Принципы нанометрологии (1027623), страница 60
Текст из файла (страница 60)
A typical CMM has twenty-onesources of geometric error. Each axis has a linear error, three rotation errorsand two straightness errors (six per axis gives eighteen). The final three errorsTraceability, calibration and performance verification of CMMsare the orthogonality errors between any two pairs of axes. These errors arealso described briefly in section 7.3.4 for scanning probe microscopes.Traditionally these errors were minimized during manufacture of theCMM. However, with the advent of modern computers CMMs can be errormapped (volumetric error compensation) with corrections to geometricerrors made in software [1,6–8].CMM geometric errors are measured in one of the four followingmanners:-using instruments such as straight edges, autocollimators and levels;-using a laser interferometer system and associated optics;-using a calibrated-hole plate [9];-using a tracking laser interferometer [10].9.3 Traceability, calibration and performance verificationof CMMsCalibration and performance verification are two issues that are oftenconfused when talking about CMMs [2].
To clarify, CMM calibration is themeasurement of the twenty-one degrees of freedom of a CMM to enablemechanical correction or error mapping of a CMM. Performance verificationis a series of tests that allows the manufacturer of the CMM to demonstratethat an individual machine meets the manufacturer’s specification. Notethat calibration can be part of the performance verification.The ISO 10360 series of specification standards defines the procedure forperformance verification of CMMs.
The series is broken down into six parts,which are briefly described.Part 1: Vocabulary. Part 1 [3] describes the terminology used to describeCMMs. It is important when describing CMMs to adhere to thisterminology.Part 2: CMMs used for measuring size. Part 2 [11] describes howa CMM should be specified and the necessary steps to show thata machine meets specification.The tests detailed in part 2 involve:-measuring a good-quality sphere at a number of positions andexamining the variation in indicated radius;269270C H A P T ER 9 : Coordinate metrology-measuring a series of lengths in a number of directions in the machinevolume and comparing the machine indication against the known sizeof the artefact.In addition part 2 describes how stable artefacts can be used for interimmonitoring of the CMM.Part 3: CMMs with the axis of a rotary table as the fourth axis. Part 3 [12]describes the extra steps necessary to performance-verify a CMM whichhas a rotary axis as the fourth axis.Part 4: CMMs used in scanning measuring mode.
Part 4 [13] containsthe tests necessary to demonstrate that the scanning capability of a CMMmeets specification.Part 5: CMMs using multiple-stylus probing systems. Part 5 [14] extendsthe probe test covered in part 2 to cover multiple-stylus probing systems.Part 6: Estimation of errors in computing Gaussian associated features.Part 6 [5] is concerned with assessing the correctness of the parameters ofcomputed associated features as measured by a CMM or other coordinatemeasuring system.9.3.1 Traceability of CMMsTraceability of CMMs is difficult to demonstrate. One of the problems isassociating a measurement uncertainty with a result straight off the CMM.The formulation of a classical uncertainty budget is impracticable for themajority of the measurement tasks for CMMs due to the complexity of themeasuring process.It used to be the case that the only way to demonstrate traceability was tocarry out ISO 10360-type tests on the machine.
However, if a CMM isperformance-verified this does not automatically mean that measurementscarried out with this CMM are calibrated and/or traceable. A performanceverification only demonstrates that the machine meets its specification formeasuring simple lengths, i.e. it is not task-specific.This task-specific nature of a CMM can be illustrated with a simpleexample. Suppose a CMM measures a circle in order to determine itsdiameter. To do this the CMM measures points on that circle. The pointscan be measured equally spaced along the circumference, but may have to befrom a small section only, for example because there is no material presentat the rest of the circle.
This is illustrated in Figure 9.3, which shows theeffect on the diameter and the centre location if measurements with thesame uncertainty are taken in a different manner. This means that even ifTraceability, calibration and performance verification of CMMsFIGURE 9.3 Illustration of the effect of different measurement strategies on thediameter and location of a circle. The measurement points are indicated in red; thecalculated circles from the three sets are in black and the centres are indicated in blue.the uncertainty for a single coordinate is known, this does not simplycorrespond to an uncertainty of a feature that is calculated from multiplepoints.A better method is described in ISO/TS 15530 part 3 [15]. This specification standard makes use of calibrated artefacts to essentially use the CMMas a comparator.
The uncertainty evaluation is based on a sequence ofmeasurements on a calibrated object or objects, performed in the same wayand under the same conditions as the actual measurements. The differencesbetween the results obtained from the measurement of the objects and theknown calibration values of these calibrated objects are used to estimate theuncertainty of the measurements.
However, this method requires independently calibrated artefacts for all its measurements, which is quite contradictory to the universal nature of a CMM.Alternative methods that are consistent with the GUM (see section 2.8.3)can be used to determine the task specific uncertainty of coordinatemeasurements. One such method that evaluates the uncertainty by numericalsimulation of the measuring process is described in ISO/TS 15530 part 4 [16].To allow CMM users to easily create uncertainty statements, CMMsuppliers and other third-party companies have developed uncertaintyevaluating software, also known as virtual CMMs [17].
Even by adoptingISO 15530 part 4, there are many different approaches to the implementation of a virtual CMM [18–20].271272C H A P T ER 9 : Coordinate metrology9.4 Miniature CMMsThe advent and adoption of the CMM greatly reduced the complexity, downtime and operator skill required for measurements in a production environment. It is difficult to imagine a modern successful automobilemanufacturing plant that does not employ CMMs. The ‘CMM revolution’has yet to come to the MNT manufacturing area. Once again many instruments are employed to measure the dimensions of MNT parts but there arenow additional problems despite their tiny size: many of the parts that needmeasuring are very complex, high-aspect-ratio structures that may be constructed from materials that are difficult to contact with a mechanical probe(for example, polymers or bio-materials).
Also, there is often a need tomeasure the surface topography of steep walls found in, for example, deepreactive ion etched (DRIE) structures used for MEMS. The only instrumentsthat are available are those which essentially ‘measure from above’ and weretraditionally used to measure surface topography. These instrumentsgenerally lack traceability for surface topography measurements (see section6.10). Therefore, it is difficult to measure with any degree of confidence thecomplex structures that are routinely encountered in MNT products.In recent years many groups have developed ‘small CMMs’, typically withranges of tens of millimetres and tens of nanometres accuracy in the x, y and zdirections.
These miniature CMMs come in two forms: those that aredeveloped as stand-alone CMMs and those that are retrofitted to macro-scaleCMMs. One of the first miniature CMMs of the latter form was the compacthigh-accuracy CMM developed at NPL [21]. This CMM used the movementscales of a conventional CMM with a retrofitted high-accuracy probe with sixdegrees of freedom metrology. This CMM had a working volume of 50 mm by50 mm by 50 mm with a volumetric accuracy of 50 nm. Retrofitted CMMs willnot be discussed in detail as they are simply a combination of conventionalCMMs (see section 9.1) and micro-CMM probes (see section 9.5).One technical challenge with probing MNT structures arises due to theinability to actually navigate around the object being measured without somelikelihood of a collision between the probe and the part being measured.Typical miniature probing systems are less robust than on larger CMMs thatincorporate collision protection.
Future research must concentrate on thesedifficult technical issues associated with micro-CMMs if they are to becomeas widely used as conventional CMMs. However, the technical barriersassociated with mechanical contact of probes at the micro-scale may forceresearchers to look into completely novel approaches such as SEM-basedphotogrammetry or x-ray computed tomography [22].Miniature CMMs9.4.1 Stand-alone miniature CMMsOnly two examples of stand-alone miniature CMMs are given here becausethey are the only two that are both currently commercially available and forwhich quite extensive information is available in the open literature. Twofurther examples are the Mitutoyo Nanocord and the IBS ISARA (based onwork by [23] but with roller as opposed to air bearing slideways). There aremany instruments that are at the research stage (see for example [24]) andsome that were developed but are not currently commercially available (seefor example [23,25,26]).9.4.1.1 A linescale-based miniature CMMThe F25 is a miniature CMM based on a design by the Eindhoven Universityof Technology (TUE) [27] and is available commercially from Carl Zeiss.
TheF25 has a unique kinematic design that helps to eliminate some of thegeometric errors inherent in conventional CMMs. The basic kinematiclayout is shown schematically in Figure 9.4. The red arms are stationary andfirmly attached to the machine. The blue arms form the x and y measurement axes and are free to move.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
