Inconsistent behaviour of VL53L1X ToF Sensors

[ACher.4]

Hi, We have been testing the VL53L1X for a while. The primary intend is to measure liquid level. Liquid can be water, coffee, windshield washer fluid, etc. We are seeing some issues that we cannot seem to figure out. Hoping for any insights.

Premise.

1. We are using Arduino platform for testing.
2. We have tried multiple libraries (sparkfun, polulu, etc)
3. Test involves a large container filled with liquid (say 10 inches), and sensors are placed above the liquid at 3-5 inches from the surface of the liquid (13-15 inches from the bottom of the container). Readings are taken every few seconds.

Concerns.

1. When we have multiple identical sensors placed over the liquid, We observe a variation in measurement of upto an inch in some sensors, which is a huge percentage error. Is this normal? Just need to set offset adjustment?
2. We did read somewhere that liquids can absorb some light. However, when we start the test, it looks like the sensor readings make sense. ie IT measures around 4 inches. (disregarding offsets). But after a week or so, some sensors start to pick up 10 to 14 inches. The only thing we can think of is that the light is bouncing from the bottom? Nothing has changed as far as the setup and liquid content goes. What could be causing this sudden difference?
3. Say if a sensor has an offset of 10mm, is it necessary/mandatory that we do the offset calibration on the sensor itself, or can we get the reading and correct it at a later stage in software? In other words, is this just an offset adjustment or a calibration where there is some kind of calibration curve that is being adjusted?

[ACHOU.18]

Hi ,

Liquid level measurment is tricky one because photons are travelling through Air and Water. As a result, light path is being impacted because of their different refractive indices. In other words, there is a possibility of part of the signal reflected from the surface, partly travelled through water and returned from the container’s base and partly multi bounce from the water itself. Thus, the final ranging distance is impacted with respect to accuracy.

So there is a possiblity that when the liquid level is less then half sensor may give over ranging as signal may reflect from suface as well. if liquid level is more then half then may get under ranging because partly signals absorbed by water.

So you could do post offset correction for over ranging part and for underranging (refractive index of liquid) + offset correction to get good accuracy.

When I say offset correction its not offset calibration , its post offset correction of recived ranging measurment.

Regards,

Aditya

[Eleon BORLINI]

Hi @ACher.4​ ,

any feedback for Aditya answer?

-Eleon

[ACher.4]

Hi Aditya, Eleon,

Sorry I could not respond earlier due to other commitments. We did play around with the timing budget and intermeasurement period (we only need very slow rates) and saw better results. Tests are still running, however, we got pushed away from this sensor for a while, but we plan on getting back to this shortly. I will update you with my findings thereafter. Thank you for the follow up.

A. Cherian