Method of constructing a 3-dimensional map of a measurable quantity using three dimensional coordinate measuring apparatus

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BACKGROUND OF THE INVENTION

This invention relates generally to three dimensional coordinate measuring machines (or CMM's). More particularly, this invention relates to a new and improved three dimensional CMM which is portable and provides improved accuracy and ease of use; and the application of this CMM to a novel method for constructing 3-dimensional maps of a measurable quantity such as temperature, coating thickness, density or the like.

It will be appreciated that everything in the physical world occupies volume or space. Position in a space may be defined by length, width and height which, in engineering terms, is often called an X, Y, Z coordinate. The X, Y, Z numbers represent the dimensions of length, width and height or three dimensions. Three-dimensional objects are described in terms of position and orientation; that is, not just where an object is but in what direction it points. The orientation of an object in space can be defined by the position of three points on the object. Orientation can also be described by the angles of alignment of the object in space. The X, Y, and Z coordinates can be most simply measured by three linear scales. In other words, if you lay a scale along the length, width and height of a space, you can measure the position of a point in the space.

Presently, coordinate measurement machines or CMM's measure objects in a space using three linear scales. These devices are typically non-portable, expensive and limited in the size or volume that can be easily measured.

FARO Technologies, Inc. of Lake Mary, Fla. (the assignee of the present invention) has successfully produced a series of electrogoniometer-type digitizing devices for the medical field. In particular, FARO Technologies, Inc. has produced systems for skeletal analysis known as METRECOM.RTM. and systems for use in surgical applications known as SURGICOM.TM.. Electrogoniometer-type devices of the type embodied in the METRECOM and SURGICOM systems are disclosed in U.S. Pat. Nos. 4,670,851 and 5,251,127 and 5,305,203 all of which are assigned to the assignee hereof and incorporated herein by reference.

While well suited for their intended purposes, the METRECOM and SURGICOM electrogoniometer-type digitizing systems are not well suited for general industrial applications where three dimensional measurements of parts and assemblies are often required. Therefore, there is a continuing need for improved, accurate and low cost CMM's for industrial and related applications.

SUMMARY OF THE INVENTION

The above-discussed and other problems and deficiencies of the prior art are overcome or alleviated by the three dimensional measuring instrument (e.g., electrogoniometer) of the present invention. In accordance with the present invention, a novel, portable coordinate measuring machine comprises a multijointed (preferably six joints) manually positionable measuring ann for accurately and easily measuring a volume, which in a preferred embodiment, comprises a sphere preferably ranging from six to eight feet in diameter (but which may also cover diameters more or less than this range) and a measuring accuracy of preferably 2 Sigma .+-.0.0005 inch (and optimally 2 Sigma .+-.0.001 inch). It will be appreciated that "Sigma" means "one standard deviation". In addition to the measuring arm, the present invention employs a controller (or serial box) which acts as the electronic interface between the arm and a host computer.

The mechanical measuring arm used in the CMM of this invention is generally comprised of a plurality of transfer housings (with each transfer housing comprising a joint and defining one degree of rotational freedom) and extension members attached to each other with adjacent transfer housings being disposed at right angles to define a movable arm preferably having five or six degrees of freedom. Each transfer housing includes measurement transducers and novel bearing arrangements. These novel bearing arrangements include prestressed bearings formed of counter-positioned conical roller bearings and stiffening thrust bearings for high bending stiffness with low profile structure. In addition, each transfer casing includes visual and audio endstop indicators to protect against mechanical overload due to mechanical stressing.

The movable arm is attached to a base or post which includes (1) a temperature monitoring board for monitoring temperature stability; (2) an encoder mounting plate for universal encoder selection; (3) an EEPROM circuit board containing calibration and identification data so as to avoid unit mixup; and (4) a preamplifier board mounted near the encoder mounting plate for transmission of high amplified signals to a remote counter board in the controller.

As in the prior art METRECOM system, the transfer casings are modular permitting variable assembly configurations and the entire movable arm assembly is constructed of one material for ensuring consistent coefficient of thermal expansion (CTE). Similarly as in the METRECOM system, internal wire routing with rotation stops and wire coiling cavities permit complete enclosure of large numbers of wires. Also consistent with the prior art METRECOM system, this invention includes a spring counterbalanced and shock absorbed support mechanism for user comfort and a two switch (take/accept) data entry device for allowing high precision measurements with manual handling. Also, a generalized option of the type used in the prior art METRECOM system is provided for the measurement of variables in three dimensions (e.g., temperature may be measured in three dimensions using a thermocouple attached to the option port).

The use of a discrete microprocessor-based controller box is an important feature of this invention as it permits preprocessing of specific calculations without host level processing requirements. This is accomplished by mounting an intelligent preprocessor in the controller box which provides programmable adaptability and compatibility with a variety of external hosts (e.g., external computers). The serial box also provides intelligent multi-protocol evaluation and autoswitching by sensing communication requirements from the host. For example, a host computer running software from one manufacturer will generate call requests of one form which are automatically sensed by the controller box. Still other features of the controller box include serial port communications for standardized long distance communications in a variety of industrial environments and novel analog-to-digital/digital counter boards for simultaneous capture of every encoder (located in the transfer housing) resulting in highly accurate measurements.

Efficient on-site calibration of the CMM of the present invention is improved through the use of a reference ball positioned at the base of the CMM to obviate potential mounting complications to system accuracy evaluation. In addition, the CMM of this invention includes means for performing a volumetric accuracy measurement protocol on an interim basis, preferably using a novel cone ballbar device.

In accordance with still another feature of this invention, a transducer is positioned at the end of the measurement arm for construction of a map of some measurable quantity as a function of 3-dimensional position. For example, a temperature probe may be positioned at the end of the arm and the arm than scanned along a path of an item of interest. The temperature probe information is stored with respect to 3-dimensional position and orientation generate a 3-dimensional map of the item's temperature. Similarly, appropriate measuring probes may be used to generate 3-dimensional maps of temperature, pressure, coating thickness, hardness, density, color, bonding and surface roughness.

The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS:

Referring to the drawings, wherein like elements are numbered alike in the several FIGURES:

FIG. 1 is a front diagrammatic view depicting the three dimensional measuring system of the present invention including a coordinate measuring machine, a controller box and a host computer;

FIG. 2 is a side elevation view depicting the host computer mounted on the serial box, which is in turn, mounted on a maneuverable arm;

FIG. 3 is a side elevation view of the three dimensional measuring system of the present invention mounted on a theodolite stand;

FIG. 4 is a rear elevation view of the CMM shown in FIG. 1;

FIG. 5 is a longitudinal view, partly in cross-section of the CMM of FIG. 1;

FIG. 6 is an exploded, side elevation view of a transfer housing used in the CMM of FIG. 1;

FIGS. 6A and 6B are views along the lines 6A--6A and 6B--6B, respectively, of FIG. 6;

FIG. 7 is a cross-sectional elevation view of two assembled, transversely orientated transfer housings;

FIG. 8 is an enlarged, side elevation view of a counterbalanced spring device used in the CMM of FIG. 1;

FIGS. 9A and 9B are fop and bottom plan views depicting the handle/probe assembly of FIG. 1;

FIGS. 10A and 10B are respective side elevation views of a ball probe and a point probe;

FIG. 11 is an enlarged front view of the controller box of FIG. 1;

FIG. 12 is an enlarged rear view of the controller box of FIG. 1;

FIG. 13 is a schematic view of the electronic components for the three dimensional measuring system of FIG. 1;

FIG. 14 is a side elevation view of the CMM of FIG. 1 depicting a probe tip calibration system;

FIG. 15 is a schematic top plan view showing a method of calibrating the probe tip;

FIG. 16 is a side elevation view of the CMM of FIG. 1 being calibrated with a ballbar;

FIGS. 17 and 18 are side elevation views of the CMM of FIG. 1 being calibrated by a novel cone ballbar device;

FIG. 19 is a side elevation view depicting a method for optimizing the CMM of FIG. 1 using an optimization jig;

FIGS. 20A-E are respective front, rear, top, right side and left side elevation views of the precision step gauge used in the jig of FIG. 19; and

FIG. 21 is a schematic view showing a method of optimizing the CMM of FIG. 1 utilizing the apparatus of FIG. 19;

FIG. 22 is a diagrammatic view of the CMM of FIG. 1 equipped with a transducer for constructing 3-dimensional maps of a quantity measurable by the transducer;

FIG. 23 is a diagrammatic view of the CMM of FIG. 22 scanning a section of a wing for a measurable quantity; and

FIG. 24 is a graphical display of the wing section of FIG. 23 depicting the size and position of the measured quantity.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the three dimensional measuring system of the present invention generally comprises a coordinate measuring machine (CMM) 10 composed of a manually operated multijointed arm 12 and a support base or post 14, a controller or serial box 16 and a host computer 18. It will be appreciated that CMM 10 electronically communicates with serial box 16 which, in turn, electronically communicates with host computer 18.

As will be discussed in more detail hereinafter, CMM 10 includes transducers (e.g., one transducer for each degree of freedom) which gather rotational positioning data and forward this basic data to serial box 16. Serial box 16 provides a reduction in the overall requirements of host computer 18 to handle certain complex calculations and provides certain preliminary data manipulations. As shown in FIG. 2, serial box 16 is intended to be positioned under the host computer 18 (such as the notebook computer shown in FIG. 2) and includes EEPROMS which contain data handling software, a microcomputer processor, a signal processing board and a number of indicator lights 20. As mentioned, basic transducer data is sent from CMM 10 to serial box 16. Serial box 16 then processes the raw transducer data on an ongoing basis and responds to the queries of the host computer with the desired three-dimensional positional or orientational information.

Preferably, all three components defining the three dimensional measuring system of this invention (e.g., CMM 10, serial box 16 and host computer 18) are mounted on either a fixed mounting surface using a rigid plate and/or a standard optical measurement instrument thread followed by mounting on a known and standard theodolite mobile stand such as shown at 22 in FIG. 3. Preferably, theodolite stand 22 comprises a part no. MWS750 manufactured by Brunson. Such a mobile stand is characterized by a stable rolling platform with an extendable vertical tower and with common attachments and locking mechanisms. As shown in FIGS. 2 and 3, support base 14 of CMM 10 is threaded or otherwise attached onto a vertical support member 24 of stand 22 while serial box 16/host 18 is supported on a shelf 26 pivotally connected at a first joint 28 to an arm 30 which is pivotally connected to a second joint 32. Connecting member 34 interconnects joint 32 to a swivel connection 36 attached to a cap 38 mounted over the top of member 24.

Referring now to FIGS. 1 and 4-9, CMM 10 will now be described in detail. As best shown in FIG. 5, CMM 10 comprises a base 14 connected to a first set of two transfer housings including a first transfer housing 40 which, in turn, is connected to a second transfer housing 42 (positioned transverse to housing 40). A first extension member 44 is rigidly attached to a second set of two transfer housings including a third transfer housing 46 transversely attached to a fourth transfer housing 48. First extension member 44 is positioned perpendicularly between transfer housings 42 and 46. A second extension member 50 is aligned with an rigidly attached to transfer housing 48. Rigid extension member 50 is rigidly attached to a third set of two transfer housings including a fifth transfer housing 52 transversely attached to a sixth transfer housing 54. Fifth transfer housing 54 has attached thereto a handle/probe assembly 56.

In general (and as will be discussed in more detail hereinafter), position sensing transducers are mounted in each of the six transfer housings 40, 42, 46, 48, 52 and 54. Each housing is comprised of bearing supports and transducer compartments which are made to then cylindrically attach to each other using 45.degree. angled attachment screws (FIG. 6). At the base 14 is a counterbalanced spring device 60 for support of arm 12 in its standard vertical configuration (FIG. 8).

Turning now to FIGS. 6 and 7, a detailed description will be made of a transfer housing and its internal components. It will be appreciated that FIG. 6 is an exploded view of a transfer housing, while FIG. 7 shows an enlarged view of the transversely oriented and attached transfer housings (e.g., housings 46 and 48). Each housing includes an internal carrier 62 and an external casing 64. Mechanical stability between internal carrier 62 and external casing 64 is provided by two counter-positioned (e.g., oppositely disposed) conical roller bearings 66, 68 positioned to compress against their respective conical races, 70, 72. Conical races 70 and 72 are permanently affixed into the external transfer casing 64. Carrier includes a shaft 122 extending therefrom and terminating at threading 74. Conical bearings 66, 69 are preferably made from hardened steel while races 70, 72 are also made from hardened steel.

During assembly of transfer casing 48, a compressional force is applied using a nut 73, which is tightened to a specific torque on threads 74, providing a prestressed bearing situation resulting in no motion other than axial rotation under typically applied loads. Because of the necessity of a low profiled or such an arm during manual handling and the attendant reduction in the overall stiffness, it is preferable and, in fact required in certain applications, to also install a thrust bearing 76 at the interface between carrier 62 and casing 64. Thrust bearing 76 provides further mechanical stiffening between carrier 62 and casing 64 of the transfer housing. Thrust bearing 76 comprises five elements including thrust adjustment ring 300, flat annular race 302, roller bearing and cage 304, annular race 306 and opposing thrust cover 308. Thrust bearing 76 is adjusted through a series of set screws 78 and provides for high bending stiffness. The transducer, (preferably an encoder 80 such as is available from Heindenhain under the designation Mini-Rod, part no. 450M-03600), is mounted to a universal mounting plate 82 for mounting into the transfer casing. Universal mounting plate 82 is important in satisfying possible component availability problems such that a change in manufacture of transducer 80 and, hence, the change in mounting screw configuration can be accommodated through modifications in the mounting plate 82. Mounting plate 82 is shown in FIG. 6A as a triangular shaped plate having rounded corners. FIG. 6A also depicts threaded members 88 and 90, a pin 86 and a coupler 84 (all of which are discussed hereinafter).

High accuracy rotational measurements using encoders 80 require that there should be no loads applied to the encoders and that motion of the transfer casing be accurately transmitted to the encoder despite small misalignments of the axis of the transfer casing and axis of the encoder. The angular transfer errors are well known to those skilled in the art from the published encoder literature. Communicating with encoder 80 is a coupler 84 such as is available from Rembrandt under the designation B1004R51R. An extension shaft 86 is utilized for ultimately connecting encoder 80 to the transfer casing 64. Shaft 86 is attached both to coupler 84 and to the end of carrier 62 at threading 74 using set screws 88, 90 (see FIG. 7). In accordance with an important feature of this invention, an electronic preamplifier board 92 is positioned in close proximity to encoder 80 and is mounted (via screws 94) on the inside of cap cover 96. Cap cover 96 is attached to casing 64 via screw 97. A transition housing 98 interconnects cap cover 96 to casing 64 via screw 97 and screws 100. Sealing of the transfer housing to the environment is accomplished at the joint using an O-ring groove 102 in which is mounted a standard rubber O-ring groove 104. A rotational endstop 106 (to be discussed hereinafter), is best shown in FIG. 6B and comprises a square shaped metal housing having an opening therethrough which is mounted onto casing 64 using bolt 108 threaded through the opening of the housing. Wire pass through grommets to stop abrasion over long term use are mounted on both carrier 62 and casing 64 at 110 and 112. A location pin 114 is received by a complimentary shaped recess 116 in carrier 62 for the purpose of maintaining relative orientation of two adjacent transfer casings.

Referring to FIG. 7, for environmental and other reasons, it is important that all wire be completely hidden from sight and, therefore, contained within the arm 12. FIG. 7 depicts two assembled transfer housings 46, 48 mounted perpendicularly to each other and demonstrating the passage of wires. It will be appreciated that during use of CMM 10, the encoder information from encoder 80 is passed to its processor board 92 through wire 118 which is then amplified and passed through the arm by machined passageways 120. Wire 118 then passes through a channel 120 in the shaft 122 of the internal carrier 62 of the transfer casing 46 and through a grommetted hole 124 at which time it passes into a large cavity 126 machined on the external casing 64 of transfer housing 46. Cavity 126 permits the coiling of the wire strands during rotation of the transfer casing and is configured so as not to produce any wire abrasion and a minimum of wire bending. However, because the wire limits the overall ability to fully rotate, an incomplete spherical groove 128 is created in which is positioned an endstop screw, 130 which limits the full rotation, in this case to 330.degree.. It will be appreciated that the pass through channel 120 and wire coiling cavities 122 are subsequently repeated in each transfer casing allowing the wires to progressively make their way down to the connector mounted at the base 14, resulting in no exposed wiring.

Turning now to FIG. 8, the construction of the aluminum arm as well as the various bearings and transducers results in an accumulated weight of approximately 10 to 15 pounds at the probe handle assembly 56 of CMM 10. Under normal circumstances, this would create a significant amount of fatigue during use and, hence, must be counterbalanced. Weight counterbalances are not preferred since they would significantly increase the overall weight of the device when being considered for transportability. Therefore, in a preferred embodiment counterbalancing is performed using counterbalance device 60 which comprises a torsional spring 132 housed in a plastic casing 134 and mounted at transfer housing 42 at base 14 for providing a lift for arm 12. Coiled torsional spring 132 can be mounted in a variety of positions affecting the overall pretension and, hence, may be usable on a variety of lengths and weights of arms 12. Similarly, due to the weight of arm 12 and the effect of the recoiled spring, significant shock loads may occur when repositioning the arm to the storage position. To prevent significant shocking of the arm upon retraction, air piston shock absorber 134 is also configured into plastic housing 142 of counterbalance spring device 60. This results in an absorption of the shock load and slow relaxation into the rest position. It will be appreciated that FIG. 8 depicts the shock absorber 134 in a depressed configuration while FIGS. 16-18 depict shock absorber 134 in a fully extended position.

In FIGS. 9A and 9B, top and bottom views of probe handle assembly 56 are shown. Probe handle assembly 56 is meant to be held as either a pencil or pistol grip and possesses two switches (items 150 and 152 in FIG. 9A) for data taking, a connector (item 154 in FIG. 9B) for the attachment of optional electronics and a threaded mount 156 for receiving a variety of probes. Because the CMM 19 is a manual measurement device, the user must be capable of taking a measurement and then confirming to CMM 10 whether the measurement is acceptable or not. This is accomplished through the use of the two switches 150, 152. The front switch 150 is used to trap the 3-dimensional data information and the back switch 152 confirms its acceptance and transmits it to the host computer 18. On the back of the switch enclosure 158 (housing 150, 152) is connector 154 which possesses a number of voltage lines and analog-to-digital converter lines for general attachment to a number of options such as a laser scanning device or touch probe.

A variety of probes may be threaded to handle assembly 56. In FIG. 10A, hard 1/4 inch diameter ball probe 158 is shown while in FIG. 10B, a point probe 160 is shown. Both probes 158, 160 are threadably mounted to mount 156 (using male threaded member 157), which in turn, is threadably mounted to probe housing 58. Mount 156 also includes a plurality of flat surfaces 159 for facilitating engagement and disengagement of the probes using a wrench.

Turning now to FIGS. 11 and 12, a description of the controller or serial box 16 now follows. FIG. 11 shows the front panel face 162 of the controller or serial box 16. From panel 162 has eight lights including power indicator light 164, error condition light 166, and six lights 20, one for each of the six transducers (identified as items 1-6) located in each transfer housing. Upon powering up, power light 164 will indicate power to the arm 12. At that time, all six transducer lights will indicate the status of each of the six transducers. In a preferred embodiment of this invention, the transducers are incremental digital optical encoders 80 and require referencing. (In a less preferred embodiment, the transducers may be analog devices). Hence, upon start up, each of the six joints (e.g., transfer housings) must be rotated to find the reference position at which time the six lights shall turn off.

In accordance with an important feature of the present invention, during usage, should any of the transducers approach its rotational endstop 106 from within 2 degrees, a light and an audible beep for that particular transducer indicates to the user that the user is too close to the end stop; and that the orientation of the arm should be readjusted for the current measurement. The serial box 16 will continue to measure but will not permit the trapping of the data until such endstop condition is removed. A typical situation where this endstop feature is necessary is the loss of a degree of freedom by the rotation of a particular transducer to its endstop limit and, hence, the applications of forces on the arm causing unmeasured deflections a