Patent Application
20180130586
The invention relates to devices and systems for deflecting laser beams.
DE 27 18 532 A1 discloses a device for deflecting a laser beam. In the device disclosed in DE 27 18 532 A1, a permanent magnet, viewed in a direction perpendicular to a pivot axis of a mirror and to a winding axis, is arranged centrally in relation to a coil, and a height of a cross section of the permanent magnet is approximately one-third the height of the coil cross section.
Furthermore, a device for deflecting a laser beam based on MEMS technology comprising electromagnetic actuators is disclosed, for example, from Ataman et al., J. Micromech. Microeng., 23 (2013) 025002 (13 pages) or from T. Iseki et al., OPTICAL REVIEW, Vol. 13, No. 4 (2006) 189-194.
Devices for deflecting a laser beam, in particular in arrangements for laser material processing, generally offer a high dynamic response with respect to angular velocity and angular acceleration and also a high accuracy with respect to angle setting and reproducibility. At the same time, the laser beams used are continuously becoming higher power and the installation spaces are gradually becoming smaller. Scanners based on MEMS (micro-electromechanical systems) and in particular devices that are based on electromagnetic actuators offer a technology of interest here.
Ataman et al. discloses a MEMS scanner, in which the coils of the actuator are designed as flat coils.
Iseki et al. discloses a gimbal-mounted deflection mirror, the electromagnetic actuator of which is based on the interaction of a permanent magnet with coil arrangements.
However, in the deflection devices disclosed by the aforementioned references, the achievable angular accelerations and angular velocities are not sufficiently high to achieve cost-effective processing speeds in applications for laser processing. This is true in particular if the processing requires not only a constant angular velocity of the mirror, but rather a rapid change between various angular velocities. The setting and repetition accuracy of the laser beam deflection decisively determines the quality of the processing result and therefore the process reliability of the processing process. The known concepts also do not meet the requirements for industrial use here.
The present disclosure relates to devices and systems for deflecting laser beams configured such that the dynamic response is increased with respect to angular velocity and angular acceleration and also the accuracy is improved with respect to angle setting and reproducibility.
In one aspect, the devices include a mirror, a frame, in which the mirror is pivotably mounted, a permanent magnet rigidly connected to the mirror and the north and south poles of which define a magnet axis, and at least one coil for exerting a magnetic deflection force on the permanent magnet. The permanent magnet is arranged offset in the direction toward the mirror in relation to a central position with respect to the coil. The at least one coil has a winding axis around which the coil turns are wound. The at least one coil is rigidly connected to the frame and the permanent magnet is arranged in the magnetic field of the coil. The magnet axis and the winding axis of the coil do not extend in parallel, viewed in the direction perpendicular to the pivot axis of the mirror and to the winding axis. The height of the cross section of the permanent magnet is less than or equal to twice the height of the coil cross section and the cross sections of permanent magnet and coil at least partially overlap.
As current flows through the coil mentioned at the outset, it generates a magnetic field {right arrow over (B)}, which has a region of high field strength close to the end face, in which the field lines are oriented parallel to the coil axis. At a greater distance from the coil end face, the field lines change the direction thereof and the field strength sinks. The following equations apply for the torque {right arrow over (T)}, which engages on a magnet having the magnetic moment {right arrow over (m)} in a homogeneous magnetic field:
{right arrow over (T)}={right arrow over (m)}×{right arrow over (B)} and |{right arrow over (T)}|=|{right arrow over (m)}×{right arrow over (B)}|=|{right arrow over (m)}∥{right arrow over (B)}| sin θ,
wherein θ denotes the angle between the two vectors. The permanent magnet may be arranged in the region of high field strength, so that the cross sections of the coil and the permanent magnet at least partially overlap. It is to be ensured in this case that the magnet axis and the winding axis of the coil do not extend parallel to one another. Accordingly, {right arrow over (m)} and {right arrow over (B)} are also not parallel and sin θ≠0.
Permanent magnets consist, for example, of SmCo or NdFeB. Such materials have a high density. To keep the moment of inertia of the moving parts of the device low, the volume of the permanent magnet is advantageously concentrated on the region in which the magnetic moment of the permanent magnet can interact with a significant external magnetic field. This is achieved in that the height of the cross section of the permanent magnet is limited to twice the height of the coil cross section, and the permanent magnet is arranged in the magnetic field of the coil so that the cross sections of the coil and the permanent magnet at least partially overlap. A high force and/or a high torque of the actuation can contribute to a high dynamic response of the deflection device, according to implementations of the invention.
According to certain implementations of the invention, the permanent magnet is arranged offset in relation to a central position with respect to the coil. The torque engaging on the permanent magnet is thus reduced, since a lower external magnetic field, because it is inhomogeneous, acts on the permanent magnet in its position offset in the direction of the mirror. However, a force component engaging in addition to the torque on the permanent magnet arises due to the magnetic field, which is now inhomogeneous. According to particular implementations of the invention, the permanent magnet is arranged offset in the direction toward the mirror. The moment of inertia is thus reduced in relation to off-center positioning in the opposite direction. In total, an increase of the torque acting at the rotational axis on the mirror results, so that higher angular velocities and angular accelerations are achievable.
In certain implementations, the magnet axis and the winding axis intersect and enclose an angle not equal to zero with one another. In this case, the dynamic response plays out in the plane spanned by the magnetic moment and the coil axis, and the deflection device according to the invention therefore has positive properties for the actuation. The angle not equal to zero may be 90°. In implementations where the angle is 90°, the absolute value of the torque that acts on the magnet when current flows through the coil is maximal.
The height of the cross section of the permanent magnet is between 0.3 times and 0.6 times the height of the coil cross section, in particular implementations. This configuration enables the greatest possible overlap of the cross sections of the permanent magnet and the at least one coil.
In certain implementations, the height of the cross section of the permanent magnet is less than the height of the coil cross section. This enables an optimized positioning of the permanent magnet in the region of high field strength, without the overlap of the cross sections of the coil and the permanent magnet decreasing.
The overlap of the cross sections of permanent magnet and coil influences the dimension of the torque that acts on the permanent magnet in the magnetic field of the coil. The height overlap of the cross sections of the coil and the permanent magnet is therefore greater than or equal to 50% of the lesser of the two heights, in certain implementations. The height overlap of the cross sections of the coil and the permanent magnet is equal to 100% of the lesser of the two heights, in particular implementations.
It is apparent from the preceding statements that a suitable positioning of the permanent magnet in the magnetic field of the coil plays an important role for the dimension of the torque that can be generated and therefore for the achievable angular acceleration and angular velocity in the deflection of a laser beam. It is also apparent that the effects on the inertial mass of the moving parts of the device have to be taken into consideration in the selection of the dimensions of the permanent magnet. In certain implementations, a spacer made of nonferromagnetic material, via the length of which the spacing between the center point of the permanent magnet and the winding axis of the at least one coil can be defined, is arranged between the permanent magnet and the connecting element, by which the mirror is pivotably mounted in the frame.
The device comprises multiple coils each having a winding axis, in particular implementations. The winding axes of the coils can be non-collinear, for example, so that forces can act from various directions on the permanent magnet and the mirror is pivotable in more than one direction. In certain implementations, at least two coils having the same winding direction are provided on a winding axis and the permanent magnet is arranged between the at least two coils. In this case, both attractive and also repulsive forces can contribute to the torque. In particular embodiments, the device comprises two winding axes having coils, which are perpendicular in relation to one another and to the magnet axis. Such embodiments are particularly advantageous with respect to a simple actuation during a deflection of the mirror about various pivot axes.
The force that a current-conducting coil can exert on a permanent magnet in its magnetic field increases with the inductance L of the coil. The inductance of a coil may be increased by introducing a core of a ferromagnetic material into the coil. If a voltage is applied to a coil, the current in the coil first results gradually because of an opposing induction voltage, wherein the time constant increases with the inductance of the coil. As a result, a higher inductance of the at least one coil results in a more sluggish reaction of the device to the deflection of a laser beam upon a change of the specification signal and therefore has a negative effect on the achievable deflection velocity and angular acceleration. An increase of the inductance via the introduction of a ferromagnetic coil core furthermore results in additional losses due to repeated re-magnetization of the coil core in the temporally changeable field of the moving permanent magnet. Furthermore, ferromagnetic coil cores display a nonlinear response to a change of the current strength in the coil up to hysteresis. The accuracy of the angle setting and the repetition accuracy of the angle setting in the device for beam deflection is thus reduced. In particular embodiments of the device, by way of the dimensioning and arrangement of permanent magnet and the at least one coil, the torques are sufficiently large and the moments of inertia are sufficiently small that a coil core made of nonferromagnetic material is provided for the at least one coil, and the inductance of the at least one coil is sufficiently small that sufficient accuracies are achieved by an actuation using a simple, for example, linear, regulation.
Unavoidable power losses during the operation of the deflection device arise, for example, because of the ohmic resistance of the at least one coil and, without further measures, result in an at least local temperature increase. Such a temperature increase is undesirable in the sense of accurate and reproducible deflection of the mirror. A discharge of occurring power loss to the frame, for example, is therefore advantageous. The at least one coil may be wound around a coil core, which can consist of a copper alloy, an aluminum alloy, aluminum oxide, or aluminum nitride, in accordance with certain implementations. These materials have very good heat conduction properties. Because the magnet axis and winding axis are not parallel, the occurrence of eddy currents in electrically conductive coil cores is substantially reduced, since the magnetic field strength acting on the coil core is substantially lower than in the case of parallel magnetic field and winding axes. Losses in electrically conductive coil cores are thus reduced and the actuation capability of the deflection device is improved, since the induction of eddy currents acts like additional damping. Coil cores made of aluminum oxide and aluminum nitride are advantageous in particular, since no losses due to eddy currents are induced therein.
To counteract a deformation of the mirror in the event of a temperature increase, the mirror can be provided on both sides with a coating, e.g., of equal thickness. The coating can be a metallic-dielectric hybrid coating, in certain implementations.
The permanent magnet is spaced apart closer to the mirror than the coil in particular implementations and may overlap on 50% to 100% of its height with the cross section of the coil.
In another aspect, the present disclosure provides deflection device systems. The deflection device systems include a deflection device, a controller for actuating the deflection device, and a sensor for measuring the pivot angle of the mirror and for generating an input signal for the controller. The deflection device includes a mirror, a frame, in which the mirror is pivotably mounted, a permanent magnet that is rigidly connected to the mirror and the north and south poles of which define a magnet axis, and at least one coil for exerting a magnetic deflection force on the permanent magnet. The permanent magnet is arranged offset in the direction toward the mirror in relation to a central position with respect to the coil. The at least one coil has a winding axis around which the coil turns are wound. The at least one coil is rigidly connected to the frame and the permanent magnet is arranged in the magnetic field of the coil, wherein the magnet axis and the winding axis of the coil do not extend in parallel, wherein, viewed in the direction perpendicular to the pivot axis of the mirror and to the winding axis, the height of the cross section of the permanent magnet is less than or equal to twice the height of the coil cross section and the cross sections of the permanent magnet and the coil at least partially overlap. The controller sets the current that flows through the at least one coil of the device via an output signal. The output signal is determined using a setpoint value and a transfer function of the system.
In certain embodiments, the device displays a characteristic curve that is linear in a good approximation, i.e., the pivot angle of the mirror and the value of the specification signal are linearly dependent on one another in a good approximation. The requirements of modern laser processing can then be fulfilled by the arrangement according to embodiments of the invention using a simple regulation, which is based on the principles of linear system theory.
Further advantages and advantageous embodiments of the subject matter of the invention result from the description, the claims, and the drawings. The above-mentioned features and the features to be set forth hereafter can also each be used per se or together in arbitrary combinations. The embodiments shown and described are not to be understood as an exhaustive list, but rather have exemplary character for the description of the invention.
Exemplary embodiments of the invention are schematically illustrated in the drawing and will be explained in greater detail hereafter with reference to the figures of the drawing.
In the following description of the drawings, identical reference signs are used for identical or functionally identical components.
A permanent magnet 8 is connected via a spacer 7, for example, made of a nonferromagnetic material, to the lower side of the connecting element 5 and is thus also rigidly connected to the mirror 2 arranged on the connecting element 5. The permanent magnet 8 is designed as a cylinder having a north pole and a south pole, wherein these magnetic poles are arranged on the cylinder upper side and lower side and define a magnet axis 9, which is coincident with the cylinder axis and/or the axis of symmetry of the permanent magnet 8. The permanent magnet can also have other geometries.
The permanent magnet 8 is arranged between two coils 10, 10′, which are each rigidly connected to the frame 6 via a coil core 11 (for example, made of a nonferromagnetic material) and have a common winding axis 12, around which the coil turns are wound. They have the same winding direction with respect to a current flow in the coils 10, 10′. The magnet axis 9 and the winding axis 12 do not extend in parallel, but rather intersect at an angle not equal to zero, namely at an angle of 90° in
Heat loss, for example, because of the ohmic resistance of the coils 10, 10′, is dissipated via the contact of the coil cores 11 to the frame 6. Coil cores 11 made of material having a high thermal conductivity, for example, aluminum alloys, aluminum oxide, or aluminum nitride, are particularly suitable for this purpose.
The permanent magnet 8 is arranged between two coil pairs 10, 10′ and 14, 14′, which are each rigidly connected to the frame 6 via a coil core 11 (for example, made of a nonferromagnetic material). Each coil pair has a common winding axis 12, 13, about which the coil turns are wound. The coils of each coil pair have the same winding direction with respect to a current flow in a coil pair. The two winding axes 12, 13 are not collinear, but rather are each perpendicular to one another and to the magnet axis 9 of the permanent magnet 8. If current flows in the coils 10, 10′ and 14, 14′, the permanent magnet 8 is arranged in each case in the magnetic field generated by the coils and is thus deflected about the axes A, B against the restoring force of the connecting element 5. The deflection of the mirror rigidly connected to the permanent magnet 8 results accordingly from the superposition of the deflection about the axes A, B.
In the device 1 shown in
In the device 1 shown in
It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15175387.8 | Jul 2015 | EP | regional |
This application is a continuation of and claims priority under 35 U.S.C. § 120 from PCT Application No. PCT/EP2016/065255 filed on Jun. 30, 2016, which claims priority from European Application No. EP 15 175 387.8, filed on Jul. 6, 2015. The entire contents of each of these priority applications are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2016/065255 | Jun 2016 | US |
| Child | 15862766 | US |
