Adjustable magnetic spring for actuator

1. An electrodynamic actuator ( 1 a , 1 b ), which is designed to be connected to a backside of a plate like structure ( 2 ) opposite to a sound emanating surface (S) of the plate like structure ( 2 ), and which comprises:

a coil arrangement ( 3 a , 3 b ) with at least one voice coil ( 4 a , 4 b ), which has an electrical conductor in the shape of loops running around a coil axis (C) in a loop section;

a magnet system ( 5 ), comprising a static magnetic circuit part ( 6 a . . . 6 F), which is arranged in fixed relation to the coil arrangement ( 3 a , 3 b ), and a movable magnetic circuit part ( 7 , 7 a . . . 7 f ), wherein the magnet system ( 5 ) is designed to generate a magnetic field (B 1 , B 2 ) transverse to the conductor in the loop section; and

a spring arrangement ( 12 ) coupling the static magnetic circuit part ( 6 a . . . 6 F) to the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) and allowing a relative movement between the static magnetic circuit part ( 6 a . . . 6 F) and said movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in an excursion direction (z) parallel to the coil axis (C),

wherein both the spring arrangement ( 12 ) and the magnet system ( 5 ) provide a total restoring force (F T ) directed towards an idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ), which idle position (P 0 ) is defined as the position of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) when no current (I) flows through the at least one voice coil ( 4 a , 4 b ),

wherein a ratio between a differential total restoring force (ΔF T ) and a differential excursion (Δz) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) is defined as a total restoring force gradient (ΔF T /Δz), and

wherein a part of the total restoring force gradient (ΔF T /Δz) caused by the magnet system ( 5 ) is at least 10% of the total restoring force gradient (ΔF T /Δz) in said idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ).

2. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a part of the total restoring force gradient (ΔF T /Δz) caused by the magnet system ( 5 ) is at least 10% of the total restoring force gradient (ΔF T /Δz) in the maximum excursion position of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ).

3. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the part of the total restoring force gradient (ΔF T /Δz) caused by the magnet system ( 5 ) in said idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) is smaller than in a position of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) displaced from said idle position (P 0 ).

4. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the part of the total restoring force gradient (ΔF T /Δz) caused by the magnet system ( 5 ) in said idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) is higher than in a position of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) displaced from said idle position (P 0 ).

5. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a width (b) of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) measured in a direction perpendicular to said coil axis (C) is constant along a direction parallel to said coil axis (C).

6. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a width (b) of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) measured in a direction perpendicular to said coil axis (C) changes or varies along a direction parallel to said coil axis (C).

7. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) changes stepwise in a direction parallel to said coil axis (C).

8. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) changes continuously in a direction parallel to said coil axis (C).

9. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in cross sectional planes comprising the coil axis (C) stays the same along an annular course of the airgap around the coil axis (C).

10. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in cross sectional planes comprising the coil axis (C) changes or varies along an annular course of the airgap around the coil axis (C).

11. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a width (d) of the at least one voice coil ( 4 a , 4 b ) in a cross sectional plane comprising the coil axis (C) measured in a direction perpendicular to said coil axis (C) is constant along a direction parallel to said coil axis (C).

12. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a width (d) of the at least one voice coil ( 4 a , 4 b ) in a cross sectional plane comprising the coil axis (C) measured in a direction perpendicular to said coil axis (C) changes or varies along a direction parallel to said coil axis (C).

13. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of the at least one voice coil ( 4 a , 4 b ) in a cross sectional plane comprising the coil axis (C) changes stepwise in a direction parallel to said coil axis (C).

14. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of the at least one voice coil ( 4 a , 4 b ) in a cross sectional plane comprising the coil axis (C) changes continuously in a direction parallel to said coil axis (C).

15. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the magnet system ( 5 ) comprises a center magnet ( 8 , 8 a . . . 8 c ), a bottom plate ( 9 , 9 a . . . 9 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ), a top plate ( 10 , 10 a . . . 10 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ) and opposite of the bottom plate ( 9 , 9 a . . . 9 c ) and an outer plate arrangement ( 6 a . . . 6 F) surrounding the top plate ( 10 , 10 a . . . 10 c ), wherein the outer plate arrangement ( 6 a . . . 6 F) comprises a groove ( 18 a , 18 b ) facing the top plate ( 10 , 10 a . . . 10 c ).

16. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the magnet system ( 5 ) comprises a center magnet ( 8 , 8 a . . . 8 c ), a bottom plate ( 9 , 9 a . . . 9 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ), a top plate ( 10 , 10 a . . . 10 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ) and opposite of the bottom plate ( 9 , 9 a . . . 9 c ) and an outer plate arrangement ( 6 a . . . 6 F) surrounding the top plate ( 10 , 10 a . . . 10 c ), wherein the outer plate arrangement ( 6 a . . . 6 F) comprises a ridge ( 19 a , 19 b ) facing the top plate ( 10 , 10 a . . . 10 c ).

17. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the magnet system ( 5 ) comprises a center magnet ( 8 , 8 a . . . 8 c ), a bottom plate ( 9 , 9 a . . . 9 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ), a top plate ( 10 , 10 a . . . 10 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ) and opposite of the bottom plate ( 9 , 9 a . . . 9 c ) and an outer plate arrangement ( 6 a . . . 6 F) surrounding the top plate ( 10 , 10 a . . . 10 c ), wherein the outer plate arrangement ( 6 a . . . 6 F) comprises holes ( 20 a , 20 b ) facing the top plate ( 10 , 10 a . . . 10 c ).

18. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the magnet system ( 5 ) comprises a center magnet ( 8 , 8 a . . . 8 c ), a bottom plate ( 9 , 9 a . . . 9 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ), a top plate ( 10 , 10 a . . . 10 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ) and opposite of the bottom plate ( 9 , 9 a . . . 9 c ) and an outer plate arrangement ( 6 a . . . 6 F) surrounding the top plate ( 10 , 10 a . . . 10 c ), wherein an airgap between the top plate ( 10 , 10 a . . . 10 c ) and the outer plate arrangement ( 6 a . . . 6 F) is larger than besides thereof.

19. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein the magnet system ( 5 ) comprises a center magnet ( 8 , 8 a . . . 8 c ), a bottom plate ( 9 , 9 a . . . 9 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ), a top plate ( 10 , 10 a . . . 10 c ) arranged adjacent to said center magnet ( 8 , 8 a . . . 8 c ) and opposite of the bottom plate ( 9 , 9 a . . . 9 c ) and an outer plate arrangement ( 6 a . . . 6 F) surrounding the top plate ( 10 , 10 a . . . 10 c ), wherein an airgap between the top plate ( 10 , 10 a . . . 10 c ) and the outer plate arrangement ( 6 a . . . 6 F) is smaller than besides thereof.

20. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) is symmetric with respect to an axis (E) perpendicular to the coil axis (C) at the idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ).

21. The electrodynamic actuator ( 1 a , 1 b ) as claimed in claim 1 , wherein a profile contour of an airgap between the static magnetic circuit part ( 6 a . . . 6 F) and the movable magnetic circuit part ( 7 , 7 a . . . 7 f ) in a cross sectional plane comprising the coil axis (C) is asymmetric with respect to an axis (E) perpendicular to the coil axis (C) at the idle position (P 0 ) of the movable magnetic circuit part ( 7 , 7 a . . . 7 f ).

22. An output device ( 17 ), comprising a plate like structure ( 2 ) with a sound emanating surface (S) and a backside opposite to the sound emanating surface (S) and comprising an electromagnetic actuator ( 1 a , 1 b ) connected to said backside, characterized in that the electromagnetic actuator ( 1 a , 1 b ) is designed according to claim 1 .

23. The output device ( 17 ) as claimed in claim 22 characterized in that the plate like structure ( 2 ) is embodied as a display and that the electromagnetic actuator ( 1 a , 1 b ) is connected to the backside of the display.

24. The output device ( 17 ) as claimed in claim 22 characterized in that an average sound pressure level of the output device ( 17 ) measured in an orthogonal distance of 10 cm from the sound emanating surface (S) is at least 50 dB_SPL in a frequency range from 100 Hz to 15 kHz.