Sphere based registration is not accurate, I need some help

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Augusto 3D
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

So here is the crazy thing...
If I run c2c everything registers but the spheres seem "moved".

What does that tell you guys?
It tells me that maybe the spheres arent all that great quality wise regarless of what the vendor claims, or I dont know.
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Re: Sphere based registration is not accurate, I need some help

Post by jorgen.olsson66 »

Hi Augusto,
Please check the "overlap" area in one of your scans. The area were the scanner start and end, the easiest way to check it is against a flat area like a wall on 20-40 meters. If this overlap is not even does it mean that the scanner measure wrong and this could explain your result.
I have a scanner in past which had that behavour and it was really a pain...
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Re: Sphere based registration is not accurate, I need some help

Post by yankoch »

Hello,

When I read the registration report, it seems that you have only 2 spheres between Exterior 001 and 003.

It also seems that Sphere and Sphere 2 in Exterior 001 create the highest tensions.

As Leandre wrote above, you should have 3 spheres between 2 scans, otherwise you have to rely on 2 spheres + inclinometer.
I don't know the distance you have between 2 stations, but the inclinometer can also create some tensions between scans.
So, one of the first thing to do when you have high tensions is to uncheck the inclinometer in some scans. But in your case, Exterior 001 and 003 won't be able to register correctly. In this case, try to create a plane in addition to the spheres.
Create 2 new clusters: the first with Exterior 001 only, and the second with all the other scans. Then, start the registration of the second cluster. If everything's fine, then you have a problem with Exterior 001.

So, when I see a report in which the same scan or the same spheres are always at the top of the tension list:
- I check that the sphere hasn't moved (as Lieven said, measure the distance between 2 spheres in 2 different scans),
- I uncheck the inclinometer of the scan, then I start the registration again,
- I isolate the scan in a new cluster and start the registration of the other scans again,
- if I don't have enough spheres between the scans, I try to create reliable planes.
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

yankoch wrote: Tue Jun 21, 2022 9:29 am Hello,

When I read the registration report, it seems that you have only 2 spheres between Exterior 001 and 003.

It also seems that Sphere and Sphere 2 in Exterior 001 create the highest tensions.

As Leandre wrote above, you should have 3 spheres between 2 scans, otherwise you have to rely on 2 spheres + inclinometer.
I don't know the distance you have between 2 stations, but the inclinometer can also create some tensions between scans.
So, one of the first thing to do when you have high tensions is to uncheck the inclinometer in some scans. But in your case, Exterior 001 and 003 won't be able to register correctly. In this case, try to create a plane in addition to the spheres.
Create 2 new clusters: the first with Exterior 001 only, and the second with all the other scans. Then, start the registration of the second cluster. If everything's fine, then you have a problem with Exterior 001.

So, when I see a report in which the same scan or the same spheres are always at the top of the tension list:
- I check that the sphere hasn't moved (as Lieven said, measure the distance between 2 spheres in 2 different scans),
- I uncheck the inclinometer of the scan, then I start the registration again,
- I isolate the scan in a new cluster and start the registration of the other scans again,
- if I don't have enough spheres between the scans, I try to create reliable planes.
thank you for your detailed response (and everyone else). I think that the culprit here was the positioning of the spheres and the fact that only a handful was used. I am going to run some more tests and try to do better planning next time. Maybe mix them with paper targets since the number of spheres is usually limited. Luckily this time I was able to fix the registration via C2C and using some planes as suggested.

From what I am gathering, it sounds like it is safe to mix spheres and checkers for target-based registration?
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Re: Sphere based registration is not accurate, I need some help

Post by landmeterbeuckx »

Augusto 3D wrote: Thu Jun 23, 2022 5:05 pm
yankoch wrote: Tue Jun 21, 2022 9:29 am Hello,

When I read the registration report, it seems that you have only 2 spheres between Exterior 001 and 003.

It also seems that Sphere and Sphere 2 in Exterior 001 create the highest tensions.

As Leandre wrote above, you should have 3 spheres between 2 scans, otherwise you have to rely on 2 spheres + inclinometer.
I don't know the distance you have between 2 stations, but the inclinometer can also create some tensions between scans.
So, one of the first thing to do when you have high tensions is to uncheck the inclinometer in some scans. But in your case, Exterior 001 and 003 won't be able to register correctly. In this case, try to create a plane in addition to the spheres.
Create 2 new clusters: the first with Exterior 001 only, and the second with all the other scans. Then, start the registration of the second cluster. If everything's fine, then you have a problem with Exterior 001.

So, when I see a report in which the same scan or the same spheres are always at the top of the tension list:
- I check that the sphere hasn't moved (as Lieven said, measure the distance between 2 spheres in 2 different scans),
- I uncheck the inclinometer of the scan, then I start the registration again,
- I isolate the scan in a new cluster and start the registration of the other scans again,
- if I don't have enough spheres between the scans, I try to create reliable planes.
thank you for your detailed response (and everyone else). I think that the culprit here was the positioning of the spheres and the fact that only a handful was used. I am going to run some more tests and try to do better planning next time. Maybe mix them with paper targets since the number of spheres is usually limited. Luckily this time I was able to fix the registration via C2C and using some planes as suggested.

From what I am gathering, it sounds like it is safe to mix spheres and checkers for target-based registration?
ALways try to have at least 5 in every scan.
Paper targets are imho only of use to connect to a coordinate system. You'll have to scan nearly perpendicular to them to avoid errors.
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Augusto 3D
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

landmeterbeuckx wrote: Thu Jun 23, 2022 7:15 pm
Augusto 3D wrote: Thu Jun 23, 2022 5:05 pm
yankoch wrote: Tue Jun 21, 2022 9:29 am Hello,

When I read the registration report, it seems that you have only 2 spheres between Exterior 001 and 003.

It also seems that Sphere and Sphere 2 in Exterior 001 create the highest tensions.

As Leandre wrote above, you should have 3 spheres between 2 scans, otherwise you have to rely on 2 spheres + inclinometer.
I don't know the distance you have between 2 stations, but the inclinometer can also create some tensions between scans.
So, one of the first thing to do when you have high tensions is to uncheck the inclinometer in some scans. But in your case, Exterior 001 and 003 won't be able to register correctly. In this case, try to create a plane in addition to the spheres.
Create 2 new clusters: the first with Exterior 001 only, and the second with all the other scans. Then, start the registration of the second cluster. If everything's fine, then you have a problem with Exterior 001.

So, when I see a report in which the same scan or the same spheres are always at the top of the tension list:
- I check that the sphere hasn't moved (as Lieven said, measure the distance between 2 spheres in 2 different scans),
- I uncheck the inclinometer of the scan, then I start the registration again,
- I isolate the scan in a new cluster and start the registration of the other scans again,
- if I don't have enough spheres between the scans, I try to create reliable planes.
thank you for your detailed response (and everyone else). I think that the culprit here was the positioning of the spheres and the fact that only a handful was used. I am going to run some more tests and try to do better planning next time. Maybe mix them with paper targets since the number of spheres is usually limited. Luckily this time I was able to fix the registration via C2C and using some planes as suggested.

From what I am gathering, it sounds like it is safe to mix spheres and checkers for target-based registration?
ALways try to have at least 5 in every scan.
Paper targets are imho only of use to connect to a coordinate system. You'll have to scan nearly perpendicular to them to avoid errors.
That is true, but sometimes it hard to place balls everywhere - even if I had that many.
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Re: Sphere based registration is not accurate, I need some help

Post by Phill »

Augusto

Check my post on this feed

https://www.laserscanningforum.com/foru ... 783#p86783

There is a report there. I did a whole stack of testing with spheres. Two things I noticed were - the laser will penetrate a small amount into the translucent objects (styrafoam, plastic) which is what a lot of these balls are made from. Basically if any light gets through it will give you an error similar to the one you are seeing as there is some amount of penetration into the object. I found that the only way to fix this was a layer of matt black paint then a layer of white over the top. The also changes depending on the laser wavelength (the C10 was different to the P40)

The other thing - not listed there but from experience when I use the spheres is quite often when I register with spheres the over spray at the edge of the sphere is included in the calculation and pushes the calculated target point back into the sphere slightly. I normally fix this by using a fence select and not selecting the edges of the spheres.

Saying that, I only use the spheres when I don't have access to my tilt and turns, and only for small amounts of scans. I also use Leica software and scanners (P40/Cyclone) so I cannot speak for Faro and Scene, but it might be worth investigating if these issues are affecting your target registration.

Hope this helps

Phill
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Re: Sphere based registration is not accurate, I need some help

Post by Augusto 3D »

Thanks for the info and links. I am using to Koppa sphere so I don't think that they are bad in this case.

We scanned another job this week and used a mix is spheres and self printed paper targets. I ran a quick registration and noticed that the most error was coming from 2 or 3 paper targets (0.5").

The spheres I believe were at 0.10 or so which is acceptable. (Less than 3mm). I am going to check the registration report again and post it here.

I did have to unassociate some spheres between scans and turn off the inclinometer (although I dont thing the later made a difference)

Speaking of home printed targets, I a matte paper better than a eggshells type?

I am going to order 2 or 3 seko tilt and turns.
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