JPH0426382A - Driving method for ultrasonic motor - Google Patents

Abstract

PURPOSE:To stabilize the status of an ultrasonic motor at the time of start and during the operation and improve the efficiency of the motor by a method wherein a driving current which is applied to one of a plurality of driving electrodes is controlled so as to be a required predetermined value in a frequency range not lower than the resonant frequency of a piezoelectric transducer unit by controlling the frequency of a driving voltage. CONSTITUTION:An oscillation frequency can be controlled by a voltage control oscillator 7 in accordance with a control voltage applied to its control terminal C. A dividing circuit 8 which divides the output AC signal of the voltage control oscillator 7 into two driving signals having a predetermined phase difference between each other is provided. A driving current which is applied to at least one of two driving electrodes of a piezoelectric transducer unit is controlled so as to be a required predetermined value in a frequency range not lower than the resonant frequency of the piezoelectric transducer unit by controlling the frequency of a driving voltage. With this constitution, the start and operation of an ultrasonic motor 11 in an unstable range can be avoided and, further, the current applied to the driving electrode can be stabilized and the stable start and operation of the motor can be realized.

Description

[Detailed Description of the Invention] Industrial Field of Application The present invention relates to a method for driving an ultrasonic motor that generates driving force by exciting elastic waves in a vibrating body. The conventional technology of an ultrasonic motor and its driving method will be explained.

FIG. 5 ζ Table is a cutaway perspective view of an annular ultrasonic motor, in which a vibrating body 3 is constructed by laminating an annular piezoelectric ceramic 2 as a piezoelectric body to one of the annular surfaces of an annular elastic substrate 1. 4 is a friction material made of a wear-resistant material, and 5 is an elastic body. Even if they are pasted together to form a movable body 6, the movable body 6 is vibrated by a pressurizing means (not shown) via the friction material 4. FIG. 6 shows the drive electrode structure formed on the piezoelectric body 2, which is installed in pressure contact with the piezoelectric body 2.
Even if two sets of drive electrodes A and B are formed with a positional shift of 1/4 wavelength, even if drive electrodes A and B are each made up of a group of small electrodes with a length equivalent to 1/2 wavelength, electrodes C and D
are the lengths equivalent to 3/4 wavelength and 1/4 wavelength, respectively.
Even if drive electrodes A and B are formed to create a positional shift of 1/4 wavelength, if two AC voltages with a phase difference of 90 degrees are applied to drive electrodes A and B, the voltage as shown in Figure 7 will be generated. Radial primary/circumferential 3 with radial displacement distribution
Even if a traveling wave of bending vibration of the following order or more is excited in the vibrating body 3, the movable body 6 is frictionally driven and rotates due to the lateral component of the wave crest of this traveling wave.

The vibrating body exhibits resonance and anti-resonance characteristics when viewed from the drive terminal in the same way as a single piezoelectric body, and has low impedance near the resonance frequency, so if it is driven near the resonance frequency, it can be driven efficiently with low voltage. Therefore, it is important to drive the ultrasonic motor stably near the resonant frequency of the vibrating body. However, as shown in Figure 8, the admittance of the vibrating body of the ultrasonic motor exhibits a unique frequency characteristic due to the nonlinear effect.In other words, as shown by the arrow in Figure 8, the There is an unstable operation region (region from f2 to fI near the resonance frequency) where the admittance exhibits hysteresis in the sweep direction of the drive frequency, and driving an ultrasonic motor in this frequency region causes the problem of unstable operation. This also solves the problem that if the ultrasonic motor is started so that the drive frequency passes through the above unstable region, the moving object will not move or the speed will not be stable at the time of startup. Means for solving the above problem is as follows: The drive current flowing into at least one of the two sets of drive electrodes of the piezoelectric body is controlled by the frequency of the drive voltage to a frequency higher than the resonant frequency of the vibrating body. By controlling the frequency of the driving voltage so that it reaches a predetermined set value in the region, when starting the ultrasonic motor, the frequency of the driving voltage starts sweeping from a frequency higher than the resonant frequency of the vibrating body. When the drive current flowing into at least one of the two sets of drive electrodes reaches a predetermined set value, the frequency sweep of the drive voltage is stopped and the drive voltage is adjusted so that the drive current reaches the predetermined value. By controlling the frequency, the arm effect is achieved.According to the above method, the motor startup and operation are performed avoiding this unstable region, and the current flowing to the drive electrode is stabilized, making stable motor startup and operation possible. be.

EXAMPLES Hereinafter, examples of the present invention will be described in detail with reference to the drawings.

Fig. 1 is a control block diagram in one embodiment of the ultrasonic motor driving method of the present invention, and Fig. 2 shows the frequency characteristics of the drive current flowing into one drive terminal when the ultrasonic motor is driven at a constant voltage. It is.

In Figure 1, 7 is a voltage controlled oscillator, and the oscillation frequency can be controlled by the control voltage at control terminal C.
8 is a dividing circuit that divides the output AC signal of the voltage controlled oscillator 7 into two drive signals having a predetermined phase difference. The output of the dividing circuit 8 is amplified by power amplifiers 9 and 10 to a level sufficient to drive the ultrasonic motor 11, and when applied to each of the two drive terminals of the ultrasonic motor 11, the current flows into one of the drive terminals. is detected by the resistor ]5 and the current detector 12. 13 is an oscillation controller that controls the oscillation frequency of the voltage controlled oscillator 7. Figure 2 shows the frequency characteristics of the drive current corresponding to the frequency characteristics of the admittance of the vibrating body, and the drive current is near the resonance frequency of the vibrating body. The speed of a large ultrasonic motor is proportional to the vibration amplitude, that is, proportional to the drive current, so in order to drive an ultrasonic motor efficiently and stably, it is necessary to It is necessary to always drive in the operating region near the resonant frequency, which is higher than the resonant frequency excluding the region indicated by r2 to f3. Determining the operating speed of the motor Once the operating speed of the ultrasonic motor is determined, the first set value of the corresponding drive current (i+ in Figure 2) in the above drive range is determined, and it is necessary to flow this current value. The driving voltage is determined.

When the ultrasonic motor 11 is started, a start signal is input to the sweep indicator 14, and the sweep indicator 14 controls the voltage controlled oscillator 7 via the oscillation controller 13. It starts oscillating at a frequency sufficiently higher than the stable frequency range, for example f- in the same figure.Then, after that, it sweeps in the direction of the lower driving frequency.
The oscillation controller 13 controls the control voltage at the control terminal C of the voltage controlled oscillator 7.

When the output of the current detector 12 becomes equal to the first drive current setting value 11 which is the normal operating current, the sweep indicator 14
sends a sweep stop signal to the oscillation controller 13, and the oscillation controller 1
3 stops the drive frequency sweep. Then, the oscillation controller 13 controls the voltage controlled oscillator 7 so that the output of the current detector 12 is always equal to the set current 11. Although the ultrasonic motor cannot be driven stably in the operating range near the resonance frequency, which is higher than the resonance frequency, when a sudden overload is applied to the ultrasonic motor, the impedance seen from the drive terminal of the vibrating body suddenly increases and the drive current decreases. In this case, the sweep indicator 1 indicates that the drive current has become smaller than the second set value 11.
4 is detected ('L) The sweep indicator 14 sends a sweep start signal to the oscillation controller 13, and the voltage controlled oscillator 7 (component) oscillates at a frequency sufficiently higher than the unstable frequency region and sweeps the drive frequency lower. When the output of the current detector 12 becomes equal to the first drive current set value 11 which is the normal operating current, the sweep indicator 14 stops sweeping the drive frequency via the oscillation controller 13. The oscillation controller 13 controls the voltage controlled oscillator 7 so that the output of the detector 12 is always equal to the second set current 11 (in this embodiment, the output is always higher than the resonant frequency near the resonant frequency). The characteristics of the ultrasonic motor 11, as shown by the broken line in Figure 2, change due to changes in temperature and load. In response to fluctuations in Set the frequency to fl. 1℃ At this time, due to changes in temperature and load, the characteristics of the ultrasonic motor change as shown by the dotted line in Figure 2, L. The drive frequency changes from fl to f+a. Paper drive current is constant value 1
1, and no problems occurred - In the above embodiment, the force used is the total current flowing through the drive electrode of the piezoelectric material constituting the vibrating body (similarly, it is also easy to use the mechanical arm current). Figure 3 is an equivalent circuit seen from the drive terminal of the vibrating body near the resonance frequency.In the figure, CI in the electric arm is a capacitive element, and C1, Ll, and R1 in the mechanical arm are elastic elements.
It is an equivalent element that represents loss. When a drive voltage is applied to the drive terminal, a current l flows into the drive terminal. This current l is divided into a current i in the electric arm and a current i in the mechanical arm. Since the amplitude is proportional to the current l in the mechanical arm, a more accurate ultrasonic motor can be realized by controlling the mechanical arm current i to a constant value.
A resistor 21 is connected to the drive terminal of 1, and a current 1 is detected by a differential amplifier 16.

Also, a capacitive element 17F whose value is equal to the value of the capacitive element Cs of the electric arm of the ultrasonic motor 11, and a resistor 1 whose value is equal to the resistor 21.
8 are connected in series, and a differential amplifier 19 calculates the electric arm current i.
The mechanical arm current i- is detected by subtracting the current i from the current l using the differential amplifier 20.The mechanical arm current detector in FIG. If used, a more precise ultrasonic motor can be realized as well, but the force may be two or more sets of driving electrodes (although there were two sets in the embodiment), and even in that case, the force flows into at least one electrode. Good operation of the ultrasonic motor can be achieved by controlling the current in the same manner as in the embodiment. Effects of the Invention As described above, the driving method of the present invention allows the motor to start at a driving frequency sufficiently higher than the unstable region. By always driving the ultrasonic motor at a drive frequency in a frequency range in which the characteristics of the vibrating body are stable, and by driving it with a constant current, it is possible to provide a highly efficient ultrasonic motor that is stable during startup and operation.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a drive circuit according to an embodiment of the present invention's drive method. Figure 2 is an example of a detection circuit for the ultrasonic motor mechanical arm current. 8...Dividing circuit 9...Power amplifier lO...
・Power amplifier 11...Ultrasonic power amplifier 12...Current detector 13...Oscillation side # unit 14...Sweep indicator 15...Resistor 16...Differential amplifier 17...Capacitor , 18...Resistor 19...Differential amplifier 2
0... Differential amplifier 21... Name of resisting agent Patent attorney Shigetaka Awano Haka 1 person Figure q@ is amplifier start 1 wing Figure II! Moat scattering diagram diagram diagram diagram diagram diagram diagram diagram is black :X 9

Claims (3)

[Claims] (1) a vibrating body; a moving body that presses into contact with the vibrating body;
An ultrasonic motor comprising a plurality of drive electrodes formed on the vibrating body, applying a drive voltage having a phase difference to the drive electrodes to excite a traveling wave in the vibrating body, and moving the movable body. In the driving method, the driving current flowing into at least one of the plurality of driving electrodes is controlled to a first set value in a frequency range equal to or higher than the resonant frequency of the vibrating body by controlling the frequency of the driving voltage. A method for driving an ultrasonic motor, characterized by controlling the frequency of a driving voltage so that (2) When starting the ultrasonic motor, start sweeping the frequency of the drive voltage from a frequency higher than the resonant frequency of the vibrating body,
When the drive current flowing into at least one of the plurality of drive electrodes reaches a first set value, the frequency sweep of the drive voltage is stopped so that the drive current reaches the first set value. Claim 1 characterized in that the frequency of the drive voltage is controlled.
The method of driving the ultrasonic motor described. (3) The second set value for the drive current is smaller than the first set value.
When the drive current becomes equal to or less than the second set value, the frequency sweep of the drive voltage is started again from a frequency higher than the resonant frequency of the vibrating body, and the frequency of the drive voltage is started again from a frequency higher than the resonant frequency of the vibrating body. The invention is characterized in that when the drive current flowing into the drive current reaches a first set value, the sweep of the drive frequency is stopped and the frequency of the drive voltage is controlled so that the current reaches the first set value. The method of driving an ultrasonic motor according to claim 1.