Abstract:
The present invention relates to a linear actuator that maximizes the flux density in the air gap where work is to be done by increasing the lines of flux that are captured while keeping the cost of production and mass relatively low. This is achieved by an improved linear actuator, which is characterized by a keeper comprised of a cross-shaped piece of ferromagnetic material bent such that the four ends of the cross are located perpendicular to the longitudinal axis of the magnet.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates generally to mass flow rate and density measuring apparatus, and more particularly to an improved flow rate sensor having improved sensitivity.  
         PROBLEM  
         [0002]    It is known to use Coriolis effect mass flowmeters to measure mass flow and other information pertaining to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. Flowmeters have one or more conduits of a straight, curved or irregular configuration. Each conduit has a set of natural vibration modes which may be of a simple bending, torsional, or twisting type. Each material filled conduit is driven to oscillate at resonance in one of these natural modes. The natural vibration modes are defined in part by the combined mass of the flow conduits and the material within the flow conduits. If desired, a flowmeter need not be driven at a natural mode.  
           [0003]    Material flows into the flowmeter from a connected material source on the inlet side. The material passes through the conduit or conduits and exits the outlet side of the flowmeter.  
           [0004]    A drive mechanism applies force to oscillate the conduit. When there is no material flow, all points along a conduit oscillate with an identical phase in the first bending mode of the conduit. With material flow, Coriolis accelerations cause each point on the conduit to have a different phase with respect to other points on the conduit: the phase on the inlet side of the conduit lags the driver; the phase on the outlet side leads the driver. Pickoffs are placed on the conduit to produce sinusoidal signals representative of the motion of the conduit. The phase difference between two sensor signals is divided by the frequency of oscillation to obtain a delay which is proportional to the mass flow rate of the material flow.  
           [0005]    The drive mechanism of the Coriolis flowmeter is affixed to the conduit(s) and oscillates the conduit(s) in response to a signal from driver control circuitry. A conventional drive mechanism for a Coriolis flow meter has a magnetic circuit comprising a keeper, magnet and pole piece mounted in opposition to a coil. The driver control circuitry applies an electric current or drive signal to the coil of the drive mechanism. The current flowing through the coil generates electromagnetic forces between the drive coil and the magnet thereby causing the conduits to vibrate.  
           [0006]    The design and implementation of a drive mechanism is important because the greater the amount of power a drive mechanism can produce, the better the performance of the flow meter in high damping applications.  
           [0007]    Past drive mechanism designs have focused on reducing cost and mass while doing very little to increase the power output. This design focus, coupled with the industry&#39;s desire to lower the cost and size of Coriolis flow meters, magnifies the difficulty in drive system design.  
           [0008]    A typical drive design is developed based on the following two equations: 
             P   disapated =−2 *ω*ξ*K*A   2   (1) 
           [0009]    where:  
           [0010]    ω=angular velocity of the system  
           [0011]    ξ=critical damping ratio of system  
           [0012]    K=system stiffness  
           [0013]    A=system amplitude  
           [0014]    P=system power  
           [0015]    and 
             P   delivered =2 *ω*I*B*L*A   (2) 
           [0016]    where:  
           [0017]    ω=angular velocity of the system  
           [0018]    I=available current  
           [0019]    B=total flux  
           [0020]    L=length of wire on coil  
           [0021]    A=system amplitude  
           [0022]    P=system power  
           [0023]    Equation (1) represents the power dissipated by the Coriolis flow meter and Equation (2) represents the power delivered to the flow meter by the drive mechanism. In some cases, depending on the application and location that the flow meter will be placed in, the amount of power delivered to the flow meter is limited by area approval agencies (i.e. UL, CENELEC, TIIS).  
           [0024]    In normal operation, frequency and conduit amplitude are pre-defined resulting in equations (1) and (2) being equal. However, many factors can cause a flow meter to deviate from normal operation. Such factors include entrained air, high viscosity fluids, and material flow comprising large amounts of solids. The deviation from normal operation results in damping of the vibrational characteristics of the system, thus requiring an increase in the power supplied to the flow meter to return the meter to normal operation. In order to ensure continued operation of the sensor during occasions when a flow meter deviates from normal operation, designers design in “overhead” or “reserve power”. “Overhead” is defined as the maximum power available to the sensor divided by the power needed to drive the system during normal operation.  
           [0025]    In order to generate the overhead needed by a sensor, a drive mechanism designer must strive to increase the power available to the sensor. However, of the variables comprising equations (1) and (2), variables K, ω and ξ are determined by the geometry of the sensor and I is limited by the area approval agencies, leaving only B, L and A available to the designer.  
           [0026]    From equations (1) and (2), it is clear that increasing conduit amplitude, A, would result in power being dissipated faster than power being supplied. Increasing the length of wire on a coil would increase power, however, an increase in the length of wire would increase the resistance and thereby reduce the delivered power. Additionally, there are additional safety restraints imposed by the approval agencies on the relationship between a coil&#39;s inductance and resistance. However, the flux, B, can be increased without impacting the power dissipated nor affecting those variables constrained by an approval agency.  
           [0027]    The total flux, B, represents how closely packed (i.e. the “density”) the flux lines are that compose the magnetic field. In order to efficiently utilize the magnetic field, a “keeper” is placed around the magnet. The keeper is a piece of ferromagnetic material, such as carbon steel, that acts as a conductor for the lines of flux. The flux lines are concentrated in the steel keeper, as a ferromagnetic material will support a greater concentration than will air. In addition to serving as a conductor for the flux lines, the keeper also channels the lines of flux so as to create the maximum flux density in an air gap where work will be done. In the case of a magnet/coil driver, the coil is positioned in the air gap and orientated to maximize the cross product between the flux and current vectors.  
           [0028]    One prior art design uses a strip of metal bent into an open channel (FIG. 1). The channel design is relatively inexpensive to build but surrounds only a small portion of the magnet, failing to “capture” a large number of flux lines. Another prior art design utilizes a cup-shaped keeper (FIGS. 2A &amp; 2B). This cup-shaped keeper design maximizes the flux lines that are captured, due to its 360 degree conductive area, however, the design is costly to produce and extremely weight prohibitive.  
         SOLUTION  
         [0029]    The object of the invention is a linear actuator that maximizes the flux density in the air gap where work is to be done by increasing the lines of flux that are captured while keeping the cost of production and mass relatively low. The object is achieved by an improved linear actuator, which is characterized by a keeper comprised of a cross-shaped piece of ferromagnetic material bent such that the four ends of the cross are located perpendicular to the longitudinal axis of the magnet.  
           [0030]    The keeper increases the total flux available to the drive mechanism without negatively impacting other variables in the system. In addition, the keeper is light weight and easy to manufacture.  
           [0031]    One possible preferred exemplary embodiment of the linear actuator according to the invention is characterized by the keeper being composed of a ferromagnetic material, preferably steel. The keeper is manufactured by forming a piece of material into a cross-shape with end portions of the legs contoured to closely match the exterior contour of the magnet. The legs are bent to form two 90 degree angles resulting in the end portions being perpendicular to the longitudinal axis of the magnet.  
           [0032]    Another possible preferred embodiment of the linear actuator according to the invention is characterized by a mounting bracket composed of a ferromagnetic material. In some cases the flux density generated by the magnet exceeds the keeper&#39;s capacity to carry, resulting in flux saturation of the keeper. When the keeper becomes saturated, any additional flux generated by the magnet goes through the air. In order to capture the additional flux, a ferromagnetic mounting bracket is located at a pole of the magnet.  
           [0033]    The invention allows a majority of the lines of flux to be captured by the linear actuator and channeled through the coil at a preferential angle. The invention also significantly reduces the gap between the magnet while nearly surrounding the magnet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a magnet keeper according to the prior art;  
         [0035]    [0035]FIG. 2A is an additional magnet keeper according to the prior art;  
         [0036]    [0036]FIG. 2B is a section view of the keeper in FIG. 2A;  
         [0037]    [0037]FIG. 3 discloses a magnet keeper in accordance with the present invention;  
         [0038]    [0038]FIG. 4 is an assembly view disclosing a linear actuator design in accordance with the present invention;  
         [0039]    [0039]FIG. 5 is an exploded view of FIG. 4;  
         [0040]    [0040]FIG. 6 discloses an alternative magnet keeper in accordance with the present invention;  
         [0041]    [0041]FIG. 7 discloses another alternative magnet keeper in accordance with the present invention; and  
         [0042]    [0042]FIG. 8 discloses another additional alternative magnet keeper in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]    Description of FIG. 1  
         [0044]    [0044]FIG. 1 is a previously known magnet keeper  100  incorporated into linear actuator design. The magnet keeper  100  includes a substantially flat portion  110  configured to mate to a mounting structure (not shown), two side portions  120 , and two top portions  130 . The two top portions have an arcuate surface  140  shaped to closely accommodate the exterior portion of a magnet (not shown). The magnet sits on the interior surface of flat portion  110  with a resulting gap between the magnet exterior and the arcuate surface  140  of the top portions  130 . The gap allows for the passing of a coil (not shown) between the keeper and the magnet. Keeper  100  is formed by bending sheet metal to the described shape.  
         [0045]    Description of FIGS. 2A &amp; 2B  
         [0046]    [0046]FIGS. 2A &amp; 2B disclose another priorly known keeper  200  adapted to be incorporated into a linear actuator design. Keeper  200  is referred in the art as a cup keeper. The cup-shaped design comprises a bottom circular flat portion  210 , a circular side wall  220  and a circular top portion  230  forming a lip on top of the side wall  220 . The top portion  230  has an interior surface  240  which is shaped to closely conform to the exterior shape of a magnet (not shown). The magnet sits on the interior surface of flat portion  210  with a resulting gap between the magnet exterior and the surface  240  of the top portions  230 . The gap allows for the passing of a coil (not shown) between the keeper and the magnet. Keeper  200  is formed by casting, bending, or machining.  
         [0047]    Description of FIG. 3  
         [0048]    [0048]FIG. 3 describes a keeper  300  according to a preferred embodiment of the invention. Keeper  300  is comprised of a substantially flat base portion  305  with a longitudinal axis  350  and four legs  315  extending from flat base portion  305 . Each leg  315  is comprised of a lower portion  310  extending from the flat base portion  305 , a side portion  320  extending at a right angle from the lower portion  310  and a top portion  330  extending at a right angle from the side portion  320 . The top portion  330  has an arcuate interior surface  340  shaped to closely conform to the exterior shape of a magnet (not shown). The magnet sits on the interior surface of flat lower portion  305  with a resulting gap between the magnet exterior and the arcuate surfaces  340 . The gap allows for the passing of a coil (not shown) between the keeper  300  and the magnet. Keeper  300  is formed by bending sheet metal to the described shape.  
         [0049]    Description of FIG. 4  
         [0050]    [0050]FIG. 4 depicts a linear actuator design  400  affixed to conduits  411  and  413  of a Coriolis flow meter. The linear actuator design is comprised of two sections, a magnet section  410  and a coil section  460 .  
         [0051]    Magnet portion  410  comprises a magnet mounting bracket  412  for mounting magnet  418  and keeper  300  to conduit  413 . Keeper  300  is attached to mounting bracket  412  by screws  416  and washers  417 . Magnet assembly  418  is held in place on keeper  412  using a combination of an adhesive, locating features and the magnetic attraction between the magnet assembly  418  and keeper  412 .  
         [0052]    Coil portion  460  comprises a coil mounting bracket  462  for mounting coil  464  to conduit  411 . The coil  464  is attached to the coil mounting bracket  462  by screws  466  and washers  467 . When mounted, coil  464  is positioned in the gap between the interior surface  340  (shown in FIG. 3) of the keeper  300  and the exterior of the magnet assembly  418 .  
         [0053]    In operation, power is applied to the coil  464  via electronics (not shown) through terminals  468 . Once power is applied, the coil&#39;s polarity is reversed at intervals which cause either attraction or repulsion of the magnet, resulting in an oscillation motion of the conduits  411  and  413 .  
         [0054]    Description of FIG. 5  
         [0055]    [0055]FIG. 5 is an exploded view showing further detail the liner actuator of FIG. 4. As discussed above, a linear actuator  400  is composed of two sections, a magnet section  410  and a coil section  460 .  
         [0056]    The magnet section  410  comprises mounting bracket  412  attached to conduit  413 , keeper  300  coupled to mounting bracket  412  by screws  416  and washers  417 , and magnet assembly  418 . Magnet assembly  418  is further comprised of a lower magnet portion  518 , an upper magnet portion  519  and a pole piece  530  encompassed in a magnet sleeve  527 . Magnet sleeve  527  has two protrusions  524  extending from its bottom portion that fit into corresponding slots  526  on the keeper. Protrusions  524  and slots  526  ensure the proper alignment of the magnet assembly  418  in the magnet section  410 .  
         [0057]    The coil section  460  comprises a coil mounting bracket  462  attached to conduit  411  and a coil  464  attached to the coil mounting bracket  462  by screws  466  and washers  467 . The assembled coil  464  resides in a gap between keeper  300  and magnet assembly  418 .  
         [0058]    Description of FIG. 6  
         [0059]    [0059]FIG. 6 describes an alternative keeper  600 . Keeper  600  is comprised of a flat base portion  605  and curved side legs  615 . Each leg  615  is terminated by an arcuate interior surface  640  shaped to closely conform to the exterior shape of a magnet (not shown).  
         [0060]    Description of FIG. 7  
         [0061]    [0061]FIG. 7 describes an another alternative keeper  700 . Keeper  700  is comprised of a flat base portion  705  and three curved side legs  715 . Each leg  715  is comprised of a lower portion  710  extending from the flat base portion  705 , a side portion  720  extending at a right angle from the lower portion  710  and a top portion  730  extending at a right angle from the side portion  720 . The top portion  730  has an arcuate interior surface  740  shaped to closely conform to the exterior shape of a magnet assembly (not shown). The magnet sits on the interior surface of flat lower portion  705  with a resulting gap between the magnet exterior and the arcuate surfaces  740 . The gap allows for the passing of a coil (not shown) between the keeper  700  and the magnet.  
         [0062]    Description of FIG. 8  
         [0063]    [0063]FIG. 8 describes an another additional alternative keeper  800 . Keeper  800  is comprised of a substantially flat base portion  805  with a longitudinal axis  850  and four legs  815  extending from flat base portion  805 . Each leg  815  is comprised of a lower portion  810  extending from the flat base portion  805 , a side portion  820  extending at a right angle from the lower portion  810  and a top portion  830  extending at a right angle from the side portion  820 . The top portion  830  has an arcuate interior surface  840  shaped to closely conform to the exterior shape of a magnet (not shown). A disc member  860  is affixed to top portions  830  of legs  815 . The addition of disc member  860  allows for complete encirclement of the circumference of the magnetic while keeping the cost of manufacture low. The magnet sits on the interior surface of flat lower portion  805  with a resulting gap between the magnet exterior and arcuate surfaces  840  and disc member  860 . The gap allows for the passing of a coil (not shown) between the keeper  800  and the magnet (not shown).  
         [0064]    In summary, it can be seen from the foregoing that the provision of a linear actuator with an improved keeper design can significantly improve the performance of a drive system by increasing both the efficiency and manufacturability. It is to be expressly understood that the claimed invention is not to be limited to the description of the preferred embodiment but encompasses other modifications and alterations.