Silent stick-slip piezo motor

Abstract

A stick-slip piezo motor. At least one voltage source is connected to a piezo motor. The piezo motor has at least one oscillating piezo element and at least one moving friction element connected to the oscillating piezo element. The moving friction element moves in a desired travel direction. A computer is programmed to control the voltage source to deliver voltage to the piezo motor at a predetermined frequency and amplitude to control the speed of the piezo motor. The computer is programmed to hold the frequency constant while varying the amplitude to adjust the speed of the piezo motor. In a preferred embodiment the computer is programmed to hold the frequency constant at an ultrasonic frequency. In another preferred embodiment the computer is programmed to hold the frequency constant at a value of 15 kHz or higher.

Claims

1. A stick-slip piezo motor system, comprising: A. at least one voltage source, B. a piezo motor electrically connected to said at least one voltage source, said piezo motor comprising: i. at least one oscillating piezo element, ii. at least one moving friction element connected to said at least one oscillating piezo element, said at least one moving friction element for moving in a desired travel direction, and C. a computer programmed to control said at least one voltage source to deliver voltage to said piezo motor at a predetermined frequency and amplitude to control a speed of said piezo motor, wherein said computer is programmed to hold said predetermined frequency constant while said amplitude is varied to adjust the speed of said piezo motor.

2. The stick-slip piezo motor system as in claim 1, wherein said predetermined frequency is an ultrasonic frequency.

3. The stick-slip piezo motor system as in claim 1, wherein said predetermined frequency is greater than or equal to 15 kHz.

4. The stick-slip piezo motor system as in claim 1, wherein said predetermined frequency is audible and pleasant to a human.

5. The stick-slip piezo motor system as in claim 1, wherein said piezo motor is a single-phase motor.

6. The stick-slip piezo motor system as in claim 5, wherein said piezo motor first accelerates during an acceleration mode, then operates at a constant speed during a constant speed mode, and then decelerates during a deceleration mode, wherein said computer is programmed to operate at a fixed amplitude and a variable frequency during at least a portion of said acceleration mode, wherein said computer is programmed to operate at a variable amplitude and a fixed frequency during said constant speed mode, and wherein said computer is programmed to operate at a fixed amplitude and a variable frequency during a portion of said deceleration mode.

7. The stick-slip piezo motor system as in claim 1, wherein said piezo motor is a multi-phase motor and wherein said at least one oscillating piezo element is a plurality of oscillating piezo elements and wherein said at least one moving friction element is a plurality of oscillating friction elements.

8. The stick-slip piezo motor system as in claim 7, wherein said piezo motor first accelerates during an acceleration mode, then operates at a constant speed during a constant speed mode, and then decelerates during a deceleration mode, wherein said computer is programmed to operate at a fixed amplitude and a variable frequency during at least a portion of said acceleration mode, wherein said computer is programmed to operate at a variable amplitude and a fixed frequency during said constant speed mode, and wherein said computer is programmed to operate at a fixed amplitude and a variable frequency during a portion of said deceleration mode.

9. The stick-slip piezo motor system as in claim 7, further comprising a plurality of alternating voltage sources, wherein each of said plurality of alternating voltage sources provides alternating voltage to one of said plurality of oscillating piezo elements so that each one of said plurality of oscillating piezo elements oscillates out of phase with respect to the other of said plurality of oscillating piezo elements.

10. The stick-slip piezo motor system as in claim 1, wherein said at least one oscillating piezo element is two piezo elements.

11. The stick-slip piezo motor system as in claim 10, wherein said two piezo elements are connected to the same side of a piezo housing, wherein both of said two piezo elements are expanding together or contracting together when both are operating in a stick phase.

12. The stick-slip piezo motor system as in claim 10, where said two piezo elements are connected to the opposite side of a piezo housing, wherein one of said two piezo elements is expanding while the other of said two piezo elements is contracting when both are operating in a stick phase.

13. The stick-slip piezo motor system as in claim 1, wherein said desired travel direction is linear.

14. The stick-slip piezo motor system as in claim 1, wherein said desired travel direction is rotational.

15. The stick-slip piezo motor system as in claim 1, wherein said piezo motor allows planar movement of said at least one moving friction element.

16. The stick-slip piezo motor system as in claim 1, wherein said at least one oscillating piezo element is at least three piezo elements.

17. The stick-slip piezo motor system as in claim 1, wherein said plurality of piezo friction elements are ceramic friction elements.

18. The stick-slip piezo motor system as in claim 1, wherein said at least one moving friction element is a sliding friction element.

19. The stick-slip piezo motor system as in claim 1, wherein at least one moving friction element is a rotational friction element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows a prior art single phase stick-slip piezo motor.

(2) FIG. 1B shows a graph depicting the resultant motion caused by a prior art stick-slip piezo motor.

(3) FIG. 2 shows a prior art multi-phase stick-slip piezo motor.

(4) FIGS. 3A-3E shows a prior art depiction of resultant motion of the sliding friction element of the piezo motor of FIG. 2.

(5) FIG. 4 shows a prior art graphical representation describing the resultant motion of the sliding friction element of the piezo motor of FIG. 2.

(6) FIG. 5 shows a prior art linearized graphical representation of the resultant motion of the sliding friction element of the piezo motor of FIG. 2.

(7) FIG. 6 shows a preferred embodiment of the present invention.

(8) FIGS. 7A and 7B show a graphical comparison of the effects of variable amplitude.

(9) FIG. 8 shows a preferred embodiment of the present invention.

(10) FIGS. 9 and 10 show a graphical comparison of the effects of variable amplitude.

(11) FIGS. 11 and 12 show slip back when operating at low amplitude.

(12) FIG. 13 shows fixed amplitude/variable frequency mode when speed is zero.

(13) FIGS. 14-15 show a mixed frequency/amplitude mode operation during acceleration, constant speed and deceleration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(14) Variable amplitude/fixed frequency (single phase motor) FIG. 6 shows a preferred embodiment of the present invention. Computer 414 is programmed to hold the frequency of voltage source 415 constant at an ultrasonic level (15 kHz or higher). The speed of stick-slip piezo motor 416 is adjusted by varying the amplitude of the voltage of voltage source 415.

(15) For example, FIGS. 7A and 7B shows waveforms at the same frequency (20 kHz), which is not audible by the human ear. Waveform 420 has an amplitude of 10V, whereas waveform 421 has an amplitude of 20V. Therefore, the resulting piezo expansion of waveform 421 is twice that of waveform 420. In other words, doubling the amplitude while holding the frequency constant has allowed the operator to double the speed of piezo motor 416. This method allows speed regulation of the piezo motor through amplitude adjustment rather than frequency adjustment. Therefore, the motor is not audible at any speed.

Variable Amplitude/Fixed Frequency (Multi-Phase Motor)

(16) FIG. 8 shows another preferred embodiment of the present invention. Computer 414 is programmed to hold the frequency of voltage source 415 constant at an ultrasonic level (15 kHz or higher). The speed of multi-phase stick-slip piezo motor 426 is adjusted by varying the amplitude of the voltages of voltage sources 415 and 425.

(17) For example, FIGS. 9 and 10 shows waveforms at the same frequency (20 kHz), which is not audible by the human ear. Waveforms 450 have an amplitude of 10V, whereas waveforms 451 have an amplitude of 20V. Therefore, the resulting piezo expansions of waveforms 451 is twice that of waveforms 450. In other words, doubling the amplitude while holding the frequency constant has allowed the operator to double the speed of piezo motor 426. This method allows speed regulation of the piezo motor through amplitude adjustment rather than frequency adjustment. Therefore, the motor is not audible at any speed.

Mixed Frequency/Amplitude Mode (Single Phase Motor)

Fixed Frequency Mode

When Speed=0

(18) A drawback of the fixed frequency mode is that the piezo motor is always driven at a fixed frequency and therefore always draws current. This holds true even when the piezo motor is standing still and even though the amplitude of the voltage is very small. This is due to the fact that there is always some slip back as part of the saw tooth waveform (FIGS. 11-12). Therefore, at an average speed of zero, there needs to be the same forward motion as is exhibited during slip back. This causes the motor to heat approximately to the square of the average current.

Fixed Amplitude Mode

When Speed=0

(19) Fixed amplitude/variable frequency is preferred when operating at very slow speeds or when speed is at zero. In order to achieve an average speed of zero in fixed amplitude mode, the voltage to the motor is held constant and the motor draws negligible current. Therefore, negligible power is consumed (FIG. 13).

(20) Another preferred embodiment of the present invention is shown in FIGS. 14-15. FIG. 14 shows a graph that depicts the speed of motor 426 (FIG. 8) as a function of time and FIG. 15 shows flowchart 530. As shown, motor 426 accelerates until it gets to constant speed and then decelerates when the motor is in position and comes to a stop. In the preferred embodiment shown in FIGS. 14-15, computer 414 is programmed to operate motor 426 so that the silent nature of the fixed frequency mode is combined with the low power and high stability of the fixed amplitude mode. Computer 414 is programmed to allow not only adjustment of the fixed frequency to optimize motion at higher speeds based on piezo characteristics, but also an adjustable switch over speed to allow for fixed amplitude mode operation during the beginning of acceleration and then the end of the deceleration.

(21) In FIGS. 14-15 computer 414 is programmed to operate at fixed amplitude and variable frequency (audible mode) during the first portion of the acceleration mode. Computer 414 is also programmed to operate at variable amplitude and fixed frequency (silent mode) during the second portion of the acceleration mode, the constant speed mode, and during the first portion of the deceleration mode. Computer 414 is also programmed operate at fixed amplitude and variable frequency (audible mode) during the last portion of said deceleration mode.

(22) As shown when using trapezoidal velocity profile 480, computer 414 switches from fixed amplitude mode at 0.5 mm/s to fixed frequency and back. This not only makes the motor more efficient at stand still, but also creates a more stable control loop due to the elimination of constant back and forth motion at zero average speed caused by the effect described above and shown in FIGS. 11-12.

Frequency Level is Programmable

(23) The above preferred embodiments discussed utilizing computer 414 to operate piezo motors at a constant frequency that is ultrasonic (i.e., at a frequency level that is higher than the upper audible limit of human hearing). It should be noted that the specific frequency utilized is variable and programmable and may be chosen and modified as appropriate. In one preferred embodiment the frequency of voltage source 415 is held constant at 20 kHz. In another preferred embodiment the frequency of voltage source 415 is held constant at 15 kHz.

(24) Other preferred embodiments recognize that although some frequencies may be audible, the frequency sound heard is not unpleasant or irritating. Therefore, the piezo motor may still be used in the presence of a human without causing distress and irritation. For example, in another preferred embodiment computer 414 is programmed to hold the frequency of voltage source 415 constant at 13.5 kHz. The frequency emitted is audible to a human nearby, however it is neither irritating nor unpleasant.

Piezo Motor Types

(25) The present invention may be utilized with a variety of stick-slip piezo motor types. For example, in addition to the single phase and multi-phase motors discussed above, the present invention may also be utilized with:

(26) 1) a piezo motor where two piezo elements are connected to the opposite side of the piezo housing,

(27) 2) a piezo motor where the travel direction is rotational,

(28) 3) a piezo motor where the piezo motor allows for planar movement of the moving friction element,

(29) 4) a piezo motor having three or more piezo elements,

(30) 5) a piezo motor where the friction elements are ceramic friction elements,

(31) 6) a piezo motor where the moving friction element is a sliding friction element, and

(32) 7) a piezo motor where the moving friction element is a rotational friction element. Examples of these embodiments are clearly described in U.S. Pat. No. 8,593,033. This list is illustrative and not all-inclusive. The above described invention may be utilized with other types of stick-slip piezo motors also.

(33) Although the above-preferred embodiments have been described with specificity, persons skilled in this art will recognize that many changes to the specific embodiments disclosed above could be made without departing from the spirit of the invention. Therefore, the attached claims and their legal equivalents should determine the scope of the invention.