denotes a 90.degree. phase shifter, OP.sub.1 and OP.sub.2 denote operational amplifiers, R.sub.1 to R.sub.4 denote resistors, C.sub.1 denotes a capacitor and numerals 1a and 1b denote electrostrictive devices.

The oscillator 11 comprising R.sub.1 to R.sub.3, 1a and OP.sub.1 is usually called a Meacham circuit which is known as a circuit used with a crystal resonator having a high frequency stability.

In accordance with the present invention, the Meacham circuit is applied to the drive circuit for the vibration wave motor which is driven by the travelling vibration wave so that the drive circuit is implemented with a simple construction.

The operation of the oscillator 11 will now be explained. In the oscillator 11, a voltage V+ at a + input terminal of the operational amplifier OP.sub.1 is given by

wherein Vout is an output voltage of the operational amplifier OP.sub.1. Similarly, a voltage V- at a - input terminal of the operational amplifier OP.sub.1 is given by

where z is an impedance of the electrostrictive device 1a.

The impedance z of the electrostrictive device changes with a vibration frequency f. A frequency characteristic thereof is shown in FIG. 4, in which fr denotes a resonance frequency and fa denotes an anti-resonance frequency. By differentiating the formula (2) with the frequency f, we get ##EQU1## accordingly, a voltage difference between the input terminal voltages, Vin=(V+)-(V-) increases and decreases with the frequency f as shown below.

as seen from the above table, the input voltage difference Vin is maximum at the resonance frequency fr of the electrostrictive device. Accordingly, the resistors R.sub.1 to R.sub.3 are selected to meet the above relation. The quality (Q) is given by Ao.multidot.Qo where Qo is the quality of the electrostrictive device and Ao is an amplification factor of the operational amplifier OP.sub.1.

In the 90.degree. phase shifter 12, the phase of the output voltage of the operational amplifier OP.sub.1 is retarded by an integraton circuit comprising the operational amplifier OP.sub.2, the resistor R.sub.4 and the capacitor C.sub.1 and the phase-retarded signal is applied to the electrostrictive device 1b.

FIG. 5 is a block diagram of another embodiment of the drive circuit. It shows only an oscillator, and the phase shifter is omitted because it may be identical to that of the previous embodiment. In the present embodiment, blocks Z.sub.13 to Z.sub.16 may adopt various impedance circuits so that various modifications may be made.

Block Z.sub.14 is the electrostrictive device to be resonated, blocks Z.sub.12, Z.sub.13 and Z.sub.15 are resistors and block Z.sub.16 is shorted.

Block Z.sub.12 is the electrostrictive device to be resonated, block Z.sub.16 is a series circuit of inductor L1 and capcitor C.sub.2 shown in FIG. 6A, where 2.pi.fr=l/L1. C.sub.2, and blocks Z.sub.13, Z.sub.14 and Z.sub.15 are resistors. The feedback voltage to the operational amplifier OP.sub.1 is maximum at the resonance frequency fr. As the frequency departs from the resonance frequency fr, the impedance of the block Z.sub.16 increases and the feedback voltage at the input terminal of the operational amplifier OP.sub.1 decreases, and hence the amplification factor decreases. As a result, the resonance at the frequency distant from the preset resonance frequency fr is suppressed.

Block Z.sub.12 is a parallel circuit of inductor L.sub.3, capcitor C.sub.4 and electrostrictive device 1a, where 2fr=l/L3.multidot.(C.sub.4 +Cd), Cd is a static capacitance of the electrostrictive device in the resonance state, blocks Z.sub.13 to Z.sub.15 are resistors and block Z.sub.16 is shorted. The impedance of the resonance circuit comprising L.sub.3 and C.sub.4 +Cd is maximum at the resonance frequency fr and the impedance decreases as the frequency deviates from the resonance frequency fr. Accordingly, the voltage is applied only at the frequency in the vicinity of the resonance frequency fr.

In the present embodiment, the electrostrictive device 1a of the devices 1a and 1b provided for generating the travelling vibration wave is inserted into the bridge circuit of the oscillator so that the oscillation frequency is automatically changed as the impedance of the electrostrictive device changes. In the present embodiment, however, it is difficult to increase the drive voltage because the electrostrictive device 1a is directly driven by the output of the oscillator.

Referring to FIGS. 7 to 9, another embodiment of the present invention is explained, in which an electrostrictive device 1c for detecting the impedance change of the vibration motor is provided separately from the electrostrictive devices 1a and 1b, an oscillator whose oscillation frequency is varied in accordance with a signal derived from the electrostrictive device 1c is operated, and the output of the oscillator is amplified and applied to the electrostrictive devices 1a and 1b so that a large drive power is applied to the electrostrictive devices. FIG. 7 shows an arrangement of the electrostrictive devices and FIG. 8A shows a plan view thereof. The electrostrictive device 1c for detecting the impedance change of the vibration wave motor is arranged at a position which vibrates in phase with the electrostrictive devices 1a. Namely, the electrostrictive device 1c is arranged at the same pitch as the electrostrictive devices 1a and adjacently to the electrostrictive device 1a. The electrostrictive device 1c need not be arranged adjacently to the electrostrictive device 1a. FIG. 9 is a circuit block diagram of a drive circuit in the present embodiment. The like elements to those shown in FIG. 3 are designated by the like numerals. Numerals 14 and 15 denote power amplifiers and numeral 13 denotes a band-pass filter which passes only frequencies between fa(n-1) and fa(n) shown in FIG. 10.

In accordance with the present embodiment, since the electrostrictive element 1c is provided to detect the impedance change of the vibration wave motor, the output of the oscillator 11 can be amplified by the amplifiers 14 and 15 and the amplified voltages can be applied to the driving electrostrictive devices 1a and 1b. Accordingly, large power can be applied to the electrostrictive devices and the drive power of the vibration wave motor can be increased. The electrostrictive devices 1a and 1b are driven with 90.degree. phase difference therebetween by the phase shifter.

The vibration wave motor of the present embodiment has resonance frequencies fr(n-1), fr(n) and fr(n+1) for (n-1)-order resonance mode, n-order resonance mode and (n+1)-order resonance mode, respectively. The impedance of the electrostrictive device is low at those resonance frequencies, as shown in FIG. 10.

When the circuit shown in FIG.3 is used to control the drive frequency so that it is brought to the resonance frequency as the impedance changes due to the resonance of the electrostrictive device. The drive frequency may be brought to fr(n-1) or fr(n+1) in spite of the fact that the highest efficiency vibration mode of the vibration wave motor is the n-order resonance mode due to the structure of the vibration wave motor, and the vibration wave motor may be vibrated in the lower efficiency (n-1)-order or (n+1)-order resonance mode.

In the present embodiment, because of the provision of the band-pass filter 13, the vibration wave motor resonates only in the highest efficiency n-order resonance mode to generate the travelling surface wave and does not resonate in the low efficiency (n-1)-order and (n+1)-order resonance modes. Accordingly, a high efficiency drive is attained.

If the travelling vibration wave is generated in the resonance state, the displacement of a point on the electrostrictive device in the direction of wire thickness changes trigonometrically with respect to time, as explained with reference to FIG. 2. The oscillation output may be of rectangular shape due to factors of the circuit configuration (circuit and constants of components). In such a case, the resonance may occur in other than the desired resonance mode. Namely, the resonance in other than the highest efficiency resonance mode may occur by harmonics included in the rectangular oscillation waveform. In accordance with the present embodiment, such a phenomenon is prevented by the provision of the band-pass filter 13.

The oscillation output for driving the electrostrictive devices 1a and 1b is generated based on the signal generated in the electrostrictive device 1c. Accordingly, the oscillation output of the oscillator 11 is in phase with the signal from the electrostrictive device 1c, that is, a vector sum of the voltage difference across the electrostrictive device 1c due to the impedance of the electrostrictive device 1c created when the voltage is applied to the electrostrictive device 1c and a counterelectromotive device 1c is displaced by the travelling vibration wave generated by the electrostrictive devices 1a and 1b when the travelling vibration wave is propagated through the area in which the electrostrictive device 1c is arranged.

If the size or the position of the arrangement of the electrostrictive device 1c is not specifically related to the arrangement of the electrostrictive device 1a or 1b, it is necessary for generating the travelling vibration wave in the vibration wave motor that the electrostrictive devices 1a and 1c vibrate in accordance with the positional phase difference. For example, when a portion of the electrostrictive device 1a is in a loop of the vibration, the electrostrictive device 1c vibrates with a phase difference .phi. with respect to the loop of the vibration. In accordance with the drive circuit of the present invention, since, in order to meet the above condition, the electrostrictive devices 1a should be driven in phase with the vibration of the area in which the electrostrictive device 1c is mounted, if the electrostrictive device 1c is positioned without having any positional relationship to those of the electrostrictive devices 1a and 1b, the travelling vibration wave can not be generated. However, the above stated embodiment satisfies the above stated condition since the travelling vibration wave can be generated in the motor.

The mounting position of the electrostrictive device 1c is determined such that the vibration in the mounting area of the electrostrictive device 1c is in phase with the mounting area of the electrostrictive devices 1a.

More specifically, as seen from FIG. 8A, the mounting position of the electrostrictive device 1c is determined such that the phase difference between the vibration in the area of the electrostrictive devices 1a and the vibration in the area of the electrostrictive device 1c is substantially zero. Accordingly, in accordance with the present embodiment, the travelling vibration wave is certainly generated.

In the present embodiment, the size of the electrostrictive device 1c is same as that of the electrostrictive devices 1a as shown in FIG. 8A and smaller than that of the electrostrictive devices 1a as shown in FIG. 8B. The size of the electrostrictive device 1c is therefore not limited.

The vibration wave motor is efficiently driven by the friction force generated by the press-contact between the movable member 3 and the vibration member 2. If the pressure changes by any reason such as an article mounted on the movable member 3, the impedance of the electrostrictive device 1c changes and the oscillation frequency of the oscillator 11 changes to other lower efficiency vibration mode. An embodiment which resolves the above problem is explained below.

FIG. 11 shows a sectional view of a vibration wave motor having a strain gauge. The like elements to those shown in FIG. 1 are designated by the like numerals. Numeral 16 denotes a support member for supporting the vibration member 2 and the electrostrictive element 1 and numeral 17 denotes a bolt to which a nut 18 is threaded. By threading the nut 18, the vibration member 2 and the movable member 3 are press-contacted. In order to prevent the rotation of the movable member 3 from being impeded by the nut 18, lubrication oil is applied on the contact surfaces of the nut 18 and the movable member 3 or a bearing is provided. R32 denotes a strain gauge which is mounted between the electrostrictive element 1 and the support member 16. The impedance of the strain gauge increases in proportion to the applied pressure.

The drive circuit for the vibration wave motor having such strain gauge is now explained.

The impedance Z(fr) of the electrostrictive element 1 changes with respect to the applied pressure F between the vibration member 2 and the movable member 3 in the following manner.

where Z(fr)F=0 is the impedance of the electrostrictive element 1 when the pressure F=0, and

The Z(fr) changes by approximately several percents although it depends on the change of pressure. As a result, the operating point of the bridge comprising the resistors R.sub.1, R.sub.2 and R.sub.3 of the oscillator 11 and the electrostrictive device 1c may be shifted. In the present embodiment, the strain gauge R.sub.32 which is a load converter and the resistor R.sub.31 are connected in series as shown in FIG. 12 and the series circuit is used in place of the resistor R.sub.3 of the oscillator 11 shown in FIG. 9.

The impedance of the strain gauge R.sub.32 is represented by R.sub.32. Thus,

where b is a constant, F is the applied pressure.

If the constant b is determined such that ##EQU2## then the operating point of the bridge of the oscillator 11 shown in FIG. 4 does not change even if the applied pressure F changes, and the oscillator 11 produces the voltage which can drive the vibration wave motor in the most efficient way irrespectively of the applied pressure F. Although, in the above explanation, we explained the use of the electrostrictive device, it's apparent that a piezoelectric devices can be used, instead of the electrostrictive devices.