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Claims  |
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What is claimed is:
1. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing, said transfer housing including a housing shoulder and a shaft shoulder for housing a position transducer, said transducer producing a position signal;
a support base attached to said first end of said movable arm;
a probe attached to said second end of said movable arm;
an electronic circuit for receiving said position signals from said transducer and providing a digital coordinate corresponding to the position and orientation of said probe in a selected volume;
a groove in both the transfer housing shoulder and the transfer shaft shoulder to allow a preselected rotation of each transfer housing; and
a shear shuttle for preventing mechanical overload due to mechanical stressing of said transfer housing.
2. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing, said transfer housing including a housing shoulder and a shaft shoulder for housing a position transducer, said transducer producing a position signal;
a support base attached to said first end of said movable arm;
a probe attached to said second end of said movable arm;
an electronic circuit for receiving said position signals from said transducer and providing a digital coordinate corresponding to the position and orientation of said probe in a selected volume;
a groove in the transfer housing shoulder to allow a preselected rotation of each transfer housing; and
an endstop in said transfer housing shaft positioned in said groove for preventing mechanical overload due to mechanical stressing of said transfer housing.
3. The measuring system of claim 2 wherein said preselected rotation is 330.degree..
4. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing for housing a position transducer, at least one of said rotational transfer housings including a pair of counterpositioned layered bearings, said transducer producing a position signal;
a support base attached to said first end of said movable arm;
a probe attached to said second end of said movable arm; and
an electronic circuit for receiving said position signals from said transducer and providing a digital coordinate corresponding to the position and orientation of said probe in a selected volume.
5. The measuring system of claim 4 wherein at least one adjustable strut is provided to provide rigidity and stability between the said support base and an object to be measured.
6. The measuring system of claim 4 wherein means are provided in the probe assembly to insure that an end effector is automatically identified by the CMM.
7. The measuring system of claim 6 wherein means are provided to attach to the end effector a marking probe.
8. The measuring system of claim 7 wherein said means are threaded means.
9. The measuring system of claim 7 wherein said marking probe comprises a mounting system and a marking pen.
10. The measuring system of claim 7 wherein said mounting system includes a mounting stem having a shank matable with a bore in said movable arm on a first end of said stem and a threaded arrangement on a second end of said stem for mating with
said probe.
11. The measuring system of claim 6 wherein means are provided to attach to the end effector a force sensitive transducer probe.
12. The measuring system of claim 6 wherein means are provided to attach to the end effector a drill probe.
13. The measuring system of claim 12 wherein means are provided to attach an external power source to the drill probe.
14. The measuring system of claim 6 wherein means are provided to attach to the end effector a continuity probe.
15. The measuring system of claim 4 including seven degrees of freedom.
16. The measuring system of claim 4 wherein the signals are transmitted by telemetry which are received by the host computer via a telemetry signal.
17. The measuring system of claim 4 wherein said support base further includes a strut extending therefrom to a support member for an item to be measured, said strut providing increased stability between the moveable arm and the item to be
measured.
18. The measuring system of claim 17 wherein said strut comprises at least two segments attached axially by at least one adjustable member, said strut having a pivoting attachment at each of a first and second end of said strut said first end
being pivotally attached to said support base and said second end being pivotally attached to said support member for the item to be measured.
19. The measuring system of claim 18 wherein said at least one adjustable member is in threaded connection with said at least two segments.
20. The measuring system of claim 18 wherein said second end pivotal attachment to said support member includes a clamp to grip said support member and said clamp is pivotally secured to said second end.
21. The measuring system of claim 20 wherein said clamp is a c-clamp.
22. The measuring system of claim 4 wherein the system includes a serial box and a host computer which are sequentially operably connected to said moveable arm.
23. The measuring system of claim 22 wherein said moveable arm, serial box and computer are interconnected via a telemetry signal transmitting device and a telemetry signal receiver.
24. The measuring system of claim 4 wherein said probe is a force sensing probe.
25. The measuring system of claim 24 wherein said probe is a force sensing probe including an end tip selected from the group consisting of spherical, flat bottom, ball and point.
26. The measuring system of claim 25 wherein said force sensing probe comprises at least one internally mounted strain gauge which is operably connected to the measuring system circuity.
27. The measuring system of claim 4 wherein said probe is a contact probe.
28. The measuring system of claim 27 wherein said contact probe provides an automatic switch.
29. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm possesses seven degrees of freedom and is movable within a selected volume, each
of said joints comprising a transducer, said transducer producing a position signal;
a support base attached to said first end of said movable arm;
a probe attached to said second end of said movable arm; and
an electronic circuit for receiving said position signals from said transducer and providing a coordinate corresponding to the position and orientation of said probe in a selected volume.
30. The measuring system of claim 29 wherein at least one adjustable strut is provided to provide rigidity and stability between the said support base and an object to be measured.
31. The measuring system of claim 29 wherein means are provided in the probe assembly to insure that an end effector is automatically identified by the CMM.
32. The measuring system of claim 31 wherein means are provided to attach to the end effector a marking probe.
33. The measuring system of claim 32 wherein said means are threaded means.
34. The measuring system of claim 32 wherein said marking probe comprises a mounting system and a marking pen.
35. The measuring system of claim 34 wherein said mounting system includes a mounting stem having a shank matable with a bore in said movable arm on a first end of said stem and a threaded arrangement on a second end of said stem for mating with
said probe.
36. The measuring system of claim 31 wherein means are provided to attach to the end effector a force sensitive transducer probe.
37. The measuring system of claim 31 wherein means are provided to attach to the end effector a drill probe.
38. The probe of claim 37 wherein means are provided to attach an external power source to the drill probe.
39. The measuring system of claim 31 wherein means are provided to attach to the end effector a continuity probe.
40. The measuring system of claim 29 including seven degrees of freedom.
41. The measuring system of claim 29 wherein the signals are transmitted by telemetry which are received by the host computer via a telemetry signal.
42. The measuring system of claim 29 wherein said support base further includes a strut extending therefrom to a support member for an item to be measured, said strut providing increased stability between the moveable arm and the item to be
measured.
43. The measuring system of claim 42 wherein said strut comprises at least two segments attached axially by at least one adjustable member, said strut having a pivoting attachment at each of a first and second end of said strut said first end
being pivotally attached to said support base and said second end being pivotally attached to said support member for the item to be measured.
44. The measuring system of claim 43 wherein said at least one adjustable member is in threaded connection with said at least two segments.
45. The measuring system of claim 43 wherein said second end pivotal attachment to said support member includes a clamp to grip said support member and said clamp is pivotally secured to said second end.
46. The measuring system of claim 45 wherein said clamp is a c-clamp.
47. The measuring system of claim 29 wherein the system includes a serial box and a host computer which are sequentially operably connected to said moveable arm.
48. The measuring system of claim 47 wherein said moveable arm, serial box and computer are interconnected via a telemetry signal transmitting device and a telemetry signal receiver.
49. The measuring system of claim 29 wherein said probe is a force sensing probe.
50. The measuring system of claim 49 wherein said probe is a force sensing probe including an end tip selected from the group consisting of spherical, flat bottom, ball and point.
51. The measuring system of claim 49 wherein said force sensing probe comprises at least one internally mounted strain gauge which is operably connected to the measuring system circuity.
52. The measuring system of claim 29 wherein said probe is a contact probe.
53. The measuring system of claim 52 wherein said contact probe provides an automatic switch.
54. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing, said transfer housing including a housing shoulder and a shaft shoulder for housing a position transducer, said transducer producing a position signal;
a probe attached to said second end of said movable arm;
an electronic circuit for receiving said position signals from said transducer and providing a coordinate corresponding to the position of said probe in a selected volume;
a groove in both said transfer housing shoulder and said transfer shaft shoulder to allow a preselected rotation of each transfer housing; and
a shuttle positioned in the groove in said transfer housing shoulder and in the groove in said transfer shaft shoulder.
55. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing, said transfer housing including a housing shoulder and a shaft shoulder for housing a position transducer, said transducer producing a position signal;
a probe attached to said second end of said movable arm;
an electronic circuit for receiving said position signals from said transducer and providing a coordinate corresponding to the position of said probe in a selected volume;
a groove in said transfer housing shoulder to allow a preselected rotation of each transfer housing; and
an endstop in said transfer housing shaft positioned in said groove.
56. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm is movable within a selected volume, each of said joints comprising a rotational
transfer housing for housing a position transducer, at least one of said rotational transfer housings including a pair of counterpositioned layered bearings, said transducer producing a position signal;
a probe attached to said second end of said movable arm; and
an electronic circuit for receiving said position signals from said transducer and providing a coordinate corresponding to the position of said probe in a selected volume.
57. A three dimensional coordinate measuring system comprising:
a movable arm having opposed first and second ends, said arm including a plurality of joints with each joint corresponding to a degree of freedom such that said arm possesses seven degrees of freedom and is movable within a selected volume, each
of said joints comprising a transducer, said transducer producing a position signal;
a probe attached to said second end of said movable arm; and
an electronic circuit for receiving said position signals from said transducer and providing a coordinate corresponding to the position of said probe in a selected volume. |
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Description |
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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.
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. No.
4,670,851, 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 or seven joints) manually positionable measuring arm for accurately and easily measuring a volume, which in a preferred embodiment, comprises a sphere preferably
ranging from six to twelve feet in diameter (but which may also cover diameters more or less than this range) and a measuring accuracy of preferably 2 Sigma .+-.0.0003 inch (and optimally 2 Sigma .+-.0.001 inch). 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, six or seven 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 or alternatively, standard duplex bearings for high bending stiffness with a low profile structure. In addition,
each transfer casing includes physical 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 or integrated controller 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 volummetric accuracy measurement protocol on an interim basis, preferably using a novel cone ballbar device.
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 top and bottom plan views depicting the handle/probe assembly of FIG. 1;
FIGURES 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;
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 front elevation view of the measuring arm of the present invention depicting strut supports for stability between the measuring arm and the object being measured;
FIG. 23A is a diagrammatic front elevation view of the measuring arm of the present invention representing a preferred embodiment of the six degrees of freedom in a 2-2-2 configuration;
FIG. 23B is a diagrammatic front elevation view of the measuring arm of the present invention representing a second preferred embodiment of the six degrees of freedom in a 2-1-3 configuration;
FIG. 23C is a diagrammatic front elevation view of the measuring arm of the present invention representing a third preferred embodiment of the seven degrees of freedom in a 2-2-3 configuration;
FIG. 24A is a front diagrammatic view of the CMM of FIG. 1 with the CMM, serial box and host computer interconnected with cables;
FIG. 24B is a front diagrammatic view of an alternative CMM where the serial box circuitry has been miniaturized and mounted directly to the side (or base) the measuring arm and where both the serial box and host computer can send and receive
signals by telemetry;
FIGS. 25A and 25B are a cross-sectional elevation views along the center line of a portion of the transfer housing of FIG. 6 depicting a preferred alternative bearing designs of the present invention;
FIG. 26A and 26B are respective side elevation views, partially in cross-section, of two different probes fitted with machined holes of different depth for receiving a probing shaft for automatic identification of the correct probe;
FIG. 26C is a center line cross section elevation view of a transducer probe shaft mount capable of automatic sensing of the machined hole depth of the probes of FIGS. 26A and 26B;
FIG. 26D is a front elevation view of a marking pen probe with the probe mount shown partially in cross-section in accordance with the present invention;
FIG. 26E is a cross sectional view along the line 26E--26E of FIG. 26D;
FIG. 26F is a front elevation view, partially in cross section of an automatic punch probe in accordance with the present invention;
FIG. 26G is a front elevation view, partially in cross section of a force sensing probe in accordance with the present invention;
FIG. 26H is a front elevation view, partially in cross section of a drill mounting probe in accordance with the present invention;
FIG. 26I is a diagrammatic elevation view of a contact probe which makes a measurement upon grounding in accordance with the present invention;
FIG. 27A is a partial diagrammatic view partly in cross section of a second preferred alternative stop arrangement of the transfer housing and shaft that permits 660.degree. of rotation at each freedom of movement in the CMM in accordance with
the present invention;
FIG. 27B is a cross-sectional view along the line 27B--27B of FIG. 27A before rotation;
FIG. 27C is a cross-sectional view along the line 27C--27C of FIG. 27A before rotation;
FIG. 27D is a cross-sectional view along the line 27D--27D of FIG. 27A after counterclockwise rotation of 330.degree.;
FIG. 27E is a cross-sectional view along the line 27E--27E of FIG. 27A after clockwise rotation of 330.degree.;
FIG. 28A is a top plan view of a shuttle in accordance with the present invention; and
FIG. 28B is a front elevation view of a shuttle in accordance with the present invention.
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.
A second preferred alternative bearing arrangement will be discussed hereinafter after the first preferred embodiment is fully described herein for reasons of clarity.
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 Renbrandt 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.. A second preferred alternative end stop design which permits 660.degree. of
rotation will be discussed hereinafter after the first preferred embodiment is fully described herein for reasons of clarity. 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 t 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.
Six additional preferred alternative probe designs will be described hereinafter after the rest of the elements of the first preferred embodiment of CMM 10 have been fully discussed for reasons of clarity.
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. Front 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
and inaccuracies in the measurement.
At any time during the measurement process, a variety of communication and calculation errors may occur. These are communicated to the user by a flashing of the error light and then a combination of lights of the six transducers indicating by
code the particular error condition. It will be appreciated that front panel 162 may alternatively utilize an alphanumeric LCD panel giving alphanumeric error and endstop warnings.
Turning to FIG. 12, the rear panel 168 of serial box 16 includes a variety of standard PC connectors and switches including a reset button 170 which resets the microprocessor; an AC input fan 172 for air circulation; a connector 174 for a
standard PC AT keyboard, connector 176 for an optional VGA board for monitoring of the internal operations of serial box 16, connector 178 for receiving the variety of signal lines for the CMM data, and connector 180 for the standard RS232 connector for
the host 18.
Serial box 16 is responsible for monitoring the temperature of the CMM and in real time modifying the kinematics or mathematics describing its motion according to formulas describing the expansion and contraction of the various components due to
changes in temperature. For this purpose, and in accordance with an important feature of this invention, a temperature monitoring board 182 (which includes a temperature transducer) is positioned at the location of the second joint 42 on the interior of
a cover 184 (see FIGS. 4 and 5). CMM 10 is preferably constructed of aircraft grade aluminum externally and anodized. Preferably, the entire arm 12 is constructed of the same material except for the mounting screws which are stainless steel. The same
material is used throughout in order to make uniform the expansion and contraction characteristics of arm 12 and make it more amenable to electronic compensation. More importantly, the extreme degree of stability required between all parts through the
large temperature range requires that there be no differential thermal expansion between the parts. As mentioned, the temperature transducer 182 is preferably located at transfer housing 42 because it is believed that this location defines the area of
highest mass and is therefore the last area to be stabilized after a large temperature fluctuation.
Referring now to FIG. 13, the overall electronic schematic layout for CMM 10 and serial box 16 is shown. Six encoders 80 are shown with each encoder having an amplifier board 92 located in close proximity to it for the minimization of noise on
signal transfer. An option port 154 is shown which is a six pin connector available at the handle 56 for the attachment of a variety of options. Two control buttons 150 and 152 for indicating to serial box 16 the measurement process, are also shown.
The temperature transducer is associated with a temperature circuit board 182 which is also located in arm 12 as shown in FIG. 13. In accordance with still another important feature of this invention, the temperature board 182 comprises an
EEPROM board. The EEPROM is a small computerized memory device (electrically erasable programmable read only memory) and is used to contain a variety of specific calibration and serial number data on the arm (see discussion regarding FIGS. 19-21). This
is a very important feature of this invention which permits high quality control of CMM 10 and importantly, precludes the inadvertent mixup of software and arms. This also means that the CMM arm 12 is a stand alone device not requiring specific
calibration data to reside in controller box 16 which may need to be separately serviced and/or switched with other machines.
The electronic and pulse data from the arm electronics is then transmitted to a combined analog-to-digital converter/digital counting board 186 which is a paired set comprising a 12 bit analog to digital converter and a multi channel 16 bit
digital counter. Board 186 is positioned on the standard buss of the controller box. The counting information is processed using the core module 188 (comprising a commercially available Intel 286 microprocessor such as a part number CMX-286-Q51
available from Ampro) and programs stored on an EEPROM also residing in the controller box. Subsequent data is then transmitted through the serial communication port 189.
The microprocessor-based serial box 16 permits preprocessing of calculations specific to CMM 10 without host level processing requirements. Typical examples of such preprocessor calculations include coordinate system transformations; conversion
of units; leap-frogging from one coordinate system to another by using an intermediary jig; performance of certain certification procedures, including calculations of distance between 2 balls (such as in ANSI B89 ballbar); and outputting data in specific
formats required for downloading to a variety of hosts and user programs.
The serial box is configured to communicate with a variety of host formats including PC, MSDOS, Windows, Unix, Apple, VME and others. Thus, the serial box processes the raw transducer data on an ongoing basis and responds to the information
requests or polling of the host computer with the desired three dimensional positional or orientational information. The language of the serial box is in such a form that drivers or computer communication subroutines in microprocessor 188 are written in
the language of the host computer so as to drive the serial port and communicate with CMM 10. This function is designated the "intelligent multi-protocol emulation and autoswitching" function and works as follows: A variety of host programs may be
installed on the host computer. These host programs will poll the serial port with a variety of requests to which the serial box must respond. A number of protocols have been preprogrammed into the serial box to respond to polls or inquiries on the
serial port for a variety of different, popular software. A polling request by a software requires a specific response. The serial box will receive the polling request, establish which protocol it belongs to, and respond in the appropriate manner.
This allows transparent communication between CMM 10 and a wide variety of application software such as computer aided design and quality control software, e.g., AutoCad.RTM. from Autodesk, Inc., CADKEY.RTM. from Cadkey, Inc., and other CAD programs;
as well as quality control programs such as GEOMET.RTM. from Geomet Systems, Inc. and Micromeasure III from Brown and Sharpe, Inc.
The three dimensional CMM of the present invention operates as follows. Upon power up, the microprocessor 188 in the serial box 16 undergoes start up self-checking procedures and supplies power through the instrument port to arm 12 of CMM 10.
The microprocessor and software residing on EEPROM 182 determines that upon initial power up none of the encoders 80 have been initialized. Hence, the microprocessor 188 sends a signal to the display board lighting all the lights 20, indicating a need
to be referenced. The user will then mechanically move the arm which will cause the transducers to individually scan their range, at which time a reference mark is passed. When the reference mark is passed, the digital counter board 186 responds by
trapping its location and identifying to the front display board 20 that the transducer has been referenced and the light is extinguished. Once all transducers have been referenced, the system establishes serial communication with the host and waits for
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