ST VL6180X - Manual Offset Calibration Procedure; VL6180 X Signal Rate Analysis

Typical ranging output AN4545
12/27 DocID026571 Rev 1
4.1.1 Manual offset calibration procedure
This section describes a procedure for calibrating offset.
1. Clear the system offset
a) Write 0x00 to SYSRANGE__PART_TO_PART_RANGE_OFFSET {0x24}.
2. Place a target at 50mm away from VL6180X.
a) Using a target with 17% reflectance or higher is recommended.
b) When calibrating with cover glass we recommended using a target with
reflectance of 88% or higher to minimise error from cross talk, cross
3. Collect a number of range measurements with the target in place and calculate mean
of the range results.
a) For a reliable measurement, take at least 10 measurements.
4. Calculate the offset required:
5. Apply offset:
a) Write the calculated offset value to
SYSRANGE__PART_TO_PART_RANGE_OFFSET {0x24}.
Note: {0x24} is in 2s complement representation. For offset value 0 to127, write 0x00 to 0x7f.
For offset value -1 to -128, write 0xff to 0x80, e.g -30 = 0xe2
4.2 Signal rate
Figure 6 shows the return signal rate output from VL6180X for different targets at different
distances. The return signal rate can provide useful information on how optimised VL6180X
settings to perform more efficiently and reliably.
The return signal rate from a target will generally follow the inverse square law as the target
moves away from VL6180X. Assume the target fills the VL6180X’s field of view
(d)
and is at a
distance >10mm.
As target distance drops below 10mm, the target will start to block off the optical path
between the VCSEL and the return array. A target at <10mm can reduce the signal to a level
that can prevent the VL6180X from completing a range measurement.
Note: Cover glass can alter the return signal rate characteristics. We therefore recommend the
signal rate be re-characterized once the VL6180X has been integrated into the final system
before attempting to perform any system optimisation.
Offset Target Distance [mm] Average range [mm]–=
Offset 50 Average range [mm]–=
d. The field or view of VL6180X is approximately ±12.5°.