Abstract:
A sensor for measurement of small-angle or small-displacement position of a rotational rheometer incorporates multiple independent capacitors in a symmetric relationship. The device presents its output as a standard bridge configured differential signal, which can be interpreted and measured using conventional electronic demodulation means. The device includes an excitation array, a measurement array and an active rotor array. The active rotor array is coupled to a drive shaft of the rotational rheometer and measured relative to the measurement and excitation arrays. The active rotor array is driven by an electrical signal that is precisely matched to signals detected by the measurement array. By driving the active array with signals sensed by the measurement array, the sensor allows for reduced sensitivity to unwanted signals not in the measurement direction.

Description:
BACKGROUND 
   1. Field of the Invention 
   Embodiments of the present invention relate to systems and methods for capacitive measurement using a rheometer. More particularly, embodiments of the present invention relate to systems and methods for measuring the rheological properties of a sample on a rotational rheometer, while exhibiting a reduced sensitivity to motions that are not in the direction or axis of the desired measurement. 
   2. Background of the Invention 
   Rotary rheometers, viscometers or viscosimeters are typically used to measure rheological properties of materials, such as their viscosity, compliance, and modulus, by rotating, deflecting or oscillating a measuring geometry in a material, either by applying a torque and measuring the resultant velocity or displacement, or by applying a velocity or displacement and measuring the resultant torque. The torque and velocity/displacement are used in conjunction with measuring geometry factors to determine the properties of the material. 
   As used herein, the term “rheometer” includes rheometers, viscometers, viscosimeters and similar instruments that are used to measure the properties of fluid or similar materials (see list below). Viscosity is an internal property of a fluid that offers resistance to flow (i.e., it concerns the thickness of a liquid). 
   The term “material,” as used herein, includes liquids, oils, dispersions, suspensions, emulsions, adhesives, biological fluids, polymers, gels, pastes, slurries, melts, resins, powders or mixtures thereof. Such materials are also referred to herein as “fluids.” More specific examples of materials include asphalt, chocolate, blood, drilling mud, lubricants, oils, greases, photoresists, liquid cements, elastomers, thermoplastics, thermosets and coatings. 
   A common use for a rheometer is to determine fluid properties of a material. One technique is to apply a torque developed by a motor in the presence of the material, and measure the resultant velocity or displacement. The torque and velocity/displacement are used in conjunction with measuring geometry factors to determine the properties of the material. Thus, the rheometer requires a position sensor that is extremely accurate, linear, stable and consistent. The position sensor must operate over a very small range of motion with a high resolution of position. 
   Unfortunately, current rotary rheometers use sensors that suffer from gain error. Specifically, mechanical motions that are not in the axis of the primary measurement cause parasitic capacitance that can be reported erroneously as a change of the primary measurement axis. Thus, it is desirable to create small-angle or small-displacement capacitive sensors that have greatly reduced sensitivity to typical sources of mechanical positioning error. 
   BRIEF SUMMARY OF THE INVENTION 
   A device for measuring movement of a rheometer according to the present invention includes three components: an excitation array, an active rotor array, and a measurement array. The active rotor array is positioned between the measurement and excitation arrays and mechanically coupled to a drive shaft of the rheometer. The position of the active rotor array element is measured relative to the measurement and excitation arrays. Particularly, the measurement array senses a signal from the excitation array, which is affected as the active rotor array moves with the drive shaft. 
   In a preferred embodiment of the invention, sensitivity may be increased by using a plurality of elements in each array of the device. For example, the excitation array may have a plurality of first emitters and a plurality of second emitters. Each first emitter emits a first sinusoidal signal, and each second emitter emits a second sinusoidal signal, with the second sinusoidal signal out of phase with the first sinusoidal signal. 
   Similarly, the measurement array has a plurality of first detectors and a plurality of second detectors. Each first detector senses a first voltage of the first sinusoidal signal and the second sinusoidal signal, and each second detector senses a second voltage of the first sinusoidal signal and the second sinusoidal signal. Likewise, the active rotor array has a plurality of movable electrodes, wherein movement of the plurality of movable electrodes varies the first voltage and the second voltage sensed by the measurement array. 
   In a preferred embodiment of the present invention, each array (e.g., the excitation array, measurement array, and active rotor/linear array) uses fifty blades, increasing the resulting sensitivity by a factor of twenty-five. 
   In a preferred embodiment of the invention, the active rotor array is driven by the voltages sensed by the measurement array. Driving the active array with the voltages of the measurement array causes rejection of unwanted signals that are not in the measurement direction. Thus, for angular measurement of the movement of the drive shaft, axial motion of an active rotor does not result in an amplitude change in the differential output signal. Lateral translation of an active rotor does not simulate an angle change due to the plurality of capacitive elements and the resultant cancellation of capacitance changes due to averaging effects. 
   Multiple capacitive elements of the sensor that combine into a single composite measurement provide increased sensitivity. Similarly, driving the active array with voltages from the measurement array results in reduced sensitivity to other mechanical motions that are not in the direction of measurement. Although the invention is described in a rheology application, the invention may be used in any application in which small angles or small distances are measured. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cutaway view of a small portion of a measurement device according to a preferred embodiment of the present invention. 
       FIGS. 2A ,  2 B and  2 C are schematic diagrams of an exemplary excitation array, an exemplary active rotor array and an exemplary measurement array, respectively according to a preferred embodiment of the present invention. 
       FIG. 3  is a schematic diagram of a capacitive bridge formed by the elements of the measurement device according to the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A rheometer according to the present invention includes a measurement device having an excitation array, an active rotor array, and a measurement array. The active rotor array is positioned between the measurement and excitation arrays and mechanically coupled to a rotating shaft of the rheometer so that it rotates with the shaft. The position of the active rotor array is measured relative to the measurement and excitation arrays. Particularly, the measurement array senses a signal from the excitation array, which is affected as the active rotor/linear array moves from side to side. 
   When the active rotor is turned relative to the excitation and measurement arrays, a signal is produced on the measurement array that is proportional to the change in angular position. Changes to the position of the active rotor that are not angular in nature produce greatly reduced output signal changes. For example, axial motion of the active rotor does not result in an amplitude change in the differential output signal. Similarly, lateral translation of the active rotor does not simulate an angle change. The measurement device also has increased sensitivity by a factor of 20 or more. 
     FIG. 1  is a cutaway view of a small portion of a small angle measurement device used in a rotational rheometer according to a preferred embodiment of the present invention. Measurement device  100  comprises an excitation array  110 , an active rotor array  120  and a measurement array  130 . Active rotor array  120  is positioned between excitation array  110  and measurement array  130 . Although not shown in  FIG. 1 , active rotor array  120  mechanically couples, for example, to a drive shaft of a motor in a rotational rheometer. Thus, active rotor array  120  moves with the drive shaft of the rheometer. 
   Each array of the measurement device  100  (i.e., excitation array  110 , active rotor array  120  and measurement array  130 ) has a plurality of elements or conductive areas, shown in more detail in  FIGS. 2A–2C  described below. 
   In a preferred embodiment of the invention, measurement device  100  operates with a maximum linear range of plus or minus 20 milliradians (mRad), which is used for a 3.5 to 5.0 mRad transducer. Measurement device  100  has an outer diameter of three inches for the assembly using PC board technology. More particularly, each array has a clear hole with a minimum diameter of 1.0 inches for a hub and wiring. In a preferred embodiment, the clear hole is 1.5 inches. Additionally, each array has an outer diameter of 2.5 inches. 
   Excitation array  110  is the electrically driven element of the measurement device  100 . Particularly, excitation array  110  provides a sinusoidal signal to measurement array  130 . Emitter A  112  and emitter B  114 , elements of excitation array  110 , emit sine signals that are 180 degrees out-of-phase. 
   Active rotor array  120  is a conductive element of the measurement device  100  used as a shadowing element to block the electrostatic field generated by excitation array  110 . Arrows  125  indicate the side-to-side motion of active rotor array  120  in the desired axis of measurement. For position determination, active rotor array  120  is mechanically coupled to the moving shaft of the rheometer, whose motion is being measured. Unlike conventional capacitive measurement devices, active rotor array  120  is driven by electric signals C′  122  and D′  124  to reduce the parasitic capacitance that causes gain error. 
   Measurement array  130  senses a signal from excitation array  110 , which is transferred by a capacitive coupling across the gap between the two arrays. The elements of measurement array  130  include detector C  132  and detector D  134 . Buffer amplifiers (not shown) are connected to nodes C  132  and D  134  to drive the C′  122  and D′  124  signals, respectively. Accordingly, nodes C′  122  and D′  124  are low impedance sources precisely matched to the signals appearing on nodes C and D. 
     FIGS. 2A ,  2 B and  2 C are schematic diagrams of excitation array  110 , active rotor array  120  and measurement array  130 , respectively, according to a preferred embodiment of the present invention. In the example illustrated in these figures, excitation array  110 , active rotor array  120  and measurement array  130  each have 50 elements or conductive areas called blade  210 A,  210 B and  210 C. Each blade measures 0.1256637 radians. In addition, each blades  210 C of measurement array  130  has two traces or double guards  220  that supply drive signal C′  122  and D′  124  to the active rotor array  120 . The trace width and spacing between the blades is 0.005 inches. 
   Although the illustrative example discloses the use of 50 blades, one skilled in the art will recognize that the present invention is not limited as such. For example, in another embodiment, only 20 blades per array may be used. Alternatively, using thin-film and other small geometry fabrication methods, arrays with a greater density than 25 bridge elements (50 blades) can be achieved. 
   In the embodiment described herein, the use of 50 blades increases the resulting sensitivity by a factor of 25. Particularly, the capacitive elements of excitation array  110  and measurement array  130  form multiple capacitive bridges, described below in reference to  FIG. 3 . Each bridge includes the following nodes: emitter A, emitter B, detector C, detector D, displacement element C′ and displacement element D′. The use of 50 excitation and measurement blades results in 25 such bridges composed of 100 capacitors, each bridge increasing the sensitivity of the measurement device. 
     FIG. 3  is a schematic diagram of a capacitive bridge formed by the elements of the measurement device according a preferred embodiment of the present invention. Capacitive bridge  300  includes nodes A, B, C and D, as well as displacement elements C′ and D′, variable capacitors AC  310 , AD  320 , BD  330  and BC  340 , drive signals  350  and  360 , and buffer amplifiers Ac  370  and Ad  380 . 
   Drive signals  350  and  360  represent the sinusoidal signals emitted from emitter A  112  and emitter B  114  that are 180 degrees out-of-phase. Nodes C  132  and D  134  each detect the two signals  350  and  360  that are emitted from emitters A and B. The detected signals vary in strength and phase as the position of active rotor array  120  changes, and can be described as variable capacitors AC  310 , AD  320 , BD  330  and BC  340 . 
   Variable capacitor AC  310  represents the capacitance between emitter A  112  and detector C  132  of  FIG. 1 . Similarly, variable capacitor AD  320  represents the capacitance between emitter A  112  and detector D  134 . Variable capacitor BD  330  represents the capacitance between emitter B  114  and detector D  134 , and variable capacitor BC  340  represents the capacitance between emitter B  114  and detector C  132 . Each of the capacitors AC  310 , AD  320 , BD  330  and BC  340  are described as variable because their capacitance values vary with the movement of active rotor array  120 . 
   For example, if active rotor array  120  of  FIG. 1  moves to the left, then the elements of rotor array provide less shadowing between detector C  132  and emitter A  112  and less shadowing between detector D  134  and emitter B  114 . Because shadowing is reduced by movement of active rotor array  120  to the left, detector C  132  receives greater exposure to emitter A  112  and detector D  134  receives greater exposure to emitter B  114 . Accordingly, moving active rotor array  120  of  FIG. 1  to the left increases the capacitance of capacitor AC  310  and BD  330 , by increasing the effective surface area of the capacitive elements (i.e., the portion of the emitter and detector elements that are not shaded by the conductive rotor elements). 
   Similarly, moving active rotor array  120  of  FIG. 1  to the left increases the shadowing provided by active rotor array  120  between emitter A  112  and detector D  134  as well as between emitter B  114  and element C  132 . Accordingly, moving active rotor array  120  of  FIG. 1  to the left decreases the capacitance of capacitor AD  320  and BC  340 . 
   Returning to  FIG. 3 , as active rotor  120  moves to the left, bridge capacitors AC  310  and BD  330  increase in capacitance while bridge capacitors AD  320  and BD  340  decrease a proportional value in capacitance. The change in capacitance values of the bridge capacitors  310 ,  320 ,  330  and  340  causes bridge  300  to become unbalanced. Sense points or detectors C  132  and D  134  detect the differential signal of the unbalanced bridge  300 . Further, the variation of capacitance is linear with respect to the horizontal displacement of the effective surface area of the various capacitive elements changes linearly. 
   Each capacitive bridge configuration formed by the various physical elements of  FIG. 1  increases the sensitivity of the system to capacitance changes. Particularly, the use of multiple bridges multiplies the sensitivity of measurement device  100  by the number of array elements used. Particularly, as described above in reference to  FIGS. 2A ,  2 B and  2 C, a preferred embodiment of the present invention includes 50 blades for each array in the measurement device (i.e., excitation, active rotor and measurement arrays), resulting in 25 bridge elements composed of 100 capacitors connected in an array. Thus, the resulting sensitivity of the measurement device is multiplied by a factor of 25. 
   Although using more array elements increases sensitivity of the measuring device, it also decreases its full-scale angular or linear range. For example, if a single-capacitor rotary position sensor has a maximum full-scale range of 180 degrees, then an array of 25 bridge elements is reduced to a maximum full-scale range of 7.2 degrees (e.g., 180/25=7.2). However, physical limitations of fabrication size, alignment, the gap between plates, edge effects and other necessary design compromises limit the range of the sensor even further than the theoretical 7.2 degrees maximum full-scale range. The embodiment depicted in  FIGS. 2A ,  2 B and  2 C, comprising 50 blades per array achieves small angle measurement of plus or minus 20 mRad in limited range applications of 5 mRad or less. 
   Parasitic capacitances (e.g., introduced by movement in the non-measurement direction) could skew the signals detected at nodes C and D in the absence of corrective measures. For example, motion of the rotary electrode that is not along the primary measurement path can introduce additional capacitances that are parasitic to the function of the sensor, causing an error in gain and a reduced sensitivity. Particularly, low impedance nodes A and B would remain relatively unaffected by any changes in capacitance due to parasitic capacitance. However, high impedance nodes C and D that are extremely sensitive to capacitive loading would change their values if a parasitic capacitance were introduced, thereby resulting in a gain error in the bridge. 
   To reduce or eliminate any skewing, active rotor array  120  is electrically driven by elements C′  122  and D′  124 . Elements C′  122  and D′  124  are low impedance elements that are relatively insensitive or immune to changes in capacitance due to capacitance loading. Buffer amplifiers  370  and  380  supply the detected signals from measurement array  130  to active rotor array  120 . In this way, drive signals C′  122  and D′  124  are precisely matched to the signals appearing on nodes C  132  and D  134 , respectively, of stationary measurement array  130 . 
   Thus, parasitic capacitances formed by the arrangement of  FIG. 1  include AC′, AD′, BC′, BD′, CC′, CD′, DC′ and DD′. However, A, B, C′ and D′ nodes are emitter elements, which are low-impedance sources that remain relatively unaffected by any change of capacitance. Thus, although parasitic capacitances AC′, AD′, BC′ and BD′ are real capacitances that are affected by gap distance, any change in these capacitances will leave nodes A, B, C′ and D′ relatively unaffected. 
   Similarly, parasitic capacitances CD′ and DC′ are also minimized by their placement in the system. Particularly, the CD′ and DC′ capacitance is minimized by placing nodes C′  122  and D′  124  above the center of nodes C and D, respectively. Thus, even when active rotor array moves to the left or right the amount of any overlap between nodes C and D′ or between nodes C′ and D are minimized. 
   Most importantly, parasitic capacitances CC′ and DD′ are effectively zero. Notably, because node D′ is driven from node D, the two sources have the same potential voltage at all times. Thus, the capacitance DD′ between node D and D′ is effectively zero. The same protection (“electrostatic guarding”) exists between nodes C and C′, resulting in a zero capacitance. The effective zero value for these two capacitors is not changed in any way by the gap between the two electrodes, which may change due to movement in a direction other than the measurement direction. Because these capacitances are normally the source of gain errors in a measurement device, this source of error is effectively eliminated from the measurement by the present invention. 
   In summary, the present invention increases sensitivity in the measurement direction and reduces error due to motion in the non-measurement direction. As described, the various physical elements of  FIG. 1  form multiple capacitive bridges, each bridge increasing the sensitivity of the measurement device. When 50 blades are used, the resulting sensitivity of the measurement device is increased by a factor of 25. 
   In addition, the measurement device is improved to reduce the parasitic capacitance introduced by movement in the non-measuring direction by driving the active rotor array with an electric signal. 
   Accordingly, using the described multiple bridge technique and capacitive guarding of rotor elements, it is possible to fabricate a highly precise small-angle capacitive position sensor for use in a rheometer that is relatively insensitive to non-measurement-axis motions. Using traditional printed circuit board techniques it is possible to achieve an array of 25 elements in a reasonable operating diameter. As described above, higher density arrays can be achieved using thin-film and other small geometry fabrication methods. 
   The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents. 
   Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.