Abstract:
In a method and an apparatus for eliminating noise generated by a piezo actuator drive which includes in a housing a drive member, a drive shoe disposed adjacent the drive member, vibrating actuators forming drive elements and being connected to the drive shoe for engaging the drive shoe with the drive member and moving the drive shoe for driving the drive member, at least one additional actuator is disposed in the housing and operated at a frequency which is phase shifted with respect to at least the clamping actuators so as to cancel the noise waves generated during operation of the apparatus.

Description:
[0001]    This is a continuation-in-part application of international application PCT/EP99/06921 filed Sep. 18, 1999 and claiming the priority of German applications 198 43 004.3 filed Sep. 21, 1998 and 199 38 140.8 filed Jun. 19, 1999.  
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a method for compensating acoustic waves which are generated by electric-actuator drives, particularly piezo actuator drives which include vibrating actuators arranged in a housing and which form a drive element including at least two actuators arranged at a right angle with respect to each other, and to an apparatus for performing the method.  
           [0003]    It is known that sound waves can be extinguished by phase-shifted counter waves, wherein the counter waves are phase shifted with respect to the sound waves by a so-called phase frequency, generally 180° such that the maxima and the minima of the sound waves and respectively, counter waves overlap. It is however necessary that the counter wave generator follows accurately the sound or vibration frequencies to be compensated.  
           [0004]    EP 0 552 344 B1 and DE 94 802 001 disclose electro-actuator drives which generate rotational or linear movements using vibrating piezo actuators. These or similar drives have the disadvantage that they generate undesirable noises which limit their applications.  
         SUMMARY OF THE INVENTION  
         [0005]    In a method and an apparatus for eliminating noise generated by a piezo actuator drive which includes in a housing a drive member, a drive shoe disposed adjacent the drive member, vibrating actuators forming drive elements and being connected to the drive shoe for engaging the drive shoe with the drive member and moving the drive shoe for driving the drive member, at least one additional actuator is disposed in the housing and operated at a frequency which is phase shifted with respect to at least the clamping actuators so as to cancel the noise waves generated during operation of the apparatus.  
           [0006]    Advantageous embodiments of the invention will be described below on the basis of the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a side view of an electro-(piezo) actuator motor as disclosed in principle in DE 94 19 802.0U1 including however a separate acutator for sound compensation,  
         [0008]    [0008]FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1,  
         [0009]    [0009]FIG. 2 a  is a cross-sectional view taken along line B-B of FIG. 1,  
         [0010]    [0010]FIG. 3 is a side view of an embodiment including a noise compensating electro-actuator,  
         [0011]    [0011]FIG. 4 is a cross-sectional view taken along line A-A of FIG. 3,  
         [0012]    [0012]FIG. 5 is a side view of an embodiment with a noise-compensating second electro-actuator,  
         [0013]    [0013]FIG. 5 a  is a cross-sectional view taken along line B-B of FIG. 5,  
         [0014]    [0014]FIG. 6 is a cross-sectional view taken along line A-A of FIG. 5,  
         [0015]    [0015]FIG. 7 is an overview for the control of the individual actuators of the motors for noise compensation,  
         [0016]    [0016]FIG. 8 shows the sine wave-like sound waves phase shifted by 90° so that they cancel each other,  
         [0017]    [0017]FIG. 9 shows a preferred embodiment of the drive arrangement, and  
         [0018]    [0018]FIG. 10 is a view corresponding to that of FIG. 9 for the explanation of another mode of operation.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    Based on a motor as disclosed in DE 94 19 802.0U1 a drive element  1  as shown in FIG. 1 comprises a pair of drive piezo elements H 1 , H 2  and a pair of clamping piezo elements K 1 , and K 2  (FIG. 2) firmly engaged each in a cage  4 , and a cage  4 ′, respectively (FIG. 9), which extend rectangularly with respect to each other. In accordance with the embodiment shown in FIG. 2 four clamping piezo elements are disposed in an axial side-by-side relationship in a common drive element body  5  to form the drive element  1 . The piezo element body  5  is continuous in outer bridge areas  6  and  6 ′ but is radially separated by slots  8  in the other areas, so that, in the embodiment as shown in FIGS.  1 - 4 , four sections  14  are formed which can vibrate independently from one another actuated by the respective drive piezo elements H 1  and H 2 . The bridge  6  for the clamping piezo elements K 1  and K 2  is biased by a wedging structure  7 ,  7 ′ with an intermediate rubber insert  11  in the direction toward the drive drum  10  and abuts the housing  9 . The four operating shoes  12  of the four sections  14  of the drive element body  5  are engaged with the drive drum  10  in a cyclical manner.  
         [0020]    With the cyclical energization of the individual clamping and drive piezo elements in accordance with the diagram shown in FIG. 7,that is, the piezo elements indicated by the designations K 1 , H 1 , K 2 , H 2 , the operating shoes  12  of the drive element body  5  sequentially engage the drive drum  10  and move the drive drum  10 , whereby the drive drum  10  is rotated. The clamping piezo elements K 1  and K 2  alternately form an air gap between the drive drum  10  and the operating shoe  12 . In this way, the engagement force for engaging the clamping piezo elements K 1  and K 2  with the drive drum  10  is generated once by the piezo element K 1  and then, in the next cycle, by the piezo element K 2 . When a gap  13  is formed between the drive drum  10  and the operating shoes  12 , the operating shoes  12  can freely oscillate back by energization of the respective drive piezo elements H 1  and H 2  respectively. At the same time, the clamping piezo elements K 1  and, respectively, K 2 , which then are in engagement with the drive member  10 , are moved in the drive direction by energization of the respective drive piezo elements H 1  or respectvely H 2 . In this way, a movement is achieved similar to the human walking which drives the drive drum  10 . In particular, this movement is achieved as follows; When the clamping piezo elements K 1  bias the operating shoes  12  of their two sections  14  into engagement with the drum  10  these two sections  14  are moved in drive direction by the associated drive piezo elements H 1 . At the same time, the operating shoes  12  of the two other sections  14  are disengaged from the drum  10  so that a gap  13  is formed between these operating shoes  12  and the drum while they move counter to the drive direction. In the next following tact, the situation is reversed.  
         [0021]    The noises generated in this procedure can be compensated by an additional piezo element  15  disposed in the housing  9 . However, this kind of compensation is optimal only with an optimally operating electronic control arrangement which senses and evaluates the sound waves essentially without time delay and converts that information into an energization of the piezo element for compensation of the emitted sound waves. The sound waves are generated primarily by the operating shoes hitting the drive drum  10 . Preferably, the compensation piezo element  15  is energized so as to produce an opposite-phased sound wave. The effective operating direction of the piezo element  15  is preferably in the operating direction of the clamping piezo elements K 1  and K 2 . The additional piezo element  15  may be energized at a frequency, which is twice the energization frequency of a clamping piezo element. Preferably, the phase of energization of the additional piezo element  15  is adjustable with respect to that of the clamping piezo elements.  
         [0022]    As shown in FIGS. 3 and 4, two drive elements  16  and  17  are provided. The two drive elements  16  and  17  are the same as the single drive element  1  shown in FIG. 1. They are controlled in accordance with the diagram of FIG. 7 in a phase-shifted manner, that is, phase-shifted by about 90° so that the sound waves generated by the two drive elements cancel each other substantially. This means that the clamping piezo element K 3  of the drive element  17  is phase shifted by 90° relative to the clamping piezo element K 1  of the drive element  16 . The same applies to H 3  with respect to H 1 , to K 4  with respect to H 1 , to K 4  with respect to K 2  and to H 4  with respect to H 2 . In this arrangement, which is applicable to rotational drives as well as to linear drives and wherein the drive element  16  is displaced by 180° with respect to the drive element  17 , however, the sound is not completely cancelled. The reason herefor is that the two drive elements are arranged at a distance from each other so that the sound cannot be eliminated right where it is generated. With the unavoidable spacing between the drive elements some sound waves will escape and can not be cancelled.  
         [0023]    In the arrangement as shown in FIG. 5, the drive elements  1   a  and  1   a ′ are joined so that they operate essentially without spatial separation from each other. In this way, a highly effective sound cancellation can be achieved. To make such an arrangement, the clamping and drive piezo elements are relatively narrow (half as wide as in the other arrangements shown in FIG. 1 and FIG. 2). Sections of one piezo actuator element may be disposed adjacent to, or between, sections of another piezo actuator element, that is the piezo actuator elements may be intertwined. The sections  14 , which are formed each by a clamping piezo element K and a drive piezo element H with associated engagement shoes  12 , are separated by the slots  8 . In accordance with diagram  7 , the piezo elements K 1 , K 2 , H 1 , H 2  are energized so as to vibrate at a 90° phase shift with respect to the piezo elements K 3 , K 4 , H 3 , H 4 . In this way, an optimal noise cancellation at the point of noise generation is achieved, that is complete noise cancellation is achieved in accordance with FIG. 8. No additional piezo element for the purpose of noise cancellation is needed.  
         [0024]    [0024]FIG. 9 shows a drive element  5  in a cross-section taken along a slot  8 . The cage  4  for the clamping piezo element K is elastic in the operating direction of the clamping piezo element and has the shape of an O. The cage  4  is connected to the drive shoe  12  by way of two thin webs  20 . The webs  20  are arranged in a slightly wedge-like pattern such that they are closer together adjacent the drive shoe  12 .  
         [0025]    The cage  4 ′ of the drive piezo element also has the shape of an O and is elastic in the operating direction of the drive piezo element H. The end of the cage  4 ′ remote from the bridge  6 ′ is disposed adjacent another O-shaped cage  21 , which is also elastic in the operating direction of the drive piezo element H. This additional cage  21  is connected to the drive shoe  12  by way of a thin web  22 . It is also possible to connect the cage  4 ′ by means of the web  22  directly to the drive shoe  12 .  
         [0026]    In the embodiment as shown in FIG. 10, each column of the drive piezo element pairs H 1  and H 2  is divided into the halves HA and HB, which are controlled independently with a phase frequency corresponding to the desired drive speed. The piezo elements of the halves HA and HB may have different polarities. All piezo elements of the halves HA and HB vibrate in resonance with each other at their full drive stroke but at different phases. The phase shift between the piezo elements of the two halves is adjustable, whereby the effective total stroke length is adjustable. The effective stroke length is the sum of the strokes of a first half HA or respectively, HB, plus the stroke of the other half HB or respectively, HA reduced by the phase difference. The mechanical stroke length can therefore be adjusted by changing the phase difference. In this way, all piezo elements can be operated at a frequency above 20 KHz, which is not audible. One of the halves HA or respectively, HB can therefore be considered to be the additional drive actuator element  15 .  
         [0027]    The phase shift between HA and HB may be one or several vibration periods so that, with subsequent strokes of for example HA, the stroke HA is doubled by the stroke HB or is cancelled out so that the overall stroke is adjustable between a doubled stroke length and zero.  
         [0028]    It is possible in this way that all the piezo actuator elements vibrate at a frequency above the human hearing threshold of 20 kHz with full stroke when the drive piezo actuator elements are operated in a skipping fashion, that is, they are energized only with every second, third, fourth, etc. vibration of the clamping piezo actuator element.