Abstract:
Detection of excessive negative offset of a condition responsive sensor such as a pressure responsive full Wheatstone bridge element ( 10 ) and circuitry associated therewith is obtained by taking the sensor&#39;s output signal, preferably after the signal has been compensated for both gain and offset and comparing (Q 1 ) the signal (V x ) with a reference voltage (V REF1 ) selected to reflect an unobtainable stimulus input condition and driving the compensated signal to a fault level when the compensated signal exceeds the reference voltage.

Description:
This application claims priority under 35 USC Section 119 (e) (1) of provisional application No. 60/234,092 filed Sep. 20, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the detection of excessive negative offset shift of a sensor signal, particularly, a signal conditioned sensor signal. 
     BACKGROUND OF THE INVENTION 
     Systems interpreting sensor outputs, herein simply called System, often can not detect if a sensor&#39;s output offset (i.e., sensor output at minimum stimulus input) has shifted negatively more than an acceptable amount due to one or both of the following: 
     1. The shifted sensor&#39;s output may be outside the System&#39;s input range, or 
     2. The sensor may clamp outputs below a certain level thereby masking the actual negative offset shift. 
     SUMMARY OF THE INVENTION 
     An object of the invention is the provision of a sensor which overcomes the above noted prior art limitations. Another object is the provision of a method and apparatus for sensitively detecting negative offset shifts and providing a sensor output by the System indicative of a sensor fault. Yet another object is the provision of such a method and apparatus which provide detection of offset shifts due either to drift in the sensor per se or in the compensation of associated circuitry. Another object is the provision of a method and apparatus for sensitively detecting a negative offset shift of a piezoresistive Wheatstone sensor output and providing a sensor output by the System indicative of a fault. 
     Briefly in accordance with the invention, the offset compensated signal within a signal conditioning circuit is compared to a reference voltage based on an unobtainable stimulus input condition to determine a fault status. In the event that the fault status is set, the output is driven to an output level interpreted by the System as a fault. The sensor electronic elements include a sense element coupled to an ASIC (application specific integrated circuit) having signal conditioning circuitry including offset and gain compensation. 
     Although it is not necessary to compensate gain prior to the fault status comparison, superior negative offset drift detection sensitivity can be achieved by basing the comparison on a both offset and gain compensated signal due to ASIC tolerances (i.e., changes in ASIC gain may influence offset). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, advantages and details of the novel and improved apparatus and method of the invention appear in the following description of the preferred embodiment of the invention, the detailed description referring to the drawings in which: 
     FIG. 1 is a schematic diagram of a Wheatstone bridge sense element coupled to an ASIC in which is shown certain signal conditioning electronics, and an excessive negative offset shift detection circuit. The full Wheatstone bridge sense element outputs are inputs to the ASIC which provides offset compensation and gain compensation to yield an output with a specific output relation versus input stimulus. 
     FIG. 2 is a plot of output vs. stimulus (e.g., pressure) and shows the variation in the output versus stimulus relation as well as the variation in the excessive negative offset shift detection circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A simplified circuit diagram showing one embodiment is shown in FIG. 1. A full Wheatstone bridge  10  is shown as one form of a sensor element used for sensing a pressure stimulus to which the invention applies comprising piezoresistive resistors R 1 , R 2  in one half bridge and R 3 , R 4  in the other half bridge. The full Wheatstone bridge is the parallel connection of the two half bridges and the bridge is connected between a voltage supply V BRG  and ground RTN and has first and second outputs INP, INM. Outputs INP, INM serve as inputs to an ASIC having signal conditioning circuitry including offset and gain compensation. As shown in FIG. 1, ASIC inputs INP, INM are connected to a conventional offset compensation section  12  having a coarse adjustment  12   a,  a fine offset adjustment section  12   b,  and an analog ground  12   c.  The output  14  of section  12  is connected to a conventional gain compensation section  16  having a coarse gain setting  16   a,  a fine gain setting  16   b  and analog ground  16   c.    
     The output V x  of section  16  is connected to a selected voltage reference V REF1 . The output of comparator Q 1  is connected to a controllable switch such as FET M 1 . The compensated signal V x  is also connected to resistor R 5  which in turn is serially connected to the negative input to amplifier Q 2 . The positive input to amplifier Q 2  is connected to a selected reference voltage V REF2  and the output of the amplifier is fed back to the negative input thereof through feedback resistor R 6 . The output of switch M 1  is connected to the negative input of amplifier Q 2 . 
     In the circuit shown, compensation causes signal V x  to equal an analog ground voltage at minimum input stimulus. A comparison between the voltage at V x  and V REF1  determines the fault status. The reference voltage, VREF 1  is equal to an analog voltage plus (for the circuit shown) a tolerance term delta, Δ. For the circuit shown, if the voltage V x  exceeds a threshold value, V REF1 , the output of Q 1  will be a logic “1” which turns on controllable switch FET M 1 , which in turn pulls down the negative input to amplifier Q 2  causing Q 2  to drive the sensor output to the high supply rail, V PWR . The tolerance term delta, Δ, should be made as small as possible to maximize detection sensitivity without causing nuisance fault reporting. Although one specific embodiment has been shown, the invention relates to any sensor conditioning circuit that provides access to a voltage following offset trimming which is sufficiently within the supply rails to permit voltage comparisons between the offset corrected signal and a reference voltage between the initial offset corrected signal and supply rail. 
     In FIG. 2, plot line “a” reflects the maximum stimulus sensitivity of a stimulus responsive sensor while plot line “c” reflects the minimum stimulus sensitivity. Line “d” reflects the minimum realistic stimulus. At zero stimulus the output at the maximum stimulus offset is shown at “e” and the output at the minimum stimulus output is shown at “f”. The output at “g” and “h” reflect the maximum and minimum excessive offset shift detection thresholds selected to avoid nuisance faults and to have maximum and minimum detection thresholds required to due to manufacturing tolerances. 
     An example of a circuit made in accordance with the preceding description employs an analog ground voltage equal to 0.70 Vpwr. The partially signal conditioned signal, which serves as the output stage inverting input, will nominally equal the analog ground following offset and gain compensation and at zero stimulus. A fault detecting circuit having a threshold of signals at the inverting input of the output stage greater than {0.70 Vpwr−0.05 Vpwr/Output stage voltage gain} will achieve detection of signals nominally 5% below the nominal output at zero stimulus following sensor calibration. A typical application may set the fault threshold at 3.667V plus a manufacturing tolerance of 10 mV given Vpwr equal to 5.0V and an output stage voltage gain of −1.5V/V. 
     Compensation for offset and gain enables the definition of the internal range of behavior of the electronics resulting in a known slope and offset of the conditioning signal and, in conjunction with pressure sensing applications, a reference voltage is selected to reflect zero absolute pressure, a pressure which can not be obtained. 
     It should be noted that the comparison between the voltage at V x  and V REF1  is made prior to the output since the output is limited by certain factors, such as supply voltage and external load drive capability. 
     Among the faults which can be detected by the invention are faults relating to a sense element electrical parameter, such as 0.05 Vpwr/Output stage voltage gain, as well as faults in the conditioning electronics. 
     It will be understood that the invention can be used with sensors responsive to various stimuli in addition to pressure, such as acceleration and that the invention includes all modifications and equivalents of the described embodiment falling within the scope of the appended claims.