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System Software Testing of Laser Tracker Leica AT401

1. Beale 780 nm Breica 19 50 985 811 58 0 1 0002679940 1 0002677165 1 0002680009 1 0002679940 23 30 984 373 46 8 1 0002641449 1 0002638716 1 0002641519 1 0002641449 2 3 Improper updates of the group refractive index of air This error improper updates of the refractive index of air in the memory of the instrument s EmScon server is considered by the author to be the most tricky and serious one It was never acknowledged by the technical support as a real error rather it was called the stan dard behaviour of the system It is everyone s choice which behaviour of the instrument is metrologically acceptable but it should be always transparent and predictable When measuring with AT Control software AT401 is always asked to return actual refractive indices used for calculated corrected measured distances The indices are then saved beside other measured data for any post processing purposes The author soon noticed that the group refractive index remains unchanged even if atmospheric parameters differ in time Only quite large jumps of the refractive index were registered from time to time During a simple test instrument s external NTC temperature sensor was heated by hand to simulate significant changes of temperature and refractive index Table 3 Improper updates of the group refractive index of air Atmospheric conditions Group refractive index of air Temperatu
2. impacts of errors depend on ambient atmospheric conditions and its gradients as well as the length to the target point Both extremes Min Max of these error intervals are evaluated in the tables The max error is derived as the maximum possible influence for the whole instrument s working range lt 0 160 m Geoinformatics FCE CTU 13 2014 54 DvoR CEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 distance lt 0 40 gt C temperature lt 500 1100 gt hPa atmospheric pressure lt 0 95 gt relative humidity In laboratory conditions stability of temperature 0 25 C at 20 C and 30 m length are assumed A reduced range of temperature lt 0 30 gt C is used for the outdoor evaluation Notice that an error in the group refractive index of air causes an error of about the same amount in the measured distance km Each of the described issues itself potentially exceeds the manufacturer s specification about the accuracy of the distance measurement 5 pm 2 Table 4 Impacts of AT40x firmware errors ppm Mus edor Estimated common error Error Laboratory Outdoor Min Max Min Max Min Max Refractive model 0 01 0 64 0 07 0 12 0 01 0 32 Wavelength 0 31 0 27 0 28 0 28 0 28 0 27 Updating n 0 50 0 50 0 25 0 25 0 50 0 50 All together 0 80 0 87 0 46 0 09 0 77 0 55 Table 5 Impacts of AT40x firmware er
3. qualities of the instrument itself RIGTC VUGTK has been regularly using Leica AT401 for several years and may recommend this instrument to other users On the other hand if technical support does not reflect desired changes in the system software spreading serviceable information among end users is what needs be done The majority of customers are satisfied with the functionality of the instrument as it came from production However if someone wants to fully take advantage of the potential of such a great hardware significant attention should be dedicated not only to the hardware but to the firmware and software as well The presented results of the paper are not dependent on the used testing software and can be verified by any user programmed controlling application which enables saving of group refractive indices Contact the author of the paper in order to get the AT Control software for free for further non commercial scientific use Geoinformatics FCE CTU 13 2014 56 DVORAGEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 References 1 10 1 12 13 GASNER G a R RULAND Instrument Tests with the New Leica AT401 online Stan fort CA USA SLAC 2011 cit 3 5 2014 Available http www slac stanford edu cgi wrap getdoc slac pub 14300 pdf Leica Geosystems Leica Absolute Tracker AT401 ASME B89 4 19 2006 Specifications Leica Geosystems 2010 12 14 2010 JOKELA Jorma a Pasi HAK
4. resolution of the International Geodetic Association IAG in 1999 in Birmingham 10 Leica s modified formula showed no significant difference from Edl n s Although the testing parameters were quite extreme up to 35 C air temperature and 100 relative humidity they are not unrealistic concerning the instrument s operating range up to 40 C and 95 11 Group Refractive Index of Air Ciddor amp Hill 1999 others t 35 C p 1000 hPa x 400 ppm CO Barrell amp Sears 39 40 UGG 63 30 Edl n 66 Owens 67 Peck amp Reeder 72 Group Refractive index 108 n 1 10 Birch amp Downs 94 0 DP Ciddor 96 AG 99 10 9 Ciddor 02 20 0 45 75 100 Relative humidity 96 Figure 1 Comparison of methods for evaluating the group refractive index of air There is a half done solution in the system software prepared by Leica 12 By changing Weather Monitor Status from Calculate default to Read only or Off the user may implement their own computation procedure of the refractive index of air This is exactly the way the complex Ciddor amp Hill procedure and also others were added to the ATControl software to offer the possibility to make a choice Unfortunately there is doubt that ordinary users Geoinformatics FCE CTU 13 2014 50 DvoR CEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 are able to apply this functionality because available commerci
5. LI On Tracebility of Long Distances Lisbon Portugal International Measurement Confederation 2009 cit 28 5 2014 Available http www imeko2009 it pt Papers FP 100 pdf LECHNER Ji et al Nov esk st tn etalon velk ch d lek Kostice V GTK 2007 cit 29 5 2014 Available http www vugtk cz odis sborniky jine geos07 paper 32_lecher_cervinka_umnov_kratochvil paper 32_lecher_cervinka_umnov_ kratochvil pdf Leica Geosystems Formula for Calculating the Refractive Index of Ambient Air used for the Leica AT401 of Hexagon Metrology 2013 02 13 2013 DVO EK Filip Nep m ur en indexu lomu vzduchu pro v po et fyzik ln redukce elektronick ch d lkom r Geodetick a Kartografick Obzor 2013 vol 101 no 10 s 253 266 ISSN 1805 7446 EDL N B The refractive index of air online Metrologia 1966 vol 2 no 2 s 71 80 cit 28 November 2012 Available http www scopus com inward record url eid 2 s2 0 34250009147kpartner ID 40 amp md5 769ccc0fb3a3cd511ba00asb8cfb7e38 CIDDOR P E Refractive index of air New equations for the visible and near infrared online Applied Optics 1996 vol 35 no 9 s 1566 1572 cit 28 11 2012 Available http www scopus com inward record url eid 2 s2 0 0030404182 amp partnerID 40 amp md5 cc21b031fafd06b6e8cd384decd7a103 CIDDOR P E a R J HILL Refractive index of air 2 Group index online Applied Optics 1999 vol 38 no 2 9 s 1663 1667 ci
6. System Software Testing of Laser Tracker Leica AT401 Filip Dvo ek Department of Special Geodesy Faculty of Civil Engineering Czech Technical University in Prague Thakurova 7 166 29 Prague 6 Czech Republic filip dvoracekOfsv cvut cz Abstract The article introduces a group of instruments called laser trackers and specifically focuses on one of them Leica AT401 At the Research Institute of Geodesy To pography and Cartography the instrument has been tested both in laboratory and outdoor conditions Several significant errors in the instruments system software have been found mostly due to the creation of user programmed controlling appli cation called ATControl The errors are related to a selection a computation and an evaluation procedure of the refractive index of air Finally notes of the new measurement modes of Leica AT40x are given and a range of distance measure ment is discussed Keywords laser tracker absolute distance measurement Leica AT401 system software er ror group refractive index of air 1 Introduction A group of instruments called absolute laser trackers has caused a small revolution in the field of length metrology Although these devices are meant to be mainly used in industrial metrology car and aircraft industry the impact of laser trackers is far wider and also effects engineering geodesy Relative precision as well as overall absolute accuracy of distance mea surement has brought about new
7. al software solutions do not implement it Therefore only the default setting is usually available for measuring 2 2 Wavelength of the ADM In the paper from Leica 5 you can find that AT401 operates on 795 nm wavelength In the instrument s manual 780 nm is declared 11 A technical support reacted that the mistake is in the manual To confirm this a simple test including measuring with recording atmospheric parameters and refractive indices and theoretical calculations of given equations 1 and 2 was performed Unfortunately the measured and calculated results did not fit together and therefore more research had to be done 1 10 P 0 613 0 010T TT BR 10742373 06609 i 1 0 0036610 T Ner_ppm AP N Gr_ppm 2 NOR Gr 000000 Both 780 and 795 nm was now considered along with a variety of constants A and B Table 1 given by Leica and calculated as stated in a presentation 13 of Munich University 3 4 Constant B for 780 nm trackers which has been received from Leica is not in congruence with the equations but this has a negligible impact on the results and will not be discussed here anymore 4 25 03792 0 16647 25 03792 0 166467 A 86 8109 107 A p 4 3 130 53 389 55 130 4 389 54 13 71 B 5722 5 4 Table 1 Constants of the refractive model for Leica AT401 EREN For trackers 780 nm For trackers 795 nm Given by Leica Calculated Given by Lei
8. ca Calculated A 0 2917349 0 2917349 0 2914269 0 2914269 Bx10 9 556 68 549 67 550 51 550 51 On the other hand an incorrect substitution of 780 nm instead of 795 nm has much more serious consequences The values of the refractive index calculated by Leica AT401 is in perfect match with the manually calculated refractive index if 780 nm wavelength and B constant given by Leica is employed At this stage the issue is very complex because all documents including Leica s support answer are somehow wrong The manual is wrong about 780 nm the paper is wrong about the way AT401 computes the refractive index and the technical support ensures about validity of the sent paper In the end the manufacturer recognized that Leica AT401 physically uses 795 nm wavelength and the programmed default computation of the group refractive index of air is wrong It can be estimated by error analyses that a mistake of 15 nm in the wavelength causes an error of the group refractive index by approximately 0 3 ppm This analyses was confirmed also by practical calculations see the 0 28 ppm difference in Table 2 Geoinformatics FCE CTU 13 2014 51 DvoR CEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 Table 2 Analyses of the AT401 refractive model Atmospheric conditions Group refractive index of air Temperature Pressure Humidity Calculated by Manually calculated C hPa 96 AT401 795 nm 780 mm
9. d with the proper refractive index Because of the facts stated above it can be believed that ATConrol is one of a few or maybe the only software solution which deals with the error and provides metrologically correct distance measurement with Leica AT40x instruments 2 4 Resolution of reading atmospheric parameters A temperature reading from the internal and external temperature sensor can be usually obtained only to a 0 1 C resolution For internal temperature sensor which is heated up by electronics in the controller and gives up to 3 C wrong results it is understandable For external NTC 2 sensor it is not the same case In manual 11 Leica describes the accuracy as 0 3 C expanded uncertainty The same sensor NTC 82 can be purchased from Hexagon Metrology but surprisingly with only 0 2 C expanded uncertainty This makes only the 0 1 C standard deviation of the absolute reading and the relative repeated reading is most likely even smaller For the external sensor it seems reasonable to enable the reading of the temperature to be with the 0 01 C resolution Besides jumps in measured distances caused by rough resolution of obtained digits are observed To be faultless it has to be said that temperature can be read to hundreds of C when the absolute temperature drops below 10 C 9 99 or lower It seems only 3 valid digits are reserved in data type to store the value An answer from Leica was that a change is not needed and t
10. fractive index immediately is the command SetEnvironmentParams In that case meteostation ATC400 is turned off atmospheric param eters are inserted manually and the device will compute the refractive index automatically by the default Edl n s procedure However these circumstances are not met very often during real measurement and this setting is inconvenient for practical use By neglecting changes in the group refractive index of 0 5 ppm simultaneously the changes in air temperature up to approximately 0 6 C are not taken into account This can be illustrated on the graph Fig 2 Temperature changes in time were simulated by spontaneous heating of the AT Controller 400 internal temperature sensor and the same length 31 m was measured repeatedly Obvious jumps of distance measurement were caused by the discussed firmware error which is demonstrated by the size of jumps about 0 5 ppm Leica AT401 Distance 31 m and temperature measurement gt Improper updates of refractive index of air A Slope distance um A Temperature C 0 00 0 15 0 30 0 45 1 00 1 15 1 30 1 45 2 00 2 15 Time hour min Figure 2 Improper updates of the refractive index of air by Leica AT401 Up to a 1 ppm difference between two measurements of the same distance can be found due to improper updates of the refractive index of air The results depend on the fact if the refractive index or temperature respectively is rising or dropping and what is the
11. hat it is simply how their system works It is partly true because by default they neglect everything smaller than 0 5 C by not updating the refractive index But in spite of this behaviour atmospheric pressure is read to 0 001 hPa and relative humidity to 0 1 The expanded uncertainties of the installed atmospheric sensor and the humidity sensor are only 1 0 hPa and 5 It proves that the programmed firmware does not reflect the way atmospheric parameters effect refractive indices measured distances respectively If the system was really designed to work with 0 5 C error in temperature also 1 5 hPa of atmospheric pressure and 30 of relative humidity errors would be acceptable In the author s opinion better resolution to 0 01 C for temperature could be beneficial for measurement On the other hand the resolutions concerning pressure and humidity could be lowered to 0 1 hPa and 196 without any noticeable change in measurement results In user programmed applications this issue cannot be resolved without an interference to the system software firmware which is out of the user s reach 2 5 Summary of described firmware errors To demonstrate that the discovered errors are significant and should be taken into account by all current and potential users a summary has been made The purpose of the tables below Table 4 Table 5 is to show how the errors may effect measuring with AT40x in ordinary conditions laboratory and outdoor The
12. ng Geodesy deals with both the laboratory and field testing of Leica AT401 in order to employ this instrument in the calibration process of the Czech State Long Distances Measuring Standard KoStice 4 and National Geodetic Baseline Hv zda This article describes the system software errors found by the author and introduces some practical issues which occur during field measurement Because of the lack of suitable geodetic software solutions on the market the user programmed Mathworks Matlab application called ATControl is used for communication with Leica AT401 during both testing and measuring 2 System software errors 2 1 The computation procedure of the group refractive index of air If you want to know the way the refractive index is computed by the instrument you have to contact the manufacturer From Leica a document describing this procedure was obtained on 5t May 2013 5 According to this paper and practical testing AT401 uses equations derived from Edl n s formula A study was made in which 10 most popular procedures of indirect computation of group refractive index of air is compared and analyzed 6 All formulae were derived from primary information sources Edl n s equation 7 originally published in 1966 and nowadays recognized by the community as inaccurate in terms of humidity posted the worst result in the study It reached nearly a 0 5 ppm difference from the Ciddor amp Hill procedure 1996 8 1999 9 recommended by a
13. possibilities for surveyors and metrologists in determining the fundamental length unit Laser trackers have become parts of laboratory equipment all over the world 1 as they are tools for creating standards and etalons and they provide metrological traceability to the definition of the meter The Leica AT401 instrument was introduced to the public in 2010 Its successor AT402 with similar technical specifications and the same system software followed in 2013 The letters AT stand for Absolute Tracker signifying that instruments implement a new technology called Absolute Distance Measurement It is currently capable of superior accuracy stated by the manufacturer as a standard deviation of 5 ym over the whole 160 m working range 2 Parameters of all up to date EDM electronic distance measurement devices integrated in geodetic total stations are not even close to this point During past decades ageing predecessor Kern Mekometer ME5000 with its accuracy 0 2 mm 0 2 ppm x D km D measured distance had held the status of the most powerful instrument for long distance measurement 3 But recently laser tracker systems have taken over this role and stand in the focus of current interest and research Geoinformatics FCE CTU 13 2014 doi 10 14311 gi 13 6 49 DVORAGEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 Research Institute of Geodesy Topography and Cartography RIGTC VUGTK Depart ment of Metrology and Engineeri
14. previous value of the refractive index stored in the instrument s memory An answer from Leica s technical support in Switzerland stated that all this had been programmed in order to speed up the whole system and eliminate delays The author of the article assumes that it cannot be the real reason Every time a new set of atmospheric parameters is observed 20 sec the refractive index has to be computed anyway but usually it is not saved as it should be Promises of the customer support to pass this issue onto the developer department had been made but one and a half years later no change in the instrument s system software has taken place In the AT Control application it was possible to overcome this error without any noticeable delay during the measurement process In addition it had to be arranged in a much more complicated way then Leica could do it Correcting the error is possible only in manual Geoinformatics FCE CTU 13 2014 53 DvoR CEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 modes Read only and Off of the meteostation After obtaining atmospheric parameters from ATC400 or elsewhere wrong refractive indices group and phase are set They have to differ from the correct ones at least 0 5 ppm Afterwards correct refractive indices are sent to the server and because they differ so much they are accepted as new values And so when the measurement starts the first velocity correction is ensured to be compute
15. re Pressure Humidity Change PC hPa 76 ng ppm Update 22 6 984 354 48 3 1 000264774151 22 8 984 361 48 0 1 000264774151 0 00 no 22 9 984 383 47 7 1 000264774151 0 00 no 23 0 984 335 47 5 1 000264774151 0 00 no 23 3 984 373 46 8 1 000264144921 0 63 yes 23 7 984 314 45 8 1 000264144921 0 00 no 22 8 984 170 49 0 1 000264719968 0 58 yes 23 0 984 200 48 4 1 000264719968 0 00 no 23 3 984 165 47 6 1 000264079414 0 64 yes From the results Table 3 it was derived that the refractive index is updated only if a new value differs from an old one by 0 5 ppm or more As investigated this fulfils all measurements in automatic Calculate mode but also all measurement in manual modes Read Only and Off In practice if the refractive index is computed and handed over to the instrument it is expected that the distance will be corrected by using this specific value Nevertheless in most cases the instrument will use some old number stored in its memory and no information about it is provided Such behaviour was confirmed by the technical support in Leica s testing software called Geoinformatics FCE CTU 18 2014 52 DVORAGEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 TPIAnalyzer A print screen from this testing is available Similar explicit testing could have been achieved in AT Control as well but there was no need to do that anymore The only functionality which serves well and updates the re
16. rors um Estimated common error Max error Error Laboratory Outdoor 160 m 30 m 160 m Min Max Min Max Min Max Refractive model 2 102 2 4 2 51 Wavelength 50 43 8 8 45 43 Updating n 80 80 8 8 80 80 All together 128 139 14 3 123 88 The question occurs of how it is possible that no one has noticed or corrected the issues Some answers can be found Firstly errors of refractive model and faulty wavelength have opposite mathematic signs and therefore partly eliminate each other in results Finally testing and measuring with laser trackers is mostly done in world class laboratories equipped with effective air conditioning A well controlled environment with a stable temperature during the whole measurement process fully or at least largely reduces the error of improper updates of the group refractive index 3 Measurement modes system Laser trackers are still a relatively new family of products which are undergoing a rapid development and innovations Under these circumstances minor errors in functionality will not be criticized and discussed here A frequency of about a half year of issuing new firmware to Leica AT40x instruments is sometimes too long if an error is desired to be fixed Since the time of a major firmware upgrade to version 2 0 0 5053 new measurement modes have been introduced and all applications should have implemented them The precise s
17. t 28 11 2012 Available http www scopus com inward record url eid 2 s2 0 0000063725 amp partnerID 40kmd5 cb84d76662da432a853142d88c93bd9Ff RUEGER J M Refractive Indices of Light Infrared and Radio Waves in the Atmo sphere University of New South Wales 2001 ISBN 9780733418655 Leica Geosystems Leica AT401 User Manual V 2 0 2013 Leica Geosystems emScon 3 8 Leica Geosystems Laser Tracker Programming Interface Programmers Manual Revision 3 ed Switzerland 2013 07 27 2013 HENNES Maria et al State of the art baseline measurements by means of laser tracking results from an interlaboratory comparison Praha EURAMET 2011 Geoinformatics FCE CTU 13 2014 57
18. tan dard fast or outdoor mode can be chosen Only a brief general description of profiles is given by Leica no specifications are available Users may only guess that the difference consists of Geoinformatics FCE CTU 13 2014 55 DvoR CEK F SYSTEM SOFTWARE TESTING OF LASER TRACKER LEICA AT401 measurement time 0 5 5 s and also a tolerance to changing environmental conditions Ac cording to the author s experience the precise mode was useable in the laboratory only up to 30 m even though the description said it was meant to be used in uncontrolled environment The outdoor mode dedicated to use in harsh environment in field conditions was practi cally not useable outdoors at all because it was not capable of determining longer distances On the other hand the standard mode which should be used in a controlled environment worked the best outside the laboratory Leica s support answer was that if the users were fine with the standard mode in field con ditions they should have used it The main idea of the outdoor mode is To make it less sensitive to disturbing light sources like reflections of the sunlight So with firmware 2 0 0 5053 2 0 1 5449 and 2 1 0 4864 the standard mode was used but since upgrading to the recent version 2 2 0 5975 a decrease of the distance range was observed The standard mode stopped working for longer distances The outdoor mode has operated up to about 120 m but it is far less than it
19. was able to measure before about 180 m From our reseller NMS Slovakia noncontact measuring systems information was given that this firmware version dealt with unwanted reflections and so it might have affected also the sensitivity to the measuring signal Users do not have an option to make a downgrade in the Tracker Pilot application and therefore Hexagon Metrology service centre in Prague had to be asked to do it Unfortunately even after downgrading to version 2 1 similar problems with long distances above 100 m are present It is believed that extended calibration of the instrument which makes all laser beams perfectly coaxial will return back the required capability of measuring long distances Measuring long distances in field conditions with the AT401 is always challenging The changeable atmosphere and the current hardware state make it difficult enough Additional uncertainties in the measurement mode system make it even more complicated to recognize what is the problem during measurement The description of the error no 113316 Distance measurement failed General error upon a distance acquisition is not much helpful Re aiming to the target in order to receive a stronger reflected signal increases the chance for a successful measurement Author s note The purpose of the article is far from an assumption that the author wanted to cause any harm to Leica Geosystems Hexagon Metrology NMS Slovakia or somehow discredit the

System Software Testing of Laser Tracker Leica AT401

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