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Author Topic: Time of Flight >>> Rounded Corners !!!  (Read 3942 times)

Gareth

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Time of Flight >>> Rounded Corners !!!
« on: November 07, 2018, 03:46:01 AM »


I placed the Sensor rig into a cardboard box just to get a feeling for the data.

This is two passes of the probe, so you can see the slight variance in data.

Overlaying the raw data for 180°.... this was with GIMP so its just approximate

all looks kinda good except Hmmmm.... rounded corners cannot be explained...

another scan with DVD's :-



maelh

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #1 on: November 11, 2018, 05:25:15 AM »
I am also looking to use this sensor, so I wonder what causes the rounded corner. From what I remember it has a limited resolution when things are close by, it might be caused by that.

MEgg

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #2 on: November 12, 2018, 01:01:49 PM »
How fast does this turn?
What happens if you turn slower (if possible at all)?
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

Gareth

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #3 on: November 13, 2018, 03:45:41 AM »
What happens if you turn slower (if possible at all)?
I have experimented and there is no difference by slowing it down. (My mind is boggling how it can measure the time it takes for light to travel 5cm)
From the specs the measurement acquisition time is around 40ms ... my current scan rate for 360° is 3 seconds.

Gareth

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #4 on: November 13, 2018, 03:53:46 AM »
so I wonder what causes the rounded corner.
The sensors is darn amazing if you ask me.... there are many hidden register codes which allow you to tune it to your way of using it..
At the moment I am still none the wiser with regards to the rounding.
one guess is that it is picking up reflections (after all it is a IR laser) so a lot of scattering of light is possible, it has good resistance to ambient light (compared to the Sharp sensors (anyone still use them?)).
Another guess is the the laser is TXed in a looooong cone shape and also the received detector receives as a cone. (this does not help I am speculating with my first guess above).

Gareth

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #5 on: November 13, 2018, 04:07:31 AM »
By the Way the Module will pick up a 3mm vertical BBQ stick at distance, not many sensors have this capability  ???

MEgg

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #6 on: November 14, 2018, 05:57:24 PM »
What happens if you turn slower (if possible at all)?
I have experimented and there is no difference by slowing it down. (My mind is boggling how it can measure the time it takes for light to travel 5cm)
From the specs the measurement acquisition time is around 40ms ... my current scan rate for 360° is 3 seconds.

Shouldn't that mean that for each 1° of the 360° you need at least 40ms, which is
40ms * 360° ->  14.4s ?
If it is possible to scan per 1°.
« Last Edit: November 14, 2018, 06:03:29 PM by MEgg »
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

MEgg

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #7 on: November 14, 2018, 06:26:08 PM »
Also from the central position of sensor S:



from the sensor S to the edge of the front line of the cuboid from my understanding
the beam 1 would be > beam 2 > beam 3 < beam4
and so on, if it goes straight.
Just a simplification but I guess this draws a curve over 1,2,3,4
I guess the cone shape you are talking about gives a bit uncertainty also
which seems to be quite narrow from your data.
The distance should be a line if you drive the sensor from left to right in the same distance.
I think this would be the same with all central stationary ToF sensors, which rotate but do not move from left to right in this case.
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

Gareth

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #8 on: November 15, 2018, 03:13:11 PM »
Here are the specs for the TOF showing the cones detect/emit (which I think contributes to the miss detect)
tof.png
*tof.png (87.1 kB . 914x561 - viewed 178 times)

Gareth

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #9 on: November 15, 2018, 03:21:51 PM »
Shouldn't that mean that for each 1° of the 360° you need at least 40ms, which is
40ms * 360° ->  14.4s ?
If it is possible to scan per 1°.
ahha... I should explain that the stepper is cheating at the moment meaning 360°=400 steps of the stepper.... and I am operating 2 sensors at same time, one for front 180° and second for back 180°.
Indeed yes when the stepper is placed into 1/16 step mode.... which means 360°= 6400 steps so you can achieve quite a resolution (at the expense of speed)

BaldwinK

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Re: Time of Flight >>> Rounded Corners !!!
« Reply #10 on: December 04, 2019, 05:46:08 PM »
I know I am late to the party but firstly thank you for introducing me to the VL53L0X laser rangefinder, hiding behind an uninformative descriptor.

I liked your development approach to the LIDAR project. A radio data link to avoid slip rings, electronic compass for direction etc. At that stage perhaps self powering the rotating assembly and spinning on a geared motor?

I had planned to mount the chip vertically and rotate a mirror to see how it scanned but having eventually taken delivery of a couple of devices that still hasn’t happened.

I, too, wondered about the rounded corners of the point map and note the discussion on radiated cones. This video clip shows the comparison against the HC-SR04 ultrasonic unit which I have used many times. You can see how viewing the laser via an electronic camera shows the cone of pulsed light.


Pursuing the rounded corners issue, there is an accuracy problem that others have reported. This video clip shows similar experiments to yours and the humorous but unhappy conclusion.


So instead I have used my time to delve deeper into the VL53L0X chip itself because it is so much smaller than the ultrasonic units and it would be nice to have a simple replacement for collision detection. Findings in the Tutorial section.

 

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