Torsional rheometer for granular materials slurries and gas-solid mixtures and related methods
This is a division of application Ser. No. 8/464,325, filed Jun. 5, 1995. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides a system for rheologically evaluating a wide range of materials and, more specifically, it provides such a system which has positive means for effecting more efficient measurement of the rheological properties of the specimen. 
     2. Description Of The Prior Art 
     It has long been known that various proxies of certain materials can be measured effectively using rheometers. 
     U.S. Pat. No. 5,209,108 disloses a helical screw rheometer wherein a computer processes signals received from pressure sensing means and speed sensing means. 
     U.S. Pat. No. 5,269,190 discloses an axial rheometer wherein a microprocessor causes axial displacement of the sample holder to deform the sample. 
     U.S. Pat. No. 5,277,058 discloses an adjustable gap rheometer wherein the dimension between a stationary section and a movable section is altered to facilitate determination of shear rate and shear stress for varying deformation rates. 
     U.S. Pat. No. 5,315,863 discloses measurement of a fluid or a slurry in a continuous on-line measurement detecting rheological properties. An inverted U-tube is employed with pressures being compared at a point in each leg. U. S. Pat. No. 5,347,852 also discloses m on-line rheological measurement apparatus wherein a portion of the melt is diverted to a capillary passage by means of a first metering pump and is returned by means of a second metering pump. 
     It has been known to measure dynamic shear properties proxies employing a rheometer wherein a stationary plate and a rotatable plate are provided with a specimen therebetween. The rotatable plate is subjected to oscillating movement relative to the stationary plate. This system measures torque dung a predetermined angular rotation and employs a transducer and computer to calculate the rheological properties at different temperatures. See U.S. Pat. No. 5,357,783 and also the two surface oscillating plate of related U.S. Pat. No. 5,271,265. 
     U.S. Pat. No. 5,303,578 also discloses a dynamic rheometer. This patent discloses shear waves impinging on two spaced shear receiving surfaces with a transducer being provided for each surface. 
     U.S. Pat. No. 5,302,878 discloses a high-feed acoustic rheometer wherein a torsional-mode transducer in a cylinder interacts with a sensor in a rod with the respective diameters of the torsional-mode transducer and the sensor being such that the coefficient of reflection at the interface of the transducer-sensor is zero. 
     U.S. Pat. No. 5,167,143 discloses a rheometer having a motor driven chamber which is said to be usable for measuring fluids, slurries and finely divided particulated solids. An annular gap is provided between the motor driven chamber and spindle with an associated transducer being employed. The system is said to convert shear into viscosity. 
     U.S. Pat. No. 5,349,847 discloses a rotatable rheometer wherein the rotatable plate, specimen and stationary plate are adapted to be separated from the thick support which supports the stationary plate with magnetic means employed to effect retention. This is said to facilitate more efficient operation of the equipment which can be switched from operation in a rotational mode to operation in an oscillating mode. 
     In spite of this known system, there remains a very real and substantial need for an improved type of rotary rheometer which will effect more efficient interengagement between the specimen and the plates in order to provide more accurate results. 
     SUMMARY OF THE INVENTION 
     The present invention has met the hereinbefore described needs. The present invention provides a torsional rheometer and associated method for determining rheological properties of a specimen with the system operating either in a pure rotational mode or in an oscillatory mode. In the pure rotational mode, the shear properties of the material are determined in a steady state manner. In the oscillating mode, intrinsic properties are determined during oscillation over a predetermined arc. 
     The apparatus includes a stationary plate and a generally coaxial rotatable plate which are structured to receive a specimen therebetween. At least one and, in the preferred embodiment, both of the plates have the surface facing the specimen toughened in order to provide for more efficient interengagement with the specimen to resist undesired slippage of the specimen. Motor means are provided for effecting rotation or oscillation of the rotatable plate which is preferably positioned in overlying relationship with respect to the stationary plate. Transducer means monitor forces applied to the stationary plate during rotation of the rotatable plate and emit responsive signals to computer means which may be a microprocessor, miniprocessor, personal computer, or main frame computer. The computer means employs the transducer signals to determine the desired rheological properties. 
     Control means are provided for determining whether the rotary motion will be repeated rotary motion over a 360° arc or will be oscillating motion within a predetermined angle of oscillation. The degree of roughness of the roughened surface is preferably related to the nature of the material which will be subjected to rheological evaluation. The associated method provides for producing specimens between the rotatable plate and stationary plate and effecting either 360° repeated rotation or oscillating rotation such that the forces applied to the stationary plate during motion can be converted by a transducer into a corresponding electrical output signals which a processor receives and converts into the desired rheological properties. 
     It is an object of the present invention to provide rotary rheological apparatus and an associated method which will provide more efficient interengagement between the specimen contacting plates and the specimen. 
     It is another object of the present invention to provide such a system wherein a wide variety of materials, including granular materials, slurries, highly viscoelastic materials and gas-solid mixtures may be evaluated rheologically. 
     It is a further object of the present invention to provide such a system which is compatible with existing equipment and may provide the benefits thereof without requiring major changes in apparatus or methods. 
     It is a further object of the present invention to provide such a system which can test gas-solid mixtures as a mixture as distinguished from as individual components. 
     In another embodiment of the invention, a stationary plate having a base formed as a capacitor may be provided. 
     These and other objects of the invention will be more fully understood from the following detailed description of the invention on reference to the illustrations appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic elevational view of a form of apparatus of the present invention. 
     FIG. 2 is a fragmentary elevational view showing the rotatable and stationary plates of the present invention in relative open position. 
     FIG. 3 is a view similar to FIG. 2, but showing the plates in relative closed position. 
     FIG. 4A is a schematic illustration showing a portion of the rotatable plate with the toughened portions integrally formed. 
     FIG. 4B is similar to 4A, but shows the roughened portions applied as a coating. 
     FIG. 4C through 4G show cross-sectional examples of shapes and spacing of projections usable in the roughened surfaces of the present invention. 
     FIG. 5 is a fragmentary cross-sectional illustration showing apparatus similar to FIG. 1, but with some features uniquely designed to make it employable with air-solid mixtures. 
     FIG. 6 is a plan view of a form of porous plate usable in FIG. 5. 
     FIGS. 7 and 8, respectively, are cross-sectional illustrations of another embodiment of the present invention wherein local forces normal to the bottom plate may be measured. 
     FIGS. 9A and 9B show alternate configurations for the upper rotatable plates. 
     FIG. 10 is a flow diagram of a method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As employed herein, the term &#34;roughened surface&#34; will refer to a surface of either a rotatable plate or stationary plate which is adapted to contact a specimen to be rheologically examined with the surface being discontinuous either due to (a) outwardly projecting elements of the plate material, or (b) pores which pass through the entire plate. 
     The apparatus shown in FIG. 1 may have a conventional support frame consisting of a pedestal 2 with four vertical posts 4, 6 and two spaced behind plates 4 and 6 (not shown), which may be of the same shape as those shown, supporting an upper platform 8. The upper and lower plate system 12 which will be discussed hereinafter as a stationary plate 18 which receives a rotatable plate 20. Transducer 22 underlies and is operatively associated with stationary plate 18 such that when axial rotation is effected in one of the directions indicated by arrow A, either in repeated 360° degree cycles of rotation in the same direction, or in an oscillating movement, the stresses will result in the stationary plate 18 receiving through the specimen forces which may be projected in three orthogonal directions and torque. This series of forces applied to transducer 22 will result in a plurality of output signals over lead 24 to processor 26 which may be any sort of computer means. The processor 26 converts the output of transducer 22 which is the nature of a charge to a voltage and emits an output signal over lead 28 to data acquisition and control unit 30 which delivers the signals to computer means 32 for processing and storage over lead 33. The computer means 32 will determine the rheological properties viscosity of the material. Such properties may be viscosity and stresses in a direction normal to the rotatable plate 20. These properties may be stored, computer enhanced, displayed or made the subject of hard copy as desired. Among the suitable transducers, usable as transducer 22, is a piezoelectric four component transducer available from Kistler Instrument Corporation. Suitable software for use in the computer means 32 is that sold by National Instruments under the trade designation LabVIEW. The wide range capability in respect of loads on the transducer 22 facilitates use of the system of the present invention with a wide range from granular materials, such as coal and rice, for example, to thick sludge similar to bread making dough, for example. 
     The transducer 22 will generally provide output signals which are responsive to the torque and axial force. If a coordinate system has the x and y axes in the plane of shear, which is generally parallel to the plates 18, 20, with the z axis directed upwardly, the transducer will provide a reading of (a) the axial force along the z axis, i.e., normal to the shear plane and (b) two forces that are perpendicular to one another in the plane of shear. The torque represents a moment about the z axis. When the rotatable plate 20 and the stationary plate 18 are coaxial the two forces normal to each other in the plane of shear during axial 360° rotation or oscillation will generally be zero, i.e., when the rheometer is in a torsional configuration. The forces monitored to determine rheological properties are, therefore, the normal or z axis force and the moment force which represents torque. 
     A motor 40, which is energized over lead 45 by a suitable power supply 42 through lead 43 to control means 44 is shown overlying the plate assembly 12. The motor is adapted to effect rotary movement of rotatable plate 20. It will be appreciated that the control means 44 receives control signals from computer means 32 through a data acquisition and control unit 30 over lead 34. This facilitates electrical switching to effect operation of the motor 40 in a first mode which provides continuous 360° axial rotation of rotatable plate 20 and in a second mode which provides for angular oscillation of rotatable plate 20 through a desired arc which may be on the order of +90° to -90° at 15 Hz, +35° to -35° at 25 Hz and +90° to -90° at 50 Hz, for example. The output of the motor 40 is converted to the desired speed through an appropriate gear box 48 and delivers rotary motion to output shaft 50. In order to facilitate relative separating movement between stationary plate 18 and rotary plate 20, the plate 20 is adapted to be translated axially moving with an assembly that rides on liner bearings 54. Once the specimen is in place, axial movement of the plate 20 in the reverse direction, i.e., toward the stationary plate 18 is also effected to effect initial compaction of the specimen. Control of operation of this function is effected by computer means 32 by its LabVIEW software by emitting appropriate signals through acquisition and control unit 30 over lead 60 to control means 61 which may be a three-axis controller of the type available from NuLogic. Power supply 63 has output over lead 65 enter control means which emits a signal over lead 67 to servo motor 66 and through a gear box 68 to ball screw and nut 62 which moves shaft 70 vertically in an up and down path. Shaft 70 is fixedly secured to upper support 72 which has four depending supports with 74 and 76 being shown, secured to lower support 80 which, in turn, is secured to the liner bearings 54. The motor shaft 50 is coupled to plate shaft 90 using a low or no backlash coupler 84. The vertical orientation of the plate shaft 90 is achieved with assistance of bearing assembly 86 which, in the form shown, has two tapered roller bearings. The rotatable upper plate 20 is mounted on shaft 90 and may easily be changed, if desired, for the particular specimen being tested. 
     Referring to FIG. 2, there is shown a more detailed view of rotary plate 20 and stationary plate 18, as well as transducer 22. It will be noted that the rotatable plate 20 is fixedly secured to annular flange 92 of shaft 90 with bolts 100, 102 securing an upper portion 104 of the rotatable plate to a lower portion 106 of the rotatable plate. The stationary plate 18 has an upwardly projecting annular wall and is generally cupshaped. Plate 18 defines an upwardly open chamber 110 which is of greater diameter than rotatable plate 20 so that the rotatable plate can be received within chamber 110. The lower plate has two elements 120, 122 with a pair of bolts 124, 126 securing the assembly. In essence, this approach provides means for replacing plate elements 106 and 122, which are generally circular, after wear or in the event of damage. An important aspect of the present invention is that the lower surface 130 of plate dement 106, which is the portion of the plate which faces the specimen which will be received in chamber 110, is roughened. The degree of toughening may be varied depending on the material being analyzed. Similarly, surface 132 of stationary plate portion 122 has its surface, which will face and contact the specimen, roughened. It will be appreciated that the transducer 22 has an annular plate 140 fixedly secured to the upper end thereof which, in turn, is secured to the lower surface of stationary plate 18 such that forces applied to the stationary plate 18 will be transferred to the transducer 22 to facilitate an electrical output signal over lead 24 to processor 26. A pedestal supporting base 142 is secured to the lower surface of transducer 22. 
     FIG. 3 is similar to FIG. 2, except shows the plates 20, 18 in relative closed position such that chamber 110 is relatively small. A specimen would normally be placed into the chamber with the roughened surfaces 130, 132 being in intimate contact therewith and penetrating the same to provide firm interengagement. The specimen, which would fill chamber 110, has not been shown for clarity of illustration. 
     Referring now to FIG. 4A, there is shown by way of an example, a part of lower portion 106 of the rotatable plate wherein a plurality of integrally formed projections 160, which preferably cover the entire surface of lower portion 106. The projections 160 have a height &#34;h&#34; which may generally be in the range of about 10 microns to 1 cm. The projections 160 preferably will have a shape which has alternating peaks and grooves. The size and shape may vary, as to granular materials, to correspond generally to the size and shape of the granular materials and will be uniformly spaced from each other such that the entire roughened surface will have projections of generally uniform shape and dimension with the identical relative spacing. It will be appreciated that the roughened surface 132 (FIG. 3) of stationary plate element 122 may have substantially identical configuration as plate element 106. The plates may preferably be made of steel, stainless steel, cast iron, copper, brass, aluminum, resinous plastic or plexiglas with the roughened projections 160 being integrally formed therein. An alternate embodiment is shown in FIG. 4B wherein the plate element 106&#39; has a plurality of similarly configurated projections 160&#39; provided by a coating which may consist of abrasive containing materials, or materials such as sand, coal, powder, grain or other suitable materials glued or otherwise fixed to the surface of the plate. 
     FIG. 4C shows a portion of a roughened surface of the present invention wherein the surface 164 cooperates with a plurality of generally rectangular projections 166, 168 having a rectangular shape in plan and defining a plurality of grooves 170 therebetween. FIG. 4D has a plurality of projections 172 cooperating with surface 174 to define a plurality of grooves, such as 176. 
     FIG. 4E has a plurality of projections 180 having curved sidewalls 182 defining grooves 184. FIG. 4F has a plurality of projections 186 generally similar to those of FIG. 4A, but having a larger base 188 between projections 186. Grooves 190 which, in the form shown, are generally U-shaped may be of any desired shape. FIG. 4G has a plurality of projections 192 with grooves 194 therebetween. 
     It will be appreciated from FIGS. 4A through 4G that various projection shapes, sizes and relative spacing provided are examples of different size grooves and different roughened surfaces. These roughened surfaces, if desired, may be provided by coatings or be integrally formed. 
     When granular material is subjected to rheological analysis by the system of the present invention, it is preferred that the average size and shape of the granules approximate the height and shape of the projections 160. In the case of a slurry, thick sludge, bread making dough, or the like, the surface projections will preferably be large enough so that slipping between the test material and the sample does not occur. In general, these projections are in the shape shown in FIG. 4A with the depth of the projection being about 5 mm to 10 mm depending upon the consistency of the sample. Even in test materials, such as light grease, which adhere to the surface of the plate, the plate surfaces need to be roughened to resist stick slip. While initially there might be no slipping, once the rotatable plate moves above a certain shear rate, the material starts to slip without use of the projections. 
     Referring to FIGS. 5 and 6, a further embodiment of the present invention will be considered. This embodiment is adapted for use with gas-solid mixtures. While it has been known to attempt to do studies on gas having entrained solids, there has not been an effective means of rheologically evaluating the mixture of both. In this embodiment of the invention, the lower plate 218 is generally cup-shaped and provides an upwardly open chamber 210 which receives the specimen. The rotatable upper plate 220 is not only mounted for axial rotation as indicated by arrow B, either through repeated 360° cycles or by oscillation through an arc, which may be on the order of +90° to -90° at 15 Hz, +35° to -35° at 25 Hz, and +9° to -9° at 50 Hz, but also may be subjected to axial movement to separate plates 218, 220 to facilitate sample insertion and relative closing movement to permit plate 220 to enter cavity 210 to provide the desired specimen receiving volume between the circular plates 218, 220. It will be noted that underlying stationary plate element 230 is a chamber 232 which is adapted to receive air. Similarly, overlying plate element 240 is a chamber 242 which is adapted to receive air. As is shown in FIG. 6, upper plate 240 has a plurality of pores 244 which will cover essentially the entire plate with all of the pores 244 being essentially the same size and generally equally spaced from each other. The pore sizes of both plates 230, 240 and spacings are preferably identical for most specimens. The pore openings 244 should be such that they are smaller than the average size of the solid particulate material entrained in the gas to resist clogging of the pores by the solid particles. In the event the process is employed to determine rheological properties of fine powders, the plates may be made of sintered material of various pore sizes to obtain micro-pores. 
     The mechanics of performing the rheological testing may be essentially as that described with respect to FIG. 1 with certain supplemental apparatus and practices being employed. A compressor 250 provides air through tube 252 and air filter 254 which is adapted to remove material entrained within the air. Tube 252 is in communication with chamber 232 and introduces air into the chamber which air passes upwardly through the pores in plate 230 to impinge upon the sample which is a mixture of gas and solids and maintain the solids distributed within the gas. (If the solids in the specimen are mixed with a gas other than air, it may be desirable to provide an airtight chamber around the plates 218, 220 and employ circulating gas in lieu of air in order to resist contamination of the gas by air.) The air then flows out of chamber 210 through pores in plate 240 into chamber 242. The air in chamber 242 is exhausted through tubes 260, 262 and, respectively, air filters 264, 266 so as to remove any undesired entrained particles. When the apparatus is not in operation, nozzle 270 may be employed to pump air into chamber 242 to clear any clogged pores that may be in the plate in plate 240 after the experiment. 
     It will be apparent that, if desired, the plates 230, 240 may be substituted for by plates having different porosity for particular types of samples or experimental needs. 
     By providing for more effective interengagement relatively slip-free interaction between the rotary plate and the stationary plate, on the one hand, and the specimen, more accurate results regarding the rheological properties of the specimen, such as viscosity, may be obtained. Normal stress differences may be determined based on the mathematical model to be employed to describe the behavior of the specimen being tested. 
     The integrally formed roughened surfaces of the present invention may be created by any desired means, such as milling machines, electrochemical machines or for very small projections, chemical etching techniques. 
     Referring to FIGS. 7 and 8, a further embodiment of the present invention will be considered. This embodiment is suitable for use with materials where the normal forces exhibited by the material under shear are small or where there is a need to measure local forces normal to the plane of shear, such as in the case of biological fluids, polymer solutions, and the like. In this embodiment of the invention, the stationary lower plate 271 is generally cup-shaped and provides an upwardly open chamber 272 which receives the specimen. The overlying rotatable plate 280 is structured to be received within chamber 272. 
     The bottom surface of the lower plate 271 is made out of a flexible membrane 273, the underside of which consists of an electrically conducting membrane 274. FIG. 8 shows the details of the stationary bottom plate 271 which consists of a unique transducer. The flexible membrane 273 is separated from the rigid bottom surface 275 by means of a relatively small fluid gap 276, which may be filled with air or any non-conducting liquid or gas. The rigid bottom surface 275 has embedded in it a plurality of small metallic plates 277 which may be the size of a pin head and which are connected to an electronic circuit 285. The electronic circuit 280 is electrically connected to conductive membrane 274 by lead 278 and to the rigid bottom surface 275 by lead 279. The output signal from electronic circuit is transferred by lead 281 to capacitive measurement unit 282 and which outputs signals over lead 283 to data acquisition unit 284. The embedded metallic plates 277 and the conducting membrane 274 form the two plates of a series of capacitors. In testing a specimen, when a load is applied to the flexible membrane 273, the gap between the conducting membrane and the embedded metallic plates 277 changes which, in turn, changes the capacitance at each of the locations where the embedded metallic plates 277 are located. Knowing the capacitance at each location, one can determine the deflection of the membrane 273 from its initial position and the force required to achieve this deflection. Before the stationary plate 271 is placed in the instrument, it is calibrated using a known load and known gap between the conducting membrane 273 and the embedded metallic plates 277. The top plate 280 can be either a plate 290&#39; cooperating with a stationary plate 271&#39; or a cone 290&#34; cooperating with a stationary plate 271&#34;, for example, as shown in FIGS. 9A and 9B. This transducer is ideal for measuring the forces in small sample volumes as is the case with bio-fluids, such as blood, plasma, polymer solutions, and the like. In addition, knowledge of the local normal forces will greatly enhance the ability to characterize the rheological properties of the samples. 
     Referring to FIG. 10, a flow chart of the processing of data received within computer means, such as computer 32 in FIG. 1, will be considered. The measured axial force 300 and torque 302 pass through data acquisition unit 306 and to computer 308. Also introduced into the computer 308 is the mathematical material model 312 which is processed by the axial force and torque equation 314 with the output going to computer 308. The computer 308 processes the actual measured data on the axial force and torque and the mathematical material model employing data analysis and regression employing the least squares method. The output of the computer 308 provides the material parameters 310 and statistical analysis 312. The final output 316 provides the mean, standard deviation, and variance of the material parameters based on a comparison of the actual data entering computer 308 from data acquisition unit 306 with the mathematical model predictions of axial force and torque supplied to computer 308 by axial force and torque equation unit 314. 
     It will be appreciated, therefore, that the present invention provides an effective means for enhancing the efficiency of rotary rheometers in working with a wide variety of specimen materials. In one embodiment, this is accomplished by providing &#34;toughened surfaces&#34; on at least one of the two plates and, preferably, on both with the degree of roughness being adjusted according to the nature of the material being processed. All of this is accomplished in a manner which is readily adaptable to existing rotatable rheometers merely by changing the nature of the plate. In another embodiment, a deformable capacitive sensor is provided within the base of the stationary plate. 
     Whereas particular embodiments of the invention have been described herein for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as set forth in the appended claims.