What is the Advantage and Disadvantage of How to Measure Profile

Measurement of small-scale features on surfaces

Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface fractality, and surface finish (including surface roughness) are the parameters most commonly associated with the field. It is important to many disciplines and is mostly known for the machining of precision parts and assemblies which contain mating surfaces or which must operate with high internal pressures.

Surface finish may be measured in two ways: contact and non-contact methods. Contact methods involve dragging a measurement stylus across the surface; these instruments are called profilometers. Non-contact methods include: interferometry, digital holography, confocal microscopy, focus variation, structured light, electrical capacitance, electron microscopy, photogrammetry and non-contact profilometers.

Overview

[

edit

]

The most common method is to use a diamond stylus profilometer. The stylus is run perpendicular to the lay of the surface.[1] The probe usually traces along a straight line on a flat surface or in a circular arc around a cylindrical surface. The length of the path that it traces is called the measurement length. The wavelength of the lowest frequency filter that will be used to analyze the data is usually defined as the sampling length. Most standards recommend that the measurement length should be at least seven times longer than the sampling length, and according to the Nyquist–Shannon sampling theorem it should be at least two times longer than the wavelength of interesting features. The assessment length or evaluation length is the length of data that will be used for analysis. Commonly one sampling length is discarded from each end of the measurement length. 3D measurements can be made with a profilometer by scanning over a 2D area on the surface.

The disadvantage of a profilometer is that it is not accurate when the size of the features of the surface are close to the same size as the stylus. Another disadvantage is that profilometers have difficulty detecting flaws of the same general size as the roughness of the surface.[1] There are also limitations for non-contact instruments. For example, instruments that rely on optical interference cannot resolve features that are less than some fraction of the operating wavelength. This limitation can make it difficult to accurately measure roughness even on common objects, since the interesting features may be well below the wavelength of light. The wavelength of red light is about 650 nm,[2] while the average roughness, (Ra) of a ground shaft might be 200 nm.

The first step of analysis is to filter the raw data to remove very high frequency data (called "micro-roughness") since it can often be attributed to vibrations or debris on the surface. Filtering out the micro-roughness at a given cut-off threshold also allows to bring closer the roughness assessment made using profilometers having different stylus ball radius e.g. 2 µm and 5 µm radii. Next, the data is separated into roughness, waviness and form. This can be accomplished using reference lines, envelope methods, digital filters, fractals or other techniques. Finally, the data is summarized using one or more roughness parameters, or a graph. In the past, surface finish was usually analyzed by hand. The roughness trace would be plotted on graph paper, and an experienced machinist decided what data to ignore and where to place the mean line. Today, the measured data is stored on a computer, and analyzed using methods from signal analysis and statistics.[3]

• The effect of different form removal techniques on surface finish analysis

• Plots showing how filter cutoff frequency affects the separation between waviness and roughness

• Illustration showing how the raw profile from a surface finish trace is decomposed into a primary profile, form, waviness and roughness

• Illustration showing the effect of using different filters to separate a surface finish trace into waviness and roughness

Equipment

[

edit

]

For a full list of standardized instruments, see ISO 25178 § Instruments

Stylus-based contact instruments have the following advantages:

• The system is very simple and sufficient for basic roughness, waviness or form measurement requiring only 2D profiles (e.g. calculation of the Ra value).
• The system is never lured by the optical properties of a sample (e.g. highly reflective, transparent, micro-structured).
• The stylus ignores the oil film covering many metal components during their industrial process.

Technologies:

• Contact Profilometers – traditionally use a diamond stylus and work like a phonograph.
• Atomic force microscope are sometimes also considered contact profilers operating at atomic scale.

Optical measurement instruments have some advantages over the tactile ones as follows:

• no touching of the surface (the sample can not be damaged)
• the measurement speed is usually much higher (up to a million 3D points can be measured in a second)
• some of them are genuinely built for 3D surface topography rather than single traces of data
• they can measure surfaces through transparent medium such as glass or plastic film
• non-contact measurement may sometimes be the only solution when the component to measure is very soft (e.g. pollution deposit) or very hard (e.g. abrasive paper).

Vertical scanning:

Horizontal scanning:

Non-scanning

Choice of the right measurement instrument

[

edit

]

Because every instrument has advantages and disadvantages the operator must choose the right instrument depending on the measurement application. In the following some advantages and disadvantages to the main technologies are listed:

• Interferometry: This method has the highest vertical resolution of any optical technique and lateral resolution equivalent to most other optical techniques except for confocal which has better lateral resolution. Instruments can measure very smooth surfaces using phase shifting interferometry (PSI) with high vertical repeatability; such systems can be dedicated for measuring large parts (up to 300mm) or microscope-based. They can also use coherence scanning interferometry (CSI) with a white-light source to measure steep or rough surfaces, including machined metal, foam, paper and more. As is the case with all optical techniques, the interaction of light with the sample for this instruments is not fully understood. This means that measurement errors can occur especially for roughness measurement.[4][5]
• Digital Holography: this method provides 3D topography with a similar resolution as interferometry. Moreover, as it is a non-scanning technique, it ideal for the measurement of moving samples, deformable surfaces, MEMS dynamics, chemical reactions, the effect of magnetic or electrical field on samples, and measurement of the presence of vibrations, in particular for quality control.:
• Focus variation: This method delivers color information, can measure on steep flanks and can measure on very rough surfaces. The disadvantage is that this method can not measure on surfaces with a very smooth surface roughness like a silicon wafer. The main application is metal (machined parts and tools), plastic or paper samples.
• Confocal microscopy: this method has the advantage of high lateral resolution because of the use of a pin hole but has the disadvantage that it can not measure on steep flanks. Also, it quickly loses vertical resolution when looking at large areas since the vertical sensitivity depends on the microscope objective in use.
• Confocal chromatic aberration: This method has the advantage of measuring certain height ranges without a vertical scan, can measure very rough surfaces with ease, and smooth surfaces down to the single nm range. The fact that these sensors have no moving parts allows for very high scan speeds and makes them very repeatable. Configurations with a high numerical aperture can measure on relatively steep flanks. Multiple sensors, with the same or different measurement ranges, can be used simultaneously, allowing differential measurement approaches (TTV) or expanding the use case of a system.
• Contact profilometer: this method is the most common surface measurement technique. The advantages are that it is a cheap instrument and has higher lateral resolution than optical techniques, depending on the stylus tip radius chosen. New systems can do 3D measurements in addition to 2D traces and can measure form and critical dimensions as well as roughness. However, the disadvantages are that the stylus tip has to be in physical contact with the surface, which may alter the surface and/or stylus and cause contamination. Furthermore, due to the mechanical interaction, the scan speeds are significantly slower than with optical methods. Because of the stylus shank angle, stylus profilometers cannot measure up to the edge of a rising structure, causing a "shadow"or undefined area, usually much larger than what is typical for optical systems.

Resolution

[

edit

]

The scale of the desired measurement will help decide which type of microscope will be used.

For 3D measurements, the probe is commanded to scan over a 2D area on the surface. The spacing between data points may not be the same in both directions.

In some cases, the physics of the measuring instrument may have a large effect on the data. This is especially true when measuring very smooth surfaces. For contact measurements, most obvious problem is that the stylus may scratch the measured surface. Another problem is that the stylus may be too blunt to reach the bottom of deep valleys and it may round the tips of sharp peaks. In this case the probe is a physical filter that limits the accuracy of the instrument.

Roughness parameters

[

edit

]

The real surface geometry is so complicated that a finite number of parameters cannot provide a full description. If the number of parameters used is increased, a more accurate description can be obtained. This is one of the reasons for introducing new parameters for surface evaluation. Surface roughness parameters are normally categorised into three groups according to its functionality. These groups are defined as amplitude parameters, spacing parameters, and hybrid parameters.[6]

Profile roughness parameters

[

edit

]

Parameters used to describe surfaces are largely statistical indicators obtained from many samples of the surface height. Some examples include:

Table of useful surface metrics Parameter Name Description Type Formula Ra, Raa, Ryni arithmetic average of absolute values Mean of the absolute values of the profile heights measured from a mean line averaged over the profile Amplitude

R a = 1 n ∑ i = 1 n | y i | {\displaystyle R_{a}={\frac {1}{n}}\sum _{i=1}^{n}\left|y_{i}\right|}

Rq, RRMS root mean squared Amplitude

R q = 1 n ∑ i = 1 n y i 2 {\displaystyle R_{q}={\sqrt {{\frac {1}{n}}\sum _{i=1}^{n}y_{i}^{2}}}}

Rv maximum valley depth Maximum depth of the profile below the mean line with the sampling length Amplitude

R v = min i y i {\displaystyle R_{v}=\min _{i}y_{i}}

Rp maximum peak height Maximum height of the profile above the mean line within the sampling length Amplitude

R p = max i y i {\displaystyle R_{p}=\max _{i}y_{i}}

Rt Maximum Height of the Profile Maximum peak to valley height of the profile in the assessment length Amplitude

R t = R p − R v {\displaystyle R_{t}=R_{p}-R_{v}}

Rsk Skewness Symmetry of the profile about the mean line Amplitude

R s k = 1 n R q 3 ∑ i = 1 n y i 3 {\displaystyle R_{sk}={\frac {1}{nR_{q}^{3}}}\sum _{i=1}^{n}y_{i}^{3}}

Rku Kurtosis Measure of the sharpness of the surface profile Hybrid

R k u = 1 n R q 4 ∑ i = 1 n y i 4 {\displaystyle R_{ku}={\frac {1}{nR_{q}^{4}}}\sum _{i=1}^{n}y_{i}^{4}}

RSm Mean Peak Spacing Mean Spacing between peaks at the mean line Spatial

R S m = 1 n ∑ i = 1 n S i {\displaystyle RS_{m}={\frac {1}{n}}\sum _{i=1}^{n}S_{i}}

This is a small subset of available parameters described in standards like ASME B46.1[7] and ISO 4287.[8] Most of these parameters originated from the capabilities of profilometers and other mechanical probe systems. In addition, new measures of surface dimensions have been developed which are more directly related to the measurements made possible by high-definition optical gauging technologies.

Most of these parameters can be estimated using the SurfCharJ plugin [1] for the ImageJ.

Areal surface parameters

[

edit

]

The surface roughness can also be calculated over an area. This gives Sa instead of Ra values. The ISO 25178 series describes all these roughness values in detail. The advantage over the profile parameters are:

• more significant values
• more relation to the real function possible
• faster measurement with actual instruments[

  clarification needed

  ] possible (optical areal based instruments can measure an Sa in higher speed then Ra.

Surfaces have fractal properties, multi-scale measurements can also be made such as Length-scale Fractal Analysis or Area-scale Fractal Analysis.[9]

Filtering

[

edit

]

To obtain the surface characteristic almost all measurements are subject to filtering. It is one of the most important topics when it comes to specifying and controlling surface attributes such as roughness, waviness, and form error. These components of the surface deviations must be distinctly separable in measurement to achieve a clear understanding between the surface supplier and the surface recipient as to the expected characteristics of the surface in question. Typically, either digital or analog filters are used to separate form error, waviness, and roughness resulting from a measurement. Main multi-scale filtering methods are Gaussian filtering, Wavelet transform and more recently Discrete Modal Decomposition. There are three characteristics of these filters that should be known in order to understand the parameter values that an instrument may calculate. These are the spatial wavelength at which a filter separates roughness from waviness or waviness from form error, the sharpness of a filter or how cleanly the filter separates two components of the surface deviations and the distortion of a filter or how much the filter alters a spatial wavelength component in the separation process.[7]

See also

[

edit

]

• Flatness (manufacturing) – Geometric condition for workpieces and tools
• Profilometer – Measuring instrument for surface profile and roughness
• Range imaging – Technique which produces a 2D image showing the distance to points in a scene from a specific point

References

[

edit

]

by GD&T Basics on February 12, 2021.

Ever wonder how surface finish is measured? For inspection, we use a device called a Profilometer.  In this article, we will show you how profilometers work and highlight their advantages and limitations.

What is a Profilometer & Why is it Used?

A profilometer is an instrument used to measure the profile and surface finish of a surface.  On a small scale, surfaces can be composed of a series of peaks and valleys with varying height, depth, and spacing.  Subtle differences in these features determine if the surface feels smooth or rough, looks matte or glossy, can form a seal, or is suitable for a wear surface.   In industries where mechanical parts are produced, surface roughness or surface finish requirements are commonly specified on technical drawings, and profilometers are used to verify that the requirements have been met.

Types of Profilometers

Profilometers come in many shapes and sizes, but they can be divided into two basic types – contact and optical.  Contact profilometers measure surface profile by physically tracing the surface with a stylus. In contrast, optical profilometers use reflections of various types of light to measure surface features in a line or area.  Operating methods, capabilities, limitations, and typical applications for contact and optical profilometers are discussed in detail below. 

Contact Profilometers

Contact profilometers measure surface profile by lightly dragging a stylus across the surface.  The tip of the stylus rides in a line across the surface, moving vertically over the peaks and valleys.  Changes in the stylus’ height are registered electrically and tracked against position as the stylus moves, creating a measured profile.  Stylus tips are conical, with a spherical radius on the bottom.  The cone angle and tip radius determine the smallest features that a stylus can trace.  Stylus tips are typically made of hard, wear-resistant materials such as diamond or sapphire. 

Diagram Showing a Contact Profilometer Contact Profilometer Tip Shape

Handheld contact profilometers are commonly used in machine shops for measuring surface finish on machined parts.  These instruments are placed on the workpiece to be measured, and the stylus is traversed automatically at rates somewhere around 1 millimeter per second.  Tip radius for handheld profilometers can be as small as a few microns, and they can accurately measure Ra down to 0.005 µm and Rz down to 0.02 µm.  This type of instrument is frequently available with several measurement ranges, depending on the smoothness of the surface to be measured.  

Contact profilometers have many advantages.  For example, they are not as sensitive to dirt and oil as their optical counterparts, and their accuracy is not dependent on surface optical characteristics.  They are also less costly than optical profilometers, but they do have a few limitations.  Stylus tips can create scratches in soft material, especially when measurements are repeated.  Over time, stylus tips wear and need to be replaced.  A worn stylus tip or scratched material can result in inaccurate profile data. 

Optical Profilometers

Optical profilometers include 1-D, 2-D, and 3-D profiling devices.  These devices use light to measure features on a surface, and their operation can be based on a number of different principles, including optical interference, use of confocal apertures, focus detection, and pattern projection.  Despite the breadth of this group of instruments, they share points of commonality.  Optical profilometers are relatively large instruments that consist of a light source, optical lenses, and image sensors.  They require the surface to reflect the light being used, so many of these instruments will have trouble measuring translucent or highly reflective surfaces.  Also, for the reflection to accurately characterize the surface, it must be free of debris and contaminants such as dirt, water, and oil.  Since light travels very quickly, measurements can be taken faster than with contact profilometers.  With some instruments, millions of readings can be collected in seconds, making it practical to model a relatively large area’s surface topography. 

Selecting a Profilometer

Selecting the correct profilometer for your application can seem like a daunting task.  The first step is to determine which parameters you are interested in measuring, the approximate range of those parameters, and the required measurement accuracy.  Next, the size and shape of the surface to be measured must be considered.  Finally, the number of measurements and cycle time for each measurement must be taken into account.

There is no “one size fits all” solution.  For example, a machine shop that occasionally manufactures large parts with varying Ra or Rz requirements may be best served by a handheld contact profilometer with a large measuring range. In contrast, a semiconductor manufacturing plant may prefer to integrate an optical profilometer into their processing line or inspection cell.

Profilometer Brands

Below is a list of some commonly available profilometer brands, along with links to their respective websites.  You will note that most manufacturers of optical profilometers require registration on their website before range and accuracy data can be downloaded. 

Mitutoyo manufactures contact profilometers.

Taylor-Hobson manufactures a wide range of contact profilometers.

Bruker manufactures both Stylus and Optical profiling equipment.

Zygo is a manufacturer of 3D Optical Profilometers.

Polytec also manufactures optical profilometers.

Key Take-Aways

1. Profilometers are used to measure surface roughness or surface finish.
2. Contact Profilometers use a stylus to measure one point at a time.
3. Optical Profilometers use reflected light for measurement, and some can measure multiple points simultaneously. 
4. All profilometers are not the same, be sure to select one with the measuring range and features best suited to your application.

Sources:

Keyence Surface Roughness Technical Guide: https://www.keyence.com/mykeyence/?ptn=001&gclid=Cj0KCQiAtOjyBRC0ARIsAIpJyGOxsUrwVPABft8RBi8pIldjSGWzWEy-Eypnhz3B3uERQfwtPHl20_MaApgTEALw_wcB&k_clickid=967d6efe-0e0b-48ad-8f98-e9b3b78199d3&aw=google-kaenvk-sitelink 

https://www.keyence.com/ss/products/microscope/roughness/equipment/line_01.jsp

https://www.pce-instruments.com/us/measuring-instruments/test-meters/profilometer-kat_162617.htm

https://www.keyence.com/ss/products/measure/select/application/shape_profile.jsp

https://www.nanoscience.com/techniques/optical-profilometry/

https://www.zygo.com/?/met/profilers/

Interested in learning more about GD&T?

Learn GD&T at your own pace and apply it with confidence in the real world.

Click Here For More Information