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A fresh look at optical gaging.

A fresh look at optical gaging

As computer and optical technology rapidly improve in both cost and performance, the optical CMM is challenging its touch-probe cousin in an increasing number of three-dimensional measurement applications. Although each retains certain areas of exclusivity--optics is best at analyzing flat surfaces while touch systems exceed at probing depths and interiors--optical CMMs are getting better at depth, and the ultimate solution may well be a joining of forces, as a growing number of ambidextrous CMMs indicate.

Here are some insights into the latest optical technology from a wide variety of recently introduced noncontact gaging products.

The optical CMM

The IQ-3000 video coordinate measuring machine from Optical Gaging Products is a good example of leading-edge noncontact measurement technology. Although designed primarily for inspecting large, flat sheet-metal or plastic parts, it is increasingly moving into heavier castings and machined parts as depth features improve.

Their largest and most sophisticated CMM, it is a planar, bridge-type unit with XYZ travel of 36" x 48" x 6". The air-bearing worktable moves on the long axis (6 ips), the optical assembly on the overhead cross axis. Worktable surface is glass, limiting load-bearing capacity to 75 lb.

Image processing

In all optical systems, image processing speed is critical. It takes many millions of operations to process each measurement or "snapshot." The OGP IQ-3000 has a 20 mops (million operations per second) processing system.

The processor must look at each pixel, discarding those that show no light/dark transition. Pixels with transition are analyzed further, using a sequence of proprietary algorithms to assess a gray-scale value and assign XY coordinates. Adjacent points are then combined to create primitive features such as arcs and lines. Sizes, locations, and orientations of these features are stored and connected with similar features to form geometries of the viewed part. Algorithms of edges or key features can be varied to suit the type of illumination or surface feature being measured.

To be useful, all of this must take place in an "instant." The more data that can be gathered and analyzed, the more edge and feature data that can be processed. Although this means a greater processing requirement for the computer, it leads to the capability to do 100% inspections as parts move by on a conveyor. In OGP's case, up to 1000 features can be extracted from a single snapshop in one second.

Double vision

In OGP's approach, two magnifications are produced simultaneously, 1X and 8X, and fed to a single camera. Which image the camera "sees" is controlled by electronic shutters. This permits the user to switch magnification at any time, with no loss in calibration or datum references. Both magnifications are parfocal (in focus) and telecentric (object size does not change as focus moves through depth of field), features essential for maintaining dimensional accuracy.

Transport requirements

In OGP's approach, a granite base plate provides a stable platform for mounting the axis guideways and optics. Granite isolates the system from vibration and is less affected by temperature change than steel or aluminum. Granite provides stiffness to prevent axis motions from twisting the structure and creating squareness errors.

Air bearings guide X and Y table motions on granite or ceramic guides. The bearings are frictionless, self-cleaning, need no periodic adjustment, and reduce the need for precision lapping of the guides for straightness.

Operating modes

The IQ 3000 is designed for automatic inspection, fully programmable with minimal operator setup. The machine can be programmed by measuring the first part manually and copying the operator's movements, or by creating a program from converted dimensional information from a CAD database through a DMIS interface, which, says OGP, requires only a minimum of operator editing.

Its measuring software works like a CAD system. As features are programmed for measurement, they are drawn on the computer screen, showing relationships of features and datums. Dimensions and tolerances are also displayed. All programming is done with a mouse; a keyboard is seldom needed.

Calibration

Calibration and certification of video inspection systems are not as well understood as for touch-type CMMs. OGP has developed a set of certified standards and a sophisticated automatic program to measure them and create nonlinear interpolative look-up tables for calibration.

Both optical field of view and stage travel are calibrated by measuring a number of features throughout their range and creating a correction map for each zone, correcting for mechanical errors in straightness and squareness. Differences in pixel size, symmetry, and response from one area of the camera's array to another can also be accounted for with this calibration software.

New developments

Among new technologies OGP has developed are newer weakedge measurement algorithms, expanded computer-memory modules, and higher-capacity mass-storage units. An integrated statistical-analysis package is being added, and the DMIS interface tailored to user needs.

A new optical assembly offers three magnification levels: 1/8X, 1X, and 4X. The 1/8-reduction option reduces apparent part size, enabling a 5-sq-in area to be viewed and measured in one snapshot. The 1X and 4X magnifications offer greater precision for small features, and the Z-axis travel of 12" makes this optical assembly ideal for large-part measurement.

An autoranging laser in this trimag optical assembly tracks the part surface, automatically adjusting focus. This is particularly useful for warped or bowed parts, avoiding the slowdown of a discrete autofocus operation. The laser also provides "intelligence" in that it does not require prior knowledge of part height.

Also new in the tri-mag assembly is the integration of a touch probe. A touch probe brings two things to vision-based CMMs: direct tactile measurement of features such as cones and cylinders that cannot be measured optically, and use as an optical interposer--i.e., it can be placed against a wall or edge that cannot be viewed optically, and then the probe itself is viewed and measured optically.

New processing software

Just as the computer brought amazing new levels of interpretation to NASA's moon photos, new analytical algorithms have boosted the accuracy of optical measurement by splitting the pixel. Using Fourier interpolation, spatial filtering, and normalized correction techniques, Itran (and others) can now offer measurements down to a fraction of a pixel (as low as 1/16).

According to Itran, the chief benefits of sub-pixel tools are greater accuracy and repeatability, inspection of larger parts with finer tolerances, and much higher ratios of part tolerance to measurement resultion (restoring the 10:1 rule of mechanical measurement). It also means faster gaging-cycle times and reduced demands on the optics, permiting fewer, smaller, and less expensive cameras and lens systems.

Measurement options

To illustrate some of today's options in optical-gaging software, here from Itran's catalog are some of their software packages for making specific measurements:

* Caliper. Measures (to a single pixel) internal or extenal dimensions between parallel part edges and determines if they are within tolerance. * Defect finder. Detects presence or absence of a specific part feature by determining the number of edge pixels and average light intensity in a specific area. * Template. Measures similarity between a part feature and a reference part, examining edge patterns, shape integrity, and presence or absence of features to determine if tolerances have been exceeded. * Pixel counter. Measures the presence or absence of features within a specific part area where light intensity indicates process variation (assembly positioning, for example). It counts the number of pixels above or below a certain level of intensity. * Tape measure. Measures the distance between two icons placed on the image of part features. Used to measure hole center distances, diagonals from opposite corners, or distances between multiple camera mages (i.e., long parts). * Protractor. Similar to above, uses three icons to measure angles and determine if tolerances have been exceeded in gear teeth, bolt circles, or corner squareness. * Locator pin. Determines a central reference point, even when no visually distinguishable feature exists, based on other distinguishing features. Then, other features can be referenced radially or angularly from this point.

The caliper, tape-measure, and template functions are also available in sub-pixel versions for much greater resolution measurements.

OTHER NONCONTACT INSTRUMENTS

Radiant temperature

Maxline infrared temperature-measurement system can use one or two optical-pyromoter sensing heads for real-time monitoring and control. Noncontact radiance response is 25 msec. Features include digital communication, auto calibration, self diagnostics, and analog and digital outputs. Models cover temperatures from 0 to 6500 F (3600 C), including a two-color (ratio) unit.

Optical CMM option

OMC 850 CMM combines optical and touch probing in a single probe head. Operators can switch between the two methods in a single measuring run. Optical probing can speed some inspections, such as the sizing and positioning of small circles in 1.5 sec, while optically inaccessible features can be probed mechanically. Both probes feature quick-change toolholders, are collision protected, and can be calibrated to a common zero point. Sub-pixeling provides resolutions to 0.7 micron. Measuring envelope is 32" X, 43" Y, and 15-19" Z.

3-D video inspection

Prism 3-D video inspection CMM can measure large, heavy parts as well as small components with a repeatability of [+ or -] 0.0002" for each independent axis. Parts remain stationary as the camera moves around them collecting data. This open architecture permits on-line inspection.

Faster interferometry

Mark IVxp interferometer offers improved repeatability, processing speed, and graphics. Processing speed is ten times faster than previously, due to the Hewlett Packard computer platform and optimization algorithms. MetroPro software uses pull-down menus, icons, and windows; and high-resolution graphics are used to produce solid-model displays of results.

Microscopic CMM

Optical MicroVal noncontact three-axis CMM uses an optical/video package to zoom in for viewing in the 10X to 250X range. Fine adjustments on each axis align measurement points with a crosshair on the monitor, and vertical alignment is accomplished by focusing. Part alignment is done electronically. Measuring envelope is 18" x 20" x 16". Built-in computer performs up to 29 geometric routines.

3-D profiling

TDP 1000 profiler combines laser triangulation gages with an X-Y stage to automatically map surfaces, generating ASCII data compatible with graphics and data-analysis packages. Depending on the stage, gages have measurement ranges of 5 mm to 76 mm, with resolutions ranging from 2 to 13 microns. Automatic exposure control allows the profiling of stainless steel, metallic, or black-painted objects without operator intervention or special filters.

Vibration analysis

Noncontact vibration sensor uses a laser Doppler displacement meter to measure vibrational velocities and accelerations with accuracies exceeding those of interferometers. Using an IBM-compatible 386 PC, the system measures and records displacements to a resolution of 1 nanometer at data rates to 800,000/sec. Frequency response is 50 kHz, with a sensitivity of 0.01 g at 1000 Hz. Data analysis includes viewing, differentiation, and power-spectra density.

Position sensing

Optocator noncontact measurement sensors feature increased measuring ranges and stand-off distances, a laser-based sweeping-beam sensor for surface profiling, a seam-finder for robotic welding, and a optoelectronic motion-measurement camera. Probe weights range from 2 to 15 lb, and are available with measurement ranges of 0.3 to 40" and stand-off distances from 3.7 to 48".

Spindle alignment course

A 32-hr training course is offered to provide theory and hands-on training in use of the laser-based L-800 spindle-alignment system. Included are laser-measurement techniques for determining motion straightness, spindle-axis parallelism, slide-assembly squareness, and straight-move misalignment.

Portable laser calibrator

Model ML10 laser interferometer is PC-compatible and rugged enough to be checked as airline baggage. It is used to calibrate machine tools and CMMs. Beam diameter is 6 mm, resolution is [+ or -] 0.001 micron, and distance accuracy is [+ or -] 0.025 micron to 40m (131 ft). Optical components permit measuring positional accuracy, velocity, angular deviation/rotation, straightness, squareness, parallelism, and flatness.

Calibration tool

Laseruler has a measuring range of 8" and a resolution of one microinch. Applications include calibration of gage blocks, plug and thread gages, pin gages, material characterization studies, and inspection of precision parts. Unit can be linked to computerized SPC systems via an RS 232X serial port.

Long video probe

LongSteer articulating probe for remote visual inspection is 1/4" dia and available in 15- to 25-ft lengths. It is used with the VideoProbe 2000 visual-inspection system, and articulation is controlled by a joystick.

Portable X-ray system

Series of three portable X-ray inspection systems are used to inspect aerospace assemblies, welded materials, and in-service aircraft. Lightweight tube head and end-grounded configuration provide ease of positioning in limited-access areas. The smallest, the SPX 160, has a 1.2-mm focal spot. The SPX 200 can penetrate 1.5" of steel, and the SPX 300, 3" of steel.
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Publication:Tooling & Production
Date:Oct 1, 1990
Words:2086
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