Abstract:
The present invention provides a differential-pressure transducer having two sensors cross-coupled and independently excited. A first Wheatstone Bridge pressure sensor has a first sensitivity and is excited by a first voltage. A second Wheatstone Bridge pressure sensor has a second sensitivity and is independently excited by a second voltage different from said first voltage. The excitation voltages are independently adjusted to increase or decrease the sensitivities of the sensors to substantially match. The outputs of the sensors are cross-coupled to each other to reduce the offset difference errors between the pressure sensors. Sensitive electronics are isolated within the sealed housing to protect them from harsh surrounding media. The transducer is configured to provide either a four pressure differential pressure measurement or a three pressure gauge differential pressure measurement.

Description:
RELATED APPLICATIONS  
       [0001]    The present invention is related to U.S. patent application Ser. No. 09/704,376 filed on Nov. 2, 2000 by the present inventor which is incorporated by reference in its entirety into the present disclosure. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a pressure transducer, and more particularly to a differential-pressure transducer having two sensors cross-coupled and independently excited.  
           [0004]    2. General Background and State of the Art  
           [0005]    Differential pressure sensors are used to measure pressure differences between two pressure sources. It is known to use separate Wheatstone Bridge arrangements of interconnected resistances as the pressure sensors for measuring each of the two pressure sources. Ideally, in order to provide an accurate differential pressure measurement, the output voltage versus pressure characteristics for each of the bridge pressure sensors should be similar and should remain similar despite factors such as changing temperature and acceleration. U.S. Pat. No. 6,023,978 to Dauenhauer et al. electrically cross-couples the bridge pressure sensors to compensate for temperature and acceleration induced errors. However, Dauenhauer suffers from the disadvantage of requiring that the two sensors have substantially similar or substantially identical error characteristics in order for the cross-coupling to compensate for errors. In practice it is difficult to find sensors with such closely matching error characteristics. This results in reduced accuracy when using a design such as described by Dauenhauer. It would be desirable to provide compensation for the sensors so that they do not need to have substantially similar or substantially identical error characteristics for use as differential pressure sensors.  
           [0006]    One important application for differential pressure sensors is in the automobile industry, where they are used to measure the pressure difference between an engine&#39;s exhaust and intake manifolds. In such applications, the sensors are exposed to harsh contaminants. It is desirable to isolate the sensor from the surrounding contaminants while still accurately measuring pressure. U.S. Pat. No. 6,023,978 to Dauenhauer does provide isolation for the pressure sensors used in a differential pressure sensor layout, but in a rather bulky package. It would be desirable to isolate the sensors from harsh contaminants by enclosing them in a compact package.  
         INVENTION SUMMARY  
         [0007]    A general object of the present invention is to provide a differential pressure transducer that will provide accurate differential pressure measurements over a range of pressures and temperatures. Another objective is to protect the electronics from contaminants while packaging the transducer within a compact housing. These goals are achieved by the present invention comprising a first pressure sensor having a first sensitivity and excited by a first voltage, a second pressure sensor having a second sensitivity different from the first sensitivity and excited by a second voltage different from the first voltage, and wherein the first and second voltages are independently adjustable to increase or decrease the sensitivities of the first and second sensors to substantially match each other, and wherein the outputs of the sensors are cross-coupled to each other to reduce the offset difference errors between the pressure sensors.  
           [0008]    These goals are also achieved by a method for matching the output characteristics of a first and a second pressure sensor comprising the steps of, applying a first excitation to the first pressure sensor, applying a second excitation different than said first excitation to the second pressure sensor, independently adjusting the first and second excitations to increase or decrease the sensitivities of the first and second sensors to substantially match each other, and cross-coupling the outputs of the sensors to reduce the offset difference errors between the pressure sensors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is an electrical schematic diagram illustrating the electrical connections of two Wheatstone Bridge pressure sensors for implementing an embodiment of the independent-excitation cross-coupled differential-pressure transducer of the present invention.  
         [0010]    [0010]FIGS. 2A, 2B,  2 C are curves representing the output voltage versus detected pressure for the high and reference pressure sensors.  
         [0011]    [0011]FIG. 3 is a perspective view of the cross-coupled differential-pressure transducer of the present invention.  
         [0012]    [0012]FIG. 4 is a perspective view of a hybrid assembly resting in a top portion of the housing of the transducer of FIG. 3.  
         [0013]    [0013]FIG. 5 is an exploded perspective view of the transducer of FIG. 3.  
         [0014]    [0014]FIG. 6 is a semi-diagrammatic cross-sectional view of a portion of the hybrid assembly showing a chimney with a pressure sensor mounted inside.  
         [0015]    [0015]FIG. 7 is a semi-diagrammatic cross-sectional view also showing the structure of the pressure sensor mounted on the hybrid assembly.  
         [0016]    [0016]FIG. 8 is a semi-diagrammatic perspective view of an engine exhaust system using the cross-coupled differential-pressure transducer of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    [0017]FIG. 1 is an electrical schematic diagram illustrating the electrical connections of two Wheatstone Bridge pressure sensors  102  and  104  for implementing an embodiment of the independent-excitation cross-coupled differential-pressure transducer  106  (see FIG. 3) of the present invention. When used in the differential-pressure transducer  106 , the pressure sensor  102  can be used to measure a high pressure while the pressure sensor  104  can be used to measure a low, or reference pressure.  
         [0018]    Resistors R 1 , R 2 , R 3 , and R 4  form a first Wheatstone Bridge that comprises the high pressure sensor  102 . Resistors R 5 , R 6 , R 7 , and R 8  form a second Wheatstone Bridge that comprises the reference pressure sensor  104 . A voltage or current excitation source  108  for the sensor  102  is provided at an input node  11 O. Another voltage or current excitation source  112  for the sensor  104  is provided at an input node  114 . In a preferred embodiment, the sources  108 ,  112  provide independent voltage excitations to the sensors  102 ,  104  and are supplied from a signal conditioner network  138 . As is known in the art, signal conditioner networks serve to process a signal so as to make it compatible with a given device. Conditioning operations can include filtering, amplification, isolation, integration, differentiation, and rectification, for example. One skilled in the art will appreciate that although particular polarities of the power supplies and output signals are illustrated in FIG. 1, the circuit functions in the same manner if all of the polarities are reversed. Nodes  116  and  118  are coupled, through a node  120 , to a reference voltage, which is typically ground. Nodes  122  and  124  are coupled together to provide a −V out  output at node  126 . Nodes  128  and  130  are connected together at node  132  to provide a +V out  output. As is evident from FIG. 1, the Wheatstone Bridges that comprise sensors  102 ,  104  are connected in a cross-coupled fashion. That is, for the polarities of voltages illustrated, the positive output node  128  and the negative output node  132  are connected together and the negative output node  122  and the positive output node  124  are connected together. The −V out  and +V out  outputs of the nodes  126 ,  132  are electrically connected to the signal conditioner network  138  at signal conditioner network inputs  134 ,  136 , respectively. The signal conditioner network  138  can include a differential amplifier  139  and the −V out  and +V out  outputs of the nodes  126 ,  132  can be connected to the negative and positive inputs of the differential amplifier to provide a high level amplified differential measurement of the pressure outputs. The differential output can be output as a differential voltage V diff  at an output  137  of the signal conditioner network  138 .  
         [0019]    The excitation sources  108 ,  112  are controlled by the signal conditioner network  138 . The signal conditioner network  138  can adjust the voltages from excitation sources  108 ,  112  based on the −V out  and +V out  output signals of the nodes  126 ,  132 . The signal conditioner network  138 , excitation sources  108 ,  112  and differential amplifier  137  and signal conditioner network  138  can all be implemented on a single or on multiple ASIC chips.  
         [0020]    For a differential pressure transducer to work properly, the slopes of the output voltage versus measured pressure curves for each pressure sensor should substantially match. These slopes represent the sensitivity of the pressure sensors. In U.S. Pat. No. 6,023,978 to Dauenhauer et al, the sensitivities are matched by choosing sensor die that have been formed adjacent or next to each other on the wafer. In the present invention, the sensitivities of the sensors  102 ,  104  are matched by independently adjusting the voltages supplied by the excitation sources  108 ,  112 , respectively.  
         [0021]    The principal of sensitivity (slope) and offset correction in the present invention is explained with reference to FIGS. 2 a ,  2   b ,  2   c . The equation of the output voltage versus detected pressure for the high pressure sensor  102  is illustrated by a curve  140  in FIG. 2 a  and is described by the equation:  
         
       V 
       1 
       =b 
       1 
       +m 
       1 
       P 
       1 
     
         [0022]    where V 1  is the output voltage at the node  110  and is measured along the y-axis. P 1  is the pressure to be measured and is measured along the x-axis. FIG. 2 a  also illustrates the offset voltage b 1  where the curve  140  intercepts the y-axis. The slope m 1  of the curve  140  represents the sensitivity of the high pressure sensor  102  and is illustrated by Δy/Δx in FIG. 2 a . The equation of the output voltage versus detected pressure for the reference pressure sensor  102  is illustrated by a curve  142  in FIG. 2 a  and is described by the equation:  
         
       V 
       2 
       =b 
       2 
       +m 
       2 
       P 
       2 
     
         [0023]    where V 2  is the output voltage at the node  114 , P 2  is pressure to be measured, b 2  is the offset voltage and m 2  is the slope. The resulting differential output is then:  
           V   1   −V   2 =( b   1   −b   2 )+( m   1   P   1   −m   2   P   2 ) 
         [0024]    The signal conditioner network  138  controls the excitation sources  108 ,  112  to independently adjust the slopes of the curves  140 ,  142  to substantially match (m 1 ≈m 2 ≈m), as illustrated in FIG. 2 b . The differential output then becomes:  
           V   1   −V   2 =( b   1   −b   2 )+ m (P 1   −P   2 ) 
         [0025]    Here, “b 1 -b 2 ” is the induced offset pressure error or static line pressure error  144  of the curves  140 ,  142 . Independently changing the excitations simultaneously changes the offset between the pressure sensors  102 ,  104 . This causes static line pressure errors as the pressure changes. The electronic circuit at the output nodes  122 ,  124 ,  128 ,  130  needs the ability to correct the induced offset to properly calibrate the differential sensing system. When large excitations are needed to match the sensitivities of the pressure sensors  102 ,  104 , the offset difference error can exceed the circuit&#39;s correction ability.  
         [0026]    The present invention cross-couples the pressure sensors  102 ,  104  to minimize the offset difference errors and cancel noise when using separate excitation sources. This cross-coupling of the fully active Wheatstone Bridges also doubles the sensitivity of the system, thus requiring less amplification of the differential signal resulting in less noise. As illustrated in FIG. 2 c , the offset difference errors between the curves  140 ,  142  have been substantially eliminated and the equation for the differential output voltage becomes:  
           V   1   −V   2   =m ( P   1   −P   2 ) 
         [0027]    Thus, the differential output voltage V diff  of the two sensors  102 ,  104  at the output  137  is directly proportional to the difference between the high pressure and the reference pressure.  
         [0028]    The sensitivities and offsets of the pressure sensors  102 ,  104  should also be matched over a range of operating temperatures. The signal conditioner network  138  modulates the excitation voltages applied by the excitation sources  108 ,  112  to the input nodes  110 ,  114  to substantially match the sensitivities of the pressure sensors  102 ,  104 , over a range of operating temperatures. Ambient temperature is measured by circuitry  141  of the signal conditioner network  138 . The conditioner  138  then injects first and higher-order error-correcting signals to compensate the offset and span signal errors of each of the sensors  102 ,  104 .  
         [0029]    [0029]FIG. 3 is a perspective view of the cross-coupled differential-pressure transducer  106  of the present invention. The system  106  is enclosed in a housing  146 . Electrical connections to the system are made through an opening formed by walls  148 . The system has a high-pressure intake  150  and a low-pressure intake  152  extending outwardly from the housing. The intakes  150 ,  152  can be nozzles as illustrated in FIG. 3, or other suitable intake structures.  
         [0030]    In one application, illustrated in FIG. 8, the system  106  is used in an automobile engine for measuring differential pressure in the exhaust system  810 . The high pressure intake  150  can be connected via a hose or conduit  812  to measure the pressure at the engine&#39;s exhaust manifold  816  while the low pressure intake  152  can be connected via a conduit  814  to measure the pressure at the engine&#39;s intake manifold  818 . The exhaust manifold  816  typically provides a pressure of between 0-19 psi to the high-pressure intake  150  while the intake manifold  818  typically provides a pressure of between 0-15 psi to the low-pressure intake  152 . This measurement is necessary for the exhaust gas recirculation valve (EGR).  
         [0031]    [0031]FIG. 4 is a perspective view of a hybrid assembly  410  resting in a top portion  412  of the housing  146 . The hybrid assembly  410  can be arranged on a ceramic substrate  414 . Chimneys  416 ,  418  are bonded to the substrate  414  to form a substantially fluid-tight connection. The chimneys are preferably ceramic, but can be made of other materials as well. The chimneys  416 ,  418  can be substantially cylindrical as illustrated in FIG. 4, or can have other shapes.  
         [0032]    [0032]FIG. 5 is an exploded perspective view of the cross-coupled differential-pressure transducer  106 . Formed in a lower housing portion  510  are grooves  512 ,  514 . Gaskets  516 ,  518  fit between the chimneys  416 ,  418  and the grooves  512 ,  514  to form a substantially fluid-tight seal between the chimneys  416 ,  418  and the intakes  150 ,  152 . The hybrid assembly  410  is secured to the top portion of the housing  412  and the top portion of the housing  412  is sealed to the lower housing portion  510  to provide a substantially fluid-tight connection. In the present application, fluid-tight is used to mean substantially impermeable by a fluid. In some applications the fluid is a gas while in others the fluid is a liquid. Thus, when the housing  146  is assembled, the space within the chimneys  416 ,  418  is open to the outside of the housing  146  through the intakes  150 ,  152 . The parts of the hybrid assembly  410  outside the chimneys  416 ,  418  are sealed within the housing  412 ,  510 , isolated from the surroundings.  
         [0033]    [0033]FIG. 6 is a semi-diagrammatic cross-sectional view of a portion of the hybrid assembly  410  showing one of the chimneys  416  with one of the pressure sensors  102  mounted inside. FIG. 7 is a more detailed semi-diagrammatic cross-sectional view also showing the structure of the pressure sensor  102  mounted on the hybrid assembly  410 . It should be noted that each of the sensors  102 ,  104  are similarly mounted, and thus the descriptions with respect the mounting of sensor  102  illustrated in FIGS. 6 and 7 apply equally to the mounting of sensor  104 .  
         [0034]    The configurations illustrated in FIGS. 6 and 7 differ in that FIG. 7 illustrates an embodiment in which a hole passes through the substrate  414  into a chamber  622 , while in FIG. 6 the chamber  622  is partially evacuated and sealed. The configuration of FIG. 6 is used to provide a differential pressure measurement of the pressures supplied through the intakes  150 ,  152  to the pressure sensors  102 ,  104 , respectively. A differential pressure measurement based on four different pressures is provided since, in general, different pressures are supplied through the intakes  150 ,  152  and the partially evacuated and sealed chambers  622  of each sensor  102 ,  104  have slightly different pressures, typically in the range of between 0-½ psi. Of course, the pressures in the chamber  622  of the pressure sensors  102 ,  104  can be substantially the same. Also, at times the intakes  150 ,  152  can supply pressures substantially the same which will result in a differential pressure of substantially zero. The configuration of FIG. 6 is advantageous in that it isolates the chamber  622  from outside contaminants and from water vapor.  
         [0035]    The configuration of FIG. 7 is used in a gauge differential pressure configuration. In the gauge differential pressure configuration, a hole formed by walls  722  passes through the substrate  414  into the chamber  622 . The same reference pressure  724 , typically atmospheric pressure, is provided to the chambers of each of the pressure sensors  102 ,  104 . Thus an additional inlet can pass through the housing  146  to provide atmospheric pressure to the chamber  622 . In this open configuration, water vapor can freely enter the chamber  622 . If the working environment of the system  106  becomes very cold, the water vapor can freeze, pushing on the insides of the chamber  622 . Such pushing can lead to cracking of the pressure sensors  102 ,  104  or breaking away from the substrate  414 . This problem can be prevented by making the holes passing into the chamber  622  of the sensors  102 ,  104  sufficiently large. By making the holes sufficiently large, the ice formed from the water vapor does not have as much surface to push against, resulting in significantly less pushing force against the chamber walls of the sensors  102 ,  104 .  
         [0036]    [0036]FIGS. 6 and 7 both show the sensor  102  surrounded by gel  612 . The gel  612  protects the pressure sensors  102 ,  104  from contaminants introduced through the intakes  150 ,  152 . Here “gel” is defined as a colloidal suspension of a liquid in a solid, forming a jellylike material in a more solid form than a solution. The gel is specially selected to accurately transmit pressure  616  while isolating the pressure sensor electronics from harsh surrounding conditions. The gel can be, for example, Shin-Etsu, which stays soft over the −40 Celsius to 135 Celsius temperature range and does not exert extra pressure on the sensor. In some applications, for example when relatively clean gas is being measured for pressure, or when the pressure measuring system is to be used only temporarily and then discarded, the gel can be disposed of and the pressure sensor can be exposed directly to the gas rather than being exposed to the gas through the gel.  
         [0037]    The pressure sensor  102  can be mounted to the substrate  414  using a rubber adhesive layer  624 . The other chimney  418  has the pressure sensor  104  similarly situated within. Wirebonds  618  electrically connect the pressure sensors  102 ,  104  to traces  620  on the substrate  414  and carry a relatively high voltage. The pressure sensor  102  illustrated in FIGS. 6 and 7 is not drawn to scale. In particular, the chamber  622  is drawn to a greatly exaggerated scale relative to the pressure sensor  102 .  
         [0038]    The pressure sensors  102 ,  104  can be made of silicon as is known in the art. Alternatively, the sensors can be have thin-film, foil gauge or bulk silicon gauge designs. A thin diaphragm  712  is formed in the pressure sensor  102 . Several resistances  714 , corresponding to the resistances R 1 -R 4  illustrated in FIG. 1, are formed by injecting, for example, boron into the silicon of the diaphragm  712 . A PYREX cap  710  of borosilicate glass with a low coefficient of thermal expansion, and high chemical, heat shock, and thermal resistance is electrostatically bonded to the pressure sensor  102 . Materials other than PYREX can also be used. The chamber  622  is formed by the cap  710  sealed to the pressure sensor  102 . In the embodiment illustrated in FIG. 6, it is the cap  710  sealed to the pressure sensor  102  which forms the partially evacuated and sealed chamber  622 . In the embodiment illustrated in FIG. 7, the hole formed by the walls  722  passes through the substrate  414 , the rubber adhesive layer  624 , and the cap  710 . The cap  710  attaches the pressure sensor  102  to the substrate  414  by way of the rubber adhesive layer  624 . At the end of the pressure sensor  102  opposite the cap  710  is the thin diaphragm  712 . A silicon dioxide (SiO 2 ) layer  716  covers the diaphragm  712  and the resistances  714 . Covering the silicon dioxide layer  716  is a silicon nitride layer  718 , which is fairly impervious to contaminants passing through, for protecting the resistances  714  from contamination.  
         [0039]    The resistances  714  can have their values changed by contaminants. For example, ions from the electrostatic bonding region between the sensor  102  and the cap  710  can travel through the gel  612  to the region between the diaphragm  712  and the silicon dioxide layer  716 . The ions then combine with the boron doped silicon to change the value of the resistances  714 . This problem can be prevented by covering the silicon nitride layer  718  with an aluminum layer  720 . The aluminum layer  720  is connected to a high potential  722  which prevents the ions from migrating to the boron injected silicon region.  
         [0040]    While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept.