Abstract:
A dual sided self-oscillation circuit for driving an oscillatory actuator with high efficiency and high response speed. The actuator has a winding to receive a periodical supply current from a power source and oscillates in a predetermined resonant frequency. The self-oscillation circuit includes a bandpass filter for receiving a back electromotive force voltage (Vbemf) developed across the winding and producing a sine wave output signal, a comparator for comparing the sine wave output signal with a threshold voltage and producing two drive pulses per cycle of the resonant frequency, and a switch connected in series with the winding to connect or disconnect the power source to the winding in response to the drive pulses, thereby flowing electric current in two directions at each cycle.

Description:
FIELD OF THE INVENTION 
     This Invention relates to a self-oscillation circuit for driving an oscillatory actuator, and more particularly to a dual sided self-oscillation circuit for driving a linear or rotational oscillatory actuator which causes drive current in two directions in the actuator within a resonant cycle of the actuator for improving performances and controllability of the actuator. 
     BACKGROUND OF THE INVENTION 
     A linear oscillatory actuator has been utilized in many electrical devices such as a reciprocatory shaver, and normally comprises a stator carrying a winding and a reciprocator carrying a permanent magnet. The winding is connected to receive a periodical electric current from a power source to generate a magnetic flux which interacts with the permanent magnet to cause a mechanical resonance of the reciprocator around its natural frequency, thereby forcing the reciprocator to move linearly relative to the stator. This invention is directed to a self-oscillation circuit for driving a linear oscillatory actuator or other types of actuator such as a rotational oscillatory actuator. Although a reciprocator is part of an actuator, within the context of this invention, the terms “actuator” and “reciprocator” may interchangeably be used. 
     U.S. Pat. No. 6,133,701 discloses a system for driving the linear oscillatory actuator with a self-oscillation circuit, one of circuit diagrams therein is shown in FIG.  1 . The circuit is connected to receive a back electromotive force voltage signal developed across the winding (actuator coil) in a positive feedback manner to generate a drive pulse. The electric current generated by the drive pulse is periodically supplied to the winding for continuing the mechanical resonance of the reciprocator. With this scheme, however, the reciprocator is difficult to keep the consistent oscillation without being considerably damped when subjected to a heavy load. 
     Further, in the prior art example shown in FIG. 1, various performances of the circuit such as power control, self start oscillation and etc. are not sufficient for next generation circuit design. The present invention has been accomplished in view of the above background to provide an improved self-oscillation circuit for driving a linear or rotational oscillatory actuator around its resonant frequency. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a dual sided self-oscillation circuit for driving an oscillatory actuator which is able to generate two drive pulses to cause electric current flowing in positive and negative directions in the actuator per resonant cycle of the actuator. 
     It is another object of the present invention to provide a dual sided self-oscillation circuit for driving an oscillatory actuator for improving the performance of the actuator such as reduction of power consumption and increase in response speed. 
     In the present invention, the self-oscillation circuit for driving an oscillatory actuator which has a winding to receive a periodical supply current from a power source and oscillates in a predetermined resonant frequency, includes a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, and a power amplifier for receiving the sine wave output signal from the bandpass filter and producing two drive pulses in each cycle of the resonant frequency of the actuator to cause the periodical supply current flowing in positive and negative directions through the winding. 
     In another aspect of the present invention, the power amplifier is realized by a comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing the drive pulse when the sine wave exceeds the threshold voltage, and a switch connected in series with the winding to connect or disconnect the power source to the winding in response to the drive pulse, thereby causing the periodic supply current flowing in positive and negative directions through the winding. 
     In a more particular implementation of the present invention, the self-oscillation circuit is configured with a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a first comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing a first drive pulse when the sine wave exceeds the threshold voltage in a first half cycle of the resonant frequency, a second comparator for comparing a sine wave which is inverted in polarity from the sine wave output signal from the bandpass filter with the threshold voltage and producing a second drive pulse when the sine wave exceeds the threshold voltage in a second half cycle of the resonant frequency, and an H-bridge switch circuit having four switches with the actuator connected in a middle portion thereof and connected to the power source. The H-bridge switch circuit connects or disconnects the power source to the winding in response to the first and second drive pulses, thereby causing the periodic supply current flowing in the positive and negative directions through the winding. 
     Preferably, the threshold voltage of the first and second comparators is different from a bias voltage of the bandpass filter to produce the drive pulses with duty ratio less than 50—50. The self-oscillation circuit additionally includes means for instantaneously changing the threshold voltage of the first and second comparators to be the same as the bias voltage of the bandpass filter at a start-up process of oscillation of the actuator. 
     In a further aspect, the self-oscillation circuit is comprised of a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a comparator for comparing the sine wave output signal from the bandpass filter with a threshold voltage and producing a drive pulse with a positive voltage swing in a first half cycle of the resonant frequency and with a negative voltage swing in a second half cycle of the resonant frequency produced every time when the sine wave crossing the threshold voltage, and a push-pull switch circuit having two switches with the actuator connected in a middle portion thereof to a ground and connected to positive and negative power sources. The push-pull switch circuit connects or disconnects the positive and negative power sources to the winding in response to the drive pulse, thereby causing the periodic supply current flowing in the positive and negative directions through the winding. 
     In a further aspect, the self-oscillation circuit includes a bandpass filter whose center frequency is adjusted to the resonant frequency for receiving a back electromotive force voltage (Vbemf) developed across the winding in the actuator and producing a sine wave output signal representing the Vbemf, a first comparator for comparing the sine wave output signal from the bandpass filter with a first threshold voltage and producing a first drive pulse when the sine wave exceeds the first threshold voltage in a first half cycle of the resonant frequency, a second comparator for comparing the sine wave output signal from the bandpass filter with a second threshold voltage and producing a second drive pulse when the sine wave exceeds the second threshold voltage in a second half cycle of the resonant frequency, and a push-pull switch circuit having two switches with the actuator connected in a middle portion thereof to a ground and connected to positive and negative power sources. The push-pull switch circuit connects or disconnects the positive and negative power sources to the winding in response to the first and second drive pulses, thereby causing the periodic supply current flowing in the positive and negative directions through the winding. 
     In the self-oscillation circuit of the present invention, the double sided drive method is used in which the drive pulse occurs two times per cycle of the mechanical resonance frequency for causing electric current in positive and negative directions in the actuator winding. This method requires a substantially smaller amount of electric power for driving the actuator than that required in the conventional technology. Further, the double sided drive method of the present invention introduces other benefits such as quick response to external loads, because the repetition rate of the drive pulse is two times higher than that of the single sided drive method. 
    
    
     These and still other objects and advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a conventional self-oscillation circuit for driving a linear oscillatory actuator which is described in U.S. Pat. No. 6,133,701. 
     FIGS. 2A and 2B are schematic diagrams showing a difference between a single sided drive method and a double sided drive method in a self-oscillation circuit. 
     FIG. 3 is a schematic diagram showing a basic circuit structure of a self-oscillation circuit for driving a linear oscillatory actuator configured with a bandpass filter and an amplifier in accordance with the present invention. 
     FIG. 4 is a schematic diagram showing a self-oscillation circuit incorporating a comparator and a switch as a power amplifier of FIG.  3 . 
     FIG. 5 is a waveform chart showing input and output waveforms of the comparator in the self-oscillation circuit of FIG.  4 . 
     FIG. 6 is a circuit diagram showing an example of structure in the self-oscillation circuit for driving a linear oscillatory actuator with an H-bridge circuit and one bias voltage configuration in the present invention. 
     FIG. 7 is a circuit diagram showing another example of structure in the self-oscillation circuit for driving a linear oscillatory actuator with an H-bridge circuit and two bias voltage configuration in the present invention. 
     FIGS. 8A-8D are waveform charts illustrating rectangular drive pulses, supply current from a power source, and displacement of the actuator involved in the operation of the embodiments of FIGS. 6 and 7. 
     FIG. 9 is a circuit diagram showing an example of structure in the self-oscillation circuit for driving a linear oscillatory actuator with a push-pull circuit and one bias voltage configuration in the present invention. 
     FIGS. 10A-10E are waveform charts illustrating rectangular drive pulses, feedback voltage representing Vbemf, supply currents from plus and minus power sources, and displacement of the actuator involved in the operation of the embodiment of FIG.  9 . 
     FIG. 11 is a circuit diagram showing an example of structure in the self-oscillation circuit for driving a linear oscillatory actuator with a push-pull circuit, two comparators, and two bias voltage configuration in the present invention. 
     FIGS. 12A-12E are waveform charts illustrating rectangular drive pulses, feedback voltage representing Vbemf, supply currents from power sources, and displacement of the actuator involved in the operation of the embodiment of FIG.  11 . 
     FIGS. 13A-13D are waveform charts illustrating a timing relationship between drive voltages and oscillation waveform of the actuator when the phase of the drive pulses is in a non-optimum setting. 
     FIGS. 14A-14D are waveform charts illustrating a timing relationship between drive voltages and oscillation waveform of the actuator when the phase of the drive pulses is in an optimum setting. 
     FIGS. 15A-15C are schematic diagram showing a process of charging and discharging the reverse current involved in the H-bridge circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 2 and 3, there is shown a basic concept of a self-oscillation circuit for driving an oscillatory actuator in accordance with the present invention. The present invention is described in a case for driving a linear oscillatory actuator for an illustration purpose. However, it should be noted that the present invention is also applicable to other types of actuator such as rotational oscillatory actuator. 
     The self-oscillation circuit is connected to receive a back electromotive force voltage (Vbemf) appearing across the winding in the actuator in a positive feedback manner to generate a drive pulse which is supplied to the actuator. The drive pulse causes the electric current to be periodically supplied to the winding in both plus and minus directions for continuing the oscillation of the actuator (reciprocator). The oscillation frequency of the actuator is a mechanical resonance frequency or natural frequency of the reciprocator. 
     In the self-oscillation circuit of the present invention, it is designed that the drive pulse occurs two times per cycle of the mechanical resonance frequency to flow electric current in the actuator winding in two (positive and negative) directions, i.e., a double sided drive method. In contrast, the self-oscillation circuit in the conventional technology generates one drive pulse per resonant cycle to flow electric current in the actuator winding in one direction, i.e., a single sided drive method. 
     FIGS. 2A and 2B show the difference between the single sided drive method and double sided drive method in the self-oscillation circuit. FIG. 2A illustrates a waveform of the single sided drive in the conventional technology in which a drive pulse with a current amplitude  2 I excites the actuator in a resonant cycle T of the actuator. FIG. 2B illustrate a waveform of the double sided drive of the present invention where two drive pulses with opposite polarities each having a current amplitude I excites the actuator during the resonant cycle T. 
     In FIGS. 2A and 2B, depending on the amount of power needs to maintain the reciprocal movement of the actuator, a pulse width will be regulated by the positive feedback loop of the self-oscillation circuit. In other words, pulse width modulation is performed in the circuit with a degree of modulation or modulation factor m (0≦m≦1). Suppose R denotes overall resistance in the path flowing the electric current I or  2 I above, overall power consumption P in the single sided drive is expressed as P=I 2 R·2m while overall power consumption P in the double sided drive is expressed as P=I 2 R·m. This relationship dictates that the single sided drive has twice as much power loss as that of the double sided drive. 
     Therefore, the double sided drive in the present invention requires a substantially smaller amount of electric power for driving the actuator. Since the linear oscillatory actuator is applied to electrical devices such as a reciprocatory shaver which is battery operated, the reduction of power achieves a significant advantage. Further, the double sided drive method introduces other benefits including quick response to external loads, because the repetition rate of the drive pulse is two times higher than that of the single sided drive method. 
     To establish the double sided drive, the basic configuration of the self-oscillation circuit is composed of a bandpass filter and a power amplifier as shown in FIG.  3 . The center frequency (pass band frequency) of the bandpass filter is set to the mechanical resonant frequency of the reciprocator. The input of the bandpass filter is biased with a predetermined voltage such as a ground potential and is provided with a feedback voltage which represents the back electromotive force voltage (Vbemf) detected at the winding of the actuator. Thus, the output of the bandpass filter is a sine wave signal with a positive curve in the first half cycle and a negative curve in the last half cycle. The power amplifier receives the sine wave signal from the bandpass filter and generates drive pulses with positive and negative directions such as shown in FIG.  2 B. 
     The power amplifier of FIG. 3 is realized by, for example, a comparator and a switch as shown in FIG.  4 . The waveforms at the input and output of the comparator are shown in FIG.  5 . The comparator receives the sine wave signal at its one input and a threshold voltage (reference voltage) at its another input. Thus, the input sine wave is wave-shaped to a rectangular wave (drive pulse) which serves as a control signal for the switch provided between the power source and the winding. As shown in FIG. 5, the threshold voltage determines a reference point of the input sine wave at which the output of the comparator changes its state. Therefore, the threshold voltage to the comparator determines a pulse width, i.e., a duty ratio of the drive pulse. 
     The first preferred embodiment is shown in FIG. 6 which is a self-oscillation circuit for driving a linear oscillatory actuator with an H-bridge circuit configuration. This example is provided with one bias voltage Vb for the bandpass filter and the comparators. Four switches and the actuator are connected to form an H-bridge circuit which is connected to a voltage source Vd to introduce a supply current to the actuator. The actuator (reciprocator) with the winding is connected at the middle section of the H-bridge circuit. In this example, the four switches are implemented by metal oxide semiconductor field effect transistors (MOSFET) M 1 -M 4 , although other type of switches can also be used. 
     The bandpass filter is configured with an operational amplifier  31 , resistors R 1 , R 2 , R 3 , Ra and Rb, and capacitors C 1  and C 2 . Such an active bandpass filter is known in the art, thus, no specific explanation regarding the bandpass filter is given here. The example of FIG. 6 includes a pair of comparators  33  and  37  each being formed with an operational amplifier. The comparator  33  receives the output of the amplifier  31  in the bandpass filter and the comparator  37  receives an inverted output of the amplifier  31  through an inverter formed with an operation amplifier  35  and resistors R 4  and R 5 . 
     The output (drive pulse) Vo 1  of the comparator  33  has a rectangular waveform such as shown in FIG.  5  and is supplied to the gates of the MOSFETs M 1  and M 2 . Similarly, the output (drive pulse) Vo 2  of the comparator  37  has a rectangular waveform and is supplied to the gates of the MOSFETs M 3  and M 4 . A feedback voltage Vf representing the back electromotive force voltage (Vbemf) across the winding is detected and supplied to the input of the bandpass filter, thereby forming a positive feedback loop. The bandpass filter is provided with the bias voltage Vb at the non-inverting input of the operational amplifier  31 . 
     The comparators  33  and  37  are provided with the same threshold voltage (bias voltage) Vb which is the same voltage as the bias voltage Vb supplied to the bandpass filter. Thus, each of the comparators  33  and  37  compares the input signal with the threshold voltage Vb. When the input signal level exceeds the threshold voltage Vb, the output of the comparator changes from one voltage level to another voltage level, thereby creating the drive pulses Vo 1  and Vo 2  of rectangular waveform noted above. 
     Since the comparator  37  receives the inverted output signal of the bandpass filter through the inverter, the drive pulses Vo 1  and Vo 2  are phase shifted with one another by 180° as shown in FIGS. 8A and 8B. Further, since the threshold voltage Vb for the two comparators  33  and  37  is the same, and thus changes the output states at the same input voltage, the drive pulses Vo 1  and Vo 2  have a 50—50 duty ratio. Since the drive pulses Vo 1  and Vo 2  are applied to the MOSFET switches M 1 -M 4 , the supply current Id flows through the switch M 2 , actuator, and switch M 1  in the first half cycle of the resonant frequency, and through the switch M 4 , actuator, and switch M 3  in second half cycle. Consequently, the self-oscillation circuit of the present invention drives the actuator by electric current Ia in two directions (double sided drive) caused by two drive pulses for each cycle of the resonant frequency. 
     Another preferred embodiment is shown in FIG. 7 which is a self-oscillation circuit for driving a linear oscillatory actuator with the H-bridge similar to the embodiment of FIG.  6 . The example of FIG. 7 includes two bias voltages Vb 1  and Vb 2 . The bias voltage Vb 1  is provided to the bandpass filter (operational amplifier  31 ) and the inverter (operational amplifier  35 ). The bias voltage Vb 2  is provided to the comparators  33  and  37 , respectively, as a threshold voltage. 
     Thus, depending on the amount of difference between the bias voltage Vb 1  and the threshold voltage Vb 2 , the duty ratio in the drive pulses Vo 1  and Vo 2  will be adjusted to less than 50—50, for example, 40—40. In FIG. 7, such adjustment can be made by changing resistance Ra and/or Rb. When the duty ratio is 50—50 as in the example of FIG. 6, the self-oscillation circuit is able to produce the drive pulses Vo 1  and Vo 2  even when the output sine wave of the bandpass filter is small. In contrast, the example of FIG. 7 requires the output sine wave having an amplitude larger than the voltage difference between the bias voltages Vb 1  and Vb 2  to produce the drive pulses for the actuator. However, the example of FIG. 7 has a better controllability for attaining optimum performance of the actuator by selecting the bias voltages as well as other parameters. 
     The example of FIG. 7 further includes a resistor network, i.e., a voltage divider, formed of resistors Ra and Rb where capacitor C 3  is connected across the resistor Rb. The voltage divider determines the bias voltages Vb 1  and Vb 2 . The purpose of this arrangement is to facilitate a start-up operation in the self-oscillation circuit for driving the actuator. As noted above, when two bias voltages Vb 1  and Vb 2  are used in the embodiment of FIG. 7, the output voltage of the bandpass filter must be larger than the difference between the two voltages. 
     At the start-up of the self-oscillation circuit, it is necessary that the comparators  33  and  37  can produce drive pulses even when an output voltage of the bandpass filter, i.e., the back electromotive force voltage (Vbemf) is small. To facilitate such a start-up operation, the capacitor C 3  is provided to instantaneously short circuit the resistor Rb, at power on, so that the two bias voltages Vb 1  and Vb 2  become the same or similar to one another, thereby increasing the voltage sensitivity of the comparators  33  and  37 . 
     The waveform charts of FIGS. 8A-8D illustrate rectangular drive pulses, supply current from the power source, and displacement of the actuator involved in the operation of the embodiments of FIGS. 6 and 7. FIG. 8A shows a waveform of the drive pulse Vo 1  and FIG. 8B shows a waveform of the drive pulse Vo 2 , which are phase shifted with one another by 180°. FIG. 8C shows a waveform of the supply current Id flowing from the power source Vd to the actuator. FIG. 8D is a waveform showing the displacement of the actuator (reciprocator) which is the oscillation amplitude of the actuator at the mechanical resonant frequency. In this example, the peak-to-peak oscillation amplitude is about 1.2 millimeters. 
     FIG. 9 shows a further embodiment of the present invention in which the self-oscillation circuit for driving a linear oscillatory actuator is configured with a push-pull switch circuit. The bandpass filter is the same as that shown in the foregoing embodiments of FIGS. 6 and 7. In this example, a single comparator (operational amplifier)  33  is connected to the bandpass filter. The output of the comparator  33  is connected to the push-pull circuit. 
     The operational amplifier  31  of the bandpass filter and the comparator  33  are commonly biased with about zero voltage. Although not shown, the operational amplifier  31  and the comparator  33  are provided with power sources +Vs and −Vs. Thus, the comparator  33  produces a drive pulse Vg of a positive voltage at one half cycle and a negative voltage at another half cycle of the resonant frequency. Since the threshold voltage of the comparator  33  is zero voltage as noted above, the drive pulse Vg is a rectangular waveform of 50—50 duty ratio. 
     The push-pull circuit is configured with MOSFETs M 11  and M 22  as switch elements, although other types of semiconductor switches are also feasible. In this example, the MOSFET M 11  is an n-type MOSFET and the MOSFET M 22  is a p-type MOSFET connected in series. Positive and negative power sources +Vd and −Vd are respectively supplied to the push-pull circuit in the manner shown in FIG.  9 . The actuator is connected between the common source of the MOSFET M 11  and M 22  and the ground. A feedback voltage Vf representing the back electromotive force voltage (Vbemf) detected at the actuator winding is provided to the input of the bandpass filter to establish a positive feedback loop. 
     In this configuration, when the drive pulse Vg is a positive pulse in the first half cycle, the MOSFET M 11  is switched ON so that a supply current Id flows from the power source Vd, the MOSFET M 11 , the actuator, and to the ground. Conversely, when the drive pulse Vg is a negative pulse in the second half cycle, the MOSFET M 22  is switched ON so that a supply current −Id flows from the ground, the actuator, the MOSFET M 22 , and to the power source −Vd. Consequently, the self-oscillation circuit of the present invention drives the actuator by two drive pulses to that electric current flows in the two directions (double sided drive) for each cycle of the resonant frequency. 
     The waveform charts of FIGS. 10A-10E illustrate the rectangular drive pulses Vg, the feedback voltage Vf, supply currents from power sources +Vd and −Vd, and displacement of the actuator in the operation of the embodiment of FIG.  9 . FIG. 10A shows a waveform of the drive pulse Vg which alternately swings in the positive direction in the first half cycle and in the negative direction in the second half cycle. FIG. 10B shows a waveform of the feedback voltage Vf representing the back electromotive force voltage (Vbemf) induced across the actuator winding. FIG. 10C shows a waveform of the supply current Id flowing from the power source +Vd to the actuator. FIG. 10D shows a waveform of the supply current −Id flowing from the actuator to the power source −Vd. FIG. 10E shows a waveform of the displacement of the actuator (reciprocator) which is in the mechanical resonant frequency of the actuator. 
     Another preferred embodiment is shown in FIG. 11 which is a self-oscillation circuit for driving a linear oscillatory actuator with the push-pull circuit similar to the embodiment of FIG.  9 . The example of FIG. 11 includes two comparators  33  and  37  and two threshold voltages +Vb and −Vb. The bias voltage (threshold voltage) −Vb is provided to the comparator  33  and the bias voltage (threshold voltage) +Vb is provided to the comparator  37 . The output of the bandpass filter (operational amplifier  31 ) is commonly connected to the inputs of the comparators  33  and  37 . The comparator  33  produces a drive pulse Vgn which is supplied to the gate of the n-type MOSFET M 11 . The comparator  37  produces a drive pulse Vgp which is supplied to the gate of the p-type MOSFET M 11 . 
     In this configuration, when the drive pulse Vgn is a positive pulse, the MOSFET M 11  is switched ON so that a supply current Id flows from the power source Vd, the MOSFET M 11 , the actuator, and to the ground. Conversely, when the drive pulse Vgp is a negative pulse, the MOSFET M 22  is switched ON so that a supply current −Id flows from the ground, the actuator, the MOSFET M 22 , and to the power source −Vd. Consequently, the self-oscillation circuit of FIG. 11 drives the actuator by two drive pulses so that electric current flows through the actuator in positive and negative directions (double sided drive) for each cycle of the resonant frequency. 
     The operational amplifier  31  of the bandpass filter is biased with zero voltage, i.e, ground potential. As noted above, the threshold voltage −Vb is provided to the comparator  33  and the threshold voltage +Vb is provided to the comparator  37 . Thus, depending on the degree of voltage difference between the threshold voltage ±Vb and the ground potential, the duty ratio in the drive pulses Vgn and Vgp will be adjusted to smaller than 50—50, for example 40—40. This adjustment can be made by changing the resistance Ra and/or Rb. Preferably, the duty ratio is adjusted by changing the threshold voltages +Vb for optimum performance of the self-oscillation circuit. 
     Similar to the embodiment of FIG. 7 described above, the example of FIG. 11 further includes a resistor network (voltage divider) formed of resistors Ra and Rb where capacitor C 3  is connected across the resistor Rb. The voltage divider determines the bias voltages −Vb and +Vb. The purpose of this arrangement is to facilitate a start-up operation in the self-oscillation circuit for driving the actuator. At the start-up of the self-oscillation circuit, it is necessary that the comparators  33  and  37  is able to produce the drive pulses even when an output voltage of the bandpass filter, i.e., feedback voltage Vf (i.e., back electromotive force voltage (Vbemf)) is small. To facilitate such a start-up operation, the capacitor C 3  is provided to instantaneously short circuit the resistor Rb, at power on, so that the two bias voltages −Vb and +Vb become the zero voltage, thereby increasing the voltage sensitivity of the comparators  33  and  37 . 
     The waveform charts of FIGS. 12A-12E illustrate the rectangular drive pulses Vg, the feedback voltage Vf, supply currents from power sources +Vd and −Vd, and displacement of the actuator in the operation of the embodiment of FIG.  11 . FIG. 12A shows a waveform of the drive pulse Vgn from the comparator  33 . The waveform of FIG. 12A alternately swings in the positive polarity in the first half cycle and in the negative polarity in the second half cycle. Because of the threshold voltage −Vb, the pulse width in the positive side is smaller than the pulse width in the negative side, and only the positive side is used for driving the MOSFET M 11 . Although not shown here, a waveform of the drive pulse Vgp from the comparator  37  has a waveform opposite to that of Vgn and is phase shifted by 180°, and only the negative side is used for driving the MOSFET M 22 . 
     FIG. 12B shows a waveform of the feedback voltage Vf representing the back electromotive force voltage (Vbemf) induced across the actuator winding. FIG. 12C shows a waveform of the supply current Id flowing from the power source +Vd to the actuator. FIG. 12D shows a waveform of the supply current −Id flowing from the actuator to the power source −Vd. FIG. 12E shows a waveform of the displacement of the actuator (reciprocator) which is in the mechanical resonant frequency of the actuator. 
     With respect to the foregoing embodiments of the present invention, the inventor has discovered that a timing relationship between the drive pulses and movements of the actuator significantly affects the efficiency in driving the actuator. Thus, it is preferable to adjust the phase of the drive pulse for achieving the maximum efficiency, i.e., the maximum oscillation amplitude of the actuator with minimum power consumption (supply current). 
     FIGS. 13A-13D show such a case where the timing relationship is inadequately set. FIG. 13A shows a waveform of the drive pulse Vo 1  such as shown in FIG.  7 . FIG. 13B shows a waveform of the feedback voltage Vf representing the back electromotive force voltage (Vbemf) induced across the actuator winding. FIG. 13C shows a waveform of the supply current Id flowing from the power source Vd to the actuator. FIG. 13D is a waveform showing the displacement of the actuator (reciprocator) which is in the mechanical resonant frequency of the actuator. In this example, the peak-to-peak amplitude of the displacement is about 1.2 millimeters. 
     FIGS. 14A-14D are waveform charts corresponding to FIG. 13A-13D in the case where the timing relationship is adequately set for the maximum efficiency. As shown in FIG. 14D, the peak-to-peak amplitude of the actuator displacement is about 2.0 millimeters. Note that the phase difference between the drive pulse Vo 1  of FIG.  14 A and the actuator displacement of FIG. 14D is different from the phase difference between the drive pulse Vo 1  of FIG.  13 A and the actuator displacement of FIG.  13 D. Also note that the supply current Id (representing power consumption) of FIG. 14C has a significantly smaller average value than that shown in FIG.  13 C. FIG. 14B shows a waveform of the feedback voltage Vf. 
     As noted above, this maximum efficiency of FIGS. 14A-14D is achieved by adjusting the timing (phase) of the drive pulses Vo 1  and Vo 2 , i.e., the sine wave applied to the comparators  33  and  37 . There are various ways of adjusting the phase, for example, by changing the resistance R 3  in the bandpass filter or by inserting a delay circuit either input or output of the bandpass filter. It is also possible to adjust the phase of the drive pulse (start and end timings of the drive pulse) by changing the threshold voltage of the comparators  33  and  38 , i.e., regulating the duty ratio of the drive pulses. 
     In the embodiments of FIGS. 7 and 11 in the foregoing, the threshold voltages for the comparators  33  and  37  are different from the bias voltage of the output sine wave of the bandpass filter, thereby creating the duty ratio smaller than 50—50 such as 40—40. When the duty ratio is smaller than 50—50, there is a time period in which none of the switches (MOSFETs) are activated by the drive pulses. The present invention is to make use of such an OFF period of the MOSFETs for discharging the magnetic energy stored in the inductor of the winding during the flow of forward current by the flow of reverse current. 
     The reverse current is a portion of the supply current Id flowing in a reverse direction as indicated in FIGS. 8C and 13C. The energy stored in the winding of the actuator by the forward flow of supply current is discharged as the reverse current after the forward flow is turned off. In the present invention, this discharge process is performed during the time period when all of the switches (MOSFETs) are OFF. 
     FIGS. 15A-15C show this process in which labels S 1 -S 4  denote the switches such as implemented by MOSFETs M 1 -M 4  in the H-bridge configuration. When the drive pulse Vo 1  is applied to the H-bridge, the switches S 1  and S 2  are ON so that the supply current Id flows through the pass indicated by the arrows shown in FIG.  15 A. Conversely, when the drive pulse Vo 2  is applied to the H-bridge, the switches S 3  and S 4  are ON so that the supply current Id flows through the pass indicated by the arrows shown in FIG.  15 B. Since the switches are ON in either situations of FIGS. 15A and 15B, the time constant involving the winding is large, the energy stored in the winding cannot efficiently be discharged. In FIG. 15C, which shows a situation where all of the switches are OFF during the time period noted above, the stored energy is quickly discharged by a discharge current I R  through, for example internal diodes of the MOSFETS. 
     As has been described above, according to the present invention, the self-oscillation circuit generates the drive pulse two times per cycle of the mechanical resonance frequency. This method requires a substantially smaller amount of power for driving the actuator than that required in the conventional technology. Further, the self-oscillation circuit is able to achieve quick response to external loads, because the repetition rate of the drive pulse is two times higher than that of the single sided drive method. 
     Although only a preferred embodiment is specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing the spirit and intended scope of the invention.