Abstract:
A pressure restrictor for use with a pressure sensing element comprising a cylindrical member of a given diameter and length having a plurality of apertures directed from a first end to a second end is disclosed. A pressure restrictor housing holds and positions the cylindrical member in close proximity to the pressure sensing element at the second end by clamping the pressure restrictor between the housing and element. The apertures provided in the pressure restrictor are the sole path for application of pressure to the sensing element. The sensing element is a double diaphragm silicon sensor where one diaphragm is exposed to pressure and the other diaphragm is not exposed to pressure. Each diaphragm has located thereon a half-bridge wherein both half bridges are connected to provide a full bridge. The full bridge provides an output strictly proportional to pressure.

Description:
FIELD OF THE INVENTION 
   This invention relates to pressure transducers and more particularly to a pressure transducer for combustion measurements employing a pressure restrictor. 
   BACKGROUND OF THE INVENTION 
   Pressure transducers are employed for measuring the combustion dynamics in an internal combustion engine. Such transducers are referred to as combustion transducers. The transducer desirably should be of a small size and have the capability of performing static and dynamic measurements. The transducer should have a high natural frequency for fast data rates and be extremely durable for the typical combustion engine environment. 
   As one can ascertain, a combustion engine is an extremely reliable device which can operate under relatively high temperatures in very different environments. The Assignee herein, namely, Kulite Semiconductor Products, Inc. has a number of patents which involve a technology referred to as silicon on insulator (SOI) leadless technology. These sensors are capable of withstanding harsh environments, extreme operating temperatures in excess, for example of 600° C. and high vibrations. 
   As will be described the device employed herein utilizes an innovative transducer design employing a vibration insensitive sensing element. This transducer employs two diaphragms with one exposed to pressure and inertial forces and the other exposed only to inertial forces. Each diaphragm is associated with a half bridge which half bridges are wired into a full bridge employing differential action where inertial forces are cancelled. 
   The Assignee, namely, Kulite Semiconductor Products has various patents as well as pending applications which show transducer arrangements. 
   It is understood that a major aspect of the present invention is the use of a pressure restrictor which can be changed according to environment and which is simple to fabricate and operates with great efficiency. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a pressure restrictor for use with a pressure sensing element. This pressure sensing element is fabricated from silicon and includes two diaphragm. Each diaphragm includes a half bridge. One diaphragm is associated with a pressure input while the other diaphragm is isolated from the pressure input. In this manner the two half bridges are wired to form a full Wheatstone bridge which full bridge will provide an output completely proportional to pressure while other effects such as inertial effects as acceleration and so on are cancelled. In order to utilize the pressure sensing element in a harsh environment such as that provided by a combustion engine, the sensing element is associated with a pressure restrictor. The restrictor consists of a cylindrical member of a given diameter and length. The member having a plurality of apertures directed from the first end to a second end. The apertures restrict the frequencies which can be applied to the pressure sensing element. 
   A pressure restrictor housing is provided for holding and positioning the cylindrical restrictor member in close proximity to said pressure sensing element at said second end, with said apertures providing the sole path for application of pressure to said sensing element. The pressure restrictor is located in an aperture of said housing for enabling said first end to receive an applied pressure at a given frequency, which applied pressure is received by only one of said diaphragms. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a cross sectional view of a combustion transducer employing a pressure restrictor device according to this invention. 
       FIG. 2  is a perspective view of a pressure restrictor according to this invention. 
       FIG. 3  is a top plan view of a pressure sensing element utilized in conjunction with this invention. 
       FIG. 4  is a cross sectional view of the pressure sensing element depicted in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , there is shown a cross sectional view of a combustion transducer according to this invention. Essentially as one can see from  FIG. 1 , there is a pressure sensing element  12  which is a silicon sensing element which operates to sense applied pressure. In any event, the pressure sensing element  12  is arranged in a front section or front adaptor section of a screw housing  17 . The housing  17  is a metallic structure and has a central aperture  18  which aperture allows leads such as  19  and  22  to extend from the pressure sensing element  12  through the screw housing  17 . The housing  17  is of a conventional design. 
   As can be seen, the leads  19  and  22  are surrounded by ceramic insulators  16 . These ceramic insulators serve to position the leads firmly within the housing and to further insulate the leads. This enables the transducers to operate at high temperatures. The ceramic insulation are stacked one against the other as seen to fully cover the leads. 
   The pressure sensing element  12  is incorporated within a header  15  through which header the leads as  19  and  22  extend. The header  15  may be fabricated from a high temperature glass. 
   As seen in  FIG. 1 , there is a mechanical pressure restrictor housing  21  which essentially has a front aperture  22 . The front aperture  22  enables one to position the pressure restrictor  20  into the front aperture with the flange  23  of the restrictor abutting against the peripheral wall of the mechanical pressure restrictor housing  21 . The pressure restrictor  20  as seen has a plurality of apertures such as  25  and  26 . 
   In any event, the novel construction of the internal combustion transducer  10  incorporates the interchangeable pressure restrictor  20 . The pressure restrictor  20  essentially is a filter and is positioned between the sensing element  12  and the pressure source P. The mechanical pressure restrictor housing  21  receives the front portion  28  of the cylindrical pressure restrictor  20 . The front portion  28  is inserted into aperture  22  of the housing  21 . The flange  23  associated with the larger diameter back portion  29  of the restrictor  20  abuts against the inner housing wall  24  and is held in place when the screw housing  17  is screwed into the mechanical restrictor housing  21  by means of screw threads  11  of housing  17  and threads  27  of housing  21 . The restrictor  20  is located as close to the sensing element as possible. This enables one to reduce the depth of the cavity. The restrictor  20  is mechanically clamped between the sensor  12  and the mounting housing  21 . This construction enables one to interchange the restrictors  20  with ease and to select the restrictor frequencies based on different restrictors to customize the frequency response of the transducer for the particular measuring application. 
   The filter or restrictor  20  design can be varied to have different size holes as  25  and  26  as well as other varying dimensions. As one can ascertain the filter can be made longer or shorter. The back end  30  of the restrictor  20  abuts almost directly at the front of the housing  31  so that it is close contact with the pressure sensing element  12 . In this manner lower frequency filters or restrictors  20  can be used for evaluation purposes. The restrictor is clamped in place by the housing  17  and the housing  21 . 
   The size and the number of apertures as  25  and  26  can be changed. Higher frequency filters can be implemented for evaluation of knocking and other phenomena associated with internal combustion engines. These phenomena are accompanied with higher frequency operation. Overall, the frequency of the transducer  10  can be adjusted from 15 KHz up to 150 kHz via the filter restrictor selection process. The filter  20  or restrictor as seen is basically cylindrical with the back flange  23  enabling it to abut against the aperture. 
   Referring to  FIG. 2 , there is shown a perspective view of a restrictor  20  or filter as depicted in cross sectional view in  FIG. 1 . 
   As seen the restrictor consists of a cylindrical front portion  40  which is contiguous with a larger back cylindrical portion  42 . There is an indented flange  23  between the cylindrical portions  40  and  41 . 
   As seen there are a number of apertures such as  25 ,  26  as depicted in  FIG. 1  and apertures  36 ,  35 ,  37  and  38 . As one can ascertain, the apertures depicted in  FIG. 2  are 6 in number. It is also understood that there can be more or less apertures depending upon the frequency response required. The diameter of the apertures as well as the configuration can be varied. 
   In any event, the apertures or filter holes as  25 ,  37  and so on extend through the restrictor  20  from the front surface  50  to the back surface  30 . This is of course clearly shown in  FIG. 1  for apertures  25  and  26 . The diameter of the apertures can be varied as well as the number and arrangement according to the designed frequency of operation. 
   Furthermore, the peripheral flange separating the front portion  40  from the back cylindrical portion  42  can be of a different depth and enables the top portion of the flange to exhibit a close coupling to the back wall of the mechanical pressure restrictor  21  due strictly to the clamping action of housings  17  and  21 . 
   The dimensions of the front portion  40  can be varied as the front section  40  can be made greater or smaller in length. The flange  23  height can be varied to enable different sized restrictors to be properly clamped. This is true of the back section  40  too as long as it is properly accommodated in the aperture  22  of the mechanical pressure restrictor  21 . 
   As is clearly seen from  FIG. 1 , the peripheral flange  23  with the extended ends abuts against the back wall  24  of the mechanical pressure restrictor  21 . The flange enables good coupling while preventing undue vibration of the restrictor element. Basically shown in  FIG. 1  the mechanical pressure restrictor housing  21  which accommodates the pressure restrictor  20  and which accommodates the screw housing  17  is positioned at any desired location within the internal combustion engine and operates to hold the pressure sensing element  12  in close contact with the restrictor  20  which as indicted is clamped in position between housing  17  and housing  21 . 
   Referring to  FIG. 3 , there is shown a top plan view of a pressure sensing element or a pressure sensing combination which is utilized in this invention. The pressure sensor depicted in  FIG. 3  is a Kulite innovation and essentially reference is made to a co pending application Kulite-103 entitled “Transducer Responsive To Pressure, Vibration/Acceleration and Temperatures and Methods of Fabricating The Same”. This application, namely Kulite 103 was filed on Dec. 3, 2004 and depicts a pressure sensing element which is the element utilized in this particular invention. 
   In any event, by referring to  FIG. 3 , there is shown a top plan view of the typical pressure sensing element. The sensing element is based on a silicon on insulator (SOI) leadless technology. These sensors are capable of light temperature operation of 500° C. and higher and withstand high vibration. As shown in  FIG. 3 , the sensor employs two deflecting diaphragms depicted by reference numerals  54  and  55 . Each diaphragm has piezoresistors located thereon and on each diaphragm one half of a Wheatstone bridge is formed utilizing two piezoresistors in series. 
   Typically, one piezoresistor of each pair increases in resistance with a positive normal stress applied to the plane of the associated diaphragm while the other decreases. One sensing diaphragm for example diaphragm  55  is exposed to the pressure media while the other diaphragm is isolated from the media. Due to the geometry of the sensing elements, both sensing diaphragms respond to shock or vibration, however only one sensing diaphragm will respond to the applied pressure signal. 
   The two half bridges from each diaphragm are electrically coupled to form a full bridge such that for a positive stress applied substantially normal to both diaphragms, the bridge output of one half bridge will subtract from the other. Thus, the signal output is responsive to the pressure applied to one diaphragm while the signal response to inertial stresses and indeed any other stress other than that due to pressure applied to both diaphragm is cancelled. 
   Thus, the sensor depicted in  FIG. 3  provides an output that is proportional to pressure only. Complete cancellation is dependent on the two deflecting diaphragms having the same size thickness and matching piezoresistive characteristics. The sensors are typically used in 100 to 500 bar range. The piezoresistive sensors themselves have an internal frequency in these pressure ranges in excess of 1 Mz. However, in the construction of the transducer there exist a cavity in front of the diaphragm and depending on its depth it produces an organ type resonance that is at a significantly lower frequency. 
   Additionally, the transducers are generally installed by being threaded into an adaptor which is itself threaded into the internal combustion engine. This is shown in  FIG. 1 . Thus, in this structure the resulting internal frequencies are determined by the organ pipe effect induced by the fitting into which the transducer is installed. Such fitting being the one that is threaded into the engine. It is desired to use the restrictor  20  located in the proximity of the transducer to avoid particle contamination at a defined tuned frequency of the transducer mounting adaptor. Thus, the sensing arrangement shown in  FIG. 3  is particularly suitable for use as a combustion transducer. This is due to pressure in which the restrictor  20  can be changed. 
   In  FIG. 4 , there is shown a cross sectional view of the sensing device depicted in  FIG. 3 . 
   As one can ascertain from  FIG. 4 , there is shown a top cover  200  which may be conventionally formed of glass or silicon as coupled to a silicon wafer  100  by means of a conventional seal. The silicon wafer  100  is treated by conventional photolithography techniques to produce a central boss area and a diaphragm section which is a thin section. The diaphragm section contains the sensing resistors such as  70 ,  80 ,  91 ,  92 . Each of the diaphragms as  54  and  55  is associated with appropriate sensing resistors. There are apertures associated with each of the diaphragms which enable connection to metal contacts such as  52  and  53  depicted both in  FIG. 3  and  FIG. 4 . 
   Reference is also made to U.S. Pat. No. 5,955,771 entitled “Sensor For Use In High Vibrational Applications And Method For Fabricating Same, issued Sep. 21, 1999 to A. D. Kurtz et al. The entire disclosure of that patent is incorporated herein by reference. 
   The patent teaches a hermetically sealed device which can be used with the present invention. It is understood that other structures can be used as well. The resulting structure is a sensor without external leads being suitable for high temperature mounting and referred to as leadless sensors. 
   In any event, as seen in  FIG. 4 , pressure is only applied to the diaphragm  54  while no pressure is applied to diaphragm  55 . In this manner, diaphragm  54  produces an output indicative of pressure and external forces while diaphragm  55  produces an output indicative of only external forces. The external forces are inertial as vibration and acceleration. These other external forces are also applied to diaphragm  54 . 
   Thus, as one can ascertain, diaphragm either  54  or  55  can measure pressure as having the aperture  85  associated therewith. The other diaphragm as either  54  and  55  has no aperture and is closed off by the glass cover plate and therefore is not responsive to pressure. 
   While it is understood that in  FIG. 3 , diaphragm  54  was the diaphragm having an active area not measuring pressure, it is also understood that the diaphragms could be reversed depending on which diaphragm is associated with the aperture  85 . In any event in conjunction with the above-noted restrictor and the mechanical mounting for the restrictor one now has a transducer which is utilized in combustion applications requiring high pressures and high temperatures, with pressure frequency restrictor. 
   It should be apparent to those skilled in the art that various other alternate embodiments can be discerned and all are deemed to be encompass within the breadth and scope of the claim appended hereto.