Abstract:
A method for measuring multiple pressures and a pressure sensing system for accomplishing the same. The method for measuring a plurality of pressures includes, exposing each of a plurality of pressure sensors to a corresponding plurality of environments each having a corresponding pressure to be measured, determining how frequently to measure each of the plurality of pressures, determining a sequence for utilizing the pressure sensors to measure the corresponding plurality of pressures, the sequence being dependent upon the determined frequency for each of the plurality of pressures and selectively utilizing each of the plurality of pressure sensors according to the determined sequence to measure the pressure to which it is exposed.

Description:
FIELD OF INVENTION 
     The present invention relates to pressure transducers, and more particularly to a method and pressure sensing system which corrects for errors of multiple pressure transducers each of which can be exposed to a unique environment of varying degrees of hostility. 
     BACKGROUND OF INVENTION 
     In general, the use of piezoresistive, and in particular Wheatstone bridge, structures as pressure transducers is well known. Further, piezoresistive pressure sensors are used in many applications where they are exposed to fluctuating pressures to be measured at extreme temperatures. However, as is also known, as temperature fluctuates, so may the output of the transducer as the gage factor and resistivity of the sensor are both functions of temperature. Where these fluctuations are substantial enough to introduce a non-negligible error for the intended application, it is desirable to compensate for, or correct the induced error. A typical application where the temperature-induced error becomes non-negligible is aerospace applications, where a sensor can be subjected to extremely high pressures and extreme temperatures. 
     It is well known in the art the resistance of a Wheatstone bridge increases with increasing temperature and that the gage factor, which is the percentage change of resistance with increasing strain, decreases with increasing temperature. Thus, for a constant voltage applied to the bridge, the decrease in gage factor with increasing temperature leads to a decrease in bridge output at a given pressure. However, by putting a non-temperature varying resistor in series with the bridge, as the temperature of the bridge increases its resistance increases, and more of the supply voltage appears across the bridge. However, not only is the drop of the gage factor and thus the inherent change of the bridge output with constant bridge-excitation non-linear with increasing temperature, but the basic compensation technique of using a resistance divider is somewhat non-linear leading to a not-perfect compensation. 
     These problems can be overcome using an approach such as that illustrated in commonly assigned U.S. Pat. No. 4,192,005, entitled COMPENSATED PRESSURE TRANSDUCER EMPLOYING DIGITAL PROCESSING TECHNIQUES, the entire disclosure of which is hereby incorporated by reference. However, in the previous work a single digital correction circuit was required for each transducer and it was assumed each circuit would be attached to each transducer. Thus, the prior art approaches yield undesirably increased weight and cost for temperature compensated, or corrected, devices. It is desirable and an object of the present invention to provide a single error correcting system for use with multiple pressure sensors, and more particularly piezoresistive bridge pressure sensors, each of which can be exposed to unique environment. Each of the unique environments are at an associated temperature and have an associated pressure to measured by the sensor. In the preferred embodiment a degree of hostility varies between the unique environments. It is a further object of the present invention that the single error correcting system be functional with a number of devices which are not necessarily identical, and which are preferably adapted to measure different pressure ranges at different temperatures at different frequencies of measurement (i.e. some more often than others). 
     SUMMARY OF INVENTION 
     A method for measuring multiple pressures and a pressure sensing system for accomplishing the same. The pressure sensing system adapted for measuring a plurality of pressures includes: a plurality of pressure sensing assemblies each adapted to measure a corresponding pressure and be exposed to a respectively associated environment; a microcontroller; and, means for selectively coupling each of the plurality of pressure sensing bridge assemblies to the microcontroller in a predetermined sequence, wherein the means for selectively coupling are responsive to the microcontroller. 
     The method for measuring a plurality of pressures includes: exposing each of a plurality of pressure sensors to a corresponding plurality of environments each having a corresponding pressure to be measured; determining how frequently to measure each of the plurality of pressures; determining a sequence for utilizing the pressure sensors to measure the corresponding plurality of pressures, the sequence being dependent upon the determined frequency for each of the plurality of pressures; and, selectively utilizing each of the plurality of pressure sensors, according to the determined sequence, to measure the pressure to which it is exposed. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a block diagram of a pressure sensing system according to the present invention. 
     FIG. 2 illustrates a block diagram of a pressure sensing system according to the present invention in a first mode of operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Basically, according to the instant invention, a plurality of pressure transducers each adapted to be exposed to a respectively associated environment are coupled to a single remote processor. It should be understood that as each of the transducers are adapted to be exposed to what may be radically different environments, they may have very different ranges of proper operation. For example, aerospace applications often call for several pressure transducers each of which is exposed to a unique environment and is designed to measure a particular pressure range. In such an application some pressure transducers may be present in an engine where they will need to monitor pressures at very high temperatures relatively frequently, while others having completely disparate measurement ranges are positioned so as to measure hydraulic pressures at relatively low temperatures relatively infrequently for example. Each transducer may have an input to the remote processor, or preferably, the transducers are sequentially coupled to the processor using at least one switch controlled by the remote processor which alternates an input between the transducers. One of the advantages of this configuration is that a resistor can be serially coupled to each pressure sensor (i.e. Wheatstone bridge). Thus, by measuring the voltage drop across the resistor, or alternatively the bridge directly, the temperature of the bridge can be determined. Because the processor is remotely located, the transducer can function in a very high temperature environment (i.e., greater than 600° F. for example) and the processor can not only operate to monitor the output of the transducer but, by knowing the temperature to which it is exposed, can essentially correct for errors introduced due to exposure to the high temperatures. 
     Referring now to the figures, wherein like references refer to like elements of the invention, FIG. 1 illustrates a block diagram of a pressure sensing system  10  according to the present invention. The system  10  includes a plurality of piezoresistive bridge networks  20 ,  20 ′,  20 ″ and  20 ′″. The particularly illustrated system  10  includes four (4) piezoresistive bridge networks, however it should be understood the number of piezoresistive bridge networks which can be utilized in the system  10  is not limited thereto, and that the choice of four (4) is solely for purposes of illustration. The system  10  further includes switches  30  and  40  and microcontroller  50 . Each of the piezoresistive networks is respectively coupled to a first lead  60 ,  60 ′,  60 ″ and  60 ′″ and a second lead  70 ,  70 ′,  70 ″, and  70 ′″. The leads  60 ,  60 ′,  60 ″ and  60 ′″ are further coupled to a first side of switch  40  and leads  70 ,  70 ′,  70 ″ and  70 ′″ are further coupled to a first side of switch  30 . The switches  30 ,  40  each include a second side respectively coupled to inputs  100 ,  110  of the microcontroller  50  via leads  80  and  90 . Each of the switches  30 ,  40  are respectively electronically coupled to microcontroller  50  and responsive to a control signal output from the microcontroller  50  via control line  120 . In this way, the microcontroller  50  controls which of the transducers  20 ,  20 ′,  20 ″ or  20 ′″ an output is measured from via the leads  80 ,  90  (by selectively coupling the transducers thereto with the switches  30 ,  40 ). The embodiment  10  of the present invention is further illustrated to include analog to digital (A/D) converters  130 ,  140  respectively serially interconnected between the switches  30 ,  40  and the microcontroller  50 . These A/D converters  130 ,  140  serve to digitize the output of the transducers  20 ,  20 ′,  20 ″ and  20 ′″ for input into the microcontroller  50 . Alternatively, if the microcontroller  50  is adapted to directly receive analog signal inputs the A/D devices  130 ,  140  could be omitted. 
     According to the present invention, the microcontroller  50  may be programmed to sequentially measure pressures which the transducers  20 ,  20 ′,  20 ″ and  20 ′″ are exposed to, in that order (or for that matter, any other order as well). To effectuate such, the microcontroller  50  first outputs a first control signal on the control line  120  which is received by the switches  30 ,  40 . Referring now also to FIG. 2, responsively thereto, the switch  30  couples lead  70  to lead  80  and switch  40  couples lead  60  to lead  90 , thereby electronically coupling the first transducer  20  to the microcontroller  50 . After appropriate measurements and calculations have been performed for the first transducer  20  as are well known (by measuring the output of the bridge  20  and determining the temperature of the bridge  20  using the voltage drop across the resistor  150  (or bridge  20 ) for example), a second control signal is output by the microcontroller  50  on the control line  120 . 
     This second control signal is received by the switches  30 ,  40  which now couple lead  70 ′ to lead  80  and lead  60 ′ to lead  90 , thereby inputting signals from the second transducer  20 ′ into the microcontroller  50 . After appropriate measurements and calculations have been performed for the second transducer  20 ′, a third control signal is output by the microcontroller  50  on the control line  120 . This third control signal is received by the switches  30 ,  40  which now couple lead  70 ″ to lead  80  and lead  60 ″ to lead  90 , thereby inputting signals from the third transducer  20 ″ into the microcontroller  50 . After appropriate measurements and calculations have been performed for the third transducer  20 ″, a fourth control signal is output by the microcontroller  50  on the control line  120 . This fourth control signal is received by the switches  30 ,  40  which now couple lead  70 ′″ to lead  80  and lead  60 ′″ to lead  90 , thereby inputting signals from the fourth transducer  20 ′″ into the microcontroller  50 . After appropriate measurements and calculations have again been performed for the fourth transducer  20 ′″, the microcontroller  50  outputs the first control signal on the control line  120  and the process repeats, for example. 
     Advantageously, this permits programming of the microcontroller to provide measurements of the different transducers at different frequencies of time. For example, in the above example each of the transducers are measured with a same frequency, i.e. 1-2-3-4-1-2-3-4-1 . . . . However, if the pressure to which the first transducer is exposed to should be measured more often than the pressures applied to the second, third and fourth transducers, this can be easily accomplished by reprogramming the controller to measure 1-2-1-3-1-4-1 . . . by outputting the first control signal, then the second, then the first, then the third, then the first, then the fourth, then the first . . . . As set forth, this may be highly advantageous in aerospace applications for example. 
     It should be recognized that each of the transducers  20 ,  20 ′,  20 ″ and  20 ′″ can be respectively exposed to a separate environment. This environment may be a high-temperature and high-pressure environment. Thus, the desired correction of the output of one transducer  20 ,  20 ′,  20 ″,  20 ′″ may be drastically different from that of another, as each of the transducers may be exposed to drastically different temperatures and pressures which cause different errors. 
     Basically, the temperature measurement can be effected through use of a resistor serially coupled to each bridge  20 ,  20 ′,  20 ″,  20 ′″, for example temperature compensation resistors  150 ,  150 ′,  150 ″ and  150 ′″, switch  160  and optionally A/D converter  170 . 
     When the transducers  20 ,  20 ′,  20 ″ and  20 ′″ are individually exposed to a respective pressure to be measured at a respective temperature, as set forth each transducer  20 ,  20 ′,  20 ″ and  20 ′″ individually exhibits a variance in the gage factor and resistivity. By calculating the voltage drop across the bridge  20 ,  20 ′,  20 ″ and  20 ′″ which is coupled to the microprocessor  50 , the temperature to which it is exposed can be determined and corrected for. Because the bridge excitation voltage V source  is known, the voltage drop across each bridge  20 ,  20 ′,  20 ″ and  20 ′″ can be either directly measured or calculated by measuring V tc =V source −V tc ). In one embodiment, to calculate the respective temperature to which a bridge ( 20 ,  20 ′,  20 ″ and  20 ′″) is exposed to, so as to be able to properly correct the output of that bridge, the voltage drop (V source −V tc ) for that bridge is calculated by the microcontroller  50  by inputting the voltage V tc  to a first side of switch  160 . The second side of switch  160  is input to the microcontroller  50 . The switch  160  is controlled by the microcontroller  50  by also coupling it to the control line  120  so it is fed the same control signal as was provided to the switches  30  and  40 . Therefore, when the microcontroller  50  outputs the first control signal on the control line  120 , V tc  is supplied to the microcontroller  50  using switch  160  so the temperature to which bridge  20  is exposed can be determined and properly compensated for (See FIG. 2 also). Likewise, as the switch  160  is coupled to the control line  120 , the appropriate voltages V tc ′, V tc ″, V tc ′″ are sequentially applied to the microcontroller for each bridge  20 ′,  20 ″ and  20 ′″. As with the leads  80 ,  90 , A/D converter  170  is serially interconnected between the switch  160  and microcontroller  50  to enable the voltages V tc , V tc ′, V tc ″, V tc ′″ to be applied to the microcontroller  50 . Alternatively, if the microcontroller  50  is adapted to directly receive analog signals, the A/D converter  170  can be omitted. 
     It should further be recognized, other errors can be corrected according to the present invention as well, for example non-linear responses of the bridges, etc., by proper programming of the microprocessor. Such induced errors can include for example a zero error, a span error, an environment induced zero error and an environment induced span error. For example, a lookup table could be provided for each transducer, and a corresponding table utilized when a particular transducer&#39;s output is being read by the microprocessor. In the illustrated form, resistors  150 ,  150 ′,  150 ″,  150 ′″ are respectively serially coupled to each of the bridges  20 ,  20 ′,  20 ″ and  20 ′″. Alternatively, a single resistor  150  can be coupled to the source voltage V source  and switchably connected to the bridges  20 ,  20 ′,  20 ″ and  20 ′″ such that the bridges  20 ,  20 ′,  20 ″ and  20 ′″ are sequentially excited responsively to the microcontroller  50  when an output is to be read therefrom and the corresponding voltage V tc , V tc ′, V tc ″, V tc ′″ is simultaneously provided to the microcontroller  50 . 
     Although the invention has been described and pictured in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example, and that numerous changes in the details of construction and combination and arrangement of parts may be made without departing from the spirit and scope of the invention as hereinafter claimed. It is intended that the patent shall cover by suitable expression in the appended claim, whatever features of patentable novelty exist in the invention disclosed.