Apparatus for varying a focal point of an optical system in a dental 3D-scanner and dental 3D-scanner

1. Apparatus for varying a focal point of an optical system in a dental 3D-scanner, comprising:

a lens unit with a lens being movable between a front reversal position and a rear reversal position to vary a position of a focal point with respect to a scan object;

a guide unit for guiding a movement of the lens unit between the front reversal position and the rear reversal position along a guide axis being parallel to an optical axis of the lens; and

a drive unit for driving the movement of the lens unit, said drive unit including a linear motor with an anchor and a stator, said anchor being movable along a drive axis of the drive unit that is parallel to the guide axis, said stator being affixed to the guide unit

wherein the drive unit includes a coupling arrangement for coupling a movement of the anchor and the lens unit so that a movement of the anchor in a first direction is transferred to a movement of the lens in a second direction opposite to the first direction.

2. Apparatus as claimed in claim 1 , wherein

the coupling arrangement includes a flexible connection element for connecting the anchor and the lens unit to transfer a force from the anchor to the lens unit.

3. Apparatus as claimed in claim 2 , wherein

the coupling arrangement includes a tension element for tensioning the flexible connection element.

4. Apparatus of claim 1 , wherein

the coupling arrangement includes an inverting element for inverting the movement of the anchor.

5. Apparatus as claimed in claim 1 , wherein

the guide unit includes a position sensor for determining a position of the lens unit between the front reversal position and the rear reversal position.

6. Apparatus as claimed in claim 5 , wherein the drive unit is configured to control the movement of the lens unit based on a sensor signal of the position sensor.

7. Apparatus as claimed in claim 1 , wherein

the guide unit includes a linear recirculating ball bearing guide; and

the lens unit includes an engaging element for engaging into said linear recirculating ball bearing guide.

8. Apparatus as claimed in claim 1 , wherein a mass of the lens unit is equal to a mass of the anchor to compensate reaction forces resulting from accelerations of the lens and the anchor.

9. Apparatus as claimed in claim 1 , wherein a centerline of mass of the lens unit parallel to the guide axis corresponds to a centerline of mass of the anchor parallel to the guide axis.

10. Apparatus as claimed in claim 1 , wherein

the linear motor is a brushless 3-phase linear servomotor; and

the linear motor preferably includes a hall sensor for measuring a position of the anchor with respect to the stator.

11. Apparatus as claimed in claim 1 , wherein a maximum displacement of the anchor is equal to a distance between the front reversal position and the rear reversal position.

12. Apparatus as claimed in claim 1 , wherein the drive unit is configured to drive the movement of the lens unit to oscillate between a selectable front oscillation position and a selectable rear oscillation position at an oscillation frequency of 2 to 20 Hz.

13. Dental 3D-scanner for scanning a three-dimensional scan object, comprising:

an apparatus as claimed in claim 1 ;

a detector for detecting a light signal from the scan object passing through the lens; and

a handheld housing for manually guiding the 3D-scanner around the scan object.

14. Dental 3D-scanner as claimed in claim 13 , comprising a control unit for controlling the drive unit.

15. Apparatus of claim 2 , wherein said connection element includes a steel strip.

16. Apparatus of claim 3 , wherein said tension element includes a spring.

17. Apparatus of claim 4 , wherein said inverting element includes a ball bearing.

18. Apparatus of claim 5 , wherein the position sensor includes an optical distance measurement sensor.