Optics: measuring and testing – Position or displacement
Reexamination Certificate
2000-09-05
2003-02-11
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Position or displacement
Reexamination Certificate
active
06519043
ABSTRACT:
BACKGROUND OF THE INVENTION
The performance or accuracy of a CNC (computer numerical control) machine tool or a coordinate measuring machine (CMM) is determined by the linear displacement errors, straightness errors, squareness errors, angular errors and non-rigid body errors of the machine tool spindle movement (to generalize the moving body which could be something other than a machine tool spindle the term “body” will be often used instead of spindle). A complete measurement of these errors is very complex and time consuming. Diagonal measurements of the body movement taken continuously along the diagonals of a volumetric space have been recommended for a quick check on the volumetric performance of the machine. However, with this measurement system there is not enough information to identify the error sources for accurate machine quality assessment and error correction.
The characterization of a machine tool body movement is very complex. The machine tool body movement errors are referred to broadly as linear error, angular errors, and squareness errors. The linear errors break down into what are referred to as linear displacement errors and vertical straightness and horizontal straightness errors. Angular errors include pitch, yaw and roll errors. Thus for each of the X, Y and Z axis of motion, there are 6 recognized errors, 3 linear, and 3 angular errors plus 1 squareness error. Thus, for a 3 axis machine, there are a total of 21 of these error.
For a non-rigid body there are many more errors. Existing methods of measuring these errors for calibration and accuracy evaluation using laser interferometer and other means has been very difficult and time consuming until the present invention was developed.
The machine accuracy can be improved by measuring all the above referred to errors and then compensating these errors, providing that the machine tool body movement is repeatable. The key is how to measure these errors accurately and quickly. The inferior previous methods for measuring these errors include the methods disclosed by G. Zhang, R. Ouyang, B. Lu, R. Hocken, R. Veale, and A. Donmes, in an article entitled “A displacement method for machine geometry calibration”, Annals of the CIRP Vol. 37, No. 1. 1988, pp 515-518, and W. L. Beckwith, Jr. in U.S. Pat. No. 4,939,678, grated Jul. 3, 1990 entitled “Method for calibration of coordinate measuring Machine”. These errors for calibration and accuracy evaluation using laser interferometer and other means has been very difficult and time consuming until the present invention was developed.
One example of a laser interferometers which have been used for measuring linear displacement is for example, model HP 5529, manufactured by Hewlett-Packard, Palo Alto, Calif. The straightness accuracy of the body movement using these prior measurement techniques can be measured, for example, by a laser interferometer using angular optics or a quad-detector as, for example, model MCV-3000, manufactured by Optodyne, Inc., of Compton, Calif. The prior art squareness measurements and these other measurements are carried out by directing a laser beam parallel to a selected diagonal of the space in which the body involved can be moved (referred to hereafter as the volumetric space) and the body is moved intermittently along the diagonal as measurements are taken. This process is duplicated for at least one other diagonal and preferably all four diagonals of this space. These laser interferometer diagonal measurements are recommended in the ASME B5.54 standard (section 5.9.2 in Methods for Performance Evaluation of Computer Numerically Controlled Machining Centers, ASME B5.54-1992, American Society of Mechanical Engineers, New York, N.Y.) for the check of volumetric performance.
These intermittent diagonal measurement techniques have heretofore been assumed to be a quick check of the machine accuracy. This was because the diagonal measurement is sensitive to all the errors. Hence, if the diagonal measurement shows the error is small, good machine accuracy has been assured. On the other hand, if the intermittent diagonal measurement shows the error is large, it has been rather difficult to determine the cause of this large error. As above indicated, these prior art techniques are time consuming, particularly if all of the errors are measured. Thus, they need as many as 18 separate setups and measurements respectively for the displacement, straightness, angular and squareness measurements. This can take two of three days to complete which can result in substantial undesired down-time of the machine involved.
Disclosed here is a new measurement method, referred to herein as a vector measurement method. It can measure all these errors, using a simple and portable laser interferometer or a laser Doppler displacement meter (LDDM), in 4 settings and within a few hours. Also, this method is so simple and easy to carry out that in-house personnel of a typical machine tool shop can readily take the measurements and then compensate for the errors involved.
SUMMARY OF THE INVENTION
The present invention involves apparatus and a method of using such apparatus which provides a vector measurement technique which enables the movement error components above described to be determined more accurately and efficiently than the prior art. It does so by directing a beam of reflectable energy in a direction non-parallel to the direction of movement of the body along paths having measuring points. (In contrast, the prior art error measurement techniques moved the body in the same direction, that is, along the beam path.) A reflector reflects the energy back to a measuring apparatus which measures the distance between the apparatus and the reflector at these different measuring points. These reflector distance measurements vary with the body position and they are then compared with predetermined ideal reflector distances measurements for these points on the assumption that the body is moved without error to these points by the computer control system involved. This comparison produces reflector distance error values from which the actual body position error components described above can be computed.
Because the prior art error measurement technique moved the body along the beam path and took its measurements at only points along the path, it cannot directly measure the straightness and other error components in directions perpendicular to the direction of body movement.
As in the prior art, the vector measurement technique of the present invention preferably uses a laser beam reflector and a measuring apparatus which measures the distance between the measuring apparatus and the point on the reflector where the laser beam is reflected. However, the broad aspects of the invention encompass the use of reflectable energy other than laser beam energy. Thus it includes the use of such energy sources as radar frequency or other electromagnetic energy sources or an acoustic energy source with measuring apparatus which measures the distance between the source of such energy and the point where a beam of such energy is reflected by a reflector back to the measuring apparatus.
Also, while it is highly preferred that the body to be moved carries the reflector and the measuring apparatus is stationary and directs a beam of the energy involved along a diagonal (or other less preferred direction) in the two or three dimensional space in which the body is moved, the positions of the apparatus and the reflector could be reversed without deviating from the broader aspects of the invention.
In the preferred invention summary now to be made and in the drawings and the description thereof to follow, a much larger than normal laser beam reflector is carried by the body being moved and a stationary laser measuring apparatus directs the laser beam along a diagonal of a three dimensional space. The reflector is sufficiently large that it will intercept the laser beam even when the body is spaced from and is moved along paths which extend a substantial distance from the diagonal. The body is
Hattis Russell E.
Optodyne, Inc.
Stafira Michael P.
LandOfFree
Vector measurement for coordinate measuring machine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Vector measurement for coordinate measuring machine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Vector measurement for coordinate measuring machine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3163711