Making Sense of Multi-Sensor MeasurementAuthor: Mr. Fred Mason OGP inc. In fact, Optical Gaging Products anticipated the user benefits of a single measurement system incorporating several data acquisition technologies in the 1980s.With so many claims about multi-sensor measurement from so many sources, it is important to understand what it all means, and most importantly, what the benefits are over dedicated, single technology measurement systems. Why Multi-Sensor?
 Multi-sensor measurement machines exist in response to changes in manufacturing technology. In an effort to reduce part counts, manufacturers use computer-aided design and engineering tools to design parts with unique, complex shapes and tight dimensional tolerances. New multi-axis machine tools, EDM technology, advances in injection molding, and other manufacturing technologies are turning out sophisticated parts with numerous physical features that have critical shapes, sizes, and locations. Often, tiny features must have accurate relationships with bores or surfaces that are inches away or on opposite surfaces. Proper performance of the final product depends on each part meeting the design specifications in the CAD file.
Complexity of the part is one thing, but it is not the only thing. No matter the product or market, manufacturers also face increased pressure to reduce costs. To aid in this, machine tools now incorporate automatic tool changers, pallet loading systems, and rotary indexers to reduce setup and handling times and increase throughput. However, some companies strive to achieve economies in manufacturing throughput but then subject their parts to traditional serial measurement practices.
The same considerations that go into reducing manufacturing time apply to reducing measurement time such as minimizing part handling, reducing the number of fixtures, cutting foot traffic between measurement machines, and freeing up valuable floor space. Since one machine can do most of the manufacturing, wouldn’t it be great if one machine could do most of the measurements? That’s what multi-sensor measurement machines are all about.
At this point you might think, “That’s what my CMM is for.” Unfortunately it’s not that simple. It’s important to understand what particular sensors can and cannot do before jumping to conclusions about what measurement tool to use.
Technological Advancements
Competition is a good thing. It forces manufacturers to continually improve their products to stay ahead. You see examples of this with advancements in technology and software. In video measurement, cameras have more pixels and better signal-to-noise ratios so imaging is improved. Innovative LED illumination improves edge detection. DSP technology speeds image processing. In laser technology, laser diodes have reduced the size and cost of sensors. Improved detectors and electronics have extended laser measurement ranges. Contact probes come in a wide range of sizes and sensitivities, and change racks allow for probe changes during a measurement routine. Today, innovative systems based on these technologies incorporate these advancements to extend their measurement versatility and provide more value. However, even with these advancements in individual technology, each sensor can do things the others cannot. Stated another way, none of these individual technologies can do all the measurements the parts of today require.
Technological plusses and minuses
Video measurement systems excel at non-contact measurement of edges. A video measuring system uses a camera sensor to acquire images of a part that are then analyzed with software. Automatic video systems have precision stages to move each feature into the imaging field of view. This can mean moving the part to the optics or the optics to the part. This requires accurate stage motion and a rigid structural design in order to maintain the spatial (XYZ) relationships of all the features. High-quality optics magnify features without distortion. Their ability to change magnification allows them to provide high data density for both small and large features. CNC stage control allows video measurement to trace the edge of a part that is hundreds of times larger than the optics field of view. Autofocus makes sure features present the sharpest image to the camera sensor. A variety of illumination techniques are used to get the best contrast image for every measured feature. Edges and through-holes can be backlit. Programmable ring lights (rings and segments of LEDs) allow control of angle and direction of the light to highlight surface details as edges. Video measuring systems employ special edge-detection algorithms to identify and measure edges, including powerful weak-edge algorithms to find and analyze faint or subtle features while ignoring extraneous debris. And video measurement is fast. Hundreds of data points around the circumference of a hole can be acquired in a fraction of a second, for example. The thousands of OGP SmartScopes® in use around the world attest to their appeal.
In spite of this impressive range of capabilities, video cannot measure everything. To measure an edge, the system must be able to image the edge. Although rotary indexers can present features on the part to the optics, sometimes the working distances of the optics or characteristics of the part prevent proper imaging. In a simple example, a counterbore may be deeper than the focus depth of the optics. This is where use of other sensors can help.
Lasers also perform non-contact measurement. There are several laser measurement techniques that all require capturing reflected laser light. Depending on the method, the reflected laser light strikes a detector that determines the image position, intensity, or light distribution. That detected data corresponds to the position of the surface relative to the sensor. As the surface is scanned, either by moving it past the laser, or moving the laser over the part, variations in surface height are detected and presented as a surface profile. This means lasers are good for contouring and shape measurement.
 Lasers have limitations too. For example, they can detect edges, but only as the spot is scanned over the edge. This means that the edge is only known at the point where the beam passed over it. Compare this to the video measurement of an edge where hundreds of points along the entire edge are captured simultaneously. The benefits of combining the two technologies are obvious from this simple example, but there are other aspects to their complementary relationship. A simple example – you can use video to see where you are going to scan the laser.
Touch probes collect data points about a surface from any point they contact. This means that as long as a feature is accessible, it can be probed. Probes are available in a wide variety of tip sizes and stylus lengths. A small probe on a long stylus can reach the bottom of a bore that may be beyond the range of both video and laser.
Touch probes have several limitations that prevent them from doing the entire measurement job. The probe can be physically larger than the feature being measured, perhaps too big to access the desired area. Traditional probing measures a single point at a time. The probe must approach the surface of the part, come in contact, and back away to acquire each data point. As you might imagine, this can be time consuming when collecting data points about a complicated part.
Simple Multi-sensor Application Example [Figure 1]
 Consider the part in Figure 1 with two chamfers (B, C) around a hole (E). They are concentric, and have edges that can be imaged and easily measured with video. This quickly provides the width and concentricity of each ground surface relative to the center of the bore. Now consider measuring the angular relationships of those chamfers (B, C) relative to the plane (A). Edge measurement alone does not provide that information. Touch probes are physically too large to supply the required data point density. And although video can do this measurement with focus points, a laser scan is the best tool for the job. Scan the laser across the plane and into the hole. The small spot size and rapid data acquisition speed provide sufficient data points so the resultant profile can be analyzed with software tools to measure the angles of the chamfers. The advantage of the laser in this case is that it acquires data across the surfaces, not just from edges. Run multiple laser scans in parallel and get a map of the entire area. The laser’s small spot size allows it to acquire numerous data points across each chamfer – far more than would be possible with a contact probe.
The complementary aspect of a touch probe becomes apparent on this simple part. Consider the walls of the bore (D). Since video measures edges, it can determine the edge of the bore near the top surface (where C & D meet) and bottom (where D & E meet). However, video may not be able to resolve possible changes in the shape of the bore along its depth. For example, is the bore perpendicular to the surface plane (A)? A laser cannot scan the walls of the bore (D) since the laser light cannot strike and reflect from its surface. Now consider a touch probe. It can extend into the bore and acquire points throughout its depth. Such a measurement can be very important if that bore is the reference against which all the other measurements are dependent.
This example of a simple manufactured part shows how the combination of sensors in a multi-sensor measurement system can combine to fully characterize important dimensions in a way that would be difficult or impossible for any one sensor-based system alone. Apply these concepts to parts with multiple complex geometries and the need is even more apparent.
How Systems are Different
As explained so far, it is possible to integrate data from a variety of sensors in a single measurement machine. Orchestrating the acquisition of those data points and analyzing them is vitally important to its efficient operation. On the acquisition side, ease of programming is the consideration. An operator should be able to call upon any sensor at any point in a routine. The system software should acquire and retain the data from those sensors for subsequent analysis or output. Well-designed multi-sensor measurement systems make it easy to create routines that automatically change sensors, rotate the part, and track the data so any operator can do the measurements. On the analysis side, well-designed software provides powerful numeric and graphic representations of the part. CAD import and export, form fitting, graphic models that can be manipulated, data transfer to reports or spreadsheets, and more, should all be easily integrated so they happen automatically during the measurement process.
Advances in Technology ContinuedThis article is a top-level overview about video, laser, and touch probe technologies - just three of the sensors that can be used in a multi-sensor measurement system. Extensions and enhancements of all these sensor technologies continue. For example, new metrology zoom lens designs incorporate aspheric optics, making them fully telecentric throughout their zoom range. This offers accuracy advantages along with the versatility of a wide range zoom lens that provides the proper resolution for every measured feature. There are several laser measurement technologies available that are either offset from the optical axis, or work through-the-lens (TTL) of the video system. Each excels at particular applications. In touch probe technology, there is now continuous contact scanning. Similar to a laser scan of a surface, rather than probe a single point at a time, continuous contact acquires surface contours. It can work on surfaces that scatter laser light, or are simply inaccessible to laser or video. Motorized probe heads add another level of flexibility to access critical part features. And new sensor technologies will add to the versatility of multi-sensor measurement systems as they evolve to meet the changing needs of today’s manufacturers.
Video measurement continues to meet the needs of manufacturers worldwide. Additional sensors expand its measurement capabilities to meet the demands of producing increasingly complex parts. Multi-sensor measurement is the multi-purpose tool for the 21st century.
Multi-sensor measurement systems are available in a range of sizes from compact, bench-top models to large format systems.
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