Methods and systems for lidar walk error correction
View Patent ↗A method for walk error correction in a lidar system. A preliminary peak location of a received laser pulsed is compared to a threshold. If the preliminary peak location is within the threshold, walk error correction is applied. A pulse starting point is obtained by locating a predetermined number of consecutive increasing intensity values. A corrected peak location is obtained using the pulse staring point, and it is used for time of flight calculation. There are other embodiments as well.
1 . A method for range determination, the method comprising:
transmitting, by a transmitting end, a pulsed laser at a first time, the pulsed laser being characterized by an initial total rise time between a pulse starting point and a pulse peak;
receiving an optical signal reflected by a target object;
converting the received optical signal into an analog electrical signal by a pixel circuit;
converting the analog electrical signal into histogram data using a time-to-digital converter (TDC), the histogram data comprising n intensity values corresponding to n time bins;
storing the histogram data in a memory module;
calculating a preliminary peak location using the histogram data, the preliminary peak location being associated with a target object distance, the preliminary peak location being associated with a highest peak intensity value of and a corresponding time bin location of the histogram data;
comparing the preliminary peak location to a threshold value, the threshold value being associated with a threshold distance;
if the preliminary peak location is less than the threshold value, determining the pulse starting point by locating m consecutive rising intensity values, and calculating a corrected peak location by adding a total rise time value, m being at least three; and
calculating a time of flight (TOF) value using the first time and a second time, the second time being based on the corrected peak location or the preliminary peak location; and
determining the target object distance.
2 . The method of claim 1 the total rise time value is determined using a reference total rise time associated with a reference received laser pulse, the reference received laser pulse having a reference peak location greater than the threshold value.
3 . The method of claim 1 further comprising applying a moving average filter to the histogram data.
4 . The method of claim 1 wherein the second time is based on the preliminary peak location if the preliminary peak location is greater than the threshold value.
5 . The method of claim 1 wherein the pulsed laser is characterized by a predetermined rise time that is optimized for detection of the pulse starting point.
6 . The method of claim 1 further comprising updating the histogram stored in the memory module based on the corrected peak location.
7 . The method of claim 1 further comprising applying an offset to the second time, the offset being based on a device characterization of the pixel circuit.
8 . The method of claim 1 wherein each of differences among adjacent m consecutive rising intensity values is greater than a threshold intensity value.
9 . The method of claim 1 wherein each of the m consecutive rising intensity values is greater than a threshold intensity value.
10 . The method of claim 1 wherein the threshold value corresponds to a walk error distance range, the walk error distance range being associated with an accuracy requirement or an intensity of the pulsed laser.
11 . The method of claim 1 wherein the preliminary peak location comprises a time bin value or a distance value.
12 . The method of claim 1 further comprising determining the preliminary peak location using a match filter.
13 . The method of claim 1 further comprising comparing a difference between the corrected peak location and a second peak location to a threshold, the second peak location being calculated using a match filter.
14 . A lidar system comprising:
a laser source configured to generate a pulsed laser at a first time, the pulsed laser being characterized by total rise time between a pulse starting point and a pulse peak;
an optical module configured receiving a reflected laser signal;
a pixel circuit configured to generate an electrical output based on the reflected laser signal;
a time-to-digital converter (TDC) configured to generate a histogram data using at least the electrical output, the histogram data comprising n intensity values corresponding to n time bins;
a memory device configured to store the histogram data; and
a processor module configured to:
calculate a preliminary peak location using the histogram data, the preliminary peak location being associated with a target object distance;
compare the preliminary peak location to a threshold value, the preliminary peak location being associated with a highest peak intensity value of and a corresponding time bin location of the histogram data;
if the preliminary peak location is less than the threshold value, determine the pulse starting point by locating m consecutive rising intensity values, and calculate a corrected peak location by adding a total rise time value to the pulse starting point, m being at least three; and
calculate a time of flight (TOF) value using the first time and a second time, the second time being based on a time bin location of the corrected peak location or the preliminary peak location; and
determine the target object distance.
15 . The system of claim 14 wherein the pixel circuit comprises a single-photon avalanche diode (SPAD).
16 . The system of claim 14 further comprising an optical splitter coupled to the laser source and a control module, the control module being configured to activate the TDC in response to the pulsed laser.
17 . The system of claim 14 wherein the processor module comprises a digital signal processor (DSP).
18 . The system of claim 14 further comprising a diffractive optical element for output configured to replicate the pulsed laser.
19 . The system of claim 14 further comprising a data medium configured to store instructions, the processor module being configured to execute the instructions.
20 . The system of claim 14 wherein the laser source is characterized by an output intensity optimized for a predetermined range, the threshold value being calibrated based on the predetermined range.