Piezoelectric stick-slip motor and control method thereof

Abstract

The present invention relates to a piezoelectric stick-slip motor and a control method thereof. In order to enable the speed of a piezoelectric stick-slip motor to be changed while suppressing noise generation, the present invention provides a control method for a piezoelectric stick-slip motor as set forth in claim 1, comprising the following steps: Step A: applying a periodic sawtooth waveform drive voltage signal having a constant frequency to the motor, the drive voltage (V) increasing to and decreasing from a peak voltage (Vp) to operate the motor at a constant speed; Step B: In each subsequent sawtooth waveform drive voltage signal cycle (C), while keeping the frequency of the drive voltage signal constant, gradually increasing or decreasing the slope (dV / dt) of the increase in the drive voltage (V) to the peak voltage (Vp), thereby changing the speed of the motor.

Description

[Technical field]

[0001] The present invention relates to a piezoelectric stick-slip motor and a control method thereof. [Background technology]

[0002] Piezoelectric stick-slip motors are known, for example, from US 2015 / 0076965, WO 2018 / 134637 A1 or EP 3120449 B1.

[0003] In the case of a piezoelectric stick-slip drive, a piezoelectric actuator element is charged with a periodic voltage, in particular a high-frequency sawtooth voltage, which causes high-frequency expansion and contraction of the actuator element, which is transferred to the friction body via a friction element arranged on the actuator element, so that in the stick phase, the friction body moves during the deflection of the actuator element, and in this phase there is static friction between the friction element and the friction body, whereas in the slip phase there is sliding friction between the friction element and the friction body, and the friction body is not driven or is driven only slightly by the movement of the friction element.

[0004] During the stick phase, the acceleration or the movement speed of the actuator element is in this case configured such that the forces acting in the frictional contact between the friction element and the friction body result in no or only very little sliding friction, so that the friction body is in any case driven by the friction element, whereas in the slip phase, the acceleration or the movement speed of the actuator element is so high that the forces in the frictional contact between the friction element and the friction body are no longer sufficient for the friction element to drive the friction body, and a relative movement (i.e. sliding) occurs between the friction element and the friction body due to the inertia of the friction body.

[0005] Accompanying Figure 1 shows two cycles of a typical periodic stick-slip drive signal according to the prior art in a voltage over time diagram. Phase 1 represents the move or stick phase, phase 2 represents the pause after the move, phase 3 represents the slip phase and phase 4 represents the pause after the slip. Vb is the base voltage, Vp is the peak voltage and dV / dt is the slope that increases the drive voltage from the base voltage Vb to the peak voltage Vp in phase 1 of cycle C.

[0006] Phase 3 (the "slip phase") should be as short as possible (typically <2 μsec), while phases 2 and 4 (the "pause after move" and "pause after slip") are typically between 3 and 10 μsec. Pause times that are too short or too long will adversely affect the efficiency of the motor, resulting in a slight loss of speed.

[0007] The effective speed of the motor is determined primarily by the frequency and peak voltage of the signal. Since phases 2 ("pause after move"), 3 ("slip phase") and 4 ("pause after slip") are relatively constant, the frequency of the signal is determined primarily by the length of the move phase.

[0008] In a typical stick-slip controller, the peak voltage Vp is held constant and the motor speed is controlled by the frequency of the signal. During acceleration, deceleration, and low speeds, the piezoelectric motor must then operate at a lower audible frequency. This causes the stick-slip motor, and therefore the motion device, to produce an audible noise. This noise can be particularly disturbing to the user, since during closed loop, the servo loop constantly changes frequency to compensate for the varying tracking error.

[0009] Some controllers try to avoid generating noise by only operating at a high frequency, typically 20 kHz. This is a major limitation, however, since it does not allow for acceleration, deceleration or lower speeds. However, when operating in a closed loop, the frequency needs to be changed further, and this frequency modulation generates further disturbing sounds. Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to enable speed changes in a piezoelectric stick-slip motor while reducing noise generation. [Means for solving the problem]

[0011] The object of the invention is achieved by the method of claim 1. Preferred embodiments are claimed in the dependent claims.

[0012] The method for controlling a piezoelectric stick-slip motor as claimed in claim 1 includes the following steps.

[0013] Step A: applying a periodic sawtooth waveform drive voltage signal having a constant frequency to the motor, the drive voltage increasing and decreasing to a peak voltage to operate the motor at a constant speed.

[0014] Step B: Varying the speed of the motor by gradually increasing or decreasing the slope at which the drive voltage increases to a peak voltage with each subsequent sawtooth waveform drive voltage signal cycle while keeping the frequency of the drive voltage signal constant.

[0015] Using this motor control, the speed of a piezoelectric stick-slip motor can be changed without generating significant noise because the frequency of the drive voltage signal is kept constant and only the slope at which the drive signal voltage rises to a peak voltage is changed.

[0016] Each sawtooth waveform driving voltage signal cycle in step A is - a first stage representing the stick (movement) stage where the driving voltage increases from a base voltage to a peak voltage; a second stage, representing a pause after the first stage, during which the drive voltage is maintained at the peak voltage; a third stage, representing a slip stage during which the drive voltage decreases from the peak voltage to the base voltage, and lasting preferably for a period of time less than 2 μs; It will prove advantageous if it comprises a fourth stage, representing a pause after the third stage, during which the drive voltage is maintained at the base voltage, and lasting preferably for a period of between 3 and 10 μs.

[0017] Such drive voltage signal cycles are relevant to operating a piezoelectric stick-slip motor.

[0018] The operating characteristics of the piezoelectric stick-slip motor can be improved if step B includes gradually increasing or decreasing the peak voltage in each subsequent drive voltage signal cycle, preferably until the peak voltage passes a threshold voltage level at which the motor starts or stops operating. By gradually decreasing the slope at which the drive voltage increases to the peak voltage in each subsequent sawtooth waveform drive voltage signal cycle, the motor can be smoothly decelerated and stopped. By gradually increasing the slope at which the drive voltage increases to the peak voltage in each subsequent sawtooth waveform drive voltage signal cycle, the motor can be smoothly started and accelerated.

[0019] The operating performance of the piezoelectric stick-slip motor can also be improved if the claimed method includes maintaining a constant ramp rate and / or ramp rate of the drive voltage between the base voltage and the peak voltage within each drive voltage signal cycle in step A and / or step B.

[0020] In order to reduce the speed of the motor, step B includes at least one of the following sub-steps of modifying the sawtooth waveform driving voltage signal compared to step A:

[0021] Sub-step B1: In each subsequent sawtooth waveform drive voltage signal cycle, the first stage is lengthened by gradually decreasing the slope at which the drive voltage increases to the peak voltage while keeping the peak voltage constant, while the second stage is shortened by the same amount to compensate for the lengthening of the first stage.

[0022] Sub-step B2: In each subsequent sawtooth waveform drive voltage signal cycle, the peak voltage is gradually decreased and the slope at which the drive voltage is increased to the peak voltage is gradually decreased, thereby lengthening the first stage, while eliminating the second stage and possibly shortening the third stage to compensate for the lengthening of the first stage.

[0023] In step B1, reducing the slope of the increase in drive voltage to the peak voltage is done by lengthening stage 1 without lowering the peak voltage, and the lengthening of stage 1 is compensated for by shortening stage 2 by the same amount. In step B2, stage 1 is lengthened so that it is longer than stages 1 and 2 of step A combined, so that stage 2 is omitted entirely and stage 3 follows immediately after stage 1.

[0024] The operating performance of the piezoelectric stick-slip motor can also be improved if the claimed method includes in step A and / or step B maintaining a constant slope of the decrease in the drive voltage from the peak voltage to the base voltage for each subsequent drive voltage signal cycle.

[0025] If the claimed method includes in step A and / or step B keeping the duration of the fourth stage of each subsequent drive voltage signal cycle constant, the operational performance of the piezoelectric stick-slip motor can be further improved.

[0026] When a driving voltage signal of a periodic sawtooth waveform having a constant frequency of 20 kHz or more is applied to the motor, it is possible to prevent noise from being generated in the piezoelectric stick-slip motor.

[0027] If the claimed method includes, in step A and / or step B, keeping the base voltage constant for each subsequent drive voltage signal cycle, the operational performance of the piezoelectric stick-slip motor can be further improved.

[0028] The speed control and position accuracy of a piezoelectric stick-slip motor can be improved when the claimed method involves operating the motor in a closed loop and / or servo loop.

[0029] When the claimed method involves adjusting the peak voltage in real time at the servo clock rate, the operating characteristics of the piezoelectric stick-slip motor can be beneficially affected.

[0030] Another aspect disclosed herein relates to a piezoelectric stick-slip motor comprising a driven element, a stator having a friction element, a controller and at least one piezoelectric actuator configured to deform and impart movement to the friction element upon application of a drive voltage signal from the controller to drive the driven element by stick-slip contact, and the controller configured to perform a method as claimed in any one of the preceding claims.

[0031] Further preferred embodiments result from combinations of the features disclosed in the claims, the drawings and the description. [Brief description of the drawings]

[0032] [Figure 1] FIG. 2 illustrates the shape of a typical stick-slip sawtooth waveform drive voltage signal over two successive drive voltage signal cycles. [Diagram 2]FIG. 1 shows the relationship between peak voltage and speed. According to this relationship, speed is not directly proportional to peak voltage; the motor will stop moving below a certain peak voltage value, and this relationship between peak voltage and speed cannot be precisely defined. [Diagram 3] FIG. 1 illustrates the shape of a stick-slip sawtooth waveform drive voltage signal according to the claimed invention, where stage 3 ("slip stage") and stage 4 ("pause after slip") remain relatively constant, while stage 2 ("pause after move") is absorbed by stage 1 ("movement stage"); the loss of stage 2 ("pause after move") reduces motor efficiency and facilitates the desired speed reduction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present application relates to a piezoelectric stick-slip motor comprising a driven element, a stator having a friction element, a controller and at least one piezoelectric actuator configured to deform and impart movement to the friction element upon application of a drive voltage signal from the controller, thereby driving the driven element by stick-slip contact. As described below, the controller is configured to carry out a method according to any one of the accompanying claims.

[0034] The claimed method primarily allows the speed of a piezoelectric stick-slip motor to be varied while keeping the motor drive signal at a constant high frequency and noise-free by varying the peak voltage of the signal.

[0035] The method includes the following steps. Step A: applying a periodic sawtooth waveform drive voltage signal having a constant frequency to the motor, where the drive voltage V increases and decreases to a peak voltage Vp to operate the motor at a constant speed.

[0036] Step B: With each subsequent sawtooth waveform drive voltage signal cycle C, the speed of the motor is changed by gradually increasing or decreasing the slope dV / dt that increases the drive voltage V up to a peak voltage Vp while keeping the frequency of the drive voltage signal constant.

[0037] The motor is preferably controlled in a closed loop, with the servo loop varying the peak voltage with high precision and in real time at the servo clock rate.

[0038] During acceleration of the piezoelectric stick-slip motor, the gradient dV / dt by which the drive voltage V increases to a peak voltage Vp is gradually increased such that the drive voltage is greater in the second drive voltage signal cycle C than in the first drive voltage signal cycle C and is greater in the third drive voltage signal cycle C than in the second drive voltage signal cycle C.

[0039] During deceleration of the piezoelectric stick-slip motor, the gradient dV / dt by which the drive voltage V increases to a peak voltage Vp is gradually decreased so that the drive voltage in the second drive voltage signal cycle C is less than the first drive voltage signal cycle C and the drive voltage in the third drive voltage signal cycle C is less than the second drive voltage signal cycle C.

[0040] Deceleration of a piezoelectric stick-slip motor will now be described in relation to FIG. As shown in FIG. 3 by a solid line (steps A and B of the periodic sawtooth waveform driving voltage signals are superimposed in FIG. 3), the sawtooth waveform driving voltage signal in step A is a first phase 1, which represents the stick / move phase in which the drive voltage V increases from a base voltage Vb to a peak voltage Vb; a second phase 2, representing a pause after the first phase 1, during which the drive voltage V is maintained at a peak voltage Vp; a third phase 3, representing the slip phase during which the drive voltage V decreases from a peak voltage Vp to a base voltage Vb; a fourth phase 4, representing a pause after the third phase 3, during which the drive voltage V is maintained at the base voltage Vb.

[0041] A periodic sawtooth waveform drive voltage signal is applied to the motor 1 at a constant frequency, typically 20 kHz or greater. In this example, the third phase 3 lasts for a time period of less than 2 μsec and the fourth phase 4 lasts for a time period of between 3 and 10 μsec.

[0042] In order to reduce the motor speed, step B includes the following sub-steps, in particular, modifying the sawtooth waveform drive voltage signal, as compared to step A:

[0043] Sub-step B1: In each subsequent sawtooth waveform driving voltage signal cycle C, while keeping the peak voltage Vp constant, the first stage 1 is extended by gradually decreasing the slope dV / dt that increases the driving voltage V to the peak voltage Vp, while the second stage 2 is shortened by the same amount to compensate for the extension of the first stage 1. As shown by the dotted line in Figure 3, the slope dV / dt in stage 1 of the cycle of step B is smaller than the slope dV / dt in stage 1 of the cycle of step A. The peak voltage VpB1 of the first cycle of step B is the same as the peak voltage VpA of step A.

[0044] Sub-step B2: In each subsequent sawtooth waveform driving voltage signal cycle C, the first stage 1 is extended by gradually decreasing the peak voltage Vp and gradually decreasing the gradient dV / dt for increasing the driving voltage V to the peak voltage Vp, while eliminating the second stage 2 and shortening the third stage 3 to compensate for the extension of the first stage 1. As shown in FIG. 3, the gradient dV / dt and peak voltage VpB2 of stage 1 in the second cycle of step B are smaller than the gradient dV / dt and peak voltage VpB1 of stage 1 in the first cycle of step B, respectively. In addition, the gradient dV / dt of stage 1 and the peak voltages VpB3, VpB4, and VpB5 are gradually decreased in each subsequent sawtooth waveform driving voltage signal cycle C until the peak voltage VpB5 passes (passes below) the threshold voltage level Vt at which the motor 1 stops operating.

[0045] In fact, the peak voltage Vp and the gradient dV / dt of increasing the drive voltage V to the peak voltage Vp are changed from one sawtooth waveform drive voltage signal cycle C to another sawtooth waveform drive voltage signal cycle. However, the gradient dV / dt of increasing the drive voltage V from the base voltage Vp to the peak voltage Vb and the gradient dV / dt of decreasing the drive voltage V from the peak voltage Vp to the base voltage Vb are constant throughout each drive voltage signal cycle C in steps A and B. In addition, the base voltage Vb and the duration of the fourth phase 4 are constant in each subsequent drive voltage signal cycle of steps A and B.

[0046] As shown in Figure 2, motor speed is not directly proportional to peak voltage. Below a certain peak voltage value, the motor will stop moving. The relationship between the two cannot be precisely defined.

[0047] This presents no problem in a closed loop because the servo loop adjusts the peak voltage Vp to the required value to produce the desired speed. The slope or gradient dV / dt of phase 1 (the "travel phase") is adjusted in real time at the servo clock rate from the ideal shape required for high speeds to a lower value where the motor stops moving.

[0048] In step B, phase 3 ("slip phase") and phase 4 ("pause after slip") are held relatively constant, while phase 2 ("pause after move") is absorbed into phase 1 ("move phase"). The omission of phase 2 ("pause after move") reduces motor efficiency and facilitates the desired speed reduction. [Explanation of symbols]

[0049] Reference sign 1 First phase (of the drive voltage signal cycle) 2 Second phase (of the drive voltage signal cycle) 3 Third phase (of the drive voltage signal cycle) 4 Fourth phase (of the drive voltage signal cycle) C Drive voltage signal cycle dV Voltage increment dt Time increment t time V Drive voltage Vb Base voltage Vp Peak voltage Vt Threshold voltage

Claims

1. 1. A method of controlling a piezoelectric stick-slip motor, the method comprising: a. Step A: applying a periodic sawtooth waveform drive voltage signal having a constant frequency to the motor, the drive voltage (V) of the drive voltage signal increasing and decreasing to a peak voltage (Vp) to operate the motor at a constant speed, the method comprising: b. Step B: while keeping the frequency of the drive voltage signal constant, varying the speed of the motor by gradually increasing or decreasing a slope (dV / dt) by which the drive voltage (V) increases to the peak voltage (Vp) in each of at least two subsequent sawtooth waveform drive voltage signal cycles (C) and by gradually increasing or decreasing the peak voltage (Vp) for each of the at least two subsequent sawtooth waveform drive voltage signal cycles.

2. Each sawtooth waveform driving voltage signal cycle (C) in step A is a. a first phase (1) representing a stick phase during which the drive voltage (V) increases from a base voltage (Vb) to the peak voltage (Vp); b. a second phase (2) representing a pause after the first phase (1), during which the drive voltage (V) is maintained at the peak voltage (Vp); c. a third phase (3) representing a slip phase during which the drive voltage (V) decreases from the peak voltage (Vp) to the base voltage (Vb), the slip phase lasting for a time period of 2 μsec or less; d) a fourth step (4) representing a pause after said third step (3), during which said drive voltage (V) is maintained at said base voltage (Vb), said fourth step (4) lasting for a time between 3 and 10 μs.

3. 3. The method of claim 2, comprising gradually increasing or decreasing the peak voltage (Vp) in each subsequent drive voltage signal cycle until the peak voltage (Vp) passes a threshold voltage level (Vt) at which the motor (1) starts or stops moving.

4. 4. The method according to claim 2 or 3, characterized in that within each drive voltage signal cycle (C) in step A and / or step B, the gradient (dV / dt) of increasing and / or decreasing the drive voltage (V) between the base voltage (Vb) and the peak voltage (Vp) is kept constant.

5. In order to reduce the speed of the motor, the step B includes the following sub-steps B1 and B2 of modifying the sawtooth waveform driving voltage signal compared to the step A, the sub-steps being: a. Sub-step B1: in each subsequent sawtooth waveform driving voltage signal cycle (C), while keeping the peak voltage (Vp) constant, gradually decreasing the slope (dV / dt) of increasing the driving voltage (V) to the peak voltage (Vp), thereby lengthening the first stage (1), while shortening the second stage (2) by the same amount to compensate for the lengthening of the first stage (1); and b. sub-step B2: gradually decreasing the peak voltage (Vp) and gradually decreasing the slope (dV / dt) of increasing the drive voltage (V) to the peak voltage (Vp) in each subsequent sawtooth waveform drive voltage signal cycle (C) to extend the first stage (1), while eliminating the second stage (2) and possibly shortening the third stage (3) to compensate for the extension of the first stage (1).

6. 6. The method according to claim 2, characterized in that in step A and / or step B, the slope (dV / dt) of the decrease of the drive voltage (V) from the peak voltage (Vp) to the base voltage (Vb) of each subsequent drive voltage signal cycle (C) is kept constant.

7. A method according to any one of claims 2 to 6, characterised in that in step A and / or step B, the duration of the fourth phase (4) of each subsequent drive voltage signal cycle is kept constant.

8. 8. The method according to claim 1, further comprising applying to the motor the periodic sawtooth waveform drive voltage signal having a constant frequency of 20 kHz or more.

9. A method according to any one of claims 1 to 8, characterized in that in step A and / or step B, the base voltage (Vb) of the drive voltage (V) for each subsequent drive voltage signal cycle is kept constant.

10. Method according to any one of the preceding claims, characterized in that the motor is operated in a closed loop and / or a servo loop.

11. 11. The method of claim 10, wherein the peak voltage (Vp) is adjusted in real time at a servo clock rate.

12. 12. A piezoelectric stick-slip motor comprising: a driven element, a stator having a friction element, a controller, and at least one piezoelectric actuator configured to deform and impart movement to the friction element upon application of a drive voltage signal from the controller, thereby driving the driven element by stick-slip contact, and wherein the controller is configured to perform a method according to any one of claims 1 to 11.

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