Profilometer
Encyclopedia
Profilometer is a measuring instrument
used to measure a surface's
profile, in order to quantify
its roughness.
While the historical notion of a profilometer was a device similar to a phonograph
that measures a surface as the surface is moved relative to the contact profilometer's stylus
, this notion is changing with the emergence of numerous non-contact profilometery techniques.
Contact and pseudo-contact methods
Advantages of contact profilometers:
), confocal microscopy (used for profiling of very small objects), low coherence interferometry and digital holography
.
Advantages of optical profilometers
Fiber-based
optical profilometers
Fiber-based optical profilometers scan surfaces with optical probes which send light interference signals back to the profilometer detector via an optical fiber. Fiber-based probes can be physically located hundreds of meters away from the detector enclosure, without signal degradation. Here are the additional advantages of using fiber-based optical profilometers:
s, as used in the famous 1958-1960 AASHO Road Test
) use a distance measuring laser (suspended approximately 30 cm from the pavement) in combination with an odometer
and an inertial unit (normally an accelerometer
to detect vehicle movement in the vertical plane) that establishes a moving reference plane to which the laser distances are integrated. The inertial compensation makes the profile data more or less independent of what speed the profilometer vehicle had during the measurements, with the assumption that the vehicle does not make large speed variations and the speed is kept above 25 km/h or 15 mph. The profilometer system collects data at normal highway speeds, sampling the surface elevations at intervals of 2–15 cm (1–6 in), and requires a high speed data acquisition system capable of obtaining measurements in the kilohertz range.
The data collected by a profilometer is used to calculate the International Roughness Index (IRI) which is expressed in units of inches/mile or mm/m. IRI values range from 0 (equivalent to driving on a plate of glass) upwards to several hundred in/mi (a very rough road). The IRI value is used for road management to monitor road safety and quality issues.
Many road profilers are also measuring the pavements cross slope
, curvature
, longitudinal gradient
and rutting. Some profilers take digital photos or video
s while profiling the road. Most profilers also record the position, using GPS technology. Another quite common measurement option is cracks
. Some profilometer systems include a ground penetrating radar, used to record asphalt
layer thickness.
Another type of profilometer is for measuring the surface texture of a road and how it relates to the coefficient of friction
and thus to skid resistance. Pavement texture is divided into three categories; megatexture (roads)
, macrotexture
, and microtexture
. Microtexture cannot currently be measured directly, except in a laboratory. Megatexture is measured using a similar profiling method as when obtaining IRI values, while macrotexture is the measurement of the individual variations of the road within a small interval of a few centimeters. For example, a road which has gravel spread on top followed by an asphalt seal coat will have a high macrotexture, and a road built with concrete slabs will have low macrotexture. For this reason, concrete is often grooved or roughed up immediately after it is laid on the road bed to increase the friction between the tire and road.
Equipment to measure macrotexure currently consists of a distance measuring laser with an extremely small spot size (< 1 mm) and data acquisition systems capable of recording elevations spaced at a mm or less apart. The sample rate is generally over 32 kHz. Macrotexture data can be used to calculate the speed-depending part of the friction number between typical car tires and the road surface. The macrotexture also give information on the difference between dry and wet road friction. However, macrotexture cannot be used to calculate a relevant friction number, since also microtexture affects the friction.
Lateral friction and cross slope are the key reaction forces acting to keep a cornering vehicle in steady lateral position, while exposed to exciting forces from speed and curvature. Since friction is strongly dependent on macrotexture, and texture, cross slope as well as curvature can be measured with a road profiler, road profilers are very useful to identify improperly banked curves that are posing a great risk to motor vehicle accidents.
Measuring instrument
In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item...
used to measure a surface's
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
profile, in order to quantify
Quantify
Quantify may refer to:* Quantification, the act or process of quantifying* in computing: IBM Rational Quantify, a profiling software, part of IBM Rational Purify...
its roughness.
While the historical notion of a profilometer was a device similar to a phonograph
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
that measures a surface as the surface is moved relative to the contact profilometer's stylus
Stylus
A stylus is a writing utensil, or a small tool for some other form of marking or shaping, for example in pottery. The word is also used for a computer accessory . It usually refers to a narrow elongated staff, similar to a modern ballpoint pen. Many styli are heavily curved to be held more easily...
, this notion is changing with the emergence of numerous non-contact profilometery techniques.
Types
Optical methods- Vertical Scanning Interferometry/White-light interferometer
- Phase Shifting Interferometry
- Differential interference contrast microscopyDifferential interference contrast microscopyDifferential interference contrast microscopy , also known as Nomarski Interference Contrast or Nomarski microscopy, is an optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples...
(Nomarski Microscopy) - Focus detection methods
- Intensity Detection
- Focus variationFocus variationFocus variation is a method to calculate a sharp image and to measure the depth with an optics with limited depth of field.-Algorithm:The algorithm work as follows:# at first images with difference focus are captured...
- Differential Detection
- Critical Angle Method
- Astigmatic Method
- Focault Method
- Confocal microscopyConfocal microscopyConfocal microscopy is an optical imaging technique used to increase optical resolution and contrast of a micrograph by using point illumination and a spatial pinhole to eliminate out-of-focus light in specimens that are thicker than the focal plane. It enables the reconstruction of...
- Pattern projection Methods
- Fringe projection
- Fourier ProfilometryFourier profilometryFourier profilometry is a method for measuring profiles using distortions in periodic patterns. The method uses Fourier analysis to determine localized slopes on a curving surface....
- Moire
Contact and pseudo-contact methods
- Stylus profilometer (mechanical profilometer)
- Atomic Force Microscopy
- Scanning Tunneling Microscopy
Contact profilometers
A diamond stylus is moved vertically in contact with a sample and then moved laterally across the sample for a specified distance and specified contact force. A profilometer can measure small surface variations in vertical stylus displacement as a function of position. A typical profilometer can measure small vertical features ranging in height from 10 nanometres to 1 millimetre. The height position of the diamond stylus generates an analog signal which is converted into a digital signal stored, analyzed and displayed. The radius of diamond stylus ranges from 20 nanometres to 25 μm, and the horizontal resolution is controlled by the scan speed and data signal sampling rate. The stylus tracking force can range from less than 1 to 50 milligrams.Advantages of contact profilometers:
- Acceptance: Most of the world's surface finish standards are written for contact profilometers. To follow the prescribed methodology, this type of Profilometer is often required.
- Surface Independence: Contacting the surface is often an advantage in dirty environments where non-contact methods can end up measuring surface contaminants instead of the surface itself. However, because the stylus is in contact with the surface, this method is not sensitive to surface reflectance or color.
- Resolution: The stylus tip radius can be as small as 20 nanometres, significantly better than white-light optical profiling.
- Direct Technique: No modeling required.
Non-contact profilometers
An optical profilometer is a non-contact method for providing much of the same information as a stylus based profilometer. There are many different techniques which are currently being employed, such as laser triangulation (triangulation sensorTriangulation sensor
Optical Triangulation Sensors are commonly used to provide door mounted safety detection on swinging automatic doors.In North America, these types of sensors are very common and manufactured in very high volumes...
), confocal microscopy (used for profiling of very small objects), low coherence interferometry and digital holography
Digital holography
Digital holography is the technology of acquiring and processing holographic measurement data, typically via a CCD camera or a similar device. In particular, this includes the numerical reconstruction of object data from the recorded measurement data, in distinction to an optical reconstruction...
.
Advantages of optical profilometers
- Resolution: Vertical resolution is usually in the nanometre level, lateral resolution is usually poorer, limited by the wavelength of the light.
- Speed: Because the non-contact profilometer does not touch the surface the scan speeds are dictated by the light reflected from the surface and the speed of the acquisition electronics.
- Reliability: optical profilometers do not touch the surface and therefore cannot be damaged by surface wear or careless operators. Many non-contact Profilometers are solid-state which tends to reduce the required maintenance significantly.
- Spot size: The spot size, or lateral resolution, of optical methods ranges from a few micrometres down to sub micrometre.
Fiber-based
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
optical profilometers
Fiber-based optical profilometers scan surfaces with optical probes which send light interference signals back to the profilometer detector via an optical fiber. Fiber-based probes can be physically located hundreds of meters away from the detector enclosure, without signal degradation. Here are the additional advantages of using fiber-based optical profilometers:
- Flexibility: Thanks to the small diameter of certain probes, surfaces can be scanned even inside hard-to-reach spaces, such as narrow crevices or small-diameter tubes.
- Long profile acquisition: Since these generally probes acquire one point at a time and at very high sample speeds, acquisition of long (continuous) surface profiles is possible.
- Ruggedness: Scanning can take place in hostile environments, including very hot or cryogenic temperatures, or in radioactive chambers, while the detector is located at a distance, in a human-safe environment.
- Ease of incorporating into industrial processes: fiber-based probes are easily installed in-process, such as above moving webs or mounted onto a variety of positioning systems.
Road pavement profilometery
Road pavement profilometers (aka profilographProfilograph
The profilograph is a device used to measure pavement surface roughness. In the early 20th century, Profilographs were low speed rolling devices. Today many Profilographs are advanced high speed systems with a laserbased height sensor in combination with a inertial system that creates a large...
s, as used in the famous 1958-1960 AASHO Road Test
AASHO Road Test
The AASHO Road Test was a series of experiments carried out by the American Association of State Highway and Transportation Officials to determine how traffic contributed to the deterioration of highway pavements...
) use a distance measuring laser (suspended approximately 30 cm from the pavement) in combination with an odometer
Odometer
An odometer or odograph is an instrument that indicates distance traveled by a vehicle, such as a bicycle or automobile. The device may be electronic, mechanical, or a combination of the two. The word derives from the Greek words hodós and métron...
and an inertial unit (normally an accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
to detect vehicle movement in the vertical plane) that establishes a moving reference plane to which the laser distances are integrated. The inertial compensation makes the profile data more or less independent of what speed the profilometer vehicle had during the measurements, with the assumption that the vehicle does not make large speed variations and the speed is kept above 25 km/h or 15 mph. The profilometer system collects data at normal highway speeds, sampling the surface elevations at intervals of 2–15 cm (1–6 in), and requires a high speed data acquisition system capable of obtaining measurements in the kilohertz range.
The data collected by a profilometer is used to calculate the International Roughness Index (IRI) which is expressed in units of inches/mile or mm/m. IRI values range from 0 (equivalent to driving on a plate of glass) upwards to several hundred in/mi (a very rough road). The IRI value is used for road management to monitor road safety and quality issues.
Many road profilers are also measuring the pavements cross slope
Cross slope
Cross slope is a geometric feature of pavement surfaces; the transversal slope [%] with respect to the horizon. It is a very important safety factor. One task is to make water run off the surface to a drainage system, as Cross Slope is the main contributor to Pavement Drainage gradient. Very low...
, curvature
Curvature
In mathematics, curvature refers to any of a number of loosely related concepts in different areas of geometry. Intuitively, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line, but this is defined in different ways depending on the context...
, longitudinal gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
and rutting. Some profilers take digital photos or video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
s while profiling the road. Most profilers also record the position, using GPS technology. Another quite common measurement option is cracks
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
. Some profilometer systems include a ground penetrating radar, used to record asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
layer thickness.
Another type of profilometer is for measuring the surface texture of a road and how it relates to the coefficient of friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
and thus to skid resistance. Pavement texture is divided into three categories; megatexture (roads)
Megatexture (roads)
Megatexture is a family of wave-shaped road surface characteristics. While vehicle suspension deflection and dynamic tyre loads are affected by longer waves , road texture affects the interaction between the road surface and the tyre footprint. Megatexture has wavelengths from 50 mm up to 500 mm...
, macrotexture
Macrotexture
Macrotexture is a family of wave-shaped road surface characteristics. While vehicle suspension deflection and dynamic tyre loads are affected by longer waves , road texture affects the interaction between the road surface and the tyre footprint. Macrotexture has wavelengths from 0.5 mm up to...
, and microtexture
Microtexture
Microtexture is the collative term for a material's crystallographic parameters and other aspects of microstructure: such as morphology, including size and shape distributions; chemical composition; and crystal orientation and relationships...
. Microtexture cannot currently be measured directly, except in a laboratory. Megatexture is measured using a similar profiling method as when obtaining IRI values, while macrotexture is the measurement of the individual variations of the road within a small interval of a few centimeters. For example, a road which has gravel spread on top followed by an asphalt seal coat will have a high macrotexture, and a road built with concrete slabs will have low macrotexture. For this reason, concrete is often grooved or roughed up immediately after it is laid on the road bed to increase the friction between the tire and road.
Equipment to measure macrotexure currently consists of a distance measuring laser with an extremely small spot size (< 1 mm) and data acquisition systems capable of recording elevations spaced at a mm or less apart. The sample rate is generally over 32 kHz. Macrotexture data can be used to calculate the speed-depending part of the friction number between typical car tires and the road surface. The macrotexture also give information on the difference between dry and wet road friction. However, macrotexture cannot be used to calculate a relevant friction number, since also microtexture affects the friction.
Lateral friction and cross slope are the key reaction forces acting to keep a cornering vehicle in steady lateral position, while exposed to exciting forces from speed and curvature. Since friction is strongly dependent on macrotexture, and texture, cross slope as well as curvature can be measured with a road profiler, road profilers are very useful to identify improperly banked curves that are posing a great risk to motor vehicle accidents.
External links
- 3D Profilometer Manufacturer, AEP Technology- Contact and Optical Profilometer Manufacturer
- The Little Book of Profiling, the University of Michigan about road profiling
- Health and Safety Issues, the EU Roadex III project on health and safety issues raised by poorly maintained road networks