Making Better Medical Devices with Multisensor Met
By Nate J. Rose , Chief Applications Engineer, Optical Gaging Products (OGP)
Multisensor metrology is becoming a preferred quality control technology for manufacturers to develop, maintain, and improve the quality of medical devices.
Articles about multisensor measurement machines often explain the types of sensors available and what they do. In all cases,measuring sensors acquire data pointsfroma part under test. Those data points are used inthe system oftware to determine geographic relationships that are compared to the part design drawing. Some sensorsare more efficient than others at acquiring data points fromcertain areason,across,orwithina part. This article describessens or sinthe context of medical devices,butit goes a step further bydescribing what canbe done with the clouds of data points acquired by any or all sensorsona multisens or measurement machine. Sincethe goal of any measurement is comparisonto the design drawings, a 3D fit ofallthe data points compared to the CAD designdata canactually make decision making based onlarge setsofdata points easier than interpreting printoutsof individual measured values. Although it may seem counterintuitive,using multiple sensorstoget lotsofdata can actually make decisions about parts and processes easier.
Specialized parts demand specialized measurement Medical devices and their component parts are usuall yextremely specialized in form and function. For example, they may be very small, like the parts that form middle ear prostheses for ENT applications. Regardless of size,medical device parts are also almost always abricated to extremely tight tolerances. Measurement systems that characterize these medical device parts must be capable of high precision, often to the submicron level.
Orthopedic implants, such as prosthetic hip ball joints, or tibialand femoralknee and ankle implants, are a challenge to measure accurately. The surfaces that define their shape are higher-order curves made up of NonUniform Rational BSplines( NURBS)or point coefficients. The forms they embody are often variable and organically curved, because the parts must fit with mating prosthetic parts and even mate with parts inside the human body. Thei rcomplex 3D curves make it difficult to measure all surfaces froma single direction, making it simply impossible for certain types of sensorsto measure them.
Video based measurement systems are wellsuited to measure prismatic parts, which bydefinition contain intersecting planes. Where planes intersect, there are edges, and edges are easy to measure with video. Orthopedic implants,however,are often made up of regular continuouscurves(hip components) or complexcontouring surfaces (knee components) — shapes that mimic the organic contoursofnatural body parts. These types of surfaces have few orno planes or intersecting edges. Since video sensors excelat measuring edges, video measurement of these parts would be limited to widths ofouter edges illuminated frombehind. Video can measure surface points,but using multiple focuspointsto gather enoughdata to support evena linear sectionofa contouredsurface would be laborious and impractical. Atouchtrigger probe would have a similar limitationbecause eachsingle point requires approach, trigger, and back off — possible but impracticalin a manufacturing environment.
A case in point - measuring a replacement knee A good way to verify that the organically curved profile of a replacement knee matches its designis with a laser. Laser sensors in multisensor systems work by projecting light towards a surface, collecting the reflected and/or scattered light ona dedicated sensor, and automatically calculating the distance ofa measured point between the laser and the part in3D space. Laser measurement can be accomplished for a single point, or alternatelya series o fdata points can be gathered and calculated as the part is translated beneath the laser or the laser ismoved over the part. The point spacing and sampling ratecanbe userspecified. Metrology software continuously calculates the distance between the laser and the part surface as the laser beam moves acrossthe part, keeping the laser sensor within its capture range through closed loop positioning of the multisensor system’sZaxis stage control. By keeping the laser sensor within its capture range, precise point positions can be collected quickly. Laser focus is faster and more accurate than video autofocus, and since it is noncontact, it avoids potential damage to the part surface and contamination of sterile parts.
In most instances, the user maynot be able to fixture the knee replacement to as sure a direct line of sight between allitscriticalsurfacesand the laser sensor. Inthose cases,mounting the replacementin a rotaryindexer may provide a solution, as wellas anopportunityto speed measurement throughput by eliminating multiple positional fixtures while cutting downonmanualpart handling. Typicallya datum isestablished fromsurfacesonthe back ofthe knee witha touchprobe. Then,the rotaryindexer rotates the knee replacementtopresentitsdesired surface to the laser sensorformeasurement. Since the opposite side ofthe measured surfaces defines the datum, it is imperative that the metrology system be equipped with fully 3D capable metrology software that rotatesthe coordinate system whenthe indexer rotates. Inthis way, every data point captured bythe laser canbe tracked in3D space bythe metrology software, regardless ofthe rotaryindexer position.
Adifferent method of measuring the complexcontoursofa prosthetic knee implant is witha multisensor systemequipped witha Renishaw . SP25 scanning touchprobe. Like the laser, the user specifies start and end points for the scanonthe knee replacement, but inthis case, the probe tip maintains constant contact withthe implant surface as the systemmoves it along the part surface acquiring data pointsasitgoes. Unlike touchtrigger probesthatmustapproachthe surface,trigger,then back off,the SP25 scanning probe maintainsconstantcontact. Aswitha laser,the datapointdensity and scanrateare userdefinable. The multisensorsystemmustbe configured forthe SP25,and also must be equipped with3Dcapable metrologysoftware to trackthe data points inXYZ space.
There are other ways to measure a knee replacement that is fixtured ina rotaryindexer. As mentioned above, a linear laser or contact probe scancan be performed across the rotaryfixtured knee’s topsurface. Since sucha line scanrepresents a sectionacross the 3D part, that section may be measurable as anedge using a video sensor. Byrotating the knee 90°, that “section” becomes a distinct edge whenthe part is illuminated frombehind. This technique requires a goodmetrologylens system that has a long working distance and limited influence fromthe knee’s steep surfaces. Since the “section” is larger thananopticalfield ofview, functions suchas “Edge Trace” are goodfor this application where the system automatically follows the edgeover multiple fields, acquiring points at each position.
When mounted ina rotaryindexer, the knee’s entire surface canalso be measured byrotating it incrementally, a few degrees at atime, and performing multiple linear scans (or edgetraces). Withtight point sampling density, the collectionofallthe points fromthese multiple scans will yield point clouds ofdata. These point clouds can be imported into 3Dcapable fitting software, which, byknowing the center of rotation, canshow how allthe part data coincide withthe CAD modelof the part. Some fitting software even has the capacityto performa GD&T analysis ofthe point cloud data satisfying simultaneous requirements, and showing graphically anydeviations fromthe design file. Not onlycanthis informationbe used for acceptance testing ofeach part, the manufacturing engineer canuse the informationto make changes to manufacturing processes to enhance accuracyand/or efficiency for subsequent parts.
GD&T analysis ofpoint cloud datacould potentiallysupport a criticalconclusionthat would not be immediatelyobvious otherwise. For example, point cloud data mayshow that two perpendicular linear laser scans are within spec, but GD&T analysis ofthe bothofthose scans takenas a whole, mayshow the entire part to be out oftolerance.
A custom video solution At the other end ofthe spectrumof medical devices, seemingly simple plastic syringe bodies require numerous dimensional measurements. Sometimes a custom solution is the best wayto measure batches of medical deviceparts like this. Asa minimum,the lengthand outside diameter of each of the tubular syringes is measured. This could easily be accomplished by fixturing a syringe horizontally on the stage ofa multisensor system, backlighting it, and using a video sensor to measure the edges that define its outer diameter and length to determine the distance between those edges. (SeeFig.1) The syringe bodies could be measured one at atime as shown here, but could be more efficiently measured if mounted ina multipart fixture, better supporting production volumes and lowering the cost per syringe.
The above solutionworks for the outer cylinder, but for these parts, diameters are important too. Video measurement ofthese perpendicular diameters cannot be gathered while the syringe bodyis in a horizontalposition. The user could remount the syringes vertically in a separate fixture, but that would add parts, labor costs, and time to the process. It would also be possible to mount eachsyringe, one at a time,ona rotaryindexer. That way,the lengthand outside diameter canbe measured withthe syringe bodyinone position, thenrotated 90° so its important diameters canbe imaged and measured. (See Fig. 2) Thiswould automate the measurementprocess,butagainitisnot efficient. Eachsyringe bodymust be loaded and unloaded, and measured one at a time, and indexer rotationtakestime, too.
A speciallydesigned multipart fixture solves this measurement problem. Withsyringes mounted horizontallyrelative to the video optics (inthe system’s XY plane), a 45°mirror could allow direct imaging and measurement ofthe diameters withthe part ina fixed position. Byfixturing a number of syringe bodies side byside, the video optics canperformallthe measurements withquick XY position movesand autofocus. Apart routine created forone syringe caneasilybe copied and reused forall others to speed throughput, and since video measurement is noncontact, potentialpart deformationof the soft plastic syringe bodies will not be a problem.
This customvideo measurement technique willonlywork if the video sensor optics have a long enoughworking distance to focusonthe part after reflecting the imaging path. Asanoptical technique, this type of fixture canwork witha TTL laser measurement sensor. Like the optics, the TTL laser would require a long working distance, as wellas minimalbeam triangulation. Some throughthelens laser systems, particularly the TeleStar . TTL lasers fromOpticalGaging Products (OGP . , Rochester, NY), fulfillthose requirements. An application study, where the measuring system manufacturer or representative measures the dimensions of interest on the manufacturer’s actual parts, can be very helpful. The study helps the measuring equipment company specify the appropriate system and sensors that they know meet the requirements on the part drawing. The application study provides pre-investment assurances to the manufacturer that the most appropriate multisensor system has been configured for their specific requirements.
The name of the game Medicaldevice manufacturers are required to have documented, controlled manufacturing processes that include the inspectionequipmentused forqualitycontroland monitoring. Multisensor measurement systems are capable of verifying manyofthe important dimensions of medicaldevices quickly, accurately, and with minimalpart handling —this article has presented onlytwo examples of the myriad of medical devicescurrentlymeasured by multisensor systems. Verifying thatmanufactured parts meet designspecifications is the name ofthe game. The finaloutcome affects the healthof medical device manufacturers’ balance sheets — and ultimately, the healthofthe patient.
In the case of internal features such as bores into a part, the probe length must be able to move across the part to the bore and then extend into the bore. For example, to measure the top of a 6-inch part requires a measurement system with more than 6 inches of travel range.
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