����������������

Filters, Outliers and the Scanning CMM

�������

Introduction

This document will describe the use of Filters,Outliers and Evaluation Methods for the Scan-ning CMM. Although Calypso has been used forall instructions, plots and output, the techniquesdescribed here apply to UMESS as well. Also, itmust be understood, the following techniquesapply to the Scanning CMM. Actually not only isit “advisable” to use scanned features butactually impossible to apply filters to featureswith low data density.

In addition to data density, other areas ofdiscussion will be:

• Application of Filters and outlierswithin the Calypso measurementplan.

• Why and when to apply filtersand outliers.

• What’s the Standard?

• Error messages.

• Evaluation Methods.

• Centers for Out-of-Roundness.

• Instrument Response. Cutoff,UPR and Lambda c.

• Filter types, Gauss and 2-RC.

• Low and High pass filters.

• Connect Segments.

• Elimination of Outliers.

• Range of Data Reduction.

The information in this document is from avariety of resources including the Calypsomanual and on-line help.

Defining Filters and Outlier Elimination

There are several different points in Calypso at whichyou can parameterize and activate filtration and outlierelimination:– For an individual characteristic– For an individual feature– As defaults for the characteristic groups and thereferences and alignment elements of the coordinatesystems. The setting for the characteristic always takespriority. Iffiltration/outlier elimination is not activated forthe characteristic, the setting for the feature applies. Iffiltration/outlier elimination is not activated for the feature,Calypso refers to the default settings for the characteris-tic groups.

A feature obtained by recall from a feature filteredbeforehand is automatically not filtered subsequently,even if filtration is activated.

Default Settings

To set the Default Filter and Outlier settings, select Re-sources > Filter/Outlier Elimination.

Settings at this level are not applied to existing characteris-tics, only those subsequently created. Features containingfilter and/or outliers settings will override these defaults.

Feature Settings

The next level at which Filters and Outliers can beset is within the Feature. Open the feature windowand select Evaluation.

These settings will be automatically applied foreach characteristic in which the feature is used.This overrides the default settings.

Characteristic Settings

From within the characteristic select thefeature button.

From this widow you can override settingsfrom either the feature or default. Also it ispossible to unselect settings applied by thefeature or default.

Why Use Filters

From the Calypso on-line help:

Filters can segregate the waviness profile of ageometric feature from the effects of surfaceroughness. Filters, therefore, are of assistance inmaximizing measuring accuracy. The filter methodssupported are Gaussian (ISO 11562) and 2 RC(ISO 4291), while the filter types are lowpass andhighpass. The new statistical data of thegeometric feature are obtained from thefiltered measuring data after compensa-tion.

What’s the Standard?

Often filters are specified on the print orinspection plan. In some cases, roundnessfor instance, a filter might be implied by thestandard:

From ANSI B89.3.1 – 1972 Measurement ofOut-Of-Roundness:

section 3.2

...If complete measurement conditionshave not been specified the:

Method of Assessment—Minimal RadialSeparation

Instrument Response—50 Cycles perRevolution

In other words:

Implies:

That is: this surface shall be round within .02mm asassessed by the MZC method with 50 cycles per revolu-tion response.

If the print is specific regarding the filter it might be shownas thus:

That is: this surface shall be round within .02mm asassessed by the LSC (Gauss) method with 150 cyclesper revolution response. Standards are available on the internet at: global.ihs.com

���

��� ��� ��

��� ��� ���

Using Filters in Calypso:

The above feature control frame requires the evaluation ofRoundness (also called Circularity or Out-Of-Roundness)

Data Density:

It is important to understand that the use Filters and Outliersare really best applied to scanned features.

From the Calypso on-line help:

It is advisable to restrict the use of filters to instances inwhich the number of points is high, in other words theyshould be used only for scanned features. All featuresmeasured using scanning methods can be filtered.

Calypso also requires a minimum of 5 (soon to be changed to7) points per Lambda c (cutoff) or Undulations Per Revolution(UPR) frequency.

Use of insufficient data density will result in the filter not beingapplied and a message printed in the default printout.

If for instance you choose to evaluate roundness with 100data points and a UPR of 50, no filter will be applied, and amessage will be reported in the default printout:

Incorrect filter value (Lambda_c or UPR)

Increasing the data to a range of 101 to 250 points and onceagain no filter will be applied. The default printout message:

Filtering with less than 5 points per undulation.

Increasing the number of points to 251 or greater and a filterwill be applied. A message reporting the filter type and cutoffwill be reported in the default printout:

Low-pass (Form):Gauss Undulations Per Revolution:50

Other Messages

• The Compact Protocol shows when a filter has beenapplied, but no error messages.

• If a filter is selected but not applied, the plot windowshows “none”, otherwise it reports the filter beingused.

• The Custom Printout never shows filter messages.

��� ��� ��

Setting Filters in Calypso

��� � ��

Select the Roundness Characteristic andscan the required feature. Check the box toapply the filter.

Notice the evaluation method is shown nextto the check box.

In this case the default setting matches theStandard. That is, Minimum Zone Centerwith a Gauss filter at 50 UPR.

Of course, all of these settings are userdefinable. They can be specified by theoperator either here, in the characteristic,within the feature or as a Calypso defaultfilter setting.

The above feature control frame specifiesthe method to be used for the evaluation isa Minimum Zone Center (MZC). Let’s lookat the other possibilities.

Evaluation Methods

Click on the Evaluation Method Parametersbutton to open the Evaluation Methodswindow.

This displays several selections:

• LSQ Element (Standard)… aka, Gauss.• Minimum-Element… aka, MZC• Minimum Circumscribed Element• Maximum Inscribed Element• Inner Tangential Element• Outer Tangential Element

It is of interest here to consider the use of the word “Ele-ment” in this dialog window. For example, one might thinkthat the selection Maximum Inscribed Element might betterbe called Maximum Inscribed Circle. However, Calypso canapply filters and outliers to a variety of features beyond thecircle we are considering here.

From the Calypso on-line help:

Individual measuring points of the geometric features 2dstraight, plane, circle, cone, cylinder and sphere can bepurged from the measured values as Outliers.

Centers for Out-of-Roundness Measurement(4 possibilities)

The centers of the measured polar profile which may be usedto determine the Out-of-Roundness value when specified arethose related to one of the following alternative methods ofOut-of-Roundness assessment:

2.8.1 Minimum Radial Separation (, MRS, Minimum ZoneCenter, MZC, Tschebyscheff)

This center is that for which the radial difference betweentwo concentric circles which just contain the measuredpolar profile is minimum.This is also known as the center for minimal TotalIndicator Reading (TIR). The British Standards Institutionpublication 3730:1964 refers to it as Minimum ZoneCenter (MZC).

2.8.2 Least Squares Center (LSC, Gauss)

This Center is that of from which the sum of the squaresof the radial ordinates of the measured polar profile has aminimum value.

2.8.3 Maximim Inscribed Circle (MIC)

This center is that of the largest circle which can beinscribed within the measured polar profile.This is also known as the plug gage center and isgenerally used for internal diameters.

2.8.4 Minimum Circumscribed Circle (MCC)

This center is that of the smallest circle which will justcontain the measured profile.This is also known as the ring gage center and is gener-ally used for external diameters.

2.9 Preferred Center

The center from which the out- of-roundness value shallbe determined unless specified otherwise is the MinimalRadial Separation Center.

In Calypso the MZC or Tschebyscheff is correctly used bydefault for Roundness evaluation. Other centers are userselectable.

In Calypso versions 3.0 and higher, within the characteristicsfeature selection window, there is a button for EvaluationParameters.

Minimum Radial Separation (, MRS,Minimum Zone Center, MZC, Tschebyscheff)

Least Squares Center (LSC, Gauss)

Maximim Inscribed Circle (MIC)

Minimum Circumscribed Circle (MCC)

Instrument Response (Cutoff)

Cycles Per Revolution (Undulations PerRevolution, UPR)

If all of the radial deviations of a circular crosssection were fully and completely represented bya measured profile, the presence of high fre-quency surface irregularities could mask anylobing conditions or the form of the profile.

In the ANSI standard the term used is CyclesPer Revolution Response, in Calypso it is calledUndulation Per Revolution.

A UPR filter of 50 means that the measuredprofile has been attenuated by a filter which hasreduced by a percentage (Gauss 50%, 2RC75%) the amplitude of the sinusoidal lobingwhich occurred at a regular interval of 50 lobesper revolution. Note that the 50 UPR frequency isbase on angular displacement rather than time.

It might be expressed that the cut-off of 50UPR represents 50segments of the circle.

Reducing the number of UPR will tend to smooth out smallscale irregularities while higher frequency are more inclusiveof total surface texture.

At least 5 measuring points are required per undulation.

Wavelength Lc (Lambda cutoff)

Flatness and straightness are evaluated with a cut-off de-scribe by a linier dimension. The default for Calypso is2.5mm.

A Wavelength Lc of 2.5 means that the measured profile hasbeen attenuated by a filter which has reduced by a percent-age (Gauss 50%, 2RC 75%) the amplitude of the sinusoidalprofile which occurred at a regular interval of 2.5mm.

Profiles at three different UPR settings

Filter Methods:

Gauss

The Gaussian filter has been adapted as the industrystandard for the measurement of roundness andsurface geometry. It is a digital, phase corrected filter.Its quick frequency response allows the accurateassessment of the surface profile.

The Gaussian filter is defined to have 50% transmis-sion at the cutoff wavelength. In other words; a mea-sured sine with 50 UPR and an amplitude of 1.0 hasafter filtering with gauss filter (50 UPR) an amplitude of0.5.

2-RC-Filter (2-CR)

The 2RC (or 2CR) filter is an older standard filter usedin surface roughness measurements. It Is typicallyimplemented as an analog electrical filter, 2 RC filtersin series, separated by a buffer. It Is not phase-correct,and it has a frequency transmission of 75% at thecutoff wavelength. The 75% level was apparentlychosen because the 2RC filter has a long “tail” in itsfrequency response, and, therefore, much of the longwavelength components above the 50% cutoff remainsin the roughness. The cutoff at 75% more accuratelyretains the intuitive sense of being the wavelengthboundary between roughness and waviness.

Some refer to the above filter as a 2CR filter becausethe capacitor comes before the resistor in each pair.However, electrical engineers refer to either order asan RC filter and distinguish between the two by callingone a high pass filter and one a low pass filter. “Pass”refers to what frequencies make it through the filter.

Both the Gauss and 2RC filters separate roughnessfrom the form error.

Gauss

2RC

Comparrison of Gauss vs. 2RC frequency responce

Surface Profile

Waviness Form

Lowpass, Highpass Filter Types

You may select a Lowpass or a Highpass filter. TheCalypso default is Lowpass.

A Lowpass filter type reduces the short-wave surfacephenomena (High frequency) and does not disrupt thelong-wave (Low frequency) effects of waviness andgeometric deviation.

Caution Should be Used!!

Caution should be used when changing from the defaultLowpass to Highpass filter. Selecting the Highpass filterwill remove all waviness (what you’re trying to evaluatewith roundness, flatness, straightness, etc.) leaving onlythe high frequency surface roughness. Rather than usinga CMM, surface texture might be better evaluated with adedicated surface finish gage such as those in our TSKproduct line.

That said, an experienced metrologist might glean insightinto a process with the cautious use of the Highpass filter.

In the instance where the measured feature has extremelypoor geometry the low frequency deviation will prevent theuse of a adequate magnification factor for plotting. Re-moving the low frequency form deviation will allow a muchhigher magnification, possibly revealing, machining, toolpath or other process problems.

Example of a Out-of-Round feature with a form error of4.8mm. A Lowpass plot must be reduced to a magnificationof two or it’s off the chart. This obscures all high frequencyform deviations.

By selecting a Highpass the magnification can be increasedto 400 revealing tool marks and chatter.

However, as this custom printout shows, care must betaken. Two evaluations of the same feature produce twoextremely different results!!

Connect Segments

From time to time it may be necessary to scan afeature with interrupted paths. Possibly keyways insidea bore. Because of the limitations of 5 data points percutoff (UPR or Wavelength Lc) filtering might not work.

If you activate this checkbox the segments of aninterrupted contour are connected to form a continuouscontour during filtration with as many points as possiblebeing taken into account. This means that you canselect a stronger filter.

Eliminating Outliers

A measured point is tagged as an outlier if it is furtherthan a defined threshold (threshold = factor * standarddeviation) from the computed Gaussian element. Outliersare geometric points that differ significantly from thegeometric form yielded by the other measured points andas such, they can produce a large error when the com-pensatory element is calculated. An error of this natureeasily propagates through the actual-value determinationof the corresponding characteristic.

Why is Outlier Elimination Necessary?

If, for instance, you are checking roundness on adedicated form tester, you are probably in a inspection labdoing the work. If the characteristic is out of tolerance theinspector might clean and recheck the feature.

Often, with CMM’s, there may not be this opportunity. Withoutlier elimination, and user definable parameters, thefeature data can be “cleaned” by the software.

Additionally, you do not want to make a process changebased on a piece of dirt or imperfection in part material.

Selecting Outlier Elimination In Calypso

As with filters there are several different points in Calypso atwhich you can parameterize and activate outlier elimination:

– For an individual characteristic.– For an individual feature.– As defaults for the characteristic groups.

At each of these points the Outlier Mode dialog window canbe accessed.

Factor For Outlier

Sets the outlier factor (factor * standard deviation) for outlierelimination. This factor can be set, individually, for inside andoutside the material.

Range of Data Reduction

Only Outlier – Removes only the outlier then recalculatesthe feature. Additional outliers may be eliminated in subse-quent iterations.

Include Adjacent Points – Operator can determine andnumber of points on each side of the outlier to also beeliminated.

To Computed Feature – Eliminates the outlier and all pointto the computed (Gaussian) feature.

Outlier Elimination is reported on the Default Printout,Custom Protocol (see next page) and graphically repre-sented on plots by the absence of deviation lines (below).

============================================================================================== C A R L Z E I S S / C A L Y P S O 3.2.-beta30 COMPACT PROTOCOL -1-———————————————————————————————————————————————Measurement Plan Operator Date Part NumberFilters and Outliers Master July 21, 2001 32CONTURA———————————————————————————————————————————————Names Description Actual Nominal Utol Ltol Deviat. Histogr.==============================================================================================

Plane4(Only Outlier) Minimum Zone Plane #P (222)S = 0.0020 Min = (3) -0.0030 Max = (57) 0.0030 Form = 0.0061 Z 0.0373 0.1000 X -83.5389 -82.5000 Y 4.8318 5.0000 A1- X/Z -0.0171 0.0000 A2- Y/Z -0.1664 -0.1000Outlier Elimination Inside Workpiece : 2 Outside Workpiece : 2No Filter Only Outlier GDT Flat 0.0100 0.0061 |—

______________________________________________________________________________________________Plane4(Outlier & Adjacent Points 4) Minimum Zone Plane #P (217)S = 0.0018 Min = (3) -0.0029 Max = (57) 0.0029 Form = 0.0057 Z 0.0376 0.1000 X -83.5389 -82.5000 Y 4.8317 5.0000 A1- X/Z -0.0169 0.0000 A2- Y/Z -0.1580 -0.1000Outlier Elimination Inside Workpiece : 0 Outside Workpiece : 9No Filter AP4 Outlier GDT Flat 0.0100 0.0057 |—

______________________________________________________________________________________________Plane4(Outlier to Computed Feature) Minimum Zone Plane #P (217)S = 0.0015 Min = (3) -0.0026 Max = (57) 0.0026 Form = 0.0051 Z 0.0381 0.1000 X -83.5390 -82.5000 Y 4.8315 5.0000 A1- X/Z -0.0166 0.0000 A2- Y/Z -0.1443 -0.1000Outlier Elimination Inside Workpiece : 4 Outside Workpiece : 5No Filter CF Outlier GDT Flat 0.0100 0.0051 |—

______________________________________________________________________________________________

Calypso Compact Protocal - Range of Data Reduction example

One plane with three evaluations. See bold print.