Patent Application
20080036309
An embodiment of the present invention will be described below with reference to the drawings.
A steerable mirror 10 is fixed to one end portion of a rotating shaft 20 with a mirror mount 11. The rotating shaft 20 is supported by a ball bearing 21 and a ball bearing 25 so that smooth rocking motions can be performed.
The ball bearing 21 and the ball bearing 25 are retained in a bearing housing 22 and a bearing housing 26 respectively. A waved washer 23 urges an outer ring of the ball bearing 21 to the right in
A cylindrical permanent magnet 30 has an inner diameter which is a little (several μm) larger than the outer diameter of the rotating shaft 20. The permanent magnet 30 is fixed with an adhesive or the like to a position which is coaxial with the rotating shaft 20 and which is predetermined in an axial direction of the rotating shaft 20. The permanent magnet 30 has three circumferential grooves 300, 301 and 302. The grooves will be described in detail later.
An encoder plate 60 having a slit (not shown) in its surface is fixed to the other end portion of the rotating shaft 20 through a hub 61. A sensor head 62 is disposed in opposition to the slit of the encoder plate 60. The sensor head 62 is retained on a sensor base 63. The sensor base 63 is fixed to the bearing housing 26. The encoder plate 60 and the sensor head 62 form a rotary encoder for feedback control of the angular displacement of the steerable mirror 10.
A collar 24 and a collar 31 which are shoulders of the inner race of the ball bearing 21 are disposed between the ball bearing 21 and the permanent magnet 30. A collar 27 and a collar 32 which are shoulders of the inner race of the ball bearing 25 are disposed between the ball bearing 25 and the permanent magnet 30.
A coil 33 and a yoke 34 are disposed in opposition to the permanent magnet 30 through an air gap 39 and coaxially with an axis O of the rotating shaft 20. In order to suppress an eddy current, the yoke 34 is composed of iron-based laminated disks having high magnetic permeability and soft magnetism and laid in the direction of the axis O. The outer diameter of the yoke 34 is a little (several μm) smaller than the inner diameter of a casing 41 (housing). The yoke 34 is retained in the casing 41 so that one end portion (right end portion in
The outer diameter of the yoke stop ring 35 is a little (several μm) smaller than the inner diameter of the casing 41. The yoke stop ring 35 is formed out of a highly thermal conductive material (aluminum-based or copper-based). A heat transfer bypass unit 51 formed out of a highly thermal conductive material (copper in this embodiment) is disposed between the casing 41 and the bearing housing 26 while a heat transfer bypass unit 50 formed out of a highly thermal conductive material is disposed between the bearing housing 22 and the yoke stop ring 35. Each heat transfer bypass unit 50, 51 has a sectionally T-shaped cylindrical shape. The outer diameter of its large-diameter portion is a little (several μm) smaller than the inner diameter of the casing 41, and the outer diameter of its small-diameter portion is a little (several μm) smaller than the inner diameter of the coil 33.
The thickness of the large-diameter portion of the heat transfer bypass unit 50 is a little larger than the distance obtained from subtracting the thickness of the yoke 34 and the thickness of the yoke stop ring 35 from the distance from the left end of the casing 41 to the left end of the flange 41a. The thickness of the large-diameter portion of the heat transfer bypass unit 51 is a little larger than the distance from the right end of the casing 41 to the right end of the flange 41a. That is, the sum of the thicknesses of the heat transfer bypass unit 50, the yoke stop ring 35, the yoke 34, the flange 41a and the heat transfer bypass unit 51 in the direction of the axis O is larger than the length of the casing 41 in the direction of the axis O. Accordingly, when the bearing housing 22 and the bearing housing 26 are fixed to the casing 41 by not-shown bolts, the bearing housing 22, the heat transfer bypass unit 50, the yoke stop ring 35, the yoke 34, the flange 41a, the heat transfer bypass unit 51 and the bearing housing 26 come into tight contact in the direction of the axis O. In addition, the rotating shaft 20 is positioned in the direction of the axis O by the collar 24 and the collar 31 disposed between the bearing housing 22 and the permanent magnet 30 and the collar 32 and the collar 27 disposed between the bearing housing 26 and the permanent magnet 30.
Highly thermal conductive materials (for example, grease) are applied thinly between the outer circumference of the yoke 34 and the inner circumference of the casing 41, between the coil 33 and each heat transfer bypass unit 50, 51, between the large-diameter portion of each heat transfer bypass unit 50, 51 and the casing 41, between the heat transfer bypass unit 50 and the yoke stop ring 35, between the outer circumference of the yoke stop ring 35 and the inner circumference of the casing 41, between the yoke 34 and the yoke stop ring 35 and between the yoke 34 and the flange 41a so as to suppress thermal resistance in contact surfaces among the respective constituent parts. In addition, a sleeve 64 and a cover 65 are disposed in an end portion of the bearing housing 26 in order to protect the rotary encoder from dust.
A cooling jacket 43 formed out of a highly thermal conductive material (copper in this embodiment) is removably disposed on the outer circumference of the casing 41 with a thermal conductive sheet material 42. The cooling jacket 43 is constituted by a cooling jacket 43R, a cooling jacket 43L and a hinge 44. The cooling jackets 43R and 43L are symmetrical with respect to the diameter of the cooling jacket 43. When the cooling jackets 43R and 43L are closed on the hinge 44, the cooling jacket 43 comes into tight contact with the outer circumference of the casing 41 with the thermal conductive sheet material 42. A flow channel shown by the broken line is formed in the cooling jacket 43. Cooling water supplied from a not-shown cooling water supply unit is introduced into the cooling jacket 43 from a duct 45 connected to both the cooling jackets 43R and 43L. After the cooling water cools the cooling jacket 43, the cooling water is discharged through a duct 46 connected to both the cooling jackets 43R and 43L.
As shown in
Next, the coil 33 will be described.
As shown in
Next, the heat transfer path will be described.
The yoke 34 is composed of an iron-based material which is low in thermal conductivity. In the present invention, however, the coil 33 is connected to the heat transfer bypass unit 50, the yoke stop ring 35 and the heat transfer bypass unit 51 which are high in thermal conductivity. The heat transfer bypass unit 50, the yoke stop ring 35 and the heat transfer bypass unit 51 are in contact with the casing 41. Accordingly, the Joule heat QC is transferred to the casing 41 rapidly, and removed to the outside of the actuator through the cooling water. That is, the path which can transfer the Joule heat of the coil 33 to the cooling means is composed of parallel branches, that is, a branch via the yoke 34 and a branch via the heat transfer bypass unit 51, the heat transfer bypass unit 50 and the yoke stop ring 35. Thus, the thermal resistance of the path becomes low. As a result, the temperature rise of the coil 33 is low. In this case, when the casing 41 is brought into contact with the bearing housing 22 and the bearing housing 26 in the direction of the axis O, the heat generated in the coil 33 flows to the casing 41 through the bearing housing 22 and the bearing housing 26. Thus, the temperature rise of the coil 33 can be made lower.
When the inner diameter of the flange 41a is made a little (several μm) larger than the outer diameter of the coil 33 and highly thermal conductive grease is applied between the both, the cooling efficiency can be more improved.
Next, the grooves 300, 301 and 302 provided in the permanent magnet 30 will be described.
The magnetic fluxes change inside the permanent magnet 30 due to an AC component of a current applied to the coil 33. Accordingly, an eddy current is generated in the permanent magnet 30 so that an eddy current loss QE appears as heat. It is believed that the eddy current loss QE appearing in the permanent magnet 30 escapes to the air around the rocking actuator through the rotating shaft 20. Thermal resistance 310 of the air gap 39 is so high that the heat of the eddy current loss QE cannot be expected to be cooled very well by the cooling jacket 43. For the eddy current loss QE, it is therefore necessary to reduce the eddy current itself.
The eddy current appears in a so-called skin. When the grooves are made deeper than the skin, the eddy current can be disconnected so that the loss can be reduced.
Depth d of the skin can be calculated by a calculation expression of skin depth known in the electromagnetism, as shown in Expression 1. When 6 designates the depth (distance) from the surface, the current density in the depth δ is attenuated to e(−δ/d) with respect to that in the surface.
skin depth d=√(2ρ/ωμ) (Expression 1)
Here, ρ and μ are material constants of the permanent magnet, that is, ρ designates the resistivity and μ designates the magnetic permeability. On the other hand, e designates the base of the natural logarithm and Ω designates the angular frequency of flux reversal.
More specific description will be made next.
When short-stroke positioning motions in one and the same direction are performed continuously, one cycle of the current waveform consists of maximum acceleration, maximum deceleration and stop in this order. When the current waveform is Fourier-expanded, the angular frequency of flux reversal can be expressed as ω=2π/Ts [rad/s] since the fundamental frequency component is expressed as the reciprocal 1/Ts [Hz] of the period Ts. According to the aforementioned calculation expression, the lower the angular frequency ω is, the larger the skin depth d is. It is rational to design the depth of each groove in conformity to the fundamental frequency component of the current.
For example, when a neodymium iron boron magnet is used as the permanent magnet 30:
ρ=1.5×10−6 [Ωm]; and
μ=1.4×10−6 [H/m]
Therefore, when the permanent magnet 30 is a neodymium iron boron magnet and the positioning responsiveness is 1/Ts=2 kHz, the skin depth d becomes 13 mm. In this case, the grooves 300-302 are designed to be as deep as or deeper than 13 mm. As for the width of each groove, it will go well if the width is 0.5 mm or smaller.
In this case, when grooves are provided intensively in a place where a plenty of an eddy current will flow easily, the eddy current can be reduced efficiently. That is, if the grooves 300, 301 and 302 were not provided in
The eddy current loss can be reduced as the number of grooves is increased. However, the torque constant is reduced in accordance with the width of each groove. In order to suppress the reduction of the torque constant, a magnet having a high electric resistance, such as a bonded magnet, may be charged into each groove.
When the thickness of the permanent magnet 30 is smaller than the skin depth d, it will go well if the reduction of the torsional rigidity of the permanent magnet 30 is suppressed not to affect the positioning of the steerable mirror (for example, the distance between the bottom of each groove and the rotating shaft 20 is set to be 10-30% of the thickness of the permanent magnet 30).
In place of the circumferential grooves 300, 301 and 302, grooves 331 and 332 may be provided in the direction of the axis O so that the loop of an eddy current 30A can be disconnected. In this case, as shown in
In this embodiment, the axial gaps among the respective constituent parts are set as very small distances. It is therefore possible to reduce the thermal resistance.
When the thermal conductivity of a member applied into the gaps is high, the axial gaps among the respective constituent parts may be increased to large values.
The outer diameter of the yoke 34 may be a little larger than the inner diameter of the casing 41. In this case, the yoke 34 is pressed into the casing 41.
A flow channel may be provided in the casing 41 so as to cool the casing 41 directly without externally connecting the cooling jacket 43 to the casing 41.
A medium to be supplied to the cooling jacket 43 is not limited to water, but gas may be used as the medium.
As described in the aforementioned BACKGROUND OF THE INVENTION, in a laser machining apparatus for laser drilling a printed circuit board in a manufacturing process thereof, a laser beam is positioned in a machining position by an optical scanner constituted by a rocking actuator including a steerable mirror serving as a load element and fixed to a rotating shaft thereof, and a servo controller for controlling the mirror so that the angle of the mirror can follow a command value. Accordingly, when a rocking actuator according to the present invention is used in such a laser machining apparatus, highly reliable machining can be performed without degrading machining throughput or hole position accuracy even if the steerable mirror is positioned by rapid and continuous motions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-216988 | Aug 2006 | JP | national |
