Patente US5640270 - Orthogonal-scanning microscope objective for vertical-scanning and phase ...

ORTHOGONAL-SCANNING MICROSCOPE OBJECTIVE FOR VERTICAL-SCANNING AND PHASE-SHIFTING INTERFEROMETRY

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention is related in general to the field of inter

ferometry and, in particular, to a novel device for measuring the surface roughness of cylinder walls in internal combustion engines.

2. Description of the Related Art

The surface roughness of cylinder walls in internal combustion engines is critical to good performance and durability. When the surface is too smooth, engine oil is thoroughly cleaned away by the recurring scraping action of piston rings over the cylinder walls, thereby greatly reducing lubrication to the parts and correspondingly increasing wear and tear. On the other hand, when the surface is too rough, the rings abut unevenly against the cylinder walls creating nonuniform lubricant distribution, which results in lubricant losses and nonuniform pressure distribution between the abutting surfaces, in turn also producing excessive wear and tear. In the ideal situation, the interior walls of cylinders have a very uniform surface with sufficient microscopic roughness to retain lubricant particles within uniformlydistributed recesses in the wall, such that the surface is never fully depleted of lubricant by the scraping action of the rings. Experience has shown that an average roughness of about 0.2 to 1.5 microns is desirable for standard automobile and truck internal-combustion engines.

In view of this objective, engine manufacturers inspect cylinder walls and measure their surface roughness as normal quality-control steps. The instruments used consist of borescopes, optical devices capable of showing small details of internal features, including imperfections and cracks, and stylus profilometers, mechanical devices for measuring the topography of the cylinder wall, typically with a limited three-dimensional resolution. Boroscopes are not very useful in testing for surface roughness because their resolution is not sufficient to identify problem spots; in addition, they provide a view of the cylinder wall but do not produce a quantified profile of the tested surface. The stylus instruments operate on contact, thereby affecting the tested surface, and have no ability to perform three-dimensional measurements.

Accordingly, there is a need for a better instrument for the purpose of rapidly, reliably and non-destructively measuring the roughness of cylinder walls. This invention is directed at providing a novel instrument and procedure based on noncontact interferometric principles for measuring the surface roughness of cylinder walls.

BRIEF SUMMARY OF THE INVENTION

One primary goal of this invention is an interferometer suitable for scanning a test surface placed orthogonally from, or at an angle with, the direction of motion of the scanning mechanism

Another important object of the invention is a non-contact instrument and method for mapping the surface profile of a cylinder wall over a large scanning range, preferably up to several millimeters.

Another basic objective is a technique for cylinder walls based on optical measurements that permit higher vertical resolution than currently available with stylus-profilometry technology.

Another objective is a device and a method for measuring surface-roughness of cylinder walls more rapidly and reliably than with prior-art procedures.

Another goal is an instrument that makes it possible to 5 obtain three-dimensional measurements of the profile of a test surface.

Finally, another goal is a method and apparatus that are suitable for implementation with relatively minor modifications to existing interferometric surface profilers.

10 Therefore, according to these and other objectives, the principle of interferometry, in particular vertical scanning interferometry (VSI). is applied to measuring the relative height of peaks and valleys in the surface of a cylinder wall to produce a topographic map of the tested area. The

15 invention consists preferably of a broad-bandwidth interferometric device or probe adapted for longitudinal insertion into a cylinder cavity to produce irradiance signals at multiple vertical-scanning positions as a function of optical path differences between a reference mirror incorporated in

20 the probe and the cylinder-wall surface. The light-source beam is passed through an objective lens placed longitudinally in the cylinder and then divided by a beam splitter disposed in fixed relation to the cylinder wall to produce a test beam directed radially to the wall and a reference beam

25 directed axially to a reference mirror disposed in fixed relation to the lens. During scanning, the objective lens and reference mirror are translated together, while the beam splitter remains stationary with respect to the cylinder wall, thereby varying the position of the focal point of the test

30 beam and providing the vertical-scanning effect required to produce interference fringes and a corresponding map of the tested cylinder surface. In order to reduce the length of the instrument, the reference beam may be folded to the side by a reflective surface and the reference mirror may be posi

35 tioned perpendicularly to the main axis of the instrument In that case, the lens, fold mirror and reference mirror are all translated together, while the beam splitter remains stationary and fixed with respect to the test surface. Various other purposes and advantages of the invention

40 will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings,

45 fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.

5Q BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic representation of a conventional vertical-scanning interferometer.

FIG. 2 is a schematic representation of the interference microscope objective portion of a cylinder-bore scanner

55 according to one embodiment of the present invention, wherein the reference mirror is translated vertically to perform vertical-scanning interferometric measurements on a stationary cylinder wall placed at right angle to the axis of the stationary objective lens and beam splitter.

60 FIG. 3 is a schematic representation of the interference microscope objective portion of a cylinder-bore scanner according to the preferred embodiment of the present invention, wherein the objective lens and reference mirror are translated vertically to perform vertical-scanning inter

65 ferometric measurements on a stationary cylinder wall placed at right angle to the axis of the objective lens through a stationary beam splitter.

3 4

FIG. 4 is a schematic representation of the optical effect in coaxial alignment with the objective 22, so that two light

of scanning the device of FIG. 3. beams produce interference fringes as a result of the optical

FIG. 5 is a variation of the device of FIG. 3 comprising Pa* difference between the reference mirror and the test

a fold mirror reflecting the reference beam to the side of the su?a<;e & The imaging array 28 normally consists of indi

objective's optical axis in order to shorten the length of the 5 vidual charge-coupled-device (CCD) cells or otiier sensing

. JL. t apparatus adapted to detect and record a two-dimensional

men' array of signals corresponding to interference effects pro

FIG. 6 is another variation of the device of FIG. 3, duced by me interferometer as a result of light reflected at

wherein the bean splitter is rotatable about the optical axis individual x-y-coordinate pixels in the surface S and

of the instrument, fliereby enabling it to view and test sample ^ received at corresponding individual cells in the array,

surfaces in all radial directions. Appropriate electronic hardware is provided to process the

DESCRIPTION OF THE PREFERRED signals generated by each cell and transmit digitized light

EMBODIMENTS OF THE INVENTION intensity data D to a microprocessor for processing. The

reference arm 20, containing the microscope objective 22,

The heart of this invention lies in the recognition that beam splitter 24 and reference surface 26 incorporated vertical-scanning interferometry can be performed on the within it, is adapted for vertical movement (along the z surface of the interior wall of an internal-combustion cyl- coordinate) to focus the image of the sample surface S on the inder by modifying a conventional interference microscope detector array 28. Thus, an interference-fringe map is genobjective so that the objective lens and the reference mirror erated by detecting the intensity of the light signal received are translated together along the optical axis in the longitu- 2Q in each cell of the array 28.

dinal direction of the cylinder, while the beam splitter is kept ^ conventional vertical-scanning interferometry, a profile

stationary with respect to the test surface, that is, the cylinder Gf me surface S is produced by repeating irradiance mea

wall, which is disposed orthogonally to the direction of surements at different, normally constant-interval OPD's

scanning. Such configuration results in vertical scanning of between the microscope objective 20 and the test surface S

the wall surface even though no radial movement occurs 25 (mat ^ at different elevations of the scanning unit), so as to

within the cylinder. provide information concerning the variation of light inten

The invention is applicable to both vertical scanning- sity at each pixel as the corresponding optical path difference

interferometry (VST) and phase-shifting interferometry is varied systematically with respect to an initial reference

(PSI), but it is mostly practical only when a rough surface is point Thus, the position of the scanning unit corresponding

being measured; that is, when VSI is the suitable interfero- 30 to maximum interference at each pixel is determined and

metric method. Therefore, the invention will be described in used, based on the distance from the reference point, to

terms of vertical-scanning interferometry but it is under- calculate the height of the surface at that pixel. Either the

stood that it should not be so limited. interference microscope assembly 20 or the test surface S is

Vertical scanning interferometry is a technique where moved vertically (vertical scanning) to produce these broad bandwidth light, such as white light, is used as a light 35 repeated measurements. It is noted that this type of intersource in an interferometer and the degree of modulation, or ference microscope conforms to the configuration of a contrast, of interference fringes produced by the instrument special type of Michelson interferometer, is measured for various distances between a test surface and The prior art discloses various ways by which VSI may be the reference surface of the interferometer (each distance implemented to determine surface height by calculating the corresponding to a different optical path difference, OPD) to 40 degree of fringe modulation, or contrast, of the interference determine surface height The method typically involves fringes produced at the light detector for various OPD's vertical scanning of a reference arm of the interferometer between the test surface and the reference surface of the with respect to a stationary sample and calculation of the interferometer. All methods involve vertical scanning of the relative modulation of the intensity signal as a function of interference microscope assembly (or its equivalent vertical position. 45 components, in the case of other types of interferometers)

As illustrated in simple schematic form in FIG. 1, typical with respect to a stationary sample, or viceversa. The

interferometric equipment 10 comprises a light source 12 components of the interference microscope objective 20

directing a beam L of light through an illuminator 14 toward (lens 22, beam splitter 24 and reference mirror 26) are kept

a beam splitter 16, which reflects the light downward in the in fixed relation and move together to maintain a fixed direction of a test surface S. The light reflected by the beam 50 focused optical path to the reference mirror while the sample

splitter 16 passes through an objective lens 22 focused on the S is scanned. This arrangement allows large vertical scans

test surface S. The objective lens 22, a beam splitter 24 and (in the order of hundreds of microns, up to several

a reference mirror together 26 constitute an interference millimeters, depending on the scanning mechanism

microscope assembly 20 adapted for relative movement with utilized); this scanning range ensures that different portions respect to the test surface, so that two light beams LR and 55 of the sample S, successively appearing within the focal

LT are generated for producing interference fringes. The depth of the lens 22, produce observable interference fringes

reference beam LR is focused on the surface of the reference during the entire scan.

surface 26, while the test beam LT is focused on the test or In order to measure the roughness of a cylinder wall with

sample surface S. Note that the test beam LT is shown for an interferometric device adapted to scan vertically along simplicity as coaxial with the optical axis of the objective 60 the cylinder's main axis and test a surface disposed orthogo

lens 22, but in practice it may be shifted either by design or nally thereto, the positions of the test and reference surfaces

by optics imperfections to a parallel or slightly angular in FIG. 1 necessarily would have to be inverted, as shown in

position by the beam splitter 24, as would be obvious to one the schematic view of FIG. 2, to reflect the coaxial position

skilled in the art. The beams reflected from the reference of the cylinder wall with respect to the optical axis A of the mirror 26 and the test surface S pass back up through the 65 objective lens 22. According to one aspect of the present

optics of the objective lens 22 and upward through the beam invention, such a configuration (shown for simplicity with a

splitter 16 to a solid-state detector array 28 in a camera 30 single objective lens 22 receiving collimated light L) could 5 6

be used to perform vertical scanning interferometry by arrows A2 (FIG. 3) and the relative height of each surface

translating the reference surface 26 in the direction of arrows pixel is measured by estimating the peak of the correspond

Al along the optical axis of the objective lens using a ing modulation envelope from the intensity measurements

scanning mechanism 40 fixedly mounted on a support collected during scanning. Note that the curvature of the test

surface 42, but the scanning range of such a device would be 5 surface (the internal wall of a cylinder) is accounted for and

limited to a fraction of the depth of focus of the lens 22. eliminated by appropriate software that is conventionally

When scanning outside the focal depth, in practice no useful used when interferometric measurements are performed on

fringes are produced because the vertical resolution is curved surfaces. It is also noted that the interferometric

greatly degraded. Therefore, while viable as a solution for arrangement of FIG. 3 can be used for phase-shifting mea

measuring a relatively smooth surface (for example, an F/4 10 surements as well when a smooth surface is being tested. In

objective lens will provide a useful scanning range of about such cases> though, conventional configurations would be

10 microns with visible tight), the configuration of FIG. 2 is Just as effective and probably preferred,

not practical for cylinder walls with greater peak-to-valley As those skilled in the art would readily understand, the

roughness concept of this invention could be applied as well in a

The main aspect of the present invention concerns the 15 system where is• projected at an angle (which

.. „ ~ . .. , , . , t. A. may or may not be 90 degrees) from the optical axis A of the

idea of performing vertical scanning by translating the „■ , A *. *u • A- t +u

.. ^. K „„ , x, „ .b L,. ... objective lens and from the scanning direction of the

objective lens 22 and the reference mirror 26 together while ia^eIomete^ and where me test beanfis then focused on

keeping the beam splitter 24 fixed with respect to the test a ^ surface kced ... t0 me test ^

surface S (i.e.. with respect to the cylinder wall). As shown prmcipies of me mvention apply to any such configuration

in the schematic representation of FIG. 3, the objective-lens/ 20 as well because a vertical translation of the objective-lens/

mirror assembly 44 is translated in the direction of arrows mirror assembly 44 in the direction of arrows A2 will

A2 along the optical axis of the objective lens using a produce the same amount of translation of the focal plane 46

scanning mechanism 40 fixedly mounted on a support Biong the test beam's axis, such that height measurements

surface 42. Because of the fixed distance between the lens 22 can be made of a test surface lying within that plane. In

and the reference mirror 26, the mirror remains in focus 25 practice, though, if the angle between the optical axis of the

during the entire range of translation. At the same time, as objective lens and the test beam is small (measured with

the distance between the lens 22 and the splitter 24 varies, respect to the direction of the lens' focal point), the test and

so will the position of the focal plane of the test beam IT, reference surfaces will be too close to each other for

thereby providing a variable OPD for producing fringes practical implementation. In addition, the beam splitter

during scanning. 30 would have to be very large to reflect the test beam at a small

FIG. 4 illustrates the effect of the simultaneous and equal angle with respect to the reference beam If the angle is

translation of the objective lens 22 and the reference mirror large, the test beam will be reflected back toward the

26 along the direction of arrows A3. The initial position, objective lens, with comparable undesirable effects,

shown in solid line, results in a test focal plane 46 deter- Therefore, for optimal performance the test beam should be

mined by the initial distance between the lens and the beam 35 close to orthogonal to the optical axis of the objective lens,

splitter 24. As the lens 22 is moved closer to the splitter 24 In practice, it is expected that any angle between about 25

by a distance d (the result is shown in broken line), the and 155 degrees could be implemented in a straightforward

reference mirror 26 remains in the reference focal plane manner for most applications. Smaller and larger angles

because the distance between the lens and the mirror is would require special design configurations that may be

unchanged, but the test focal plane is shifted away. Thus, 40 harder to implement and, therefore, not preferred,

during the translation corresponding to the two lens and The concept of the invention could also be implemented

reference mirror positions indicated in FIG. 4, the focal by directing the reference beam LR out of the beam splitter

plane of the test beam travels through the same distance d 24 at an angle with respect to the optical axis A of the

and successively produces interference fringes correspond- objective lens 22. This configuration would require coordi

ing to points found in focus on the scanned surface. 45 nating the translation of the objective lens and of reference

Therefore, the scanning range can be extended as needed to mirror in such a way that the reference mirror would remain

cover the roughness of the sample surface. in focus throughout the scanning procedure, while the beam

According to this principle, the preferred embodiment of splitter would remain stationary. As well understood in the

the present invention consists of an interferometric probe art, this could be accomplished by translating the objective

adapted for longitudinal insertion in the cavity of a cylinder 50 lens and the reference mirror vertically by different amounts

and comprising a microscope objective lens and a reference (the relative motion would depend on the geometry and

surface disposed in fixed relation to one another and adapted optical characteristics of the beam splitter), or by translating

to translate together along the cylinder's axis during vertical each along its respective axis by the same amount The

scanning. The probe also comprises a stationary beam relationship between the vertical movement of the objective

splitter to produce a test beam projecting radially 55 lens and the vertical movement of the reference beam's focal

(orthogonally to the optical axis of the objective lens) and plane is a function of the structural configuration of the beam

focused on the cylinder wall. In operation, the probe of the splitter 24 and is well understood in the art. Accordingly,

invention is inserted longitudinally into the cavity of a appropriate correction would be made as a function of the

cylinder such that the optical axis of the objective lens 22 is angle between the reference beam and the optical axis of the

parallel to the main axis of the cylinder. As would be obvious 60 objective lens to account for the proportionate (but unequal)

to those skilled in the art of interferometry, the probe must vertical displacement of the focal plane with respect to that

be rigidly secured to a supporting structure so that its of the objective lens. Note that the term "vertical" is used

position relative to the target surface can be finely adjusted. here to indicate the direction of scanning of the

The probe is placed in such a way that the target surface on interferometer, but any other scanning direction would be

the wall of the cylinder is within the focal depth of the test 65 equivalent for the purposes of this invention,

beam LT. Conventional VSI is then carried out by translating FIG. 5 illustrates a refinement of the concept of the the objective-lens/mirror assembly 44 in the direction of present invention which may be preferred for applications 10

where the length of the probe is crucial and needs to be as compact as possible to conform to limited space requirements. In order to shorten the tip of the probe, a fold mirror 48 is placed within the reference beam LR to divert it to the side (preferably, but not necessarily, at 90 degrees). This allows the placement of the reference mirror 26 at the focal length in a direction appropriate for shortening the physical length of the probe. Obviously, this embodiment of the invention is optically equivalent to the one illustrated in FIG. 3.

In still another embodiment of the invention illustrated schematically in FIG. 6, the beam splitter 24, which is stationary along the optical axis A of the objective lens 22, is made rotatable around such axis (arrows A4) so as to provide flexibility with respect to the radial direction of the test beam LT. This feature enables use of the probe of the invention to project the test beam in any direction on a plane perpendicular to the lens' optical axis and scan any surface placed in parallel to the probe. For example, it can be used to test the entire circumference of the inside wall of a cylinder, as shown in the figure where dotted and solid lines correspond to two alternative positions of the beam splitter 24. Note that mis feature is illustrated in FIG. 6 with reference to the embodiment of FIG. 3, but it could be implemented in equivalent fashion with the probe of FIG. 5.

Thus, the present invention exploits the optical effect of translating a microscope objective lens and a reference mirror together along the lens' optical axis while maintaining a beam splitter in stationary position to reflect the test beam radially therefrom to carry out vertical scanning interferometry on a test surface disposed in parallel to the direction of scanning. This procedure is not limited by the focal depth of the objective lens and, therefore, it can be implemented over a wide scanning range, constrained only by the characteristics of the mechanism utilized to effect the scanning translation. With the motorized devices recently introduced in the art, the present invention has been used over a scanning range of about one millimeter and it is expected that VSI will soon be carried out reliably over a scanning of several millimeters.

Various changes in the details, steps and components that have been described may be made by those skilled in the art within the principles and scope of the invention herein illustrated and defined in the appended claims. Therefore, while the present invention has been shown and described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent processes and products.

We claim:

1. In a device for executing interferometric measurements at multiple distances between a reference surface and a test surface disposed in aligned optical paths to a light-intensity sensor, wherein a scanning means is used to effect a relative translation between the reference surface and the test surface to achieve said multiple distances at which light-intensity outputs are produced, wherein said light-intensity outputs are used to calculate a surface-height output corresponding to a profile of the test surface, and wherein the test surface is disposed orthogonally to a direction forming a first angle with a scanning direction of the interferometer, an interference microscope that comprises:

(a) an objective lens having an optical axis and a focal plane;

(b) a beam splitter aligned with the optical axis of the objective lens, said beam splitter producing a reference

beam and producing a test beam directed substantially along said direction forming a first angle with the scanning direction of the interferometer and toward said test surface placed substantially within the focal plane of said test beam;

(d) scanning means for translating the objective lens and the reference surface together while the beam splitter and test surface remain stationary.

2. The device of claim 1, wherein said beam splitter is rotatable around said optical axis.

3. The device of claim 1, wherein said first angle is substantially 90 degrees.

4. The device of claim 2, wherein said first angle is substantially 90 degrees.

5. The device of claim 1, further comprising a fold mirror placed between the beam splitter and the reference surface in fixed relation to the objective lens to divert the reference beam toward a direction at a second angle with said optical axis of the objective lens.

6. The device of claim 5, wherein said first angle is substantially 90 degrees.

7. The device of claim 5, wherein said second angle is substantially 90 degrees.

8. The device of claim 5, wherein said first angle is substantially 90 degrees and said second angle is substantially 90 degrees.

9. The device of claim 5, wherein said beam splitter is rotatable around said optical axis.

10. The device of claim 9, wherein said first angle is substantially 90 degrees.

11. The device of claim 9, wherein said second angle is substantially 90 degrees.

12. The device of claim 9, wherein said first angle is substantially 90 degrees and said second angle is substantially 90 degrees.

13. In a device for executing interferometric measurements at multiple distances between a reference surface and a test surface disposed in aligned optical paths to a lightintensity sensor, wherein a scanning means is used to effect a relative translation between the reference surface and the test surface to achieve said multiple distances at which light-intensity outputs are produced, wherein said lightintensity outputs are used to calculate a surface-height output corresponding to a profile of the test surface, and wherein the test surface is disposed orthogonally to a scanning direction of the interferometer, an interference microscope that comprises:

(a) an objective lens having an optical axis and a focal plane;

(b) a beam splitter aligned with the optical axis of the objective lens, said beam splitter producing a reference beam directed substantially along said optical axis and producing a test beam directed substantially along said direction forming a first angle with the scanning direction of the interferometer and toward said test surface placed substantially within the focal plane of said test beam;

(c) a reference surface disposed in fixed relation to the objective lens and substantially within the focal plane of said reference beam; and

(d) scanning means for translating the objective lens and the reference surface together along said optical axis while the beam splitter and test surface remain stationary.

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Orthogonal-scanning microscope objective for vertical-scanning and phase ...