WO2013075104A1

WO2013075104A1 - Method and apparatus for detecting granular slip

Info

Publication number: WO2013075104A1
Application number: PCT/US2012/065861
Authority: WO
Inventors: Troy Shinbrot; Thyagu NIRMAL
Original assignee: Rutgers, The State University Of New Jersey
Priority date: 2011-11-18
Filing date: 2012-11-19
Publication date: 2013-05-23

Abstract

A method and apparatus for predicting granular slip is provided. In particular, the electrical field is measured for a granular bed to detect variations in the electrical field. If the variation exceeds a predetermined threshold, a granular slip may be declared. In this way a granular slip may be predicted before the granular slip occurs. Additionally, an apparatus is provided in which a sensor detects variations in the electrical field for a granular bed. A controller analyzes the data to determine whether a granular slip has occurred or it may predict that a granular slip is imminent. Based on the determination, the controller may control aspects of a process, such as controlling a mixer used to mix materials, such as pharmaceuticals or ceramics.

Description

Method and Apparatus for Detecting Granular Slip

Priority Claim

[001] This application claims priority to U.S. Provisional Patent Application No.

61/561 ,546, filed November 18, 2011 . The entire disclosure of the foregoing application is hereby incorporated herein by reference.

Field of the invention

[002] The present invention relates to the field of detecting granular slip and methods for producing materials formed of granular elements.

Background

[003] It has been known for over a century that electrical signals are produced by material failure, for example during crack formation of crystals and glasses, or stick-slip motion of liquid mercury on glass. We describe here new experiments revealing that slip events in cohesive powders also produce electrical signals, and remarkably these signals can appear significantly in advance of slip events. We have confirmed this effect in two different experimental systems and using two common powdered materials, and in a third experiment we have demonstrated that similar voltage signals are produced by crack-like defects in several powdered materials.

[004] Powders and grains - used by the thousands of tons in the processing of polymers, catalysts, pharmaceuticals and building materials - are well known to exhibit unpredictable jamming to flow transitions. On the large scale, these transitions manifest themselves in catastrophic geological slip events, and on the small scale, they are intimately associated with fracture and material inhomogeneities, causing a documented 50% rejection rate of manufactured ceramics. In this letter, we show for the first time that electrical precursors are strongly correlated with - and often precede - any outward sign of a slip event in a powder bed.

Summary of the Invention

[005] In light of the foregoing, the present invention provides a method for predicting granular slip comprising the step of monitoring granular material for an electronic signal produced by the granular material. For instance, the electronic signal may be a voltage created by the granular material. The method further comprises the step of processing the electronic signal to evaluate whether the electronic signal is indicative of granular slip.

[006] According to another aspect, the present invention provides an apparatus for predicting granular slip. The apparatus comprises a sensor operable to monitor granular material for an electronic signal produced by the granular material. For instance, the sensor may comprise a sensor operable to detect whether the granular material produces a voltage. The apparatus also includes a processor operable to process the electronic signal to evaluate whether the electronic signal is indicative of granular slip.

[007] According to another aspect, the present invention provides an apparatus for mixing granular materials. The apparatus comprises a mixer having a chamber for receiving and mixing granular materials and a detector for detecting granular slip in the granular materials within the chamber. The detector comprises a sensor for detecting a voltage within the granular materials and the apparatus comprises a processor operable to process the detected voltage to evaluate whether the detected voltage is indicative of granular slip.

[008]

Description of Figures

[009] The foregoing summary and the following detailed description of the preferred embodiments of the present invention will be best understood when read in conjunction with the appended drawings, in which

[0010] Figures 1 (a)-(d) is a series of views illustrating: (a) a granular powder processor with a sensor for detecting granular slip; (b) a diagram illustrating cross-correlation of signals from elements of the arrangement in Fig. 1 (a); (c) a diagram illustrating output from the sensor of the arrangement in Fig, 1 (a); and (d) a diagram illustrating slip events and sensor output from the arrangement in Fig. 1 (a).

[001 1 ] Figures 2(a)-(e) is a series of views illustrating the association of sensor location and slip events, in which (a) is a diagram illustrating a cross-sectional diagram of a drum of the processor illustrated in Fig. 1 (a); (b) is a cross-section of the drum illustrated in Fig. 2(a) illustrating defects in the granular powder; (c) is a diagram illustrating output from the sensor of the arrangement in Fig. 2(a); (d) is a diagram illustrating cross-correlation of signals from elements of the arrangement in Fig. 2(a); and (e) is a diagram illustrating slip events and sensor output from the arrangement in Fig. 2(a). [0012] Figures 3(a)-(e) is a series of views illustrating an alternative apparatus for detecting granular slip, in which (a) is a diagram of the apparatus; (b) is a diagrammatic view illustrating output from the arrangement in Fig. 3(a); (c) is a diagram illustrating enlarged views of a portion of the output from the arrangement in Fig. 3(a); (d) is a diagram illustrating slip events of the arrangement in Fig. 3(a) for APAP powder; and (e) is a diagram illustrating slip events of the arrangement in Fig. 3(a) for white flour.

[0013] Figures 4(a)-(is a series of view illustrating an alternative apparatus for detecting granular slip, in which (a) is a diagram of the apparatus including multiple chambers; (b) is a diagram illustrating data corresponding to cracks opening and closing in the powder in the apparatus in Fig. 4(a); and (c) is a diagram illustrating data corresponding to cracks opening and closing in alternative materials in the apparatus in Fig. 4(a).

[0014] Figures 5(a)-(b) is a series of views illustrating diagrams for synchronizing sensors of the apparatus in Fig. 1 (a).

[0015] Figures 6(a)-(c) is a series of view illustrating an alternate embodiment of an apparatus for detecting granular slip, in which (a) is a diagram of the apparatus; (b) is an enlarged fragmentary view of sensors for the array in Fig. 6(a); and (c) is an enlarged fragmentary view of alternative sensors for the array in Fig. 6(a).

Detailed Description

[0016] Referring now to the figures in generally and to Figs. 1 (a)-(d), a system for detecting granular slip is designated 10. The system 10 comprises a sensor 30 for detecting a characteristic of a granular bed 15. Specifically, the sensor is operable to detect voltage produced by the granular bed contemporaneously with granular slip occurring within the granular bed. More specifically, the voltage may be predictive of a granular slip about to occur or the voltage may be indicative of a granular slip that already occurred.

[0017] The system 10 may be utilized in a variety of processes in which it is desirable to detect granular slip. For instance, in the field of ceramics and pharmaceuticals it is desirable to detect granular slip which impedes the homogenous mixing of the ceramic or pharmaceutical mixture. In the case of ceramics, the granular slip may create voids that lead to defects. In the case of pharmaceuticals, the granular slip may lead to improper mixing which may result in non-homogeneity that can affect the amount of one or more compounds in the dosages metered from the mixture.

[0018] Accordingly, the system 10 may be utilized in connection with a mixer 20. The mixer may be any of a variety of mixers having a compartment for receiving one or more granular materials. For instance, referring to Fig. 1 , the mixer may include a tumbler or drum 22. The mixer 20 may include elements such as augers, fins or other elements designed to agitate or mix the granular material 15. In the embodiment in Fig. 1 (a), the mixer 20 comprises a drum 22 configured to receive a bed of granular material. The drum 22 may be horizontally elongated and the mixer may include a drive mechanism, such as a motor 40 for driving the drum about a generally horizontal axis. For instance, in Fig. 1 (a), the motor 40 drives the drum around a substantially horizontal axis extending through the center of the drum. [0019] The sensor 30 is operable to detect a characteristic of the granular bed. For instance, the sensor 30 may detect a voltage or a voltage change created by the material in the granular bed. In the present instance, the sensor 30 comprises one or more field sensors operable to detect an electrical field.

[0020] Optionally, the system 10 may further include a second detector for detecting a second characteristic of the granular bed. For instance, the system may include a load cell 35 for detecting forces created by movement of the granular bed.

[0021] In the present instance, the system also includes a processor for receiving signals from the sensor 30 indicative of the sensed electronic field to determine whether the electronic field is indicative of granular slip. For specifically, the processor is a

microprocessor operable to receive signals from the sensor and determine whether the signals are indicative of granular slip. More specifically, the processor may be configured or programmed to determine whether the signals for the sensor are predictive of granular slip about to occur in the granular bed.

[0022] Referring now to Figs. 6(a)-(e) an alternate system 110 is illustrated in which an array of sensors 130 are provided for detecting an electrical field produced by granular slip. Specifically, the sensor array 130 comprises a grid of physically separated sensors 135. Each sensor 135 is operable to detect an electrical field produced by granular slip. However, as discussed above, the distance between the slip event and the sensor can impact the detected electrical signal. By providing an array of sensors 130, there is a greater chance of a sensor being adjacent the slip event so that the sensor will detect the voltage produced by the slip event. [0023] Referring to Fig. 6(b), the sensors 135 may be sensor spots each of which is connected by a separate conductor, such as a wire, to a processor. In the present instance, the processor is an electrometer. Additionally, in the present instance, each sensor spot is shielded by a grounded ITO surround. Fig. 6(c) illustrates sensors that can be used as an alternative to the sensors shown in Fig. 6(b). The sensors in Fig. 6(c) are larger-sized that then sensors in Fig. 6(b).

[0024] Additionally, as shown in Fig. 6(c), in the present instance the array 130 comprises a grid of multiple sensors 135 arranged in a plurality of generally parallel rows and columns. By using a grid of sensors, it may be possible to pinpoint the location of the granular slip based on the signal received by a plurality of the sensors. Specifically, the detected voltage is likely to be higher for the sensors closest to the slip event than for the sensors farther away from the event, and the temporal signature and signs of voltages detected may vary with sensor location. In this way, a comparison of the voltage measurements from a plurality of sensors in the array may be used to triangulate the location of the slip event.

[0025] By way of example, referring to Fig. 6(c), if the sensor at c3 (row c, column 3) and the sensor at c4 are the highest voltage measurements of the sensor and the sensor at b3 and b4 are the next two highest detected voltages, it is likely that the granular slip occurred in the portion of the grip bounded by rows b and c and columns 3 and 4, and likely closer to row c since the readings for c3 and c4 were higher that the measured readings for row b. In this way, the location of the granular slip event can be approximated based upon a comparison of the voltages measured by 2 or more sensors in the array, and in the present instance, the location is approximated based upon a comparison of the voltages measured by 3 or more sensors in the array. Moreover, if the electrical signal is propagated through a dielectric medium on the way from its origin to a sensor, the medium will induce its own charge and so modify the signal when it is received by the sensor.

[0026] Although Fig. 6(c) illustrates the use of an array of sensors to detect granular slip in small scale applications, it should be understood that a similar process can be used to detect granular slip on a large scale, such a in geological applications. In such an application, a plurality of voltage detectors are arranged in a grid. The detectors are operable to detect the voltage in a particular area of the ground produced in response to movement indicative of an earthquake.

[0027] The sensor array in Fig. 6 can also be for quality control, such as to detect flaws in an element formed of granular material. As discussed further below, the crack formed at a defect in a granular material, such as ceramic, creates a voltage when stress is applied to the crack. Accordingly, an item to be tested may be placed adjacent the sensor array and stress applied to the item. If the item has a defect then a development of a crack will cause a voltage and the sensor array will detect the voltage. Therefore, if the sensor array detects a voltage in response to the application of force, such as an applied stress or strain, the system may determine that the elements has a defect. Furthermore, the location of the defect can be detected by analyzing the voltages detected by a plurality of the sensors in the sensor array.

[0028] The configuration of the sensor array may vary depending on the application. As shown in Fig. 6(d) and (e), the sensor array may include a grounded conductive element between adjacent sensor elements. The conductive element operates as a shield to limit electrical interference between adjacent sensors, which can become more of an issue as the sensors are packed closer to one another.

Example 1

[0029] Fig. 1 (a) illustrates an experimental systems that used to investigate powder slip events. This system used a cylindrical tumbler instrumented with a load cell that accurately detects avalanches alongside a non-contact field probe that measured voltage with respect to ground. In the present example, the mixer comprised a 25 cm diameter tumbler driven at 5 rpm by an onboard DC motor. The tumbler loaded 50% by volume with powder. Avalanches are detected by load cell shown and electric fields were detected using a non-contact voltage probe, such as the probe sold by Trek, Inc. of Medina, NY as model 6000B-7C. The powder is retained in 10 cm of axial extent of tumbler by a foam-core spacer and the available space was filled 50% with powder. The powder used was a 25/75 blend of acetaminophen [APAP] and microcrystalline cellulose, such as product sold by FMC Biopolymer of Philadelphia, PA under the trade name Avicel 101 .

[0030] Data was acquired by monitoring the tumbler alongside outputs of voltage and load cell sensor with a video camera. In this first example readings were obtained from the video record twice a second from both voltage and load sensors. A cross-correlation of these voltage and load cell signals exhibited a maximum at 4.8 sec, as shown in Fig.

1 (b), and shifting the voltage signal forward in time by this amount, shown in Fig. 1 (c), produced a visible correspondence between steep voltage drops and large slip events.

Near coincidences between voltage and load cell spikes are shown as black lines; voltage drops and large slip events are shown as circles and stars respectively. Fig. 1 (d) provides a diagram with a side-by-side comparisons of large slip and voltage correspondences by moving each slip event to Time=0. The powder used was an electrically insulating pharmaceutical blend that has been well studied in powders research and exhibits discrete avalanches; the mean size of the blend measured using laser diffraction (Beckman Coulter, model LS 13 320 particle size analyzer) is 90±50 μηη .

[0031 ] To assess the reliability of the precursors, a Fisher Exact Test was performed ( hereinafter referred to as FET) of the data shown in Fig. 1 (c), comparing the number of slip events correctly predicted (within a time period 4.8±1 sec), the numbers of false positives, false negatives, and correctly identified "safe" periods (intervals of ±1 sec. not preceded by a voltage drop). Doing so, it was determined that the probability that the results were due to random chance is 1 .1 % (p=0.011 ).

[0032] It should be noted that in this experiment, the was desirable to synchronize the timescales of slip detection (from the load cell) and voltage measurements (from the probe). Second, there are known to be two types of avalanche: smaller "cascade avalanches" typically involving flow originating uphill and accumulating material as it propagates downward, and larger "slip events" (also known as slab avalanches in the snow literature), in which material failure occurs within the bulk, and most of the free surface slides downhill. Cascades may exhibit precursors, however in the examples it was noticed that the largest events were most predictable, and therefore in the Examples smaller avalanches were ignored. Correspondingly, the criterion used to establish the presence of a voltage or load cell event was if either the voltage detected or the load cell dropped by 1/4 of its dynamic range (maximum-minimum); smaller signals were

disregarded. This criterion agrees with visual observations of large avalanches from the video record, however other criteria have been tested as well and it was found that the FET results were sufficiently strong that nearly any sensible criterion identified a correspondence between slip and voltage data.

[0033] The data shown in Figs. 1 (b)-(d) were obtained by placing the voltage probe at the location shown in Fig. 1 (a), at about 5 o'clock. Placement of the voltage probe was moved to about 1 and 3 o'clock (cf. Fig. 2(a)), and it was found that the correlation between voltage and slip signals was then lost. To investigate this dependence on location within the powder bed in greater detail, the probe was moved to the front of the tumbler - again out of contact with the surface - and measured voltage and slip signals at 12 additional spatial locations shown in Fig. 2(a), chosen to approximate locations at which the strongest correlation was seen in the first experiment. In these experiments, the apparatus was instruments with an automated data collection system (LabVIEWTM), obtaining both voltage and load cell readings at 1 kHz. Additionally, to discount spurious triboelectrification of the powder or the apparatus, prior to each of these experiments, the tumbler was discharged inside and out with an active eliminator (EXAIR Ion Air Gun), and then the tumbler was loaded in 1 cm layers of fresh powder, discharging each layer after loading. Finally, throughout the duration of these experiments, exterior of the tumbler was discharged with the same eliminator.

[0034] The apparatus in Fig. 2(a) was tumbled at 5 rpm and loaded 70% by volume with powder, showing locations at which voltages were evaluated. In the video acquisition approach shown in Fig. 1 , measurements were made at R1 , R2, R3, and data from R3 is shown in Fig. 1 (b)-(c). In the automated data acquisition approach, the voltage probe was pointed axially inward at each of gridpoints d6, d7, e3-e7 and f4-f8. [0035]

[0036] In Fig. 2(c), the measured data for the load cell signal is overlapped with measured data regarding voltmeter signal taken at location e6 advanced by 3.4 sec. The thick lines in Fig. 2(c) show highly significant correlations between slip events and voltage signals several seconds before. The was taken at 1 kHz and low-pass filtered using a direct form II biquadratic filter to remove high frequency noise.

[0037] These experiments confirmed that the correlation between voltage and slip signal depends strongly on probe location. The data shown in Fig. 2(c) were taken at location e6 in Fig. 2(a). At this location, it was found that voltages precede slip events by 3.4 sec, as shown in the cross-correlation shown in the inset to Fig. 2(c). The voltage precursors at e6 corresponded almost one to one with slip events, however as before, this strong correspondence between voltages and load cell readings was not seen either at location f5 (immediately upstream of e6) or at d6 (downstream). It was concluded therefore that the voltage production mechanism is transient and does not involve persistent charging of, for example, the tumbler surface or a localized region of charge in the bed.

[0038] An FET analysis produced a 0.8% (p = 0.0075) likelihood that the correlation in this experiment was random. Additionally, experiments were performed at different speeds (3 and 5 rpm), fill levels (50% and 70%), and environmental conditions (18% - 39% RH).

Additionally, the tumbler was started and stopped for periods ranging from 1/2 to several minutes to give any charged regions time to relax. Even with these changes, it was confirmed that the predictions were not due to possible initial static charge by running the experiment continuously for an hour. Provided that the probe was suitably placed, it was found that correlations between voltage drops and large slip events persisted despite these changes.

[0039] It is noted from Figs. 2(b)-(e) that both voltage and load signals are contained in an envelope that rises and falls at close to the 12-second period of the tumbler rotation. No evident irregularity was identified in the tumbler, its rotation speed or its loading condition; nevertheless this periodicity remained in the signals measured. To validate the observed results and eliminate unwanted periodicity, an independent set of experiments were performed using a second apparatus in which the powder was prepared in a nearly uniform initial state prior to each experimental trial.

Example 2

[0040] The second apparatus, shown schematically in Fig. 3(a) consisted of an acrylic box, 46 cm long, 30 cm high, and 4.5 cm wide, that was slowly tipped to produce avalanches. Initially, the powder was stirred with a metal rod to break up any incipient clumps, the box was tipped to the left at about 45°, and the surface was gently flattened. The box was then sharply tapped 10 times each on the bottom and then the left side of the box, after which it was slowly tipped to the right (measured at about 1 .5 rpm) until avalanches began. Avalanches were recorded using a 30 frame/sec. video camera with a stopwatch in the field of view.

[0041] Numerous trials were performed with the apparatus shown in Fig. 3(a) and described above. The times at which the first indication of any kind of bed movement appears on the video were recorded and plotted Fig. 3(b) as stars alongside voltage spikes for two example trials. In Fig. 3(c), 8 typical slip events are enlarged, including an occasional false negative and a very rare false positive. Unlike the tumbling experiments, there was no evidence of periodicity here, yet as Fig. 3 (d) shows graphically using the APAP/Avicel mixture, slip events over numerous experiments were more often than not preceded by at least one voltage spike. Specifically, out of 23 slip events, all exhibited a distinct voltage spike, and 14 of these occured before the slip. The criterion used in Fig. 3(d) for a voltage spike was that the voltage must have changed by at least 15V within 1/4 sec, after which the time to the next avalanche was evaluated. Calculations showed an FET probability of 0.30% that the precursors were random.

[0042] The tipping experiment illustrated in Fig. 3 was repeated 10 additional times using common bleached flour. Flour produces larger voltage excursions and in Fig. 3(e) the criterion used was that the voltage changed by 500 V within 1/2 sec. The results showed a reliable predictions of slip events, with an FET probability of 0.41 %, and of the 18 slip events detected, 15 exhibited voltage precursors. It is noted that the precursor time for slip events in the tipping experiments was under 1 sec: significantly shorter when the bed was tapped than in the tumbler.

[0043] To aid in the understanding of these results, two facts are noted. First, solids including crystals, glasses, rocks and ice, adhesives such as on plastic tape, and liquids, for example mercury, all produce electrical signals during failure. Second, granular materials dilate before they flow. Considering these two facts together, a straightforward explanation for the results is that granular materials may also produce a voltage signal when they dilate - and since dilation precedes slip, the data may simply have unveiled electrical signals of dilation. In the context of geophysical granular materials, dilation has been used as an indicator of localized shear stress, and indeed dilation in the form of geophysical crack propagation has been proposed as a source of electrical signals.

Similarly in the above-described cohesive powder beds, crack-like defects appeared as the bed was sheared: these defects can be seen by eye in Fig. 2(b), which shows a false- colored snapshot of the tumbler during rotation. In the enlargement to the right of this panel, defects in the powder bed are identified by arrowheads, and the video record revealed that slip events occurred subsequently, once these defects approach the bed surface . Moreover, as identified in Fig. 2(b), these defects appear to form a train emanating from the location e6 also shown in the snapshot. At 5 rpm, it took about 3 seconds for regions near e6 to travel to the surface of the bed. No such defects were seen in the tipped bed, which was stirred and then tapped to remove any incipient structure.

[0044] Thus it is proposed that in the tumbler, these defects may produce slip after they have been transported to the bed surface, while in the tipped bed, longstanding defects have been prevented by consolidating the bed, so that defect formation precipitates a more immediate slip event. We note that disorder within a granular bed may increase significantly in advance of large avalanches, and the mean gap between particles may begin to grow several seconds before evidence of slip.

[0045] The role of defects in the inception of slip events has previously been documented in studies of grains and colloids, in models of earthquake dynamics, and in research into disorder transitions in a broad range of glassy materials. The hypothesis engendered by the Examples above is that the initiation of these defects precedes large slip events, and that the formation of defects in powder beds produces distinctive voltage spikes. To verify this line of reasoning, a final set of experiments were performed to confirm that the formation of defects by itself does produce the hypothesized voltage spikes. Example 3

[0046] As shown in Fig. 4(a), the third Example used a vertically oriented shear cell loaded with sifted powder. Because the cell was vertical and the powder was loosely packed, powder was able to settle downward, leaving a sizeable gap between the powder bed and the top of the upper chamber. This gap in turn allowed crack-like defects to develop as the upper and lower chambers shown in the figure were sheared left to right (see enlargement to right of Figs. 4(a)-(c)).

[0047] Individual cracks could be repeatedly opened and closed as the chambers were sheared back and forth: this is shown in the enlargements to the right of Fig. 4(b). By pointing the voltage probe at a particular crack and tracking the crack with the probe as the chambers were sheared, voltage signals were obtained as shown in Fig 4(b)-(c) for various powders. Fig. 4(b) shows voltages in chambers loaded with unbleached white flour and sheared left and right every 5 seconds. The shearing time was brief, about 1/4 sec, and no motion of either chamber occurred between shearing events. The experiments were replicated several times, and the voltage signals near cracks were invariably abrupt and persisted at relative humidities at least up to 70%. As shown in Fig. 4(b), the voltage near the crack (lower trace, labeled 'Defect') dropped abruptly every time the crack opens, and jumped abruptly every time the crack closed. A control experiment, using exactly the same system but with the probe near the center of the upper chamber where cracks seldom change, showed little voltage variation (See upper trace in Fig. 4(b), labeled 'Control'). [0048] The repeatability shown in Fig. 4(b) depended strongly on the crack morphology: in Fig. 4(b) the crack (see upper right inset) was large, well defined, nearly linear, and did not change perceptibly with multiple shearing events. In multiple trials for this and a variety of other powders, it was observed that smaller, more irregular or transient cracks produced more variable voltages. Exemplar voltage traces are shown in Fig. 4(c) for the APAP- avicel blend discussed earlier (upper trace) and for sheetrock patching compound (lower trace). Alongside these plots snapshots of the crack are shown on which the probe was focused before and after the experiment using patching compound; the APAP crack is similar in appearance. In the cases shown in Fig. 4(c), the crack was small, irregular and multifaceted, and while the voltages still spiked sharply at most shear events, more variability was seen during both shearing and stationary periods.

[0049] Notwithstanding this variability, it was observed that with few exceptions, voltages decrease abruptly during periods of crack opening and increase abruptly during closing. For large and stable cracks (e.g. Fig. 4(b)), these voltage features are repeatable, while for smaller and more irregular cracks, the features are still present, but are convolved with other voltage signals during both shearing and stationary periods.

[0050] In summary, the above experiments demonstrate that detectable and reproducible electrical signals precede slip events in powder beds. These results were confirmed in two different experimental systems using different materials, different measurement methods, different fill levels, and different rates of stress. It was also observed that voltage spikes are formed as crack-like defects in powder beds are opened and closed, and it is proposed that the appearance of these signals may be analogous to voltages produced by failures in other materials.

[0051 ] The mechanism underlying these electrical precursors remains to be definitively determined: the materials studied are not piezoelectric, the effects persist in the presence of active static elimination as well as at relative humidities up to 70%, and stresses are several orders of magnitude too small to produce chemical changes reported elsewhere to lead to measurable voltages. Likewise the effect does not seem to be associated with granular contact electrification: it was observed that large and transient voltages (over 100 V) could be measured outside of the shear zone significantly in advance (~ 1 sec.) of slip, whereas studies of contact electrification during granular shear report much smaller and lasting voltages (under 100 mV) in probes that are in contact with shearing surfaces over much shorter precursor times (under 0.1 ms).

[0052] The implications of the results observed above way be wide ranging. On one front, many common materials - ceramics in turbines, chalk in cliffs and concrete in bridges to name three - are made from grains or powders, and the results lead to speculation that failure of these materials may be also preceded by telltale electrical signals. On another front it may be possible to analyze measured changes in electrical field may be used to identify electrical precursors for earthquakes.

Synchronizing Voltage, Load Cell and Video Camera

[0053] Figures 1 -3 involve three separate signals from three different sources - voltage probe, load cell, and video camera - and as discussed above, these must be accurately synchronized to make meaningful timing measurements. Figures 1 (a)-1 (d), 2(a)-2(e), 3(a)-3(c) and 4(a)-4(c) include corrections after synchronizing these signals; the following provides a description of experiments that evaluated relative timing of the three signals.

[0054] To evaluate the timing of each signal, the load cell was tapped with a charged rod in the field of detection of both the voltage probe and the video camera, which also views a stopwatch. There was then a comparison of the instants that (1 ) the tap occurred according to the video/stopwatch combination, (2) the load cell reported an impulse and (3) the voltmeter reported a voltage spike. For technical reasons associated with how the recording software (LabVIEWTM) shares CPU time, different delays were obtained with the load-cell on (for the tumbler trials) or off (for the tipping experiments), so these timing experiments were performed in both cases.

[0055] Referring to Fig. 5(a) data is plotted in which the load cell was off. A total of 9 tapping experiments were performed, in which the load cell were tapped between 20 and

30 times. A dozen exemplar tapping responses are shown in the narrow lines, and the average of the voltmeter responses is shown as a thick gray line. Time = 0 is defined by recording the stopwatch time when the tap is seen to occur in the video record, and is estimated to be accurate to within the video frame rate, 1/30 sec. For each tap, the maximum amplitude of the signal is identified. The mean of the times at which these maxima appear is 0.32 sec, with an uncertainty of 0.04 sec. When the uncertainty associated with the frame rate is included, a delay with propagated error of 0.32 ± 0.05 sec. Was obtained. This delay is chiefly introduced by the voltmeter probe itself, which is a null detector46, meaning it adjusts a shielding voltage to nullify the capacitive voltage measured through a small hole in the shield. This produces highly accurate and drift-free data, but requires internal electronics that integrate voltages over time to produce the null result, and so involve a delay. This delay is compensated for by shifting the voltage readings in Figs. 3(b)-(e) backward in time by 0.32 sec. compared with the video record, so that the voltage and video signals are simultaneous. After synchronizing the timescales in this way, the precursor times described in the letter was computed.

[0056] In Fig. 5(b), the results with the load cell on are illustrated. Here Time = 0 is defined when the load cell detects the tap, and 10 exemplars are shown from a total of 44 tapping trials, with the average over the 10 exemplars shown as a thick line. The mean delay between the load cell and the voltmeter responses was 0.96 ± 0.1 sec. The increase in delay over the results shown in Fig. 5(a) was been found to be associated with CPU sharing between data handling loops in the data acquisition software (LabVIEWTM). This delay can be reduced through reprogramming, but is present in the data shown, so in Fig's 1 (b)-(d) and 2(c)-(d), the voltage signal was shifted in time by 0.96 sec. compared with the load cell signal. As in Figs. 3(b)-(e), the cross-correlations and FET results described above were calculated with data synchronized in this way.

[0057] Finally, it is noted that Fisher Exact Test evaluations were performed as follows.

Starting with a voltage time series such as is shown in Fig. 1 (c), two criteria were established: one for a voltage spike and a second for a slip event. These criteria depend on experimental details; for Fig. 1 , the dynamic range was calculated (difference between maximum and minimum signal) of the voltage and the load cell data, and each voltage or load cell reading that drops by more than 1/4 of that range within 0.5 sec was identified.

For Fig. 2, the criteria were defined to be that either voltage or load cell reading drops by more than 1/4 of the dynamic range within 1 .5 sec. For Fig. 3, the voltage criterion were defined to be reached whenever the voltage dropped by more than 15 V within 14 sec, and the slip criterion to be the first time any visible motion is detected on the video record.

[0058] Using these criteria, the voltage and load cell/video records were converted to sequences of times at which voltage spikes appeared and slip events appeared. The times were synchronized as described earlier above in connection with Figs. 5(a)-(b), and a total was calculated of count how many times (a) a false positive, (b) a false negative, (c) a true positive, and (d) a true negative event occurred. These were defined as follows. A false positive occurred if a voltage spike was not followed within a specified correlation time, Tc, (described in the body of this letter) with a slip event; a false negative occurred if a slip event was not preceded by a voltage spike within Tc; a true positive occurred if a slip event was preceded by a voltage spike within Tc, and a true negative was a period of Tc during which there was no voltage spike and there was no slip event. The prediction counts were then input into a single-tailed Fisher Exact Test calculator, and probability, p, that the result could have occurred by random chance was reported.

[0059] It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the claims.

Claims

1 . An apparatus for detecting slip in a granular material, comprising:

a detector for detecting granular slip in the granular materials, wherein the detector comprises a sensor for detecting a voltage within the granular materials; and a processor operable to process the detected voltage to evaluate whether the detected voltage is indicative of granular slip.

2. The apparatus of claim 1 wherein the detector comprises a plurality of sensors for

detecting a voltage within the granular material and the plurality of sensors are arranged in an array.

3. The apparatus of claim 2 wherein a grounded conductor is disposed between adjacent sensors in the array.

4. The apparatus of any of the foregoing claims the granular materials are ceramic materials.

5. The apparatus of any of claims 1 -3 wherein the processor is operable to identify the

approximate location of the granular slip by analyzing the voltages detected by the sensors.

6. The apparatus of any of the foregoing claims wherein the processor is operable to control an indicator to provide a signal indicative of granular slip.

7. The apparatus of any of the foregoing claims wherein the detector comprises a field sensor for measuring electric fields.

8. The apparatus of claim 7 wherein the detector is spaced apart from the granular materials.

9. The apparatus of any of the foregoing claims comprising a second detector operable to detect a second characteristic of the granular materials within the mixer and provide signals to the processor indicative of the second characteristic, wherein the processor is operable to evaluate the likelihood of granular slip within the mixer based on the detected voltage and the signal from the second detector.

10. The apparatus of claim 9 wherein the second detector comprises a load cell.

11 . The apparatus of claim 9 wherein the processor is operable to compare the detected

voltage against a threshold to evaluate whether the detected voltage is indicative of granular slip.

12. A method for predicting granular slip, comprising:

monitoring granular material for an electronic signal produced by the granular material; and

processing the electronic signal to evaluate whether the electronic signal is indicative of granular slip.

13. The method of claim 12 comprising the step of providing a bed of granular material;

14. The method of claim 12 or 13 comprising the step of providing an indicator that the electronic signal is indicative of granular slip.

15. The method of any of claims 12-14 wherein the step of processing the electronic signal comprises determining whether the electronic signal is an electronic precursor formed by the granular material.

16. The method of any of claims 12-15 wherein the step of processing the electronic signal comprises processing the electronic signal to evaluate whether the signal is indicative of a void produced in the granular material.

17. The method of any of claims 12-16 comprising the applying a stress to an item formed of the granular material.

18. The method of claim 17 comprising the steps of identifying the location of a defect in the item based on the step of processing the electronic signal.

19. The method of claim 18 wherein the step of forming an article comprises forming a dosage of a pharmaceutical.

20. The method of claim 18 wherein the step of forming comprises forming a ceramic element.

21 .The method of any of claims 12-20 wherein the electronic signal is indicative of a voltage produced by the granular material and wherein the step of processing the electronic signal comprises determining whether the detected voltage is indicative of granular slip.

22. The method of any of claims 12-21 wherein the step of processing comprises using a computer processor to process the electronic signal to determine whether the signal is indicative of a geologic slip event.

23. The method of any of claims 12-21 wherein the computer processor is operable to process the electronic signal to determine whether the signal is indicative of a rockburst.

24. The method of any of claims 12-23 wherein the step of processing the electronic signal comprises evaluating whether the electronic signal is indicative of the likelihood of future granular slip.

25. The method of any of claims 12-21 wherein the granular slip includes, but is not limited to one of: slip events during manufacture of ceramics and pharmaceuticals, geological slip events, rockbursts and landslides.

26. An apparatus for predicting granular slip, comprising:

a sensor array operable to monitor granular material for an electronic signal produced by the granular material; and

a processor operable to process the electronic signal to evaluate whether the

electronic signal is indicative of granular slip.

27. The apparatus of claim 26 wherein the processor is operable to provide an indicator that the electronic signal is indicative of granular slip.

28. The apparatus of claim 26 or 27 wherein the processor is operable to process the electronic signal to determine whether the electronic signal is an electronic precursor formed by the granular material.

29. The apparatus of any of claims 26-28 wherein the processor is operable to evaluate whether the electronic signal is indicative of a void produced in the granular material.

30. The apparatus of any of claims 26-29 wherein the sensor is a field sensor operable to detect a voltage produced by the granular material without contacting the granular material.

31 . The apparatus of any of claims 26-30 wherein the sensor array comprises a plurality of sensors arranged in a grid.

32. The apparatus of claim 31 wherein the sensor array comprises a grounded conductor shielding adjacent sensors from one another.