Dual axis compensator VS IMU

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Rikore
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Re: Dual axis compensator VS IMU

Post by Rikore » Wed Oct 03, 2018 3:09 pm

I believe, but might not be 100% correct, that a dual-axis compensator is used to correct for horizontal and vertical angles due to deviation of the vertical axis in the scanners as well as other surveying equipment. And IMU or known as Inertial Navigation System uses raw measurements to calculate angles and velocity relative to a global reference. Used a lot in aircraft and military vehicles. IMU's could be used in mobile scanning and drones. One disadvantage of IMU's is they accumulate error. Not sure if this helps give an overview or not, but I believe IMU's would not be as accurate as a dual-axis compensator.

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Re: Dual axis compensator VS IMU

Post by Scott.Warren » Wed Oct 03, 2018 4:11 pm

Rikore wrote:
Wed Oct 03, 2018 3:09 pm
I believe, but might not be 100% correct, that a dual-axis compensator is used to correct for horizontal and vertical angles due to deviation of the vertical axis in the scanners as well as other surveying equipment. And IMU or known as Inertial Navigation System uses raw measurements to calculate angles and velocity relative to a global reference. Used a lot in aircraft and military vehicles. IMU's could be used in mobile scanning and drones. One disadvantage of IMU's is they accumulate error. Not sure if this helps give an overview or not, but I believe IMU's would not be as accurate as a dual-axis compensator.
This ^
The IMU would most likely have the ability to detect how level the instrument is, but is not nearly as sensitive as a the Dual Axis Compensator. Where a DAC will be able to resolve how level the instrument is so such a high sensitivity/frequency that it will help remove small vibrations/deviations from data collection, the IMU would most definitely not be able to do so in such a reliable manner.

RTC360 - IMU - 3' for any tilt (3 Minutes of Arc)
P50 -Liquid sensor with real-time onboard compensation, resolution 1”, dynamic range ±5’, accuracy 1.5”

A clear difference of 3' (180") for the RTC 360 vs ~1.5" for a survey grade DAC on the P50. Kinda wish they were listed in common terms on their datasheets.

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Re: Dual axis compensator VS IMU

Post by jamesworrell » Wed Oct 03, 2018 9:19 pm

Good read with your morning coffee .. the Whitepaper PDF will tell you all you need to know:

http://blog.hexagongeosystems.com/third ... pensation/

Physical instantiation of a compensator aside - for me - a compensator allows you to avoid having external (to the scanner) survey control. So no total station required.

The compensator "levels" each line (in the case of Leica) of scan data (I believe the Z+F does each point?). For what it is worth - it does an awesome job - we had a roof - was windy as hell - we had data moving about "half-a-target" - so 6" target about 100m away - scanner was "bouncing" about 3 inches of that - but the target centre was still fine .. if that makes sense. Without the compensator I imagine the data would have been pretty ugly.

Examples: Using the really obvious one - the P50 - the laser is measuring 1km away .. a very very small error in "level" translates to a massive error 1km away.

Even the RTC can scan 100m+ away .. and a small "lean" in your scan makes a mess of things. Hence the question of the 3' IMU.

Cyclone knows when a compensator is used - so the scans are marked/known as levelled - fixing the scan in space (there are 6 degrees of freedom in 3d space) - meaning you can only move it (register) via translating (x+y axis); rotating; or along the z-axis (up/down) - it won't "orbit" in 3d space.


youtu.be/L7BX6CnvRk0

6DoF vid - https://www.youtube.com/watch?v=L7BX6CnvRk0 (first 30 seconds).

RTC or various other non-compensator equipped scanners won't have "levelled" data - so the scan needs additional registration parameters to fix it in 3d space - the IMU gets it "close". Generally speaking IMU's aren't as accurate as compensators - and the more accurate the IMU - the more $$'s .. even hundreds of thousands for a little bloody box.

Cyclone will cloud-to-cloud all the scans together - but the entire bundle could be on a lean .. the IMU is ok, but it is not a compensator.

To fix this - you would add survey control from say a total station, or reference clouds from compensator-enabled instrument - eg a P40 - so Cyclone can use this additional data to lock the bundle through all degrees of freedom in 3d space.

So if you want "engineering grade" data - have a compensator somewhere in the picture .. if you are doing a floor plan .. meh.

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Re: Dual axis compensator VS IMU

Post by pburrows145 » Thu Oct 04, 2018 1:48 pm

tbwester wrote:
Wed Oct 03, 2018 2:56 pm
If you don't know if you need it, then you don't need it :)

On Leica machines, the DAC is incredible accurate and allows for precise level over very long distances. It also can clean vibrations from the scan (if scanning on a vibrating floor).

For MOST scanning folks on MOST projects, there is a little or no difference in the two techniques. And even if there is a difference, very very very few projects ever even need the accuracy of a DAC.
Any outdoor, long range, civil-type job which is tying into control requires (IMHO) a compensator to provide truly leveled data. A scanner with a DAC you can use confidently without control (and with the Leica ScanStation even establish a control network - to a degree) if needed.

An IMU is an alternative means to ensure the data is up-right, but not levelled in the purest sense of the word.

However, having said that, I have been using the RTC360 (which uses VIS (Camera+IMU+Compass+GNSS+Altimeter)) for internal and external jobs, and so far the data has been exceptional, even when cross-checking against traditional survey data.

Put it this way, I wouldn't want to be scanning over large areas with a scanner without a compensator: this is why we (Leica hat on again, sorry) offer the BLK360, RTC360 and ScanStation so users can choose the right solution for the majority of work they will be carrying out.

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Re: Dual axis compensator VS IMU

Post by jamesworrell » Thu Oct 04, 2018 10:06 pm

pburrows145 wrote:
Thu Oct 04, 2018 1:48 pm
this is why we (Leica hat on again, sorry) offer the BLK360, RTC360 and ScanStation so users can choose the right solution for the majority of work they will be carrying out.
Since the hat is on .. from a customer perspective, shame a "P360" wasn't dropped the same time as the RTC to actually offer said choice. Realistically I would have probably preferred a "P360" - i.e. a compensator equipped RTC aka P40 with VIS - including long range targeting / traverse / adjustment .. for a "few" dollars more .. but I have to "settle" for the RTC which I get in the next week or so! (very excited still by the way). Or as I have mentioned to a few people .. an add-on accessory - some sort of compensator base - but the mounts don't look like this was contemplated ... a modular system would have been interesting.

In the meantime we "have" to send out the RTC with a total station or C10 .. and yes we grabbed a demo RTC and ran it up against a C10 and we got 3mm difference cloud-to-cloud RTC vs targeted C10 - that was from ~ 12 RTC scans in and out of a building ~ 250m2 targets about 75m apart .. and ironically biggest diff was in X-Y .. Z was 1mm .. so once we get to play with it proper, a level of comfort may creep in regardless of what the spec sheet says (i.e. 3' IMU). Right now we have a roof to scan - I wouldn't send the RTC .. it will be the C10's .. a "P360" can do everything - barring perhaps a tight spot as no doubt it will be "full size".

But the cynic in me thinks you want to sell more P40's - and we were very close to grabbing one .. but we will just wait - to be honest, we can't see why we would buy one now .. We were thinking 2 x RTC's straight away - but might wait the year or two for the "P360" .. as I am sure it is no doubt in development .. it would be a no-brainer. But is it HxGN 2019 or 2020?

As a small firm (~10 ppl) we have to be cautious with our buying - we already have and use 6 Leica scanners daily .. about to be 7. Any tech purchases need solid justification.

If we had the "choice" - your words .. might have been 2 x "P360's" straight up .. will never know.

One minor tweak to mitigate a few scenarios might be high-def window scans for targets - similar to the latest Faro firmware - still doesn't help me on a flexing roof (compensator) - but where there is poor structure for cloud-to-cloud say - a "target scan" wouldn't hurt - I want to run medium res but still acquire targets out to 60m+ say. And please add HDR on/off to the list of tweaks. I haven't even got it (RTC) yet and my storage network is running away screaming.

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Re: Dual axis compensator VS IMU

Post by danielgadowski » Mon Nov 19, 2018 1:55 pm

Ahhh... my pet topic :o
Haven't been on the forum for a year, I come back and here we go - a can of worms got opened again :lol:
So as others explained above already, there should be a massive difference in accuracy of levelness of the dataset between a traditional compensator and an IMU. It may be thou that the IMU will be pretty close by accident :P
One thing interests me thou - In cyclone you need to pick one scan as your home scan world. Then if your IMU scans connect to it and you're unlucky for that scan to be not very level, then others will have an option of increasing that error over distance. That's the scenario if you treat your scans as unlevelled. If you switch them on to be leveled, then your dataset might show shadows on flat surfaces stemming from the tilts of each of the scans. Hence the need for targets fixed with a total station to maintain the levelness of the data.
With compensated scans you don't have that problem (as long as your compensator works as it should) as each line of the scan is corrected live. Genius :)
All of the above is old school (proper ;) )way of laser scanning. These days with the IMU people just go out and hope for the best unfortunately and sometimes it bites.

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Re: Dual axis compensator VS IMU

Post by MomentEng » Tue Feb 05, 2019 4:42 am

Faro advertises the Focus S line to have "dual axis compensation"....After reading a bit of the old post on the topic, several years ago, people were pointing out that Faro was using tilt sensors and not true DAC...was this claim true (it seems to get the support of the forum) and if so, is Faro now using true DAC?

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Re: Dual axis compensator VS IMU

Post by Matt Young » Tue Feb 05, 2019 9:21 am

If you want to make something simple, then take one thing away that still allows you to do the job. Don't add two more things, and say they both make it easier.

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Re: Dual axis compensator VS IMU

Post by danielgadowski » Tue Feb 05, 2019 1:01 pm

Good find Matt!
I think these days the issue stems from the fact that people tend to "adjust" the definition of compensation :)

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Re: Dual axis compensator VS IMU

Post by Matt Young » Tue Feb 05, 2019 3:52 pm

The thing I always liked when using scanners with a DAC was the ability to free station as I was scanning. Placing two targets common between each scan. scanner makes the third point for a fix. Total station backs up some of the targets. so easy.

The IMU on the RTC360 has to be good in order to remember where the scanners is and has been. Suddenly, so easy is really easy :D Now the scanner does all the free station work and all you need is a little total station control.

The workflow with IMU - whether on a static or mobile unit, seems easier than DAC in terms of workflow - so in my own little world it's got to be better.
If you want to make something simple, then take one thing away that still allows you to do the job. Don't add two more things, and say they both make it easier.

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