Abstract:
A screening machine that includes a screen having a periphery and a central porous region is provided. A motion amplifier is substantially rigidly attached in direct contact with the periphery of the screen and not in direct contact with the central porous region. A transducer is substantially rigidly attached to the motion amplifier, wherein the transducer imparts a vibratory motion to the screen via the motion amplifier.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This Application is a division of U.S. application Ser. No. 09/792,778 filed Feb. 23, 2001, now U.S. Pat. No. 6,543,620. 

   TECHNICAL FIELD 
   The present invention relates generally to the field of physical separation of materials and, in particular, to vibrating screens. 
   BACKGROUND 
   Vibrating screens are used by a number of industries, e.g., mining, food processing, sand-and-gravel, etc., to separate a fine portion of a heterogeneous substance from a coarse portion. For example, the mining industry (e.g., taconite processing) uses vibrating screens after the ore is crushed to separate fine ore from coarse ore. Typical screening processes involve placing a heterogeneous substance that comprises fine and coarse portions atop a screen. The screen is then vibrated so that the fine portion passes through the screen and the coarse portion stays atop the screen. 
   Typically, an electric motor having a rotating unbalance vibrates the screen. Electrical unbalance motors are usually heavy and bulky and normally require considerable maintenance and a heavy support structure. Another disadvantage is that such a configuration normally involves several moving parts, many of which are heavy and bulky, and a number of bearings. These moving parts and bearings require considerable maintenance and generate heat and excessive audible noise. Moreover, a substantial portion of the energy output of the electric motor typically goes into the useless elastic deformation of the heavy support structure and the generation of audible noise and heat. 
   To put this into perspective, the use of the above-type of vibrating screens during taconite processing will be used by way of example. Many of the screening operations used during taconite processing involve a motor vibrating a load that is at least 17 times the load of taconite to be screened. Moreover, the noise generated by the vibrating screens used in taconite processing may result in work environment safety issues. The taconite industry has identified vibrating screens as being responsible for substantial maintenance costs and production losses. 
   For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for vibrating screens that are smaller and lighter, that have fewer moving parts and fewer bearings, and that consequently are less noisy, require less maintenance, have reduced downtimes, and are more energy efficient than conventional vibrating screens. 
   SUMMARY 
   The above-mentioned problems with conventional vibrating screens and other problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention provide a screening machine. 
   More particularly, one embodiment provides a screening machine that includes a screen having a periphery and a central porous region. A motion amplifier is substantially rigidly attached in direct contact with the periphery of the screen and not in direct contact with the central porous region. A transducer is substantially rigidly attached to the motion amplifier, wherein the transducer imparts a vibratory motion to the screen via the motion amplifier. 
   Another embodiment provides a screening machine that has a frame and a mesh enclosed within and attached to the frame. A motion amplifier is substantially rigidly attached in direct contact with the frame and not in direct contact with the mesh. A transducer is substantially rigidly attached in direct contact with the motion amplifier to impart a vibratory motion to the motion amplifier. The motion amplifier amplifies the vibratory motion and imparts the amplified vibratory motion directly to the frame and thereby to the mesh. A sensor is substantially rigidly attached in direct contact with the motion amplifier. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of an embodiment of the screening machine of the present invention. 
       FIG. 2  is an enlarged view of a portion of FIG.  1 . 
       FIG. 3   a  is a side view of an embodiment of a transducer for vibrating a screen. 
       FIG. 3   b  illustrates a transducer having an array of discrete components. 
       FIGS. 4   a  through  4   d  are side-view illustrations of different embodiments of a motion amplifier for amplifying vibrations imparted to a screen by a transducer. 
       FIGS. 5   a  and  5   b  are side view illustrations of other embodiments of motion amplifiers for amplifying vibrations imparted to a screen by a transducer. 
       FIG. 6  is a block diagram of an embodiment of a control apparatus for controlling vibrations imparted to a screen by a transducer. 
       FIG. 7  is a block diagram of another embodiment of a control apparatus for controlling vibrations imparted to a screen by a transducer. 
       FIG. 8  is a flow chart of a method for unclogging a screen. 
       FIG. 9  is an example of superimposed waveforms that are transmitted to a transducer during a method for unclogging a screen. 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
   Embodiments of the present invention replace the electrical motor and rotating unbalance used with conventional vibrating screens with a combination of transducers and motion amplifiers and thus the concomitant heavy support structure and numerous moving parts and bearings. The transducers can be piezoelectric patches, discrete piezoelectric components, or electromagnetic shakers. In embodiments of the present invention, these transducers are attached to a screen and are used to vibrate the screen. 
   A first embodiment of the present invention is demonstrated by the simplified top view of screen machine  100  in FIG.  1 . Screen machine  100  includes a base  101  and screen  102 . Transducers  104  are substantially rigidly attached to screen  102 . Screen  102  and transducers  104  are discussed in more detail below. The screen is used to separate fine material from course material. The screen is mounted to the base using spring-type mountings  103 . The spring-type mountings  103  allow the screen to be moved independently of the mounting base. 
   Screen  102  includes frame  106  having two opposing boundaries  108  and two opposing boundaries  110  that are perpendicular to boundaries  108 . Boundaries  108  and  110  can be solid or hollowed-out solids. Boundaries  108  and  110  have a cross-sectional shape that can be circular, rectangular, square, angular, or the like. Boundaries  108  and  110  can be fabricated from steel, plastic, ceramic, aluminum, or the like. Boundaries  108  can be attached to boundaries  110  by welding, gluing, bolting, using cap screws, or the like. Alternatively, frame  104  can be formed as a single component by casting or the like, with boundaries  108  and  110  being integral with each other. It will be appreciated by those skilled in the art that that  FIG. 1  has been simplified to focus on the present invention and numerous features are not illustrated. For example, material input and output mechanisms, and control components are not illustrated in FIG.  1 . 
   Screen  102  includes mesh  112  that is enclosed within frame  106 . Mesh  112  can be fabricated from steel, plastic, ceramic, aluminum, urethane, rubber, or the like. Mesh  112  can be attached to frame  106  by welding, gluing, bolting, using cap screws, or the like. The mesh size varies according to the size of material that is to be screened out. 
   In one embodiment, transducers  104  are of a piezoelectric material, such as a formulation of lead, magnesium, and niobate (PMN), a formulation of lead, zirconate, and titanate (PZT), or the like. In another embodiment, transducers  104  are electromagnetic shakers or unbalanced motors. In another embodiment, transducers  104  include integral transducer and sensor portions, e.g., both are piezoelectric materials. In another embodiment, transducers  104  include separate, adjacent transducer and sensor portions, e.g., the transducer portion is an electromagnetic shaker and the sensor portion is a piezoelectric material, both are piezoelectric materials, or the like. 
   When an alternating voltage is applied to a piezoelectric material, such as transducer  104 , the piezoelectric material alternately expands and contracts. When an alternately expanding and contracting piezoelectric material is attached to an object, such as screen  102 , the alternating expansions and contractions cause the object to vibrate. Conversely, when a vibrating object, such as screen  102 , exerts an alternating force on a piezoelectric material, the piezoelectric material alternately expands and contracts, and the piezoelectric material produces an alternating voltage that is indicative of the vibration. In this manner, the piezoelectric material can be used as a sensor. These facts can be used to construct transducers having sensing capabilities. For example, a transducer can include adjacent piezoelectric portions, where one portion has leads used as an input for accepting an alternating voltage and the other portion has leads used as an output for outputting voltages indicative of vibrations. 
   In another embodiment, where transducer  104  is an electromagnetic shaker attached to screen  102 , the electromagnetic shaker imparts a vibratory motion to the screen  102 . 
     FIG. 2  is an enlarged view of encircled region  114  of screening machine  100 .  FIG. 2  demonstrates that one embodiment of transducer  104  includes patches  104   a  and  104   b , each of PMN, PZT, or the like. In another embodiment at least one of patches  104   a  and  104   b  is an electromagnetic shaker. Patches  104   a  and  104   b  are substantially rigidly attached, as shown, to motion amplifier  116  by bolting, screwing, gluing, or the like and sandwich motion amplifier  116  between them. Hereinafter “substantially rigidly attached” will be referred to as “attached” and will include these methods of attachment and others recognized as suitable equivalents by those skilled in the art. The transducers apply lateral forces to the screen as shown by arrow  107 . These forces may be amplified, as described below, to provide vibration to the screen. 
   In one embodiment, patches  104   a  and  104   b  respectively include electrical leads  104   c  and  104   d . In one embodiment, leads  104   c  and  104   d  are used to input an alternating voltage that causes the respective patch to impart a vibratory motion to motion amplifier  116 . In another embodiment, one of leads  104   c  and  104   d  is used to output a voltage that is indicative of the vibratory motion of motion amplifier  116  and thus the corresponding patch acts as a sensor. 
   Piezoelectric and electromagnetic shaker construction and operation are well known to those in the art. A detailed discussion, therefore, of specific constructions and operation is not provided herein. It will be understood, with the benefit of the present description, that transducers  104  are electrically controlled to provide physical movement. As described below, using multiple transducer elements in unison and/or placing an amplifier between the transducer elements and the screen can enhance the physical movement. 
   Frame  106  can include an optional extension  118  adjacent each of its corners. A motion amplifier  116  is attached to frame  106  at each extension  118 . In other embodiments, frame  106  includes extensions  118  at locations intermediate to the corners of frame  106  (not shown). In these embodiments, a motion amplifier  116  can be attached to the frame at each of these extensions  118 , with each motion amplifier having a transducer(s)  104  attached thereto. Motion amplifier  116  can be steel, aluminum, plastic, a composite, a fiber reinforced laminate, or the like. 
   In operation, transducer  104  imparts a vibratory action to motion amplifier  116  (arrow  107 ). Motion amplifier  116  amplifies the vibration (i.e., the displacement and the acceleration of the vibration) and transmits the amplified vibration to frame  106 , thus causing screen  102  to vibrate. The amplification increases as the distance between transducer  104  and the location of attachment of motion amplifier  116  to frame  106  increases, e.g., the distance between transducer  104  and extension  118 . 
   In another embodiment, transducer  104  imparts a vibratory action to motion amplifier  116  at substantially the resonant frequency of motion amplifier  116 , in which case motion amplifier  116  may be termed a resonator. At substantially resonant conditions, motion amplifier  116  not only amplifies the displacement output of the transducer but also the energy output. 
   In another embodiment transducers  104  are used to divert energy from particular regions of screen  102  and to focus the energy at other regions where it is more useful, thus making the system more efficient. The focused energy can be used directly or after amplification to vibrate screen  102 . Detailed descriptions of how energy can be diverted from one region and focused at another region are given in U.S. Pat. No. 6,116,389 entitled APPARATUS AND METHOD FOR CONFINEMENT AND DAMPING OF VIBRATION ENERGY issued on Sep. 12, 2000 and U.S. Pat. No. 6,032,552 entitled VIBRATION CONTROL BY CONFINEMENT OF VIBRATION ENERGY issued on Mar. 7, 2000, which are incorporated herein by reference, and in pending U.S. application Ser. No. 09/721,102 entitled ACTIVE VIBRATION CONTROL BY CONFINEMENT filed on Nov. 22, 2000, which is incorporated herein by reference. 
     FIG. 3   a  illustrates a stacked embodiment of transducer  104  attached to an amplifier  116 . In this embodiment, transducer  104  comprises piezoelectric layers  104 - 1  through  104 -N stacked one atop the other. Each of piezoelectric layers  104 - 1  through  104 -N is a formulation of lead, magnesium, and niobate (PMN), a formulation of lead, zirconate, and titanate (PZT), or the like. In one embodiment, piezoelectric layers  104 - 1  through  104 -N are electrically interconnected in parallel. Stacking of layers  104 - 1  through  104 -N amplifies the vibration by multiplying the force or the vibration displacement by the number of layers. In one embodiment, one or more of layers  104 - 1  through  104 -N can be used as a sensor. That is, piezoelectric elements can be used to provide motion in response to an applied voltage, or provide a voltage in response to physical changes. 
     FIG. 3   b  is a side view of a transducer  104  attached to motion amplifier  116 . The transducer includes an array of discrete piezoelectric elements  117 . Each element provides physical movement to the amplifier, or directly to the screen, in response to applied voltages. Again, one or more of the elements can be coupled as a sensor. 
     FIGS. 4   a  through  4   d  illustrate side views of different embodiments of motion amplifier  116 .  FIG. 4   a  illustrates a straight motion amplifier  116 .  FIG. 4   b  illustrates a C-shaped motion amplifier  116 , and  FIG. 4   c  illustrates an S-shaped motion amplifier  116 . It will be appreciated by those of ordinary skill in the art that the embodiments of motion amplifier  116  illustrated in  FIGS. 4   a  through  4   c  can be combined in various ways to form other embodiments of motion amplifier  116 . For example,  FIG. 4   d  illustrates an embodiment of motion amplifier  116  that includes several C-shaped motion amplifiers linked together. 
   In the embodiments of motion amplifier  116  illustrated in  FIGS. 4   a  through  4   d , transducer  104  is attached to one of end regions  116 - 1  or  116 - 2 , and motion amplifier  116  is attached to frame  106  at the other of end regions  116 - 1  or  116 - 2 . In operation, transducer  104  imparts a vibratory motion to one of end regions  116 - 1  or  116 - 2 . Motion amplifier  116  amplifies the vibration between transducer  104  and the other of end regions  116 - 1  or  116 - 2 , where the vibration is imparted to frame  106 . 
   The embodiments of motion amplifier  116  demonstrated in  FIGS. 4   a  through  4   d  are based on a basic cantilever beam where the transducer is attached to the free end. However, the size and shape of the motion amplifier can be selected to increase or decrease movement of the screen based on engineering requirements, and the present invention is not limited to any specific size, length, cross-section shape or overall geometric configuration of amplifier. For example,  FIG. 5   a  illustrates an embodiment of motion amplifier  116  that comprises a beam that is pinned at both of its ends.  FIG. 5   b  illustrates an embodiment of motion amplifier  116  that comprises a pair of beams, each pinned at both of its ends, and a substantially rigid coupler  116 - 3  that couples the two beams together. In  FIG. 5   a , a transducer  104  is attached to the beam at a location between the end supports, and motion amplifier  116  is attached to frame  106  at region  116 - 1 . In  FIG. 5   b , a transducer  104  can be attached to at least one of the beams at a location between the end supports, and motion amplifier  116  is attached to frame  106  at region  116 - 1 . 
     FIG. 6  is a block diagram illustrating control apparatus  600  for controlling vibratory output  602  of transducer  104  and thereby the vibration of screen  102 . Power supply  606  is electrically coupled to an input of a transducer portion of transducer  104  and transmits an ac voltage to it. An output of a sensor portion of transducer  104  is electrically coupled to an input of control circuit  608  and transmits a monitoring signal indicative of the vibration of screen  102  to it. An output of control circuit  608  is coupled to an input of power supply  606  and transmits a control signal to it. In one embodiment, the control signal adjusts the voltage amplitude up or down and thereby the amplitude of output  602 . 
   In operation, power supply  606  transmits an alternating voltage to the transducer portion of transducer  104 . The alternating voltage causes the transducer portion to produce vibratory output  602  that imparts a vibratory motion to screen  102  via motion amplifier  116 . The sensor portion transmits a monitoring signal to control circuit  608  that is indicative of the vibration of screen  102 . 
   In one embodiment, the monitoring signal is indicative of the amplitude of the vibration of screen  102 . Control circuit  608  compares the amplitude to a preselected amplitude and transmits a control signal to power supply  606 . The control signal adjusts the amplitude of the ac voltage transmitted by power supply  606  to the transducer portion, thereby adjusting the amplitude of the vibration of screen  102 . In one embodiment, the preselected amplitude is the amplitude required to maintain the flow of the fine portion of the substance being screened through mesh  112 . 
     FIG. 7  is a block diagram illustrating another control apparatus  700  for controlling vibratory output  702  of transducer  104  and thereby the vibration of screen  102 . Signal-generator/amplifier  706  is electrically coupled an input of a transducer portion of transducer  104  and transmits an ac voltage to it. An output of a sensor portion of transducer  104  is electrically coupled to an input of control circuit  708  and transmits a monitoring signal indicative of the vibration of screen  102  to it. An output of control circuit  708  is coupled to an input of signal-generator/amplifier  706  and transmits a control signal to it. 
   In operation, signal-generator/amplifier  706  transmits an alternating voltage to the transducer portion of transducer  104 . The alternating voltage causes the transducer portion to produce vibratory output  702  that imparts a vibratory motion to screen  102  via motion amplifier  116 . The sensor portion transmits a monitoring signal to control circuit  708  that is indicative of the vibration of screen  102 . 
   In one embodiment, the monitoring signal is indicative of the amplitude of the vibration of screen  102 . Control circuit  708  compares the amplitude to a preselected amplitude and transmits a control signal to signal-generator/amplifier  706 . The control signal adjusts the amplitude of the ac voltage transmitted by signal-generator/amplifier  706  to the transducer portion, thereby adjusting the amplitude of the vibration of screen  102 . In one embodiment, the preselected amplitude is the amplitude required to maintain the flow of the fine portion of the substance being screened through mesh  112 . 
   In another embodiment, the monitoring signal is indicative of the frequency of the vibration of screen  102 . Control circuit  708  compares the frequency to a preselected frequency and transmits a control signal to signal-generator/amplifier  706 . The control signal adjusts the frequency of the ac voltage transmitted by signal-generator/amplifier  706  to the transducer portion, thereby adjusting the frequency of the vibration of screen  102 . In one embodiment, the preselected frequency is the frequency required to maintain the flow of the fine portion of the substance being screened through mesh  112 . 
   In another embodiment, the monitoring signal is indicative of the frequency and amplitude of the vibration of screen  102 . Control circuit  708  compares the frequency and amplitude to a preselected frequency and amplitude and transmits a control signal to signal-generator/amplifier  706 . The control signal adjusts the frequency and amplitude of the ac voltage transmitted by signal-generator/amplifier  706  to the transducer portion, thereby adjusting the frequency and amplitude of the vibration of screen  102 . In one embodiment, the preselected frequency and amplitude are the frequency and amplitude required to maintain the flow of the fine portion of the substance being screened through mesh  112 . 
   In another embodiment, apparatus  700  is used to unclog screen  102  using method  800 , exemplified by the flow chart in FIG.  8 . In the screening industry, screen clogging is termed “screen blinding.” Block  810  of method  800  includes receiving the monitoring signal from the sensor portion of transducer  104  at control circuit  708 , where the monitoring signal is indicative of the load on the screen. Block  820  includes evaluating the monitoring signal at the control circuit. The evaluation involves comparing the monitoring signal to a predetermined value indicative of a clogged screen. If the monitoring signal indicates that the load is below the predetermined value, the screen is unclogged, and method  800  proceeds along the “No” path from block  830  to block  840 , where no action is taken. On the other hand, if the monitoring signal indicates that the load is above the predetermined value, the screen is clogged, and method  800  proceeds along the “Yes” path from block  830  to block  850 . 
   Block  850  includes control circuit  708  transmitting a control signal to signal-generator/amplifier  706 . The control signal causes signal-generator/amplifier  706  to superimpose a high-energy impulsive wave onto the vibratory motion of the transducer portion of transducer  104 . This is exemplified for one embodiment in FIG.  9 . In this embodiment, y(t) represents the vibratory motion and h(t) represents the high-energy impulsive wave. In this example, h(t) has a lower frequency and higher amplitude than y(t). The high-energy impulsive wave causes the transducer portion to impart high-energy impulses to screen  102 . The high-energy impulses thus imparted shake the clogs loose from screen  102 , thus unclogging it. 
   CONCLUSION 
   Embodiments of the present invention have been described. In one embodiment, a screening machine has been described that can be used to replace loud, bulky screening machines that use unbalanced motors. The present machine uses electrically controlled transducers to vibrate a separating screen. The transducers can be piezoelectric patches, discrete piezoelectric components, or electromagnetic shakers. Further, the transducers can be coupled directly to the screen or through a vibration amplifier. Different attachment locations have been described for coupling the transducers and/or amplifiers to the screen. In one embodiment, one or more of the transducers are used as sensors to provide feedback for operation control. 
   Although specific embodiments have been illustrated and described in this specification, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. For example, the screen can have a variety of different shapes, e.g., circular, square, oval, or the like.