Optical sensor for mirror zero angle in a scanning lidar
The present disclosure relates to systems and methods that provide an accurate angle measurement of a rotatable mirror. An example method includes receiving, from a detector device, a reflected light signal. The reflected light signal is indicative of primary reflection light and secondary reflection light. The primary reflection light corresponds to a first portion of emission light that reflects directly from the reflective surface of the rotatable mirror toward the detector device. The secondary reflection light corresponds to a second portion of emission light that: 1) reflects from the reflective surface of the rotatable mirror toward a secondary mirror surface; 2) reflects from the secondary mirror surface toward the reflective surface of the rotatable mirror; and 3) reflects from the reflective surface of the rotatable mirror toward the detector device. The method also includes determining, based on the reflected light signal, the rotational angle of the rotatable mirror.
1. An optical system comprising:
a rotatable mirror, wherein the rotatable mirror is configured to rotate about a rotational axis;
a substrate;
a light-emitter device configured to emit emission light along an optical axis, such that the emission light interacts with a reflective surface of the rotatable mirror to provide reflected light;
a detector device, wherein the light-emitter device and the detector device are disposed along the substrate, wherein the detector device is configured to receive at least a portion of the reflected light;
a cylindrical lens, wherein the light-emitter device and the detector device are optically coupled to the rotatable mirror by way of the cylindrical lens, and wherein the detector device is configured to provide a reflected light signal indicative of a rotational angle of the rotatable mirror with respect to the rotational axis; and
a detector readout circuit, wherein the detector readout circuit comprises:
a transimpedance amplifier with a capacitive feedback network configured to convert a current pulse from the detector device into an analog signal; and
a digital comparator configured to provide a digital signal, wherein the digital signal comprises information indicative of rising and falling edges of the analog signal.
2. The optical system of claim 1 , wherein the cylindrical lens comprises a radius of curvature of between 1.5 mm to 2.5 mm, wherein the cylindrical lens is formed from polycarbonate.
3. The optical system of claim 1 , wherein the light-emitter device comprises a single mode vertical cavity surface emitting laser (VCSEL).
4. The optical system of claim 1 , wherein the detector device comprises a silicon PIN photodiode.
5. The optical system of claim 1 , wherein the light-emitter device and the detector device are separated along the substrate by a separation distance between 0.8 mm to 1.5 mm.
6. The optical system of claim 1 , wherein the cylindrical lens is configured to collimate the emission light and the reflected light.
7. The optical system of claim 1 , wherein the rotational angle corresponds to an orientation of the rotatable mirror such that the reflective surface of the rotatable mirror is perpendicular to the optical axis.
8. The optical system of claim 1 , wherein the rotatable mirror comprises a plurality of reflective surfaces, wherein the rotatable mirror has a triangular prism shape or a rectangular prism shape.
9. The optical system of claim 1 , further comprising:
a spacer, wherein the spacer comprises a light-emitter cavity and a detector cavity, wherein the spacer is coupled to the substrate and the cylindrical lens.
10. The optical system of claim 9 , wherein the spacer comprises a rectangular cavity with openings along a first surface of the spacer and an opposing second surface of the spacer.
11. The optical system of claim 1 , wherein the reflected light comprises primary reflection light, wherein the primary reflection light corresponds to a first portion of emission light that reflects directly from the reflective surface of the rotatable mirror toward the detector device.
12. The optical system of claim 11 , further comprising a secondary mirror surface disposed on the cylindrical lens, wherein the reflected light further comprises secondary reflection light, wherein the secondary reflection light corresponds to a second portion of emission light that: 1) reflects from the reflective surface of the rotatable mirror toward the secondary mirror surface; 2) reflects from the secondary mirror surface toward the reflective surface of the rotatable mirror; and 3) reflects from the reflective surface of the rotatable mirror toward the detector device.
13. The optical system of claim 12 , further comprising:
a controller having a processor and at least one memory, wherein the processor executes instructions stored in the at least one memory so as to carry out operations, the operations comprising:
receiving, from the detector device, the reflected light signal, wherein the reflected light signal is indicative of the primary reflection light and the secondary reflection light; and
determining, based on the reflected light signal, the rotational angle of the rotatable mirror.
14. The optical system of claim 13 , wherein the operations further comprise:
determining, based on the reflected light signal, a lens offset, wherein determining the rotational angle of the rotatable mirror is further based on the lens offset.
15. The optical system of claim 14 , wherein the secondary mirror surface is tilted at a tilt angle between 10 degrees to 20 degrees with respect to a plane perpendicular to the optical axis such that the reflected light signal comprises a primary reflection peak and a secondary reflection peak, and wherein determining the lens offset is further based on a mean angle difference between the primary reflection peak and the secondary reflection peak.
16. The optical system of claim 13 , wherein the operations further comprise:
receiving, from an angle encoder, an encoder angle corresponding to the rotatable mirror;
comparing the encoder angle to the rotational angle; and
based on the comparison, performing at least one of:
averaging the encoder angle and the rotational angle so as to provide a corrected rotational angle; or
determining an angle measurement fault.
17. The optical system of claim 1 , wherein the analog signal comprises a 1.5 volt peak-to-peak signal.
18. A method comprising:
causing a light-emitter device to emit emission light along an optical axis toward a rotatable mirror, such that the emission light interacts with a reflective surface of the rotatable mirror to provide reflected light, wherein the rotatable mirror is configured to rotate about a rotational axis;
receiving, from a detector device, a reflected light signal, wherein the reflected light signal is indicative of primary reflection light and secondary reflection light, wherein the primary reflection light corresponds to a first portion of emission light that reflects directly from the reflective surface of the rotatable mirror toward the detector device, wherein the secondary reflection light corresponds to a second portion of emission light that: 1) reflects from the reflective surface of the rotatable mirror toward a secondary mirror surface; 2) reflects from the secondary mirror surface toward the reflective surface of the rotatable mirror; and 3) reflects from the reflective surface of the rotatable mirror toward the detector device; and
determining, by a detector readout circuit and based on the reflected light signal, a rotational angle of the rotatable mirror, wherein determining the rotational angle of the rotatable mirror comprises:
converting, by a transimpedance amplifier with a capacitive feedback network, a current pulse from the detector device into an analog signal; and
providing, by a digital comparator, a digital signal comprising information indicative of rising and falling edges of the analog signal.
19. The method of claim 18 , further comprising:
determining, based on the reflected light signal, a lens offset, wherein determining the rotational angle of the rotatable mirror is further based on the lens offset.
20. The method of claim 18 , further comprising:
receiving, from an angle encoder, an encoder angle corresponding to the rotatable mirror;
comparing the encoder angle to the rotational angle; and
based on the comparison, performing at least one of:
averaging the encoder angle and the rotational angle so as to provide a corrected rotational angle; or
determining an angle measurement fault.