Patent Grant
11239736
Actuators that generate inertial forces by forced oscillation of a moving mass through the interaction of permanent and electrically generated magnetic fields are commonly known as electromagnetic shakers.
Many applications of shakers require a linear response, which means that the force output is directly proportional to the signal input. Nonlinearities result in a distorted force output, which includes harmonics of the input frequency.
U.S. Pat. No. 5,587,615 issued to Murray et al. teaches a method to linearize the output of a magnetic actuator with force generated across air gaps. Murray et al. arranges two air gaps with the total actuator force equal to the difference of the forces across them and then establishes magnetic bias flux in opposite directions in the two gaps and coil flux in the same direction in the gaps. Therefore, as coil flux increases it tends to cancel the bias flux in one gap and add to the bias flux in the other gap. The inherent force generated across an air gap is quadratic with respect to the total flux across the gap. If the bias flux is Φbias and the coil flux is Φcoil then the force in one gap can be written as F=k(Φbias±Φcoil)2, where k is a proportionality constant dependent on the geometry. The net force in the two gaps can be written as Fnet=k [(Φbias+Φcoil)2−(Φbias−Φcoil)2]. Simplifying this equation yields Fnet=4k ΦbiasΦcoil. Thus, the net output force is linear with respect to the coil flux.
Linear electromagnetic shakers may include a radial gap between the stator and the permanent magnets. This results in a radial force between the moving and stationary components, which complicates assembly and applies load to the support bearings increasing friction, and thus wear. The radial gap also adds to the reluctance of the magnet flux path reducing the bias across the axial gaps. Linear electromagnetic shakers may also include two pairs of axial gaps; one between the stator and the pole pieces, and one between the moving structure and the supporting structure. These two pairs of gaps increase the axial length of the device.
Consequently, if the radial gap between the stator and the permanent magnets were eliminated, and if one of the pairs of axial gaps were eliminated, the result would be a more compact electromagnetic shaker with improved reliability.
The present invention provides an electromagnetic inertial force generator (shaker) with improved compactness and reliability because it has no radial gaps and only one pair of axial gaps.
The preferred embodiment of the invention includes top and bottom radially polarized permanent magnet rings between and in contact with an inner flux cylinder and an outer flux cylinder. A drive coil is positioned radially between the inner and outer flux cylinders and axially between the top and bottom magnet rings. The magnet rings, flux cylinders, and drive coil are joined and move as a unit forming the moveable inertial mass of the actuator. The inertial mass is supported by bearings and springs and can move axially with respect to the top and bottom stationary flux returns and support structure. There are top and bottom air gaps between the moving inertial mass and the stationary flux returns to accommodate axial motion.
Magnetic flux from the top and bottom magnet rings pass through the outer flux cylinder, across the air gaps, through the stationary flux returns, back across the air gaps, and through the inner flux cylinder back to the magnet rings to complete a flux loop. The magnetic flux passing across the air gaps provides a bias for the actuator. The magnetic bias flux is in opposite directions across the two air gaps.
Magnetic flux from the drive coil passes from the outer flux cylinder across one air gap, through a stationary flux return, back across the air gap, through the inner flux cylinder, across the other air gap, through a stationary flux return, back across the air gap, and through the outer flux cylinder to complete a flux loop. The coil flux is in the same directions across the two air gaps.
The combination of bias flux and coil flux cancels in one gap and adds in the other gap because the bias flux is in opposite directions across gaps while the coil flux is in the same direction. Thus, there is net force on the inertial mass and an equal and opposite force on the returns. The resulting force is linear with current through the drive coil and will be in the opposite direction when the current through drive coil is reversed.
Another embodiment of the invention operates on the same principles as the preferred embodiment, but uses one radially polarized permanent magnet ring and two drive coils. The permanent magnet ring is between and in contact with an inner flux cylinder and an outer flux cylinder. Two drive coils are located radially between the inner and outer flux cylinders and on each side of the magnet ring. The two drive coils can either be in series or parallel such that the direction of current flow is the same in each coil. The magnet ring, flux cylinders, and drive coils are joined and move as a unit forming the moveable inertial mass of the actuator. The inertial mass is supported by bearings and springs and can move axially with respect to the top and bottom stationary flux returns and support structure. There are top and bottom air gaps between the moving inertial mass and the stationary flux returns to accommodate axial motion.
Magnetic flux from the magnet ring passes through the outer flux cylinder, across the air gaps, through the stationary flux returns, back across the air gaps, and through the inner flux cylinder back to the magnet ring to complete a flux loop. The magnetic flux passing across the air gaps provides a bias for the actuator. The bias flux is in opposite directions across the two air gaps.
Magnetic flux from the drive coils passes from the outer flux cylinder across one air gap, through a stationary flux return, back across the air gap, through the inner flux cylinder, across the other air gap, through a stationary flux return, back across the air gap, and through the outer flux cylinder to complete a flux loop. The coil flux is in the same directions across the two air gaps.
The combination of bias flux and coil flux cancels in one gap and adds in the other gap because the bias flux is in opposite directions across the gaps while the coil flux is in the same direction across the gaps. Thus, there is a net force on the inertial mass and an equal and opposite force on the returns. The resulting force is linear with current through the drive coil and will be in the opposite direction when the current through drive coil is reversed.
In alternate embodiments, a different number and arrangement of radial magnet rings and drive coils may be used as long as the magnetic flux across the two gaps from the magnet ring or rings is in opposite directions across the gaps and the flux from the drive coil or coils is in the same direction across the two gaps.
The top radially polarized magnet ring (101) and bottom radially polarized magnet ring (102) are preferably composed of a radial array of high strength magnet segments such as Neodymium Iron Boron (NdFeB) magnets. The inner flux cylinder (104) and outer flux cylinder (103) and top stationary flux return (106) and bottom stationary flux returns (107) are preferably made of silicon steel to provide high permeability and low hysteresis. These components may also be composed of thin laminations to reduce eddy currents. The top spring (108) and bottom spring (109) and the shaft (112) are preferably made of nonferrous material to prevent a flux path bypassing gap (113) and gap (114). Linear bearings (110 and 111) are preferably low friction linear ball bearings to prevent inertial force distortion due to friction. Drive coil (103) is preferably wound from insulated copper wire manufactured for coils known as magnet wire.
The magnetic flux passing across gap (113) and gap (114) is called the bias flux. The bias flux is in opposite directions across gap (113) and gap (114). For example, if the bias flux is upward between outer flux cylinder (104) and top flux return (106) then it is downward between outer flux cylinder (104) and bottom flux return (107). For this example, the bias flux is downward between inner flux cylinder (105) and top flux return (106) and upward between inner flux cylinder (105) and bottom flux return (107).
The magnetic flux that does not pass across gap (113) and gap (114) is called magnet leakage flux. Magnet leakage flux passes through outer flux cylinder (104), coil (103), and inner flux cylinder (105) to complete a flux loop back to permanent magnet rings (101) and (102). Minimizing magnet leakage flux reduces the magnetic material required and therefore the cost. The preferred embodiment inherently has low leakage flux because flux lines from the top magnet ring (101) and bottom magnet rings (102) repel each other within coil (103) allowing each magnet ring only half of the coil for leakage flux loops. Making the radial extent of magnet rings (101) and (102) as large as practical further reduces the magnet flux leakage.
The drive coil magnetic flux passing across gap (113) and gap (114) is in the same direction. That is, if the bias flux is upward between outer flux cylinder (104) and top flux return (106) then it is also upward between outer flux cylinder (104) and bottom flux return (107). For this example, the bias flux is downward between inner flux cylinder (105) and top flux return (106) and also downward between inner flux cylinder (105) and bottom flux return (107).
Radially polarized magnet ring (201) is preferably composed of a radial array of high strength magnet segments such as Neodymium Iron Boron (NdFeB) magnets. Inner flux cylinder (204) and outer flux cylinder (205) and top stationary flux return (206) and bottom stationary flux return (207) are preferably made of silicon steel to provide high permeability and low hysteresis. These components may be composed of thin laminations to reduce eddy currents. Spring (212), spring (213), and shaft (214) are preferable made of nonferrous material to prevent a flux path bypassing gap (208) and gap (209). Linear bearing (210) and linear bearing (211) are preferably low friction linear ball bearings to prevent inertial force distortion due to friction. Drive coil (202) and drive coil (203) are preferably wound from insulated copper wire manufactured for coils known as magnet wire.
The magnetic flux passing across gap (208) and gap (209) is called the bias flux. The bias flux is in opposite directions across gap (208) and gap (209). That is, if the bias flux is upward between outer flux cylinder (205) and top stationary flux return (206) then it is downward between outer flux cylinder (205) and bottom stationary flux return (207). For this example, the bias flux is downward between inner flux cylinder (204) and top stationary flux return (206) and upward between inner flux cylinder (204) and bottom flux return (203).
The magnetic flux that does not pass across gap (208) and gap (209) is called magnet leakage flux. Magnet leakage flux lines (900) pass from the permanent magnet ring through outer flux cylinder (205), upper drive coil (202), lower drive coil (203), and inner flux cylinder (204) to complete a flux loop back to magnet ring (201). Minimizing magnet leakage flux reduces the magnetic material required and therefore the cost. Magnet leakage flux is minimized by making the upper drive coil (202) and lower drive coil (203) as thin as practical and the radial extent of the magnet ring (201) as large as practical.
Coil flux leakage flux lines (1000) pass from inner flux cylinder (204) to the outer flux cylinder (205) and back to inner flux cylinder (204) to complete a flux loop without crossing gap (208) and gap (209). Minimizing coil leakage flux increases efficiency and reduces inductance. Coil leakage flux is minimized by making drive coils (202) and drive coil (203) as thin as practical and their radial extent as large as practical.
Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is the intent of this application to cover, in the appended claims, all such modification and equivalents. The entire disclosure and all references, applications, patents and publications cited above are hereby incorporated by reference.
This patent application is a divisional of U.S. patent application Ser. No. 15/675,901, filed on 14 Aug. 2017. The entire contents of this application are incorporated herein by reference.
The invention described herein may be manufactured and used by, or for the Government of the United States of America, for governmental purposes without payment of any royalties thereon or therefore.
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| Entry |
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| See attached Form SB08a, listing references in parent U.S. Appl. No. 15/675,901. |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15675901 | Aug 2017 | US |
| Child | 17060236 | US |
