ToF framerate vs multipath

hmlow

Hi devs

I have been encountering issues where the forward facing ToF sensor (used for both VOA and mapping) detects "phantom" obstacles in an indoor environment.

It is very repeatable and always occurs at the same few spots; hence that lead me to think it might be because of reflections/multipath. I can see the phantom obstacle reported by voxl-vision-px4 in the linescan.

However, when i adjusted the ToF framerate from the default 5Hz to 15Hz (voxl-camera-server doesnt allow me to change to 10Hz), the problem goes away although the CPU load goes up.

Have you encountered this before?
What is the drawback of increasing the fps to 15Hz? Any undesired consequences that i should know of?

Im on VOXL2 SDK-0.9

Thanks

Chad Sweet

That is super interesting

15FPS will generally have shorter range. I'm not 100% sure, but this implies the emitter will be less strong. Maybe that is helping?

hmlow

@Chad-Sweet I haven't really tested specifically for the range but I'm sure I'm still seeing up to at least 3-4m obstacles being reported by the linescan.

VOA and mapping (of my indoor space) isn't affected as well.

Would 15Hz cause any of the services to function suboptimally (voxl-vision-px4? voxl-mapper?)

Chad Sweet

They should all adapt to the new framerate fine.

Darshit Desai

@Chad-Sweet @hmlow Hi can you tell me the process of increasing the frame rate for the ToF camera?

Alex Kushleyev

@hmlow ,

I know your post was a while ago.. But I wanted to add some detail.

Going to 15Hz is going to reduce the maximum exposure (duration of emitted IR signal) to about 1/3 of the maximum which would be for 5Hz. This is due to the fact that maximum power output remains the same and increasing FPS means less power is emitted during each frame. Longer exposure / IR pulse means the sensor can achieve higher signal to noise ratio and pick up returns at longer distances.

So that would make sense that you may see less phantom obstacles at 15hz compared to 5hz, if these obstacles are indeed coming from multi-path returns (weaker / shorter IR signal has less chance of being picked up by the sensor)