Patent Publication Number: US-11035660-B2

Title: Inertial dimensional metrology

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to dimensional metrology and, more specifically, to large volume physical measurement of three dimensional (3D) objects. 
     Dimensional Metrology is the science of calibrating and using physical measurement equipment to quantify the physical size of, or distance from, any given object. Inspection is a critical step in product development and quality control. 
     Dimensional Metrology requires the use of a variety of physical scales to determine dimensions, with the most accurate of these being holographic etalons or laser interferometers. The realization of dimensions using these accurate scale technologies is the end goal of dimensional metrologists. 
     Modern measurement equipment include hand tools, Coordinate-Measurement Machines (CMMs), machine vision systems, laser trackers, and optical comparators. A CMM is based on CNC technology to automate measurement of Cartesian coordinates using a touch probe, contact scanning probe, or non-contact sensor. 
     Optical comparators are used when physically touching the part is undesirable. Optical comparators can now build 3D models of a scanned part and internal passages using x-ray technology. 
     Furthermore, optical 3D laser scanners are becoming more common. By using a light sensitive detector (e.g. digital camera) and a light source (laser, line projector) the triangulation principle is employed to generate 3D data, which is evaluated in order to compare the measurements against nominal geometries either in a scale drawing or CAD Model. 
     In some cases, the object to be measured is transported to an area with stationary measuring devices for measurement. Typically in large volume dimensional metrology, portable measuring devices are transported to the large object for measurement. 
     Large volume metrology examples include precision measurement of aircraft and spacecraft, energy generation structures and devices, and large manufacturing and assembly facilities. 
     The CMM is a very powerful measuring device used in dimensional metrology because it simultaneously produces coordinates of a point on the object being measured based on a reference location of the CMM using a suitable coordinate system like the three orthogonal axis Cartesian coordinates X, Y, and Z having a common reference origin. 
     The laser tracker is a popular portable CMM that can calculate X,Y,Z coordinates for any point on an object. This is accomplished by measuring the distance between the tracker and each target point with a laser and combining it with the horizontal and vertical angles of the laser pointing device embodied in the tracker using a common reference coordinate systems for all points in the measurement survey. 
     An optical target in the exemplary form of a Spherically Mounted Retro-reflector (SMR) is placed at the desired point on the object for the laser tracker to precisely determine laser range and fix horizontal and vertical angles of the emitted laser beam in the pointing device. 
     Other portable CMMs include theodolites, robotic total stations, and a system of camera photos called photogrammetry. They all require Line of Site (LOS) between the portable CMM and the target point on the object they are measuring. 
     Since ultimately all the desired points measured on the 3D object need to be plotted in their exact relationship with each other in a suitable coordinate system, and because the CMM will most likely not have visibility on all the desired points from one location, the LOS requirement becomes a significant problem. 
     In large volume metrology, the object being surveyed is typically large in three dimensions and typically complex in configuration, and may therefore include a significant number of recessed or obstructed target points hidden from LOS view of the CMM within the full complement of desired survey locations or points. 
     However, because this type of CMM is portable, the CMM can be relocated to a new LOS reference location, or a second CMM may be used, for providing LOS measurements of survey points previously hidden at the first CMM location. CMM measurements from both viewing or source locations will therefore include both survey points with LOS coordinate measurements thereof, and other survey points hidden from LOS view of the differently located CMMs. 
     Since the two CMM viewing locations will have different coordinate references, a mathematical work-around to the LOS requirement, such as least squares optimization, may be used to mathematically tie together the measured coordinates based on some of the common survey points having LOS visibility from both CMM viewing locations to establish a common coordinate reference system for all measured points from both viewing locations. 
     Other solutions for measuring hidden points lacking LOS visibility include special optical targets cooperating with the CMM that include touch probes that can reach the hidden points while at least some portion of the probe remains within LOS visibility of the CMM. 
     However, such optical targets probes can have various configurations including different benefits and different problems in measuring the hidden survey point. 
     Significant to large scale dimensional metrology is the typical requirement for precision measurement of the 3D object coordinate locations X,Y,Z within very small dimensional tolerances of about plus/minus 0.6 mils (0.0006 inches or 15 microns), for example. 
     The typical laser tracker CMM can achieve this high precision; and highly specialized optical targets may be used therewith for matching such high precision based on different technologies having different problems and different benefits, and at correspondingly different cost. 
     Various optical targets and probes are known for various fields of endeavor including land surveying, and vary substantially in configuration and operation, with correspondingly different accuracy of measurement. 
     Fundamental to metrology are the typical six degrees of freedom (DOF) associated with 3D objects, which can be measured in a suitable coordinate system such as the exemplary six-axis Cartesian coordinate system introduced above. Three orthogonal linear axes X, Y, and Z extend outwardly from a common origin for defining linear position therefrom; and three angular or rotary axes A, B, C define angular position or attitude around the corresponding linear axes, commonly known as roll, pitch, and yaw. 
     Various technologies are commonly known for measuring linear position and angular attitude with varying degrees of complexity and accuracy. And, such various technologies may be combined in various manners for various benefits. 
     Many common measuring technologies are based on optical measurements having various optical encoders or camera systems, which require LOS. Other technologies include the Global Positioning Satellite (GPS) system commonly used in navigation for measuring or determining location based on longitude and latitude positions, but subject to the substantial problem of GPS signal loss. 
     Still other technologies include the Inertial Measurement Unit (IMU) also commonly used in navigation in which cooperating accelerometers and gyroscopes measure relative movement of the IMU in the six DOF, but subject to the also significant problem of inherent temporal drift errors. 
     All such measuring technologies have different capabilities and different problems, and correspondingly different costs. 
     For example, fundamental to IMUs is the significant drift errors inherent therein which increase exponentially, or quadratically, with time. Accordingly, commercial inertial sensors based on IMUs have a six-order magnitude difference in price and performance in different configurations or grades thereof. 
     Four IMU grades include automotive &amp; consumer; industrial; tactical; and marine &amp; navigation having correspondingly decreasing drift errors resulting in horizontal position errors of about 7900 km/hr, 190 km/hr, 19 km/hr, and 1.6 km/hr, respectively, with cost ranging from low for consumer grade to exceedingly high for the marine grade. 
     However, one particular advantage of IMUs is their dead-reckoning capability to measure both linear and angular positions without regard to the loss of LOS or GPS signal problems. Another particular advantage of IMUs is modern advancements thereto in which the size, cost, and drift errors of IMUs continue to decrease. 
     Accordingly, one object of the present invention is to provide improved large volume dimensional metrology of an object. 
     Another object of the invention is to provide an improved method for measuring location of one or more of the full complement of survey points having blocked LOS in a measurement survey of the object. 
     Another object of the invention is to provide location measurement of the hidden point with preferential precision thereof. 
     Another object of the invention is to provide an improved method and system for conducting large volume dimensional metrology having reduced complexity and cost. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A method of performing dimensional metrology of an object includes incorporating an Inertial Measurement Unit (IMU) with an elongate probe in a portable metroprobe. A tip of the probe has an offset length from an origin of a coordinate system in the IMU and position thereof is correlated based on attitude measurement of the IMU. The metroprobe is transported in sequence to a complement of survey points on the object for measuring corresponding coordinates thereof based on measured attitude of the IMU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is an elevational isometric view of a survey system using an inertial metroprobe for performing large volume dimensional metrology on a large object. 
         FIG. 2  is a schematic view of the metrology system shown in  FIG. 1  for conducting a coordinate measurement survey of a set of points on the object. 
         FIG. 3  is a flowchart for performing the metrology survey of  FIGS. 1 and 2 . 
         FIG. 4  is an elevational isometric view of a modified metrology system additionally including a laser tracker cooperating with the metroprobe shown in  FIGS. 1 and 2  for conducting the measurement survey of the object shown in  FIG. 1 . 
         FIG. 5  is a schematic view of the modified metrology system shown in  FIG. 4  for conducting the measurement survey of the set of points. 
         FIG. 6  is a flowchart for performing the metrology survey of  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in  FIG. 1  is a metrology probe, or metroprobe,  10  specifically configured for conducting large volume dimensional metrology of an object  12 , such as the exemplary cargo aircraft. 
     In large volume dimensional metrology, a preferential complement or set of measurement points Pn are suitably selected on the object, and may have any desired quantity or number, with n ranging in value from 1, 2, 3, . . . to N, where N is the total number of measurement or survey points Pn desired. The survey points Pn correspond with various point locations on the object  12  for which precise coordinate locations thereof are desired. 
     The metroprobe  10  may be controlled and functionally operated by using a suitable controller  14 , such as a digitally programmable laptop computer, operatively joined thereto by either an electrical cord or by wireless communication using a standard Wireless Local Area Network (WLAN) having suitable WLAN adapters in both devices. 
     The metroprobe  10  includes a suitable housing or body  16  in which are structurally supported or integrated an Inertial Measurement Unit (IMU)  18  and a removable probe  20 . The probe  20  is affixed to the bottom end of the housing  16  in any suitable manner such as pin &amp; socket or bayonet mounting for joining and removing the probe as desired. 
     The metroprobe assembly  10  is relatively small and portable, and the probe  20  may have any suitable configuration and length as desired for extending the reach of the metroprobe  10  in conducting dimensional metrology. The probe  20  has a small spherical tip  22  at the distal, bottom end thereof for use in contacting or touching any of the survey points Pn during the measurement survey. 
     The dimensional metrology measurement survey is illustrated schematically in  FIG. 1  in which the metroprobe  10  is manually transported by metrologist or user conducting the survey to the various survey locations or points Pn in any desired sequence 1, 2, 3, 4, . . . N so that the probe tip  22  may be temporarily placed in contact with the desired survey point Pn for measuring or recording its 3D position or location in space based on a suitable coordinate system and suitable reference location. 
     The line-of-sight (LOS) of some of the survey points may be blocked by obstructions in or around the object  12 , which points are thereby hidden, as shown for the exemplary survey point P 4  hidden behind a structural rib of the aircraft. The user may therefore simply carry the metroprobe  10  to each survey point to obtain direct access thereto by the probe tip  22 , as long as the slender probe  20  is able to reach the desired survey point, such as the otherwise hidden point P 4 . 
     The metroprobe  10  is illustrated in more detail in  FIG. 2  and may include any suitable commercially available IMU  18  as desired, with corresponding size, performance, and cost, from relatively low to high. 
     For example, one suitable navigation-grade IMU is commercially available from distributors for Honeywell Aerospace, Phoenix, Ariz., in model HG9900 IMU having correspondingly high performance and cost. 
     Another suitable example is the industrial-grade VN-100 IMU commercially available from distributors for VectorNav Technologies, Dallas, Tex., which uses Micro Electro-Mechanical System (MEMS) sensor technology to integrate various sensors and cooperating IMU processor in a small form factor. 
     The IMU  18  is shown schematically to include three linear accelerometers  24  arranged orthogonally to each other to correspond with the three orthogonal linear axes X,Y,Z of the conventional Cartesian coordinate system having a common origin  26 , which may be the center-of-gravity of the module defining the IMU  18 . 
     The IMU  18  also includes three gyroscopes, or gyros,  28  arranged coaxially with the corresponding accelerometers  24  to correspond with three rotary or angular axes A,B,C of the Cartesian coordinate system, for measuring roll, pitch, and yaw, respectively. 
     Accordingly, the three accelerometers may be designated  24   x ,  24   y ,  24   z  to correspond with the three linear axes X,Y,Z along which they measure linear or translation movement of the IMU; and the three gyros may be designated  28   a ,  28   b ,  28   c  to correspondingly measure angular or rotary movement of the IMU around the three linear axes X,Y,Z. 
     The basic IMU  18  may include any other conventional features for operating in a stand-alone module as typically commercially available, including for example its own internal digital processor for controlling operation thereof, and having suitable or standard input and output ports for communicating with an external computer, such as the controller  14 . 
     In the exemplary VectorNav configuration identified above, the IMU may also include 3-axis magnetic sensors, a barometric sensor, and a temperature sensor cooperating with the basic three accelerometers and three gyroscopes, all operatively joined to a 32-bit microprocessor and memory device. 
     In basic conventional operation, the IMU  18  utilizes the three accelerometers  24  and three gyroscopes  28  to produce a three dimensional measurement of both specific force and angular rate or velocity. Specific force is a measure of acceleration relative to free-fall, and angular rate is a measure of rate of rotation. 
     Subtracting the gravitational acceleration in the IMU  18  results in a measurement of actual coordinate acceleration. And, by providing the IMU with a reference position, the IMU may thereafter compute its own position and velocity by mathematically integrating the linear accelerations as measured by the three accelerometers  24  suitably corrected using the angular velocities as measured by the three gyroscopes  28  in a conventional manner. 
     The ability of the IMU to measure and calculate its own position in 3D space is dependent firstly on the inherent accuracy of the accelerometers and gyroscopes themselves, as well as the computational accuracy of the mathematical processing of the data measured thereby. Calculated position accuracy is also dependent on filtering out common error sources, such as sensitivity to supply voltage variations and temperature dependent hysteresis. 
     A conventional IMU is typically calibrated over a preferred operating temperature range to determine bias, sensitivity, and cross-axis alignment of each individual component; and corresponding calibration coefficients are stored in the IMU for use during operation in filtering out the common error sources. In this way, the basic IMU  18  can measure its own coordinate location or position using the combined outputs from its accelerometers and gyros, with an accuracy and drift error as specified for the particular make and model of the IMU commercially available at a corresponding price. 
     However, understanding the different components of the inertial IMU and different performance thereof may be used to advantage in specially configuring the inertial metroprobe  10  for enhanced operation and utility in large volume dimensional metrology in contradistinction from the typical use of IMUs for ordinary inertial navigation use. 
     In navigation, location on the globe is desired, and is typically represented by latitude and longitude in a substantially planar environment represented by the typical surface map of the globe. 
     In dimensional metrology, 3D locations of the 3D object are being measured, with typically higher precision than needed for common navigation. 
     And as mentioned above, an understanding of the time-dependent, or temporal, drift errors of IMUs can be used to advantage in performing dimensional metrology. 
     Drift error is a general term representing the many errors inherent in the typical IMU based in large part on the mathematical double integration required for the three accelerometers to determine linear translation. 
     An IMU starts operation at an initial location and initial time, and then measures linear acceleration along the three linear axes X,Y,Z and angular velocity along the three rotary axes A,B,C. By mathematically integrating the measured acceleration over time, velocity can be obtained, and by further integrating the velocity, displacement or linear movement or translation along the three linear axes X,Y,Z can also be obtained. 
     In this manner, the IMU operates by dead reckoning from a known starting location by measuring linear movement therefrom along the three linear axes X,Y,Z to the present location of the IMU. Various drift errors accumulate over time in calculating the location of the IMU as it moves in space, which errors may increase exponentially, or quadratically, over time. 
     As indicated above, such drift errors can accumulate so that the present location of the IMU may be incorrect after an exemplary hour of travel by 1.6 km to about 7900 km depending on the grade of IMU. 
     Of course, such position errors would be unacceptable where higher precision is appropriate, and therefore various techniques can be used to reduce or accommodate drift errors in an IMU. 
     One conventional example for accommodating drift errors in an IMU is the integration therewith of a GPS device to provide an external measurement of position, subject to the inherent positional errors of GPS, at a corresponding increase in complexity and cost. 
     However, the errors in an IMU are different between the gyroscopes and accelerometers, with gyro errors being substantially less than accelerometer errors due to their different configuration and operation in the IMU. This difference is typically specified as Gyro Bias Error as distinct from Accelerometer Bias Error in specifications presented for commercial IMUs. 
     Accordingly, the probe  20  illustrated in  FIG. 2  is structurally integrated with the IMU  18  in a suitable configuration so that the probe tip  22  can be fixed relative to the IMU  18 , and suitably correlated to the origin  26  of the six-axis Cartesian coordinate system X,Y,Z,A,B,C. 
     For example, the probe  20  has a first offset length L measured from the origin  26  to the bottom of the distal probe tip  22 . The tip  22  itself may have a spherical configuration like a typical touch probe, with a suitably small radius R. 
     The probe  20  is coaxially aligned with the vertical Z-axis of the IMU  18  in the exemplary configuration shown in  FIG. 2 , or may have any other orientation as desired. 
     The particular orientation and offset length of the probe  20  is therefore fixed and known relative to the IMU coordinate system so that location and angular orientation or attitude of the IMU correspondingly controls location and attitude of the fixedly attached probe  20 . 
     By correlating position of the probe tip  22  to the origin  26  of the IMU  18 , any change in location and angular attitude of the IMU  18  corresponds directly with location and attitude of the affixed probe  20 , and its tip  22  in particular. 
     In this way, a basic method of performing dimensional metrology of the object  12  includes the simple correlation in position of the probe tip  22  having the first offset length L from the origin  26  of IMU coordinate system based on attitude measurement of the IMU itself. 
     The metroprobe  10  is merely hand-carried or transported by the user during the survey from an initial reference location, such as P 1  for example, in a suitable sequence to simply touch or directly contact each of the desired survey points Pn on the object  12  for inertially measuring corresponding coordinates X,Y,Z of the survey points Pn based in part on measured attitude of the affixed IMU  18 , which attitude is measured by the gyros  28  of the IMU  18  itself. 
     A suitable record button  30  is provided in the metroprobe  10  for actuation by the user to record in the controller  14  the specific location and attitude of the metroprobe  10  when the probe tip  22  contacts each desired survey point. 
     The IMU  18  then measures its own linear travel or translation along the three axes X,Y,Z, to the recorded survey point, which linear travel is identical to the linear travel or translation of the probe  20  and its tip  22 . 
     Quite significantly, the metroprobe  10  can also rotate in 3D spherical space, about the origin  26  for example, to have any orientation or attitude within the full 360 degrees of rotation along the three rotary axes A,B,C. 
     In  FIG. 2 , the metroprobe  10  is shown at the left in an exemplary vertical attitude, with the IMU  18  positioned vertically atop the coaxial probe  20  at the bottom end thereof. Full rotary movement of the metroprobe  10  allows infinite attitudes in space from IMU-side up, or upside-down with the probe  20  up and the IMU  18  down, or at any attitude therebetween. The metroprobe  10  is therefore operable without attitude limitation, including full attitude motion from horizontal to vertical, and all inclination attitudes therebetween. 
     The three gyros  28  accurately measure angular rate of rotation or velocity, which may be mathematically integrated in the IMU processor to accurately calculate, or measure, angular orientation or attitude of the IMU  18 , with the attached probe  20  experiencing the same angular movement and attitude. 
     Accordingly, the position of the probe tip  22  may be mathematically established by combining the linear translation and angular attitude of the probe  20  as it travels with the metroprobe  10  in any suitable orientation for accessing any survey point, including otherwise hidden survey points. 
     Simple trigonometry is used to resolve the three components of the offset length L of the probe  20  along the three linear axes X,Y,Z which are then added to the measured coordinates X,Y,Z of the IMU  18  at its origin  26  to correspondingly establish the linear position and coordinates X,Y,Z of the probe tip  22  itself. 
     For example, the linear coordinates of the probe tip  22  may be expressed as P 22 (XYZ)=P 26 (XYZ)+V 22 (XYZ); where V 22  represents the coordinate vector from the origin  26  to the probe tip  22 . 
     The tip vector V 22  can be resolved by trigonometry for obtaining the respective components of the probe length L from the origin  26  as represented by L(XYZ) which is a function of the attitude A,B,C of the probe  20 . 
     In the special configuration of the coaxially aligned IMU  18  and probe  20 , changes in attitude in the yaw C-axis do not affect the coordinates of the probe tip  22  and simplifies the trigonometry. 
     For the exemplary vertical attitude A,B,C=(0,0,0) of the metroprobe  10  shown at the left in  FIG. 2 , the coordinates P 22 (XYZ) of the probe tip  22  are simply the measured coordinates P 26  of the IMU origin  26  minus the offset length L, or P 22 (XYZ)=(X,Y,Z-L), which represents the linear coordinates of measured first survey point P 1 . 
     In another exemplary vertical attitude of the metroprobe  10  shown at survey point P 4 , the metroprobe  10  has an attitude inclination angle dB, which is the differential or delta angle in the −B pitch direction only, i.e. (A,B,C)=(0,−dB,0). The −dB attitude angle inclines the probe  20  counterclockwise solely in the X-Z plane, and correlates with the coordinate position of the probe tip P 22 (XYZ)=(X+L×Sine(dB), Y, Z−L×Cosine(dB)) relative to the new P 26 (XYZ) coordinate position of the IMU origin  26  from which the probe contacts the fourth survey point P 4  whose measured coordinates equal the new P 22 (XYZ) coordinates. 
     The vector V 22  may be similarly resolved in the YZ plane, or in any attitude corresponding with the changing attitude of the metroprobe  10  during operation. 
     Since the metroprobe  10  can be hand-held and properly operate at any angular attitude, it is not constrained in use and may be freely moved to access any survey point without regard to LOS obstructions as long as suitable access is provided to the probe  20  itself. 
     Accordingly, the probe  20  can be removable and replaced with various custom configurations for accessing any desired survey points requiring short or long lengths, or straight or curved paths, with the probe tip  22  nevertheless being simply correlated to the IMU origin  26  for establishing its linear X,Y,Z coordinate position relative thereto. 
     Because the IMU  18  includes the three accelerometers  24  and three gyros  28  for correspondingly defining the three orthogonal linear axes X,Y,Z and three respective angular axes A,B,C, the linear coordinate position X,Y,Z of the probe tip  22  can be readily correlated to the Cartesian X,Y,Z coordinate system and its origin  26  based on the angular attitude A,B,C of the IMU  18  as measured by its own gyros  28 . 
     And, as indicated above, gyro bias errors are substantially small in conventional IMUs, and much smaller than accelerometer bias errors, and therefore allow increased accuracy in measuring coordinate position of the probe tip  22  in the specially configured metroprobe  10 . 
       FIG. 3  shows a flowchart depiction of the basic method of performing large volume dimensional metrology using the dedicated inertial metroprobe  10  in a relatively simple configuration integrating the conventional IMU  18  with the suitable probe  20  and controlled by a suitable controller  14  in the simple form of the typical laptop computer configured with suitable control and measurement software. 
     Since the typical IMU measures relative movement of the IMU itself, the measurement survey preferably begins by initially transporting the metroprobe  10  to any suitable reference location to establish at the probe tip  22  three linear reference coordinates relative to the three linear axes X,Y,Z and three angular reference coordinates relative to the three angular axes A,B,C, all based on the reference origin  26  in the six-axis Cartesian reference coordinate system. 
     For example, the first survey point P 1  itself may be used as the first reference location to establish nominal X,Y,Z reference coordinates such as (0,0,0) for the IMU or coordinate system origin  26 , or for the probe tip  22  itself as desired. This may also establish the nominal A,B,C reference coordinates such as (0,0,0) for the attitude of the IMU  18  or probe  20  as well. 
     Accordingly, as the metroprobe  10  is hand-carried in series to the remaining survey points Pn, the respective linear X,Y,Z coordinates thereof may be measured by the metroprobe  10  relative to the (0,0,0) reference coordinates of the reference location of the metroprobe  10 . 
     With the linear coordinate position X,Y,Z of the probe tip  22  correlated to the attitude of the IMU  18  as described above, transport of the metroprobe and re-orientation thereof to reach subsequent survey points will readily establish and record corresponding or correlated linear coordinates X,Y,Z at each survey point Pn upon simply depressing the record button  30  on the metroprobe  10 . 
     Since the probe tip  22  may have any suitable offset position in 3D space, its position is preferably correlated to the coordinate system origin  26  based on corresponding angular position or attitude of the probe  20  relative to the three linear axes X,Y,Z. The probe tip  22  may be offset based on any one, or more, of the three axes X or Y or Z, but in all cases simply trigonometry can correlate the linear coordinates of the tip  22  in the IMU coordinate system. 
     In the simple configuration shown in  FIG. 2 , the probe  20  is coaxially aligned solely with the Z-axis in the X-Z plane, and its tip  22  has a single offset from the origin  26  measuring L in length, without any offset of the tip along the X &amp; Y axes. 
     In the exemplary method illustrated in  FIGS. 1-3 , the metroprobe  10 , alone, is transported by the user in a sequential survey of the multiple survey points Pn, at which the linear positions X,Y,Z of the IMU  18  are measured and recorded by depressing the record button  30 . 
     Simultaneously, attitude of the IMU  18  in the angular axes A,B,C is also recorded and used to correlate therewith the corresponding attitude of the probe  20 . 
     This survey is recorded and controlled by the laptop computer  14  which is used to establish or calculate the corresponding linear coordinates X,Y,Z of the probe tip  22  in contact with the survey points Pn as correlated with the measured linear positions X,Y,Z of the IMU  18 . 
     Since the IMU  18  includes the accelerometers and gyroscopes which define the corresponding three orthogonal linear axes and three respective angular axes, the angular attitude A,B,C of the IMU  18  in three axes is measured by the gyros  28  in the IMU  18 , and the linear positions X,Y,Z of the IMU  18  in three axes are measured by both the three accelerometers  24  and three gyroscopes  28  in a conventional manner. 
     In a preferred and basic embodiment, the metroprobe  10  is autonomous in performing the dimensional metrology of the object  12 , and is operatively joined to the controller  14  for establishing the linear coordinates X,Y,Z of the probe tip  22  at the many survey points Pn relative to the three orthogonal linear axes X,Y,Z based on the measured three-axis linear position X,Y,Z and three-axis attitude A,B,C of the IMU  18 . 
     Precision or accuracy of the measured coordinate locations X,Y,Z for each of the full complement of survey points Pn is therefore based solely on the inherent accuracy of the conventional IMU used in the metroprobe  10 , based in part on the bias errors of the accelerators and based in additional part on the bias errors in the gyros. 
     However, as indicated above, the gyro bias errors are substantially smaller than the accelerometer bias errors which improves the overall precision and accuracy of the metroprobe  10  in its special configuration for conducting large volume dimensional metrology in the exemplary method disclosed above. 
     Since the IMU  18  is subject to temporal drift errors, the linear coordinates X,Y,Z at the survey points Pn can be suitably corrected to reduce the drift errors. 
     For example, the method may be modified to include transporting the metroprobe  10  between two suitable reference locations in surveying the survey points Pn, and establishing a position error vector  32  as shown in  FIG. 2  based on a difference in linear positions X,Y,Z measured at the two reference locations, and then mathematically resolving the error vector to reduce corresponding errors in the linear coordinates X,Y,Z at the applicable survey points Pn. 
       FIG. 2  illustrates schematically how the IMU drift error increases with time during transportation of the metroprobe  10  during the measurement survey. Since the survey can take minutes to hours to complete depending on the particular survey, the accumulation of drift error can be small or large, especially since the accelerometer drift error can increase exponentially with time. 
     In  FIG. 2 , the drift error increases in time for each of the three accelerometers  24 , and collectively result in the total drift error vector  32  which has increased size at subsequent survey points. 
     The total error in the linear coordinates X,Y,Z can be represented by the single error vector  32  at any suitable survey point and can be mathematically resolved into constituent components in the three linear axes X,Y,Z, but then requires suitable distribution or attribution to all previous survey points from which it was made. 
     Since the drift error is known to accumulate exponentially according to IMU performance, knowledge of that IMU performance can be suitably used to correspondingly reduce more error in the linear coordinates at subsequent survey points. 
     In other words, the drift error accumulates according to known performance of the IMU, and therefore can be resolved and distributed in reverse sequence over the relevant time period. 
     Since the first survey point, P 1  for example, initiates the process at the reference coordinates (0,0,0) it establishes a reference zero drift error condition. 
     If the metroprobe  10  is periodically returned to the first survey point P 1  it will effect a total error vector  32 , like that shown for the fifth survey point P 5  in  FIG. 2  where P 5  may also represent return to the first survey point P 1 . 
     By resolving the total error vector  32  based on the duration of the survey, and based on actual time intervals measured between the sequential survey points, corresponding error corrections can be made at each of the intervening survey points between the first point P 1 , and the return thereto. 
     A similar correction in drift error can be performed by establishing the total drift error vector from the initial reference location P 1  and any subsequent reference location, such as fifth point P 5 , which can have a separately determined known position, just as the first reference location P 1  is assumed to have the known (0,0,0) reference coordinates. 
     By understanding and knowing the form of the specific drift error over time, various corrections therefor may be mathematically effected in suitable software in the laptop computer  14  to suitably correct the measured survey coordinates X,Y,Z at each of the survey points during which the drift error is experienced. 
     Another method for correcting drift error of IMUs includes suitably introducing a zero velocity (V=0) update in the integration of acceleration as conventionally known. Since a significant component of drift error is attributed to the double integration of acceleration to obtain displacement in the IMU, periodically introducing the zero velocity update restarts a portion of the integration process which establishes displacement along the three linear coordinates X,Y,Z. 
     Although, the concept of zero velocity update is conventionally known, there are different methods of introducing such update, yet again having different advantages and problems. 
     In view of the special hand-held configuration of the metroprobe  10  illustrated in  FIGS. 1 and 2 , it may be periodically placed in a stationary cradle  34  at one or more of the survey points Pn, upon which a zero velocity (V=0) may then be introduced to update the IMU  18  and reduce the drift errors in all three linear coordinates for subsequent survey points. 
     Since the metroprobe  10  is typically hand-held during the survey process, it is difficult to actually hold still to establish in reality zero velocity thereof. 
     The cradle  34  can be specifically configured to provide a complementary seat or socket for temporarily rigidly locking therein the metroprobe  10  to ensure that introduction of the zero velocity update into the processor controlling IMU operation occurs in fact when the metroprobe  10 , and integrated IMU  18 , are in fact stationary with zero movement or velocity. 
     In  FIGS. 1 and 2 , the cradle  34  can be temporarily affixed or hot-glued at any suitable location for conducting the survey and/or introducing the zero velocity update. 
     For example,  FIG. 2  illustrates schematically that the cradle  34  can be affixed in the object  12  initially at the first survey point P 1  to ensure an accurate initial calibration of the IMU  18  for the reference or starting coordinates X,Y,Z, at which the IMU  18  will have no movement or motion, and therefore should record an accurate reference coordinate location. 
       FIG. 2  also illustrates that another cradle  34  can be affixed to the object  12  at the exemplary fifth survey point P 5 , at which the metroprobe  10  will again be held stationary with no movement or motion, or velocity, and therefore the zero velocity (V=0) update can be accurately introduced in the IMU processor so that drift errors will be temporarily reduced, after which the drift errors will again (re)accumulate as the survey continues. 
     During the measurement survey, the metroprobe  10  is transported in turn to each of the survey points Pn at which the record button  30  is depressed for recording in the laptop computer  14  the six IMU coordinates X,Y,Z,A,B,C for each of the survey points. This survey process is repeated for each survey point, and depending on the duration of the survey, and the need for zero velocity update, such update may be introduced as desired or required. 
     And, if desired, the cradles  34  may be temporarily affixed or hot-glued at each of the intended survey points Pn for temporarily affixing thereat the portable metroprobe  10  to ensure no movement thereof when the coordinate measurements are being made. 
     Alternatively, the user may be instructed to manually hold still the metroprobe  10 , without using the cradle, at various survey points Pn within the physical ability to do so to minimize movement of the metroprobe  10  when the record button  30  is depressed to maximize accuracy of the recorded position Pn. 
     At the end of the survey, or at convenient intervals therein, the metroprobe  10  may be returned to the original reference point, P 1  for example, for subsequently establishing a total or interim error vector, which may then be resolved as disclosed above for suitably correcting the measured survey coordinates X,Y,Z for the respective survey points Pn. 
     Error correction and introduction of zero velocity updates can be applied either singly or in combination as desired for any particular measurement survey; and may also be applied at suitable intervals in performing the survey depending upon the expected duration of the survey. 
     However, measurement precision in the metrology survey can be further improved by using a modified metroprobe system  36  for performing large volume dimensional metrology of the object  12  as initially shown in  FIG. 4 . 
     The metroprobe system  36  includes a suitable coordinate measurement machine (CMM) in the exemplary form of a conventional laser tracker  38  having a variable horizontal (H) and vertical (V) field of view through which a laser beam  40  is aimed or directed toward the various survey points Pn for accurately measuring the distance D thereto. 
     For example, one suitable laser tracker  38  is the FARO Laser Tracker ION™ commercially available from distributors for FARO Technologies Inc, Lake Mary, Fla., and has a horizontal field of view of +/−270°; and a vertical field of view of +75° and −50°, with an exemplary accuracy or precision of about 15 microns (0.0006 inches) at 18 meters. 
     Another example of the laser tracker  38  is the Leica Absolute Tracker AT402 commercially available from distributors for Leica Geosystems, Norcross, Ga., and has a horizontal field of view of +/−360°; and a vertical field of view of +/−145°, with an exemplary accuracy or precision of about 15 microns (0.0006 inches) per meter for the measured distance. 
     Laser trackers typically operate with a cooperating reflective target in the exemplary form of a Spherically Mounted Retro-reflector (SMR). 
     Accordingly, the portable metroprobe  10  illustrated in  FIG. 4  is preferably modified so that the inertial measurement unit (IMU)  18  is integrated with both the elongate probe  20  as previously described, and a spherically mounted retro-reflector (SMR) target  42  specifically configured to reflect back to the laser tracker  38  the laser beam  40  for accurately measuring the distance D therebetween. 
     The SMR target  42  may have any conventional configuration, and is typically commercially available in paired or matched configuration with the specific laser tracker, such as the FARO or Leica examples presented above, for maximizing accuracy of measurement. Fundamentally, the SMR target includes a precision reflector for reflecting back the laser beam to the laser tracker for precisely measuring the distance therebetween. 
     The SMR reflector is typically configured as a corner cube having three orthogonal mirrors joined together at a common target corner from which the laser beam  40  is reflected back to the laser tracker  38  for measuring the distance D thereto. 
     The spherical coordinates H,V for the laser beam  40  and the measured distance D to the matched target  42  may then be resolved or converted to corresponding linear coordinates X,Y,Z to define the 3D position or location of the target  42  based on a suitable reference location. 
     The laptop controller  14  is operatively joined to both the laser tracker  38  and the IMU  18  for controlling operation thereof. 
     As shown in  FIG. 4 , any suitable communication between the controller  14  and the laser tracker  38  and IMU  18  may be used, such as a wired tether, or wireless communication using standard WLAN adapters  44  integrated therewith using standard input/output ports as shown schematically in  FIG. 5 . 
     The laptop controller  14  is suitably configured in software to control operation of the laser tracker  38  and IMU  18  in conducting the measurement survey, and in particular is used to correlate position and attitude of both the probe  20  and the target  42  to the reference X,Y,Z,A,B,C Cartesian coordinate system in the IMU  18 . 
     The laser tracker  38  is configured in conjunction with the metroprobe  10  for establishing position coordinates for the probe  20  at the plurality of survey locations or points Pn on the object  12  based on coordinate location X,Y,Z of the target  42  as measured by the laser tracker  38  and based also on attitude A,B,C of the IMU  18  as measured by the IMU  18 . 
     A modified method of performing large volume dimensional metrology on the object  12  may therefore include measuring linear positions X,Y,Z of the IMU  18  by the independent coordinate measurement machine (CMM) having direct line-of-sight (LOS) with the metroprobe  10 ; and then establishing the linear coordinates X,Y,Z of the probe tip  22  at the corresponding survey points Pn as correlated to the linear positions X,Y,Z of the IMU  18  as measured by the CMM. 
     In a preferred configuration, the CMM comprises the laser tracker  38  having the variable horizontal (H) and vertical (V) field of view, and the cooperating target  42  is integrated with both the IMU  18  and the probe  20  in the common metroprobe  10 . 
     As shown in  FIG. 5 , the target  42  has a second offset length L 2  as measured between its reflective target corner and the origin  26  of the common X,Y,Z,A,B,C Cartesian coordinate system in the IMU  18 . The position of the target  42  at its target corner, as measured by the laser tracker  38 , is similarly correlated to the coordinate system origin  26  in the IMU  18  based on the A,B,C attitude measurement of the IMU  18  for correspondingly establishing the linear position X,Y,Z of the IMU  18 , and in turn establishing the linear coordinates X,Y,Z of the probe tip  22  at the various survey points Pn. 
     The IMU  18  provides a common reference for correlating movement of both the probe tip  22 , as described above, and the attached target  42  during operation based on the measured attitude A,B,C of the IMU  18 . 
     In the autonomous embodiment of the metroprobe  10  described above, the position of the probe tip  22  is correlated to the position of the common origin  26  as measured by the IMU  18 . 
     In the SMR target  42  modification of the metroprobe  10 , the more accurate location of the target  42  as measured by the laser tracker  38  is substituted for the less accurate location of the IMU origin  26  as measured by the IMU itself, and a similar correlation is used between the target  42  and the probe tip  22  but still based on the common origin  26  of the integrated IMU. 
     In the exemplary configuration illustrated in  FIG. 5 , the probe  20  and IMU  18  are coaxially aligned with the common Z-axis in the same configuration illustrated in  FIG. 2 , but the SMR target  42  is further introduced in a special configuration additionally coaxially aligned with the common Z-axis. In this special configuration, the measured corner reflector of the target  42  and origin  26  of the IMU  18  and the probe tip  22  are all coaxially aligned in a straight line having a total length of L+L 2 . 
     In this modified correlation, the linear coordinates of the probe tip  22  may be generally expressed as P 22 (XYZ)=P 42 (XYZ)+V 26 (XYZ)+V 22 (XYZ); where P 42 (XYZ) represents the measured coordinates of the target  42 , V 26 (XYZ) represents the coordinate vector from the measured target  42  to the IMU origin  26 , and V 22 (XYZ) again represents the coordinate vector from the origin  26  to the probe tip  22 . 
     The tip vector V 22  is the same as that described above. 
     The origin vector V 26  can be similarly resolved by trigonometry for obtaining the respective components of the target offset length L 2  from the measured target  42  as represented by L 2 (XYZ) which is yet again the same function of attitude A,B,C of the IMU  18  and probe  20 . 
     In the special configuration of the coaxially aligned target  42 , IMU  18 , and probe  20 , changes in attitude in the yaw C-axis do not affect the coordinates of the probe tip  22  and simplifies the trigonometry. 
     For the exemplary vertical attitude A,B,C=(0,0,0) of the metroprobe  10  shown at survey point P 4  to the right in  FIG. 5 , the coordinates P 22 (XYZ) of the probe tip  22  are simply the measured coordinates P 42 (XYZ) of the target  42  minus the total offset lengths L 2 +L, or P 22 (XYZ)=(X,Y,Z−(L 2 +L)), which represents the linear coordinates of measured survey point P 4 . 
     In another exemplary vertical attitude of the metroprobe  10  shown at survey point P 3  in  FIG. 5 , the metroprobe  10  again has an attitude inclination angle dB in the −B pitch direction only, i.e. (A,B,C)=(0,−dB,0), which again inclines counterclockwise the metroprobe  10  solely in the X-Z plane, and correlates with the new coordinate position of the probe tip P 22 (XYZ)=(X+(L 2 +L)×Sine(dB), Y, Z−(L 2 +L)×Cosine(dB)) relative to the new P 42 (XYZ) coordinate position of the target  42  as measured by the laser tracker  38 . The probe tip  22  contacts the third survey point P 3  whose coordinate position therefore matches the so calculated new tip coordinates P 22 (XYZ). 
     The vectors V 26  and V 22  may be similarly resolved in the YZ plane for any roll inclination angle dA in the roll rotary axis-A, or in any attitude corresponding with the changing attitude of the metroprobe  10  during operation. 
     In other configurations of the metroprobe  10  in which the target  42 , IMU  18 , and probe tip  22  are not coaxially aligned, corresponding vectors V 26  and V 22  still define the corresponding offset length L 2  between the target  42  and origin  26 , and offset length L between the origin  26  and probe tip  22 , and similar vector analysis may be used to correlate coordinate location P 22 (XYZ) of the probe tip  22  to the origin coordinates P 26 (XYZ) and target coordinates P 42 (XYZ). 
     Accordingly, the linear X,Y,Z coordinate position of the probe tip  22  may be mathematically correlated and established by combining the linear translation of the metroprobe  10  as measured at the target  42  with the angular attitude of the metroprobe  10  as measured by the IMU  18  in any suitable orientation or attitude for accessing any survey point, including otherwise hidden survey points. 
     As described above, the target  42  preferably comprises the spherically mounted retro-reflector (SMR), which is suitably affixed or mounted atop the housing  16  containing the IMU  18  in the metroprobe  10 . Other than the addition of the SMR target  42 , the metroprobe  10  is identical in configuration and function to the one described above, including the use of the three accelerometers  24  and three gyroscopes  28  for correspondingly defining the same three orthogonal linear axes X,Y,Z and the same three respective angular axes A,B,C. 
     In the basic measurement survey illustrated in  FIG. 6 , the laser tracker  38  is used to precisely measure linear position of the target  42  by directing the laser beam  40  directly along the line-of-sight (LOS) therewith to in turn establish the linear coordinates of the probe tip  22  as correlated with the target  42  through the IMU  18  as presented above. 
     By also measuring attitude (A,B,C) of the IMU  18  using the three gyroscopes  28  therein, suitable correlation with attitude of both the probe  20  and the target  42  having the corresponding offset lengths L,L 2  may then be used to correlate the linear position of the probe tip P 22 (XYZ) to the coordinate system origin  26  in the IMU  18  as also described above. 
     However, when line-of-sight (LOS) to the target  42  is blocked by some obstruction  46 , such as a portion of the survey object  12  itself, the linear position of the IMU  18  is instead measured using the three accelerometers  24  therein to in turn establish the linear coordinates of the probe tip P 22 (XYZ) as correlated with the IMU  18  in the manner described above. 
     The survey process may be further modified by placing the metroprobe  10  in a stationary cradle  34  at one of the survey points, like point P 4  shown in  FIG. 5 , when line-of-sight (LOS) to the target  42  is blocked by the obstruction  46 . 
     By using the cradle  34  to temporarily immobilize the metroprobe  10  during the survey, the zero velocity update (V=0) may be introduced in the IMU  18  in the same manner described above to reduce drift errors in all three linear coordinates (X,Y,Z) in sequential or subsequent survey points in which line-of-sight (LOS) to the target  42  may also be blocked. 
     IMU performance can be improved by updating it with known position information whenever possible. Since the precise location of the SMR target  42  as measured by the laser tracker  38  is continually updated during the survey, the correlated and equally precise location of the IMU  18  may also be continually updated by the controller  14 . 
     The introduction of the zero velocity update in the IMU  18  is appropriate whenever the IMU  18  is known to be stationary since the drift errors continuously accumulate, and the update may be introduced manually through the controller  14  or manually upon depressing the record button  30  when the IMU  18  is mounted in the stationary cradle  34 . 
     The zero velocity update may even be introduced by programming the controller  14  to detect and recognize in the IMU  18  either measured acceleration or velocity below a defined low-value threshold, which threshold could be a function of the error-time performance as specified for a particular IMU. 
     A particular advantage of the metroprobe  10  is its configuration and total length L 2 +L as measured from target-to-tip to improve access to various survey points Pn, especially those points partially or fully hidden by various obstructions from the field of view of the laser tracker  38 . 
     Most of the metroprobe  10  can be hidden from LOS access, as long as the target  42  remains visible within the LOS of the laser tracker  38 , and a precise measurement of the hidden survey point, such as point P 3 , may still be made. The precision or accuracy of the measured coordinates at the probe tip P 22 (XYZ) will closely match the specified precision of the laser tracker  38  itself due to the low bias error and high precision operation of the gyroscopes  28  in the IMU  18  which are used to correlate position of the probe tip  22  to the position of the target  42  being measured. 
     When the target  42  itself is hidden from the laser tracker  38 , the coordinate position P 26 (XYZ) of the IMU origin  26  is instead measured using the IMU  18  itself and correlated to the coordinate position of the probe tip  22 , having a measurement accuracy dependent on accuracy of the IMU  18 , including the substantial accuracy component due to the low gyro bias errors inherent therein. 
     Additional advantages may accrue to the combined use of the laser tracker  38 , IMU  18 , and controller  14  as specifically configured for controlling operation thereof. 
     As described above, linear position P 26 (XYZ) of the IMU  18  at its origin  26  may be measured by the IMU using its accelerometers  24 , and similarly reverse-correlated to the position P 42 (XYZ) of the SMR target  42  so that measured location of the IMU  18  may be specially used to determine coordinate location P 42 (XYZ) of the target, independent of the location of the target as measured by the laser tracker  38 . 
     In this way, the laser tracker  38  may itself be feedback-controlled to follow or track movement of the target  42  atop the metroprobe  10  based on the linear coordinate position P 26 (XYZ) of the IMU  18  as measured by the IMU itself. 
     The IMU  18  provides a new ability for obtaining automatic tracking (Auto-Tracking) between the laser tracker  38  and its SMR target  42  within the specified field-of-view of the laser tracker. And, this automatic tracking may be accomplished using any conventional SMR target in its simplest and most inexpensive fixed form. 
     However, further improvements may be obtained by using a target  42   m  as illustrated in  FIG. 4  in the special configuration of a multiaxis motorized SMR pivotally mounted atop the IMU  18  in the metroprobe  10 . 
     Any conventional motorized SMR may be used, such as the Active Target™ commercially available through distributors for Automated Precision Inc, Rockville, Md. The SMR Active Target  42   m  has an azimuthal tracking angle corresponding with the yaw C-axis of unlimited 360°, and an elevational tracking angle corresponding with the pitch A-axis of +80° and −55°. The SMR may therefore be actively directed within its field of view toward the laser tracker source of the laser beam incident to the motorized SMR. 
     The laser tracker  38 , the motorized SMR target  42   m , and the IMU  18  are all operatively joined to the common controller  14  which is suitably configured in software for automatically guiding or tracking the laser tracker  18  to actively follow movement of the motorized SMR target  42   m , and also automatically back-tracking the motorized SMR target  42   m  to actively follow heading or direction movement of the laser tracker  38  in response to IMU-correlated position of the SMR target  42  relative to position and attitude of the IMU  18 . 
     Automatic tracking between a laser tracker and its SMR target is a common feature that ensures that the laser beam of the tracker is continuously aimed at the target as the target itself moves during a measurement survey. 
     Using a non-motorized target, the laser tracker itself must be suitably motorized and controlled to aim its laser beam toward the moving target. 
     Using a motorized target, both motorized target and laser tracker can improve automatic tracking therebetween, but such operation still requires LOS visibility therebetween. 
     By further introducing the IMU  18  in the metroprobe  10 , further improvement and tracking accuracy may be additionally obtained by communicating aiming directions to the laser tracker  38  to follow movement of the motorized target  42   m  based on its location as measured by the IMU  18 . 
     In this configuration, LOS visibility between the laser tracker  38  and SMR target  42   m  can be temporarily lost or broken by various obstructions during the survey, but the laser tracker  38  can nevertheless still be controlled to still follow the temporarily hidden SMR target  42   m.    
     By maintaining continuous and accurate tracking of the laser beam  40  emitted from the laser tracker  38  and either its non-motorized SMR target  42  or motorized SMR target  42   m , the measurement survey can be completed more accurately and with minimal, if any, interruptions, and thereby enhance the advantages associated with active tracking of the SMR target. 
     Further improvements in the large volume dimensional metrology survey may also be obtained by optionally integrating a second IMU  48  with the laser tracker  38  itself as shown in  FIG. 4 . The second IMU  48  may have any conventional configuration, such as the exemplary configurations described above for the first IMU  18  integrated in the metroprobe  10 . 
     The second IMU  48  is fixedly attached to the laser tracker  38  and again suitably operatively joined to the common laptop computer controller  14  for measuring linear position (XYZ) and attitude (ABC) of the second IMU  48 . 
     The measurement survey may then be conducted using the IMU-embedded laser tracker  38  at two different reference locations having line-of-sight with a plurality of common survey points for measuring coordinates thereof. 
     The least squares iterative optimization process introduced above in the Background section may then be conducted using the position and attitude of the second IMU  48  at the two different reference locations and the measured coordinates at the common survey points to conform all measurements from the laser tracker to a common coordinate reference system. 
     The mathematical least squares process may be used whenever appropriate to improve CMM measurement accuracy and confidence anytime multiple CMM observations occur on a measurement or common tie-in point to re-establish a common coordinate reference system. 
     The mathematical optimization process to tie-in all the measurements from all the different CMM locations is generally referred to in academia as a least squares problem. There are numerous variations of the tie-in process. However, least squares is the underlying mathematical principal. 
     When least squares is used in the metrology process, each measurement point can be treated as three variables. For example, the Cartesian location X, Y, and Z. Also, each CMM can be treated as six variables. For example, the Cartesian position of the CMM X, Y, and Z and its three orientation angles pitch (A), roll (B), and yaw (C). 
     The actual measurement data from each CMM to each point can be treated as up to three observations. In the case of the CMM laser tracker, the observations can be laser range, horizontal laser pointer angle, and vertical laser pointer angle. 
     After initial estimates of the CMM X,Y,Z position and A,B,C orientation are created, all the variables and observations for each measurement are written out in a system of equations and solved simultaneously in a least squares fashion. The least squares solution produces adjustments to the three measurement point variables and six CMM variables. 
     The adjustments should result in a better least squares fit for each of the measurement observations. The measurement observations are considered constants in the system of equations. After applying the adjustments to the variables, a new system of simultaneous equations is constructed and solved for another set of adjustments for the same variables. 
     The iterative process is repeated until the adjustments to the variables are insignificant and the optimum measurement point locations and CMM positions and orientations are obtained in a common reference system tying together all measurement points and the two or more CMM viewing locations. 
     This mathematical least squares process is merely a general description. Variations of the least squares process can be effectively applied to accommodate for scale adjustments, constraints on various measurements, confidence weighting on various measurements, and many other issues in accordance with conventional practice. 
     The first step in the mathematical optimization process provides an initial estimate of the position and orientation of the CMM at its different locations in the measurement survey. 
     Conventionally, this initial estimate is arbitrary, and may be randomly selected. 
     By embedding the second IMU  48  in the CMM and aligning it with a common position and orientation, the second IMU  48  can provide a generally accurate position and orientation for the CMM at the different locations for the initial step of the mathematical optimization process for improving that process. 
     Aligning the second IMU  48  in the CMM to a common position and orientation can be effected for one or more CMMs used in the measurement survey. When a single CMM is being used for the metrology survey, position and orientation of the single CMM on the first measurement or tie-in point observation may be used as the reference. When the CMM is moved for observations on a different measurement or tie-in point, the second IMU  48  embedded in the CMM can report its new position and orientation parameters for utilization in the least squares process. 
     For the case of multiple CMMs, the mathematical least squares optimization process may begin with simple least squares estimates of the CMM position and orientation. However, after observations for each CMM on common points are taken, the complete least squares process can be used to update the position and orientation of the second IMU  48  in each CMM for subsequent measurements. 
     The IMU updating procedure just described could precede the metrology survey as a calibration phase. The IMU updates could be repeated throughout the survey anytime a complete least squares optimization process is desired and accomplished. 
     Fundamental to the various improvements in the large volume dimensional metrology measurement survey described above in various configurations is the common use of the special metroprobe  10  which integrates the IMU  18  with a correlated touch probe  20 . The IMU itself may be used for constantly and accurately monitoring the angular attitude of the IMU and attached probe  20  within the high accuracy of the gyroscopes which typically have very low gyro bias drift error irrespective of the grade and cost for the IMU. 
     Such metroprobe  10  provides convenient access to various survey points, some of which might be hidden from the typical line-of-sight requirement for typical CMMs. 
     In the most basic configuration, the accelerometers in the IMU are used in an autonomous mode of operation of the metroprobe for measuring the linear coordinates X,Y,Z of the probe tip as precisely correlated to the reference origin in the IMU, and within the accuracy attributable to the accelerator drift error bias as specified for the IMU. 
     In an enhanced configuration, the metroprobe  10  is integrated with a CMM in the preferred form of the laser tracker  38  for precisely measuring location of the cooperating target  42 , which is precisely correlated to location of the probe tip  22  using the measured attitude of the IMU for in turn precisely measuring location of various survey points, with less regard to partially or fully hidden ones thereof. 
     Various features of the measurement survey process and apparatus therefor have been disclosed above, and may be used in various combinations and configurations consistent therewith, based on recombining any one or more of the individual features presented in the following claims which are merely representative and not limiting. 
     While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.