- Email: [email protected]
tape spacing measurement
Tribology International 33 (2000) 409–414 www.elsevier.com/locate/triboint
Simultaneous five-wavelength interferometry for head/tape spacing measurement J. Zhu a, E. Baugh b, F.E. Talke a b
a,*
CMRR, University of California, San Diego, La Jolla, CA 92093-0401, USA IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA
Abstract An improved five-wavelength interferometer with high-speed shutters in the light path was designed and implemented. The interferometer allows switching between two sets of three wavelengths, keeping one wavelength in each measurement in common. The set-up allows nearly simultaneous acquisition of fringe intensities and can be used to measure the head/tape spacing in a moving linear tape drive. The precision of the new five-wavelength interferometer was investigated and was found to be superior to the precision obtained with a three-wavelength interferometer. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Interferomertry; Head/tape interface; Tribology
1. Introduction Interferometry is widely used for the measurement of the head/medium spacing [1]. In the mid-1970s the spacing of a typical head/tape interface was on the order of 0.5 µm to 1.0 µm, and frequency-based white light interferometry was commonly used to measure head/medium separation [2]. However, as the head/tape spacing decreases, white light interferometry becomes inadequate for the measurement of spacing below 100 nm since at those spacings only various shades of grey exist. To measure spacings in the sub-100 nm range, intensitybased monochromatic interferometry must be used [3]. Since the spacing measurements using a single wavelength are multi-valued, it is necessary to use more than one wavelength for the unique determination of the head/medium spacing in the region below 100 nm. Although three-wavelength interferometry has been shown to be successful for very small spacing measurements, both experimental evidence and Monte Carlo analysis suggest that the measurement precision can be further improved by using more than three wavelengths [4]. Wahl et al. [5] developed a technique in which five wavelengths were used in sequence to measure the spac-
* Corresponding author. Tel.: +1-858-534-3646. E-mail address: [email protected] (F.E. Talke).
ing at the flexible disk/head interface. In Wahl’s work, a filter was inserted manually for each individual wavelength in order to measure the fringe pattern for each wavelength in sequence. Sequential acquisition of fringe intensities is time-consuming, and cannot be used for interfaces that change dynamically. An improved sequential interferometer was developed by Baugh [4] who used two consecutive measurements to switch two sets of three wavelengths manually. Although the switching of two sets of three wavelengths is faster than the switching of five individual wavelengths, the time for two consecutive measurements is still large. In fact, the typical time for a spacing measurement using Baugh’s method is on the order of at least sixty seconds and the time is even larger for five sequential measurements as done by Wahl. Thus, neither one of the above five wavelength interferometers can be used to measure the dynamic head/tape spacing in a moving tape drive, i.e., only steady-state head/tape interfaces can be investigated. In this paper, a five-wavelength interferometer was developed which allows the measurement of the head/tape spacing in a moving tape drive. Nearly simultaneous data acquisition of the fringe pattern from the five wavelengths was implemented with a single 3–CCD (charge-coupled device) camera using high-speed shutters. The five wavelengths were grouped into two sets of three wavelengths with one wavelength in common.
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J. Zhu et al. / Tribology International 33 (2000) 409–414
The two sets of three wavelengths were switched by two sets of high-speed shutters. During a measurement, data are first acquired for the initial three wavelengths, and then a second set of three different wavelengths is switched into the light path. By synchronizing the shutter and data acquisition with the video signal, two sets of data are acquired in the odd and even fields of the NTSC (National Television System Committee) video, respectively. A typical measurement of the image pattern of the five-wavelength interferometer takes about 35 ms. This time is much shorter than the typical time of 60 seconds of the Wahl [5] and Baugh [4] interferometers. In this paper, we first measure the head/tape spacing in a linear tape drive using the newly developed five-wavelength interferometer. Thereafter, measurements from the threewavelength interferometer and the five-wavelength interferometer are compared in terms of the standard deviation of the spacing. A marked improvement in the precision of the five-wavelength interferometer is observed compared to the three-wavelength interferometer. 2. Theory of interferometry The principle of monochromatic interferometry is shown in Fig. 1. Here, two wave trains of different phase recombine and interfere with each other after having been reflected from the bottom side of the glass head and the top side of the magnetic tape. As a result, the light intensity is modulated according to the phase difference between the two wave trains. A schematic of the light reflection at the head/tape interface is shown in Fig. 1b. The light intensity I(x,y) of a fringe is related to the spacing h(x,y), by I(x,y)⫽A(x,y)⫹B(x,y) cos
冉
冊
4ph(x,y) ⫺f l
(1)
where A(x,y), B(x,y) are constant for any single point on the head/tape interface, but vary with position, l is the wavelength, h(x,y) is the spacing and f is the phase shift. The intensity is directly measured, while l is known from the wavelength of the interference filter used. The phase shift f is inherent upon reflection from the magnetic tape and can be measured, for instance, by ellipsometer. The intensity-spacing relationship of Eq. (1) is plotted in Fig. 2. In order to solve the spacing from Eq. (1), A(x,y) and B(x,y) must be known over the entire head/tape interface. If the maximum and minimum fringe intensity Imax(x,y) and Imin(x,y) are known, the values for A(x,y) and B(x,y) can be found from A(x,y)⫽
Imax(x,y)+Imin(x,y) 2
B(x,y)⫽
Imax(x,y)−Imin(x,y) 2
Fig. 1. (a) Principle of interferometry; (b) Typical fringe pattern at head/tape interface.
(2)
Fig. 2.
Principle of envelope calibration.
The values of Imax(x,y) and Imin(x,y) are known as the calibration envelopes. An unloading method of obtaining the calibration envelopes was developed by Lacey and Talke [6]. By tilting the tape in the drive using an unloading rod, the fringe pattern is shifted across the
J. Zhu et al. / Tribology International 33 (2000) 409–414
interface. Lacey and Talke used this unloading method to record the maximum and minimum intensities at each position along the interface when various fringes pass across the interface. These experimental envelopes then provide the A(x,y) and B(x,y) values needed for Eq. (1). To improve monochromatic interferometry, polychromatic or multi-wavelength interferometry can be used. The benefits include redundancy at low spacings, robustness at fringe order changes, and unique determination of spacing by adjusting the fringe orders of the wavelengths used. If multiple wavelengths are used, the intensity-spacing relation can be written as Ii(x,y)⫽Ai(x,y)⫹Bi(x,y) cos
冉
冊
4phi(x,y) ⫺fi l
(3)
where the subscript i indicates each individual wavelength. Ai(x,y) and Bi(x,y) can be determined by detecting the calibration envelopes using the unloading technique. Inverting Eq. (3) and assuming that only the zero order fringe is of concern, the spacing value hi(x,y) can be written as
冋 冉
冊 册
A(x,y)−I(x,y) l hi(x,y)⫽ cos−1 ⫺p⫹f 4p B(x,y)
(4)
With the calibration envelopes determined, the spacing can be calculated for each wavelength as if i independent monochromatic measurements were done. On the other hand, when making any measurement, it is desirable to have a high sensitivity between the intensity Ii(x,y) and spacing hi(x,y). The sensitivity is defined as ∂Ii(x,y) the normalized derivative of Eq. (3) given by ∂hi(x,y)
| 冉
wi(x,y)⫽ sin
4phi(x,y) ⫺f li
冊|
(5)
A plot of intensity and sensitivity as a function of spacing for three wavelengths is shown in Fig. 3. As can be seen in Fig. 3, when the sensitivity of one wavelength goes to zero at the intensity peak, the other wavelengths still have good sensitivity. Using Eq. (5) as a weighting factor, the spacing corresponding to each wavelength can be combined into a best value using a weighted average
Fig. 3.
Light intensity and sensitivity as a function of spacing.
冘
411
wi(x,y)hi(x,y)
h(x,y)⫽
冘
(6) wi(x,y)
The weighted average spacing has the effect of basing the final spacing measurement h(x,y) on whichever wavelengths have the highest sensitivity at any particular spacing value.
3. Experimental setup In order to perform simultaneous five-wavelength interferometry, illumination with five specific wavelengths must be provided. The five-wavelength illumination is obtained by coupling the output of five separate 150 W white-light sources, each with a different interference filter. The five wavelengths used in the experiment are denoted by R (620 nm), G (535 nm), B (450 nm), R* (650 nm) and G* (550 nm), with R, G and B representing red, green and blue. A 3–CCD video camera is used to image the head/tape interface. Because a 3–CCD camera can acquire only three wavelengths simultaneously, in order to implement five-wavelength interferometry, one must use either two 3–CCD cameras simultaneously or switch one 3–CCD camera after the first three wavelengths have been acquired. Using two cameras, one has to align both cameras so that they are directed at exactly the same position. This is difficult to achieve. Furthermore, it is not easy to separate the five wavelengths used into the two 3–CCD cameras. Thus, the second approach of using one camera which is switched is a preferable option and is implemented in this paper. A NTSC video frame obtained with a 3–CCD camera consists of “odd” and “even” fields, with the even field being recorded after the odd field. Between the two fields, there is a blank interval of 1.3 ms, called the video vertical interval. If the five wavelengths are grouped into two sets of RGB’s with one wavelength in common, and the two sets of RGB’s are stored in the odd and even fields respectively, all five wavelengths can be acquired simultaneously within a single video frame. To implement this approach, high speed shutters must be used to switch within 1.3 ms from the first set of three wavelengths to the second set of three wavelengths. Fig. 4 shows the schematic of the simultaneous five-wavelength interferometer. The five wavelengths were grouped into two sets of three wavelengths, i.e., W1 (RGB) and W2 (R*G*B). One wavelength (B) is common to both sets. Here we define W1 as the odd set and W2 as the even set. Four shutters, two for each set, were used to open and close R, G, R* and G*. Together with the common wavelength of B, this gives the two sets of wavelengths W1 and W2, respectively. In the following, the two sets of shutters
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J. Zhu et al. / Tribology International 33 (2000) 409–414
Fig. 4.
Schematic of simultaneous five-wavelength interferometer.
are described as the odd and even set with the odd set controlling R and G, and the even set controlling R* and G*. The shutters operate during the blank interval between the odd and even field. A custom-made 5-in-1 fiber optic cable is used to couple the five wavelengths at the light source end and guide the light into the microscope. The light goes into the microscope and is directed onto the head/tape interface. The images of the two sets of three wavelengths are reflected consecutively into the odd and even fields of the 3–CCD camera. Three 12-bit A/D boards which are installed in a PC digitize the images. One board is assigned to each of the three output colours of the camera. A software package was developed to capture the fringe pattern, read the image data from the A/D board and then process it. A trigger circuit was fabricated to synchronize the data acquisition and shutter operation. Data acquisition is triggered by a two-stage process: First, a transition signal is sent from the parallel port of the PC to the trigger circuit. The trigger circuit then monitors the video signal from the camera and waits until a new frame begins; the trigger circuit then issues a trigger signal to the master data acquisition board. Because the shutters are initially set with the odd set open and the even set closed, the odd set of three wavelengths is first exposed and acquired in the odd field. Once the odd field ends, the trigger circuit activates the shutters to cause the odd set of shutters to close and the even set to open. The even set of three wavelengths is then exposed and acquired in the even field. Thus, all five wavelengths are acquired in a single frame, with the odd set of three wavelengths in the odd field and the even set in the even field. After completion of the data acquisition, the shutters are reset and the data acquisition goes to the next cycle. The process is automated and controlled by software.
4. Simultaneous spacing measurement in a tape drive To measure spacing at the head/tape interface with the tape moving, data acquisition of all five wavelengths must be done simultaneously. In this study, we have used a DLT4000 tape drive to measure the head/tape spacing while the tape is moving. As is usual for interferometric measurements, the head in the DLT4000 drive was replaced by a glass head. A prism was mounted next to the glass head to direct the light into the head/tape interface and to reflect the interface image back into the microscope. On the top side of the tape drive, a load/unload mechanism is mounted to move the fringes. Standard DLT4 tape was used. In order to measure the head/tape spacing, fringe calibration must first be performed. Because the head has a curvature, the envelopes must be detected over the entire interface. This was done by recording the fringe images while moving the fringes across the interface when the tape drive is running. Fig. 5a shows one of the calibration fringe images (odd field in this case) while Fig. 5b shows the envelopes for a wavelength of 535 nm together with an intensity scan of the moving finge pattern of Fig. 5a. The following points were considered to optimize the envelope detection process: (1) The time delay between the shutter operation and the data acquisition of the A/D boards was optimized, i.e., the time delay needed to gurantee that data acquisition is completed and that all shutters (which have slightly different charging time) operate synchronously; (2) The number of image lines to be acquired for calibration was reduced to as few as possible to shorten the data acquisition time. In this way, the recording density of the images was increased to obtain smooth envelopes. Spacing was mea-
J. Zhu et al. / Tribology International 33 (2000) 409–414
413
Fig. 6. Spacing map of typical area of 200×200 pixels (DLT4 tape, speed: 2.5 ms⫺1, tape tension: 1.2 N).
Fig. 5. (a) Calibration fringe pattern (Odd field, wavelength l=620, 535, 450 nm); (b) Envelopes and fringe intensity for line A–A of Fig. 6a (wavelength l=535 nm).
sured at a tape speed of v=2.5 ms⫺1 and a tape tension of T=1.2 N. An area of 200×200 pixels (146×118 µm) of the interface was selected. After determining the calibration envelopes and the fringe intensity, we can calculate the interface spacing. Fig. 6 shows the spacing map over the area of 200×200 pixels, indicating an average spacing of 30 nm.
5. Discussion To compare the results of the five-wavelength interferometer with those of the three-wavelength interferometer, we have first measured the spacing of a moving head/tape interface with the newly developed five-wavelength interferometer. After calculating the head/tape spacing using the intensity data from all five wavelengths, we have recalculated the spacing using only three of the five wavelengths, i.e., R, G and B. In this way, we can make a direct comparison of the two techniques without having to be concerned with systematic errors of two different interferometers, changes of the
head/tape interface with time and position, or different tape drive operating conditions. In other words, any three-wavelength measurement is a subset of a particular five-wavelength measurement, and the deletion of the data of two wavelengths of the five-wavelength measurement results in a typical three-wavelength measurement. Table 1 compares the standard deviation of the head/tape spacing over an area of 200×200 pixels in 10 measurements for the three-wavelength and the five-wavelength interferometer. We observe that the value of the standard deviation of the head/tape spacing measurement is approximately 40% smaller for the five-wavelength measurement as compared to the three-wavelength measurement. The 3s error in the spacing measurement with five wavelengths is approximately ±1.8 nm while the 3s error in the three-wavelength measurement is approximately ±3 nm. We also note that the mean spacing is larger by 4 nm with the five-wavelength measurement than with the three-wavelength measurement. It is difficult to determine which of the mean spacing measurements is closer to the “correct” physical spacing since we do not have any independent spacing calibration. However, since the calculated spacing is based on a weighted average of all the wavelengths used, it seems justifiable to conjecture that the five-wavelength measurement is the more reliable measurement. Thus, it can be concluded that the precision of the five-wavelength interferometer is superior to that of the threewavelength interferometer. Table 1 Comparison between three- and five-wavelength interferometry for spacing measurement in a DLT4000 tape drive (10 measurements) Three-wavelength Mean spacing (h, nm) Std. deviation (s, nm)
25.9 1.0
Five-wavelength 29.9 0.6
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J. Zhu et al. / Tribology International 33 (2000) 409–414
The measurement error in the implementation of the three-wavelength or the five-wavelength interferometer is mainly due to intensity fluctuations in the light source and due to uncertainties in the envelope calibration. Alternating current regulated light sources were used in our experiment. The output of these light sources is not perfectly stable, showing fluctuations of more than 1 percent. Furthermore, the light intensity decreases as the light passes through the shutters. To increase the intensity signal, the gain of the camera controller was adjusted to near its maximum. All of these factors resulted in a somewhat noisy intensity signal for each of the wavelengths. Furthermore, fringe calibration was performed using a mechanical load/unload procedure, which also introduced measurement errors. In any case, the use of two additional wavelengths improves the smoothing of the results and, therefore, gives a more reliable measurement. Comparing the three-wavelength and the five-wavelength interferometer, we note that the five-wavelength interferometer is more complicated. It requires four highspeed shutters, two additional light sources, and synch-
ronization of the shutter operation with the data acquisition. Thus, the five-wavelength interferometer should be considered as a research tool to be used only for high precision measurements. For the general case of head/tape interface studies, the three-wavelength interferometer is sufficient. References [1] Baugh E, Swenson J, Talke FE. Simultaneous three-wavelength interferometry for head/tape spacing measurement. ASME J. Tribol., 1998;120(3):549–53. [2] Vogel SM, Groom JL. White light interferometry of elastohydrodynamic lubrication of foil bearing. IBM J Res Develop 1974;18:521–8. [3] Mizukawa M, Hosaka H, Hara S. Study on spherical foil bearings. Bull JSME 1985;28(243):2105–11. [4] Baugh E. Interferomtry for head/tape spacing measurement. PhD thesis, 1988. [5] Wahl MH, Casmer S, Talke FE. Multi-wavelength intensity based interferometry for flexible head/medium interface. Tribol Trans 1995;38(3):533–40. [6] Baugh E, Talke FE. The head/tape interface: a critical review and recent results. Presented at the STLE/ASME Tribology Conference in Kissimmee, Florida, October 8–11, 1995.
Simultaneous five-wavelength interferometry for head/tape spacing measurement J. Zhu a, E. Baugh b, F.E. Talke a b
a,*
CMRR, University of California, San Diego, La Jolla, CA 92093-0401, USA IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA
Abstract An improved five-wavelength interferometer with high-speed shutters in the light path was designed and implemented. The interferometer allows switching between two sets of three wavelengths, keeping one wavelength in each measurement in common. The set-up allows nearly simultaneous acquisition of fringe intensities and can be used to measure the head/tape spacing in a moving linear tape drive. The precision of the new five-wavelength interferometer was investigated and was found to be superior to the precision obtained with a three-wavelength interferometer. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Interferomertry; Head/tape interface; Tribology
1. Introduction Interferometry is widely used for the measurement of the head/medium spacing [1]. In the mid-1970s the spacing of a typical head/tape interface was on the order of 0.5 µm to 1.0 µm, and frequency-based white light interferometry was commonly used to measure head/medium separation [2]. However, as the head/tape spacing decreases, white light interferometry becomes inadequate for the measurement of spacing below 100 nm since at those spacings only various shades of grey exist. To measure spacings in the sub-100 nm range, intensitybased monochromatic interferometry must be used [3]. Since the spacing measurements using a single wavelength are multi-valued, it is necessary to use more than one wavelength for the unique determination of the head/medium spacing in the region below 100 nm. Although three-wavelength interferometry has been shown to be successful for very small spacing measurements, both experimental evidence and Monte Carlo analysis suggest that the measurement precision can be further improved by using more than three wavelengths [4]. Wahl et al. [5] developed a technique in which five wavelengths were used in sequence to measure the spac-
* Corresponding author. Tel.: +1-858-534-3646. E-mail address: [email protected] (F.E. Talke).
ing at the flexible disk/head interface. In Wahl’s work, a filter was inserted manually for each individual wavelength in order to measure the fringe pattern for each wavelength in sequence. Sequential acquisition of fringe intensities is time-consuming, and cannot be used for interfaces that change dynamically. An improved sequential interferometer was developed by Baugh [4] who used two consecutive measurements to switch two sets of three wavelengths manually. Although the switching of two sets of three wavelengths is faster than the switching of five individual wavelengths, the time for two consecutive measurements is still large. In fact, the typical time for a spacing measurement using Baugh’s method is on the order of at least sixty seconds and the time is even larger for five sequential measurements as done by Wahl. Thus, neither one of the above five wavelength interferometers can be used to measure the dynamic head/tape spacing in a moving tape drive, i.e., only steady-state head/tape interfaces can be investigated. In this paper, a five-wavelength interferometer was developed which allows the measurement of the head/tape spacing in a moving tape drive. Nearly simultaneous data acquisition of the fringe pattern from the five wavelengths was implemented with a single 3–CCD (charge-coupled device) camera using high-speed shutters. The five wavelengths were grouped into two sets of three wavelengths with one wavelength in common.
0301-679X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 3 0 1 - 6 7 9 X ( 0 0 ) 0 0 0 6 1 - X
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J. Zhu et al. / Tribology International 33 (2000) 409–414
The two sets of three wavelengths were switched by two sets of high-speed shutters. During a measurement, data are first acquired for the initial three wavelengths, and then a second set of three different wavelengths is switched into the light path. By synchronizing the shutter and data acquisition with the video signal, two sets of data are acquired in the odd and even fields of the NTSC (National Television System Committee) video, respectively. A typical measurement of the image pattern of the five-wavelength interferometer takes about 35 ms. This time is much shorter than the typical time of 60 seconds of the Wahl [5] and Baugh [4] interferometers. In this paper, we first measure the head/tape spacing in a linear tape drive using the newly developed five-wavelength interferometer. Thereafter, measurements from the threewavelength interferometer and the five-wavelength interferometer are compared in terms of the standard deviation of the spacing. A marked improvement in the precision of the five-wavelength interferometer is observed compared to the three-wavelength interferometer. 2. Theory of interferometry The principle of monochromatic interferometry is shown in Fig. 1. Here, two wave trains of different phase recombine and interfere with each other after having been reflected from the bottom side of the glass head and the top side of the magnetic tape. As a result, the light intensity is modulated according to the phase difference between the two wave trains. A schematic of the light reflection at the head/tape interface is shown in Fig. 1b. The light intensity I(x,y) of a fringe is related to the spacing h(x,y), by I(x,y)⫽A(x,y)⫹B(x,y) cos
冉
冊
4ph(x,y) ⫺f l
(1)
where A(x,y), B(x,y) are constant for any single point on the head/tape interface, but vary with position, l is the wavelength, h(x,y) is the spacing and f is the phase shift. The intensity is directly measured, while l is known from the wavelength of the interference filter used. The phase shift f is inherent upon reflection from the magnetic tape and can be measured, for instance, by ellipsometer. The intensity-spacing relationship of Eq. (1) is plotted in Fig. 2. In order to solve the spacing from Eq. (1), A(x,y) and B(x,y) must be known over the entire head/tape interface. If the maximum and minimum fringe intensity Imax(x,y) and Imin(x,y) are known, the values for A(x,y) and B(x,y) can be found from A(x,y)⫽
Imax(x,y)+Imin(x,y) 2
B(x,y)⫽
Imax(x,y)−Imin(x,y) 2
Fig. 1. (a) Principle of interferometry; (b) Typical fringe pattern at head/tape interface.
(2)
Fig. 2.
Principle of envelope calibration.
The values of Imax(x,y) and Imin(x,y) are known as the calibration envelopes. An unloading method of obtaining the calibration envelopes was developed by Lacey and Talke [6]. By tilting the tape in the drive using an unloading rod, the fringe pattern is shifted across the
J. Zhu et al. / Tribology International 33 (2000) 409–414
interface. Lacey and Talke used this unloading method to record the maximum and minimum intensities at each position along the interface when various fringes pass across the interface. These experimental envelopes then provide the A(x,y) and B(x,y) values needed for Eq. (1). To improve monochromatic interferometry, polychromatic or multi-wavelength interferometry can be used. The benefits include redundancy at low spacings, robustness at fringe order changes, and unique determination of spacing by adjusting the fringe orders of the wavelengths used. If multiple wavelengths are used, the intensity-spacing relation can be written as Ii(x,y)⫽Ai(x,y)⫹Bi(x,y) cos
冉
冊
4phi(x,y) ⫺fi l
(3)
where the subscript i indicates each individual wavelength. Ai(x,y) and Bi(x,y) can be determined by detecting the calibration envelopes using the unloading technique. Inverting Eq. (3) and assuming that only the zero order fringe is of concern, the spacing value hi(x,y) can be written as
冋 冉
冊 册
A(x,y)−I(x,y) l hi(x,y)⫽ cos−1 ⫺p⫹f 4p B(x,y)
(4)
With the calibration envelopes determined, the spacing can be calculated for each wavelength as if i independent monochromatic measurements were done. On the other hand, when making any measurement, it is desirable to have a high sensitivity between the intensity Ii(x,y) and spacing hi(x,y). The sensitivity is defined as ∂Ii(x,y) the normalized derivative of Eq. (3) given by ∂hi(x,y)
| 冉
wi(x,y)⫽ sin
4phi(x,y) ⫺f li
冊|
(5)
A plot of intensity and sensitivity as a function of spacing for three wavelengths is shown in Fig. 3. As can be seen in Fig. 3, when the sensitivity of one wavelength goes to zero at the intensity peak, the other wavelengths still have good sensitivity. Using Eq. (5) as a weighting factor, the spacing corresponding to each wavelength can be combined into a best value using a weighted average
Fig. 3.
Light intensity and sensitivity as a function of spacing.
冘
411
wi(x,y)hi(x,y)
h(x,y)⫽
冘
(6) wi(x,y)
The weighted average spacing has the effect of basing the final spacing measurement h(x,y) on whichever wavelengths have the highest sensitivity at any particular spacing value.
3. Experimental setup In order to perform simultaneous five-wavelength interferometry, illumination with five specific wavelengths must be provided. The five-wavelength illumination is obtained by coupling the output of five separate 150 W white-light sources, each with a different interference filter. The five wavelengths used in the experiment are denoted by R (620 nm), G (535 nm), B (450 nm), R* (650 nm) and G* (550 nm), with R, G and B representing red, green and blue. A 3–CCD video camera is used to image the head/tape interface. Because a 3–CCD camera can acquire only three wavelengths simultaneously, in order to implement five-wavelength interferometry, one must use either two 3–CCD cameras simultaneously or switch one 3–CCD camera after the first three wavelengths have been acquired. Using two cameras, one has to align both cameras so that they are directed at exactly the same position. This is difficult to achieve. Furthermore, it is not easy to separate the five wavelengths used into the two 3–CCD cameras. Thus, the second approach of using one camera which is switched is a preferable option and is implemented in this paper. A NTSC video frame obtained with a 3–CCD camera consists of “odd” and “even” fields, with the even field being recorded after the odd field. Between the two fields, there is a blank interval of 1.3 ms, called the video vertical interval. If the five wavelengths are grouped into two sets of RGB’s with one wavelength in common, and the two sets of RGB’s are stored in the odd and even fields respectively, all five wavelengths can be acquired simultaneously within a single video frame. To implement this approach, high speed shutters must be used to switch within 1.3 ms from the first set of three wavelengths to the second set of three wavelengths. Fig. 4 shows the schematic of the simultaneous five-wavelength interferometer. The five wavelengths were grouped into two sets of three wavelengths, i.e., W1 (RGB) and W2 (R*G*B). One wavelength (B) is common to both sets. Here we define W1 as the odd set and W2 as the even set. Four shutters, two for each set, were used to open and close R, G, R* and G*. Together with the common wavelength of B, this gives the two sets of wavelengths W1 and W2, respectively. In the following, the two sets of shutters
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J. Zhu et al. / Tribology International 33 (2000) 409–414
Fig. 4.
Schematic of simultaneous five-wavelength interferometer.
are described as the odd and even set with the odd set controlling R and G, and the even set controlling R* and G*. The shutters operate during the blank interval between the odd and even field. A custom-made 5-in-1 fiber optic cable is used to couple the five wavelengths at the light source end and guide the light into the microscope. The light goes into the microscope and is directed onto the head/tape interface. The images of the two sets of three wavelengths are reflected consecutively into the odd and even fields of the 3–CCD camera. Three 12-bit A/D boards which are installed in a PC digitize the images. One board is assigned to each of the three output colours of the camera. A software package was developed to capture the fringe pattern, read the image data from the A/D board and then process it. A trigger circuit was fabricated to synchronize the data acquisition and shutter operation. Data acquisition is triggered by a two-stage process: First, a transition signal is sent from the parallel port of the PC to the trigger circuit. The trigger circuit then monitors the video signal from the camera and waits until a new frame begins; the trigger circuit then issues a trigger signal to the master data acquisition board. Because the shutters are initially set with the odd set open and the even set closed, the odd set of three wavelengths is first exposed and acquired in the odd field. Once the odd field ends, the trigger circuit activates the shutters to cause the odd set of shutters to close and the even set to open. The even set of three wavelengths is then exposed and acquired in the even field. Thus, all five wavelengths are acquired in a single frame, with the odd set of three wavelengths in the odd field and the even set in the even field. After completion of the data acquisition, the shutters are reset and the data acquisition goes to the next cycle. The process is automated and controlled by software.
4. Simultaneous spacing measurement in a tape drive To measure spacing at the head/tape interface with the tape moving, data acquisition of all five wavelengths must be done simultaneously. In this study, we have used a DLT4000 tape drive to measure the head/tape spacing while the tape is moving. As is usual for interferometric measurements, the head in the DLT4000 drive was replaced by a glass head. A prism was mounted next to the glass head to direct the light into the head/tape interface and to reflect the interface image back into the microscope. On the top side of the tape drive, a load/unload mechanism is mounted to move the fringes. Standard DLT4 tape was used. In order to measure the head/tape spacing, fringe calibration must first be performed. Because the head has a curvature, the envelopes must be detected over the entire interface. This was done by recording the fringe images while moving the fringes across the interface when the tape drive is running. Fig. 5a shows one of the calibration fringe images (odd field in this case) while Fig. 5b shows the envelopes for a wavelength of 535 nm together with an intensity scan of the moving finge pattern of Fig. 5a. The following points were considered to optimize the envelope detection process: (1) The time delay between the shutter operation and the data acquisition of the A/D boards was optimized, i.e., the time delay needed to gurantee that data acquisition is completed and that all shutters (which have slightly different charging time) operate synchronously; (2) The number of image lines to be acquired for calibration was reduced to as few as possible to shorten the data acquisition time. In this way, the recording density of the images was increased to obtain smooth envelopes. Spacing was mea-
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Fig. 6. Spacing map of typical area of 200×200 pixels (DLT4 tape, speed: 2.5 ms⫺1, tape tension: 1.2 N).
Fig. 5. (a) Calibration fringe pattern (Odd field, wavelength l=620, 535, 450 nm); (b) Envelopes and fringe intensity for line A–A of Fig. 6a (wavelength l=535 nm).
sured at a tape speed of v=2.5 ms⫺1 and a tape tension of T=1.2 N. An area of 200×200 pixels (146×118 µm) of the interface was selected. After determining the calibration envelopes and the fringe intensity, we can calculate the interface spacing. Fig. 6 shows the spacing map over the area of 200×200 pixels, indicating an average spacing of 30 nm.
5. Discussion To compare the results of the five-wavelength interferometer with those of the three-wavelength interferometer, we have first measured the spacing of a moving head/tape interface with the newly developed five-wavelength interferometer. After calculating the head/tape spacing using the intensity data from all five wavelengths, we have recalculated the spacing using only three of the five wavelengths, i.e., R, G and B. In this way, we can make a direct comparison of the two techniques without having to be concerned with systematic errors of two different interferometers, changes of the
head/tape interface with time and position, or different tape drive operating conditions. In other words, any three-wavelength measurement is a subset of a particular five-wavelength measurement, and the deletion of the data of two wavelengths of the five-wavelength measurement results in a typical three-wavelength measurement. Table 1 compares the standard deviation of the head/tape spacing over an area of 200×200 pixels in 10 measurements for the three-wavelength and the five-wavelength interferometer. We observe that the value of the standard deviation of the head/tape spacing measurement is approximately 40% smaller for the five-wavelength measurement as compared to the three-wavelength measurement. The 3s error in the spacing measurement with five wavelengths is approximately ±1.8 nm while the 3s error in the three-wavelength measurement is approximately ±3 nm. We also note that the mean spacing is larger by 4 nm with the five-wavelength measurement than with the three-wavelength measurement. It is difficult to determine which of the mean spacing measurements is closer to the “correct” physical spacing since we do not have any independent spacing calibration. However, since the calculated spacing is based on a weighted average of all the wavelengths used, it seems justifiable to conjecture that the five-wavelength measurement is the more reliable measurement. Thus, it can be concluded that the precision of the five-wavelength interferometer is superior to that of the threewavelength interferometer. Table 1 Comparison between three- and five-wavelength interferometry for spacing measurement in a DLT4000 tape drive (10 measurements) Three-wavelength Mean spacing (h, nm) Std. deviation (s, nm)
25.9 1.0
Five-wavelength 29.9 0.6
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The measurement error in the implementation of the three-wavelength or the five-wavelength interferometer is mainly due to intensity fluctuations in the light source and due to uncertainties in the envelope calibration. Alternating current regulated light sources were used in our experiment. The output of these light sources is not perfectly stable, showing fluctuations of more than 1 percent. Furthermore, the light intensity decreases as the light passes through the shutters. To increase the intensity signal, the gain of the camera controller was adjusted to near its maximum. All of these factors resulted in a somewhat noisy intensity signal for each of the wavelengths. Furthermore, fringe calibration was performed using a mechanical load/unload procedure, which also introduced measurement errors. In any case, the use of two additional wavelengths improves the smoothing of the results and, therefore, gives a more reliable measurement. Comparing the three-wavelength and the five-wavelength interferometer, we note that the five-wavelength interferometer is more complicated. It requires four highspeed shutters, two additional light sources, and synch-
ronization of the shutter operation with the data acquisition. Thus, the five-wavelength interferometer should be considered as a research tool to be used only for high precision measurements. For the general case of head/tape interface studies, the three-wavelength interferometer is sufficient. References [1] Baugh E, Swenson J, Talke FE. Simultaneous three-wavelength interferometry for head/tape spacing measurement. ASME J. Tribol., 1998;120(3):549–53. [2] Vogel SM, Groom JL. White light interferometry of elastohydrodynamic lubrication of foil bearing. IBM J Res Develop 1974;18:521–8. [3] Mizukawa M, Hosaka H, Hara S. Study on spherical foil bearings. Bull JSME 1985;28(243):2105–11. [4] Baugh E. Interferomtry for head/tape spacing measurement. PhD thesis, 1988. [5] Wahl MH, Casmer S, Talke FE. Multi-wavelength intensity based interferometry for flexible head/medium interface. Tribol Trans 1995;38(3):533–40. [6] Baugh E, Talke FE. The head/tape interface: a critical review and recent results. Presented at the STLE/ASME Tribology Conference in Kissimmee, Florida, October 8–11, 1995.