Device and method for using a magnetic clutch in BLDC motors

Abstract

An apparatus for coupling mechanical power between the rotor of a Brushless DC Motor and an external mechanical load comprises: a) two concentric rings; b) an equal number of magnets connected to the inner ring and to the outer ring; and c) an opposite orientation of the poles of each couple of facing magnets, wherein one magnet is placed on the inner ring, and its facing magnet is placed on the outer ring; wherein the first of said two concentric rings is rotatable around an axis by the application of a force not applied by the second ring, and wherein when said first concentric ring rotates, the second ring rotates as well by the action of magnetic forces.

Claims

1. A brushless motor comprising a) two concentric rings consisting of an inner ring constituting a rotor of a brushless DC motor and an outer ring constituting a clutch, each of said two rings having an inner edge and an outer edge; b) an equal number of circumferentially spaced magnets connected to the inner ring and to the outer ring, such that each couple of facing magnets are of opposite magnetic polarity, wherein one magnet is connected to the inner ring, its facing magnet is connected to the outer ring and magnetically coupled to said one magnet and the distance between the facing magnets of each couple is no less than 18 mm and no greater than 35 mm; c) a plurality of circumferentially spaced and stationary air-core solenoids constituting a stator of said brushless DC motor, each of said air-core solenoids encircling both said inner edge and said outer edge of the inner ring so that, through an interior of each of said air-core solenoids, the magnets of the inner ring can pass; and d) an actuator for energizing said air-core solenoids to generate an electromagnetic field that produces a torque on a magnetic field of the magnet passing through their interior to rotate the inner ring, as well as the outer ring, by means of said magnetically coupled magnets.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 shows two concentric rings, provided with magnets, according to one embodiment of the invention, in a static state;

(3) FIG. 2 shows the two rings of FIG. 1 in a dynamic state;

(4) FIG. 3 shows the measurements of the force on a single couple of magnets mounted at distance d from each other and shifted linearly;

(5) FIG. 4 shows the measurements of the force in a demo system, according to another embodiment of the invention;

(6) FIG. 5 shows exemplary physical measures of the components in a BLDC demo system, according to another embodiment of the invention;

(7) FIG. 6 shows a schematic setup of two magnets, according to another embodiment of the invention;

(8) FIG. 7 shows solenoids illustrated as consisting of a collection of infinitesimal current loops, stacked one on top of the other; and

(9) FIG. 8 shows two loops of infinitesimal thickness, each one belonging to a magnet.

DETAILED DESCRIPTION OF THE INVENTION

(10) FIG. 1 shows two concentric rotating rings 101 and 102 at rest. The inner ring 101 consists of the rotor of a BLDC motor (which can be, for example, the motor of PCT/IL2013/050253-WO/2013/140400), and the outer ring 102 is connected to a mechanical load and provides the power for it. A number of permanent magnets, equal to the number of the magnets in the rotor of the BLDC motor, are mechanically fixed on the outer ring 102 with their S-N axes oriented tangentially to the circumference.

(11) At rest, each one of the magnets 104 located on the outer ring 102, is facing the corresponding magnet 103 located on the rotor 101. The S-N axis orientation of each magnet 104 on the outer ring 102 is opposite to the S-N axis orientation of the corresponding (facing) magnet 103 on the rotor 101. As a result, the magnets 104 on the outer ring 102 are positioned with alternating polarity. Each pair of magnets 104 on outer ring 102 may be equidistantly spaced from one another by a circumferential distance of F. Alternatively, one pair of magnets 104 on outer ring 102 may be spaced from one another by a circumferential distance of F, while another pair of magnets 104 on the same may be spaced from one another by a circumferential distance of G which may be greater than F. It should be emphasized that there is no physical connection between the rotor 101 and the outer ring 102. For reasons that will be thoroughly explained later on in this description, based on the laws of magnetostatics, the relative position of the rotor 101 with respect to the outer ring 102, depends on the state of the systemif the system is in a static state or a dynamic state, as will be further described.

(12) In a static statewhen the BLCD rotor is at rest, each magnet 104 on the outer ring 102 is exactly aligned in front of the corresponding magnet 103 on the rotor 101, as shown in FIG. 1. In a dynamic statewhen the BLCD rotor 101 turns, while the outer ring 102 is connected to a load (not completely free to move), the relative position of each magnet 103 on the rotor ring 101 with respect to the corresponding magnet 104 on the load ring 102, will change and will stabilize to a new state.

(13) The corresponding magnets 103 and 104 will no longer be perfectly aligned. The relative position of the magnets will shift in a quasi-linear fashion tangentially to the circumference of the rings 101 and 102. The magnets 103 and 104 will reach an offset h as shown in FIG. 2, and will stabilize there. The offset h will depend on the opposing force exercised by the load. It will be seen that under proper conditions h will increase directly proportionally to the force needed to make the load ring 102 rotate along with the rotor ring 101.

(14) It will be presented that in the range of interest, the offset h is roughly directly proportional to the force transfer, and as long as h is not too large, the rotor ring 101 will be able to pull along the load ring 102, without the occurrence of any physical contact between the two ring 101 and 102. When the size of h approaches the width of the gap between the magnets 103 and 104, the force transferred drops. The maximal force that the rotor ring 101 will be able to apply to the load ring 102 will depend on the strength and on the geometry of the permanent magnets, on the number of magnets, as well as on the gap between the two rings 101 and 102.

(15) FIG. 3 shows the measurements of the force on a single couple of magnets mounted at distance d from each other and shifted linearly. The shaded area 301 shows the range for which the pulling force between the magnets 103 and 104 is roughly proportional to the offset h.

(16) To illustrate the order of magnitude of the forces involved, two magnets with front-to-front separation of 29 mm, can provide roughly a maximal force transfer of 140N (about 14 Kg) in direction tangential to the ring.

(17) In the BLDC motor demo system built according to one embodiment of the present invention, there are 8 magnets were provided with face-to-face separation of about 30 mm. The demo system is capable to apply a force of 1408=1120N (about 112 Kg). Since the outer ring 102 in the demo system has a radius of about 420 mm, the magnetic clutch should be able to transfer a torque of about 470N-m.

(18) In a measurement carried out on the BLDC demo system, and as shown in FIG. 4, the inventors did not try to achieve and measure the maximal power transfer, however, they showed force transfer measurements of the order of 600N, which is in good agreement with the order of magnitude of the maximal possible force (1120N) predicted by the measurements on one couple of magnets. Also it shows that the total force is proportional to the relative offset.

(19) The physical measures of the components in the BLDC demo system, according to some embodiments as provided by the inventors, are shown in FIG. 5. From the figure one can see that the system includes 8 magnets, and the separation between the rotor ring 101 and the load ring 102 is 30 mm. In some embodiments, the separation distance may be greater than 18 mm. In some embodiments, the separation distance may be less than 35 mm.

(20) A plurality of circumferentially spaced and stationary air-core solenoids 132 constituting a stator of the Brushless DC Motor 100 encircle both the inner edge 107 and the outer edge 109 of inner ring 101. When inner ring 101 is rotated, the circumferentially spaced permanent magnets 103 fixed to inner ring 101 are allowed to pass through the interior 134 of each air-core solenoid 132. The rotation of inner ring 101 is made possible by the operations of schematically illustrated actuator 124, which may be embodied by a plurality of switches controlled by a microcontroller for determining, at each instant, the polarity and magnitude of voltage applied to each solenoid, as well as the average DC level.

(21) During operation of actuator 124, the air-core solenoids 132 become energized and generate an electromagnetic field that produces a torque on a magnetic field of the magnet 103 passing through a given solenoid interior 134, causing inner ring 101 to rotate around its axis 108. As inner ring 101 rotates, outer ring 102 rotates as well under the influence of magnetic forces between each couple of magnetically facing magnets 102 and 103 of opposite magnetic polarity. Connecting means may connect the rotating outer ring 102 to a load 126. The connecting means may be, for example, a plurality of circumferentially spaced linear elements 137 that are connected to outer ring 102 and radially extend therefrom to a mechanical load 126 which is located at center of the inner 101 and outer rings 102.

(22) Magnetostatic computations are among the most difficult and complex tasks to be carried out analytically, and even when a closed-form analytical expression can be found, the resulting formulas are often too complex to provide a clear understanding of the phenomena. Moreover, most often, one can only perform computerized simulations obtained by numerically solving the field equations. Numerical solutions, however, although precise for a specific setup, do not provide an insight to the general behavior of the system.

(23) Fortunately, in the specific case under consideration, general conclusions can be drawn by means of a relatively simple mathematical analysis. This is made possible because, in the system under consideration, the magnets are free to move only along a direction tangential to their S-N axis, and they are fixed in all other directions. Therefore, it is only needed to compute the component of the force in a direction parallel to the S-N axes of the magnets, which results in major mathematical simplifications that allow us to draw conclusion regarding general system features, without the need of actually solving the complex three-dimensional integrals involved.

(24) What was analyzed is the setup shown in FIG. 6. {circumflex over (x)}, and {circumflex over (z)} are mutually perpendicular unit vectors. Two cubic magnets 601 and 602 are positioned so that their S-N axes are parallel to direction {circumflex over (z)}. Their S-N orientation is opposite, and they are displaced with an offset h in direction {circumflex over (z)}. The magnets 601 and 602 are assumed cubic, for the purpose of this exemplary analysis, however the general conclusions hold true for other shapes as well. The measurements shown in FIG. 3 have been carried out on a similar setup.

(25) Under this setup, as long as the offset h is small relatively to the physical dimension of the gap between the magnets 601 and 602, the component of the force acting on either magnet 601 and 602 in the direction {circumflex over (z)}, is directly proportional to the offset h. The size of h is relatively small, roughly when the offset h is less than of the separation d between the magnets 601 and 602. As the offset becomes larger than that, the force reaches a maximal value, and then decreases with increasing h.

(26) As a first step, by using the Amperian model, a permanent magnet with magnetization M in direction {circumflex over (z)}, may be modeled in the form of a uniform surface current density , flowing on the surface of the magnet in direction perpendicular to {circumflex over (z)}. is the net magnetic dipole moment per unit volume, and is the equivalent surface current per unit length. Therefore we may replace each magnet 601 and 601 in FIG. 6 by the equivalent solenoids shown in FIG. 7, with equal currents in opposite directions.

(27) Each solenoid 701 in FIG. 7 can be represented as consisting of a collection of infinitesimal current loops, stacked one on top of the other, carrying currents of amplitudes dl=J.sub.sdz and dl=J.sub.sdz, flowing in the plane in {circumflex over (x)} plane in opposite directions. Let us consider now, two loops of infinitesimal thickness, each one belonging to one of the magnets as shown in FIG. 8.

(28) The force caused on the left-side loop L located at vertical position z the right-side loop L located at vertical position z, is directly derived from Ampere's law of force, and is given by the expression:

(29) ${\stackrel{.fwdarw.}{F}}_{p^{}p}\left(z,z^{}\right)=-\frac{{\mathrm{dI}}^{}\mathrm{dI}}{4}{}_L{}_{L^{}}\left(d\hat{}.Math.d{\hat{}}^{}\right)\frac{{\hat{r}}_{p^{}p}}{{.Math.\hat{r}-{\hat{r}}^{}.Math.}^2}=-\frac{{\mathrm{dI}}^{}\mathrm{dI}}{4}{}_L{}_{L^{}}\left(d\hat{}.Math.d\hat{}\right)\frac{\hat{r}-{\hat{r}}^{}}{{.Math.\hat{r}-{\hat{r}}^{}.Math.}^3}$$\mathrm{where}{\hat{r}}_{p^{}p}=\frac{\hat{r}-{\hat{r}}^{}}{.Math.r-{\hat{r}}^{}.Math.},\hat{r}-{\hat{r}}^{}=\left(x-x^{}\right)\hat{x}+\left(y-y^{}\right)\hat{y}+\left(z-z^{}\right)\hat{z},.Math.\hat{r}-{\hat{r}}^{}.Math.=\sqrt{{\left(x-x^{}\right)}^2+{\left(y-y^{}\right)}^2+{\left(z-z^{}\right)}^2}$
And d{circumflex over (l)} and d{circumflex over (l)} are infinitesimal lengths in the direction of the current flow in the corresponding loops, and therefore they lie in the {circumflex over (x)} plane.

(30) Now, referring to FIG. 8, it points out several preliminary remarks:

(31) 1. We know that |yy|d and we denote R.sub.{circumflex over (x)}{square root over ((xx).sup.2+(yy).sup.2)}. It follows that R.sub.{circumflex over (x)}d. R.sub.{circumflex over (x)}(x,x,y,y) is independent from z and z, and we may write |{circumflex over (r)}{circumflex over (r)}|=R.sub.{circumflex over (x)}.sup.2+(zz).sup.2.

(32) 2. In the present setting, d is comparable to the size of the magnet, and we assume offsets small enough so that .sup.2<<d.sup.2

(33) $\left(\mathrm{for}\mathrm{instance}<\frac{d}{3}\right).$

(34) 3. Since we are interested only in the force in the {circumflex over (z)} direction, the only relevant component of {circumflex over (r)}-{circumflex over (r)} in the numerator of the integrand, is the one in direction {circumflex over (z)}. All other forces are of no interest, since the magnets cannot move in other directions. Thus, in order to compute the force acting on the magnets in {circumflex over (z)} direction, we may replace {circumflex over (r)}-{circumflex over (r)} in the numerator of the integrand by (zz){circumflex over (z)}.

(35) 4. d{circumflex over (l)} and d{circumflex over (l)} are incremental vectors in the {circumflex over (x)} plane. More precisely, in the present setting of square magnets, the scalar product (d{circumflex over (l)}.Math.d{circumflex over (l)}) is either dxdx or dydy. Therefore z and z are constant with respect to the integration variables when integrating over the path of the loops. Moreover, if dx, dx have opposite signs, their direction of integration is opposite too, and therefore, the limit of the corresponding integrals are reversed, and similarly for dy, dy. The outcome is that the sign of the integral for all the various sub-integration ranges defined by (d{circumflex over (l)}.Math.d{circumflex over (l)}) remains unchanged. Therefore the sign value of the double integral over the loop paths, is the same as the sign of the integrand.

(36) With the above understanding, the force F.sub.z in the direction {circumflex over (z)} acting on the current loop L because of the current loop L, is the result of the following integral:

(37) $F_{\hat{z}}=\frac{J_s^2{\mathrm{dz}}^{}\mathrm{dz}}{4}{}_L{}_{L^{}}\frac{\left(d\hat{}.Math.d{\hat{}}^{}\right)\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}},R_{\hat{x}\hat{y}}\sqrt{{\left({\mathrm{xx}}^{}\right)}^2.Math.{\left({\mathrm{yy}}^{}\right)}^2},{\mathrm{dI}}^{}{J}_{}{\mathrm{dz}}^{}.Math.{\mathrm{dIJ}}_{}\mathrm{dz}$

(38) The cumulative force F.sub.{circumflex over (z)},L applied by all the current loops on the right side on one single current loop L on the left side (see FIG. 8) is given by:

(39) $F_{\hat{z},L}={}_h^{h+a}{F}_{\hat{z}}{\mathrm{dz}}^{}=-\frac{J_s^2\mathrm{dz}}{4}{}_h^{h+a}\left({}_L{}_{L^{}}\frac{\left(d\hat{}.Math.d{\hat{}}^{}\right)\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}}\right){\mathrm{dz}}^{}$

(40) The total force F.sub.{circumflex over (z)}(h) acting on the magnet located at the origin is the sum of all the forces on its loops:

(41) $F_{\hat{z}}\left(h\right)={}_0^a{F}_{\hat{z},L}\mathrm{dz}=-\frac{J_s^2}{4}{}_0^a\left[{}_h^{h+a}\left({}_L{}_{L^{}}\frac{\left(d\hat{}.Math.d{\hat{}}^{}\right)\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}}\right){\mathrm{dz}}^{}\right]\mathrm{dz}$

(42) Changing the order of integration we obtain:

(43) $F_{\hat{z}}\left(h\right)=-\frac{J_s^2}{4}{}_L{}_{L^{}}\left[{}_0^a\left({}_h^{h+a}\frac{\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}}{\mathrm{dz}}^{}\right)\mathrm{dz}\right]\left(d\hat{}.Math.d{\hat{}}^{}\right)$
Noting that R.sub.{circumflex over (x)}.sup.2 is independent from z and z, and therefore is constant when integrating with respect to dz and dz, the inner integrals can be computed analytically, and yield:

(44) ${}_0^a\left({}_h^{h+a}\frac{\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}}{\mathrm{dz}}^{}\right)\mathrm{dz}=\ln \left\{\frac{\left[\left(\mathrm{.Math.}-A\right)+\sqrt{1-{\left(\mathrm{.Math.}-A\right)}^2}\right]\left[\left(\mathrm{.Math.}-A\right)+\sqrt{1-{\left(\mathrm{.Math.}-A\right)}^2}\right]}{{\left(\mathrm{.Math.}+\sqrt{1+\mathrm{.Math.}}\right)}^2}\right\}$
Where we used

(45) $A=\frac{a}{R_{\hat{x}\hat{y}}},\mathrm{and}\mathrm{.Math.}=\frac{h}{R_{\hat{x}\hat{y}}}.$

(46) Since R.sub.{circumflex over (x)}d, then if h.sup.2<<d.sup.2R.sub.{circumflex over (x)}.sup.2

(47) $\left(\mathrm{for}\mathrm{instance}h<\frac{d}{3}\right)\mathrm{then}\frac{h^2}{R_{\hat{x}\hat{y}}^2}={\mathrm{.Math.}}^{2}1,$
and we may expand the last expression in a first-order Taylor series as follows:

(48) 0${}_0^a\left({}_h^{h+a}\frac{\left(z-z^{}\right)}{{\left[R_{\hat{x}\hat{y}}^2+{\left(z-z^{}\right)}^2\right]}^{3/2}}{\mathrm{dz}}^{}\right)\mathrm{dz}=2\frac{1-\sqrt{1+A^2}}{\sqrt{1+A^2}}\mathrm{.Math.}+O\left({\mathrm{.Math.}}^3\right)2\frac{1-\sqrt{1+a^2/R_{\hat{x}\hat{y}}^2}}{\sqrt{1+a^2/R_{\hat{x}\hat{y}}^2}}.Math.\frac{h}{R_{\hat{x}\hat{y}}}=g\left(x,x^{},y,y^{}\right).Math.h$

(49) Since {square root over (1+a.sup.2/R.sub.{circumflex over (x)}.sup.2)}>1, it follows that the function g(x, x,y,y) is some negative function of (x, x, y, y), namely g(x, x,y,y)=|g (x, x,y,y). Therefore, recalling that the sign of the double integral over x, x, y, y is the same as the sign of the integrand, and setting

(50) ${}_L{}_{L^{}}.Math.g\left(x,x^{},y,y^{}\right).Math.\left(d\hat{}d{\hat{}}^{}\right)=\frac{K^2{J}_s^2}{4}h,h^2{d}^2.$
Where K is some proportionality constant. Finally, recalling that M=J.sub.s is the net magnetization per unit volume in the {circumflex over (z)} direction, and referring to FIG. 6, the force acting on the left magnet is:

(51) $F_{\hat{z}}\left(h\right)=\frac{K^2{M}^2}{4}h,h^2{d}^2$

(52) Thus, for any offset h<d/3, the force transferred by the clutch is directly proportional to the offset h and to the square magnetization per unit volume. Moreover, the force is in direction of the offset itself.

(53) All the above description has been provided for the purpose of illustration and is not meant to limit the invention in any way. The computations shown above are provided as an aid in understanding the invention, and should not be construed as intending to limit the invention in any way.