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A portable three-dimensional stylus profile measuring instrument
ELSEVIER
A portable three-dimensional stylus profile measuring instrument V. G. Badami, S. T. Smith, J. Raja, and R. J. Hocken Precision Engineering Laboratory, University of North Carolina, Charlotte, NC, USA This paper describes the development o f an inexpensive, portable threedimensional (3-D) stylus-based surface profiler with a scan range o f 4.5 m m x 5.5 m m and a m a x i m u m vertical range of 150 p m (limited by signal conditioning electronic range) with a resolution o f better than O. 1 p m and a range o f 30 p m with a resolution o f better than 30 nm at the highest vertical sensitivity. The instrument occupies a volume with dimensions o f approximately 75 × 75 x 40 m m (L x B x H) with scan speeds up to 0.5 m m s -~. Construction o f instrument components from similar material results in a thermally balanced design with a reasonably l o w thermal coefficient of 0.27 p m K -7 and a first-order time constant o f approximately 40 min. The stylus probe sensor has a free resonant frequency o f 49 Hz and a l o w damping ratio o f 0.006. When in stationary contact with a steel surface, this increases to above the transducer bandwidth o f 100 Hz. Calibration o f the probe sensor is achieved through direct comparison against a standard stylus gauge. Lateral calibration o f the specimen carriage position has been assessed by the measurement o f standard gratings and laser interferometry. Planar errors caused by the motion o f the carriage have been assessed by measuring an optical flat and inferring deviation from a perfect plane as an indication of the worstcase error, which in this case, was 0.2 p m (P-V). The design and construction o f the internal datum and the portability o f the instrument to facilitate in situ measurement of components are emphasized. Images illustrating the surface mapping capabilities o f the profiler are presented.
Introduction The effects of surface finish on the performance of many engineering mechanisms are now established across a broad range of disciplines. Stylus profilers are still the main method for quantitative measurement of the surfaces generated by engineering processes. Traditionally, surface finish parameters computed from stylus measurements have been based on a relatively short portion of the profile. Consequently, low cost, portable stylus measurement instruments presently being marketed derive their dimensional stability and surface datum information by using a skid referenced directly to the surface under examination and are limited to line (two-dimensional, 2-D) traces. HowAddress reprint requests to Mr. V. G. Badarni, Precision Engineering Laboratory, University of North Carolina at Charlotte, Highway 49, Charlotte, NC 28223, USA.
Precision Engineering 18:147-156, 1996 © Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
ever, it is increasingly being recognized that longer range features of the measured profiles can provide additional information relating to functional characteristics of the device or the process used for surface generation. In particular, three-dimensional (3-D) topographic maps enable both visual images and the parametric analysis of areal distributions and direct volume measurements. Many methods exist for the 3-D topographic measurement of surfaces such as optical, large laboratory stylus profiling instruments, and, more recently, a variety of scanning probe microscopes employing a wide range of surface proximity sensing techniques. Invariably, the former two of these methods consist of large and costly instrumentation that is neither readily portable nor robust enough for typical industrial environments. 1'2 A portable 3-D stylus has been reported, although this is limited to a scan area of 0.5-mm square. 3
0141-6359/96/$15.00 SSDI 0141-6359(95)00071-2
Badami et al.: A portable 3-D stylus profile measuring instrument Scanning probe microscopes, on the other hand, invariably are limited to small scan areas and, in the absence of expensive closed-loop monitoring, suffer from a lack of a well-defined datum. 4 Many engineering surfaces contain features of interest that have dimensions considerably larger than the scan range typical of probe microscopes and have reflectivities not suitable for optical inspection. Additionally, it is not always desirable or, sometimes, possible to transport the component to the laboratory for measurement purposes. In principle, the ability to measure a series of parallel profiles referenced to a common datum increases the capability of the stylus instrument from 2-D profiles to 3-D surface maps. For this reason, a novel, inexpensive, portable stylus profiler capable of relatively large area scans has been developed and is presented in this paper. The resultant system consists of a simple flexure-supported stylus attached to a scanning stage that slides upon a flat datum surface. To obtain a topographic map, the stylus is contacted with a specimen and subsequently raster scanned. Data are recorded at equidistant points and stored for subsequent display and analysis. A block diagram representation of this system is shown in Figure 1. Elements of the mechanical design, such as the probe, scanning stage, drives, datum reference, mounting adjustment, and system monitoring and control, are outlined below. This is followed by a performance evaluation and topographic maps to illustrate the range of features that can be measured quantitatively with such a system.
Design An exploded diagram of the complete profiler is shown in Figure 2. Key components of this system are separated to reveal key interfaces. Conceptually, the stylus profiler consists of a main platen (at the top of the diagram) that is free to slide on a flat datum surface. This has been produced by replication from a glass optical fiat using
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Moglice TM, a castable polymer. It is constrained to translate in two orthogonal directions by the surrounding frames (the main frame and the intermediate frame). Linear constraints are imposed by the two guide bars that are supported in the intermediate frame and are free to rotate about their longitudinal axis. The platen rests on the datum surface, which provides the vertical constraint. The two guide bars run along slots milled into its underside (Figure 3). Two polymer bearings (Delrin TM) are located on the side wall of one of the milled slots and slide against the guide bar. To maintain contact between the adjacent guide bar and bearings, a preload mechanism is employed. This consists of a spring-loaded preload arm that imposes the closure force on the second guide bar, again by means of a polymer bearing. Rotation of the guide bars imposes one linear and one rotary constraint, while providing four freedoms necessary for decoupling error motion of the frame from the platen (i.e., allowing constraint in the vertical axis only by the datum surface). The s e c o n d , o r t h o g o n a l , t r a n s l a t i o n is achieved using a similar mechanism. Two further guide bars are supported in the main instrument frame, and these are perpendicular to those in the intermediate frame. These guide bars serve to constrain the intermediate frame as it is translated along the second axis. Five polymer bearings are affixed on the underside of the intermediate frame to form a vee-groove and flat slideway arrangement providing one linear degree of freedom (Fig-
ure 4). The probe is attached to the bottom surface of the platen (Figure 2) and protrudes from the underside of the instrument through a hole in the datum surface. To achieve a relatively long vertical range of 1 mm while maintaining a compact overall size, a specially designed stylus probe was produced. This probe consists of a vertical rod supported by two beryllium copper flexures. These flexures were produced using chemical etching techniques common to printed circuit board manufacture. For clarity, this is also shown in an exploded view in Figure 5. A stylus probe has been bonded to the bottom end of the rod, and a magnetically permeable core is attached between the flexures to form the core of an LVDT displacement transducer. The complete transducer assembly is housed in a compact body that provides mounting holes for attaching to the platen. Translation of the carriage in the two axes is achieved by two wire drives powered by DC motor/ gearbox units (Figure 6). Each wire drive consists of a drive pulley and one or more idlers with a tensioning spring. The drive to the platen is arranged to be close to the centers of mass and friction and is colinear with the rotating guide bars. The driving force is transmitted by a cotton thread (thin sewing thread) that wraps around a drive pulley supported
APRIL/MAY 1996 VOL 18 NO 2/3
Badami et al.: A portable 3-D stylus profile measuring instrument
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by bearings in the intermediate frame. This is, in turn, powered by a drive shaft supported in the main instrument frame through a spline. The spline is needed to accommodate the translation of the intermediate frame in the other axis. An exploded view of the drive shaft and spline for this axis can be seen in Figure 2. In principle, the motor could be directly connected to this shaft. To retain the compact nature of the design, it is necessary, in practice, to drive this shaft using another pulley system that enables the motor/gearbox drive to be embedded in the base. The second drive axis is used to step the platen/intermediate frame combination after each scan. This is a relatively straightforward
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design that is connected to the intermediate frame near to the axis of the four pad vee-bearing and is driven by a second motor/gearbox combination (not shown). Position has been inferred from rotary encoders mounted directly onto the motor shafts of each drive. The zero, or start position, of each trace is established by a trigger pulse produced by an optoelectronic switch (opto-interrupter). This is generated by occlusion of an optical slit on the intermediate frame by a blade mounted on the platen (Figure 2). A photograph of the complete instrument is shown in Figure 7. A typical measurement cycle begins with the stylus being brought into contact with the surface
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using three mounting screws attached to the main instrument frame. After adjustment, these can be locked in position for additional stability, although, in practice, this has not been found to be necessary. The platen is then moved to the desired position beyond the opto-interrupter, and a constant motor voltage is applied to the traverse drive. Registration of successive profiles is ensured by start-
ing data acquisition upon receipt of each encoder pulse, or successive multiples thereof, thus making data sampling immune to changes in motor speed. By returning the platen sufficiently byfar beyond the trigger position and noting that data are collected only in one traverse direction, transient effects and backlash were not observed. Presently, each encoder pulse corresponds to a traverse distance of
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Bottom view of intermediate frame APRIL/MAY 1996 VOL 18 NO 2/3
Badami et al.: A portable 3-D stylus profile measuring instrument
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translation DT2821 acquisition board under personal computer control. To reduce problems with memory, each trace is downloaded to hard disk storage.
Performance evaluation
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Ideally, such an instrument should be capable of scanning the stylus probe in a perfectly planar space while acquiring data. In reality, error sources such as noise, repeatability, and drifts preclude such a goal. To ascertain the performance limits of this instrument, various components of the system have been assessed.
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0.5 pro, giving a maximum sample size of 10,000 data points per profile. The minimum step in the other axis is 10 IJm, giving a maximum of 500 traces. At the end of each trace, the platen is returned to the start position, incremented in the orthogonal axis, and the process is repeated until the desired area scan is complete. Sample spacing and step increment can be specified to the computer control program. Data are sampled using a data
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Traverse calibration Calibration of these axes proved to be more of a challenge than anticipated. The main reason for this was the lack of miniature, lightweight optics for laser interferometric evaluation or any alternative noninfluencing gauging with the appropriate dynamic range. Several complementary techniques were used to evaluate the relationship between the encoder output and stylus position. Results from three different approaches were evaluated and compared:
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This is calibrated by comparison with a reference LVDT sensor by placing the probes of both on the anvil of a differential micrometer (0.1 IJm resolution). To reduce the effects of Abbe offset errors, these are positioned so that they are nearly coincident on the anvil with typical separation of less than 0.5 ram. A typical calibration curve is obtained by manually adjusting the micrometer while monitoring both sensors simultaneously. From such measurements, a high degree of linearity is observed with a maximum deviation of less than 0.5% over a full range of 150 IJm. Presently, the probe range is limited by the signal conditioning electronics with lock-in amplifier saturation corresponding to a displacement of the stylus of 150 pm and with data acquisition single converter bit resolution of 66 nm when set at this minimum sensitivity. In principle, if lower sensitivities were available, it should be possible to monitor stylus output for displacements of up to 1 ram. Higher lock-in amplifier sensitivity settings are possible, and the finest resolution used with this system gave a full vertical range of 30 pm with a noise-limited resolution of approximately 30 nm.
1. direct calculation of effective resolution based on gearbox and pulley reduction; 2. calibration based on profile data of an artifact of known geometry; and 3. laser interferometric calibration of separate platen traversals. Actuation
of the p u l l e y s in each axis is 151
Badami et al.: A portable 3-D stylus profile measuring instrument
Figure 7
Photograph of complete instrument
achieved by a DC motor/gearbox combination with the thread wound around a 4-ram diameter output shaft. Considering a gearbox ratio for each drive of 2,190:1, an encoder outputs 16 pulses per revolution, and, with a secondary reduction pulley introducing a further reduction of 1.4:1, a traverse displacement of 0.5 pm per pulse is predicted. Clearly, effects of slip and drive compliances are likely to result in a slight reduction of this value, thus necessitating further calibration. In an effort to obtain a direct calibration of the stylus translation, an artifact of known geometry was sought. To achieve this, a series of gauge blocks were wrung together, and, to observe the geometry of the interface between blocks, the sides of the assembly were profiled using a Rank-TaylorHobson Form Talysurf 120L. Traverse position for
this instrument is provided by an optical grating scale. An area scan of this assembly was then taken, and corresponding distances were used to infer relative displacements. A typical area map for a set of four gauge blocks is shown in Figure 8. Clearly, the exact position of the interface is ambiguous. However, it is possible to identify features that are caused by the transition from the standard coarse finish of the nonfunctional surfaces to the radiused edge leading to the interface. These values have been obtained by visually aligning cursors at these positions and then reading the horizontal scale between them. It is felt that this will provide a consistent scale to within approximately 2%. From such a procedure, a calibration factor of 0.485 pm per pulse was determined. Further confirmation was obtained by mounting a miniature
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Area map of the side face of four gauge blocks APRIL/MAY 1996 VOL 18 NO 2/3
Badami et al.: A portable 3-D stylus profile measuring instrument retroreflector onto the probe housing near the stylus. With the laser beam parallel to the traverse direction and coincident with the stylus, a series of measurements were performed at a number of points within the traverse range. After each traverse measurement, the platen was incremented a small distance, and the procedure repeated. Based on this set of measurements, a calibration factor of 0.483 pm per pulse was obtained. Clearly, this is well within limits of experimental error. The step axis was also calibrated in a similar fashion, giving a calibration factor of 0.410 pm per pulse from the gauge block map and 0.414 IJm per pulse using the laser interferometer. It is worth noting that this calibration procedure only provides positional information at a few discrete points, and there may be other error components, such as periodicities that have not been observed.
Planar motion errors of the specimen stage Surface profiles measured with this instrument are a direct comparison between the stylus motion and the motion of the traverse mechanism. Consequently, it is necessary that the scanning stage move in a flat plane that is considerably better than the deviations of interest in our measurement. Conventional choices for such a datum's surface are optical flats. However, these flats suffer from some drawbacks; i.e., they are expensive to produce and are hard to fabricate into irregular shapes. The datum for this instrument has been produced by replication. This process involves the use of a replicant material that is applied to a master and then removed. The replica produced then closely approximates the surface of the master. A 150-mm diameter k/4 optical flat was used as a master and Moglice TM, a castable polymer, as the replicant, with polishing wax used as the release agent. For reference, a Shore hardness of 84/85 is quoted by the manufacturers. The desired datum surface was, thus, directly replicated onto the instrument base with a typical thickness of 200 IJm. Measurement of this replicated surface using Fizeau interferometry revealed a correspondingly flat surface with peakto-valley (P-V) variations of less than 0.24 pm over any 20 x 20 mm portion. As a direct assessment of the planar motion of the stylus, an optically flat specimen was mapped, and the resultant deviations of 0.2 pm (P-V) from a perfect plane were found.
Noise Possible noise sources for this system are electronic, air- and ground-borne mechanical vibrations. Mechanical noise is invariably present during scanning. Although not fully characterized as yet, observations during many surface measurements have not shown this to introduce significant additional noise with the present transducer discrimiPRECISION ENGINEERING
nation. Electronic noise has been assessed by contacting the stylus with a stationary steel surface and monitoring the output from the signal conditioning amplifier using a digital spectrum analyzer. The subsequent spectra showed a noise characteristic typical of flicker with approximately -95 dBV at 25 Hz and and increasing to approximately -60dBV at low frequency, indicating an electronic noise level of 4 nm Hz -1/2.
Repeatability To assess repeatability of the complete instrument system, a triangular calibration specimen of 100 pm spatial wavelength and peak-to-valley height of 10 IJm was repeatedly scanned with the incremental step axis drive disconnected. Theoretically, this should result in successive profiles of nominally the same track. Variations of less than 0.1 pm (P-V) were observed for over 40 repeat traverses. As a consequence, a conservative value of 0.2 tJm is chosen as representing the repeatability over the 5-mm traverse along the trace direction. This does not represent repeatability of the step axis traverse. Registration of successive traces can also be inferred from this procedure by examination of lateral shifts of successive profiles. Such shifts in the start points of successive traces are an indication of the repeatability of the opto-interrupter, whereas, variations along the profile are a measure of the encoder drive mechanism. Observations in both cases showed a nonsystematic lateral shift of less than 0.2 pm over 40 traces.
Thermal response Estimating the thermal coefficient of this system is complicated by the different time constants of individual components within the system. However, most of the large mechanical elements have been constructed from aluminum. Because the stylus contacts a flat specimen at a point that is coincident with the plane of contact of the three instrument support legs, under this circumstance, there will be an approximately equal length of materials in the upward and downward parts of the measurement loop. Consequently, if all materials were of the same thermal expansivity, then the instrument would be almost fully compensated. Thermal effects have been assessed by placing the probe in stationary contact with a steel specimen with the complete system housed in a temperature and humidity controlled laboratory. For a typical thermal variation of less than 0.1 K, corresponding height variations of less than 50 nm were observed in the stylus output over a period of 40 min. Placing the instrument in an environmental chamber and subjecting it to a rapid step change of 10 K resulted initially in a negative correlation, w h i c h then changed to a slow positive response characteristic of a first-order system having a time constant of 153
Badami et al.: A portable 3-D stylus profile measuring instrument approximately 55 rain. Assuming a steady-state linear relationship between output displacement and temperature, this gives an effective thermal coefficient of approximately 0.27 pm K-1.
tion, there was a correspondingly sharp reduction at a frequency of 27 Hz indicative of a distinct "bouncing" frequency. Results
Dynamic characteristics The objective of this design was that the instrument be capable of performing in an industrial environment. Clearly, under these circumstances, vibrations are particularly troublesome. To assess the system susceptibility, the complete instrument was mounted onto a Bruel & Kjaer 4808 vibration exciter. With the stylus free from contact, a secondorder response is observed at the output with a resonance frequency of 49 Hz and a damping ratio of 0.006. Assuming that this resonance represents a simple spring/mass system, measurement of the mass of the probe components provides an estimation of the spring coefficient. To comply with established standards, the flexure was designed using simple beam theory to have a stiffness of 35 N m -1, which was in close agreement with the experimental value of 34.6 N m -1 derived from the observed resonance and the combined mass of the springs/LVDT core of 0.365 gm. With the stylus in contact with a steel specimen, the resonant frequency shifted to above 100 Hz and outside of the instrument bandwidth. However, for large amplitudes of vibration, motion of the complete instrument was observed. Looking at the coherence func-
In this section, a number of area maps are shown to illustrate the measurement capability of this instrument. A typical 5 x 5 mm scan is obtained with a stylus traverse speed of 0.5 m m s -1, and 200 profiles are taken with 500 datapoints per profile. This leads to a scan time of approximately one hour. Figure 9 shows a 4.5 x 5.5 mm area measured on a one-cent coin and showing the face of Abraham Lincoln (maximum vertical feature size 100 pro). Although this does not include the full vertical range of the probe, it provides an easily appreciated demonstration of the mapping capability of this instrument. Figure 10shows a similar area of a rough milled surface (4.08 pm Ra). This shows both surface finish and longer range profile information that could be used for geometric and surface finish evaluation. Figure 11 shows a t o p o g r a p h i c map of a ground aluminum surface. Both the "lay" of this surface and finer submicrometer scale features can be clearly distinguished. Finally, Figure 12 shows a 2 x 2 mm map of a print roller. Both the size and shape of the individual pits in this roller have a key functional role in control of print quality. This illustrates the ability to
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Area map of Abraham Lincoln on a one-cent piece APRIL/MAY 1996 VOL 18 NO 2/3
Badami et al.: A portable 3-D stylus profile measuring instrument
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perform volumetric calculations on surfaces at scales hitherto unachievable using atomic force microscopy (AFM) profiling. Other artifacts, such as an NIST SRM 2071 sinusoidal roughness specimen, a machinists rule, a specially fabricated miniature ball plate, and a variety of other common engineering surfaces have been profiled. The range and scale of 3-D surface measurements presented in this paper represent a consid-
erable increase over that presently available with commercial microscope systems. Although some commercial profilers with mapping capability do exist, they tend to be relatively expensive and do not have the portability of our instrument. There is still some scope for improvement of this current prototype. In particular, the precision of the current probe sensor is relatively low in comparison to some modern electronic circuits; profiling speed still results in scan times of tens of minutes; and the
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Area map of a print roller
large a m o u n t of data leads to relatively long processing times with the present computer system. Other aspects of this system, such as the determination of the point of contact, specimen leveling, and stylus replacement, are currently performed manually and w o u l d require some development for commercial exploitation. These relatively straightforward advances are more than offset by difficulties associated with calibration of this device in this relatively small 3-D space. In many ways, this instrument, and modern probe microscopes, may be considered equivalent to a scaled-down coordinate measuring machine. Normally, these are calibrated using laser interferometers, ball bars, or ball plates. However, such calibration tools are not available for these smaller volumes. Other techniques, such as the measurement of gratings and steps, provide information relating linear axis calibration but do
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not provide information on errors caused by rotations and coupling between axes. The volumetric calibration of 3-D metrology systems has not received due attention and is likely to be a key enabling technology in this growing field. References
1 Sayles,R. S. and Thomas,T. R. "Mapping of a small area of surface," J Phys E: Sci Instrum 1976, 9, 855-861 2 Teague, E. C., Scire, F. E., Baker, S. M., and Jensen, S. W. "Three-dimensional stylus profilometry," Wear 1982, 83, 1-12 3 Morrison,E. "A prototype scanning stylus profilometer for rapid measurement of small surface areas," Int J Mach Tools Manufact 1995, 35, 325-331 4 Xu, Y., Smith, S. T., Atherton, P. D., Judge, T., and Jones, R. "A metrological scanning force microscope," Prec Eng, 1996, 19, in press
APRIL/MAY 1996 VOL 18 NO 2/3
A portable three-dimensional stylus profile measuring instrument V. G. Badami, S. T. Smith, J. Raja, and R. J. Hocken Precision Engineering Laboratory, University of North Carolina, Charlotte, NC, USA This paper describes the development o f an inexpensive, portable threedimensional (3-D) stylus-based surface profiler with a scan range o f 4.5 m m x 5.5 m m and a m a x i m u m vertical range of 150 p m (limited by signal conditioning electronic range) with a resolution o f better than O. 1 p m and a range o f 30 p m with a resolution o f better than 30 nm at the highest vertical sensitivity. The instrument occupies a volume with dimensions o f approximately 75 × 75 x 40 m m (L x B x H) with scan speeds up to 0.5 m m s -~. Construction o f instrument components from similar material results in a thermally balanced design with a reasonably l o w thermal coefficient of 0.27 p m K -7 and a first-order time constant o f approximately 40 min. The stylus probe sensor has a free resonant frequency o f 49 Hz and a l o w damping ratio o f 0.006. When in stationary contact with a steel surface, this increases to above the transducer bandwidth o f 100 Hz. Calibration o f the probe sensor is achieved through direct comparison against a standard stylus gauge. Lateral calibration o f the specimen carriage position has been assessed by the measurement o f standard gratings and laser interferometry. Planar errors caused by the motion o f the carriage have been assessed by measuring an optical flat and inferring deviation from a perfect plane as an indication of the worstcase error, which in this case, was 0.2 p m (P-V). The design and construction o f the internal datum and the portability o f the instrument to facilitate in situ measurement of components are emphasized. Images illustrating the surface mapping capabilities o f the profiler are presented.
Introduction The effects of surface finish on the performance of many engineering mechanisms are now established across a broad range of disciplines. Stylus profilers are still the main method for quantitative measurement of the surfaces generated by engineering processes. Traditionally, surface finish parameters computed from stylus measurements have been based on a relatively short portion of the profile. Consequently, low cost, portable stylus measurement instruments presently being marketed derive their dimensional stability and surface datum information by using a skid referenced directly to the surface under examination and are limited to line (two-dimensional, 2-D) traces. HowAddress reprint requests to Mr. V. G. Badarni, Precision Engineering Laboratory, University of North Carolina at Charlotte, Highway 49, Charlotte, NC 28223, USA.
Precision Engineering 18:147-156, 1996 © Elsevier Science Inc., 1996 655 Avenue of the Americas, New York, NY 10010
ever, it is increasingly being recognized that longer range features of the measured profiles can provide additional information relating to functional characteristics of the device or the process used for surface generation. In particular, three-dimensional (3-D) topographic maps enable both visual images and the parametric analysis of areal distributions and direct volume measurements. Many methods exist for the 3-D topographic measurement of surfaces such as optical, large laboratory stylus profiling instruments, and, more recently, a variety of scanning probe microscopes employing a wide range of surface proximity sensing techniques. Invariably, the former two of these methods consist of large and costly instrumentation that is neither readily portable nor robust enough for typical industrial environments. 1'2 A portable 3-D stylus has been reported, although this is limited to a scan area of 0.5-mm square. 3
0141-6359/96/$15.00 SSDI 0141-6359(95)00071-2
Badami et al.: A portable 3-D stylus profile measuring instrument Scanning probe microscopes, on the other hand, invariably are limited to small scan areas and, in the absence of expensive closed-loop monitoring, suffer from a lack of a well-defined datum. 4 Many engineering surfaces contain features of interest that have dimensions considerably larger than the scan range typical of probe microscopes and have reflectivities not suitable for optical inspection. Additionally, it is not always desirable or, sometimes, possible to transport the component to the laboratory for measurement purposes. In principle, the ability to measure a series of parallel profiles referenced to a common datum increases the capability of the stylus instrument from 2-D profiles to 3-D surface maps. For this reason, a novel, inexpensive, portable stylus profiler capable of relatively large area scans has been developed and is presented in this paper. The resultant system consists of a simple flexure-supported stylus attached to a scanning stage that slides upon a flat datum surface. To obtain a topographic map, the stylus is contacted with a specimen and subsequently raster scanned. Data are recorded at equidistant points and stored for subsequent display and analysis. A block diagram representation of this system is shown in Figure 1. Elements of the mechanical design, such as the probe, scanning stage, drives, datum reference, mounting adjustment, and system monitoring and control, are outlined below. This is followed by a performance evaluation and topographic maps to illustrate the range of features that can be measured quantitatively with such a system.
Design An exploded diagram of the complete profiler is shown in Figure 2. Key components of this system are separated to reveal key interfaces. Conceptually, the stylus profiler consists of a main platen (at the top of the diagram) that is free to slide on a flat datum surface. This has been produced by replication from a glass optical fiat using
CO
DATUM 1
+OLCO+OR
AND DATA ACQUISITION ELECTRONICS
/ I PICK-UP
~j
DRIVES &L
r~P.D~CK I~
t CONDITIONLNG SIGNAL
SPECM I E~
-
-
Figure 1 Block diagram representation of the complete system 148
Moglice TM, a castable polymer. It is constrained to translate in two orthogonal directions by the surrounding frames (the main frame and the intermediate frame). Linear constraints are imposed by the two guide bars that are supported in the intermediate frame and are free to rotate about their longitudinal axis. The platen rests on the datum surface, which provides the vertical constraint. The two guide bars run along slots milled into its underside (Figure 3). Two polymer bearings (Delrin TM) are located on the side wall of one of the milled slots and slide against the guide bar. To maintain contact between the adjacent guide bar and bearings, a preload mechanism is employed. This consists of a spring-loaded preload arm that imposes the closure force on the second guide bar, again by means of a polymer bearing. Rotation of the guide bars imposes one linear and one rotary constraint, while providing four freedoms necessary for decoupling error motion of the frame from the platen (i.e., allowing constraint in the vertical axis only by the datum surface). The s e c o n d , o r t h o g o n a l , t r a n s l a t i o n is achieved using a similar mechanism. Two further guide bars are supported in the main instrument frame, and these are perpendicular to those in the intermediate frame. These guide bars serve to constrain the intermediate frame as it is translated along the second axis. Five polymer bearings are affixed on the underside of the intermediate frame to form a vee-groove and flat slideway arrangement providing one linear degree of freedom (Fig-
ure 4). The probe is attached to the bottom surface of the platen (Figure 2) and protrudes from the underside of the instrument through a hole in the datum surface. To achieve a relatively long vertical range of 1 mm while maintaining a compact overall size, a specially designed stylus probe was produced. This probe consists of a vertical rod supported by two beryllium copper flexures. These flexures were produced using chemical etching techniques common to printed circuit board manufacture. For clarity, this is also shown in an exploded view in Figure 5. A stylus probe has been bonded to the bottom end of the rod, and a magnetically permeable core is attached between the flexures to form the core of an LVDT displacement transducer. The complete transducer assembly is housed in a compact body that provides mounting holes for attaching to the platen. Translation of the carriage in the two axes is achieved by two wire drives powered by DC motor/ gearbox units (Figure 6). Each wire drive consists of a drive pulley and one or more idlers with a tensioning spring. The drive to the platen is arranged to be close to the centers of mass and friction and is colinear with the rotating guide bars. The driving force is transmitted by a cotton thread (thin sewing thread) that wraps around a drive pulley supported
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Badami et al.: A portable 3-D stylus profile measuring instrument
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by bearings in the intermediate frame. This is, in turn, powered by a drive shaft supported in the main instrument frame through a spline. The spline is needed to accommodate the translation of the intermediate frame in the other axis. An exploded view of the drive shaft and spline for this axis can be seen in Figure 2. In principle, the motor could be directly connected to this shaft. To retain the compact nature of the design, it is necessary, in practice, to drive this shaft using another pulley system that enables the motor/gearbox drive to be embedded in the base. The second drive axis is used to step the platen/intermediate frame combination after each scan. This is a relatively straightforward
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design that is connected to the intermediate frame near to the axis of the four pad vee-bearing and is driven by a second motor/gearbox combination (not shown). Position has been inferred from rotary encoders mounted directly onto the motor shafts of each drive. The zero, or start position, of each trace is established by a trigger pulse produced by an optoelectronic switch (opto-interrupter). This is generated by occlusion of an optical slit on the intermediate frame by a blade mounted on the platen (Figure 2). A photograph of the complete instrument is shown in Figure 7. A typical measurement cycle begins with the stylus being brought into contact with the surface
149
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using three mounting screws attached to the main instrument frame. After adjustment, these can be locked in position for additional stability, although, in practice, this has not been found to be necessary. The platen is then moved to the desired position beyond the opto-interrupter, and a constant motor voltage is applied to the traverse drive. Registration of successive profiles is ensured by start-
ing data acquisition upon receipt of each encoder pulse, or successive multiples thereof, thus making data sampling immune to changes in motor speed. By returning the platen sufficiently byfar beyond the trigger position and noting that data are collected only in one traverse direction, transient effects and backlash were not observed. Presently, each encoder pulse corresponds to a traverse distance of
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Bottom view of intermediate frame APRIL/MAY 1996 VOL 18 NO 2/3
Badami et al.: A portable 3-D stylus profile measuring instrument
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translation DT2821 acquisition board under personal computer control. To reduce problems with memory, each trace is downloaded to hard disk storage.
Performance evaluation
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Ideally, such an instrument should be capable of scanning the stylus probe in a perfectly planar space while acquiring data. In reality, error sources such as noise, repeatability, and drifts preclude such a goal. To ascertain the performance limits of this instrument, various components of the system have been assessed.
Probe calibration
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0.5 pro, giving a maximum sample size of 10,000 data points per profile. The minimum step in the other axis is 10 IJm, giving a maximum of 500 traces. At the end of each trace, the platen is returned to the start position, incremented in the orthogonal axis, and the process is repeated until the desired area scan is complete. Sample spacing and step increment can be specified to the computer control program. Data are sampled using a data
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Traverse calibration Calibration of these axes proved to be more of a challenge than anticipated. The main reason for this was the lack of miniature, lightweight optics for laser interferometric evaluation or any alternative noninfluencing gauging with the appropriate dynamic range. Several complementary techniques were used to evaluate the relationship between the encoder output and stylus position. Results from three different approaches were evaluated and compared:
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This is calibrated by comparison with a reference LVDT sensor by placing the probes of both on the anvil of a differential micrometer (0.1 IJm resolution). To reduce the effects of Abbe offset errors, these are positioned so that they are nearly coincident on the anvil with typical separation of less than 0.5 ram. A typical calibration curve is obtained by manually adjusting the micrometer while monitoring both sensors simultaneously. From such measurements, a high degree of linearity is observed with a maximum deviation of less than 0.5% over a full range of 150 IJm. Presently, the probe range is limited by the signal conditioning electronics with lock-in amplifier saturation corresponding to a displacement of the stylus of 150 pm and with data acquisition single converter bit resolution of 66 nm when set at this minimum sensitivity. In principle, if lower sensitivities were available, it should be possible to monitor stylus output for displacements of up to 1 ram. Higher lock-in amplifier sensitivity settings are possible, and the finest resolution used with this system gave a full vertical range of 30 pm with a noise-limited resolution of approximately 30 nm.
1. direct calculation of effective resolution based on gearbox and pulley reduction; 2. calibration based on profile data of an artifact of known geometry; and 3. laser interferometric calibration of separate platen traversals. Actuation
of the p u l l e y s in each axis is 151
Badami et al.: A portable 3-D stylus profile measuring instrument
Figure 7
Photograph of complete instrument
achieved by a DC motor/gearbox combination with the thread wound around a 4-ram diameter output shaft. Considering a gearbox ratio for each drive of 2,190:1, an encoder outputs 16 pulses per revolution, and, with a secondary reduction pulley introducing a further reduction of 1.4:1, a traverse displacement of 0.5 pm per pulse is predicted. Clearly, effects of slip and drive compliances are likely to result in a slight reduction of this value, thus necessitating further calibration. In an effort to obtain a direct calibration of the stylus translation, an artifact of known geometry was sought. To achieve this, a series of gauge blocks were wrung together, and, to observe the geometry of the interface between blocks, the sides of the assembly were profiled using a Rank-TaylorHobson Form Talysurf 120L. Traverse position for
this instrument is provided by an optical grating scale. An area scan of this assembly was then taken, and corresponding distances were used to infer relative displacements. A typical area map for a set of four gauge blocks is shown in Figure 8. Clearly, the exact position of the interface is ambiguous. However, it is possible to identify features that are caused by the transition from the standard coarse finish of the nonfunctional surfaces to the radiused edge leading to the interface. These values have been obtained by visually aligning cursors at these positions and then reading the horizontal scale between them. It is felt that this will provide a consistent scale to within approximately 2%. From such a procedure, a calibration factor of 0.485 pm per pulse was determined. Further confirmation was obtained by mounting a miniature
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Badami et al.: A portable 3-D stylus profile measuring instrument retroreflector onto the probe housing near the stylus. With the laser beam parallel to the traverse direction and coincident with the stylus, a series of measurements were performed at a number of points within the traverse range. After each traverse measurement, the platen was incremented a small distance, and the procedure repeated. Based on this set of measurements, a calibration factor of 0.483 pm per pulse was obtained. Clearly, this is well within limits of experimental error. The step axis was also calibrated in a similar fashion, giving a calibration factor of 0.410 pm per pulse from the gauge block map and 0.414 IJm per pulse using the laser interferometer. It is worth noting that this calibration procedure only provides positional information at a few discrete points, and there may be other error components, such as periodicities that have not been observed.
Planar motion errors of the specimen stage Surface profiles measured with this instrument are a direct comparison between the stylus motion and the motion of the traverse mechanism. Consequently, it is necessary that the scanning stage move in a flat plane that is considerably better than the deviations of interest in our measurement. Conventional choices for such a datum's surface are optical flats. However, these flats suffer from some drawbacks; i.e., they are expensive to produce and are hard to fabricate into irregular shapes. The datum for this instrument has been produced by replication. This process involves the use of a replicant material that is applied to a master and then removed. The replica produced then closely approximates the surface of the master. A 150-mm diameter k/4 optical flat was used as a master and Moglice TM, a castable polymer, as the replicant, with polishing wax used as the release agent. For reference, a Shore hardness of 84/85 is quoted by the manufacturers. The desired datum surface was, thus, directly replicated onto the instrument base with a typical thickness of 200 IJm. Measurement of this replicated surface using Fizeau interferometry revealed a correspondingly flat surface with peakto-valley (P-V) variations of less than 0.24 pm over any 20 x 20 mm portion. As a direct assessment of the planar motion of the stylus, an optically flat specimen was mapped, and the resultant deviations of 0.2 pm (P-V) from a perfect plane were found.
Noise Possible noise sources for this system are electronic, air- and ground-borne mechanical vibrations. Mechanical noise is invariably present during scanning. Although not fully characterized as yet, observations during many surface measurements have not shown this to introduce significant additional noise with the present transducer discrimiPRECISION ENGINEERING
nation. Electronic noise has been assessed by contacting the stylus with a stationary steel surface and monitoring the output from the signal conditioning amplifier using a digital spectrum analyzer. The subsequent spectra showed a noise characteristic typical of flicker with approximately -95 dBV at 25 Hz and and increasing to approximately -60dBV at low frequency, indicating an electronic noise level of 4 nm Hz -1/2.
Repeatability To assess repeatability of the complete instrument system, a triangular calibration specimen of 100 pm spatial wavelength and peak-to-valley height of 10 IJm was repeatedly scanned with the incremental step axis drive disconnected. Theoretically, this should result in successive profiles of nominally the same track. Variations of less than 0.1 pm (P-V) were observed for over 40 repeat traverses. As a consequence, a conservative value of 0.2 tJm is chosen as representing the repeatability over the 5-mm traverse along the trace direction. This does not represent repeatability of the step axis traverse. Registration of successive traces can also be inferred from this procedure by examination of lateral shifts of successive profiles. Such shifts in the start points of successive traces are an indication of the repeatability of the opto-interrupter, whereas, variations along the profile are a measure of the encoder drive mechanism. Observations in both cases showed a nonsystematic lateral shift of less than 0.2 pm over 40 traces.
Thermal response Estimating the thermal coefficient of this system is complicated by the different time constants of individual components within the system. However, most of the large mechanical elements have been constructed from aluminum. Because the stylus contacts a flat specimen at a point that is coincident with the plane of contact of the three instrument support legs, under this circumstance, there will be an approximately equal length of materials in the upward and downward parts of the measurement loop. Consequently, if all materials were of the same thermal expansivity, then the instrument would be almost fully compensated. Thermal effects have been assessed by placing the probe in stationary contact with a steel specimen with the complete system housed in a temperature and humidity controlled laboratory. For a typical thermal variation of less than 0.1 K, corresponding height variations of less than 50 nm were observed in the stylus output over a period of 40 min. Placing the instrument in an environmental chamber and subjecting it to a rapid step change of 10 K resulted initially in a negative correlation, w h i c h then changed to a slow positive response characteristic of a first-order system having a time constant of 153
Badami et al.: A portable 3-D stylus profile measuring instrument approximately 55 rain. Assuming a steady-state linear relationship between output displacement and temperature, this gives an effective thermal coefficient of approximately 0.27 pm K-1.
tion, there was a correspondingly sharp reduction at a frequency of 27 Hz indicative of a distinct "bouncing" frequency. Results
Dynamic characteristics The objective of this design was that the instrument be capable of performing in an industrial environment. Clearly, under these circumstances, vibrations are particularly troublesome. To assess the system susceptibility, the complete instrument was mounted onto a Bruel & Kjaer 4808 vibration exciter. With the stylus free from contact, a secondorder response is observed at the output with a resonance frequency of 49 Hz and a damping ratio of 0.006. Assuming that this resonance represents a simple spring/mass system, measurement of the mass of the probe components provides an estimation of the spring coefficient. To comply with established standards, the flexure was designed using simple beam theory to have a stiffness of 35 N m -1, which was in close agreement with the experimental value of 34.6 N m -1 derived from the observed resonance and the combined mass of the springs/LVDT core of 0.365 gm. With the stylus in contact with a steel specimen, the resonant frequency shifted to above 100 Hz and outside of the instrument bandwidth. However, for large amplitudes of vibration, motion of the complete instrument was observed. Looking at the coherence func-
In this section, a number of area maps are shown to illustrate the measurement capability of this instrument. A typical 5 x 5 mm scan is obtained with a stylus traverse speed of 0.5 m m s -1, and 200 profiles are taken with 500 datapoints per profile. This leads to a scan time of approximately one hour. Figure 9 shows a 4.5 x 5.5 mm area measured on a one-cent coin and showing the face of Abraham Lincoln (maximum vertical feature size 100 pro). Although this does not include the full vertical range of the probe, it provides an easily appreciated demonstration of the mapping capability of this instrument. Figure 10shows a similar area of a rough milled surface (4.08 pm Ra). This shows both surface finish and longer range profile information that could be used for geometric and surface finish evaluation. Figure 11 shows a t o p o g r a p h i c map of a ground aluminum surface. Both the "lay" of this surface and finer submicrometer scale features can be clearly distinguished. Finally, Figure 12 shows a 2 x 2 mm map of a print roller. Both the size and shape of the individual pits in this roller have a key functional role in control of print quality. This illustrates the ability to
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Area map of a rough milled surface
perform volumetric calculations on surfaces at scales hitherto unachievable using atomic force microscopy (AFM) profiling. Other artifacts, such as an NIST SRM 2071 sinusoidal roughness specimen, a machinists rule, a specially fabricated miniature ball plate, and a variety of other common engineering surfaces have been profiled. The range and scale of 3-D surface measurements presented in this paper represent a consid-
erable increase over that presently available with commercial microscope systems. Although some commercial profilers with mapping capability do exist, they tend to be relatively expensive and do not have the portability of our instrument. There is still some scope for improvement of this current prototype. In particular, the precision of the current probe sensor is relatively low in comparison to some modern electronic circuits; profiling speed still results in scan times of tens of minutes; and the
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Area map of a fine ground surface
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B a d a m i et al.: A p o r t a b l e 3-D stylus p r o f i l e m e a s u r i n g i n s t r u m e n t
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Area map of a print roller
large a m o u n t of data leads to relatively long processing times with the present computer system. Other aspects of this system, such as the determination of the point of contact, specimen leveling, and stylus replacement, are currently performed manually and w o u l d require some development for commercial exploitation. These relatively straightforward advances are more than offset by difficulties associated with calibration of this device in this relatively small 3-D space. In many ways, this instrument, and modern probe microscopes, may be considered equivalent to a scaled-down coordinate measuring machine. Normally, these are calibrated using laser interferometers, ball bars, or ball plates. However, such calibration tools are not available for these smaller volumes. Other techniques, such as the measurement of gratings and steps, provide information relating linear axis calibration but do
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not provide information on errors caused by rotations and coupling between axes. The volumetric calibration of 3-D metrology systems has not received due attention and is likely to be a key enabling technology in this growing field. References
1 Sayles,R. S. and Thomas,T. R. "Mapping of a small area of surface," J Phys E: Sci Instrum 1976, 9, 855-861 2 Teague, E. C., Scire, F. E., Baker, S. M., and Jensen, S. W. "Three-dimensional stylus profilometry," Wear 1982, 83, 1-12 3 Morrison,E. "A prototype scanning stylus profilometer for rapid measurement of small surface areas," Int J Mach Tools Manufact 1995, 35, 325-331 4 Xu, Y., Smith, S. T., Atherton, P. D., Judge, T., and Jones, R. "A metrological scanning force microscope," Prec Eng, 1996, 19, in press
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