Abstract:
A knock sensor of the invention includes a base having a cylindrical part mounted on a vibration generating part and a flange part, an annular piezoelectric element fitted to the cylindrical part and for converting a knocking vibration of the vibration generating part into an electric signal to detect it, electrodes respectively provided to be in contact with both surfaces of the piezoelectric element, terminal plates disposed to be in contact with the respective electrodes and for extracting an output of the piezoelectric element to outside, and a hold unit for pressure holding the piezoelectric element, the electrodes and the terminal plates to the flange part, in which a conductive adhesive material having an almost equal thickness to the electrode is provided on a portion of the piezoelectric element which is not in contact with the electrode.

Description:
CROSS REFERENCE 
   This is a divisional of application Ser. No. 10/964,608 filed Oct. 15, 2004 now U.S. Pat. No. 7,007,544. The entire disclosure of the prior application, application Ser. No. 10/964,608 is considered part of the disclosure of the accompanying divisional application and is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a knock sensor and particularly to a nonresonant knock sensor mounted on, for example, a vibration generating part of an internal combustion engine or the like and for converting a knocking vibration of the internal combustion engine into an electric signal to detect it. 
   2. Description of the Related Art 
   A structure of a conventional knock sensor will be described with reference to the drawings.  FIG. 8  is a sectional view showing an inner structure of a generally known knock sensor  70 . A metal base  21  mounted on a vibration generating part of an internal combustion engine or the like by a bolt or the like is constructed of a cylindrical part  21   b  in which a bolt through hole  22  is formed and a disk-like flange part  21   a  formed at its end. A lower insulating sheet  7  is slid onto the cylindrical part  21   b  to come in contact with the flange part  21   a , and the following are successively slid thereon. 
   That is, a lower terminal plate  5 , a piezoelectric element  4  having whole surface electrodes  4   a  at its upper and lower surfaces, an upper terminal plate  6 , an upper side insulating sheet  8 , and a weight  9  are slid thereon. Next, a nut  23  is screwed onto a male screw part  21   c  threaded on the tip end outer peripheral surface of the cylindrical part  21   b , and a tool such as a torque wrench is used to tighten the component parts to the flange part  21   a  at a predetermined torque. Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding. The base  21  except the inner peripheral surface and both end surfaces of the cylindrical part  21   b  is coated with a resin mold to form a case  13 , and a connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   This kind of knock sensor is attached to, for example, an internal combustion engine by a bolt inserted in the through hole  22  provided in the axial direction of the base  21 . When a knocking vibration is generated in the internal combustion engine, the component members, such as the piezoelectric element  4  and the weight  9 , constituting the knock sensor are vibrated integrally with the knocking vibration, this vibration is converted into a voltage signal by the piezoelectric element  4 , and the detection signal is outputted from the terminal part  14  to the outside through the lower terminal plate  5  and the upper terminal plate  6  (see, for example, patent document 1). 
   [patent document 1] JP-A-2002-257624 
   The conventional knock sensor is constructed as described above, and since the electrodes  4   a  of the piezoelectric element  4  are provided on the whole surfaces in contact with the terminal plates  5  and  6 , the electrostatic capacity of the piezoelectric element  4  is determined by the thickness of the piezoelectric element and a level corresponding to the area of the whole surface electrode, and the level of the output signal extracted from the knocking vibration is also determined to a predetermined level. In order to change the level of the output signal extracted from the knocking vibration, it is necessary to change the thickness of the piezoelectric element  4  or its diameter. In this case, there also arises a necessity to change the outer shape of the knock sensor. 
   Thus, there arises a necessity to partially provide electrodes without providing the whole electrodes on the surfaces of the piezoelectric element  4  coming in contact with the terminal plates  5  and  6 . However, it has been found that when the piezoelectric element  4  is made to have partial electrodes, since a gap corresponding to the thickness of the partial electrode exists between the piezoelectric element  4  and the terminal plates  5  and  6 , there are following problems. 
   That is, in the piezoelectric element  4 , when a polarization processing of the partial electrode part is performed, the polarization action is also exerted on an electrodeless part around the partial electrode. Although electric charges generated by a pyro (pyroelectric) effect caused by environmental temperature change are successively discharged through the terminal plates  5  and  6  at portions where the electrodes exist, they are stored at portions where the electrodes do not exist. The electric charges stored in the electrodeless parts of the piezoelectric element produce an electrical breakdown by a voltage corresponding to the gap existing between the piezoelectric element  4  and the terminal plate  5  or  6 , and are instantaneously discharged to the terminal plate  5  or  6 . 
   The electric charges transferred to the terminal plate  5  or  6  by this discharge are applied to the piezoelectric element  4  in the form of a return current. At this time, the electric charge of the same polarity as the electrode polarity of the piezoelectric element  4  is applied, so that the piezoelectric element  4  is instantaneously expanded in the polarization direction, and an electric charge with reverse polarity is generated in the inside. As stated above, in the case where the electrode of the piezoelectric element is made the partial electrode in the structure of the conventional knock sensor, there is a problem that noise is superimposed on the output of the knock sensor. 
   SUMMARY OF THE INVENTION 
   The invention has been made to solve the problems as described above, and has an object to provide a knock sensor in which in a case where it is necessary to provide knock sensors having the same outer shape and different output sensitivities, the generation of output noise due to temperature change can be suppressed. 
   A knock sensor of this invention includes a base having a cylindrical part mounted on a vibration generating part and a flange part provided at a part of the cylindrical part, an annular piezoelectric element fitted to the cylindrical part and for converting a knocking vibration of the vibration generating part into an electric signal to detect it, electrodes respectively provided to be in contact with both surfaces of the piezoelectric element, terminal plates disposed to be in contact with the respective electrodes and for extracting an output of the piezoelectric element to outside, and a hold unit for pressure holding the piezoelectric element, the electrodes and the terminal plates to the flange part, wherein the electrodes are provided to be in partial contact with the piezoelectric element, and an electric insulating material having an almost equal thickness to the electrode is provided on a portion of the piezoelectric element which is not in contact with the electrode. 
   Since the knock sensor of this invention is constructed as described above, the electrostatic capacity of the piezoelectric element can be changed by changing the area of the partial electrode of the piezoelectric element, and the knock sensors having different output sensitivities can be obtained to have the same outer shape. Further, the knock sensor can prevent discharge to the terminal plate by electric charges generated in the electrodeless part of the piezoelectric element by a pyro effect caused by temperature change, and it becomes possible to obtain such a stable output that noise is not superimposed even when temperature change occurs. 
   Besides, it becomes possible to adopt a manufacturing method in which the coating of the electric insulating material is reasonably and uniformly applied to the piezoelectric element or the terminal plate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view showing an inner structure of a knock sensor of embodiment 1 of the invention. 
       FIGS. 2A and 2B  show a structure of a piezoelectric element in embodiment 1, in which  FIG. 2A  is a plan view, and  FIG. 2B  is a sectional view taken along line E—E. 
       FIG. 3  is a sectional view showing an inner structure of a knock sensor of embodiment 2 of the invention. 
       FIG. 4  is a sectional view showing an inner structure of a knock sensor of embodiment 3 of the invention. 
       FIG. 5  is a sectional view showing an inner structure of a knock sensor of embodiment 4 of the invention. 
       FIG. 6  is a sectional view showing an inner structure of a knock sensor of embodiment 5 of the invention. 
       FIG. 7  is a sectional view showing an inner structure of a knock sensor of embodiment 6 of this invention. 
       FIG. 8  is a sectional view showing a structure of a conventional knock sensor. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiment 1 
   Hereinafter, embodiment 1 of this invention will be described with reference to the drawings. 
     FIG. 1  is a sectional view showing an inner structure of a knock sensor of embodiment 1 of this invention. 
   The knock sensor  1  is constructed such that all component parts are covered with a case  13  made of synthetic resin (for example, nylon 66), and a connector part  15  for connection with a connector from an ignition timing control device (not shown) is formed on the case  13 . 
   A base  2  is made of metal, and is constructed of a cylindrical part  2   b  having a through hole  3  and a disk-like flange part  2   a  provided at its one end. Plural engagement grooves  11  and  12  for ensuring engagement with the case  13  and preventing intrusion of water from the outside are formed on the outer peripheral surface of the flange part  2   a  and the tip end (upper end in the drawing) outer peripheral surface of the cylindrical part  2   b.  An annular piezoelectric element  4  is slid onto a portion of the cylindrical part  2   b  close to the flange part  2   a . The detailed structure of the piezoelectric element  4  is shown in a plan view of  FIG. 2A  and a sectional view of  FIG. 2B  taken along line E-E. 
   That is, partial electrodes  4   b  each having a predetermined thickness (several microns to ten and several microns) are formed on both surfaces of the piezoelectric element  4  which are surfaces coming in contact with terminal plates  5  and  6 . A center hole part  4   c  is slid onto the cylindrical part  2   b , and a vibration in an axial direction transmitting through the base  2  is outputted as a voltage signal. 
   The partial electrode  4   b  of the piezoelectric element  4  is formed into a shape coming in contact with a part, not the whole surface, of the terminal plate of the piezoelectric element  4 , and is subjected to a polarization processing. The piezoelectric element  4  has an electrostatic capacity corresponding to the thickness thereof and the area of the partial electrode  4   b , and becomes an element to determine the output sensitivity of the knock sensor  1 . 
   An electric insulating material A is filled in a gap existing between an electrodeless part of the surface of the piezoelectric element  4 , that is, a portion where the partial electrode  4   b  is not provided and the terminal plates  5  and  6  and corresponding to the thickness of the partial electrode  4   b  of the piezoelectric element  4 . As the electric insulating material A, silicone grease or silicone oil is suitable. In order to apply the electric insulating material A to the electrode formation surface of the piezoelectric element  4 , there is a method in which the piezoelectric element  4  is rotated under a discharge port of a dispenser for discharging a predetermined material, and a uniform coating is formed on a predetermined coated surface, or a method in which the piezoelectric element  4  is fixed and a dispenser is moved along a predetermined locus to apply a coating. 
   Further, a method may be adopted in which a predetermined material is made to drop on rotation rollers, and a piezoelectric element  4  is made to pass through between the rotation rollers, so that a coating is applied to upper and lower electrode formation surfaces at the same time. 
   The lower terminal plate  5  is formed of a conductive metal plate, and in a state where the lower terminal plate is slid onto the cylindrical part  2   b  of the base  2 , it is in contact with the partial electrode  4   b  from the lower surface of the piezoelectric element  4 . The upper terminal plate  6  is also formed of a conductive metal plate, and in a state where the upper terminal plate is slid onto the cylindrical part  2   b  of the base  2 , it comes in contact with the partial electrode  4   b  from the upper surface of the piezoelectric element  4 . 
   A lower insulating sheet  7  of a thin plate-like insulating resin formed into a ring shape is disposed between the lower terminal plate  5  and the flange part  2   a , and insulates the lower terminal plate  5  from the flange part  2   a . Similarly, an upper insulating sheet  8  of a thin plate-like insulating resin formed into a ring shape is disposed on the upper surface of the upper terminal plate  6 , and insulates the upper terminal plate  6  from an after-mentioned weight  9 . 
   The weight  9  is for giving exciting force to the piezoelectric element  4 , is formed into a ring shape, is slid onto the cylindrical part  2   b  of the base  2 , and is provided at the upper side of the upper insulating sheet  8 . A nut  23  is screwed on to a male screw part  2   c  threaded on a tip outer peripheral surface of the tube  2   b . The nut tightens the lower insulating sheet  7  slid onto the cylindrical part  2   b , the lower terminal plate  5 , the piezoelectric element  4 , the upper terminal plate  6 , the upper insulating sheet  8 , and the weight  9  at a predetermined tightening torque, and fixes them in a state where they are pressure held between the nut and the flange part  2   a.    
   As a result, the electric insulating material A applied to the electrode formation surface of the piezoelectric element  4  flows into a gap existing between the electrodeless part of the electrode formation surface of the piezoelectric element  4  and the terminal plates  5  and  6  and corresponding to the thickness of the partial electrode  4   b  of the piezoelectric element  4 , and is filled therein. Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding, and the base  2  except the inner peripheral surface and both end surfaces of the cylindrical part  2   b  is coated with a resin mold to form the case  13 , while the connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   As stated above, in the knock sensor according to embodiment 1, the partial electrode  4   b  is formed on the electrode formation surface of the piezoelectric element  4 , and the electric insulating material A is filled in the gap existing between the electrodeless part and the terminal plates  5  and  6  and corresponding to the thickness of the partial electrode of the piezoelectric element. 
   By adopting the structure as stated above, it becomes possible to change the electrostatic capacity of the piezoelectric element by changing the electrode area of the partial electrode  4   b , and knock sensors having different output sensitivities and the same outer shape can be obtained. Further, it is possible to prevent the electric charge generated in the electrodeless part of the piezoelectric element by the pyro effect caused by temperature change from being discharged to the terminal plate, and the stable output can be obtained in which noise is not superimposed even when temperature change occurs. 
   Embodiment 2 
   Next, embodiment 2 of this invention will be described with reference to the drawings. 
     FIG. 3  is a sectional view showing an inner structure of a knock sensor  20  of embodiment 2 of the invention. 
   Although embodiment 2 has the same structure as embodiment 1, a conductive adhesive B is filled in a gap existing between an electrodeless part of an electrode formation surface of a piezoelectric element  4  and terminal plates  5  and  6  and corresponding to the thickness of a partial electrode  4   b  of the piezoelectric element  4 . As the conductive adhesive B, an epoxy adhesive containing conductive metal powder is suitable. 
   In order to apply the conductive adhesive B to the electrode formation surface of the piezoelectric element  4 , there is a method in which the piezoelectric element  4  is rotated under a discharge port of a dispenser for discharging a predetermined material, and a uniform coat is applied to a predetermined coated surface, or a method in which the piezoelectric element  4  is fixed and a dispenser is moved along a predetermined locus to apply a coating. 
   After the conductive adhesive B is applied to the electrode surface of the piezoelectric element  4 , similarly to embodiment  1 , the lower insulating sheet  7 , the lower terminal plate  5 , the piezoelectric element  4 , the upper terminal plate  6 , the upper insulating sheet  8 , and the weight  9  are successively slid onto the cylindrical part  2   b  of the base in this order from below, and are stacked up on the flange part  2   a.    
   Thereafter, a nut  23  screwed onto a male screw part  2   c  threaded on a tip outer peripheral surface of the cylindrical part  2   b  is tightened at a predetermined torque by using a tool such as a torque wrench tightening head, and the above component parts are fixed in a state where they are pressure held between the nut and the flange part  2   a . As a result, the conductive adhesive B applied to the electrode formation surface of the piezoelectric element  4  flows into a gap existing between the electrodeless part of the electrode formation surface of the piezoelectric element  4  and the terminal plates  5  and  6  and corresponding to the thickness of the partial electrode  4   b  of the piezoelectric element  4 , and is filled therein. 
   In this state, heating is performed for several hours in a drying furnace of 100° C. to 120° C., and the conductive adhesive B is dried. Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding, and the base  2  except the inner peripheral surface and both end surfaces of the cylindrical part  2   b  is coated with a resin mold to form a case  13 , while a connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   As stated above, in the knock sensor of embodiment 2, the partial electrode  4   b  is formed on the electrode formation surface of the piezoelectric element  4 , and the conductive adhesive B is filled in the gap existing between the electrodeless part of the electrode formation surface and the terminal plates  5  and  6  and corresponding to the thickness of the partial electrode of the piezoelectric element. 
   By adopting the structure as stated above, the electrostatic capacity of the piezoelectric element can be changed by changing the electrode area of the partial electrode  4   b , and knock sensors having different output sensitivities and the same outer shape can be obtained. Further, the electric charge generated in the electrodeless part of the piezoelectric element by the pyro effect caused by temperature change is transferred to the terminal plate through the conductive adhesive B at any time, and discharge does not occur between the electrodeless part of the piezoelectric element and the terminal plate. Thus, even when temperature change occurs, noise is not superimposed, and stable output can be obtained. Besides, it is possible to adopt a manufacturing method in which application of the conductive adhesive B to the piezoelectric element is reasonably and uniformly performed. 
   Embodiment 3 
   Next, embodiment 3 of this invention will be described with reference to the drawings.  FIG. 4  is a sectional view showing a structure of embodiment 3. In a knock sensor of embodiment 3, a gap existing between an electrodeless part of an electrode formation surface of a piezoelectric element  4  and terminal plates  5  and  6  and corresponding to the thickness of a partial electrode  4   b  of the piezoelectric element  4  is filled with an electric insulating material C containing a filler having a particle size smaller than the gap. Since the other structure is the same as embodiment 1, its description will be omitted. As a manufacturing method, manufacture can be made by the same manufacturing method as embodiment 1. However, since the filler in the electric insulating material C is dispersed without being collapsed in the gap, the material can be filled in the gap without damaging the physical properties of the electric insulating material C. 
   Embodiment 4 
   Next, embodiment 4 of this invention will be described with reference to the drawings. 
     FIG. 5  is a sectional view showing an inner structure of a knock sensor  40  of embodiment 4 of the invention. 
   Although the knock sensor of embodiment 4 is almost similar to the knock sensor of embodiment 1, a spring  10  is slid onto a cylindrical part  2   b  of a base  2  so that it is positioned on a weight  9 . 
   Since a lower insulating sheet  7  slid onto the cylindrical part  2   b , a lower terminal plate  5 , a piezoelectric element  4 , an upper terminal plate  6 , an upper insulating sheet  8 , and a weight  9  are pressure held between the nut  23  and a flange part  2   a  through the spring  10 , they can be firmly tightened and fixed. 
   Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding, and the base  2  except the inner peripheral surface and both end surfaces of the cylindrical part  2   b  is coated with a resin mold to form a case  13 . Further, a connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   Although  FIG. 5  shows the example in which the electric insulating material A is filled in the gap corresponding to the thickness of the partial electrode  4   b  of the piezoelectric element  4 , the conductive adhesive B may be filled therein instead of the electric insulating material A, and similar effects can be expected. 
   Embodiment 5 
   Next, embodiment 5 of the invention will be described with reference to the drawings. 
     FIG. 6  is a sectional view showing an inner structure of a knock sensor  50  of embodiment 5 of the invention. 
   Although the knock sensor of embodiment 5 is almost equal to the knock sensor of embodiment 1, instead of the nut  23 , a stopper ring  24  is slid onto a cylindrical part  2   b  of a base  2  and is caulked into a groove  25  of the cylindrical part  2   b  in a state where a weight  9  slid onto the cylindrical part  2   b,  an upper insulating sheet  8 , an upper terminal plate  6 , a piezoelectric element  4 , a lower terminal plate  5 , and a lower insulating sheet  7  are pressure held between the stopper ring and a flange part  2   a  of the base  2 . 
   Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding, and the base  2  except the inner peripheral surface and both end surfaces of the cylindrical part  2   b  is coated with a resin mold to form a case  13 , while a connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   Although  FIG. 6  shows the example in which the electric insulating material A is filled in the gap corresponding to the thickness of the partial electrode of the piezoelectric element  4 , the conductive adhesive B may be filled therein instead of the electric insulating material A, and similar effects can be expected. 
   Embodiment 6 
   Next, embodiment 6 of this invention will be described. 
     FIG. 7  is a sectional view showing an inner structure of a knock sensor  60  of embodiment 6 of this invention. 
   Although the knock sensor of embodiment 6 is almost equal to the knock sensor of embodiment 5, a spring  10  of a coned disc spring is slid onto a cylindrical part  2   b  between a caulked stopper ring  24  and a weight  9 . The weight  9 , an upper insulating sheet  8 , an upper terminal plate  6 , a piezoelectric element  4 , a lower terminal plate  5 , and a lower insulating sheet  7  are pressure held between the stopper ring and a flange part  2   a  of the base through the coned disc spring  10 . 
   Thereafter, a terminal part  14  is bonded to the lower terminal plate  5  and the upper terminal plate  6  by soldering or resistance welding, and the base  2  except the inner peripheral surface and both end surfaces of the cylindrical part  2   b  is coated with a resin mold to form a case  13 , while a connector part  15  for extracting a signal is integrally and simultaneously molded to protrude from one side surface of the case  13 . 
   Although  FIG. 7  shows the example in which the electric insulating material A is filled in the gap corresponding to the thickness of the partial electrode of the piezoelectric element  4 , the conductive adhesive B may be filled therein instead of the electric insulating material A, and similar effects can be expected.