Abstract:
A method and apparatus are provided for improving the performance of displacement sensors, including inclinometers, accelerometers and linear position transducers, by reducing hysteresis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Patent Application No. 61/341,351, entitled “Sensors with reduced hysteresis”, filed Mar. 30, 2010, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention generally relates to a method and apparatus for reducing hysteresis in displacement sensors such as, for example, inclinometers, accelerometers and linear position transducers. More particularly, the invention relates to such a method and apparatus wherein controlled and constrained motion is imparted to the sensor&#39;s sensing unit relative to the sensor base or housing during use. 
       BACKGROUND 
       [0003]    The performance of sensors, such as displacement sensors, is frequently degraded by hysteresis. Displacement sensors may be absolute sensors such as certain inclinometers and accelerometers that measure the inclination or acceleration of the housing or enclosure of the sensor with respect to the earth or other inertial reference frame. Displacement sensors may also be used to measure the relative displacement between two or more points. Such sensors may be physically connected to such points or may rely on, for example, magnetic or electric fields or electromagnetic or acoustic waves to link to such points. 
         [0004]    Displacement sensors typically comprise a base and certain sensing elements, within a sensing unit, that are immovably connected to the base. Displacement sensors, such as for example inclinometers, also contain certain sensing elements, within the sensing unit, that may move relative to the base as a result of motion that is imparted to the base. The relative motion between these two types of elements within the sensing unit is typically measured and used to determine the displacement of the base. 
         [0005]    If an ideal error-free displacement sensor, such as a single axis inclinometer with sufficient range and without hysteresis, underwent exactly a 25 degree clockwise change in inclination about its sensitive axis followed by a counterclockwise change in inclination of exactly 25 degrees about the same axis, the sensor would indicate a net change in inclination of precisely zero degrees. However, due to hysteresis, conventional displacement sensors typically cannot perform in this manner. 
         [0006]    The present applicant was a co-inventor of an invention described in U.S. Pat. No. 4,624,140 the contents of which are included herein by reference in their entirety. An inclinometer disclosed in that patent comprises a sensing unit comprising a spherical vessel, partially filled with a conductive liquid, with conductive wall segments at least one of which is coated with a thin dielectric coating. In use, when the inclination of such an inclinometer is varied, the conductive liquid covers a variable portion of at least one dielectric coated wall segment. The capacitance between the conductive liquid and the coated wall segment varies as function of the inclination of the base of the device. An alternate capacitive sensor, which uses a low conductivity liquid as the dielectric of a capacitor, is disclosed in U.S. Pat. No. 3,906,471, the contents of which are included herein by reference in their entirety. U.S. U.S. Pat. Nos. 4,912,662 and 5,083,383, the contents of which are included herein by reference in their entirety, also describe other configurations of inclinometers. Generally, the accuracy of inclinometers and other displacement sensor technologies, with and without liquid sensing elements, are limited by hysteresis. 
         [0007]    The sensing units in displacement sensors typically have components that are immovably fixed to the housing or base of the sensor and others that are free to move or have the propensity to move relative to the housing or base when the sensor is displaced. The relatively fixed elements in the sensing unit of the inclinometer disclosed in U.S. Pat. No. 4,624,140 comprise the vessel and the conductive wall segments. The conductive liquid, on the other hand, is a movable element within the sensing unit that moves relative to the housing of the sensor or the sensor base when the housing and base are displaced. 
         [0008]    Displacement sensors are typically configured to be sensitive to a single input. For example, an inclinometer is typically configured to measure only changes in inclination of its base. Although, a two dimensional sensor may be used to measure an inclination change in two dimensions, the only input that can typically be measured with such a device is change in inclination of the base or housing. 
         [0009]    U.S. Pat. No. 1,637,445, the contents of which are included herein by reference in their entirety, describes the use of a liquid filled, shaft mounted, variable capacitor attached to a turning knob of a radio. Such a device cannot be used as an inclinometer because the output of the variable capacitor is sensitive to two different inputs, namely the inclination of the base of the radio and the rotation of knob  25  in  FIG. 1  of the patent. In such a device, the output of the capacitance is the result of an indeterminate combination of the inclination of the base and the rotation of the knob. 
       SUMMARY OF INVENTION 
       [0010]    It is an object of present invention to improve the accuracy of displacement sensors by reducing or eliminating errors caused by hysteresis. 
         [0011]    It is another object of this invention to compensate for the error caused by hysteresis in the use of a displacement sensor. 
         [0012]    It is yet another object of this invention to configure a displacement sensor to produce a calibrated output that may be used to measure the displacement of its base, but which also comprises a mechanism for producing a prescribed determinate displacement, of the sensing unit of the sensor, relative to the sensor base. The induced relative motion between the sensing unit and the sensor base is more preferably of a predetermined magnitude and timing which is automatically implemented. It is further preferred, that the net induced motion of the sensing unit relative to the base is zero. Therefore, the position of the “fixed” elements with respect to the sensor base after the induced motion is the same as it was prior to it. Alternatively, if the net relative displacement is not zero, it is necessary that the net relative displacement and its effect on the sensor output be ascertainable. The base of the sensor is typically used to attach the sensor to a surface the displacement of which is to be measured. 
         [0013]    An inclinometer may, for example, be configured so that in the case of changes in inclination about a sensitive axis, clockwise changes may be determined without any induced motion between the base and the sensing unit. In the case of counterclockwise changes in inclination, mechanisms within the sensor housing may automatically cause the sensing unit to undergo a predetermined additional counterclockwise change in inclination followed by an equal amount of clockwise change relative to the sensor base. Once these predetermined induced changes are completed, the measurement of inclination is obtained. In this manner, the sensing unit of such a sensor is always moving in the same direction, i.e. clockwise, prior to a reading, regardless of the overall direction of displacement of the sensor base. The effect of hysteresis is consequently reduced or eliminated. It is preferred that the magnitude of the predetermined relative displacement internal to the sensor be at least equal to or greater than the maximum error that would otherwise be caused by hysteresis. 
         [0014]    It is a further object of this invention to reduce or eliminate the effect of hysteresis by inducing vibration or oscillation of the sensing unit relative to the base. It is preferred that the magnitude of oscillations be equal or greater in magnitude than the maximum error due to the hysteresis. It is further preferred that the vibration or oscillation be stopped prior to obtaining a reading from the sensor. 
         [0015]    It is a yet another object of this invention to induce a predetermined displacement to the normally movable components in the sensing unit relative to the sensor base prior to obtaining a reading from the sensor. For example, in the case of a liquid filled capacitive sensing unit as disclosed in U.S. Pat. No. 4,624,140, the conductive liquid may be agitated directly and independently of the motion of the sensor base. 
     
    
     
       DESCRIPTION OF FIGURES 
         [0016]      FIG. 1  is a schematic of a conventional displacement sensor with sensing unit. 
           [0017]      FIG. 2  is a schematic of an embodiment of the invention with an actuator for moving the sensing unit relative to the sensor base. 
           [0018]      FIG. 3  is a schematic of another embodiment of the invention wherein the induced motion between the sensing unit and the base is controlled and constrained by components including a spring, a damper and a stop. 
           [0019]      FIG. 4  is a schematic of the embodiment illustrated in  FIG. 3  shown undergoing clockwise and counterclockwise displacements. 
           [0020]      FIG. 5  is a schematic of a further embodiment of a displacement sensor wherein actuator is used to induce vibration of sensing unit with respect to the sensor base. 
           [0021]      FIG. 6  is a schematic of a sensing unit of an inclinometer comprising liquid filled capacitors. 
           [0022]      FIG. 7  is a schematic of the sensing unit of  FIG. 6  displaced 30° in the counterclockwise direction. 
           [0023]      FIG. 8  is a schematic of a sensing unit with agitators for agitating a liquid component contained within the sensing unit. 
           [0024]      FIG. 9  is a schematic of a still further embodiment of the invention showing a sensor comprising a sensing unit and controller. 
           [0025]      FIG. 10  is a schematic of a sensing unit comprising a servo inclination sensor with actuators configured according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0026]      FIG. 1   a  shows a conventional inclinometer  1  with base  2  and sensing unit  3  with axis of sensitivity  4  and terminals  5  for power input, ground and signal out. The base is typically used to attach the sensor to a surface of another object where the displacement of the surface is to be measured. The sensing unit is immovably attached to the base. 
         [0027]      FIG. 1   b  shows the inclinometer of  FIG. 1  undergoing a sequence of changes in inclination. It starts from a horizontal position  6 , followed by a counterclockwise (CCW) or negative change  7  of β degrees and then returns to a horizontal position  8 . Typically, the reading of the sensor output in position  8  does not return to the reading obtained in position  6  because of hysteresis. In  FIG. 1   c , the inclinometer again starts from a horizontal position  9  and undergoes a change of β degrees in the clockwise (CW) direction  10  and again returns to a horizontal position  11 . Again, the reading in position  9  produced by an inclinometer typically does not match the reading in position  11 . Even if the output of the sensor starts at the same value in positions  6  and  9 , typically, the absolute value of the reading at position  7  compared to that at position  10  and the value at position  8  compared to that at position  11  will be different due to hysteresis. In fact, if the angle β is small enough, the output of the sensor in  FIG. 1   b  may not change at all, due to hysteresis, as the sensor transitions between position  6  and  7  and  7  and  8 . 
         [0028]      FIG. 2  shows an inclinometer  20  with sensor base  21  and sensing unit  22  within assembly  22   a . Also shown is a hinge or pivot  23  that permits the sensing unit and assembly  22   a  to be moved relative to the sensor base, i.e. rotated by a controlled amount about the axis of sensitivity  24  even when the base remains fixed. It is preferred that the relative movement between the sensing unit and the housing base  21  be controlled by actuator  25  which may be, for example, a piezoelectric stack, a cam mechanism, a worm gear drive or a rack and pinion device. The actuator may also comprise a linear or rotary drive with mechanical, electric pneumatic or hydraulic jacks, or a linear motor. 
         [0029]    The actuator may be used to minimize the effect of hysteresis by, for example, causing movement of the sensing unit relative to the base for certain displacements, to always be in a predetermined direction just before a reading is taken. It is preferred that the net motion induced by the actuator not add or subtract from the total displacement of the sensing unit caused by displacement of the base. It is further preferred that the change in position induced by the actuator also be of a predetermined magnitude such that its effect on the sensing unit output is equal to or larger than the maximum error otherwise due to hysteresis. The base may be located in any convenient location with respect to assembly  22   a . For example, base  21  may be attached to the side or top of assembly  22   a.    
         [0030]    The motion of the sensing unit relative to the enclosure base may be limited by using stops so as to ensure that the net induced motion is exactly zero.  FIG. 3  shows another embodiment of an inclinometer according to the present invention. The relative motion of the sensing unit with respect to the sensor base  31  may be constrained by a hinge  30 , a damping mechanism  34 , a spring mechanism  35  and a stop  36 . Relative motion of the sensor unit  32  with respect to base  31  may be induced by actuator  33 . 
         [0031]      FIG. 4   a  shows an inclinometer with base  31 , sensing unit  32 , and axis of sensitivity  32   a . The inclinometer base has undergone a rotation, or change in inclination, of 30° in the CCW or negative direction. In this embodiment, the base  31  and the assembly  41  that holds the sensing unit  32  are held together and move as one piece during a CCW displacement. The actuator  33  remains inactive and spring  35  holds assembly  41  firmly against stop  36 . Since the base  31  and assembly  41  move as one piece, distance “y”  42  remains unchanged. 
         [0032]      FIG. 4   b  shows the inclinometer undergoing a 30° CW or positive change in inclination. However, before the reading is taken, the actuator is activated and the assembly  41  is rotated through an angle δ° in the CW or positive direction followed by an angular displacement of δ° in the CCW or negative direction such that the assembly  41  again rests against stop  36 . During this actuator induced motion, the speed of relative angular displacement may be controlled by a combination of the actuator  33 , the spring  35  and damper  34 . In this case, the distance “y”  42  increases and then returns to the same value as in  FIG. 4   a . It is preferred that the angular displacement is equal to or larger than the maximum angular error normally resulting from hysteresis when no corrective action is taken. Alternatively, the sequence of CW and CCW relative displacements of δ° of the sensing unit may be induced even if the sensor output prior to the reading is in the CW direction or no displacement is detected by the sensing unit. 
         [0033]      FIG. 5  shows an inclinometer  50  with base  51  and assembly  52  comprising the sensing unit  53 . In this embodiment, the actuator  54  oscillates or vibrates the assembly  52  with respect to the base  51  at a predetermined frequency and amplitude. Readings are preferably taken at the same point in time during the period of oscillation or vibration. In this embodiment, the actuator  54  is preferably attached to both base  51  and assembly  52 . 
         [0034]      FIG. 6   a  shows a schematic of an inclinometer sensing unit  60  that may be used in displacement sensors such as illustrated in  FIG. 2  or  FIG. 3 . The sensing unit comprises a vessel  61  partially filled with a conductive liquid  62  and dielectric coated wall segment  63 .  FIG. 6   b  shows a section view of the sensing unit. In the position shown, the conductive liquid completely covers the lower dielectric coated wall segments  63  and  64 . The conductive coated wall segments  65  and  66  are not covered by the conductive liquid. In  FIGS. 6   a  and  6   b , the capacitances between each of the wall segments  63  and  64  and the liquid are at a maximum value while the capacitances between each of the wall segments  65  and  66  and the liquid are at their minimum value. 
         [0035]    During use of this sensing unit in a displacement sensor, the vessel walls including the conductive wall segments  63 ,  64 ,  65  and  66  are preferably maintained in a predetermined or fixed relationship with respect to the sensor base when the output reading is obtained. These elements remain fixed relative to the base of the sensor unless moved by, for example, actuator  25  in  FIG. 2 . It is further preferred that the net relative movement with respect to the base caused by the actuator be zero prior to when a reading of the sensing unit output is taken. 
         [0036]      FIG. 7  shows the sensing unit of  FIG. 6   a  after it has undergone a 30° CCW angular displacement. As a result, the lower plates  63  and  64  (not shown) are partially uncovered while plates  65  and  66  (not shown) are partially covered by liquid  62 . When the sensor unit is installed in an inclinometer such as shown in  FIG. 4   a , elements such as the wall segments  63 ,  64 ,  65 , and  66  are constrained to move with assembly  41 . The conductive liquid, although a part of the sensing unit, may move relative to the wall segments, assembly  41  and base  31 . 
         [0037]    Hysteresis in liquid filled sensing units, such as shown in  FIG. 6  and disclosed in U.S. Pat. Nos. 4,624,140 and 3,906,471 are at least in part a result of surface tension of the liquid. The impact of surface tension on hysteresis may be diminished by causing the induced motion prior to or during the reading of the output to always be in the same direction. Alternatively, the effect of hysteresis may be diminished by vibrating the sensor unit as a whole. As yet another alternative, only a portion of the sensing unit, for example the conductive liquid, may be moved or agitated directly. 
         [0038]      FIG. 8   a  shows a sensing unit  80  with vessel  81  partially filled with liquid  82 .  FIG. 8   b  shows a section of the sensor with conductive wall segments  83 ,  84 ,  85 , and  86 . Also shown are two pistons  87  and  88  that may be moved axially inward with axial actuators (not shown). The axial motion of these pistons is preferably initiated before the sensor reading is obtained and after the inclinometer containing the sensing unit has reached the position where a measurement is to be obtained. The disturbance induced by pistons  87  and  88  may also be oscillatory and continue even during the period when the measurement is taken. Alternatively, the liquid may be agitated by imparting motion or oscillations to a wall of the vessel that is configured to be flexible. The use of devices to directly cause a disturbance in the liquid may be used in conjunction with using devices to induce desired motions to the sensing unit as a whole. 
         [0039]      FIG. 9  shows a schematic of an inclinometer  90  configured according to yet another embodiment of the invention, comprising a sensor base  91  and sensing unit  92  and sensing unit assembly  93 . The assembly  93  also comprises a controller  94 , terminal strip  95 , contact  96 , and an actuator  97  with plunger  98 . The movement of the assembly  93  with respect to the sensor base  91  is constrained by hinge  99 , spring  100 , and stop  101 . When assembly  93  approaches base  91  sufficiently so that contact  96  touches stop  101 , the relative motion of assembly  93  towards base  91  typically ceases. However, contact  96  may be configured so that assembly  93  may move closer than this point. 
         [0040]    The controller is connected to a power terminal  95   a  and ground terminal  95   b . The controller supplies power and monitors the sensing unit  92 , the actuator  97 , and input terminal  95   c . Based on the output of the sensor unit  92 , input commands obtained from terminal  95   c  and on board algorithms or empirical data, the controller causes the actuator to induce relative motion between the assembly  93  and base  91 . The controller obtains the output from the sensor unit  92  after or during the induced motion and supplies an appropriate signal indicative of the inclination of base  91  to the output terminal  95   d.    
         [0041]    Contact  96  may be configured so that the controller may determine if there is physical contact between the stop  101  and contact  96 . The device may also be configured so that the contact  96  may be disabled if it is desired that the assembly be moved closer to the base than the stop would otherwise allow. The contact device may be configured with a disabling mechanism  96   a  so that the controller may disable the contact device so that it does not engage the stop  101 . In the embodiment in  FIG. 9 , if contact  96  is disabled by the controller, the motion of the assembly will be constrained by only hinge  99 , actuator  97  and spring  100 . 
         [0042]    The plunger  98  of actuator  97  may be attached to base  91  so that the actuator can be used to push or pull on the base  91 . The actuator may then be used to induce vibratory relative motion between the assembly and the base. 
         [0043]      FIG. 10  shows a servo inclination sensing unit  110  comprising a pendulum mass  111 , motor  112  and proximity sensor  113  arranged in a conventional fashion. This is another example of a sensing unit that may be used in a displacement sensor built according to this invention. Conventionally, when a sensing unit  110  is inclined, the position of the pendulum mass is altered as a result of the realignment of the mass  111  with respect to the direction of the gravitational field. The motor  112  then realigns the position of the mass  111  as measured by the proximity probe so that the position is returned to the undisturbed position  114  based on commands from the controller  115 . The current supplied to the motor is proportional to and used as a measure of the displacement of the sensing unit  110 . 
         [0044]    To minimize the effect of hysteresis, an actuator (not shown) may be used to alter the angular position of the normally stationary motor  112  so that the motion of the pendulum mass always approaches the null position  114  from the same direction regardless of the direction of the overall sensor displacement. Alternatively, instead of using an actuator to modify the position of a normally fixed component of the sensing unit such as the motor  112 , actuators  116  and  117  may be used to induce added motion in the mass  111  so that it always approaches the null point from the same direction regardless of the direction of the overall sensor displacement. Actuators  116  and  117  may be used, for example, to magnetically attract a mass  111 , which may be at least partially made of iron. Alternatively, actuators  116  and  117  may be used to vibrate the mass  111  before or during the period that the output reading is obtained. 
         [0045]    The invention has been described in terms of its functional principles and several illustrative embodiments. Many variants of these embodiments will be obvious to those of skill in the art based on these descriptions. Therefore, it should be understood that the ensuing claims are intended to cover all changes and modifications of the illustrative embodiments that fall within the literal scope of the claims and all equivalents thereof.