Abstract:
An analog gauge includes a coil assembly and a permanent magnet fixedly attached to a rotatable shaft whereby magnetically orthogonal stator coils are disposed about the permanent magnet. The shaft is fixedly attached to a pointer arm that moves over a dial face. As electrical current passes through the coils, electromagnetic fields are induced which, when summed, comprise a magnetic force which is followed by the permanent magnet, shaft, and pointer arm. With reduced-parts circuitry, the flat zone responsiveness of the circuit is delayed until a zener diode is forward biased, yielding increased control over the flat zone responsiveness, without reliance upon the voltage drop across a diode connected between ground and the coil located furthest from the power source.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to analog gauges and specifically to flat response temperature circuitry for analog gauges yielding a decreased parts count and increased control over a “flat zone” of the gauge. 
     2. Description of Related Art 
     Analog gauges are commonly used to display automobile data to a driver. In a typical analog gauge, electrical current flows through wire coils disposed about a permanent magnet. The amount of electrical current flowing through each coil varies according to the value of a measurand at a remote location. 
     As current flows through each coil, a magnetic field B is induced proximal to the coil. The direction of the magnetic field is determined by the direction of the winding of the coil as given by the right-hand-rule. In general, a stronger magnetic field can be created by allowing more current to pass through a given coil. The strength and direction of the magnetic field can be represented by a vector having a magnitude corresponding to the strength of the magnetic field and a direction corresponding to the direction of the induced magnetic field. 
     The magnetic fields induced about each coil combine to create a resultant magnetic force which is, in terms of direction, followed by the permanent magnet about which the coils are disposed. The permanent magnet is attached to a rotatable shaft that is attached to a pointer arm that moves over a dial face in response to changes in the direction of the resultant force. Circuitry, attached to the coils, varies the relative current flow in each coil to change the resultant magnetic vector corresponding to the value of the measurand at the remote location. If the measurand changes, the direction of the resultant force will change and the shaft and pointer will rotate accordingly. 
     In a linear gauge, the shaft responds in a linear relationship to changes in the measurand at the remote location. For example, in a linear temperature gauge, a 20% change in temperature causes a 20% rotation in the magnet, shaft, and pointer. Alternatively, the responsiveness of the gauge can be reduced for a predetermined range of temperatures. Such a gauge is commonly referred to as a “flat response” gauge because a “flat zone” is created in which the circuitry of the gauge has a reduced level of responsiveness to changes in the measurand. 
     The prior art circuit of FIG. 1A is exemplary and provides for a power source  10  such as a DC battery and a bridge resistor  12  having one terminal connected to the power source  10  and one terminal connected to a sender resistor  14  such as a thermistor. The sender resistor  14  has its remaining terminal connected to ground. This sender resistor  14  has an operating resistance of 275−18.3 Ω. 
     Further connected to the power source  10  is a first coil L 1  having one terminal connected to the power source  10  and one terminal connected to a second coil L 2 . L 2  is in series with L 1  and has its remaining terminal connected to a third coil L 3 . L 3  has one terminal connected to L 2  and one terminal connected to a fourth coil L 4 . L 4  has its remaining terminal connected to an anode of first diode  16  whose cathode  16 C is connected to ground. 
     L 3 , L 4 , and the first diode  16  are connected in series, and L 1  is wound about a first axis, L 2  and L 3  are counterwound about the same first axis, and L 4  is counterwound about a second axis which is magnetically orthogonal to the first axis. L 1  and L 2  are formed from a single piece of uninterrupted wire having a resistance of 235.2 Ω, and L 3  and L 4  are formed from a single piece of uninterrupted wire having a resistance of 100.6 Ω. L 1  comprises 1290 turns of wire; L 2 , 490 turns; L 3 , 630 turns; and L 4 , 310 turns. 
     The prior art circuit further includes a zener diode  18  connected at its anode  18 A to the common terminal between L 2  and L 3  and at its cathode  18 C to the common terminal between the bridge resistor  12  and the sender resistor  14 . The zener diode  18  is a 3.6 V, 1 W zener diode, and, dependent on the resistance of the sender resistor  14 , it provides a current path when reverse biased or forward biased, as will be elaborated upon below. The zener diode  18 , in conjunction with the resistance of the sender resistor  14 , establishes the flat zone responsiveness of the circuit. 
     Referring now to FIG. 1B, the magnetic fields induced by the electrical currents flowing through each coil L 1 -L 4  are depicted by individual vectors B 1 -B 4 , respectively, each vector having a magnitude corresponding to the strength of the related magnetic field and a direction corresponding to the direction of the related magnetic field according to the right hand rule oriented along the appropriate winding axis. Because coils L 1 , L 2 , and L 3  are wound about the same magnetic axis, their respective magnetic fields, B 1 , B 2 , and B 3 , lie along a common axis. Stronger magnetic fields are represented by vectors having greater magnitudes along the appropriate axes, and the direction of the magnetic fields induced by coils L 2  and L 3  (i.e., B 2  and B 3 , respectively) are aligned with one another because both are wound about the same axis in the same direction, as opposed to the magnetic field induced by coil L 1  (i.e., B 1 ), which is counterwound about the same magnetic axis in the opposite direction. The magnetic field induced by coil L 1  therefore magnetically opposes the fields induced by coils L 2  and L 3 . The magnetic field induced by coil L 4  (i.e., B 4 ) is magnetically orthogonal to the magnetic field induced by coils L 1 -L 3  because L 4  is wound about a second axis which is magnetically orthogonal to the first. What is needed, however, is circuitry having magnetic fields induced in all four directions from the common origin located at the intersection of the winding axes of the coils L 1 -L 4 . 
     Finally, a resultant magnetic force acting on the permanent magnet can be represented by a resultant vector B having a magnitude and direction which is equal to the sum of the individual magnitudes and directions of the magnetic fields B 1 -B 4  induced by the coils L 1 -L 4 , respectively. The direction of the resultant vector corresponds to the direction of the resultant force and determines the amount of rotation of the permanent magnet, shaft, and pointer, which are fixedly attached to one another. 
     Unfortunately, however, traditional flat response circuitry has significant drawbacks. For example, a diode must be connected in series between ground and L 4 . That is, the coil that is furthest from the power source, in order to provide a voltage drop to allow adjusting the flat zone responsiveness of the circuit. Moreover, different manufacturers require different flat response curves for arbitrary sender resistances, and the circuitry of the prior art does not allow the flexibility required to implement different flat response curves. 
     What is needed, therefore, is circuitry allowing increased control over the flat zone responsiveness of a non-linear gauge. Such circuitry must be flexible enough to meet the demands of numerous manufacturers utilizing different sender resistors and demanding differing levels of angular displacements of the pointer arm over the dial face. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly, the circuitry of the present invention comprises a plurality of coils wound about a first axis and a plurality of coils wound about a second axis, the second axis being magnetically orthogonal to the first axis. A single zener diode is provided having its cathode connected to a common terminal between a bridge resistor and a sender resistor and its anode connected to a common terminal between the coils wound about the second axis. In an embodiment described below, the sender resistor is a thermistor and the zener diode conducts in a forward or reverse direction dependent upon the resistance of the sender resistor compared to the bridge resistor. This circuitry provides a flat zone responsiveness without a second diode. By eliminating the need for this second diode, cost and circuit space are saved and the reliability of the gauge is increased. The circuitry is simplified because the number of turns of each coil and hence the relative strength of the four magnetic fields can be readily adapted to operate with diverse senders. Thus, it is no longer necessary to individually adjust and calibrate the sender resistance based upon specified levels of flat zone responsiveness. 
     Accordingly, it is an object of the present invention to provide non-linear gauge circuitry having increased flexibility with respect to setting the flat zone responsiveness of the circuit. It is a further object of this invention to achieve this increased flexibility with a minimum number of circuit elements. It is another object of this invention to provide circuitry having magnetic fields induced in four directions from an origin located at the intersection of the winding axes of the coils. It is still another object to provide circuitry yielding increased control over the resultant force comprised of the summation of the individual magnetic fields induced about the individual coils of the circuit. 
     The foregoing and other objects, advantages, and aspects of the present invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown, by way of illustration, a preferred embodiment of the present invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must also be made to the claims herein for properly interpreting the scope of this invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1A is a schematic diagram of a prior art flat response circuit; 
     FIG. 1B is a vector diagram illustrating vectors corresponding to the magnetic fields induced by passing current through the coils of the circuit of FIG. 1A; 
     FIG. 2 is a perspective view of an analog gauge employing winding layouts in accordance with the circuitry of this invention; 
     FIG. 3A is a schematic diagram of the flat response circuit of the present invention; 
     FIG. 3B is a vector diagram illustrating vectors corresponding to the magnetic fields induced by passing current through the coils of the circuit of FIG. 3A; 
     FIG. 4A is vector diagram, superimposed on a dial face, illustrating the resultant magnetic force acting on the permanent magnet and pointer arm of FIG. 2 when the zener diode is reversed biased; 
     FIG. 4B is vector diagram, superimposed on a dial face, illustrating the resultant magnetic force acting on the permanent magnet and pointer arm of FIG. 2 when the zener diode is non-conductive; and 
     FIG. 4C is vector diagram, superimposed on a dial face, illustrating the resultant magnetic force acting on the permanent magnet and pointer arm of FIG. 2 when the zener diode is forward biased. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 2, an analog gauge  20  employing the circuitry and winding pattern of the present invention is illustrated. Although the present invention has been embodied as a temperature gauge for reference purposes, it should be understood that the apparatus and principles described herein can of course be applied to other non-linear instrument gauges as well. 
     Although various shapes and configurations of magnets are contemplated, gauge  20  includes, in a preferred embodiment, a generally cylindrical permanent magnet  22  having a longitudinal axis extending between a first end  24  of the magnet  22  and a second end  26  of the magnet  22 . The north and south poles of the magnet  22  are separated transversely across the first end  24  and the second end  26 . The magnet  22  is capable of rotation about an axis of rotation A—A that crosses a centroid of the magnet  22  running from the first end  24  of the magnet  22  to the second end  26 . 
     Protruding from and fixedly attached to the first end  24  of the magnet  22  is the first end of a shaft  28  that is concentric with the axis of rotation A—A and has a generally smaller diameter than the magnet  22 . Attached to the second end of the shaft  28  is a pointer arm  30  that lies in a plane that is generally orthogonal to the axis of rotation A—A. The pointer arm  30  is fixedly attached to the shaft  28  which is fixedly attached to the first end  24  of the magnet  22  such that the magnet  22  and shaft  28  and pointer arm  30  are capable of rotation about the axis of rotation A—A in tandem. In a preferred embodiment of the pointer arm  30 , it comprises a needle. 
     Intermediate to the magnet  22  and pointer arm  30 , and generally proximal to the latter, is a planar dial face  32  having generally equally spaced graduations thereon  32 A- 32 E which generally extend radially outwards towards the perimeter of the dial face  32 . The plane of the dial face  32  is substantially parallel to the plane of the pointer arm  30  and generally orthogonal to the axis of rotation A—A. The dial face  32  comprises graduations  32 D and  32 E at a terminal end thereof, defining what is referred to as the “red zone”  34  of operation. As the pointer arm  30  is moved over the dial face  32 , it provides a visual representation of the angular displacement of the permanent magnet  22  and shaft  28  in accordance with the value of the measurand at the remote location, as will be elaborated upon below. 
     The permanent magnet  22  is driven by a coil assembly comprising coils L 1 , L 2 , L 3 , and L 4 . Coils L 1 -L 4  are generally disposed about the permanent magnet  22  and wound on axes that are magnetically orthogonal to one another and lie in a plane that is orthogonal to the axis of rotation A—A. For example, coils L 1  and L 2  are wound about a first axis B—B which is orthogonal to a second axis C—C about which coils L 3  and L 4  are wound. The coils L 1 -L 4  can be either wound or counterwound about their corresponding axis to achieve the magnetic vector layout described below. 
     The permanent magnet  22  responds to magnetic fields induced by electrical current passing through the coils L 1 -L 4  in response to the value of a measurand such as the engine temperature of an automobile that is located remote from the permanent magnet  22 . The relationship between the currents driven through the coils L 1 -L 4  define the direction of angular displacement of the permanent magnet  22  whereby a coil that has more current flowing through it has a greater effect than the same coil having less current flow through it. 
     Via the coil assembly of the present invention, magnetic fields are induced in four directions emanating from a common origin point located at the intersection of first and second axes, B—B, C—C. The winding pattern of this invention yields better control over the permanent magnet because having electromagnetic fields extend in four directions about a common origin yields greater flexibility in setting the flat zone responsiveness of the circuit than does a winding pattern having electromagnetic fields extending in fewer directions. 
     Referring now to FIG. 3A, the circuit of the present invention provides for a power source  40  such as a DC battery and a bridge resistor  42  having one terminal connected to the power source  40  and one terminal connected to a sender resistor such as a thermistor  44 . The sender resistor  44  has one terminal connected to the bridge resistor  42  and one terminal connected to ground. In a preferred embodiment, this sender resistor  44  is a thermistor having an operating resistance of 125−15.2 Ω. The actual resistance of the sender resistor  44  is inversely proportional to its temperature whereby when the temperature is relatively low, the resistance is relatively high; when the temperature is relatively high, the resistance is relatively low. 
     Further connected to the power source  40  is a first coil L 1  having one terminal connected to the power source  40  and one terminal connected to a second coil L 2 . L 2 is in series with L 1  and has one terminal connected to L 1  and one terminal connected to a third coil L 3 . L 3  is in series with L 2  and L 1  and has one terminal connected to L 2  and one terminal connected to a fourth coil L 4 . L 4  has one terminal connected to L 3  and one terminal connected to ground. In addition, L 1  is wound about a first axis B—B, L 2  is counter-wound about the same first axis B—B, L 3 is wound about a second axis C—C which is magnetically orthogonal to the first axis, and L 4 is counter-wound about the same second axis C—C. L 1 , L 2 , and L 3  are formed from a single piece of uninterrupted wire having a resistance of approximately 144+/−6 Ω, and L 4  is formed from a single piece of uninterrupted wire having a resistance of approximately 72+/−4Ω. L 1  comprises approximately 180 turns of wire; L 2 , approximately 465 turns; L 3 , approximately 596 turns; and L 4 , approximately 670 turns. In an alternative embodiment, a single coil L 5  comprises the first and second coils L 1 , L 2 . 
     The circuit further includes a zener diode  46  connected at its anode  46 A to the common terminal between L 3  and L 4  and at its cathode  46 C to the common terminal between the bridge resistor  42  and sender resistor  44 . In a preferred embodiment, the zener diode  46  is a 3.3 V, 0.5 W zener diode, and, dependent on the resistance of the sender resistor  44 , it allows current to flow both to and from relative to the common terminal between coils L 3  and L 4 . The conductivity of the zener diode  46  depends on the resistance of the sender resistor  44  such as changes the relative voltages across the zener diode  46 . Accordingly, the zener diode  46 , in conjunction with the resistance of the sender resistor  44 , establishes the flat zone responsiveness of the circuit of the present invention. 
     At colder engine temperatures, the resistance of the sender resistor  44  is relatively high. The voltage at the cathode  46 C of the zener diode  46  exceeds the voltage at the anode  46 A by the zener voltage. Hence, the zener diode  46  is reverse biased to its zener voltage and current i 1  flows from the cathode  46 C to the anode  46 A. Thus, the zener diode  46  acts as a current path whereby the current i 3  flowing into coil L 4  exceeds the current flowing through the coils L 1 , L 2  and L 3 , namely i 3 &gt;i 2  in accordance with Kirchhoff&#39;s law. Accordingly, the magnetic field B 4  induced by the current in coil L 4  is emphasized, and the resultant force created by summing the magnetic fields B 1 -B 4  has an increased B 4  component. Thus, the permanent magnet  22  of FIG. 2 is drawn more into alignment by B 4 , and the approximate magnitude and direction of the resultant force is as depicted in FIG. 4A, in which the pointer arm  30  tends to indicate colder engine temperatures according to the graduations  32   a - 32   b  of the dial face  32 . 
     As the engine temperature begins to increase, the resistance of the sender resistor  44  begins to decrease and so does the voltage at the cathode  46 C. A flat zone in the response of the gauge starts at a temperature when the voltage at the cathode  46 C equals the voltage at the anode  46 A plus the zener voltage. During the flat zone region, no current passes through the zener diode  46  (i.e., i 1 =0 amps). The current i 2  and i 3  are the same so the permanent magnet  22 , shaft  28  and pointer arm  30  do not respond to changes of the sender resistance value. As a result in FIG. 4 b , the pointer arm  30  stays near  32 C during the normal engine operation temperature range. 
     At high engine temperature, the resistance of the sender resistor  4 C decreases to a point that voltage at anode  46 A is higher then voltage at cathode  46 C plus the zener forward voltage drop. Starting at this higher temperature, current flows from the anode  46 A to cathode  46 C. The current i 2  is thus larger than the current i 3  in FIG. 3 a . Consequently, magnetic field created by L 3  is relatively higher to the one created by L 4 . The increasing magnetic field at L 3  pulls the magnet  22  in the clockwise direction if L 2 &gt;L 1  (i.e. B 2 &gt;B 1 ) in FIG.  2 . Alternatively, the pointer is pulled in the counter clockwise direction if L 2 &lt;L 1  (i.e. B 2 &lt;B 1 ) in FIG.  3 . The pointer is very sensitive to the temperature change in this high temperature region. 
     The spirit of the present invention is not limited to any embodiment described above. Rather, the details and features of an exemplary embodiment were disclosed as required. Without departing from the scope of this invention, other modifications will therefore be apparent to those skilled in the art. Thus, it must be understood that the detailed description of the invention and drawings were intended as illustrative only, and not by way of limitation. 
     To apprise the public of the scope of this invention, the following claims are made: