Abstract:
A novel electrical circuit is disclosed that corrects for at least one undesired characteristic in an incoming electrical signal. In one embodiment, the apparatus reduces the number of wires heretofore required to correct electrical signals in a circuit. The apparatus also corrects for one or more of a second-order error in a measurement, an offset drift can be directly corrected during operation, and a span can be linearized during operation.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     This disclosure generally relates to apparatus and methods for measuring and conditioning electronic circuits, and specifically to apparatus and methods for correcting electrical signals in electric circuits.  
         [0003]     2. Related Art  
         [0004]     Piezoresistors are typically used in a Wheatstone bridge circuit. Typically, four piezoresistors are coupled, and use either constant-voltage excitation or a constant-current excitation. As is well known, piezoresistors are resistors that vary their resistance value in response to an applied strain.  
         [0005]      FIG. 1  shows a Wheatstone bridge circuit wherein piezoresistors are used in a sensed-parameter transduction scheme. Arranged as a four device Wheatstone bridge,  FIG. 1  shows a first constant voltage (“Vc”) excitation path  102  connected to the junction a first piezoresistor  110   a  and a second piezoresistor  110   b.  A first “sensed” voltage (“V 1 ”) is measured at a first sense wire  106 . The first sense wire  106  is connected to the junction of the first piezoresistor  110   a  and a third piezoresistor  110   c.  A second “sensed” voltage (“V 2 ”) is measured at a second sense wire  104 . The second sense wire  104  is connected to the junction of the second piezoresistor  110   b  and a fourth piezoresistor  110   d.  A return path is provided to the sensed-parameter transduction scheme via a return path wire  108 , which is connected to the junction of the third piezoresistor  110   c  and the fourth piezoresistor  110   d.  Although  FIG. 1  is described herein as a constant voltage excitation scheme, those skilled in the electronics art appreciate that the circuit illustrated in  FIG. 1  can readily be adapted for use in a constant current excitation scheme. In this configuration, the constant voltage excitation path  102  is replaced with a constant current (“Ic”) excitation path. In some embodiments, the piezoresistors  110   a,    110   b,    110   c,  and  110   d  have approximately identical values.  
         [0006]     A full four wire Wheatstone bridge exhibits immunity to noise. Common-mode noise coupled into the first sense wire  106  and the second sense wire  104  can be cancelled using well-known noise cancellation techniques. In the embodiments wherein all piezoresistors  110   a,    110   b,    110   c,  and  110   d  have identical resistance, (i.e.,  110   a = 110   b = 110   c = 110   d =Ro), a variation in a sensed parameter, which causes a change in resistance (±ΔRo), in the piezoresistors  110   a,    110   b,    110   c,  and  110   d,  causes a differential voltage to appear between the first sense wire  106  and the second sense wire  104 . This differential voltage is proportional to the variation in the sensed parameter. For a constant voltage excitation, the differential voltage has a value of VcΔ. For a constant current excitation, the differential voltage has a value of IcΔRo.  
         [0007]     A differential voltage which exists in the absence of a variation in the sensed parameter is commonly referred to as an “offset” voltage. This offset is typically removed by “paralleling” selected piezoresistors with trimmable (non-piezo) resistors and then production-trimming one or more of the resistors until the offset is zeroed. Although this technique provides good correction of an initial offset, it disadvantageously introduces higher-order error terms into response signals. The differential voltage becomes VccX(Δ+higher order terms), for constant voltage excitation, and Ic(ΔRo+higher order terms), for constant current excitation. Further, this trim-resistor technique cannot be used to make corrections to offset drifts that occur during operation, nor can it be used to correct for span drift. Furthermore, this technique cannot linearize the response of the bridge.  
         [0008]     Therefore, the need exists for an apparatus and method that corrects undesired signal characteristics in an electric circuit. The teachings provide such an apparatus.  
       SUMMARY  
       [0009]     An improved apparatus for correcting undesired signal characteristics in an electrical circuit is disclosed. In one embodiment, a multi-wire sensing bridge circuit is disclosed. In this embodiment, a first impedance element and a second impedance element are arranged to sense an incoming signal and subsequently correct for at least one of a plurality of potential undesired signal characteristics.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Embodiments of the present disclosure are more readily understood by reference to the following figures, in which like reference numbers and designations indicate like elements.  
         [0011]      FIG. 1  is a schematic diagram of a piezoresistor Wheatstone bridge having a constant excitation.  
         [0012]      FIG. 2A  illustrates a schematic diagram of a multi-wire harness half-bridge circuit of the present teachings.  
         [0013]      FIG. 2B  illustrates a schematic diagram of a multi-wire harness half-bridge in accordance with the present teachings.  
     
    
     DETAILED DESCRIPTION  
       [0014]     The present teachings disclose an improved method and circuit for correcting undesired characteristics in an incoming electrical signal.  
         [0015]     An improved multi-wire sensing bridge circuit is illustrated in  FIG. 2A . In one embodiment, the improved multi-wire sensing bridge comprises an exemplary three wire “half-bridge” circuit. In one embodiment, the multi-wire sensing bridge is adapted to sense an incoming electrical signal and correct undesired characteristics associated with the incoming electrical signal.  
         [0016]     In the embodiment shown in  FIG. 2A , a first electrically conductive wire  202  is connected to a first impedance element  208 . A second electrically conductive wire  204  is connected to the junction of the first impedance element  208  and a second impedance element  210 , at a node  209 . A return path is electrically connected to the second impedance element  210  via a third electrically conductive wire  206 . In some embodiments, the first impedance element  208  and the second impedance element  210  comprise piezoresistors.  
         [0017]     Referring now to  FIG. 2B , an improved multi-sensing bridge circuit is shown. A first current Ic 1  conducts through a first wire  224  to a first impedance element  228  and to a return path wire  230 . A second current Ic 2  conducts through a second wire  222  through a second impedance element  226  and to the return path wire  230 . The first wire  224  has an associated and corresponding first excitation and a first sensed parameter when the improved multi-sensing bridge  220  is in operation. The second wire  222  has an associated and corresponding second excitation and a second sensed parameter associated therewith when the improved multi-sensing bridge  220  is in operation. The first impedance element  228  and the second impedance element  226  are connected to the return path wire  230  at a node  235 . The first wire  224  and the second wire  222  comprise both excitation lines and sensed parameter lines. The impedance elements  228  and  226  are excited by separate, independent current sources, Ic 1  and Ic 2 , via the lines  224 ,  222 , respectively.  
         [0018]     The half-bridge circuits shown in  FIGS. 2A and 2B  advantageously reduce the number of excitation/sensed parameter wires required to correct undesired electrical signal characteristics. Typically, for example, in a Wheatstone bridge, four wires are required. However, only three such wires are required in the circuits of  FIGS. 2A and 2B .  
         [0019]     The first wire  224  has a first voltage (“V 1 ”) associated therewith. Similarly, the second wire  222  has a second voltage (“V 2 ”) associated therewith. In one embodiment, wherein the impedance elements  226 ,  228  comprise piezoresistors, the differential voltage is defined by the following equation: 
 
 V 2− V 1=( Ic 2)*( Ro 2)−( Ic 1)*( Ro 1);   Eq. 1 
 
         [0020]     Wherein Ro 1  comprises a resistance value associated with the first impedance element  228  and Ro 2  comprises a resistance value associated with the second impedance element  226 .  
         [0021]     If Ro 1  equals Ro 2 , for Ic 1 =Ic 2 , the measured offset is zero.  
         [0022]     If the sensed parameters are at their reference zero for the first impedance element  228 , Ro 1 =Ro 1 (0), and second impedance element  226 , Ro 2 =Ro 2 (0), are not exactly equal, but differ by an amount, “r”, such that Ro 2 (0)=Ro 1 (0)−r=Ro(0)−r. And if Ic 1 =Ic 2 =Ic, then V 2 −V 1  will be offset from zero by an amount V 2 (0)−V 1 (0)=−Ic*r.  
         [0023]     An offset in the sensed parameters can be corrected, straightforwardly, by adjusting Ic 2 (0) such that Ic 2 (0)=[Ro(0)/(Ro(0)−r)]*Ic 1 (0)=[V 1 (0)/V 2 (0)]*Ic(0).  
         [0024]     An adjusted offset ADJ[V 2 (0)−V 1 (0)] then becomes ADJ[V 2 (0)−V 1 (0)]={[[Ic(0)]*[Ro(0)]]/[Ro(0)−r]}*[Ro(0)−r]−(Ico(0))*(Ro(0))=0.  
         [0025]     If a variation in the sensed parameters of an incoming electrical signal causes Ro 2 =Ro 2 (0)(1+δ) and Ro 1 =Ro 1 (0)*(1−δ), then the result is: 
 
 V 2− V 1 ={[[Ic (0)] *Ro (0)]]/ Ro (0)− r}*[Ro (0)− r]*[ 1 +δ]−[Ic (0)]* R (0)*(1−δ)=2 *Ic (0)* Ro (0)*δ
 
         [0026]     This expression gives a simple, linear relationship between V 2 −V 1  and δ, with higher-order error terms (e.g., second order error terms) absent. As such, the improved multi-wire sensing bridge advantageously eliminates higher-order terms from a measurement of the sensed parameters.  
         [0027]     If δvaries with temperature (“T”), according to a known, or detectable linear or non-linear pattern, δ(T)=(1+B(T))*δ, then the bridge can be corrected directly for span drift by modifying Ic 1  and Ic 2 , each by a factor, (1+B(T)) −1 . Span drift is caused by two main factors; changes in temperature and sensor deterioration.  
         [0028]     The foregoing description illustrates exemplary implementations, and novel features, of aspects of an apparatus for correcting undesired characteristic associated with an incoming electrical signal. Alternative implementations are suggested, but it is impractical to list all alternative implementations of the apparatus. Therefore, the scope of the presented disclosure should be determined only by reference to the appended claims, and should not be limited by features illustrated in the foregoing description except insofar as such limitation is recited in an appended claim.  
         [0029]     While the above description has pointed out novel features of the present disclosure as applied to various embodiments, the skilled person will understand that various omissions, substitutions, permutations, and changes in the form and details of the methods and systems illustrated may be made without departing from the scope of the present teachings.  
         [0030]     Each practical and novel combination of the elements and alternatives described hereinabove, and each practical combination of equivalents to such elements, is contemplated as an embodiment of the present teachings. Because many more element combinations are contemplated as embodiments of the present teachings than can reasonably be explicitly enumerated herein, the scope of the present teachings is properly defined by the appended claims rather than by the foregoing description. All variations coming within the meaning and range of equivalency of the various claim elements are embraced within the scope of the corresponding claim. Each claim set forth below is intended to encompass any apparatus or method that differs only insubstantially from the literal language of such claim, as long as such apparatus or method is not, in fact, an embodiment of the prior art. To this end, each described element in each claim should be construed as broadly as possible, and moreover should be understood to encompass any equivalent to such element insofar as possible without also encompassing the prior art. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising.”