Selection of pulse repetition intervals for sensing time of flight
Sensing apparatus includes a radiation source, which emits pulses of optical radiation toward multiple points in a target scene. A receiver receives the optical radiation that is reflected from the target scene and outputs signals that are indicative of respective times of flight of the pulses to and from the points in the target scene. Processing and control circuitry selects a first pulse repetition interval (PRI), a second PRI, greater than the first PRI, and a third PRI, greater than the second PRI, from a permitted range of PRIs, drives the radiation source to emit sequences of the pulses at the first PRI, the second PRI, and the third PRI, and processes the signals output by the receiver in response to the first, second, and third sequences of the pulses in order to compute respective depth coordinates of the points in the target scene.
1. Sensing apparatus, comprising:
a radiation source, which is configured to emit pulses of optical radiation toward multiple points in a target scene;
a receiver, which is configured to receive the optical radiation that is reflected from the target scene and to output signals, responsively to the received optical radiation, that are indicative of respective times of flight of the pulses to and from the points in the target scene; and
processing and control circuitry, which is configured to select a first pulse repetition interval (PRI), a second PRI, greater than the first PRI, and a third PRI, greater than the second PRI, from a permitted range of PRIs, and to drive the radiation source to emit a first sequence of the pulses at the first PRI, a second sequence of the pulses at the second PRI, and a third sequence of the pulses at the third PRI, and to process the signals output by the receiver in response to the first, second, and third sequences of the pulses in order to compute respective depth coordinates of the points in the target scene.
2. The apparatus according to claim 1 , wherein the radiation source comprises an array of vertical-cavity surface-emitting lasers (VCSELs).
3. The apparatus according to claim 1 , wherein the radiation source comprises an array of emitters, which are arranged in multiple banks, and wherein the processing and control circuitry is configured to drive the multiple banks sequentially so that each bank emits respective first, second, and third sequences of the pulses at the first, second, and third PRIs.
4. The apparatus according to claim 1 , wherein the receiver comprises an array of single-photon avalanche diodes (SPADs).
5. The apparatus according to claim 1 , wherein the first PRI defines a range limit, at which a time of flight of the pulses is equal to the first PRI, and wherein the processing and control circuitry is configured to compare the signals output by the receiver in response to the first, second, and third sequences of pulses in order to distinguish the points in the scene for which the respective depth coordinates are less than the range limit from the points in the scene for which the respective depth coordinates are greater than the range limit, thereby resolving range folding of the depth coordinates.
6. The apparatus according to claim 5 , wherein the processing and control circuitry is configured to compute, for each of the points in the scene, respective first, second, and third histograms of the times of flight of the pulses in the first, second, and third sequences, and to detect that range folding has occurred at a given point responsively to a difference between two or more of the first, second, and third histograms.
7. The apparatus according to claim 1 , and comprising a radio transceiver, which communicates over the air by receiving signals in an assigned frequency band, wherein the processing and control circuitry is configured to identify the permitted range of the PRIs responsively to the assigned frequency band.
8. The apparatus according to claim 7 , wherein the processing and control circuitry is configured to store a record of multiple groups of the PRIs, to identify an operating environment of the apparatus, and to select one of the groups to apply in driving the radiation source responsively to the identified operating environment.
9. The apparatus according to claim 1 , wherein the first, second, and third PRIs are co-prime relative to a time scale used in processing the signals.
10. The apparatus according to claim 1 , wherein the processing and control circuitry is configured to select the first, second, and third PRIs so as to maximize a range of the depth coordinates while maintaining a resolution of the depth coordinates to be no greater than a predefined resolution limit.
11. The apparatus according to claim 1 , wherein the processing and control circuitry is configured to drive the radiation source to emit the first, second, and third sequences of the pulses repetitively over a succession of frames having a predefined frame duration, and to select the first, second, and third PRIs so that a cumulative duration of the first, second, and third sequences of the pulses is no more than 80% of the frame duration.
12. The apparatus according to claim 11 , wherein the processing and control circuitry is configured to define a sleep period in each frame following emission of the first, second, and third sequences of the pulses, wherein the radiation source and the receiver operate in a low-power mode during the sleep period.
13. A method for sensing, comprising:
selecting a first pulse repetition interval (PRI) and a second PRI, greater than the first PRI, and a third PRI, greater than the second PRI, from a permitted range of PRIS;
driving a radiation source to emit a first sequence of pulses of optical radiation at the first PRI, a second sequence of the pulses of the optical radiation at the second PRI, and a third sequence of the pulses at the third PRI toward each of multiple points in a target scene;
receiving the optical radiation that is reflected from the target scene and outputting signals, responsively to the received optical radiation, that are indicative of respective times of flight of the pulses to and from the points in the target scene; and
processing the signals output in response to the first, second, and third sequences of the pulses in order to compute respective depth coordinates of the points in the target scene.
14. The method according to claim 13 , wherein the first PRI defines a range limit, at which a time of flight of the pulses is equal to the first PRI, and wherein processing the signals comprises comparing the signals output in response to the first, second, and third sequences of pulses in order to distinguish the points in the scene for which the respective depth coordinates are less than the range limit from the points in the scene for which the respective depth coordinates are greater than the range limit, thereby resolving range folding of the depth coordinates.
15. The method according to claim 14 , wherein processing the signals comprises computing, for each of the points in the scene, respective first, second, and third histograms of the times of flight of the pulses in the first, second, and third sequences, and detecting that range folding has occurred at a given point responsively to a difference between two or more of the first, second, and third histograms.
16. The method according to claim 13 , wherein selecting the first, second, and third PRIS comprises identifying the permitted range of the PRIs responsively to an assigned frequency band of a radio transceiver, which communicates over the air in the assigned frequency band in proximity to the radiation source, and choosing the first PRI and the second PRI from the permitted range.
17. The method according to claim 13 , wherein selecting the first, second, and third PRIs comprises storing a record of multiple groups of the PRIS, and selecting one of the groups to apply in driving the radiation source.
18. The method according to claim 13 , wherein the first, second, and third PRIs are co-prime relative to a time scale used in processing the signals.
19. The method according to claim 13 , wherein driving the radiation source comprises causing the radiation source to emit the first, second, and third sequences of the pulses repetitively over a succession of frames having a predefined frame duration, and
wherein selecting the first, second, and third PRIS comprises choosing the first, second, and third PRIs so that a cumulative duration of the first, second, and third sequences of the pulses is no more than 80% of the frame duration.
20. The method according to claim 19 , and comprising defining a sleep period in each frame following emission of the first, second, and third sequences of the pulses, wherein the radiation source and a receiver that receives the optical radiation operate in a low-power mode during the sleep period.