Abstract:
A device for measuring relative distance between two physical objects includes an elongated inductor coil and a movable core. The movable core includes a slug of magnetically interactive material and is configured to move within the elongated inductor coil and to couple and interact magnetically with the elongated inductor coil. Electric current flowing through the elongated inductor coil generates a magnetic flux within the elongated inductor coil, and the magnetic flux is subsequently modified by moving the movable core within the elongated inductor coil and the modified magnetic flux is used to produce an electric output as a function of the position of the slug within the elongated inductor coil.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 61842603 filed on Jul. 3, 2013 and entitled POSITION SENSING DEVICE, which is commonly assigned and the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a position sensing device, and in particular to a device that uses a variable inductance sensor for measuring relative position. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are many prior art devices for measuring relative position including ultrasonic devices, optical encoders, and linear variable differential transformers (LVDT). The performance of ultrasonic devices and optical encoders are highly influenced by the medium in which they operate. The linear variable differential transformer devices are expensive, and require multiple coils in precise positions. 
         [0004]    Accordingly, a low cost position sensor that has high accuracy is desirable. 
       SUMMARY OF THE INVENTION 
       [0005]    This invention relates to sensors for measuring relative distance between two physical objects. One form of this invention relates to sensors in which magnetic coupling is used to produce an electric output as a function of distance. This is done by providing a relatively large air gap between the movable core and the shield of the unit, when a shield is used, and through the use of a precision wound helical sensing coil with corrected native linearity. 
         [0006]    In general, one aspect of the invention provides a device for measuring relative distance between two physical objects including a sensor comprising an elongated inductor coil and a movable core. The movable core includes a slug of magnetically interactive material and is configured to move within the elongated inductor coil and to couple and interact magnetically with the elongated inductor coil. Electric current flowing through the elongated inductor coil generates a magnetic flux within the elongated inductor coil, and the magnetic flux is subsequently modified by moving the movable core within the elongated inductor coil and the modified magnetic flux is used to produce an electric output as a function of the position of the slug within the elongated inductor coil. 
         [0007]    Implementations of this aspect of the invention include the following. The elongated inductor coil includes windings with a pitch that varies along the elongated inductor coil length. The slug comprises a ferromagnetic material. The movable core includes a shaft and the magnetically interactive material is attached to an outer surface of the shaft. The device further includes a drive element configured to drive the shaft of the movable core linearly within the elongated inductor coil. The magnetically interactive material is attached to the outer surface of the shaft with an adhesive, or via press-fitting. The device further includes a shield surrounding the elongated inductor coil and movable core. The shield comprises a ferromagnetic material and conducts a return magnetic flux. The elongated inductor coil comprises windings with a constant pitch and the windings begin at one end of the shield and end internal to a second end of the shield. The elongated inductor coil comprises windings with a constant pitch and the winding begin internal to one end of the shield and end internal to a second end of the shield. The elongated inductor coil comprises windings with a variable pitch and the windings begin at one end of the shield and end at a second end of the shield. The elongated inductor coil comprises windings arranged so that a time constant of the elongated inductor coil is a predetermined function of the position of the movable core. The device further includes a time constant network configured to generate an oscillation having a period proportional to a time constant of the elongated inductor coil. The device further includes a linearization network connected to an output of the time constant network and configured to generate a linear transfer function between the period of the time constant network oscillation and the time constant of the elongated inductor coil. The device further includes an output network connected to an output of the time constant network or the linearization network and configured to provide an output signal that is amplified and corrected for environmental conditions. The slug comprises a conductive material that excludes the magnetic flux. 
         [0008]    In general, another aspect of the invention provides a device for measuring relative distance between two physical objects including a sensor comprising an inductive circuit and the inductive circuit includes an inductor and a slug of magnetically interactive material. The relative distance between the inductor and the slug of magnetically interactive material is measured by varying a time constant of the inductive circuit. The inductance of the inductor varies as a function of the slug position relative to the inductor and thereby affects the time constant of the inductive circuit. The inductor includes helical windings and is encased within a ferromagnetic material. The inductor includes windings with variable pitch and the inductance of the inductor varies linearly with the position of the slug within the inductor. The inductive circuit further includes a resistor and a capacitor. The inductive circuit further includes a Colpitts oscillator. 
         [0009]    In general, another aspect of the invention provides a method for measuring relative distance between two physical objects including providing a sensor comprising an elongated inductor coil and a movable core. The movable core includes a slug of magnetically interactive material and is configured to move within the elongated inductor coil and to couple and interact magnetically with the elongated inductor coil. Electric current flowing through the elongated inductor coil generates a magnetic flux within the elongated inductor coil, and the magnetic flux is subsequently modified by moving the movable core within the elongated inductor coil and the modified magnetic flux is used to produce an electric output as a function of the position of the slug within the elongated inductor coil. 
         [0010]    Among the advantages of this invention may be one or more of the following. Magnetic linear motion sensors are useful for a variety of motion sensing tasks such as measuring the position of valves, automated assembly equipment, balancing machines, strength testing, liquid level, structure testing, actuator position sensing, valve position, thickness control, wind power generators, earth moving equipment components and hydraulic cylinders. 
         [0011]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a schematic diagram of a position sensing device according to this invention; 
           [0013]      FIG. 2  is a schematic diagram of the position sensing device of  FIG. 1  having the measuring electronics and magnetic assembly contained within one housing; 
           [0014]      FIG. 3  shows a partially schematic and partially cutaway view of the position sensing device of  FIG. 1 ; 
           [0015]      FIG. 4  is a cross sectional view of one embodiment of the position sensing device of  FIG. 1 , where the pitch of coil  26  changes as a function of position within component  25 ; 
           [0016]      FIG. 5  is a cross sectional view of another embodiment of the position sensing device of  FIG. 1 , where coil  26  begins at one end of component  25  and ends internal to the distal end of component  25 ; 
           [0017]      FIG. 6  shows an electrical diagram of an embodiment of the present invention; 
           [0018]      FIG. 7  shows an electrical diagram of another embodiment of the present invention; 
           [0019]      FIG. 8  shows an electrical diagram of another embodiment of the present invention; and 
           [0020]      FIG. 9  shows an electrical diagram of another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    A position sensing device includes a magnetic assembly positioned in relation to a slug of material which modifies the inductance of the magnetic assembly as the position of the slug changes in relation to the magnetic assembly. The magnetic assembly includes an electrical conductor and preferably magnetic conductors which guide the magnetic fields so that as the slug is displaced in relation to the magnetic assembly the inductance of the magnetic assembly changes. The sensor may be a portion of an inductive time constant circuit such that the time constant varies as a function of the position of the slug. 
         [0022]    This invention measures distance by varying the time constant of an inductive circuit. In an RL type circuit the time constant τ is equal to L/R, where L is the inductance and R the resistance of the circuit. The time constant of the inductive circuit is changed by changing the inductance of a magnetic assembly according to the relative position of a slug of ferromagnetic or conductive material. In one embodiment, the invention uses an inductive coil that is wound with a controlled pitch  99  as function of the position along the associated magnetic coil, as shown in  FIG. 5 . The pitch  99  of a coil is the distance between the centers of two adjacent windings. In this embodiment, the inductance L of the magnetic coil varies as a function of the slug  27  position and thereby affects the time constant τ of the circuit. This sensor arrangement is used for measuring the position of a slug  27  relative to the coil  26 . This sensor arrangement is simpler than LVDT position sensors and is more cost effective. This arrangement is also advantageous for measuring longer stroke lengths than the stroke lengths measured with LVDT position sensors. This arrangement also allows the sensor to be shorter than the corresponding LVDT unit. The use of time constant electronics with this sensor arrangement allows the output period of the combined sensor and electronics to have a large linear transfer function range. In one embodiment, the coil  26  is helical and is encased with a ferromagnetic material  25  which conducts the return flux. The coil&#39;s magneto-motive force generates a field within the active part of the coil  26  which is modified by the position of the slug  27 . In another embodiment, the coil is wound with a variable pitch  99  so that the inductance L varies linearly as a ferromagnetic or conductive slug modifies the flux from an increasing number of turns as the slug moves into a portion of the coil with higher turns density, as shown in  FIG. 4 . A further refinement of the invention arranges the turn&#39;s density of the coil such that the time constant of the circuit is a predetermined function of the position of the slug. 
         [0023]    Referring to  FIG. 1  and  FIG. 2 , displacement sensor  10  includes housing  11 , end caps  12  and  14 , sensor head cable  15 , signal and excitation cable  16 , together with combined measuring electronics  17  and associated power input terminal  18 , power and signal return terminal  20  and signal output terminal  19 . The simplest form of this invention includes a movable slug  27  of magnetically interactive material which interacts with elongated inductor  26  and shield  25  in the active magnetic assembly together with measuring electronics  17 , as shown in  FIG. 3 . 
         [0024]      FIG. 2  shows a variable inductance sensor with the measuring electronics contained within one housing. 
         [0025]    Referring to  FIG. 3 , sensor  10  includes a rigid housing  11 , supporting outer material  25 , coil  26  and movable slug  27 . In some embodiments, outer material  25  is ferromagnetic and conducts flux generated by coil  26 . Current flowing through coil  26  generates magnetic flux that is modified by moving slug  27  into or out of the coil  26 . Moving slug  27  relative to coil  26  changes the magnetic flux according to the relative position of the slug  27  in the coil  26 . Probe assembly  36 , includes probe active material  27 , glue  28  if required, and distal shaft  29  that is driven by shaft  13 . The glue  18 , is usually an epoxy. In other embodiments, press fit is used, instead of glue  28 . Sensor  10  also includes a time constant network  21 , a linearization network  22  and output network  23 . These networks include digital or analog components. The inductance L of this sensor construction, when the core, or slug, or probe material  27  is ferrite is almost entirely governed by the number of turns squared (N 2 ) adjacent to the core  27 . If the inductance ratio from one end of travel of the core to the other end is 3:1 then the turns ratio from one end to the other is the square root of three, or 1.732. Since obtaining the highest inductance for a given core length is useful, the turns density at the densest end is that obtainable with the turns almost touching. The density at the lowest end is 1/1.732 of that. In between the ends, the turn&#39;s density is such that the output period is linear with the motion of the core, or some other desired transfer function. The core  27  is usually made of ferrite that is stable to the desired operating range, but can be conductive material that excludes flux, thereby altering the inductance, especially in the shorter stroke sensors. The proximal shaft  13  and distal shaft  29  are typically titanium, stainless or aluminum. 
         [0026]      FIG. 4  shows a section view of material  25  outside of the helical conductor where the pitch of coil  26  changes as a function of position within material  25 . 
         [0027]    In one example, the diameter of coil  26  is 0.34 inches. This diameter can be varied easily if desired. The length of the coil is typically 10 mm to 1 meter, depending on the intended measurement range. The shield  25  is typically made of the same ferrite as the core  27 . In lower cost sensor units or in longer range sensors, shield  25  is made of permeability  1  material. 
         [0028]      FIG. 5  shows a coil  26  beginning at one end of material  25  and ending internal to the distal end of material  25 . Another embodiment includes a coil  26  beginning internal to material  25  and ending either flush with the distal end of material  25  or internal to the distal end of material  25 . 
         [0029]      FIG. 6  and  FIG. 7  show electrical circuit schematic diagrams of the present invention. An output terminal  30  is driven by a first electronic switch  31  and second electronic switch  32 . When output terminal  30  is driven to a voltage near Vexcitation  24  by switch  31  current builds in coil  26  increasing the voltage at the positive input of comparator  35  relative to the voltage at the negative comparator input. Eventually the voltage at the positive input of the comparator using the second circuit network  34  becomes higher than the voltage at the negative input of the comparator and the output of the comparator goes to a level near the excitation voltage  24 . This causes the switches  31  and  32  to change state, driving the voltage at output terminal  30  to a level near ground. The resulting change of voltages at the inputs of the comparator through the first circuit network  33  reinforces this change in state until reversal of the direction of current in coil  26  changes the state of switches  31  and  32  back to the beginning of this cycle. The comparator  35  may be made of either analog or logic elements. This operation causes the period of the resulting oscillation to be nearly proportional to the time constant t of the inductance L of the sensor  26  and the resistance R of resistor  50 .  FIG. 7  shows a variation of the electrical circuit  21  which reduces the excitation current  24  through the use of the capacitor  55  in the second circuit network. A linearization network  22  can be connected to the output of time constant network  21  if a more linear transfer function is required for an application. Alternatively, by making the turns density of the inductor a predetermined function of the position along the sensor, a wide variety of transfer functions can be obtained. An output network  23  can be connected to the output of time constant network  21  or to the output of the linearization network  22  to provide an output signal with higher amplitude, correction for environmental conditions or other signal translations. It is also possible to correct for changes in the sensing inductor separately from the other electronics by measuring the temperature of the inductor. The temperature of inductor  26  may be measured either by measuring its resistance, or by using a separate temperature sensor in close proximity to the sensing inductor  26 . Measurement of the electronic temperature to compensate for the non-inductive components may also be made to compensate for the temperature effect they have on the operation of the complete sensor. The inductance L is typically a linear function of the position of core  27 , with a non-zero starting point for the inductance. The output signal can have many formats. In many cases, a 0 to 5 volt range, or a +/−5 volt range or a 0 to 10 volt range is desired. The output signal may also be a digital format or analog format current. 
         [0030]      FIG. 8  shows an electrical circuit schematic diagram of the present invention where the inherent superior linearity obtained using the circuits of  FIG. 6  and  FIG. 7  can be sacrificed to allow lower power operation. In this embodiment a Colpitts oscillator using a bipolar transistor  50  and a current determining resistor  52  causes the transistor to stay out of a saturated collector-emitter voltage condition. This eliminates saturation delay in that transistor, allowing the circuit to faithfully operate at the resonant frequency of the combined sensor inductor and the series capacitance of the collector-emitter capacitor and the emitter-supply capacitor. The gain limitation required for any oscillator is provided by the emitter cutoff condition of the transistor during a portion of the operating cycle, which is inherently a fast mode of operation with minimal phase shift. One version of this circuit also has a constant current sink to bias the emitter of the oscillator transistor. Referring to  FIG. 9 , one version of the constant current sink uses the base-emitter voltage of a second transistor  51  with an accompanying resistor  53  to provide an essentially constant current sink for transistor  50 . Resistor  54  sinks both the oscillator current and the collector current in transistor  51 . 
         [0031]    Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.