Vibration Wave Driven Motor

1. Field of the Invention

This invention relates to a driving circuit for a vibration wave motor.

2. Related Background Art

A vibration wave motor for frictionally driving a moving member by the utilization of a travelling vibration wave is recently being put into practical use. The principle of operations of such motor will hereinafter be described with reference to FIG. 3 of the accompanying drawings.

A ring-like vibration member 3 of resilient material whose full peripheral length is an integral number of as great as a certain length .lambda., and two groups of piezoelectric elements 4 are arranged circumferentially around thereof and secured to one surface of the vibration member 3 to provide a stator 2. These piezoelectric elements 4, in each group, are arranged at a pitch of .lambda./2 and so as to be of expansion-contraction polarity and are also arranged in such a manner that there is a deviation an odd number times as great as .lambda./4 between the two groups. Electrode films are provided on the two groups of piezoelectric elements. If an AC voltage is applied only to one group (hereinafter referred to as the A phase), a standing wave (wavelength .lambda. ) of such flexural vibration that the middle point of each piezoelectric element of the A phase and a point spaced, apart by .lambda./2 therefrom are the positions of antinodes. The middle point between the positions of the antinodes is the position where a node is produced in the vibration member 3 over the full periphery thereof. If an AC voltage is applied only to the other group (hereinafter referred to as the phase B), a standing wave is produced in a similar manner, but the positions of the antinodes and node deviate by .lambda./4 relative to the standing wave. If AC voltages identical in frequency and having a time phase difference .lambda./4 therebetween are applied to the A and B phases at the same time, the standing waves of the two are combined together with a result that a travelling wave (wavelength .lambda.) of flexural vibration travelling in the circumferential direction is produced in the vibration member 3. At this time, each point on the other surface of the vibration member 3 having a thickness effects a kind of elliptical movement. Consequently, if a ring-like moving member as a rotor 1 is brought into pressure contact with said other surface of the vibration member 3, the moving member 1 is subjected to a circumferential frictional force from the vibration member 3 and is rotatively driven. The direction of rotation of the moving member can be reversed by changing over the phase difference between the AC voltages applied to the A phase electrodes 5a and the B phase electrodes 5b of the two A and B phases to the positive or the negative. What has been described above is the epitome of the principle of operation vibration wave motor of this type.

In the vibration wave motor thus constructed, there are two types of control of the number of revolutions of the vibration wave motor, i.e., the system of varying the applied frequencies of the driving AC signals applied to the A phase and the B phase, and the system of varying the voltages of the driving AC signals applied to the A phase and the B phase.

According to the former system of controlling the number of revolutions, as shown in FIG. 5 of the accompanying drawings, there is an abrupt variation in the number of revolutions on the side of the frequency lower than the mechanical resonance frequency of the vibration wave motor, and this leads to numerous problems in stably driving the motor over a wider range of the number of revolutions. The latter system of controlling the number of revolutions has no abrupt point of variation in the number of revolutions as in the former system and therefore, it is a control method suitable for a case where the system is utilized over a wide range of the number of revolutions, and in this case, it is popular that the frequency of the driving AC signal applied to the vibration wave motor is fixed at a mechanical resonance frequency which is highest in driving efficiency.

Now, the mechanical resonance frequency of the vibration wave motor varies for each individual motor and with the temperature during driving. Therefore, the mechanical resonance frequency is not always provided at a fixed frequency.

So, in the vibration wave motor of this type, there has been provided a driving circuit of the PLL control system in which, besides the piezoelectric elements of the A phase and the B phase, a vibration detecting piezoelectric element (hereinafter referred to as the S phase) is secured to the vibration plate. The frequency of the AC voltage applied to the A phase and the B phase is automatically made into a resonance frequency in conformity with the detection output from the S phase electrode 5s of the S phase, whereby the vibration wave motor can be driven most efficiently.

However, in the driving circuit for the vibration wave motor by such PLL control system, as shown in FIG. 4 of the accompanying drawings, the output of the S phase is indefinite during the start of the motor or when the driving voltage is low has led to a case where the PLL control becomes impossible.

It is an object of the present invention to provide a driving circuit for a vibration wave motor which can accomplish stable control of the number of revolutions even during the actuation of the vibration wave motor or even when the driving voltage is low.

Other objects of the present invention will become apparent from the following detailed description.

FIG. 1 is a block diagram showing an embodiment of a driving circuit for a vibration wave motor according to the present invention.

FIG. 2 is a flow chart showing the operation procedure of the driving circuit shown in FIG. 1.

FIG. 3 is a schematic view of a vibration wave motor.

FIG. 4 is a graph showing the voltage vs. number of revolutions characteristic of the vibration wave motor.

FIG. 5 is a graph showing the frequency vs. number of revolutions characteristic of the vibration wave motor.

The present invention will hereinafter be described with respect to an embodiment thereof shown in the drawings.

FIG. 1 is a block diagram showing an embodiment of a driving circuit for a vibration wave motor according to the present invention. This circuit controls the number of revolutions of the vibration wave motor by a voltage.

The reference numeral 6 designates a vibration wave motor as shown in FIG. 3, the reference numeral 7 denotes a control unit comprising a microcomputer for controlling the driving of the entire circuit, the reference character 8a designates a comparator for the S phase, the reference character 8b denotes a comparator for the A phase, the reference character 8c designates a shift register, the reference characters 8d and 8m denote edge trigger type phase comparators, the reference character 8e designates an AND circuit, the reference characters 8f and 8n denote lag lead filters, the reference characters 8g and 8p designate voltage control oscillators, the reference character 8h denotes a switch for changing over the forward rotation and the reverse rotation, the reference characters 8i and 8j designate variable voltage sources, the reference characters 8k and 8l denote matching coils, the reference character 8q designates an N-order frequency dividing circuit, the reference character 8r denotes a counter circuit, the reference character 8s designates a numerical value comparator, the reference numeral 8t denotes a closed loop/open loop change-over device, the reference character 8u designates a self-running frequency setting device for the voltage control oscillator 8g, the reference numeral 9 denotes control instructing means comprising a microcomputer for instructing the driving/stoppage, forward/reverse rotation and acceleration/deceleration of the vibration wave motor. The output signal of the numerical value comparator 8s is input to the control unit 7, and control signals are output from the control unit 7 to the forward/reverse rotation change-over switch 8h, the closed/open loop change-over-device 8t, the self-running frequency setting device 8u and the variable voltage sources 8i and 8j. The open loop means a mode in which the number of revolutions of the vibration wave motor is controlled without resorting to the signal from the S phase electrode, and the closed loop means a mode in which the number of revolutions of the vibration wave motor is controlled by the signal from the S phase electrode.

The signal from the voltage control oscillator 8g is amplified by the variable voltage source 8i and applied to the electrode 6a of the vibration wave motor through the coil 8k.

A signal shifted by +90.degree. or -90.degree. relative to the signal of the voltage control oscillator 8g is taken out by the shift register 8c which shifts the data of the signal from the A phase comparator 8b by a clock signal N times the oscillation frequency f of the voltage control oscillator 8g, is amplified by the variable voltage source 8j and is applied to the electrode 6b of the vibration wave motor through the coil 8l.

At this time, the forward/reverse rotation change-over switch 8h is operated by the instruction from the control unit 7 and the signal from the shift register 8c is selected, whereby the phase relation between the signals applied between the A phase and the B phase changes and the direction of rotation of the vibration wave motor changes.

Signals resulting from the output of the S phase comparator 8a and the signal from the A phase comparator 8b being shifted by 90.degree. by the shift register 8c are input to the phase comparator 8d. The output pulse of the phase comparator 8d is counted by the counter circuit 8r, and the value thereof is compared with the internal set value by the numerical value comparator 8s, whereby the present phase state can be detected. This is described in detail in Japanese Laid-Open Patent Application No. 63-154074 (laid open on Jun. 27, 1988 . . . corresponding U.S. application Ser. No. 131,469) and therefore need not be described in detail herein.

Also, by the control unit 7 operating the open loop/closed loop change-over device 8t, the voltage control oscillator 8g effects PLL control so that the output of the phase comparator 8d becomes constant. This is also described in detail in the aforementioned Japanese Laid-Open Patent Application No. 63-154074 and therefore need not be described in detail herein.

The operation procedure of the control unit 7 will now be described with reference to the steps (a) - (n) of a flow chart shown in FIG. 2.

At step (a), when a signal for starting the system is sent from the control instructing means 9 to the control unit 7, the control unit 7 outputs the instruction of power source off to the voltage sources 8i and 8j of FIG. 1, and renders the output voltage level L of the voltage sources 8i and 8j into L=0 and sets a minimum voltage in the voltage sources 8i and 8j. Also, the value T of a time counter (not shown) provided in the control unit 7 is set to T=0, and the initial state is set.

At step (b), the control unit 7 receives the instruction of the starting/stoppage of driving from the control instructing means 9. If the instruction is the instruction of the starting of driving, the program branches off to step (c), where the process of inputting the direction of driving is carried out. If the instruction is the instruction of the stoppage of driving, the program branches off to step (a), where the initial state is kept.

Description will be continued with it being assumed that in this case, the driving start instruction signal has been given.

At step (c), the control unit 7 further receives the instruction of the direction of driving from the control instructing means 9, and sets the forward/reverse rotation change-over switch 8h.

Description will be continued with it being assumed that in this case, the signal of the instruction of the direction of driving is a signal instructing the forward rotation.

At step (d), the control unit 7 receives the acceleration/deceleration instruction from the control instructing means 9, and if there is no instruction of acceleration/deceleration, this step branches off to step (i), where the aforementioned counter T provided in the control unit 7 is checked up. If there is the instruction of acceleration, the step branches off to step (e), where the acceleration process is carried out, and if there is the instruction of deceleration, the step branches off to step (g), where the deceleration process is carried out.

In this case, the vibration wave motor is in its actuated state and therefore, the control instructing means 9 outputs an acceleration instruction signal. Accordingly, the step proceeds to step (e).

At step (e), the check-up of the level L of the output voltages from the voltage sources 8i and 8jis executed, and if the level L of the output voltages is equal to a maximum voltage level M, the step branches off to step (i), and if L<M, the step branches off to step (f), and in this case, the vibration wave motor is in its actuated state as previously mentioned and the level L of the output voltages of the voltage sources 8i and 8j is L<M and therefore, the step proceeds to step (f), where the level L of the output voltages of the voltage sources 8i and 8j is set to (L+1). That is, setting in the direction of acceleration by one step is effected, whereafter the step proceeds to step (i).

At step (i), the comparison between the aforementioned time counter in the control unit 7 and a count value N corresponding to the actuation stabilizing time is executed, and since the value T of the time counter is set to T=0 as previously described, the result of the comparison becomes T<N. Accordingly, the step proceeds to step (k). When the step proceeds to step (k), the value T of the time counter (not shown) in the control unit 7 is increased to (T+1), whereafter the step branches off to step (m).

When the step proceeds to step (m), the control unit 7 changes over the open/closed loop change-over device 8t to the open loop side, and thereafter renders the voltage sources 8i and 8j into their ON state (see step (n)). In this state, the change-over device 8t is changed over to the open loop side and as a result, the output of the comparator 8d is cut off and therefore, independently of the output of the phase comparator 8d, the voltage control oscillator 8g is oscillating at a self-running frequency f.sub.o (higher than the mechanical resonance frequency f.sub.r of the vibration wave motor). A voltage corresponding to the aforementioned (L+1) in which the signal from the voltage control oscillator 8g is set in the voltage source 8i is applied to the electrode 6a of the vibration wave motor through the voltage source 8i and the coil 8k. Also, the signal from the oscillator 8g is shaped by the A phase comparator 8b, whereafter it is input to the shift register 8c which shifts by a clock signal N times the above-mentioned frequency f.sub.o.

Thereby, a signal shifted by .+-.90.degree. (.+-.N/4 steps) relative to the input signal is taken out of the register 8c and a voltage corresponding to the aforementioned (L+1) set in the voltage source 8j is applied to the electrode 6b of the vibration wave motor through the switch 8h and through the voltage source 8j and the coil 8l.

By the above-described operation, the output of the oscillator 8g is applied to the electrode 6a and a signal which is 90.degree. out of phase relative to the applied signal to the electrode 6a and corresponds to the aforementioned (L+1) is applied to the electrode 6b.

When frequency voltages differing by 90.degree. in phase are applied between the electrodes 6a and 6b in this manner, standing waves are formed in the A phase and the B phase, respectively, of the piezoelectric element 4, and these are combined together to create a travelling vibration wave, by which the rotor 1 is driven.

Thereafter, the step proceeds to step (b) and therefore, the step makes a round of the route of steps (b).fwdarw.(c).fwdarw.(d).fwdarw.(e).fwdarw.(f).fwdarw.(i).fwdarw.(m).fwda rw.(n). At this time, the level L of the output voltages of the voltage sources 8i and 8j is increased by one step to (L+2), and the value of the time counter in the control unit 7 is set to (T+2). Therefore, the vibration wave motor is driven by a voltage corresponding to the aforementioned (L+2), and the rotational speed of the rotor 1 is increased. Again in this case, the change-over device 8t is changed over to the open loop side and therefore, the vibration wave motor continues to be driven independently of the output from the electrode 6s. That is, the vibration wave motor continues to be driven in the open loop mode.

The step continues to go around the aforementioned route, and the value L of the voltage sources 8i and 8j increases by one step each time the step makes a round of the aforementioned route and the value T of the time counter also increases by one step each time the step makes a round of the aforementioned route, thereby indicating the lapse of time.

Thereafter, a sufficient time for the operation of the vibration wave motor to become stable elapses and the value T of the time counter in the control unit 7 becomes T=N, and when it is checked at step (i), the step branches off to step (j), where the check-up of the level L of the output voltages of the voltage sources 8i and 8j is executed. When the level L of the output voltages has reached a closed loop driving voltage level C, the step branches off to step (l).

As a result, the control unit 7 changes over the change-over device 8t to the closed loop side and therefore, the output of the phase comparator 8d is connected to the voltage control oscillator 8g through the change-over device 8t and the filter 8f, and the control of the vibration wave motor begins to be executed in the closed loop mode. That is, the control based on the output of the electrode 6s is executed.

Of course, when the level L of the output voltages is L<C, the step branches off to step (m), and then the step again returns to step (b), and at step (f), an increase in the level of the output voltages is executed In this closed loop mode, the output of the comparator 8d is input to the oscillator 8g through the change-over device 8t and the filter 8f, and the output frequency of the oscillator 8g is thereafter controlled in conformity with the signal from the output of the comparator 8d.

In the closed loop mode, the output out of the comparator 8d exhibits the output characteristic of FIG. 5. Accordingly, in the closed loop mode, if the output of the electrode 6s and the signal of the electrode 6a shift by 90.degree. or more relative to each other, the output of the comparator 8d assumes a high level to make the frequency of the oscillator 8g higher, whereby the signal of the electrode 6a is controlled in such a direction that it is more advanced in phase than the output of the electrode 6s, i.e., such a direction that the phase difference is 90.degree., and if the phase difference between the signals of the electrodes 6s and 6a is within 90.degree., the output of the comparator 8d assumes a low level to thereby reduce the frequency of the oscillator 8g and shift the phase difference of 90.degree.. By the above-described operation, in the closed loop mode, the phase difference between the signals of said electrodes is controlled so as to become 90.degree.. That is, so-called PLL control is executed.

When a deceleration instruction signal is given from the control instructing means 9 to the control unit 7 after the vibration wave motor has assumed the above-described stably driven state, the step branches off from step (d) to step (g), where the level L of the output voltages of the voltage sources 8i and 8j is lowered by one step each as shown at step (h) and thus, the vibration wave motor is gradually decelerated. When the level L of the output voltages becomes L<C, the step branches off to step (m), where the open loop mode is assumed again.

The above-described embodiment has been a case where the level L of the output voltages of the voltage sources 8i and 8j gradually rises when the vibration wave motor is actuated, but where the level L of the output voltages is set to L=C from the beginning of the actuation of the vibration wave motor, as a matter of course, a similar effect will be obtained even if the vibration wave motor is driven in the closed loop mode (the PLL control system) after the operation of the vibration wave motor has become stable, in other words, immediately after a sufficient time for the output of the S phase electrode 6s to become stable has elapsed.

As described above, the driving in the closed loop mode (the PLL control mode) which is a second driving mode is adopted only when the output of the vibration detecting electro-mechanical energy converting element is stable, and the driving in the open loop mode which is a first driving mode is adopted when said output is unstable and therefore, stable driving control has become possible even when the vibration wave motor is actuated or when the driving voltage is low.