Abstract:
A method and apparatus are provided for improving the performance of displacement sensors, including absolute displacement sensors, such as inclinometers and accelerometers, and relative displacement sensors such as linear relative position transducers, by reducing or eliminating hysteresis. During use, independent, controlled and limited displacement is induced between the sensing unit and housing or base of such sensors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation in part of U.S. application Ser. No. 13/065,789, filed Mar. 30, 2011, now U.S. Pat. No. 8,510,966 and claims priority thereto. This application also claims priority to U.S. Provisional Patent Application No. 61/341,351, entitled “Sensors with reduced hysteresis”, filed Mar. 30, 2010. The contents of the patent and patent applications, referenced above, are incorporated herein by reference, in their entirety. 
    
    
     FIELD OF INVENTION 
     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 
     The performance of sensors, such as displacement sensors, is frequently degraded by hysteresis. Displacement sensors may be absolute displacement sensors such as certain inclinometers and accelerometers that measure the inclination or acceleration of the housing or enclosure of the sensor relative to the earth or other inertial reference frame. Frequently, the same sensor may be used as an inclinometer and an accelerometer. Displacement sensors may also be used to measure the relative displacement between two or more points. Such relative displacement sensors may be physically connected to or in contact with such points or may rely on, for example, magnetic or electric fields or electromagnetic or acoustic waves to link to such points. 
     Absolute displacement sensors, such as for example inclinometers, typically comprise a base and certain sensing elements, within a sensing unit, that are immovably connected to the base. Such displacement sensors 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 sensing elements within the sensing unit is typically measured and used to determine the displacement of the base. 
     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 (CW) change in inclination about its sensitive axis followed by a counterclockwise (CCW) 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, at least due to hysteresis, conventional inclinometers typically cannot perform in this manner. 
     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 and 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 a 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. Pat. Nos. 4,503,622; 4,854,047; 4,912,662; 5,083,383; 6,490,920; 6,516,527; and 7,886,451, the contents of which are included herein by reference in their entirety, also describe other configurations of inclinometers. U.S. Pat. Nos. 3,721,010 and 5,682,682, the contents of which are included herein by reference in their entirety, describe related displacement sensors that measure the distance or the change in distance between two points. Generally, the accuracy of inclinometers and other displacement sensor technologies, with and without liquid sensing elements, is limited by hysteresis. 
     The sensing units in displacement sensors typically have components that are immovably fixed relative to the housing or base of the sensor and other components 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. 
     The contents of co-pending U.S. Pat. App. 2012/0266470 are incorporated herein by reference in their entirety. 
     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. 
     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 tuning 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 U.S. Pat. No. 1,637,445. 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 
     It is an object of the present invention to improve the accuracy of displacement sensors by reducing or eliminating errors caused by hysteresis. 
     It is another object of this invention to compensate for the error caused by hysteresis in the use of a displacement sensor. 
     Displacement sensors may be configured to measure absolute displacement relative to an inertial reference frame, such as, for example, the earth. Displacement sensors, such as inclinometers and accelerometers, are typically absolute displacement sensors. Alternatively displacement sensors may be configured to measure relative displacement between two or more objects, surfaces or points. Displacement sensors typically have sensing units that detect absolute quantities, i.e. for example absolute acceleration or inclination or relative quantities, such as for example relative distance or the change in relative distance. It is yet another object of this invention to configure a displacement sensor to produce a calibrated output that may be used to measure displacement, but which also comprises a mechanism for producing a prescribed determinate displacement, of the sensing unit of the sensor, relative to the sensor base or housing. 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 or housing, prior to obtaining the final output reading, is zero. Therefore, it is preferable that the positions of the elements of the sensing unit that are normally “fixed” relative to the sensor base are returned to their original relative positions after the induced motion. 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 an absolute displacement sensor is typically used to attach the sensor to or place the sensor against a surface the displacement or position of which is to be measured. Displacement sensors, such as inclinometers, may be placed against a surface to measure its absolute inclination. Alternatively, relative displacement sensors may be attached to two surfaces in order to measure the relative displacement between the two surfaces or points on those two surfaces. 
     An inclinometer may, for example, be configured so that in the case of changes in inclination of its base or housing about a sensitive axis, CW changes may be determined without any induced motion between the base and the sensing unit. In the case of CCW changes in inclination, mechanisms within the sensor housing may automatically cause the sensing unit to undergo a predetermined additional CCW change in inclination as determined by the sensing unit followed preferably by an equal amount of CW change relative to the sensor base. Preferably, once these predetermined induced changes are completed, a measurement of inclination of the base is obtained. In this manner, the motion of the sensing unit of such a sensor is always in the CW direction before the output reading is obtained, regardless of the overall direction of displacement of the sensor base. As a result, the effect of hysteresis is 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. 
     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 or the absolute reference frame. It is preferred that the magnitude of oscillations be equal to 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 an output reading from the sensor. 
     In an embodiment of the invention, an inclinometer is configured comprising an inclination sensing unit and a base. The base can be used to connect the sensor to a surface so that the inclination of the surface can be determined. The sensor further comprises an actuator that may be used to impart relative controlled motion between the sensing unit and the base. The motion is preferably of predetermined speed, duration, magnitude and overall direction. 
     In a further embodiment, a method for measuring the inclination of a surface is disclosed comprising: providing an inclination sensing unit, providing a base, placing the base on a surface, inducing relative motion between the sensing unit and the base, and obtaining an output measurement from the sensing unit. The sensing unit may comprise a liquid element that may be conductive. The conductive liquid may be an electrolyte. 
     It is a yet another object of this invention to directly 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 in the sensing element as a whole. 
     Various features of one or more embodiments of the invention described herein may be used singularly or in combination with other features including features not described herein. The objectives indicated are not intended to be exhaustive. 
    
    
     
       DESCRIPTION OF FIGURES 
       The foregoing summary, as well as the description of the embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the embodiments of the present inventions, and to explain their operation, drawings of preferred embodiments and schematic illustrations are shown. It should be understood, however, that the invention is not limited to the precise arrangements, variants, structures, features, embodiments, aspects, methods, advantages, improvements, and instrumentalities shown, and the arrangements, variants, structures, features, embodiments, aspects, methods, advantages, improvements, and instrumentalities shown and/or described may be used singularly in the device or method or may be used in combination with other arrangements, variants, structures, features, embodiments, aspects, methods, advantages, improvements, and instrumentalities. 
         FIG. 1   a  is a schematic of a conventional displacement sensor with sensing unit. 
         FIG. 1   b  shows the inclinometer of  FIG. 1   a  undergoing a sequence of changes in inclination.  FIG. 1   c  shows the inclinometer of  FIG. 1   a  undergoing another sequence of changes in inclination. 
         FIG. 2  is a schematic of an embodiment of the invention with an actuator for moving the sensing unit relative to the sensor base. 
         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. 
         FIG. 4   a  is a schematic of the embodiment illustrated in  FIG. 3  shown undergoing a CW displacement.  FIG. 4   b  is a schematic of the embodiment illustrated in  FIG. 3  shown undergoing a CCW displacement. 
         FIG. 5  is a schematic of a further embodiment of a displacement sensor wherein an actuator is used to induce vibration of the sensing unit relative to the sensor base. 
         FIG. 6   a  is a schematic of a side view cross section of a sensing unit of an inclinometer comprising a conductive liquid.  FIG. 6   b  is a schematic of an end view cross section of a sensing unit of an inclinometer comprising a conductive liquid. 
         FIG. 7  is a schematic of the sensing unit of  FIG. 6   a  and  FIG. 6   b  displaced 30° in the CCW direction. 
         FIG. 8   a  is a schematic of a side view cross section of a sensing unit with agitators for agitating a liquid component contained within the sensing unit.  FIG. 8   b  is a schematic of an end view cross section of a sensing unit with agitators for agitating a liquid component contained within the sensing unit. 
         FIG. 9  is a schematic of a still further embodiment of the invention showing a sensor comprising a sensing unit and controller. 
         FIG. 10  is a schematic of a sensing unit comprising a servo inclination sensor with actuators configured according to another embodiment of the invention. 
         FIG. 11  is a schematic of a further embodiment of the invention with an actuator for moving the sensing unit relative to the sensor housing. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       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 inclination of the surface is to be measured. The sensing unit is immovably attached to the base. 
       FIG. 1   b  shows the inclinometer of  FIG. 1   a  undergoing a sequence of changes in inclination. It starts from a horizontal position  6 , followed by a CCW or negative change of β degrees in position  7  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  at least partly due to hysteresis. In  FIG. 1   c , the inclinometer again starts from a horizontal position  9  and undergoes a change of β degrees in the CW direction in position  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 magnitude 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 at least 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 positions  6  and  7  and  7  and  8 . 
       FIG. 2  is a schematic of an inclinometer configured according to the invention. Inclinometer  20  comprises a base  21 , a housing  22 , and a sensing unit  23  within assembly  24 . Base  21  is configured to be attached, otherwise secured to or placed against a surface or object. Also shown is a pivot or hinge  25  which permits the assembly  24  and sensing unit  23  to be moved relative to the sensor base  21  and housing  22 . The sensing unit  23  and assembly  24  may be rotated about the axis of sensitivity  25  even when the base and the housing remain fixed. It is preferred that the relative movement between the sensing unit  23  and the housing base  21  be controlled by actuator  26  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. Terminals  28  are configured to supply electrical power and to communicate with the sensing unit. Opening  27  in housing  22  is configured to allow assembly  24  and terminals  28  to be moved by actuator  26  without being obstructed. 
     The actuator may be used to minimize the effect of hysteresis by, for example, causing movement of the sensing unit relative to the base to always be in a predetermined direction just before an output 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 as a result of the 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 the magnitude of 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 position or orientation relative to assembly  24  and housing  22 . The base  21  may be of any convenient shape for placing against or attaching to a surface of an object for measuring the object&#39;s inclination. For example, base  21  may be movably attached, for example by means of a hinge, to the side or top of assembly  24 . 
     The motion of the sensing unit relative to the base may be limited or constrained by using stops so as to ensure that the net induced relative motion is exactly zero.  FIG. 3  shows another embodiment of an inclinometer according to the present invention. The relative motion of the sensing unit  32  relative 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 sensing unit  32  relative to base  31  is induced by actuator  33 . 
       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 from horizontal position  43 . In this embodiment, the base  31  and the assembly  41  that comprise 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. 
       FIG. 4   b  shows the inclinometer undergoing a 30° CW or positive change in inclination from horizontal position  43 . However, after the 30° CW rotation, but before the final output 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 followed by CCW relative displacements of δ° of the sensing unit may be induced prior to obtaining the final reading regardless of the direction of the rotation of base  31 . 
       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  relative to the base  51  at a predetermined frequency and amplitude. Output 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 . 
       FIG. 6   a  shows a schematic of an inclinometer sensing unit  60  that may be used in inclinometers 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 segments  63  and  65 .  FIG. 6   b  shows a section view of the sensing unit. In the position shown, the conductive liquid  62  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. 
     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 relative 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 relative to the base caused by the actuator be zero prior to when a final reading of the sensing unit output is obtained. 
       FIG. 7  shows the sensing unit of  FIG. 6   a  after it has undergone a 30° CCW angular displacement from the horizontal position shown in  FIG. 6   a . 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 sensing 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  63 ,  64 ,  65  and  66 , assembly  41  and base  31 . 
     Hysteresis in liquid filled sensing units, such as shown in  FIG. 6  and disclosed in U.S. Pat. Nos. 4,503,622; 4,912,662; 5,083,383; 6,490,920; 6,516,527; and 7,886,451, is at least in part a result of surface tension of the liquid and the adhesion between the liquid and at least a portion of the wall of the vessel that contains the liquid. The impact of these attractive forces on sensor performance may be diminished by causing the induced relative 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 sensing unit as a whole. As yet another alternative, only a portion of the sensing unit, for example the conductive liquid, may be directly moved or agitated independently of the motion of the remainder of the sensing unit. 
       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 final 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 a 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. Devices to directly cause a disturbance in the liquid may be used in conjunction with devices to induce a desired relative motion to the sensing unit as a whole relative to the base. 
       FIG. 9  shows a schematic of an inclinometer  90  configured according to yet another embodiment of the invention, comprising a sensor base  91 , sensing unit  92  and 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  relative 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 be permitted to move closer to base  91  than this point. 
     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 sensing 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 sensing 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.    
     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 so that the assembly  95  may move closer to the base than the stop would otherwise permit. 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 . 
     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 . Alternatively, the actuator may then be used to induce vibratory relative motion, instead of or in addition to oscillatory motion between the assembly and the base. 
       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  relative to the direction of the gravitational field. The motor  112  then realigns the position of the mass  111  as measured by the proximity sensor  113  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 . 
     To minimize or eliminate the effect of hysteresis, an actuator (not shown) may be used to alter the angular position of the normally stationary motor  112  or the mass  111  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. 
     Typically, an inclinometer is sensitive to the total acceleration, i.e. the vector sum of the acceleration of gravity and the absolute acceleration of the base of the inclinometer. Therefore, inclinometers can be used to measure acceleration, preferably if held at a fixed inclination in the inertial reference frame. The output reading of an inclinometer undergoing acceleration in the inertial reference frame is proportional to the total acceleration vector perpendicular to the sensitive axis of the inclinometer. Therefore, inclinometers and accelerometers are typically interchangeable. Accelerometers may also be used as velocity sensors and absolute displacement sensors by integrating the output signal of the accelerometer. 
       FIG. 11  shows an illustration of a relative displacement sensor  120  configured to measure the relative distance between points on two separate objects  121  and  122 . The sensor  120  comprises a reciprocating probe  123  with a knurled contact tip  123   a , a rack  124 , a pinion  125  and a sensing unit  126 . The position of reciprocating probe  123  relative to pinion  125  is a function of distance “d”  127  between points on objects  121  and  122 . The position of probe  123  is detected by sensing unit  126 . The measurement of distance “d”  127  or a change in this distance is displayed by the sensing unit  126  by means of a display unit (not shown). The sensor further comprises an actuator  128  that is configured to move the sensing unit relative to the housing  129  or base  130  of the sensor. The actuator is further configured to move the sensing unit in such a manner so that the measurement of “d” is always changing in the same manner, for example is always decreasing, before the final reading is taken. The sensor  120  also comprises a spring  131 , a probe flange  132 , a base flange  133  and a probe stop  134  configured to limit and constrain the motion of the probe  125 . 
     The invention has been described in terms of functional principles and illustrations of specific embodiments. Embodiments described herein, including descriptions of the figures, are merely intended as exemplary, but the concept of the invention is not limited to these embodiments. The following claims are not limited to or by the described illustrative embodiments, figures, and stated objectives of the invention or the abstract. Furthermore, various presently unforeseen or unanticipated combinations of the disclosed embodiments, or their elements, or alternatives, variations or improvements which may become apparent to those of skill in the art are also intended to be encompassed by the following claims. 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.