JP5408978B2 - Ultrasonic motor speed control device and speed control method - Google Patents

Description

  The present invention relates to an ultrasonic motor speed control device and the like that can suppress power consumption even during low-speed rotation.

  An ultrasonic motor is a motor that rotates using mechanical resonance of a built-in piezoelectric element. In an ultrasonic motor, an A-phase supply signal and a B-phase supply signal having a phase difference of 90 degrees are supplied to a piezo element through a coil to generate distortion in the piezo element and convert distortion energy into rotational motion.

  Resonance occurs not only with mechanical resonance but also with electricity, and resonates when a coil and a capacitor are combined. For this reason, the piezoelectric element is expressed by the circuit shown in FIG. 13 as an electrical equivalent circuit of the piezoelectric element.

  In FIG. 13, reference numeral 100 denotes a piezo element. A capacitor 102 included in the piezo element 100 is a capacitor indicating interelectrode capacitance, and is a capacitance component that does not contribute to mechanical resonance. A series circuit of resistors, coils, and capacitors indicated by 103, 104, and 105 is called an RLC series resonance circuit. When the coil 104 of the series resonance circuit and the capacitor 105 are electrically resonated, mechanical resonance occurs and distortion occurs in the piezo element.

  The electrical resonance generated in the capacitor 105 and the coil 104 depends on the frequency of the drive signal supplied via the external coil 101. The frequency of the drive signal at which the capacitor 105 and the coil 104 resonate most is represented by f = 1 / (2π√LC).

  As an example, in the ultrasonic motor used by the applicant, the capacitor 105 is 1.2 nF and the coil 104 is 15.8 mH. The resonance frequency of the ultrasonic motor in this example is calculated to be 36.55 KHz. Hereinafter, this resonance frequency will be referred to as a series resonance frequency.

  The closer the frequency of the drive signal is to the series resonance frequency, the greater the electrical resonance, and hence the greater the mechanical resonance. Further, the ultrasonic motor rotates faster as the frequency of the drive signal approaches the series resonance frequency, and rotates slower as it moves away. The ultrasonic motor is driven at a frequency higher than the series resonance frequency. The drive signal for driving the ultrasonic motor is usually a square wave. An external coil 101 is used to smooth the square wave into a sine wave and supply it to the ultrasonic motor.

  Electrical resonance also occurs in the external coil 101 and the capacitor 102, which is a capacitance between electrodes.

  In the ultrasonic motor as an example, the capacitor 102 which is a capacitance between electrodes is 131 nF, and the external coil 101 is 56 uH. The resonance frequency in this example is 58.76 KHz when calculated by the calculation formula of f = 1 / (2π√LC). Hereinafter, this resonance frequency will be referred to as an external resonance frequency.

  FIG. 14 is a graph showing the frequency dependence of current by simulating an equivalent circuit of the piezoelectric element in the example shown in FIG. In FIG. 14, the horizontal axis indicates the frequency, which is 34 KHz to 69 KHz. The vertical axis represents the current, which is a maximum of 7A.

  110 is a current increase at a series resonance frequency of 36.55 KHz. Reference numeral 111 denotes a current increase at an external resonance frequency of 58.76 KHz.

  At a frequency higher than the series resonance frequency 110, first, the current rapidly decreases, and as the frequency increases, the current gradually increases and a current peak occurs again at the external resonance frequency. As described above, the closer to the series resonance frequency, the greater the mechanical resonance, and the ultrasonic motor rotates faster. The further away from the series resonance frequency, that is, the higher the frequency, the smaller the mechanical resonance and the slower the rotation of the ultrasonic motor.

  FIG. 18 is a graph showing the frequency dependency of the current, as in FIG. 14, in which the horizontal axis is 36 KHz to 44 KHz, the vertical axis is 1 A at maximum, and the graph of FIG. 14 is enlarged.

  For example, when the ultrasonic motor is rotated once per minute, that is, at 1 rpm, the drive frequency is around 44 KHz, and the current is about 0.47 A as indicated by 127. The current at the time of high speed rotation is about 0.15 A as indicated by 126, and the current 0.47 A at 1 rpm is three times or more.

  In an apparatus using an ultrasonic motor, a current at 1 rpm, about 0.47 A, is a large current in, for example, a battery-driven portable device and a digital camera, and therefore, a low-speed drive such as 1 rpm cannot be adopted. Alternatively, even when a power supply is supplied from a network cable, such as a monitoring camera using Ethernet (registered trademark), the current is a large current that cannot be used for low-speed driving.

  As described above, the increase in current at the time of low-speed driving is a limitation on specifications in a device using an ultrasonic motor.

  As a prior art for limiting the current at low speed rotation, in Patent Document 1, when the rotation speed is low, the frequency is fixed, and the speed is controlled by changing the voltage to increase the current due to the increase in the frequency of the drive signal. Restricted.

  In Patent Document 2, the upper limit of the drive frequency is stored, and when the upper limit frequency is reached, the pulse width of the ON time of the drive signal is decreased to enable low-speed rotation without increasing the current.

  In Patent Document 3, low-speed rotation is enabled by periodically changing the phases of a plurality of drive signals supplied to the piezo elements.

JP 09-247966 A JP 09-84369 A JP 2004-304947 A

  However, in the above prior art, in Patent Document 1, speed control is performed by changing the voltage of the drive signal, so a circuit for making the voltage variable is necessary, which causes an increase in cost.

  In Patent Document 2, by reducing the duty of the drive signal (pulse width of the ON time), the voltage waveform supplied to the piezo is distorted.

  FIG. 15 shows a simulation waveform when the duty of the drive signal is reduced to ¼. In FIG. 15, reference numeral 120 denotes a waveform of a drive signal supplied to the coil 101 shown in FIG. Reference numeral 121 denotes an output side of the coil 101 and a voltage waveform supplied to the piezo element 100.

  As shown by the voltage waveform 121, if the duty of the drive signal is too small, the voltage waveform supplied to the piezo is distorted, and the ultrasonic motor does not move or annoying noise is generated.

  In Patent Document 3, the phases of the A phase and the B phase are changed at a constant cycle, but noise is generated when the cycle of changing the phase becomes an audible frequency band.

  In addition to the above prior art, as a possible current reduction method, a method of reducing the value of the external coil 101 shown in FIG. 13 and making the external resonance frequency 111 shown in FIG.

  As described in the background art, the external resonance frequency is calculated by f = 1 / (2π√LC) by the external coil 101 in FIG. 13 and the interelectrode capacitance (of the capacitor 102). When the external coil 101 is 56 uH, the external resonance frequency is 58.76 KHz as described above. When the value of the external coil 101 is reduced to 10 uH, for example, the external resonance frequency is 139 KHz. It becomes √ (56uH / 10uH) times 58.76 KHz. When the external resonance frequency becomes high, the current in the drive frequency range shown in FIG. 18 is suppressed.

  However, if the value of the external coil 101 is simply reduced, the waveform will be distorted as in the case where the duty shown in Patent Document 2 is reduced.

  16 and 17 show voltage waveforms supplied to the piezo element 100 when a drive signal of 37 KHz is applied to the external coil 101 shown in FIG.

  FIG. 16 shows a waveform when the external coil is 10 uH, and FIG. 17 shows a waveform when the external coil is 56 uH.

  In FIG. 17, the voltage waveform 125 supplied to the piezo element by applying the drive signal 124 is substantially a sine wave. In FIG. 16, the voltage waveform 123 supplied to the piezo element becomes a voltage waveform that cannot be said to be a sine wave even though the drive signal 122 having the same frequency and the same duty as in FIG. 17 is applied. In this case, the ultrasonic motor does not move or causes annoying noise. For this reason, the value of the external coil 101 cannot simply be reduced.

  An object of the present invention is to provide an ultrasonic motor that can be driven at a low speed without increasing the current without the above-described problems.

The ultrasonic motor speed control device according to the present invention includes a PID control unit that determines a frequency signal in a response frequency range of the ultrasonic motor, and another frequency signal that is higher in frequency than the frequency signal by dividing the frequency signal. A frequency signal generating unit that generates a PWM signal having a pulse width controlled based on the other frequency signal, and a sine wave generating unit that generates a sine wave based on the PWM signal. , comprising a constant of the sine wave generating means, said external resonance frequency of the ultrasonic motor, wherein a value such that at least twice the minimum value of the response frequency range, characterized in that.

  In the present invention, it is possible to reduce the current when the ultrasonic motor is driven, particularly when driving at a low speed. For this reason, even in a device that uses a battery drive or a network cable as a power source, low-speed drive can be driven with low current consumption, and the application range of the ultrasonic motor can be expanded. Further, even in a device that already employs an ultrasonic motor, by implementing the present invention, low-speed driving can be driven with low current consumption, and there is an effect that a device considering environmental properties can be commercialized. In the present invention, a constant smaller than that of the conventional coil can be used. By reducing the coil constant, the coil can be reduced in size and price.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described. The ultrasonic motor according to the present embodiment causes electric resonance by supplying a sine wave to a built-in piezoelectric element, and the electric resonance is converted into mechanical resonance. The built-in piezo is divided into a plurality of areas, and by supplying sinusoidal signals (frequency signals) having a phase difference of 90 degrees to the divided areas, the mechanical resonance becomes a rotational motion, and the ultrasonic motor rotates.

  In the following description, a signal output from the drive circuit and supplied to the external coil (101 in FIG. 13) will be referred to as a drive signal. A signal supplied to the ultrasonic motor via the external coil will be referred to as a USM supply signal. Also, USM supply signals having a phase difference of 90 degrees will be described as an A phase supply signal and a B phase supply signal.

  Note that the USM supply signal supplied to the ultrasonic motor must be a sine wave as described above. However, the drive signal applied to the external coil 101 in FIG. 13 described in the prior art may not be a sine wave. In the prior art, it is a square wave.

  In the present embodiment, as shown in the timing chart of FIG. 2, the external coil 101 shown in FIG. 13 is supplied with a high-frequency PWM signal indicated by 32 as a drive signal and is integrated by the coil to generate ultrasonic waves. A sine wave drive signal 30 is supplied to the motor.

  In the timing chart shown in FIG. 2, 32 PWM signals are PWM signals obtained by dividing one period of time 31 into 18 parts. Will be explained.

  FIG. 1 shows a waveform simulating a drive signal generated via the external coil 101 based on a high-frequency PWM signal.

  In FIG. 1, 40 indicates a drive signal, and 41 indicates a USM supply signal. As shown in the USM supply signal 41, a substantially sine wave is generated and supplied to the ultrasonic motor. When a sine wave is supplied as the USM supply signal, the ultrasonic motor resonates, the electric resonance is converted into mechanical resonance, and the A-phase supply signal and the B-phase supply signal that are 90 degrees out of phase are converted into the ultrasonic motor. In this embodiment, rotational movement is possible.

  In FIG. 1, the simulation was performed by setting the value of the external coil 101 shown in FIG. 13 to 10 uH. In the conventional example, as shown in FIG. 16, when the coil is made small, distortion occurs in the USM supply signal. However, in the present embodiment, a sine wave can be generated almost as shown in FIG. 1, and there is no waveform distortion as shown in FIG. 16, thus solving the problem in the conventional example.

  5 and 6 show the current in the square wave driving of the prior art and the current in the driving method in the present embodiment. The horizontal axis is frequency and the vertical axis is current.

  In FIG. 5, 60 is a square wave drive, and shows the current when a 56 uH coil is used. Similar to the current curve shown in FIG. 14, there is a current peak at 58.76 KHz.

Reference numeral 61 denotes a driving method according to the present embodiment, which indicates a current when a 10 uH coil is used.
There is a current peak at 139 KHz.

  FIG. 6 is a current curve obtained by enlarging and displaying the current curve of FIG. 5 in the drive frequency range (response frequency range) from 36.7 KHz to 44 KHz. At the upper limit of the driving frequency of 44 KHz, in the square wave driving of the conventional method, it was about 0.47 A as shown by 62, but according to the driving method using this embodiment, it is about 0.23 A and halved. .

  In this embodiment, even when the frequency of the drive signal becomes high (44 KHz in the example of FIG. 6) during low-speed driving, the increase in current is reduced, and the object of the present invention is achieved.

  As described in conjunction with FIG. 16 as the problem of the prior art, in the square wave driving of the prior art, there is a problem that the waveform is distorted when the value of the external coil is reduced. When the driving frequency by square wave driving is set to a frequency smaller than ½ of the external resonance frequency, the USM supply signal starts to be distorted. For this reason, it is necessary to set the external coil (101 in FIG. 13) so that the external resonance frequency is not more than twice the minimum frequency of the drive frequency.

  According to an example of the prior art, the external resonance frequency is the resonance frequency of the coil 101 and the capacitor 102 illustrated in FIG. 13 and is 58.76 KHz.

  According to an example of the prior art, as shown in FIG. 14, the drive frequency range is 36.7 KHz to 44 KHz, and the minimum frequency is 36.7 KHz. That is, 58.76 KHz <36.7 KHz × 2 is satisfied.

  In this embodiment, even if the external coil is made small, waveform distortion does not occur as in the case of square wave driving according to the prior art. In other words, the external resonance frequency may be set to be twice or more the minimum frequency in the drive frequency range. For this reason, according to the present embodiment, the external coil is reduced to 10 uH and the external resonance frequency is set to 139 KHz. That is, 139 KHz> 36.7 KHz × 2.

  FIG. 3 is a block diagram of the ultrasonic motor speed control apparatus according to the first embodiment. In FIG. 3, reference numeral 10 denotes a power supply circuit which supplies a power supply such as 3.3 V to the logic circuit of each control unit, and supplies a motor driving voltage to the A-phase bridge circuit 16 and the B-phase bridge circuit 23. Supply.

  Reference numeral 11 denotes a clock pulse generator (CPG) that supplies a clock pulse serving as a basic timing of processing timing to each control unit. A parameter setting unit 12 obtains acceleration time, deceleration time, speed at constant speed, and the like based on target position information received from an external device, and supplies it to the PID control unit 13.

  The PID control unit 13 is a control unit that obtains a driving cycle based on parameters from the parameter setting unit 12 and speed information from the speed detection unit 21. If the speed information from the speed detection unit 21 is lower than the target speed, the PID control unit 13 performs control so that the drive cycle is lengthened and the drive frequency is lowered. If the speed is higher than the target speed, control is performed so as to shorten the driving cycle (maximum pulse width) and increase the driving frequency.

  Here, the drive cycle determined by the PID control unit 13 is a cycle determined within the drive frequency range (in the example, 36.7 KHz to 44 KHz) of FIG. 14 described in the related art.

  For example, when the driving frequency is 36.7 KHz, the driving cycle required by the PID control unit 13 is 1 / 36.7 KHz = 27.25 uS. The period dividing unit 14 is a processing unit that finely divides the driving period obtained by the PID control unit 13 to generate a high frequency.

  In the present embodiment, the drive period is divided into 32 by the period dividing unit 14. For example, when the drive period obtained by the PID control unit 13 is 27.25 uS, 27.25 uS ÷ 32 = 0.851 uS. It becomes.

  The PWM control unit 15 is a control unit that generates a subdivided PWM signal based on the duty data previously written in the 22 look-up tables (LUT). In order to distinguish the output signals of the A-phase bridge circuit 16 and the B-phase bridge circuit 23 supplied to the coils from the drive signals, the output from the PWM control unit 15 is described as a PWM signal and will be described below.

  Reference numeral 16 denotes an A-phase bridge circuit, which amplifies the PWM signal to output a drive signal and energizes the A-phase piezo element 19 of the ultrasonic motor via the coil 17. Reference numeral 23 denotes a B-phase bridge circuit, which amplifies the PWM signal and outputs a drive signal, and energizes the B-phase piezo element 24 of the ultrasonic motor via the coil 18.

  The USM supply signal energized to the A-phase piezo element 19 of the ultrasonic motor and the B-phase piezo element 24 of the ultrasonic motor is a sine wave obtained by integrating the PWM signal as indicated by 41 in FIG. Further, the B phase supply signal energized to the B phase piezo element 24 of the ultrasonic motor is a B phase supply signal that is 90 degrees behind the A phase supply signal energized to the A phase piezo element 19. It controls by the PWM control part 15 so that it may become.

  Reference numeral 20 denotes an encoder, which generates encoder pulses in accordance with the rotation of the ultrasonic motor. When the speed is low, the cycle of the encoder pulse becomes long, and when the rotation speed is high, the cycle of the encoder pulse becomes short.

  The speed detector 21 is a circuit that calculates a speed based on the encoder pulse. The processing of the PWM control unit 15 and the operations of the A-phase bridge circuit 16 and the B-phase bridge circuit 23 will be described with reference to the timing diagram of FIG. 2 and the bridge circuit diagram of FIG.

  In the bridge circuit diagram of FIG. 4, the same blocks as those in FIG. In the timing diagram of FIG. 2, 30 indicates a USM supply signal supplied to 19 ultrasonic motors connected via coils 17 and 18. A cycle 31 of 30 sine waves is a cycle obtained by the PID control unit 13.

  When the drive frequency is, for example, 36.7 KHz, the period 31 of the 30 sine waves is 27.25 uS.

  The signal 32 is a PWM signal generated by the PWM control unit 15. The pulse width is determined and changed according to the data of the LUT 22. One period (34 in the figure) of the 32 PWM signals is the period (maximum pulse width) of the PWM signal obtained by the period dividing unit 14.

  The duty of the PWM signal 32 is stored in the 22 LUTs so as to be proportional to the voltage level of the sine wave 30. When the voltage level is low, the duty is set low, the ON time is shortened, and the OFF time is set long. When the voltage level is high, the duty is increased, the ON time is set longer, and the OFF time is set shorter.

  FIG. 4 is a bridge circuit diagram showing details of the A-phase bridge circuit 16 of FIG. The B-phase bridge circuit 23 is a similar circuit. In the present embodiment, the drive signal is generated by a half bridge as shown in FIG.

  The A-phase bridge circuit 16 amplifies the PWM signal 32 of FIG. 2 and energizes the A-phase of the 19 ultrasonic motors. Based on the PWM signal 32 shown in FIG. 2, the FETs 50 and 52 which are frequency signal generating means in this embodiment for generating a high frequency signal are controlled.

  When the PWM signal 32 is at a high level, the FET 50 is turned on and the FET 52 is turned off. When the FET 50 is turned on, a current flows from the FET 50 through the coil 17 and charges the capacitor 54 which is a capacitance between the electrodes.

  In the simulation of FIG. 1, the potential of the USM supply signal is increasing. When the PWM signal 32 is at a low level, the FET 50 is turned off and the FET 52 is turned on. By turning on the FET 52, the electric charge accumulated in the capacitor 54, which is a capacitance between the electrodes, is discharged according to the low level pulse width.

  By controlling in this way, the sine wave 30 shown in FIG. 2 can be generated as a USM supply signal, and the sine wave can be supplied to 19 USM_A phases (A-phase piezo elements).

  The present embodiment has been described above. In this embodiment, it is possible to generate a sine wave to be supplied to an ultrasonic motor by generating a drive signal based on a high-frequency PWM signal and supplying it to a coil.

  Further, by using a high-frequency PWM signal as a drive signal, the constant of the external coil can be reduced, and the current during low-speed drive can be reduced.

  According to this embodiment, the period dividing unit 14 divides the drive period into 32 parts, but it may be divided into more fine parts such as 48 parts or 64 parts. Alternatively, the high frequency may be generated by superimposing a high frequency such as 1 MHz and increasing or decreasing the number of pulses.

  In this embodiment, the PWM control unit 15 refers to the LUT 22 and uses a method of setting the pulse duty. However, a method of calculating the voltage for each sine wave angle and setting the pulse duty is used. It may be used.

(Second Embodiment)
A second embodiment in which the present invention is implemented will be described below with reference to FIGS. This embodiment is an embodiment in which the half bridge circuit described in FIG. 4 is changed to a full bridge circuit in the first embodiment.

  In FIG. 8, the same elements as those in the bridge circuit shown in FIG. Similarly, in FIG. 8, the same parts as those in FIG.

  In FIG. 7, reference numeral 38 denotes a PWM signal output from the PWM control unit 15 during the period of 36. A current is passed from the FET 50 to the FET 53 shown in FIG. 8 and a drive signal generated via the external coil 17 is added. This is a PWM signal for making a potential.

  Reference numeral 33 in FIG. 7 denotes a PWM signal output from the PWM control unit 15 during the period 37, and a current is passed from the FET 51 to the FET 52 shown in FIG. This is a PWM signal for making a potential.

  The full bridge circuit of FIG. 8 will be described. When the PWM signal 38 is at a high level during the period 36 in FIG. 7, the FETs 50 and 53 are turned on and the FETs 51 and 52 are turned off. By turning on the FETs 50 and 53, a current flows from the FET 50 through the coil 17, and the capacitor 54, which is a capacitance between the electrodes, is charged to a positive potential.

  When the PWM signal 38 is at a low level, the FETs 50 and 51 are turned off and the FETs 52 and 53 are turned on. By turning on the FET 52, the electric charge accumulated in the capacitor 54, which is a capacitance between the electrodes, is discharged according to the low level pulse width.

  The PWM signal 38 generates a portion of the drive signal 30 having a potential higher than the midpoint potential 35 illustrated in FIG. When the PWM signal 33 is at a high level during the period 37 in FIG. 7, the FETs 51 and 52 are turned on and the FETs 50 and 53 are turned off. When the FETs 51 and 52 are turned on, a current flows from the FET 51 via the A-phase piezo element 19 and the coil 17, and the capacitor 54, which is a capacitance between the electrodes, is charged to a negative potential.

  When the PWM signal 33 is at a low level, the FETs 50 and 51 are turned off and the FETs 52 and 53 are turned on. By turning on the FET 52, the electric charge accumulated in the capacitor 54, which is a capacitance between the electrodes, is discharged according to the low level pulse width. The PWM signal 33 generates a portion of the drive signal 30 having a potential lower than the midpoint potential of 35 shown in FIG.

  By controlling in this way, the drive signal generated via the external coil 17 can generate a drive signal that swings positively and negatively with 35 shown in FIG. 7 as a midpoint.

  Compared to the first embodiment, the voltage swing width is doubled. In the present embodiment, when the PWM signal is at a low level, the FETs 52 and 53 are turned on and the charge accumulated in the capacitor 54 that is the interelectrode capacitance is controlled to be discharged. You may control. That is, when the PWM signal is at a low level, all FETs (51, 52, 53, 54) may be controlled to be turned off.

  Alternatively, all the FETs (51, 52, 53, 54) may be controlled to be turned off when the PWM signal is at a low level only during the period of increasing the voltage of the drive signal 30: 42, 44.

  Similarly, when the PWM signal is at a high level, the FETs 50, 53 or 51, 52 are turned on and the capacitor 54, which is the interelectrode capacitance, is controlled to be charged. You may control to. That is, it may be controlled to turn off all the FETs (51, 52, 53, 54) when the PWM signal is at a high level only during the periods 43, 45 during which the voltage of the drive signal 30 is lowered.

(Third embodiment)
Hereinafter, a third embodiment in which the present invention is implemented will be described. In the present embodiment, a circuit for controlling the maximum duty of the PWM signal is added according to the drive frequency, and the current at a low speed is further reduced.

  FIG. 11 shows a block diagram in the present embodiment. The same blocks as those in the block diagram of the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  In FIG. 11, the maximum duty setting unit 73 sets the maximum duty based on the driving cycle required by the PID control unit 13. In the PWM control unit 15, when setting the duty of the PWM signal, it is set based on the data from the LUT 22.

  In the present embodiment, the PWM duty is not set as it is based on the data of the LUT 22, but the maximum duty set by the maximum duty setting unit 73 is multiplied by the multiplier 74. Thereby, the pulse width of the PWM signal can be controlled by the drive cycle obtained by the PID control unit 13.

  FIG. 12 shows an image corresponding to the maximum duty of the PWM signal. Reference numeral 80 denotes a PWM signal when the maximum duty set by the maximum duty setting unit 73 is 100%. 81 denotes a PWM signal having a maximum duty of 75%, and 82 denotes a PWM signal having a maximum duty of 50%. As the duty of the PWM signal decreases, the voltage of the drive signal supplied to the ultrasonic motor via the external coils 17 and 18 decreases.

  The waveform of the USM supply signal is shown in FIG. In FIG. 9, 70 indicates a USM supply signal when the maximum duty is 100%, 71 indicates a USM supply signal when the maximum duty is 75%, and 72 indicates a USM supply signal when the maximum duty is 50%. Due to the maximum duty, the voltage of the USM supply signal is reduced.

  FIG. 10 shows a current curve of the piezoelectric element in the ultrasonic motor according to the frequency. In FIG. 10, the same current curves as those in FIG. In the first embodiment, the current is reduced as indicated by 63 by supplying the PWM signal to the external coil.

  In the present embodiment, by changing the maximum duty of the PWM signal according to the frequency, it is possible to reduce the current as the frequency becomes particularly high, as indicated by 75.

  The third embodiment has been described above. In the present embodiment, by specifying the maximum duty of the PWM signal, the voltage of the USM supply signal can be reduced, and the current at a low speed can be reduced more than in the first embodiment.

  Of course, the method of the present invention can be realized by a program installed in a computer or the like.

It is a timing diagram which shows the USM supply signal in the 1st Embodiment of this invention. It is an image figure which shows the PWM signal and USM supply signal in the 1st Embodiment of this invention. It is a block diagram of the speed control apparatus of the ultrasonic motor in the 1st Embodiment of this invention. It is a circuit diagram of a bridge circuit in a 1st embodiment of the present invention. It is a figure which concerns on the graph which showed the relationship between a frequency (0 to 160 KHz) and an electric current in the 1st Embodiment of this invention. It is a figure which concerns on the graph which shows the relationship between the frequency (36K to 44KHz) and an electric current in the 1st Embodiment of this invention. It is an image figure which shows the PWM signal and USM supply signal in the 2nd Embodiment of this invention. It is a circuit diagram of the bridge circuit in the 2nd Embodiment of this invention. It is the image figure which showed the voltage level of the USM supply signal in the 3rd Embodiment of this invention. It is a figure which concerns on the graph which shows the relationship between a frequency (36K to 44KHz) and an electric current in the 3rd Embodiment of this invention. It is a block diagram of the speed control apparatus of the ultrasonic motor in the 3rd Embodiment of this invention. It is the image figure which showed the PWM signal in the 3rd Embodiment of this invention. It is the figure which showed the piezoelectric element equivalent circuit used for the ultrasonic motor in a prior art. It is a figure which concerns on the graph which showed the relationship between a frequency (34K to 69KHz) and an electric current in a prior art. It is the timing figure which showed the relationship between the drive signal and the USM supply signal in the prior art. It is the timing figure which showed the relationship between the drive signal and the USM supply signal in the prior art. It is the timing figure which showed the relationship between the drive signal and the USM supply signal in the prior art. It is a figure which concerns on the graph which showed the relationship between a frequency (36K to 44KHz) and an electric current in a prior art.

Explanation of symbols

10 Power supply circuit 11 Clock pulse generator (CPG)
12 Parameter setting unit 13 PID control unit 14 Period division unit 15 PWM control unit 16 A phase bridge circuits 17 and 18 Coil 19 A phase piezo element 20 Encoder 21 Speed detection unit 22 LUT
23 B phase bridge circuit 24 B phase piezo element 50-53 FET
54 Capacitor 73 Maximum duty setting part

Claims (3)

PID control means for determining a frequency signal in the response frequency range of the ultrasonic motor;
A frequency signal generating means for generating another frequency signal having a higher frequency than the frequency signal by dividing the frequency signal;
PWM control means for generating a PWM signal whose pulse width is controlled based on the other frequency signal ;
Sine wave generating means for generating a sine wave based on the PWM signal ,
The ultrasonic motor speed control device , wherein the constant of the sine wave generating means is a value such that the external resonance frequency of the ultrasonic motor is at least twice the minimum value of the response frequency range . 2. The maximum duty setting means for determining a maximum pulse width of the PWM signal generated by the PWM control means based on a cycle of the frequency signal generated by the PID control means. Ultrasonic motor speed control device. A PID control step in which the PID control means determines a frequency signal in a response frequency range of the ultrasonic motor;
A frequency signal generating step in which the frequency signal generating means generates another frequency signal having a higher frequency than the frequency signal by dividing the frequency signal;
PWM control means for generating a PWM signal with a pulse width controlled based on the other frequency signal ; and
The sine wave generating unit, on the basis of the PWM signal, possess generating a sinusoidal drive signal, a,
An ultrasonic motor speed control method for controlling the minimum value of the response frequency range to be ½ times or less the external resonance frequency of the ultrasonic motor.

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