Geometrical instruments – Gauge – Coordinate movable probe or machine
Reexamination Certificate
2000-10-19
2003-12-30
Gutierrez, Diego (Department: 2859)
Geometrical instruments
Gauge
Coordinate movable probe or machine
C033S0010PT, C033S556000, C033S559000
Reexamination Certificate
active
06668466
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to the field of precision machining, and more specifically to an articulated coordinate measuring machine (ACMM), system, and method for providing independent, real-time position feedback control during precision machining.
Traditionally, the position of a movable machine member (e.g. tool holder or end effector) is determined indirectly by sensing motion at a large distance away from the actual point of operation. For example, the linear motion of a slide for a worktable is derived indirectly from rotational motion of a motor and lead screw combination by using a angle encoder or resolver. Another example is the use of a linear scale located adjacent to a guideway. These indirect methods have limited accuracy due to the well-known Abbe-offset error effect (e.g. comparator error). The accuracy can also be degraded by (1) thermal distortion effects induced by operation of the machine itself (e.g. motor heat from driving a tool under load, and spindle heating due to friction), or by an uncontrolled thermal environment; and (2) by progressive wear or aging of drive gears, guideways, etc. These problems become even more severe as the number of degrees of freedom (DOF) increases, such as a serially-linked 6-axis robotic arm manipulator, or a parallel-linked 6-axis Hexapod or Rotopod machine.
In-process inspection and Statistical Process Control requirements have forced the need to periodically re-certify the manufacturing process. Traceability of the machining process, and quick performance evaluation within the required workspace, can be critically important for small lot size and one-of-a-kind fabrication. The American National Standard ASME B5.54 provides rules for determining the three-dimensional positional performance of computer numerically controlled (CNC) systems. Satisfying the specifications of ASME B5.54 usually requires a multitude of expensive and sensitive equipment, applied by trained experts.
For straightness evaluation of a single machine tool axis, a laser interferometer or a series of calibrated, multiple-length gauge blocks can be used. For two-dimensional positional performance, a precision variable-length ball-bar is recommended. For three-dimensional positional performance (e.g. volumetric), the length of a fixed ball-bar should be measured in 20 different locations along the edges, face diagonals, and body diagonals of a cubical work zone. For non-cubical work zones, the number of positions can increase to 30-35 positions. A need exists, therefore, for a rapid, yet accurate multi-dimensional metrology system for calibrating precision machine tools.
Despite the need for increased accuracy and speed, such a metrology tool should remain a lightweight and portable unit. This would allow a single coordinate measuring machine (CMM) to periodically check and certify an entire shop floor having a multitude of equipment.
Statistical process control (SPC) has been successfully used for quality control of high volume manufacturing. However, as product diversity increases, and lot size decreases, the desire to minimize or eliminate independent product inspections has grown. This has created a new requirement for total process control (TPC), where all of the factors influencing the production process are detected, analyzed, and controlled, preferably in real-time. For this reason, an independent, real-time position feedback control system is needed to build Total Process Control (TPC) into the manufacturing process.
As explained above, real-time position measurement systems used on conventional precision machines suffer from a number of errors (e.g. comparator offset, thermal distortion, and wear of sliding surfaces). An ideal metrology system would directly measure the precise location of the actual cutting point of a machine tool, spray head, etc. during operation.
Laser trackers can provide non-contact, real-time measurement in three dimensions of a retroreflector sphere or cube mounted to a surface. These devices utilize a motorized, gimbal-mounted laser interferometer, which tracks the moving retroreflector (such as the SMX-4500 Laser Tracker manufactured by Spatial Metrix Corp., or a SMART310e Laser Tracker manufactured by Leica, Inc.). However, a laser retroreflector can not be practically mounted to an actual spinning drill bit or cutting tool. More importantly, if an object obstructs the laser beam, tracking can be lost. Operation must be paused to allow re-acquisition of the reflecting target mirror. Generation of cutting chips or small particles during machining operations and large volumes of cutting cooling fluids, can obstruct the laser beam and prevent useful application for real-time position control. End effectors mounted on the end of robotic arms, such as paint sprayers, thermal spray heads, plasma spray heads, sand blasters, grinders, etc. can also produce large volumes of particulates or dust that obscure laser beams, 3-D vision systems, or other non-contact sensors. 3-D laser tracking devices are generally very expensive, in the range of $ 125-250 K.
A need exists, therefore, for a position measurement device that remains in solid contact with the moving member, ideally as close as possible to the cutting tool or point of operation. If the probe tip is physically attached to the moving member, then cutting chips, particulates, and fluids should not interfere with the ACMM's operation. Ideally, the apparatus would not intrude on the work zone. Also, the device should have a low inertia, so as not to interfere with the rapid motion of the moving member. Such a device should be lightweight, low cost, have low vibrations, have a large range of motion, and a high accuracy. The data collection system of the system should be capable of processing position measurements at a sufficiently high data rate, commensurate with providing real-time feedback to a rapidly moving machine member. The accuracy of such a system should be better than 10 microns, preferably better than 3 microns. Such a system should be easily mounted on, or near, existing machines with minimal structural modifications.
If the probe tip of the ACMM is physically mounted with a pivoting joint to the moving member of the machine tool, the ACMM's probe could be automatically guided by the moving member through all extremes of the workspace. This could make the application of the ACMM simple and automatic; requiring little skill or training. Existing feedback systems, i.e. resolvers, encoders, linear scales, etc. could be used for the servo controls velocity feedback loop, while using the independent position feedback of a pivotally mounted CMM for the displacement feedback loop.
Gantry or bridge-style fixed 3-axis orthogonal (e.g. Cartesian) CMM's provide outstanding accuracy (better than 0.0001 inches), but are typically very expensive and are not portable.
Multi-axis, portable CMM's (ACMM's) are commercially available from Romer, Inc. of Carlsbad, Calif. and by Faro Technologies, Inc. of Lake Mary, Fla. However, their accuracy is limited to about 0.001-0.005 inches. These portable ACMM's have six degrees-of-freedom (one rotation axis and one swivel axis at each of the three joints, linked by two support arms). Six degrees-of-freedom (DOF's) provides the ability to easily position the probe tip underneath and behind complex shapes, without having to reposition the base. The use of rotary joints also minimizes errors due to torques and bending moments. Precision rotary transducers (e.g. angle encoders) are mounted at each of the six joints. Their data are used to calculate the probe's position in three-dimensional Cartesian space. The measuring volume is generally spherical, with the radius equal to the maximum reach of the linked arms, typically a 3-6 foot radius. The tubular support arms are typically made of a lightweight and stiff material, such as an aluminum alloy, or a carbon fiber composite.
A need exists, therefore, for a low-cost, portable ACMM that has sufficient accuracy for provi
Bieg Lothar F.
Ensz Mark T.
Jokiel, Jr. Bernhard
Watson Robert D.
Gonzalez Madeline
Gutierrez Diego
Sandia Corporation
Watson Robert D.
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