Abstract:
A piezoelectric crystal with transversal effect comprising: at least one plate; and, at least one base at an angle to the at least on plate, the at least one base projecting laterally beyond a thickness of the least one plate on at least one side of the at least one plate. A piezoelectric sensor, for detecting one or more of force, pressure, acceleration, moments and strain signals, comprising at least one of the piezoelectric crystals with transversal effects. A method for producing the piezoelectric crystals with transversal effect is disclosed.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to a piezoelectric crystal with transversal effect. Such piezoelectric crystals have changes or reactions in different planes to applied forces. Those changes generally occur transverse to the direction of the applied force. 
     Piezoelectric crystals are employed in various sensors for measuring forces, pressures, accelerations, strains and moments. For this purpose, crystals with transversal effect are cut into thin plates or rods, for example. For metrological uses, these thin plates are exposed typically to a pressure on the small end surfaces of the plate, causing an electrical charge to appear on the two large side surfaces. By placing an electrically conductive layer on the two side surfaces, which however have no electrical contact with each other, this charge is measured with an appropriate device in the sensor so that information about the pressure is obtained and may be transmitted further. Such sensors are well known. 
     What is crucial, however, is that the crystal is fitted vertically and centered on the axis of the sensor. Any slight tilt will result in a false measurement or fracture of the crystal under the influence of the forces occurring subsequently. A contact of the crystal to the edge of the sensor may lead to a short circuit or hysteresis. 
     Since the sensitivity of the crystal is proportional to the ratio of the charge pickup surface to the pressure surface, these conventional crystal plates are very thin. Hence the handling, especially the centering and aligning in the sensor, are very difficult and laborious. 
     Often the sensor is fitted with centering aids which hold the crystal in position. However the various materials of these centering aids do not tolerate very high temperatures. Consequently the application areas of the known sensors as a whole are limited to a lower maximum temperature. 
     The present invention provides for a piezoelectric crystal which can be fitted easily into a sensor without laborious centering and aligning, and without restriction to a lower temperature range. Furthermore, the crystal of the present invention can be manufactured in large quantities, at low cost and fully automatically. 
     The present invention, then, is a piezoelectric crystal with transversal effect that has at least one plate and at least one base at an angle to the at least one plate. The at least one base projects laterally beyond a thickness of the at least one plate on at least one side of the at least one plate. An embodiment of the present invention may have two such identical crystals. 
     The present invention also includes a sensor for detecting one or more of force, pressure, acceleration, moments and strain signals by using at least one of the piezoelectric crystals with transversal effect discussed above. 
     The present invention also includes a method for producing the piezoelectric crystals with transversal effect. 
     Other aspects, advantages and novel features of the present invention will become apparent from the following detail description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a side sectional view of a piezoelectric crystal fined in a sensor, according to the state of the art. 
     FIG. 1 b  is a plan view of FIG. 1 a.    
     FIG. 2 a  is a side sectional view of a piezoelectric crystal arrangement fitted in a sensor, according to the state of the art. 
     FIG. 2 b  is a plan view of FIG. 2 a.    
     FIG. 3 is a perspective view of an embodiment of a piezoelectric crystal, according to the present invention. 
     FIG. 4 a  is a sectional view of another embodiment of a crystal in a fitted position, according to the present invention. 
     FIG. 4 b  is a plan view and partial cross-sectional view of the fitted crystal of FIG. 4 a.    
     FIG. 5 is a perspective view of the production process of wafer crystals, according to the present invention. 
     FIG. 6 is a sectional view of an embodiment of a double crystal, according to the present invention. 
     FIG. 7 is a sectional view of an embodiment of another double crystal, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 a  and  1   b  show a piezoelectric crystal  11  with transversal effect in the form of a plate (not numbered), fitted in a sensor  10 , as is known from the state of the art. The crystal  11  is clamped at its ends by holding devices  12 ,  1 , to retain it in a required position. FIG. 1 b  shows the crystal  11  fitted in a sleeve  14 . The electrical charges (+, −) are taken off from two side surfaces  15 ,  16  on electrically conductive layers or electrodes  17 ,  18 , provided for this purpose. For example, one electrically conductive layer  17  leads to an upper holding device  12  that is negatively (−) charged, while the other conductive layer  18  leads to a lower holding device  13  on the opposite side that is positively charged (+). Accordingly, the two holding devices  12 ,  13  have opposite electrical charges. 
     FIGS. 2 a  and  2   b  show another embodiment known from the state of the art. In this embodiment, typically three identical crystal rods  11 , their cross sections having the form of circular segments, are disposed in a circle as shown in FIG. 2 b . The charge on the outer surface  18  of each of the crystals  11  is picked up via a sleeve  14  at or through one end of the sensor  10 , for example. The other pole or polarity electrode  17  on the inside of the crystals  11  is picked up via an electrically conducting spiral  19 , as shown in FIG. 2 a . The spiral  19  also acts as a centering aid for the crystals  11 , which in turn are held from outside on or by the sleeve  14 . 
     FIG. 3 shows a crystal  20  with transversal effect according to the present invention. This crystal  20  comprises preferentially a monocrystalline material, whose symmetry of the piezoelectric constant d corresponds to that of the point group  32 . This crystal  20  includes a base  21  which has a plate  22  attached at one end of the crystal  20 . The base  21  projects laterally beyond the thickness of at least one side  15 ,  16  of the plate  22 . The projection may be at right angles. According to the present invention, a transition surface (not identified) from an end of the plate  22  to the base  21  may have a curvature  23  to enhance the stability of the base  21  and plate  22 . Other types of transition surfaces are possible. At both an end face  24  of the plate  22  opposite from the end adjacent the base  21  and on the bottom of the base  25 , bevels  26  may be provided to prevent edges, sides and related surfaces of the crystal  20  from breaking. 
     The sides of the plate  15 ,  16  are each coated with an electrically conductive layer  17 ,  18 , making a charge transport possible. One layer  17  runs on one side  15  to the top end of the crystal plate  22 . The layer  18  on the opposite side  16  runs on over the edge of the base  28  to the bottom of the base  25 . If the crystal  20  is clamped by suitable holding devices  12 ,  13  (see FIG. 4 a ), opposed charges can be picked up on the bottom of the base  25  and on the end face  24  of the plate  22 . At the end face  24  and the base  25 , it is essential that the electrically conductive layers  17 ,  18  are insulated electrically from each other. To ensure this, an insulating bevel  29  may be provided at the end face  24 . Face  34  at the base  21  of the crystal plate  22 , may be insulated by removing all or part of its conductive layer  17  such that any connection between the electrically conductive layers  17 ,  18  is interrupted. 
     FIG. 4 a  shows a crystal  70  fitted in a sensor  10 . The bottom holding device  13  has a drilled area or recess  30  into which the base  21  can be fitted. This recess  30  must be less deep than the height of the edge of the base  28 , to ensure that the side  15  having the electrically conductive layer  17  leading upwards has no electrical contact with an edge of the recess  30 . Face  34  may be insulated by removing all or part of its conductive layer  17 . The other side  16  having the electrically conductive layer  18  must have a good electrical contact at the bottom of the recess  30 . 
     FIG. 4 b  shows a plan view and partial cross sectional view of the crystal  70  fitted into recess  30  of holding device  13 . The edge of the base  28  may be circular, at least in part, and has partially rounded contours  31  formed on the base  21 . This ensures that the crystal  70  fits into the recess  30  of the holding device  13  (See FIG. 4 a ). The curvature or contours  31  may be continuous and extend over the side faces  32  of the crystal plate  22 . The curvature  31  on the base  21  should not be continuous on side  33  parallel to the crystal plate  22 . Otherwise, the forming of curvature  31  would remove the electrically conductive layer  18  completely, which would have to be restored again to assure contact with the bottom surface of the base  25 . By forgoing a complete curvature  31  on side of base  33 , the electrically conductive layer  18  is retained and contact with the electrically conductive layer  18  of the bottom of the base  25  is assured. 
     A method or process for the mass production of crystals, such as crystal  20 , according to the present invention, is shown in FIG. 5. A crystal wafer  40  may be in rectangular form, for example, (other geometric forms are possible). The wafer  40  may be cut in a first process stage or step so that a plate  41  of a desired thickness T is obtained, with a base ledge  42  running at least along one edge of the plate  41 . Here it is essential that the transition from the plate  41  to the base ledge  42  has a curvature  23  (shown as concave) in accordance with the present invention. In a further process stage the crystal wafer  40  is coated completely with an electrically conductive layer, except for end faces  43 . After this, the electrically conductive layer is broken through, preferentially at two areas. One of these areas is on or along one edge of base ledge  44  on one side of wafer  40 . The other area may be provided on or along end face  24  diametrically and on the other side of the crystal wafer  40 . At these areas, it is advisable to provide insulating bevels  29 ,  45 . This results in two electrically conductive layers  17 ,  18  isolated electrically from each other. 
     In a further process step, the crystal wafer  40  (See FIG. 5) may be divided into two or more smaller crystals  20 , all having a base  21  and electrically conductive layers  17 ,  18  (see FIG.  3 ). Each base  21  of crystal  20  may have at least one partially rounded contour or curvature  31  on one or more of four comers of the base  21  which may extend over the sides  32  of the plate  22  without interruption (see FIG.  3 ). 
     In a further process step, each crystal  20  may be provided with bevels  26  on the bottom edge of the base  21  and along the edge of the end face  24 . However, the electrically conductive layer  17  on end face  24  must not be interrupted. The bevels  26  may be produced on the crystal wafer  40  before the electrically conductive layer is applied to the wafer  40 . 
     The crystal  20  is inserted into the recess  30  of the holding device  13  by inserting the base  21  first. Care must be taken to ensure that the recess  30  is large enough to have some play to allow insertion of the crystal  20  without breaking. The edge of the base  28  may be about twice as high as the depth of the recess  30 . The crystal  20  is not clamped in the recess  30 , but is held sufficiently rigid to allow the second holding device  12  to be fitted on the opposite end of crystal  20  without the crystal  20  being able to shift off-center or tilt. 
     The overall height of crystal  20  may be between approximately 1 and 40 mm, and preferably between 2 and 10 mm. The height of the base  21 , including the rounded contours  31  to the crystal plate  22 , may be approximately {fraction (1/10)} th  to ⅓ rd  of the overall height of the crystal  20 . The crystals  20  described herein are suited for use in metrology, and in particular, for measuring forces, pressures, accelerations, moments and strains. 
     Another embodiment of the present invention, crystal  50 , is shown in FIG.  6 . This double crystal  50  may have two or more crystal plates  22 , joined by a common base  21 . This arrangement provides approximately a double load capacity of the crystal  50  under pressure or force, with the same sensitivity and overall height as crystal  20 . This structure is formed by removing material from the center of wafer  40  (see FIG. 5) down to the base  21 . Other configurations with more than two plates  22  are also possible. With this double crystal  50  configuration or similar configurations, it must be ensured that each crystal plate  22  has an electrically conductive layer  17 ,  18  on both sides, with the two layers  17 ,  18  of a plate  22  having different holding devices  12 ,  13 , respectively, in electrically conductive contact and insulated from the other. For this, the insulating surfaces or bevels  29  must be applied. A surface or surfaces of hole  59  may be provided with an electrically conductive layer  18  in order to conduct a charge from the interior conductive layer  18  to the bottom of the base  25 . Additionally, there may be other ways to electronically connect layer  18  with the bottom of base  25 , such as connecting another surface (not shown) of crystal  22  to the base  25 . To separate electrically charged layers  17  and  18 , surface  57  may be insulated by removing all or part of its conductive layer  17  adjacent hole  59 . 
     Another embodiment of the present invention, crystal  60 , is shown in FIG.  7 . This embodiment has two identical crystal plates  22 , each having a base  21  projecting beyond each of the crystal plates  22  at one side of the plate  22  only. The sides  16  of the crystals  20  having no base can thus be placed adjacent to each other or together. Since sides  16  have the same polarity, short-circuiting upon contact is ruled out. The production of this crystal  60  is analogous to the production of crystals  20  from wafer  40  already described except only one face of the wafer  40  is shaped. Placing two crystals together gives a configuration  60  similar but not identical to that of one crystal  20  in FIGS. 3 and 4 a . The difference is that generally, with equal geometrical conditions, the same force may be applied onto the crystal or crystals  20 , but the sensitivity of the configuration with the double crystal  60  should be approximately twice as high as a configuration with the single crystal  20 . That is because a surface area of the charge pickups, as shown but not identified in FIG. 7, is approximately double what is shown but not identified in FIG.  3 . When the width of the crystal plates  22  is doubled, load capacity is approximately doubled for the same sensitivity. With regard to a self-centering capability, the double crystal  60  in FIG. 7 is equal to that of the single crystal  20  in FIG.  3 . 
     In all the embodiments of the present invention, there is no need for ancillary or additional materials or aids to facilitate centering of the crystals  20 ,  50 ,  60  in a sensor  10 . Consequently, the application range of the crystals  20 ,  50 ,  60  in a sensor  10  is subject to no restrictions due to temperature. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that this is done by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.