VL53L4CD Distance Measurement Non-Linearity of High Reflectance Objects in Close Proximity (5-30mm)

[PMT_RD]

Hello,

Our application requires detecting the presence of a low-reflectance object when inserted between the TOF sensor and a highly reflective object (polished brass). The inserted low-reflectance object is at a distance of 15mm and the high-reflectance object is at 30mm.

The measured distance when the low-reflectance object is not inserted is ~15mm instead of the actual 30mm. Replacing the highly-reflective polished brass with lower reflectance material, for example paper with shading from white, to gray, to black, provides the correct distance measurements of objects from 30mm down to 5mm.

I have observed that the TOF sensor SPAD (kcps) value returned in conjunction with the highly reflective brass is quite high being > 60,000 and can actually exceed the 16-bit range of 65,535 and roll over into values of 200 to 3000. The lower reflective material SPAD values range from 10,000 to 30,000.

Modifying the Sigma Threshold and Signal Threshold do not influence this behaviour. It would appear that the Sensor input stage is saturating as a consequence of the signal strength and, therefore, transitioning from a linear operating point to a non-linear region.

I would sincerely appreciate any insight towards resolving this issue.

Note: Timing Budget set for 200msec; Intermeasurement Time set for 0msec. 

Thank-you.

[John E KVAM]

At those distances you do not need a timing budget of 200ms, I'd suggest that 30ms would be much better. And if you only want 5 frames per second, set the inter-measurement period to 200ms. 

I think that will solve the over-flow problem. And it will save you a lot of power. 

if it does not, query the Number-of-SPADs field. With a highly reflective target you should be down to the minimum. With a dull target you should jump up quite a bit. 

(Sorry for the slow reply. I went to CES, and like a lot of my friends got pretty sick.)

[PMT_RD]

Hello John,

Thank-you very much for your kind assistance.

I have not had opportunity to implement your suggestion but will do so at the earliest opportunity and advise the results for your consideration.

Best regards,

RD