Processing system for lidar measurements

Abstract

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.

What is claimed is: 
     
         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 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 a time bin in the histogram;   a peak detection circuit configured to identify a maximum peak in at least a portion of the histogram in the plurality of first registers each time the peak detection circuit executes; and   a masking circuit configured to cause the peak detection circuit to exclude registers corresponding to peaks identified as a maximum peak during a previous execution of the peak detection circuit from being identified as a maximum peak during a subsequent execution of the peak detection circuit, such that a plurality of executions of the peak detection circuit identifies a plurality of peaks.   
     
     
         2 . The optical measurement system of  claim 1 , further comprising a plurality of second registers configured to store the plurality of peaks identified by the peak detection circuit. 
     
     
         3 . The optical measurement system of  claim 2 , wherein each of the plurality of peaks is stored as a series of time bins representing a time interval in the histogram around the peak. 
     
     
         4 . The optical measurement system of  claim 2 , wherein each of the plurality of peaks is stored with a relative time in the histogram at which the peak occurred. 
     
     
         5 . The optical measurement system of  claim 1 , wherein the optical measurement system is configured to send the plurality of peaks identified by the peak detection circuit to a processor, wherein the processor operates on a first integrated circuit chip that is distinct from a second integrated circuit chip on which the peak detection circuit operates. 
     
     
         6 . The optical measurement system of  claim 5 , wherein the plurality of peaks are sent to the processor without applying a filter to values representing the plurality of peaks. 
     
     
         7 . The optical measurement system of  claim 1 , wherein each execution of the peak detection circuit is configured to cycle through the plurality of first registers and identify a maximum value in the plurality of first registers that has not been excluded by the masking circuit. 
     
     
         8 . The optical measurement system of  claim 7 , wherein a maximum peak comprises a subset of the plurality of first registers centered around one of the plurality of first registers storing the maximum value. 
     
     
         9 . The optical measurement system of  claim 1 , wherein the peak detection circuit is configured to cycle through the plurality of first registers at least three times to identify at least three maximum peaks in the histogram. 
     
     
         10 . The optical measurement system of  claim 1 , wherein the one or more photodetectors comprises one or more single-photon avalanche diodes (SPADs). 
     
     
         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 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 a time bin in the histogram; and   identifying a plurality of peaks in the histogram in the plurality of first registers through a plurality of executions of a peak detection circuit, wherein each execution of the peak detection circuit identifies a maximum peak in at least a portion of the histogram, and wherein registers corresponding to peaks identified as a maximum peak during a previous execution of the peak detection circuit are excluded from being identified as a maximum peak during a subsequent execution of the peak detection circuit.   
     
     
         12 . 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 plurality of peaks. 
     
     
         13 . 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 plurality of peaks, wherein the matched filter corresponds to the one or more pulse trains. 
     
     
         14 . The method of  claim 11 , wherein the peaks identified as maximum peaks during previous executions of the peak detection circuit are excluded from being identified as maximum peaks during subsequent executions of the peak detection circuit by:
 masking registers in the plurality of first registers that represent the peaks previously identified as maximum peaks from the peak detection circuit during subsequent executions.   
     
     
         15 . The method of  claim 11 , wherein the peaks identified as maximum peaks during previous executions of the peak detection circuit are excluded from being identified as maximum peaks during subsequent executions of the peak detection circuit by:
 setting a threshold at or below a maximum value from the peaks previously identified as maximum peaks; and   excluding peaks during subsequent executions of the peak detection circuit that meet or exceed the threshold.   
     
     
         16 . The method of  claim 15 , wherein excluding peaks during subsequent executions of the peak detection circuit that meet or exceed the threshold comprises:
 identifying a register in the plurality of first registers with a value that meets or exceeds the threshold; and   excluding registers around the register in the plurality of first registers.   
     
     
         17 . The method of  claim 11 , further comprising excluding an initial peak in the histogram from executions of the peak detection circuit. 
     
     
         18 . The method of  claim 17 , wherein the initial peak results from a reflection of the one or more pulse trains off a housing of the optical measurement system. 
     
     
         19 . The method of  claim 17 , wherein the initial peak meets or exceeds a saturation threshold for a saturation detection circuit. 
     
     
         20 . The method of  claim 19 , further comprising masking the initial peak from the saturation detection circuit.