Processing system for LIDAR measurements
An optical measurement system may improve the accuracy with which it estimates distances to surrounding objects by upgrading various aspects of its data path. Spatial resolution may be increased by subdividing histogram buckets or integration registers based on spatial location. Saturation at any point in the data path can be detected and used to stop counting photons in individual pixels, which can then be normalized after a measurement is over. Multiple peaks can be detected using recursive or iterative techniques to identify a largest remaining peak at each stage. Instead of iterating through the histogram memory multiple times, a threshold can be pre-calculated based on an estimated ambient noise level, and peaks can be detected in a single pass.
1. An optical measurement system comprising:
a light source configured to transmit one or more pulse trains over one or more first time intervals as part of an optical measurement, wherein each of the one or more first time intervals includes one of the one or more pulse trains;
a photosensor comprising one or more photodetectors configured to detect photons from the one or more pulse trains;
a first integrated circuit comprising:
a plurality of first registers that accumulate photon counts from the one or more photodetectors received during the one or more first time intervals to represent a histogram of photon counts received during the one or more first time intervals, each of the plurality of first registers corresponding to a time bin in the histogram;
one or more integration registers that accumulate the photon counts from the one or more photodetectors over a second time interval that overlaps with at least a portion of the one or more first time intervals;
a threshold detection circuit configured to provide a threshold for identifying one or more peaks in the histogram, wherein the threshold detection circuit is configured to calculate the threshold using one or more values in the one or more integration registers; and
a peak detection circuit configured to make a pass through the plurality of first registers and identify the one or more peaks represented in the histogram using the threshold; and
a second integrated circuit comprising a processor, wherein the first integrated circuit is configured to send information describing the one or more peaks detected using the threshold to the processor on the second integrated circuit.
2. The optical measurement system of claim 1 , wherein the threshold detection circuit is configured to use an existing threshold.
3. The optical measurement system of claim 1 , wherein the threshold detection circuit is configured to calculate the threshold based on the background noise level present during the one or more first time intervals.
4. The optical measurement system of claim 1 , wherein the threshold detection circuit is further configured to calculate the background noise level by dividing a total photon count in the one or more integration registers by a duration of time during which the one or more integration registers were enabled.
5. The optical measurement system of claim 4 , wherein the duration of time during which the one or more integration registers were enabled is determined based on a total number of clock cycles during which the one or more integration registers were enabled.
6. The optical measurement system of claim 1 , wherein the threshold detection circuit is further configured to calculate the background noise level by:
identifying one or more time bins in the histogram that represent the one or more peaks; and
subtracting photon counts in the one or more time bins from a total photon count in the one or more integration registers.
7. The optical measurement system of claim 1 , further comprising a plurality of second registers configured to store the one or more peaks identified by the peak detection circuit.
8. The optical measurement system of claim 7 , wherein each peak of the one or more peaks is stored as:
a window of time bins representing a time interval in the histogram around the peak; and
a relative time in the histogram at which the peak occurred.
9. The optical measurement system of claim 1 , wherein the first integrated circuit is implemented on a first chip that is physically separate and distinct from a second chip on which the second integrated circuit is implemented.
10. The optical measurement system of claim 1 , wherein the one or more peaks are sent to the processor without applying a filter to values representing the one or more peaks in the histogram.
11. A method of using an optical measurement system, the method comprising:
transmitting one or more pulse trains over one or more first time intervals as part of an optical measurement, wherein each of the one or more first time intervals includes one of the one or more pulse trains;
detecting photons from the one or more pulse trains using one or more photodetectors;
accumulating, on a first integrated circuit, photon counts from the one or more photodetectors into a plurality of first registers to represent a histogram of photon counts received during the one or more first time intervals, each of the plurality of first registers corresponding to a time bin in the histogram;
accumulating, by one or more integration registers, photon counts from the one or more photodetectors over a second time interval that overlaps with at least a portion of the one or more first time intervals;
providing, on the first integrated circuit, a threshold for identifying one or more peaks in the histogram, wherein the threshold is calculated using one or more values in the one or more integration registers;
identifying, on the first integrated circuit, the one or more peaks represented in the histogram by making a pass through the plurality of first registers with the threshold; and
sending, from the first integrated circuit, information describing the one or more peaks detected using the threshold to a processor on a second integrated circuit.
12. The method of claim 11 , further comprising removing a background noise level from the one or more peaks represented in the histogram.
13. The method of claim 12 , further comprising determining the background noise level based on a sample of photons received by the one or more photodetectors between the one or more pulse trains.
14. The method of claim 11 , wherein providing the threshold comprises setting the threshold a predetermined interval above a background noise level.
15. The method of claim 11 , wherein providing the threshold comprises setting the threshold an interval above a background noise level that is a predetermined percentage of the background noise level.
16. The method of claim 11 , further comprising applying a low-pass filter to the histogram stored in the plurality of first registers prior to identifying the one or more peaks.
17. The method of claim 11 , further comprising applying a matched filter to the histogram stored in the plurality of first registers prior to identifying the one or more peaks, wherein the matched filter corresponds to the one or more pulse trains.
18. The method of claim 11 , wherein the one or more peaks comprises a first peak corresponding to reflections of the one or more pulse trains and a second peak that does not correspond to reflections of the one or more pulse trains.
19. The method of claim 11 , wherein identifying the one or more peaks represented in the histogram uses only a single pass through the plurality of first registers.