Driving device for ultrasonic wave motor

Abstract: There is disclosed a driving device for an ultrasonic wave motor, in which a limit detecting circuit (2,3) detects that the drive voltage for the ultrasonic wave motor (100) has reached a limit, and, in response, a drive voltage limiting circuit controls the drive voltage so as not to go beyond the limit value. Such control is conducted on the voltage or frequency of a signal for driving the ultrasonic wave motor. In another aspect of the invention, a drive state detecting circuit detects the drive state of the ultrasonic wave motor (100), and a drive frequency (or voltage) setting circuit (5,4) controls the frequency (or voltage) of a signal for driving the ultrasonic wave motor (100) so as to attain the maximum efficiency in the motor, based on the output of the drive state detecting circuit.

Description: The present invention relates to a driving device for an ultrasonic wave motor according to the preambles of independent claims 1 and 2. Conventionally there have been proposed various types of driving device for the ultrasonic wave motor. At first the structure of an ultrasonic wave motor will be briefly explained with reference to the attached drawings. Fig. 1 is a cross-sectional view of an ultrasonic wave motor, in which a rotor (movable member) is composed of a main rotor member 100-1 and a sliding member 100-2 which are mutually adhered. Also a stator is composed of an elastic member 100-3 and a piezoelectric member 100-4 which are mutually adhered. Said rotor and stator are maintained in mutual pressure contact to constitute said ultrasonic vibration motor. Fig. 2 is a view showing the arrangement of electrodes in said piezoelectric member 100-4. Input electrodes 100-4a, 100-4b receive AC drive voltages of a mutual phase difference of π/2. A common electrode 100-4c is grounded. A monitor electrode 100-4d does not contribute to the oscillation of the elastic member but provides an AC output voltage corresponding to the vibration state of the stator. Thus the ultrasonic wave motor consists of the main rotor member, sliding member, elastic member and piezoelectric member explained above. The structure and function of such motor will not be explained in detail as they are already known for example from the U.S. Patent No. 4,510,411. In brief, drive voltages are supplied to said electrodes 100-4a, 100-4b of the piezoelectric member to oscillate said member, thereby generating a travelling vibration wave in the elastic member, and driving said rotor maintained in pressurized contact with said stator by unrepresented pressurizing means. A drive control device for such ultrasonic wave motor is already disclosed for example in the Japanese Laid-open Patent Appln. Nos. 59-204477 61-251490. These drive control devices are either (1) to control the frequency of the drive voltage signals according to the voltage obtained from the monitor electrode 100-4d, or (2) to control the frequency of the drive voltage signals according to the phase difference between the wave form of the drive voltage signals applied to the piezoelectric member 100-4 and that of the voltage signal obtained from the monitor electrode 100-4d. In addition, for driving such ultrasonic wave motor, there have been proposed various methods by varying the drive voltage or by varying the drive voltage and the drive frequency, as disclosed for example in the Japanese Laid-open Patent Appln. Nos. 61-124275 and 62-19276. In the drive control of the ultrasonic wave motor, it is known that the drive speed of the motor becomes unstable or the motor generates abnormal noises if the frequency of the drive voltages applied to the drive electrodes, namely the drive frequency, is selected close to the resonance frequency specific to the ultrasonic wave motor. It is also known that the motor loses the drive speed rapidly and enters a very unstable operation state if the drive frequency is selected lower than said resonance frequency. Fig. 3 shows the relationship between the drive frequency and the drive speed of the ultrasonic wave motor, in which F1 indicates the resonance frequence specific to the ultrasonic wave motor. Bu dividing the entire frequency range into regions a and b by a drive frequency F3 slightly higher than said resonance frequency F1, the drive frequency region a suffers the above-mentioned abnormalities in the drive, while the drive frequency region b from said frequency F3 to a higher frequency F2 where the drive speed approaches to zero provides stable drive control. The conventional drive control devices for the ultrasonic wave motor effects the drive control within such frequency region b. However the present applicant has experimentally confirmed that following drawbacks are encountered even when the ultrasonic wave motor is driven with a frequency within said frequency region b allowing stable drive control: (1) When the drive voltage is increased beyond a certain value (hereinafter called upper limit), the motor shows unstable drive speed or generates abnormal noises. This is presumably due to an undesirable relationship between the stator and the rotor maintained in pressure contact by the pressurizing means, induced when the amplitude of the travelling vibration wave exceeds a certain value; (2) On the other hand, when the drive voltage is decreased beyond a certain value (hereinafter called lower limit), the operation of the ultrasonic wave motor becomes unsmooth and the motor may eventually stop. This phenomenon is presumably due to a fact that the oscillating force generated by the piezoelectric member cannot overcome the pressurizing force of the pressurizing means, due to the influence of the load, in the contact portions of the stator and the rotor, when the amplitude of the travelling vibration wave becomes smaller than a certain value. Thus, in the speed control of the ultrasonic wave motor, the drive frequency may become Positioned outside the region b providing unstable drive because of the above-mentioned drawbacks if the drive voltage exceeds the upper limit or decreases beyond the lower limit. Thus the drive frequency enters the region involving the above-mentioned unstable drive state, thus rendering the function of the ultrasonic wave motor unstable. In addition, in case the drive voltage is given to the drive electrode of the motor from a power amplifier through a coil or a transformer provided therebetween, a resonance is generated between the inductance of such coil or transformer and the capacitance of the piezoelectric member, whereby the drive voltage on the drive electrode may fluctuate depending on the drive frequency even if the output voltage of the power amplifier is maintained constant. As a result, even if the drive frequency is selected at the middle of the region b, the drive frequency may be shifted from the stable region b due to fluctuation in the drive voltage, wherebv the function of the ultrasonic wave motor may become unstable due to such unstable drive frequency. US-A-4,853,579 discloses a drive method for ultrasonic motors providing enhanced stability to rotation. More specifically, a drive frequency of the motor is controlled in accordance with a phase difference detected between the wave forms of a drive current and a drive voltage signal. As a result, the drive frequency is maintained between a resonance frequency and an antiresonance frequency in that a continuous and stable operation of the ultrasonic wave motor is attained. US-A-4,743,788 discloses an input frequency control device for surface acoustic wave motors. More specifically, the input power supply frequency is controlled in such a manner that a monitor voltage generated by the vibration of the motor reaches a predetermined value lower than a maximum voltage, corresponding to the stator of said surface acoustic wave motor. As a result, the power supply frequency is maintained higher that said resonance frequency. It is the object of the present invention to provide an improved driving device for keeping an ultrasonic wave motor in a stable manner. The object is solved by the subject matters of claims 1 and 2. According to a preferred embodiment of the invention, a driving device for an ultrasonic wave motor consists of a stator for generating a travelling vibration wave in an elastic manner by the oscillation of a piezoelectric member, and a rotor maintained in pressure contact with the stator by pressurizing means and adapted to be driven by said travelling vibration wave, and particularly to a driving device for achieving the drive control by a variable drive voltage. The afore-mentioned object can be attained by the use of a lower limit detecting means for detecting that the drive voltage has reached a lower limit. The drive voltage limiting means is adapted, in response to the detection of the drive voltage at the lower limit by said lower limit detecting means, for maintaining the drive voltage at said lower limit or increasing the drive voltage. There may be further provided an upper limit detecting means for detecting that the drive voltage has reached the upper limit, and the drive voltage limiting means may further be adapted, in response to the detection of the drive voltage at the upper limit by said upper limit detecting means, for maintaining the drive voltage at said upper limit or decreasing the drive voltage. The present invention is also applicable to a driving device for achieving the drive control by a variable drive frequency. In this case the aforementioned object can be attained by the use of a lower limit detecting means for detecting that the drive voltage of the ultrasonic wave motor has reached a lower limit, and the drive voltage limiting means is adapted, in response to the detection of the drive voltage at said lower limit by said lower limit detecting means, for controlling the drive frequency so as to maintain the drive voltage at said lower limit or to increase the drive voltage. There may be further provided an upper limit detecting means for detecting that the drive voltage has reached an upper limit, and the drive voltage limiting means may further be adapted, in response to the detection of the drive voltage at said upper limit by said upper limit detecting means, for controlling the drive frequency so as to maintain the drive voltage at said upper limit or to decrease said drive voltage. In the following there will be explained the function of the above-explained driving device for the ultrasonic wave motor. In case of voltage control of the motor, if the lower limit detecting means detects that the drive voltage is at the lower limit, the drive voltage limiting means either maintains the drive voltage at said lower limit or increases the drive voltage. If the upper limit detecting means detects that the drive voltage is at the upper limit, the drive voltage limiting means either maintains the drive voltage at said upper limit or decreases the drive voltage. In case of frequency control of the motor, if the lower limit detecting means detects that the drive voltage is at the lower limit, the drive voltage limiting means controls the drive frequency so as to maintain the drive voltage at said lower limit or to increase the drive voltage, thereby limiting the drive voltage. Also if the upper limit detecting means detects that the drive voltage is at the upper limit, the drive voltage limiting means controls the drive frequency so as to maintain the drive voltage at said upper limit or to decrease the drive voltage, thereby limiting the drive voltage. In this manner the drive voltage is controlled between the upper and lower limits, thereby driving the ultrasonic wave motor in stable manner. In the following, two preferred embodiments of the invention will be described in more detail by referring to the accompanied figures, wherein : Fig. 1 is a cross-sectional view of an ultrasonic wave motor; Fig. 2 is a view showing arrangement of electrodes in the piezoelectric member of an ultrasonic wave motor; Fig. 3 is a chart showing the relationship between the drive speed and the drive frequency of an ultrasonic wave motor; Fig. 4 is a chart showing the relationship between the drive speed and the monitor electrode voltage; Fig. 5 is a chart showing the relationship between the drive speed and the drive frequency at different drive voltages; Fig. 6 is a chart showing the relationship between the monitor electrode voltage and the drive frequency at different drive voltages; Fig. 7 is a chart showing the relationship between the drive speed and the drive voltage; Fig. 8 is a circuit diagram of a first example; Fig. 9 is a table showing the relationship between the signal input and control input of a multiplexer MPX; Fig. 10 is a circuit diagram of a second example; Fig. 11 is a circuit diagram of a third example; and Fig. 12 is a circuit diagram of a 4th example. At first there will be explained, based on the experimental results of the present inventors, a method of determining the upper and lower limits of the drive voltage for stably driving the ultrasonic wave motor. The output voltage VM of the monitor electrode is related with the amplitude of the travelling vibration wave generated in the elastic member, and increases or decreases respectively said amplitude increases or decreases. When the relationship between the drive speed and the drive frequency is investigated with the drive voltage varying at the high voltage side, there result unstable drive state and abnormal noises as described in the Related Background Art when the monitor electrode voltage VM reaches a substantially constant upper limit VM1. On the other hand, in a similar investigation with the drive voltage varying in the lower voltage side, there result unsmooth motor operation and eventual motor stoppage when the monitor electrode voltage VM reaches a substantially constant lower limit VM2 (cf. Fig. 4). The drive speed - drive frequency characteristics and the monitor electrode voltage - drive frequency characteristics at different drive voltages are respectively shown in Figs. 5 and 6. The curves 1, 2 and 3 are obtained with drive voltages increased in this order, wherein the curve 2 corresponds to the aforementioned characteristics shown in Fig. 3. As shown in these drawings, the aforementioned frequencies F2, F3 respectively move to F2H, F3H and the region b moves to a region bH when the drive voltage is made lower. On the other hand, when the drive voltage is shifted higher, these respectively move to F2L, F3L and a region bL. As the drive voltage substantially reaches the upper or lower limit respectively when the monitor electrode voltage VM is at VM1 or VM2, as explained above, a control to maintain a condition VM2 ≦ VM ≦ VM1 under detection of the monitor electrode voltage allows to limit the drive voltage between the upper and lower limits, thereby driving the ultrasonic wave motor in stable manner. This drive control method for the ultrasonic wave motor will be explained later in the 1st example. Then, in case transformers or coils are connected to the input electrodes, the upper and lower limits of the drive voltage can be determined as shown in Fig. 7 by investigating the relationship between the drive state of the motor and the drive voltage in advance, and the drive voltage can be controlled between said upper and lower limits to realize stable drive control of the ultrasonic wave motor. Such drive control method will be explained in the 2nd example. In case the motor is driven with a constant drive frequency, the drive voltage providing stable drive state at said drive frequency is determined from Figs. 5, 6 and 7. In case the motor is driven with a variable drive frequency within a limited range, the drive voltage providing stable drive state within such range is determined from Figs. 5, 6 and 7. Such drive control method for the ultrasonic wave motor will be explained in the 3rd example. [1st example] Fig. 8 is a circuit diagram of the 1st example. A voltage detecting circuit 1 is composed of a diode 11, a resistor 12 and a capacitor 13, and, being connected at the input terminal thereof to the monitor electrode, converts the AC output VM of said monitor electrode into a DC voltage V0. An upper limit detecting circuit 2 is composed of a voltage comparator 21 and a variable resistor 22, and the (+) and (-) input terminals of the voltage comparator 21 are respectively connected to the output of the variable resistor 22 and the output of the voltage detecting circuit 1. The variable resistor 22 is connected between a power supply and the ground, and the output thereof is set equal to the output voltage V01 of the voltage detecting circuit 1 when the monitor voltage VM is equal to VM1. Consequently the upper limit detecting circuit 2 releases a high-level output when the output V0 of the voltage detecting circuit 1 satisfies a condition V0 > V01, namely the monitor voltage VM meets a condition VM > VM1, but a low-level output when V0 ≦ V01 or VM ≦ VM1. A lower limit detecting circuit 3 is composed of a voltage comparator 31 and a variable resistor 32, and the (+) and (-) input terminals of the voltage comparator 31 are respectively connected to the output of the voltage detecting circuit 1 and the output of the variable resistor 32. The variable resistor 32 is connected between the power supply and the ground, and the output thereof is set equal to the output voltage V02 of the voltage detecting circuit 1 when the monitor voltage VM is equal to VM2. Consequently the lower limit detecting circuit 3 releases a low-level output when the output V0 of the voltage detecting circuit 1 satisfies a condition V0 < V02, namely when the monitor voltage VM meets a condition VM < VM2, or a high-level output when V0 ≧ V02 or VM ≧ VM2. A drive voltage setting circuit 4 is composed of a multiplexer (MPX) 41, resistors 42, 44, variable resistors 43, 45, a capacitor 46 and a variable output power supply 47. The multiplexer 41 switches signal inputs A, B, C and D according to the signal levels entered into control inputs X, Y and Z as shown in Fig. 9. Said control input terminals X, Y, Z are respectively connected to the output of the upper limit detecting circuit 2, output of the lower limit detecting circuit 3 and a start input signal. The resistor 42 is connected between the power supply and the input terminal A of the multiplexer 41 and serves to set the lower limit of the drive voltage. The variable resistor 43 is connected between the power supply and the ground and has an output connected to the input terminal B of the multiplexer 41, thereby varying the drive voltage according to the voltage set therein. The resistor 44 is connected between the input terminal C of the multiplexer 41 and the ground and serves to set the upper limit of the drive voltage. The variable resistor 45 is connected between the power supply and the ground and has an output connected to the input terminal D of the multiplexer 41, thereby setting the drive voltage at the start of the ultrasonic wave motor. The capacitor 46 is connected, at a terminal thereof, to the output of the multiplexer 41 and the input to the variable output power supply 47 and is grounded at the other terminal, and holds the output voltage of the multiplexer 41. The variable output power supply 47 releases a DC voltage proportional to the input signal level. A drive frequency setting circuit 5 is a known oscillator generating a cyclic signal, and its output frequency is set within a region b, shown in Fig. 3, where the ultrasonic wave motor can function stably. The output of said circuit 5 is supplied to a phase shifting circuit 6, which generates two cyclic signals of a mutual phase difference of π/2. A power amplifier 7 amplifies two cyclic signals supplied from the phase shifting circuit 6 to the voltage supplied from the variable output power supply 47, and applies said cyclic signals to start electrodes 100-4a, 100-4b of the ultrasonic wave motor only when the start input signal is at a high-level state. In the following there will be explained the function of the 1st example. When the start input signal is at the low-level state, the multiplexer 41 selects the input D as shown in Fig. 9, whereby the variable output power supply 47 supplies the power amplifier 7 with a drive voltage at the start of the ultrasonic wave motor set by the variable resistor 45. When the start input signal is shifted to the high-level state for starting the motor, the cyclic drive signals generated by the drive frequency setting circuit 5 and subjected to a mutual phase shift of π/2 by the phase shifting circuit 6 are amplified to the above-mentioned start voltage by the power amplifier 7 and supplied to the ultrasonic wave motor. Thus the motor is started by the starting drive voltage set by the variable resistor 45. When the ultrasonic wave motor is started, the monitor electrode 100-4d generates a monitor voltage VM, which is fed back to the voltage detecting circuit 1, converted into a DC voltage V0 therein and supplied to the upper-and lower limit detecting circuits 2, 3. If said DC voltage V0 satisfies a relation V02 ≦ V0 ≦ V01, namely if the monitor voltage VM satisfies a condition VM2 ≦ VM ≦ VM1, said detecting circuits 2, 3 both release high-level output signals, whereby the multiplexer 41 selects the input B as shown in Fig. 9. Consequently the drive voltage for the ultrasonic wave motor is determined by the voltage set by the variable resistor 43, and the drive speed of the motor can be regulated by varying said set voltage. Then, when the variable resistor 43 is shifted to a higher voltage, the drive voltage increases accordingly. When the drive voltage reaches the upper limit, the output voltage V0 of the voltage detecting circuit 1 becomes larger than V01, namely the monitor voltage VM becomes larger than VM1, so that the upper limit detecting circuit 2 releases a low-level output signal and the multiplexer 41 selects the input C as shown in Fig. 9. Consequently the potential of the capacitor 46, which is charged to the voltage of the input B, which is set by the variable resistor 43 and has been selected up to immediately before, is gradually lowered by discharge through the resistor 44, and the output voltage of the variable output power supply 47 decreases accordingly, thus reducing the drive voltage for the motor. When the monitor voltage VM is accordingly lowered to satisfy a condition VM ≦ VM1, the output voltage V0 of the voltage detecting circuit 1 satisfies a condition V0 ≦ V01, whereby the upper limit detecting circuit 2 again releases a high-level output signal. Consequently the multiplexer 41 selects the input B, or the voltage set by the variable resistor 43, whereby the drive voltage is again elevated. The voltage detecting circuit 1, the upper limit detecting circuit 2 and the drive voltage setting circuit 4 maintain the monitor voltage VM so as to satisfy a condition VM ≦ VM1 by repeating the above-explained operation, so that the drive voltage does not exceed the upper limit and the ultrasonic wave motor is stably driven. Then, when the variable resistor 43 is shifted to a lower voltage, the drive voltage decreases accordingly. When the drive voltage reaches the lower limit, the output voltage V0 of the voltage detecting circuit 1 becomes smaller than V02, namely the monitor voltage VM becomes smaller than VM2, so that the lower limit detecting circuit 3 releases a low-level output signal and the multiplexer 41 selects the input A as shown in Fig. 9. Consequently the potential of the capacitor 46, which is charged to the voltage of the input B, which is set by the variable resistor 43 and has been selected up to immediately before, is gradually elevated by charging by the power supply through the resistor 42, thus elevating the drive voltage for the motor. When the monitor voltage VM is accordingly elevated to satisfy a condition Vm ≧ VM2, the output voltage V0 of the voltage detecting circuit 1 satisfies a condition V0 ≧ V02, whereby the lower limit detecting circuit 3 again releases a high-level output signal. Consequently the multiplexer 41 selects the input B, whereby the drive voltage is again lowered. The voltage detecting circuit 1, the lower limit detecting circuit 3 and the drive voltage setting circuit 4 maintain the monitor voltage VM so as to satisfy a condition VM ≧ VM2 by repeating the above-explained operation, so that the drive voltage does not become smaller than the lower limit and the ultrasonic wave motor is stably driven. As explained in the foregoing, the control based on the detected monitor voltage VM so as to satisfy a condition VM2 ≦ VM ≦ VM1 allows to limit the drive voltage between the upper and lower limits, thereby realizing stable drive of the ultrasonic wave motor. In the above-explained 1st example, in case the monitor voltage VM becomes larger than VM1 or smaller than VM2, the input of the multiplexer 41 is switched to charge or discharge the capacitor 46, thereby varying the output voltage of the variable output power supply 47 and thus controlling the drive voltage for the ultrasonic wave motor, but it is also possible to intercept the output of the multiplexer 41, thereby retaining the immediately preceding voltage by said capacitor 46. In such case the output of the multiplexer 41 is connected again when a condition VM2 ≦ VM ≦ VM1 is restored. [2nd example] Fig. 10 is a circuit diagram of a 2nd example, which is same as the 1st example except that the input terminal of the voltage detecting circuit 1 is connected to the drive electrode 100-4a instead of the monitor electrode 100-4d and that the output of the power amplifier 7 is connected to the drive electrodes 100-4a, 100-4b through coils 8, and such same portions will not be explained further. In case of driving power supply to the drive electrodes 100-4a, 100-4b through coils, the drive voltage varies depending on the drive frequency even if the output voltage of the power amplifier 7 is maintained constant, as explained before. In such case, the control can be made as in the 1st example, by detecting the drive voltage from the drive electrode 100-4a as shown in Fig. 10 and feeding it back to the voltage detecting circuit 1. The settings of the variable resistors 22, 32 and the resistances of the resistors 42, 44 for respectively determining the upper and lower limits of the drive voltage can be suitably selected in experimental manner. In this 2nd example, the drive voltage is directly fed back to the voltage detecting circuit 1 for detecting the upper and lower limits, and is lowered or elevated respectively when the upper or lower limit is reached, Consequently the drive voltage is constantly controlled between the upper and lower limits, and the ultrasonic wave motor can be driven in stable manner (cf. Fig. 7). [3rd example] Fig. 11 is a circuit diagram of a 3rd example, wherein same components as those in the 1st example are represented by same symbols and will not be explained further. A drive frequency setting circuit 5A is composed of a multiplexer 51, resistors 52, 54, variable resistors 53, 55, a capacitor 56 and a voltage-controlled oscillator (VC0) 57. The multiplexer 51 is constructed same as the multiplexer 41 in the 1st example, and, as shown in Fig. 9, switches the signal inputs A, B, C and D according to the signal levels supplied to the control inputs X, Y and Z. The resistors 52, connected between the input A of the multiplexer 51 and the ground, sets a drive frequency for maintaining the drive voltage for the motor at the lower limit or elevating said drive voltage. The variable resistor 53 is connected between the power supply and the ground, and has an output connected to the input B of the multiplexer 51, thereby varying the drive frequency according to the set voltage. The resistor 54, connected between the power supply and the input C of the multiplexer 51, sets a drive frequency for maintaining the drive voltage for the motor at the upper limit or decreasing said drive voltage. The variable resistor 55 is connected between the power supply and the ground, and has an output connected to the input D of the multiplexer 51, thereby varying the drive frequency at the start of the ultrasonic wave motor. The capacitor 56 is connected, at a terminal thereof, to the output of the multiplexer 51 and the input of the voltage-controlled oscillator 57, and, at the other terminal, to the ground, and retains the output of the multiplexer 51. The voltage-controlled oscillator 57 generates a cyclic signal of a frequency proportional to the input voltage. The power amplifier 7 amplifies the cyclic output signals of the phase shifting circuit 6 to a constant drive voltage, and applies thus amplified signals to the drive electrodes 100-4a, 100-4b only when the start input signal is at the low level state. Also if the relationship between the output voltage of the variable output power supply and the drive voltage or the monitor voltage is experimentally investigated in advance, there may be detected the output voltage of the variable output power supply instead of the drive voltage or the monitor voltage. In the following explained is the function of the 3rd example. When the start input signal is at the low-level state, the multiplexer 51 selects the input D as shown in Fig. 9, whereby the voltage-controlled oscillator 57 supplies the phase shifting circuit 6 with a cyclic signal for motor starting : set by the variable resistor 55. In response the phase shifting circuit 6 generates two cyclic signals of a mutual phase shift of π/2 and said two signals to the power amplifier 7. The variable resistor 55 is set at a stable drive frequency in the region b shown in Fig. 3. When the start input signal is shifted to the high-level state for starting the motor, the power amplifier 7 amplifies the cyclic signals from the phase shifting circuit 6 and applies the amplified signals to the drive electrodes 100-4a, 100-4b. Thus the motor is started by the starting drive voltage set by the variable resistor 55. When the ultrasonic wave motor is started, the monitor electrode 100-4d generates a monitor voltage VM, which is fed back to the voltage detecting circuit 1, converted into a DC voltage V0 therein and supplied to the upper and lower limit detecting circuits 2, 3. If said DC voltage V0 satisfies a relation V02 ≦ V0 ≦ V01, namely if the monitor voltage VM satisfies a condition VM2 ≦ VM ≦ VM1, said detecting circuits 2, 3 both release high-level output signals, whereby the multiplexer 51 selects the input B as shown in Fig. 9. Consequently the drive frequency for the ultrasonic wave motor is determined by the voltage set by the variable resistor 53, and the drive speed of the motor can be regulated by varying said set voltage. Then, when the variable resistor 53 is shifted to a lower voltage, the drive frequency decreases and the drive speed increases accordingly. Also the monitor voltage VM increases in proportion to the drive speed. When the monitor voltage VM exceeds VM1, namely when the output voltage V0 of the voltage detecting circuit 1 exceeds V01, the upper limit detecting circuit 2 releases a low-level output signal, whereby the multiplexer 51 selects the input C as shown in Fig. 9. Consequently the potential of the capacitor 56, charged to the voltage of the input B selected immediately before, namely the voltage set by the variable resistor 53, is gradually elevated by charging from the power supply through the resistor 54, whereby the voltage-controlled oscillator 57 accordingly increases the output frequency. Consequently the drive frequency for the ultrasonic wave motor is increased and the drive speed thereof is reduced. As the monitor voltage VM is lowered to a state VM ≦ VM1 by the reduction of the drive speed, the output voltage V0 of the voltage detecting circuit 1 reaches a stateup V0 ≦ V01, whereby the upper limit detecting circuit 2 again releases a high-level output. Consequently the multiplexer 51 selects the input B, or the voltage set by the variable resistor 53, whereby the drive frequency is lowered and the drive speed of the motor is increased. The voltage detecting circuit 1, the upper limit detecting circuit 2 and the drive frequency setting circuit 5A maintain the monitor voltage VM in a state VM ≦ VM1 by repeating the above-explained procedure, so that the drive voltage does not exceed the upper limit. When the variable resistor 53 is shifted to a higher voltage, the drive frequency increases and the drive speed decreases accordingly. Also the monitor voltage VM decreases in proportion to the drive speed. When the monitor voltage VM reaches a state VM < VM2 or when the output voltage V0 of the voltage detecting circuit 1 reaches a state V0 < V02, the lower limit detecting circuit 3 releases a low-level output, whereby the multiplexer 51 selects the input A. Consequently the potential of the capacitor 56, charged to the voltage of the input B selected immediately before, namely voltage set by the variable resistor 53, is gradually lowered by discharge through the resistor 52, whereby the voltage-controlled oscillator 57 accordingly decreases the output frequency. Consequently the drive frequency for the ultrasonic wave motor is decreased and the drive speed thereof is increased. As the monitor voltage VM is elevated to a state VM ≧ VM2 by the increase of the drive speed, the output voltage V0 of the voltage detecting circuit 1 reaches a state V0 ≧ V02, whereby the lower limit detecting circuit 3 again releases a high-level output. Thus the multiplexer 51 selects the input B, or the voltage set by the variable resistor 53, and the drive frequency increases while the drive speed decreases. The voltage detecting circuit 1, the lower limit detecting circuit 3 and the drive frequency setting circuit 5A control the monitor voltage VM so as to maintain a state VM ≧ VM2 by repeating the above-explained procedure, so that the drive voltage does not becomes smaller than the lower limit. As explained in the foregoing, the drive frequency is controlled so as to maintain the drive voltage between the upper and lower limits, so that the ultrasonic wave motor can be stably driven. [4th example] The drive device of the present invention for the ultrasonic wave motor is also applicable in feedback speed control by detecting the speed of the rotor 100-1 of the ultrasonic wave motor. In such case, the variable resistor 43 of the 1st example shown in Fig. 8 is replaced by a pulse generator 130 and a speed setting circuit 9 shown in Fig. 12. The pulse generator 130 is coupled with the rotor 100-1 of the ultrasonic wave motor and generates pulses in response to the speed of said rotor 100-1. The drive speed setting circuit 9 is composed of an F/V converter 91, a reference voltage supply 92, and an amplifier 93. The F/V converter 91 converts the pulse signal from the pulse generator 130 into a voltage signal proportional to the frequency of said pulse signal and constituting a speed feedback voltage signal, for supply to the amplifier 93. The reference voltage supply 92 releases a constant voltage, corresponding to the drive speed instruction for the ultrasonic wave motor. The amplifier 93 determines and amplifies the difference between the voltage signal from the F/V converter 91 and that from the reference voltage supply 92. In the following explained is the function of the above-explained circuit. When the monitor voltage VM is in a state VM2 ≦ VM ≦ VM1, namely when the drive voltage of the motor is between the upper and lower limits, the multiplexer 41 selects the input B. The pulse generator 130 sends a pulse signal of a frequency corresponding to the drive speed of the motor to the F/V converter 91, which converts said pulse signal into a voltage signal, namely speed feedback voltage signal for supply to the amplifier 93. Said amplifier 93 calculates the error between the speed instruction signal from the reference voltage supply 2 and the above-mentioned speed feedback signal, and amplifies said error. The amplified error signal is supplied through the multiplexer 41 to the variable output power supply 47 whereby the drive voltage for the ultrasonic wave motor is controlled in the same manner as in the 1st example. In case the monitor voltage VM is in a state VM > VM1 or VM < VM2, the drive voltage for the ultrasonic wave motor is controlled between the upper and lower limits by the voltage detecting circuit 1, the upper limit detecting circuit 2 and the lower limit detecting circuit 3 as in the 1st example, whereby the motor is driven in stable manner. In the foregoing examples, the upper limit detecting means is constituted by the voltage detecting circuit 1 and the upper limit detecting circuit 2; the lower limit detecting means by the voltage detecting circuit 1 and the lower limit detecting circuit 3; the first voltage limiting means by the multiplexer 41, resistor 42, capacitor 46 and variable output power supply 47; the second voltage limiting means by the multiplexer 41, resistor 44, capacitor 46 and variable voltage power supply 47; the third voltage limiting means by the multiplexer 51, resistor 52, capacitor 56 and voltage-controlled oscillator 57; and the fourth voltage limiting means by the multiplexer 51, resistor 54, capacitor 56 and voltage-controlled oscillator 57.

Claims: A driving device for an ultrasonic wave motor (100) comprising a stator in which a vibration wave is generated in its elastic member (100-3) by oscillation of a piezoelectric member (100-4), and a relative motion member maintained in pressure contact with said stator by pressurizing means and adapted to be driven by said vibration wave characterized by upper limit detecting means (1,2) for detecting that a drive voltage has reached an upper limit; lower limit detecting means (1,3) for detecting that the drive voltages has reached a lower limit; and drive voltage limiting means (4) for controlling, responsive to a detection of said lower or upper limit, the drive voltage of the ultrasonic wave motor (100) in such a manner that it does not exceed said limits. A driving device for an ultrasonic wave motor (100) comprising a stator in which a vibration wave is generated in its elastic member (100-3) by oscillation of a piezoelectric member (100-4); a relative motion member maintained in pressure contact with said stator by pressurizing means and adapted to be driven by said vibration wave; drive frequency setting means (5,5a) for controlling a drive frequency of the ultrasonic wave (100); and upper limit detecting means (1,2) for detecting that a drive voltage has reached an upper limit; characterized by lower limit detecting means (1,3) for detecting that the drive voltage has reached a lower limit; and wherein the drive frequency is controlled, response to a detection of said lower or upper limit, in such a manner that the drive voltage does not exceed said limits.

Inventors: SUGANUMA RYOICHI, SUGANUMA, RYOICHI
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1991
Main classification: H02N2
IPC codes: H01L41, H02N2