ABCs of Multisensor Measurement

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More and more companies are offeringmultisensor measurement systems, yetmost of the available information

about such systems is with regard to sensor tech-nologies. Although it is helpful to know how dif-ferent sensors work, it is more important isknow how to make use of more than one sensorto improve manufacturing productivity.

What is it?There may be some confusion about what amultisensor measurement system actually is. Allmeasurement systems use at least one type ofsensor technology. The sensor collects data

points from a part to obtain a measurement. It isthe probe, or detector, that senses the part.

Sensor technologies can be grouped in twomajor categoriescontact and noncontact.Contact sensors are those that apply any pressureto a part to obtain a measurement. Coordinatemeasuring machine (CMM) touch probes arethe most common type of contact sensors.

Noncontact sensors do not require physicalcontact with the part. There is a broad range ofnoncontact sensor technologies that use cam-eras for optical imaging. Laser scanners useoptics and detectors to capture laser light

reflected from a surface. New microprobingtechnologies use resonance technology or spec-tral analysis of light related to the distancebetween the probe and the surface. Any combi-nation of contact and noncontact sensors on asingle measurement machine makes it a multi-sensor measurement machine.

The three sensors commonly referred toare touch probe, video and laser. Touch probetypically refers to a touch-trigger probe such asthose commonly found on a CMM. Videorefers to video measurement, as with dedicatedmeasurement machines with lighting, optics

and software for measurement of video imagesof a part. Laser means a light source to illumi-

Knowing how to

make use of

multiple sensors

can improve

productivity.

BY WILLIAM VERWYS

ABCs of Multisensor

Measurement

QUALITY MEASUREMENT

All measurement systems use at least one type of sensor technology.

The sensor collects data points from a part to obtain a measurement.

Data points can be collected by numerous sensor technologies

such as a touch probe, video, laser and microprobes.

TECH TIPS

A video measurement of a cast metal part is

illuminated by an LED ringlight.

Photo: Optical Gaging Products

A star touch probe measures a cast metal part on

a multisensor measurement machine.



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nate a specific part of the surfaceand the associated detector to col-lect the reflected or scattered light.But other sensor technologies arein use, especially those in the realmof microsensors.

Sensors in useAn important factor in sensor selec-tion is in how it is to be used. At thesimplest level, sensors are used inone of two wayssingle point at atime, touching the surface of anobject to gather data, or continuousscanning of the surface to collect amultitude of data points. In touch-trigger probing, the probe is movedto individual locations on a part, point-to-point. The sequence is to move to alocation, approach the surface, make a

single contact and collect that datapoint, back away, move to another loca-tion and repeat the process.

It is possible to use a scanning probeon some multisensor metrology sys-tems. To scan, the probe makes contactwith the surface and maintains contactas it moves across the part. While scan-ning the surface,data points are contin-uously collected. Scanning probesacquire more data points more quickly

than touch-trigger probing, but theyhave the added complexity of stage con-trol requirements to maintain surface

contact across surface deviations as theprobe or part moves. In addition, sys-tem software must retain the spatialrelationships of all the data points.

New versions of most sensors arecapable of scanning. Laser scannersmove a laser point or line across a part,continually collecting data points.

By scanning a part while collectingfocus points in the optical field of view,a video measuring machine can devel-

op a three-dimensional contourmap of the surface. Edge scanning,or tracing, uses a video systemsedge-detection capabilities andclosed-loop stage control to scan theperimeter of a part, even if theperimeter extends outside the field

of view. A video measuring machinecan continually acquire focus pointsas the part or optics are moved.Zoom optics allow scans to vary inr es ol ut io n, t o a cc om mo da techanges in surface texture and fea-ture sizes of a part.

Sensor dataIn multisensor measurement sys-

tems, data points can be acquired bynumerous sensor technologies. Forexample, a touch probe can access single

points at the base and sides of a counter-bore.Video can trace the outer perimeterof the entire part. A laser can scan anundulating surface. Microprobes cancollect data points in a critical orifice toosmall for traditional probes. Analysis ofall the points from all these sensors isbased on where those points exist in thesystems measurement volume.

Acquired data points are used tod ete rmi ne di me ns io ns o r a ng le s

Multisensor metrology system. A meas-

urement system with more than one sen-

sor technology that allows those sensors

to be used in a single measurement rou-

tine with a single part setup.

Multisensor. Two or more different sen-

sor technologies.

Scanning. Moving the part or the sensor to

acquire data points on a continuous basis.

Sensor. The technology that detects data

about a part. Commonly categorized as

contact or noncontact, sensors use a

number of technologies, each with

strengths for particular applications.

Glossary of terms

A resonant microprobe prepares to be used to follow

the contour of a deformable object.Photo: Optical Gaging Products

A spectral-analysis microprobe scans a small plastic part.



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between the points. It makes no difference whether thosepoints were acquired by single-point probing or scanningadata point is a data point. This means that a multisensormetrology system can use a sensor best suited for specific partattributes. It can produce measurements of complex dimen-sional forms and surface relationships simply not possiblewith single sensor systems.

Why use multisensor systems?Once a measurement routine is created, computer numericalcontrol measurement machines follow that routine automat-ically. So, no matter how many sensors are used, how manymeasurement steps are involved or how many times sensorsare changed in a routine, that process is followed every timethat measurement routine runs. The automation of multi-sensor measurements leads to a fundamental benefit of mul-tisensor metrology systemsthey can save time.

The value of a multisensor metrology system depends onthe complexity and mix of products to be measured.Complex products can benefit from the ability to use multi-

ple sensors to more fully characterize parts than is possible onany traditional single-sensor measurement system. On theother hand, a mix of different parts, with varying feature sizesand dimensional tolerances, can benefit from the availabilityof sensors ready for every situation.

Choosing a systemWhen deciding on a multisensor measuring system, start byunderstanding which sensors are available on each systemand what they can do. Then, look at how tightly the differentsensors are integrated in each systems measurement soft-ware. Many systems are actually single-sensor systems withother sensors added. Such systems are optimized for the pri-mary sensor, but not for the others. True multisensor systemshave software functions that optimize the performance of allthe sensors.

Another consideration in system selection is how wellthe system supports sensor changes during a measurementroutine. Most people are familiar with a CMM using achange rack for switching probes during a measurementroutine. The CMM drops off one probe and picks up anoth-er under program control. Similar mechanisms make it easyto switch between sensors on a multisensor metrology sys-tem without operator involvement. Some designs retractsensors into the body of the system, automatically deploying

them only when called on by the measurement routine.Such designs speed access to other sensors, while reducingthe potential for sensor damage because unused sensors areout of the way.

Total solution?Many offices have a multifunction machine that prints, scansand faxes. They replace separate printers, scanners and faxmachines that specialize in each function. The separatemachines do the job, but each takes up space, has utilityrequirements, needs service and training and makes someonehandle the paper several times to get all the jobs done.

Multisensor measurement machines are similar to multi-

function office machines. One measurement system takes upless space, uses fewer utilities, requires one training and serv-

ice obligation, and minimizes part handling and fixturing. Inthis era of continual productivity improvements and costreductions, a multisensor metrology system can be an impor-tant way to meet those objectives. Q

QUALITY MEASUREMENT

William Verwys is applications engineering manager for Optical

Gaging Products Inc. (Rochester, NY). He can be reached at (585)

544-0400, ext. 321.

Through-the-lens (TTL) laser measurement of a surface.


Contact Noncontact

Touch Probes Video

Laser

Resonant (Microprobe)

Spectral Analysis (Microprobe)

Categories of Sensors

Sensor Single Point Strength Scanning Strength

Touch Probe Surface points ContoursVideo Edges PerimetersLaser Surface points Contours

Resonant Surface points ContoursSpectral Surface points Contours

Sensor Applications

Reprinted from Quality, Copyright December 2004.

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ABCs of Multisensor Measurement