Seatbelt payout measuring device and system

1. A seatbelt payout measuring device, comprising:

a rewind spring;

a dielectric material adjacent to the rewind spring;

an electrolytic coil adjacent to the dielectric material, such that the rewind spring is electrically isolated from the electrolytic coil; and

a capacitive measuring device connected to a first end of the rewind spring and connected to a first end of the electrolytic coil.

2. The device of claim 1 , wherein the electrolytic coil and the rewind spring are made of a same material.

3. The device of claim 1 , wherein the electrolytic coil is made of aluminum, copper, or a malleable metal in the form of a flexible thin film or foil.

4. The device of claim 1 , wherein the electrolytic coil is a metalized coating on a dielectric plastic film.

5. The device of claim 1 , wherein the dielectric material and the electrolytic coil are arranged in a dielectric-electrolytic layer.

6. The device of claim 1 , wherein the electrolytic coil has a length and a width that are one of:

shorter than a respective length and width of the rewind spring; and

roughly equal to the respective length and the width of the rewind spring.

7. The device of claim 1 , wherein a first end of the electrolytic coil is closer to an outer periphery of a rewind spring housing than a center spool in the rewind spring housing.

8. The device of claim 1 , wherein a first end of the electrolytic coil is closer to a center spool in a rewind spring housing than an outer periphery of the rewind spring housing.

9. The device of claim 1 , wherein the dielectric material includes one of Teflon, mylar, polyvinyl chloride (PVC), polypropylene (PP), polyethylene naphthalate (PEN), polyethylene terephthalate (PET) or any other suitable dielectric material.

10. The device of claim 1 , wherein:

the first end of the rewind spring is connected to a center spool;

the rewind spring has a second end connected to a rewind spring housing;

the dielectric material has a first side adjacent to the rewind spring, the dielectric material and the rewind spring forming a first layer;

the electrolytic coil has both sides adjacent to the dielectric material, such that the rewind spring is electrically isolated from the electrolytic coil; and

the capacitive measuring device has a first lead connected to the first end of the rewind spring and a second lead connected to the first end of the electrolytic coil.

11. A system, comprising a computer including a processor and a memory, the memory storing instructions executable by the computer to:

apply, to a capacitor, a frequency signal based at least in part on a predetermined frequency value;

determine a capacitive value of the capacitor based at least in part on a measured voltage of the capacitor; and

determine a length of an extracted seatbelt based at least in part on the capacitive value.

12. The system of claim 11 , further comprising a second computer including a second processor and a second memory, the second memory storing instructions executable by the second computer to:

receive the length of the extracted seatbelt from the computer;

communicate with at least one vehicle sensor; and

communicate with a triggering mechanism.

13. The system of claim 12 , wherein the second memory further includes instructions to:

receive data from the at least one vehicle sensor; and

upon detecting an impact based at least in part on the received data, instruct the triggering mechanism to activate a pyrotechnic pretensioner.

14. The system of claim 12 , wherein the memory further includes instructions to send the measured voltage of the capacitor to the second computer, and wherein the second memory further includes instructions to:

determine a capacitive value of the capacitor from the voltage; and

determine a length of an extracted seatbelt from the capacitive value.

15. The system of claim 12 , wherein the computer, the second computer and at least one vehicle sensor communicate via a controller area network bus.

16. The system of claim 11 , wherein the memory further includes instructions to:

retrieve from the memory a predetermined current value and a time period;

measure a voltage of the capacitor over the time period;

determine a capacitive value of the capacitor from the voltage measurements over the time period; and

determine a length of an extracted seatbelt from the capacitive value.

17. The system of claim 16 , wherein the memory further includes instructions to send the voltage measurements to the second computer, and wherein the second memory further includes instructions to:

determine a capacitive value of the capacitor from the voltage measurements over the time period; and

determine a length of an extracted seatbelt from the capacitive value.