Position controlled electrodynamic linear motor

A linear drive for a miniaturized optical system, as used for example in an endoscope, includes a stator and an armature. The stator has a coil with two stator pole shoes arranged in axial direction, and two magnetic field sensors arranged at the outer side of the stator pole shoes. The armature has permanent magnets which are polarized in opposite directions, and a center armature pole shoe between the two permanent magnets, and an armature pole shoe at each side of the permanent magnet, opposite to the center armature pole shoe in axial direction. The magnetic field of the outer armature pole shoe goes completely or only in part, dependent from the armature position, through the magnetic field sensor and thus generates a position-dependent signal. This signal can be used for measuring and/or controlling the position of the armature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to pending German Application No. 102014107297.9 filed on May 23, 2014.

FIELD OF THE INVENTION

The invention relates to a linear motor also called linear drive, in particular for optical systems. Such optical systems are used for example in endoscopes. In modern video endoscopes, a camera chip together with a lens system are integrated in the endoscope tip. A miniaturized motor is required to adjust the focal length and the focus point of the lens system.

BACKGROUND OF THE INVENTION

Endoscopes as known from prior art, for example, for minimally invasive surgery, guide an image by means of rod lenses from an intracorporeal objective lens to an extracorporeal occular. Due to the rod lenses, the system is rigid and limited in optical quality. Modern video endoscopes use a camera chip in the endoscope tip. Such an endoscope is disclosed in U.S. Pat. No. 7,365,768 B1. This has a rigidly arranged lens in front of the camera chip. An adjustment of the focal length of the lens is not possible.

DE 103 23 629 A1 discloses a moving field linear motor which includes at least three stator coils. A phase-shifted current supply to the coils produces a moving field which effects a displacement of the armature with axial permanent magnets.

From DE 10 2008 038 926 A1, a linear drive including two axially polarized permanent magnets in an armature is known. The armature is deflected in axial direction by the current supply to the stator coils. Additionally, stable positions of the armature are realized by pole shoes mounted to the stator, allowing a continuous displacement of the armature in a guiding tube.

In DE 10 2010 000 582 A1, a further linear drive is disclosed, which has an axially polarized permanent magnet in the armature, and one or two axially polarized permanent magnets in the stator.

These three linear drives each include a stator and an armature. The armatures are construed of one or several permanent magnets. For deflection and for generation of the electromagnetic flux in defined directions, rings of soft magnetic iron are disposed at the permanent magnets (pole shoes). One or more coils generate Lorentz-forces in the stator. Additional permanent magnets and rings of soft magnetic iron serve partially for generation of reluctance forces. The stator is enclosed by a soft-magnetic sleeve which constitutes a reflux yoke for the magnetic flux. In a current less state, the armature is in a so-called idle position due to resetting reluctance forces. Due to supplying the coils with electric current of constant current strength, Lorentz-forces are generated which lead to a continuous deflection of the armature from the rest position. By generating a force balance of Lorentz-forces in the DE 103 23 629 A1, or of Lorentz-forces and reluctance forces in DE 10 2008 038 926 A1 and DE 10 2010 000 582 A1, the armature stays in a deflected position. Thereby it is preferred to have a predetermined relationship between the magnitudes of currents to positions of the armature. Normally, this in achieved by calibration after setup of the drives.

External forces which are difficult to control, such as for example friction forces or gravity, lead to positioning inaccuracy. By predefining fixed magnitudes of current, the position of the armature can be determined only in a limited manner.

DE 196 05 413 A1 discloses a linear drive with position measurement. Here, the drive winding is at the same time used as measurement winding. By such a position measurement, a higher positioning accuracy can be achieved. However, the preciseness of the measurement system and thus of the control system is limited due to the minor change of the coil inductivity during movement of the armature.

In U.S. Pat. No. 5,747,952, a three-phase linear drive is disclosed, where the magnetic field is measured by a Hall sensor between coil and armature, and the amplitude of the control signal is held at a constant value.

SUMMARY OF THE INVENTION

The embodiments are based on the object of improving a linear drive such that the armature can be brought in defined positions, and a defined and preferably linear characteristic curve exists between the control signal and the armature's position. Furthermore, the position of the armature should preferably be independent to a large extent from an external load. The linear drive should be miniaturizable to such an extent that it may be inserted into an endoscope.

In an embodiment, a linear drive comprises a stator and an armature which is linearly displaceable thereto. Preferably, the linear drive is built mainly rotationally symmetrical. Thereby, the components are largely formed ring-shaped. The movement of the armature is performed along an axis, which is preferably parallel to the center axis of the rotationally symmetrical arrangement, and which is most preferably on the center axis. The stator has one or two coils, wherein at least one stator pole shoe is arranged preferably in axial direction, laterally to the coils, as well as a magnetic member is arranged in radial direction at the outside. This magnetic member is at least largely parallel to the movement direction, and preferably encloses said one or two coils, and more preferably encloses at least one stator pole shoe. Preferably, the magnetic member comprises a soft magnetic material and most preferably a material comprising iron or ferrite. The armature is at least partially enclosed by the coil in radial direction, and has a first permanent magnet and optionally a second permanent magnet. Preferably, an armature pole shoe, respectively, is arranged laterally in axial direction at each of the permanent magnets. In the case of two permanent magnets, one further pole shoe is arranged between the two permanent magnets. The armature pole shoes allow a defined flow of the magnetic fields of the permanent magnets through the coil towards the magnetic member. Basically, the arrangement also may work without pole shoes. However, by means of the pole shoes, the force of the motor can be increased by more than one magnitude. In the case of two permanent magnets, these are magnetized axially and are aligned in their polarization such that either the north poles or the south poles are located opposite to one another. The armature and in particular the permanent magnets as well as the pole shoes are preferably hollow-cylindrical, that is they have the form of a cylindrical sleeve. The beam path of an optical system can then go through the sleeve. In particular, a lens or another optical element can be positioned in the sleeve. Consequently, the focal length and/or the focus point of the optical system can be adjusted by a displacement of the sleeve.

The pole piece and/or the magnetic member preferably comprise ferromagnetic and/or soft magnetic materials. Most preferably, these materials comprise iron or ferrite.

The linear motor may easily be miniaturized as far as to a size of a few millimeters external diameter.

Preferably, the coils may be wound onto a coil form or without a coil form, as desired. It may also be multi-part, i.e. it may comprise of a plurality of windings.

For indirect determination of the position of the armature, one or two magnetic field sensors, herein also called field sensors, are provided. Basically, also a higher number of field sensors may be provided. The magnetic field sensors are arranged laterally in axial direction next to the at least one winding. A part of the magnetic flux through the permanent magnets is, dependent from the position of the armature, through these magnetic field sensors. By determining the magnetic flux and the corresponding magnetic flux density B, the position of the armature can be concluded. Due to a one-phase current supply of the coils, the control electronics can be easily realized. The simple construction allows for miniaturization of the drive, such that it is suitable for use in minimal-invasive instruments.

Preferably, the at least one magnetic field sensor is arranged laterally next to at least one stator pole shoe. Alternatively, at least one magnetic field sensor can also be integrated in a stator pole shoe. The evaluation of the signal difference of at least two magnetic field sensors is particularly advantageous. Thereby, improved independence of external influences, such as temperature, as well as higher measurement accuracy may be achieved.

In a preferred embodiment a coil and on both sides of the coil in axial direction stator pole shoes are provided, wherein the coil and the stator pole shoes are enclosed by a magnetic member. Two magnetic field sensors are provided laterally next to the stator pole shoes, or are integrated into the stator pole shoes, respectively. The armature has two permanent magnets which are polarized in opposite directions, and preferably magnetized axially, wherein between these two permanent magnets, an armature pole shoe is provided, and at the ends of the permanent magnets two further armature pole shoes are provided. A further embodiment of the linear drive has only one stator pole shoe and only one field sensor. The armature may be simply comprise only one permanent magnet and preferably has two armature pole shoes at the ends of the permanent magnet.

In a further embodiment, two coils are arranged laterally in axial direction at two sides of a stator pole shoe. Here, at least one magnetic field sensor, preferably two magnetic field sensors are integrated in the stator pole shoe. Preferably, here an armature with two oppositely polarized permanent magnets and one armature pole shoe between the permanent magnets as well as at the outer ends of the permanent magnets is provided.

For measuring the magnetic field, various sensor types may be used. The most common sensors are Hall effect sensors, GMR sensors, and AMR sensors.

For the linear drive no balance state between reluctance force and Lorentz-force is required for the positioning, as it is necessary for the linear drives known from the prior art. This results in comparatively significantly higher drive and actuating forces at equal electrical power. Therefore, it can also be used in surgical instruments.

Further embodiments of the linear drive may also be realized with an ironless stator. Here, the stator pole shoes and the magnetic member would consist of non-ferromagnetic material or would even be omitted. Due to the absent magnetic field conductive materials in the stator, the magnetic flux density in the magnetic circuit is reduced. Thereby, also the driving Lorentz-force by an electrical current flow in the coil is reduced.

In a further advantageous embodiment, there is a sliding layer between the stator and the armature. This sliding layer can be implemented as a sliding sleeve, in particular in the case of a rotationally symmetric arrangement. In order to influence the magnetic fields as little as possible, the sliding layer preferably comprises of non-magnetic-field-conductive material, particularly a non-ferromagnetic material. Its surface preferably comprises a material with low friction coefficient, for example PTFE (polytetrafluoroethylene), silicon nitride, silicon carbide, poly-para-xylylene polymers, or DLC (diamond like carbon), like for example disclosed in U.S. Pat. No. 5,478,650.

The sliding layer may compensate for unevenness on the side of the stator facing the armature.

In an alternate embodiment, the linear drive can be realized with a flat stator, e.g. having a plate-shaped structure and likewise flat or plate-shaped pole piece of the armature. Alternatively, a plurality of linear drives disposed around a cylinder or a polygonal body may also be provided. A stable guidance is obtained, for example, in the case of a uniform arrangement of linear motors around a cylinder.

In another embodiment, the linear drive can also consist of solid material and have a plunger at one end for the nano-positioning of instruments. Such a device can preferably be used in molecular biology, microelectronics or neurosurgery.

Due to the simple arrangement, the linear drive may be realized very compact with respect to axial building length. Therefore, the linear drive is well-suited for an embodiment with a hollow armature in optical systems.

The linear drive may be integrated directly into the objective lens of an endoscope camera. The controlled adjustment of zoom and focus functions with measurement of the position of the moveable optical lens groups is therefore possible. Due to the small size, also stereo cameras with two single objectives lenses for 3D systems may be integrated in a space-saving manner into conventional camera housings. The linear drive preferably is integrated twice in identical construction manner for the movement of lens groups along the optical axis. Due to the controlled positioning of the lens groups by the linear drives, the images projected on the image sensors can be displayed clearly.

In chip-on-the-tip-objective lenses, the complete optics, including a camera chip may be integrated into the tip of the endoscope. Thereby, optical lens groups are moved, for example, along the optical axis, in order to allow a focusing or zooming of the image. The controlled linear drive may be used in miniaturized form also in video endoscopes. By the inventive drive, it is possible to control the position of the lens groups by means of feedback of the positioning signals. By measurement of the armature position by means of the magnetic field sensors, the position of the lens groups is known. In a drive for focus adjustment, with knowledge about the optical system, the object distance to focused objects can thereby be determined. With help of the inventive linear drive, also a simple coupling of two or more movable lens groups is possible. In stereo endoscopes in chip-on-the-tip-embodiment, two neighbored optical lens systems at the endoscope tip are often used for the stereo imaging. If here, lens groups should be axially movable for focus or zoom function, respectively, two inventive drives may be used for this purpose.

By use of the drive in a surgical instrument it is possible to control the aperture angle Phi of a jaw section. The armature can be positioned in the stator by means of the magnetic field sensors, as described above. If a pull-push-rod, which initiates opening and closing of the jaw section by linear movement, is mounted in the drive, the aperture angle Phi of the instrument can be adjusted by the position of the armature. By the controlled operation, the aperture angle Phi can be maintained or adjusted, independent of the clamp force of the jaw section. As the inventive motor is a motor driven by Lorentz force, the axially acting drive force can be determined through the relation of current strength in the coil. Thereby, also the clamping force of the jaw section can be calculated. This relationship plays an important role especially in novel force-feedback-systems, in which the operating physician receives feedback about the clamping force of the jaw section.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1schematically shows a linear drive in a longitudinal section. According to a preferred embodiment, the linear drive is arranged mainly rotationally symmetric around the center axis30. It comprises a stator10and an armature20.

The stator10has a coil14, which preferably is enclosed by a cylindrical magnetic member11in radial direction. The coil14is enclosed by a first stator pole shoe12and a second stator pole shoe13in axial direction. Preferably, also these stator pole shoes are enclosed by the magnetic member in radial direction. Furthermore, a first magnetic field sensor15is arranged at the side of the first stator pole shoe12in axial direction next to the coil, and a second magnetic field sensor16is arranged at the side of the second stator pole shoe13in axial direction next to the coil14. Preferably, at least one magnetic field sensor is arranged in an opening of a stator pole shoe. More preferably, this opening extends in axial direction, as shown in this Figure, but it may also extend in radial direction. The opening may be continued into the magnetic member11, in order to offer sufficient mounting space for a bigger field sensor. Preferably, the stator pole shoes and/or the magnetic member are ring-shaped.

The armature has a first permanent magnet21and a second permanent magnet22, which are polarized in opposite directions and preferably parallel to the center axis. A center armature pole shoe24is arranged between the two permanent magnets. At the sides opposite to the center armature pole shoe24in axial direction, a first outer armature pole shoe23is arranged towards the first pole shoe21, and a second outer armature pole shoe25is arranged towards the second pole shoe22. Preferably, the armature is hollow, more preferably hollow-cylindrical. Preferably, the permanent magnets and/or armature pole shoes are ring-shaped. In some embodiments, a sliding sleeve90may be arranged between the stator10and the armature20.

Preferably, the two magnetic field sensors15,16are arranged in the same plane through the center axis30, but they may also be arranged in other planes.

Preferably, the drive is construed such that within the moving distance, no axially directed reluctance forces act in a currentless state. If the coil14is energized, a Lorentz-force is generated, which acts on the armature, independently of its position.

Basically, this embodiment, as well as all other embodiments illustrated in this specification, may be realized with an ironless stator. Thereby, the stator pole shoes12,13as well as the magnetic member11would consist of non-ferromagnetic material or would even be omitted. Due to the absent magnetic field conductive materials in the stator, the magnetic flux density in the magnetic circuit is reduced. Thereby, also the driving Lorentz-force by an electrical current flow in the coil is reduced.

InFIG. 2, a view towards the center axis30is shown. Here, the preferred concentric arrangement of the components is illustrated in detail. In this embodiment, the first field sensor15is arranged in an opening, which protrudes through the magnetic member11into the first stator pole shoe.

InFIGS. 3, 4 and 5, the magnetic field profiles in different positions of the armature20relative to the stator10are shown. InFIG. 3, the armature20is shown displaced to the left relative to the center position, which is shown inFIG. 4. InFIG. 5, it is displaced to the right. Basically, a first magnetic circuit41is generated, starting from the first permanent magnet21. The magnetic field goes through the first outer armature pole shoe23via the first field sensor15and the first stator pole shoe12, respectively, continuing through the magnetic member11, via the coil14and further continuing to the center armature pole shoe24and back to the first permanent magnet21. Accordingly, the second magnetic circuit42is oriented in opposite direction.

The lines41and42symbolically illustrate the magnetic field curve. In fact, the magnetic field spreads, for example, over the whole front side of the permanent magnets21,22. Similarly, the magnetic field spreads in radial direction out of the armature pole shoes over the surface.

InFIG. 3it can be seen that a main part of the magnetic field goes through the first field sensor15, while the negligible part goes through the first stator pole shoe12. The magnetic field starting from the second armature pole shoe25mainly goes over the second stator pole shoe13into the magnetic member11. Only a minimal and negligible part will run through the second field sensor16.

InFIG. 4, a part of the magnetic field goes out of the first armature pole shoe23, via the first field sensor15, and parallel thereto via the first stator pole shoe12into the magnetic member11. Similar holds true for the magnetic field out of the second armature pole shoe25. Here, the field divides as well between the second field sensor16and the second stator pole shoe13.

InFIG. 5, the main part of the magnetic field goes out of the first armature pole shoe23via the first stator pole shoe12into the magnetic member11, while the main part of the magnetic field out of the second armature pole shoe25goes through the second field sensor16.

InFIG. 6, a further embodiment is shown. In this embodiment, the stator has only one stator pole shoe12, and the armature has only one permanent magnet21. A first field sensor17and optionally a second field sensor18are provided. Preferably, integrated field sensors17,18with smaller construction size are used, which can be integrated into the first stator pole shoe12. Preferably, these are inserted into a recess of the first stator pole shoe12. The field sensors17,18are disposed opposite to each other, in axial direction. Preferably, they can also be arranged in different planes through the rotation axis. In this embodiment, they are arranged in the same plane, on different sides of the rotation axis. Dependent from the position of the armature20relative to the stator10, the magnetic flux flows, starting from the first outer armature pole shoe23, through one of the field sensors. In the illustrated position, a major part of the magnetic flux flows through the second field sensor18, while the first field sensor is in a space free of fields.

In the arrangement shown here, the magnetic field sensor can only lie in the magnetic field of a pole shoe. Thereby, simplified magnetic field sensors can be implemented, which deliver an output signal independent of the direction of the magnetic field. Such sensors are, for example, GMR—(Giant Magneto Resistance) sensors. In the prior art, it is often necessary to use direction-sensitive magnetic field sensors, such as Hall sensors, in order to achieve an accurate position determination. Such sensors are in most cases bigger, more expensive, and require a more complex control and evaluation circuitry.

The integration of the magnetic field sensors in the stator pole shoe allows a significantly improved exploitation of space, in particular in miniature motors. This embodiment is at the same time more robust, as the magnetic field sensors are supported mechanically by the stator pole shoe. As a result, a separate housing for the magnetic field sensors can be omitted.

Basically, in this embodiment of a linear drive, also an arrangement with a first magnetic field sensor15next to a first stator pole shoe12, as in the embodiment ofFIG. 1, may be implemented. Similarly, the embodiment ofFIG. 1may be realized with a first field sensor17which is integrated into the first stator pole shoe12, according to this embodiment. Preferably, then also the second field sensor16of the embodiment ofFIG. 1is replaced by second field sensor18, which is integrated into the second stator pole shoe13.

InFIG. 7, a further embodiment of the invention is shown. The armature20corresponds to the armature of the first embodiment, as shown, for example, inFIG. 1. Here, the stator has a first coil14and a second coil19. The coils may be operated single-phased (identical current strength) or double-phased (different current strength in both coils). A stator pole shoe12is arranged between the two coils. The coils and the stator pole shoe are enclosed by the magnetic member11. The field sensors17,18are disposed one to another, in axial direction, and are integrated into the stator pole shoe12, or are received by the recesses of the stator pole shoe. The magnetic field of the middle armature pole shoe24goes as a whole or partly—depending on the position of the armature—through the first field sensor17or the second field sensor18. The output signals of the field sensors17,18correspond approximately to the curves61and62ofFIG. 8. Also here, a signal according toFIG. 9can be achieved by subtraction of the signals.

FIG. 8shows the signal curve of the field sensors, for example according to the illustrations inFIGS. 3, 4 and 5. The diagram shows on the horizontal axis the distance relative to the zero position “0” in millimeters, which corresponds, for example, toFIG. 4. A deflection of −2 mm corresponds toFIG. 3, and a deflection of +2 mm corresponds toFIG. 5. On the vertical axis, the amplitudes of the sensor signals are scaled and indicated from 0-150. The curve61shows the signal of the first field sensor15, while the curve62shows the signal curve of the second field sensor16.

Curve61shows on the left side, a maximal amplitude at a deflection of −2 mm, which corresponds to the maximal magnetic flux density through the first magnetic field sensor15. This is achieved by the position of the armature as shown inFIG. 3. At the same time, the second field sensor16is in a nearly field free space, such that the sensor signals according to curve62are nearly zero. In the center position according toFIG. 4, the flux density of both field sensors is approximately equal, such that also both curves61and62have the same amplitude at position “0”. At the right position at +2 mm, the sensor signals behave in a reversed manner as in the left position. Here, the maximal magnetic flux density is through the first magnetic field sensor16, while the first field sensor15lies in a nearly field free space.

FIG. 9shows the sum of curves61and62ofFIG. 8in curve63. This curve can be well approximated by linear approximation64. A measurement signal results, which is approximately proportional to the position of the armature. This measurement signal generally can be input into a control loop, such that the position of the armature can be kept constant, in dependence of a setpoint value.

FIG. 10shows the signals of the field sensors in an arrangement according toFIG. 6. Also here, the horizontal axis shows the displacement towards the center axis, while the vertical axis indicates the amplitude of the sensor signals. Curve65indicates the signal amplitude of the first field sensor17, while the second curve65indicates the signal amplitude of the second field sensor18. As a result, the exact position can be determined by evaluating which sensor outputs a signal, in combination with the signal amplitude of the sensor. In this example, only two sensors are shown. Of course, any higher number of magnetic field sensors may be used, in order to increase the resolution and/or maximal path length.

FIG. 11shows an endoscope80, in which a linear drive84is implemented for adjustment of the viewing angle φ. The endoscope has a distal shaft81as well as a proximal ocular83. Optionally, connections82for light input or for input of fluids and gases may be provided. The endoscope has an axis88, which preferably is also the optical and/or mechanical axis. At the distal end of the shaft81, a prism85is arranged pivotably around the bearing86. The prism allows a deflection of the optical beam path, such that light entering into the distal end of the endoscope under various viewing angles φ can be detected. Adjustment of the prism is conducted by means of a linear drive84via a push/pull-rod87.

LIST OF REFERENCE NUMERALS

15first field sensor

16second field sensor

17first integrated field sensor

18second integrated field sensor

41first magnetic circuit

42second magnetic circuit

61signal curve first field sensor

62signal curve second field sensor

63difference of signal curves

65signal curve first integrated field sensor

66signal curve second integrated field sensor