Seat belt retractor

1. A seat belt retractor comprising:

a spool for winding and unwinding a seat belt webbing;

a measurement mechanism for determining a measure of the amount of seat belt webbing unwound from the spool, the measurement mechanism including

a rotating cycloidal gear including a plurality of openings and wherein the rotation of the cycloidal gear is driven by rotation of the spool;

a rotating secondary gear, wherein the secondary gear includes a plurality of pins and wherein each of the pins is located within one of the plurality of openings in the cycloidal gear and wherein rotation of the secondary gear is driven by rotation of the cycloidal gear;

a rotating sensing gear, wherein the sensing gear is engaged with the secondary gear, and wherein rotation of the sensing gear is driven by the secondary gear;

a magnet mounted to the sensing gear; and

a sensor for detecting movement of the magnet, and wherein the sensor is configured to provided a signal to a controller for determining a measure indicating the amount of rotation of the spool and the amount of webbing withdrawal based on the movement of the magnet.

2. The seat belt retractor of claim 1 , further comprising a second rotating cycloidal gear, wherein the second rotating cycloidal gear includes a plurality of openings and wherein the rotation of the second cycloidal gear is driven by rotation of the spool, and wherein each of the pins of the secondary gear is located within one of the plurality of openings in the secondary cycloidal gear and wherein rotation of the secondary gear is driven by rotation of the second cycloidal gear.

3. The seat belt retractor of claim 1 , wherein the retractor is configured so that the sensing gear rotates less than a full revolution.

4. The seat belt retractor of claim 3 , wherein the magnet is located proximate the center of the sensing gear.

5. The retractor of claim 1 , further comprising an automatic locking (ALR) mechanism that includes a lever that is configured to engage the secondary gear.

6. The retractor of claim 5 , wherein the ALR mechanism is configured so that when the webbing is fully extracted from the spool, and wherein the lever is engaged with the secondary gear thereby preventing further rotation of the spool in the unwinding direction.

7. The retractor of claim 6 , wherein the secondary gear includes a retracting projection and a locking projection for engaging the lever and wherein the retracting projection engages the lever during retraction of the webbing to move the lever away from an ON position (in which extraction of the webbing is blocked) to an OFF position in which extraction of the webbing is permitted.

8. The retractor of claim 7 , wherein the retracting projection is in a first plane and the locking projection is in a second plane and wherein the first and second planes to each other and perpendicular to the axis of rotation of the spool.

9. The retractor of claim 1 , further comprising a second magnet configured to rotate with the spool and a second sensor for detecting movement of the second magnet, and wherein the second sensor provides a signal to the controller so that the controller can determine the measure indicating the amount of rotation of the spool and the amount of webbing withdrawal based on the movement of the second magnet.

10. The retractor of claim 9 , wherein the second magnet is mounted to rotate at the same rate as the spool so that the second sensor can detect the angular position of the spool.

11. The retractor of claim 10 , further comprising a shaft connected to the spool and extending through the gears of the measurement mechanism and a rewinding spring for biasing the spool in the winding direction, wherein the rewinding spring is connected to a bushing that is connected to the shaft and the second magnet is mounted to the bushing.

12. The retractor of claim 10 , wherein the signals from the first and second sensors are provided to the controller to that the controller can determine the following additional measures indicating the direction of rotation of the spool, the velocity of rotation of the spool, and the acceleration of the webbing.

13. A seat belt retractor comprising:

a spool for winding and unwinding a seat belt webbing;

a measurement mechanism for determining a measure of the amount of seat belt webbing unwound from the spool, the measurement mechanism including a cycloidal gear transmission for transferring the rotational movement of the spool to rotational movement of a sensing gear, wherein the cycloidal gear transmission includes a primary gear including holes that are engaged with pins on a secondary gear so that at least one of the primary and secondary gears rotates eccentrically;

wherein rotational movement of the sensing gear is detected by a first sensor, and wherein the first sensor is configured to provide a signal to a controller for determining a measure indicating the amount of rotation of the spool and the amount of webbing withdrawal based on the movement of the sensing gear.

14. The seat belt retractor of claim 13 , wherein the secondary gear rotates proportionally slower to the spool, wherein the retractor includes an automatic locking (ALR) mechanism that includes a lever that is configured to engage the secondary gear.

15. The retractor of claim 14 , wherein the secondary gear includes a retracting projection and a locking projection for engaging the lever and wherein the retracting projection engages the lever during retraction of the webbing to move the lever away from an ON position (in which extraction of the webbing is blocked) to an OFF position in which extraction of the webbing is permitted.