Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2000-12-29
2003-05-13
Kim, Robert H. (Department: 2882)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S500000
Reexamination Certificate
active
06563569
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser tracking interferometric length measuring instrument that uses a laser beam to track the traveling distance of retroreflectors traveling in X, Y, Z three-dimensional space while performing trilateration, and to a method of measuring length and coordinates using the length measuring instrument.
2. Description of the Prior Art
There are known measuring instruments using optical interferometry. These instruments were developed for triangulation applications, and incorporate an optical interferometry system in the lens-barrel of the instrument's telescope to measure the traveling distance of mirrors. This technology is used in construction engineering works and other areas of industry that employ the triangulation method, with laser interferometry being used for the measurements that were formerly performed using a tape measure, because optical interferometry provides a higher measurement accuracy than tape measures.
Measurement objects include large-scale coordinate measuring machines, industrial robots, aircraft and other large structures, and general triangulation objects. In recent years, coordinate measuring machines have made dramatic improvements in accuracy, with some systems being capable of measuring length to an accuracy in the order of 1 &mgr;m per meter. In endeavoring to construct this type of accuracy in a coordinate measuring machine, it is said to be desirable for the gage used to have an accuracy that is one-fifth to one-tenth that of the instrument being calibrated. Therefore, if the accuracy of a coordinate measuring machine is 1 &mgr;m, a gage with an accuracy that is one-fifth to one-tenth gm is desirable. However, with the existing level of technology, there is no gage having that kind of precision. Moreover, large-sized coordinate measuring machines have appeared that have a measurement volume capability of 10 meters cubed, within which the measurement accuracy is in the order of a few tens of micrometers.
As described above, it is generally desirable for a calibration gage to be calibrated to an accuracy of one-fifth to one-tenth the accuracy of the machine being calibrated. Interferometric measurement using a frequency-stabilized He—Ne laser is a method suited to measurement of such a calibration gage, but it entails many problems that still need to be resolved.
With respect to the accuracy of single-axis measurement of a coordinate measuring machine along the X axis, Y axis and Z axis, interferometric measurement using the He—Ne laser is possible, but the coordinate measurement resulting from the X, Y, Z travel gives rise to an anti-Abbe's error from the perpendicularity between the three axes and the yawing and pitching. Measuring all error takes far too long to be practical.
The prior art includes laser tracking interferometric length measuring instruments for aiming a laser beam at a moving object.
FIG. 8
shows an example of such aninstrument (JP-A-HEI 7-332922 and 7-332923). In this laser interferometric length measuring instrument, a mirror
610
can be rotated around the X axis and the Y axis by rotators
614
and
616
, so the laser beam can be projected onto a retroreflector (not shown) attached onto a moving object. That is, the rotator
614
that supports the mirror
610
is rotatably supported by bearings
612
, thereby allowing the rotator
614
to rotate freely around the X axis, relative to the rotator
616
, while the rotator
616
is rotatably supported by bearings
620
, enabling the rotator
616
to rotate freely around the Y axis, relative to a base plate
618
.
The laser beam emitted by a laser source (not shown) is split by a polarizing cube beamsplitter
622
affixed to the base plate
618
, with one of the split beams falling incident on a retroreflector
624
such as a corner cube prism or cat's eye, whereby the beam is reflected as a reference beam, and passes via the polarizing cube beamsplitter
622
and falls incident on a detector
622
. The other laser beam is reflected along the Y axis by a prism
628
, and then along the X axis by prisms
630
and
632
, to thereby fall incident on the mirror
610
.
Thus, the laser beam reflected by the mirror
610
is rotated when the rotator
616
rotates about the Y axis, and is moved vertically when the rotator
614
rotates about the X axis. This makes it possible to direct the laser beam at a retroreflector attached onto a moving object by controlling the rotation of the rotators
614
and
616
. Because of the factors mentioned above, it is preferable for origin of measurement eccentricity arising from the rotation not to exceed 1.0 &mgr;m. However, with the configuration of a prior art measuring instrument, keeping the eccentricity to not more than 1.0 &mgr;m is difficult. The reasons for this can be explained with reference to the arrangement of
FIGS. 8 and 9
, as follows.
1. The axis
601
of the laser beam reflected by the mirror
610
is an imaginary axis of a cylinder having a certain sectional area, so mechanical contact is not possible.
2. It is difficult to have the three axes comprised by the X-Y two-axis rotation center
602
of the gimbal mount and the laser beam axis
601
intersect with one another at one point.
As the interferometric origin of measurement, there should be no movement at the point at which the three axes intersect one another. If the attitude of the telescope is changed by error at the point of intersection of the three axes, the approximate 3-axis intersection point will move eccentrically, giving rise to a primary error in the length measurement.
An object of the present invention is to provide a laser tracking interferometric length measuring instrument and method that enable eccentricity arising from rotation of the origin of interferometric measurement to be kept to not more than 1.0 &mgr;m, even when a change in the attitude of the laser beam axis results in displacement of the origin of interferometric measurement, that is, a slight movement of the center of the reflector of an articulating optical lever.
Another object of the invention is to provide a method of measuring coordinates that can utilize the calibration of a large-sized coordinate measuring machine having a measurement volume capability of meters cubed.
A further object of the invention is to provide a laser interferometric length measuring method that can utilize the calibration of a high-accuracy coordinate measuring machine capable of measuring length to an accuracy in the order of 0.1 &mgr;m per meter.
SUMMARY OF THE INVENTION
To attain the above object, the present invention provides a laser tracking interferometric length measuring instrument comprising: a laser source; a tracking articulating optical lever provided on an optical path of an interferometric optical system, the articulating optical lever reflecting an incident laser beam from the laser source to a retroreflector that is a measurement object, the beam being reflected by the retroreflector to fall incident on the optical lever; a quadrant photodiode having a neutral point on which falls incident a reflected beam from the articulating optical lever; and attitude control means for controlling an attitude of the articulating optical lever to position a reflected laser beam from the optical lever on the neutral point of the photodiode.
The above object is also attained by the laser tracking interferometric length measuring instrument according to claim
1
, wherein the articulating optical lever comprises: a hemisphere with a mirror surface; a ball stylus one end of which is connected to a spherical surface portion of the hemisphere via a connecting rod, and another end of which is fixed to a table that can be moved along X and Y axes; and a three-ball seat into which the spherical surface portion of the hemisphere is urged.
The above object is also attained by a laser tracking interferometric length measuring method comprising: directing a laser beam from a laser source to fall incident on a center of an
Hong Jiang
Kurosawa Tomizo
Noguchi Hironori
Osawa Sonko
Takatsuji Toshiyuki
Agency of Industrial Science & Technology, Ministry of Internati
Kim Robert H.
Suchecki Krystyna
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