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Walk error calibration

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iliasam edited this page May 12, 2022 · 11 revisions

This calibration in needed to compensate walk error - error in time measurement, that appears because of changing amplitude of received signal.

To compensate this error, MCU can calculate a special correction, based on measured pulse width (pulse width is correlated with pulse amplitude).
See this description: https://github.com/iliasam/OpenTOFLidar/wiki/TDC-data-processing

Collecting calibration data

Firstly you will need to collect data to process.
Data must be collected with stopped mirror.
Calibration is correct only for LIDAR parameters that was set during calibration (laser voltage, APD voltage, comparator threshold voltage).
All measurement must be done at the same distance, 1.5m for example.

  • You need to run "LidarTestingUtility", open connection, confirm that distance is measured stabile.
  • Set "Auto Request Batch" switch.
  • Confirm that received signal is really high (see histogram width and "AVR WIDTH" value).
  • Press "START" button in "Slow Recording" panel.
  • Begin to close slowly the receiving lens with something opaque. Confirm that light from laser can't reflect from that opaque object, and this object is not closing laser beam.
  • Confirm that you do not touch LIDAR, so distance must be stable during measurements.
  • Press "STOP" button in "Slow Recording" panel when received signal will be low (when histogram will be too wide and unstable). You need to collect at least 20-30 points with different amplitude using this method.

Processing calibration data

  • You need to open "Calibration" page in "LidarTestingUtility"
  • Load calibration data using "Last File" or "Select File" buttons in "Open" group. Collected data should look so (every captured point is marked by small cross):

    Horizontal axis is "WIDTH" in BINs, vertical axis is "START" offset (minimal "START" value from the log is used as zero level).
  • You need to move "Fit Start" slider to set start position of the fitting process and press "Fit" button.
    The resulting curve would be shown like at the picture above (red line). If this resulting curve looks fine, you can load its coefficients to the MCU by pressing "Send Coeffs. to MCU" button.
  • Press "Save to Flash" at "Main" page in "LidarTestingUtility".
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