Abstract:
An adaptive driver circuit which uses a modified conventional current mirror circuit to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit. The adaptive driver circuit according to the present invention is a unique modification of a current mirror circuit in that an SMR device is used as the reference resistor and a fixed resistor in the mirrored circuit to generate an output voltage. A modification is also possible whereby two adaptive driver circuits are used in a differential mode.

Description:
TECHNICAL FIELD 
     The present invention relates to semiconductor magnetoresistive (SMR) devices, also known in the art as magneto-resistors (MR), employed in position and speed sensors, and more particularly to a method and apparatus to achieve increased sensitivity of SMR devices at high temperatures and large air gaps. 
     BACKGROUND OF THE INVENTION 
     It is well known in the art that the resistance modulation of SMR devices can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). 
     The shortcoming of SMR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of SMR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive SMR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source. 
     To compensate for the SMR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the SMR device, it is also desirable to make the current of the current source automatically increase with the SMR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the SMR device. It is also known that air gap variations between the SMR device and ferromagnetic materials or objects will affect the resistance of SMR devices with larger air gaps producing less resistance and decreased output signals. 
     What is needed is a less complicated method and apparatus having the features of a current source and employing an automatically adjustable current to compensate for decreased SMR sensitivity at high temperatures and large air gaps. 
     A circuit of interest in this regard, well known in the art, is a conventional current mirror circuit  10  (often referred to simply as a current mirror), shown in FIG.  1 . In current mirror circuit  10 , the reference resistor R has a fixed value and, in conjunction with the constant voltage source V SS and transistor Q 1 , determines the magnitude of the reference current I R . The electronic operation of the current mirror circuit  10  dictates that the current I 0  will have approximately the same magnitude as the reference current I R  provided that transistors Q 1  and Q 2  are matched. Thus, the reference current I R  is mirrored to be the collector current I 0  of transistor Q 2 . In FIG. 1, I 0  is conventionally called the mirror current or mirrored current and the “mirrored portion” of the current mirror circuit  10  will be designated the mirrored circuit  12 . 
     Accordingly, it would be desirable if somehow the current mirroring feature of a mirror circuit could be adapted to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit. 
     SUMMARY OF THE INVENTION 
     The present invention is an adaptive driver circuit which uses a modified conventional current mirror circuit, as shown in FIG. 1, to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit. The adaptive driver circuit according to the present invention is a unique modification of the current mirror circuit  10  of FIG. 1, in that an SMR device is used as the reference resistor and a fixed resistor in the mirrored circuit  12  of FIG. 1 to generate an output voltage. 
     In operation of the adaptive driver circuit according to the present invention, as the resistance of the SMR device decreases due to temperature increases or air gap increases between the SMR device and ferromagnetic materials or objects, the reference current increases. This produces an increase in the mirror current which increases the value of the output voltage across the fixed resistor, thereby maintaining the peak of the output voltage close to a known fixed saturation voltage. This offers a simple fixed threshold approach in converting the output signal into a digital signal. 
     A modification of the present invention is also possible whereby two adaptive driver circuits are used in a differential mode. 
     Accordingly, it is an object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage. 
     It is an additional object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage as the resistance of an SMR device change due to temperature variations of the SMR device. 
     It is still another object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage as the resistance of an SMR device changes due to air gap variations between the SMR device and ferromagnetic materials or objects. 
     It is yet another object of the present invention to provide an output voltage representing a differential signal voltage between SMR devices. 
     These, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a circuit schematic of a conventional current mirror. 
     FIG. 2 depicts an example of the preferred environment of use of the present invention. 
     FIG. 3 shows the preferred embodiment of the present invention, wherein a PNP transistor is used. 
     FIG. 3A shows the preferred embodiment of the present invention, wherein a NPN transistor is used. 
     FIG. 4 shows an alternative embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Drawing, FIGS. 2 through 4 depict the adaptive driver circuit  100  according to the present invention. 
     FIG. 2 is a schematic representation of an exemplar automotive environment of using of the adaptive driver circuit  100  according to the present invention, wherein a sensor wheel  102  is rotating, such as for example in unison with a crankshaft, a drive shaft or a cam shaft, and the rotative movement thereof is to be sensed. Rotative movement of the sensor wheel  102  is sensed using an SMR device  104 , which is biased by a permanent magnet  106 , wherein the magnetic flux  108  emanating therefrom is represented by the dashed arrow. The magnetic flux  108  passes from the permanent magnet  106 , through the SMR device, and through an air gap  115  to the sensor wheel  102 . The sensor wheel  102  is made of a ferromagnetic material typically, but not restrictively, toothed, having teeth  110  and slots  112  therebetween. The sensor wheel  102  is located near the SMR device  104  as indicated in FIG.  2 . 
     A constant voltage source  114 , a current conditioner  116 , the SMR device  104 , and a resistance (R 0 )  118 , represent a modified conventional current mirror according to the present invention. The output voltage (V out )  120  is developed across resistor (R 0 )  118 . It is to be understood that all voltages are measured with respect to ground unless otherwise indicated herein. The current through the SMR device  104  is a function of its resistance, which can vary due to a variety of parameters. Typical parameters that can vary resistance of the SMR device  104  can be attributed to, but not limited by, variations in the air gap  115  between the SMR device and the sensor wheel  102  due to variations in the distance between the teeth  110  and the SMR device, and/or variations in the distance between the bottom of the slots  112  and the SMR device, and/or variations in the air gap as a result of irregular rotation of the sensor wheel, and/or temperature variations of the SMR device. 
     FIG. 3 depicts the preferred embodiment of the adaptive driver circuit  100  according to the present invention, which is a modified conventional current mirror circuit (see current mirror circuit  10  at FIG.  1 ). 
     The current conditioner  116  of FIG. 2 is represented by a dashed block  116  in FIG.  3 . The resistors (R)  124   a ,  124   b  have the same resistance value and are preferred to be present in order to serve as a limit for variations of the output voltage (V out )  120  from a predetermined fixed saturation voltage; however these resistors may alternatively be replaced by respective short circuits, if desired. Electrical power for the adaptive driver circuit  100  is supplied by a constant voltage source (V SS )  114 . 
     The adaptive driver circuit  100  uniquely modifies a conventional current mirror circuit by using the SMR device  104  as a current reference resistor and a fixed resistor (R 0 )  118  to generate the output voltage (V out )  120 . The resistance of the SMR device  104  determines the current (I)  126  which is mirrored as mirror current (I 0 )  128 . If the resistance of the SMR device  104  decreases due to an increase in the air gap  115  or due to an increase in the temperature of the SMR device, the current (I) 126  will increase, causing an increase in the mirror current (I 0 )  128 . This increase in the mirror current (I 0 )  128  boosts the magnitude of the output voltage (V out )  120  so as to thereby maintain the peak of the output voltage (V out ) close to a predetermined fixed saturation voltage which, in turn, offers a simple fixed threshold approach in converting the output voltage (V out ) into a digital signal. The variation in resistance of the SMR device  104  due to air gap variation from a tooth to a slot  112  or from a slot to a tooth produced by rotation of the sensor wheel  102 , is sufficient enough such that a threshold can be set at approximately the midpoint between the peak and valley values of the output voltage which corresponds to, respectively, the tooth and the slot. The resistance value of the fixed resistor (R 0 )  118  cannot exceed the minimum resistance value of the SMR device  104 , for transistor biasing reasons. Accordingly, fixed resistor (R 0 )  118  can be selected to have a negative temperature coefficient of resistance smaller, however, than that of the SMR device  104  so as to always satisfy the condition (R 0 ) min &lt;(SMR) min  where (SMR) min  represents the minimum resistance of the SMR device in a particular application. 
     FIG. 4 depicts an alternative embodiment of an adaptive driver circuit  100 ″, wherein a twofold utilization of the adaptive driver circuit  100  of FIG. 3 is utilized. In FIG. 4, first and second SMR devices  104 ,  104 ′(designated respectively as SMR 1 , and SMR 2 )are matched SMR devices, wherein the first SMR device  104  (SMR 1 ) determines a current (I 1 )  138  which is mirrored as a first mirror current (I 01 )  140  and the second SMR device  104 ′ (SMR 2 ) determines a current (I 2 )  142  which is mirrored as a second mirror current (I 02 )  144 . In FIG. 4, the fixed resistors (R 01 )  118  and (R 02 )  118 ′ are matched resistors having the same resistance value. Also in FIG. 4, the output voltage (V out )  150  as measured between terminals  146  and  148 , represents the differential voltage of fixed resistors (R 02 )  118 ″ and (R 01 )  118 , and is proportional to the differential voltage of the first and second SMR devices  104 ,  104 ′. 
     While the above embodiments of the adaptive circuit according to the present invention show usage of PNP transistors, it is within ordinary skill in the art to substitute NPN transistors for the PNP transistors, wherein routine adjustments of the embodiments of FIGS. 3 and 4 are made, as exemplified by FIG. 3A with respect to FIG.  3 . 
     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.