Patent Application
20080192379
The instant nonprovisional patent application claims priority to Japanese Patent Application No. 2007-013733 filed Jan. 24, 2007 and which is incorporated by reference in its entirety herein for all purposes.
In recent years, magnetic disk drives (HDDs) have been widely used not only with computer devices but also with household electric appliance, such as video recorders. Such a magnetic disk drive includes a magnetic disk and a magnetic head slider. While flying over the magnetic disk, the magnetic head slider magnetizes the magnetic disk or reads a magnetized state of the magnetic disk and thereby executes recording to playback of information. For example, in writing information, as the distance between the magnetic disk medium and the magnetic head slider becomes narrower, the expansion of a magnetic field formed by the magnetic head can be reduced to be smaller, whereby the area size to be magnetized on the magnetic disk medium is reduced to be smaller. More specifically, for increasing the record density of the magnetic disk drive, the distance between the magnetic disk medium and the magnetic head, that is, the fly height of the magnetic head slider is sought to be reduced.
As one conventional technique for reducing the fly height of the magnetic head slider, Japanese Patent Publication No. 2005-135501 discloses a technique in which a heater formed from a thin film resistor or the like is mounted in the vicinity of the recording/playback elements. A part of the magnetic head slider is heated and thermally expanded to thereby bring the recording/playback elements to be close to the side of the magnetic disk. In an application of the technique, fly heights of respective magnetic heads are tested and stored during a pre-shipment testing process. Then, in the a product usage event, an amount of heating of the heater is controlled corresponding to any one of appropriate amounts of electricity specific to the magnetic head sliders and usage conditions (such as usage environment temperature, usage environment pressure, zone of a magnetic disk medium targeted for recording/playback, and operation modes such as recording and playback modes).
As described above, the fly heights of the respective magnetic heads have to be tested during pre-shipment testing of the magnetic disk drive. To do this, it is effective to use a method in which the electricity supply amount for the heater is eventually increased, and the phenomenon of contact of the magnetic head slider with the information recording medium is detected and recorded, and the original fly height is inversely calculated from a contact-event amount of electricity and a proportionality coefficient between the amount of electricity and amount of fly height variation.
As a method for detecting contact between the magnetic head slider and the magnetic disk medium, there is a simplest method that does not need the provision of an additional hardware device such as an acoustic emission (AE) sensor, and that detects a vibration from a variation in a magnetic playback signal.
However, substantially no methods have as yet been proposed for calculating the variation in the magnetic playback signal. In addition, in the event that the time interval of sampling of the magnetic playback signal falls just in an integer multiple of a wavelength of a contact vibration, the contact vibration cannot be successfully captured.
Embodiments of the present invention provide a process that monitors a magnetic playback signal while gradually increasing an electricity supply amount for a heater to thereby determine contact between a magnetic head slider and a magnetic disk medium.
In a manufacturing method for a magnetic disk drive, in which fly heights of magnetic head sliders are respectively regulated have to be set before shipment. According to the particular embodiments disclosed in
a)-8(C) are conceptual views for explaining a measuring method for the index indicative of contact of the magnetic head slider in accordance with the first embodiment.
Embodiments of the present invention relate to a magnetic disk drive in which a part of a magnetic head slider is heated by heater to control a distance (fly height) from a magnetic disk medium to a magnetic head, thereby to improve recording and playback performances.
Embodiments of the present invention are made in view of the above situations, and an object of certain embodiments is to provide a practical method regarding how to calculate a variation in a magnetic playback signal by using what types of parameters. Further, taking the frequency of contact vibration into account, embodiments of the invention provide a highly reliable contact detecting method that does not fail in detection of a contact state. Further, embodiments of the invention provide a manufacturing method for a magnetic disk drive that sets an appropriate amount of electricity of a respective magnetic head slider by using the aforementioned methods before product shipment.
In order to solve the problems described above, a manufacturing method for a magnetic disk drive in accordance with embodiments of the present invention includes a process that monitors a magnetic playback signal while gradually increasing an electricity supply amount for a heater to thereby determine contact between a magnetic head slider and a magnetic disk medium. An index of contact is set in accordance with an increase in variation within magnetic playback signal amplitude data at a plurality of portions split along a circumferential direction, that is, an increase in spatial variation in the magnetic playback signal amplitude. As a parameter indicative of the magnetic playback signal amplitude, gain of a variable gain amplifier (VGA), for example, can be used.
Preferably, the magnetic playback signal amplitude data in the respective measurement portion is a value obtained by subtracting a value, which has been measured in a state where an electricity supply amount for the heater is sufficiently small and the magnetic head slider and the magnetic disk medium are not in contact with one another, from a value a value measured in the state of electricity supply.
Further, preferably, a distance between each of the plurality of portions for measurement is non-constant or variable, and a minimum alteration unit thereof is smaller than a distance obtained by dividing a peripheral speed of the magnetic disk medium by 300 kHz.
According to embodiments of the present invention, in the manufacturing method for a magnetic disk drive, the appropriate electricity supply amount for the heater of the respective magnetic head slider can be set by detecting contact between the magnetic head slider and the magnetic disk medium from the magnetic playback signal.
A first embodiment of the present invention will be described with reference to drawings.
The spindle motor 2 rotates one or a plurality of magnetic disk media 3. The carriage assembly 4 is rotated by the voice coil motor 8, thereby to relatively move the magnetic head slider 5, which is attached to a lead end portion of the carriage assembly 4, substantially along a radial direction on the magnetic disk medium 3. With reference to
Upon receipt of an input signal indicative of record information, the preamplifier 7 amplifies and supplies the signal to the recording element 5a of the magnetic head slider 5. Further, the preamplifier 7 amplifies and outputs a playback signal output from the playback element 5b. Further, upon receipt of an input of a specification of an amount of current for output to the heater 5c, the preamplifier 7 supplies the specified amount of current (or voltage or power) to the heater 5c.
In a midway of a path electrically connecting together the magnetic head slider 5 and the read/write channel 11, there is a provided a “flexible printed cable” 6 (FPC) formed of a flexible wireline that absorbs the rotational motion by the voice coil motor 8 as deflection. The preamplifier 7 is attached onto the FPC 6 in a manner such as soldering.
The temperature sensor 9 detects an environmental temperature in the vicinity the magnetic head slider 5, and outputs a signal indicative of the detected temperature. The temperature sensor 9 may be disposed on the FPC 6, for example. Alternatively, similarly as the HDC 13 and the control portion 14, the temperature sensor 9 may be disposed on a base board (card). The base board (card) is mounted to a reverse face of the housing 1 shown in
The read/write channel 11 outputs to the preamplifier 7 a signal formed by code modulation of targeted record data. Further, the read/write channel 11 performs code demodulation of a playback signal output from the preamplifier 7, and outputs to the HDC 13 data obtained by the code demodulation.
The motor driver 12 outputs a driving current to, for example, the spindle motor 2 or the voice coil motor 8 in accordance with a specification input from the control portion 14, thereby to operate the spindle motor 2 or the voice coil motor 8.
The HDC 13 receives, for example, record data or command transferred from an external host 20, or transfers playback data output from the read/write channel 11 to the host 20.
The control portion 14 controls a respective portion, such as control of the motor driver 12, to perform position control of the magnetic head slider 5. The control portion 14 is a program control device, such as a microcomputer, and operates in accordance with self-contained programs and/or programs stored in the storage portion 15. According to the present embodiment, the control portion 14 provides to the preamplifier 7 the specification of the amount of current for supply to the heater 5c. Operation information and the like information of the control portion 14 will be described below.
The storage portion 15 contains, for example, programs for execution by the control portion 14 and data necessary for execution of the programs. The storage portion 15 further contains values (control parameters corresponding to electricity supply amount for the heater 5c) that the control portion 14 references when controlling the heater 5c and sets into heater control registers of the heater 5c. Examples of the control parameters will be described below. The storage portion 15 is a nonvolatile memory, such as an EEPROM (electrically erasable programmable read only memory). Depending on the case, the storage portion 15 may further include a partial area of the magnetic disk medium 3. In this case, in manufacture, the control parameters are stored in the magnetic disk medium 3; and in use, after power-on the control parameters are first copied into a memory accessible at high speed from the magnetic disk medium 3, and are then referenced in control of the heater 5c.
A general forming process for the magnetic head slider 5 will be described hereinbelow. To begin with, large numbers of, for example, heaters 5c, playback elements 5b, recording elements 5a, and wirelines connecting thereto are laminated by a thin-film process on a wafer 5d of an alumina-titanium-carbide sintered compact (“AlTiC,” hereinbelow). The heaters 5c are disposed between the AlTiC portion 5d and the playback elements 5b. Then, the structure in the wafer state is cut and separated by dicing into a bar state, and is further cut and separated into discrete sliders. Before or after the process, an air bearing surface 5f in the bar or slider state is polished to be smooth, thereby to form a carbon protection film. Further, a step bearing in a shape for effectively generating air pressure is formed on the air bearing surface 5f.
Upon start of an appropriate electricity estimation process (step 100), a first one of the plurality of magnetic head sliders 5 is positioned on a specific track of specific zone of the magnetic disk medium 3 (step 101). At the outset, the electricity supply amount for the heater 5c is set to an initial value (step 102). The initial value is a value at which the magnetic head slider 5 and the magnetic disk medium 3 do not contact with one another, and the electricity amount is zero (0). Subsequently, at steps 103 to 106, the electricity supply amount for the heater 5c is gradually increased, in which a phenomenon in which the magnetic head slider 5 vibrates upon contact with the magnetic disk medium 3 is detected from a variation in the magnetic playback signal.
When the magnetic head slider 5 vibrates upon contact with the magnetic disk medium 3, the distance from the playback element 5b to the magnetic disk medium 3 increases and decreases in operative association with the vibration. As a result, the amplitude (intensity) of the magnetic playback signal varies. In this case, the magnetic signal should be recorded in a state the magnetic head slider 5 and the magnetic disk medium 3 are not in contact with one another. As shown in
The value of the magnetic playback signal amplitude after contact varies greater than that before contact, so that the standard deviation corresponding to the items of data significantly increases. First, a value obtained by performing a multiplication of the standard deviation by a fixed multiplication factor, such as 3 (times) or 5 (times), of a standard deviation in an obvious noncontact state is set as a threshold value. Then, the electricity supply amount for the heater 5c is gradually increased, in which a state where the standard deviation corresponding to the items of data has exceeded the threshold value, the state is determined to be a contact state. Even when contact is not caused, when the magnetic head slider 5 approaches the disk medium 3 thereby to cause an increase of the value itself (average value) of the playback signal amplitude, also the standard deviation naturally increases. In order to accurately detect only the increase of the standard deviation caused by the contact vibration, the standard deviation can be calculated after being normalized by the average value. By execution of the above-described process, an electricity amount for initiating contact between the head and the zone in the playback mode under conditions (a normal pressure and a room temperature) for performing the pre-shipment testing can be known.
The process described above is executed for respective one of all the magnetic head sliders on one or a plurality of radial zones (steps 107 to 109). It is possible that the appropriate-electricity amount estimation process is executed on only one zone on the outer circumference, but compensation is provided on the other zone by a typical radial fly variation profile obtained by, for example, fly height modeling or sample testing. However, it is desirable that the appropriate-electricity amount estimation process be carried out on, for example, three, outer, inbetween, and inner circumferential zones. Of course, when an even more accurate appropriate electricity amount estimation process data on respective one of a larger number of zones, such as 30 zones, for example, even more accurate appropriate electricity amount data can be obtained. However, when head damage due to contact is taken into account, about three portions are most appropriate.
In the process described above, as a parameter indicative of the amplitude of the playback signal, a gain regulation parameter (gain of variable gain amplifier (“VGA,” hereinbelow)), for example, is used. The preamplifier 7 amplifies a playback signal played back by the playback element 5b, and sends the amplified playback signal to the read/write channel 11. The read/write channel 11 amplifies the playback signal, which has been sent from the preamplifier 7, to a constant or fixed amplitude, extracts data from the received playback signal, and provides a decoding process thereon. A parameter used for the amplification is the gain of VGA. As the gain of VGA is larger, the source signal is less intense—which is indicative that the distance between the playback element 5b and the magnetic disk medium 3 (i.e., the fly height of the playback element 5b) is large. In contrast, as the gain of VGA is smaller, the source signal is more intense—which is indicative that the distance between the playback element 5b and the magnetic disk medium 3 is small.
The gain of VGA includes two types, namely, a gain for servo VGA (servo VGA gain) for servo data and a gain for data VGA (data VGA gain) for user data. Servo data fields include not only fields, such as a field storing servo sector numbers (servo IDs) and a field storing position error signals (PES), but also a field storing signals called as preamble signals. Preamble signals are recorded as signals having a fixed frequency and uniform in terms of the radial direction for attaining frequency synchronism. The servo VGA gain is a parameter that is calculated by the read/write channel 11 from an amplitude with which the preamble signal has been read out.
The data VGA gain is a parameter to be calculated by the read/write channel 11 from a playback signal amplitude of data, which has a constant frequency and which is preliminarily recorded, in order to make a gain for reading user data to be constant. As such, before measuring of the data VGA gain, single-frequency signals similar to preamble signals but different from innate user data have to be preliminarily recorded the data sector (not only in a sync portion but in the whole).
Using any one of the two types, i.e., servo VGA gain and data VGA gain, enables effects of embodiments of the present invention to be obtained. The servo VGA gain has an advantage of enabling measurement not only during playback operation but also during recording operation. However, utilizing the data VGA gain enables provision of technical measures for successful detection or capture of contact vibrations.
b) and 8(c), respectively, are conceptual views representing a situation in which contact vibrations are occurring in the event magnetic data stored at a constant amplitude and a single frequency. However, small vibration waveforms are shown by being enlarged along the horizontal axis (time axis) to be larger than those actually formed. The frequency of actual record data is in the range of from several tens to several hundreds of kilohertz (KHz), the small vibration waveforms as shown in
With reference to
In
While the frequency of contact vibration varies corresponding to the vibration mode, the frequency of a pitch direction vibration of the magnetic head slider 5 is highest as being ranged from some latter half of 100 KHz to some 200 kHz. The frequency of a sway direction vibration along an in-plane direction from a base of the suspension 4a that supports the magnetic head slider 5 and exerts a force of pressing it onto the magnetic disk medium 3 is lower than the frequency of pitch direction vibration. A smallest one of contact vibration wavelengths corresponds to a distance obtained by dividing the peripheral speed of the magnetic disk medium by 300 kHz. As such, the distances along a plurality of portions for measuring the amplitudes of the playback signal are non-constant or variable, and the minimum alteration unit D is smaller than a distance obtained by dividing the peripheral speed of the magnetic disk medium by 300 kHz.
As described above, according to the first embodiment, in the final step of the manufacture of the magnetic disk drive, the appropriate amount of electricity can be determined and set by detecting contact of the magnetic head slider from the variation in the magnetic playback signal. In this case, the contact state can be securely detected by taking the frequency of contact vibration of the magnetic head slider into account.
In addition to the above-described contact detection method using variation in the playback signal amplitude, a contact detection method different from the above-described method may be additionally used. For example, when the electricity supply amount for the heater 5c is gradually increased, the playback signal amplitude is substantially linearly increased as the distance between the playback element 5b and the magnetic disk medium 3; however, after start of contact, variation in the playback signal amplitude deviates from the linear transition. The contact detection method may be based on the phenomenon. However, in many cases, such a phenomenon of deviation of variation in the playback signal amplitude is detected after passage through a true contact point (inflection point), and is detectable later than in the “contact detection (method) using the playback signal amplitude variation” described in the embodiment described above. As such, instead of total replacement of “contact detection (method) using the playback signal amplitude variation” according to the embodiment described above, it can be additionally used as a backup.
In addition, a method is available that measures an off-track component of contact vibration by monitoring position error signals (PES). While the method has a problem in that sensitivity is different depending on the radial position and is low on an inbetween circumference portion, the method can be used as a backup of the “contact detection (method) using the playback signal amplitude variation.”
The electricity supply amount for the heater 5c, which regulates the fly height, is regulated corresponding to the operating head, zone, operating temperature, or operating mode. More specifically, taking fly height variation into account, the step bearing is designed so that, even when there are added other overlapping low fly conditions, such as operating temperature and mode, at a lowest air pressure corresponding to a highest latitude predetermined as a specification, the magnetic head slider 5 does not contact with the magnetic disk medium 3.
Compensation for fly height variance of the discrete or respective head slider will be described hereinbelow. The fly height is different depending on the respective head slider. In a pre-shipment testing process, when contact detection is carried out by gradually increasing the heater-actuating electricity supply amount on a zone or respective zone, a respective distance (clearance) leading to contact of the respective head slider can be obtained. In the product, values each obtained by subtraction of a reliability margin from the heater-actuating electricity supply amount leading to contact are preliminarily recorded in the storage portion 15 in units the respective head slider. Upon receipt of a pre-recording/playback seek command seek command from the host 20, a value in the heater control register of the preamplifier 7 is appropriately updated in accordance with recording/playback head number information.
Another method is available in which the clearance respective head slider is not obtained by the contact detection. According to the method, a recording/playback performance examination is carried out for an error rate or the like by eventually increasing the heater-actuating electricity supply amount in a zone or respective zone in a pre-shipment testing process, and an amount of electricity in the event a desired value is reached is adopted as a specific amount of electricity for the corresponding head slider. In this case, a process called “clearance checking process” needs to be performed. In this process, a value obtained by adding an electricity amount corresponding to the reliability margin to the electricity amount determined in the above-described method is actually applied, thereby to verify that contact dose not occur.
Compensation for fly height variance associated with a zone of the magnetic disk medium will be described hereinbelow. The fly height is different depending on the zone. As a profile thereof, an average profile of the zones can be known through design values or sample tests carried out in a laboratory. Alternatively, in the pre-shipment testing process, the respective profile corresponding to the respective head slider can be obtained through contact detection carried out by gradually increasing the appropriate heater-actuating electricity supply amount corresponding to the respective zone. In the product, per-zone appropriate heater-actuating electricity supply amounts are preliminarily recorded in the form of tables in the storage portion 15. Upon receipt a seek command from the host 20, the value in the heater control register of the preamplifier 7 is appropriately updated in accordance with recording/playback zone information.
A method for storing control parameters by splitting the magnetic disk medium 3 into a large number of zones is most accurate. However, a method in which common control parameters for the whole of the magnetic disk medium 3 (that is, the number of zones is only one) may be employed. Alternatively, control may be provided in the manner that control parameters are stored by being separated into a small number of zones, such as three zones corresponding to an outer, inbetween, and inner circumferential portions, in which ranges thereamong may be controlled by being interpolated through, for example, a primary or secondary expression.
Compensation for fly height variance associated with the environmental temperature will be described hereinbelow. When the environmental temperature is high, the fly height is reduced due to the effects of thermal protrusion caused due to a difference between the linear expansion coefficients of the recording/playback elements material and the peripheral material. In contrast, when the environmental temperature is low, the fly height is increased. According to embodiments of the present invention, in the product, an appropriate heater-actuating electricity supply amount(s) corresponding to an environmental temperature zone(s) is recorded as a single numeric value (coefficient) or a plurality of numeric values (table) in the storage portion 15. The respective value is obtained in accordance with the result of preliminarily investigation of effects of the environmental temperature on the fly height in a laboratory. When a seek command is received from the host 20, the value in a heater control register of the preamplifier 7 is appropriately updated in accordance with information received from the temperature sensor 9.
Control may be provided in the manner that a range between upper and lower limits of operation guaranteeing temperatures is split into a large number of temperature zones, and all control parameters corresponding to the respective temperature zones are stored. Alternatively, control may be provided in the manner that only control parameters corresponding to a limited number of temperature zones, such as three temperature zones corresponding to a low, normal, and high temperatures are stored, in which ranges thereamong are interpolated through, for example, a primary or secondary expression.
Compensation for fly height variance associated with an operation mode, such as mode of record or playback operation, will be described hereinbelow. Recording current in the recording operation works similar to heater current to thereby cause thermal expansion deflection, such that the fly height during the record operation is reduced relative to that in the playback operation. The amount of reduction (amount of fly variation associated with write protrusion) can be known through design values or sample tests carried out in a laboratory. Alternatively, in the pre-shipment testing process, the respective amount of fly variation associated with write protrusion corresponding to the respective head slider can be obtained through a comparison between a playback signal wavelength amplitude immediately after consecutive writes and a playback signal wavelength amplitude not associated with write. In the product, heater-actuating electricity supply amounts for compensation of write protrusion are preliminarily recorded in the storage portion 15. Upon receipt of a pre-recording/playback seek command from the host, a value in a heater control register of the preamplifier 7 is appropriately updated in accordance with operation mode information.
In a summary of the heater-actuating electricity supply amount setting methods described above, electricity amount tables as shown in
For controlling the heater of the preamplifier 7, three methods, namely, methods for power control, voltage control, and current control are used. The heater-induced fly height variation is substantially proportional to the power, and is proportional to the square of the voltage or current value. As such, the level of heating of the heater is first calculated based on the power. For the power control, a simple addition operation is sufficient. More specifically, the addition operation is carried out to add together amounts of power respectively corresponding to the fly height variances associated with the respective head slider and the zone, the fly height variance associated with the environmental temperature, and the fly height variance associated with the operation mode. Thereby, the total amount of power is calculated. For calculation by using voltage or current values, it is necessary that the sum of squares of the voltage or current value for compensating for the respective single fly height variance, thereby to obtain the total amount of voltage or current.
An example of operation of the magnetic disk drive 10 manufactured as described above will be described herebelow. For description, it is contemplated that values (control parameters) for controlling the amounts of electricity for supply to the heater 5c in the events of recording and playback are preliminarily stored and set in the storage portion 15 in correspondence to the respective heads, temperature zones, and zones.
Upon receipt of a data recording command and recording-targeted data from the host 20, the HDC 13 outputs the recording-targeted data to the read/write channel 11, and outputs to the motor driver 12 a specification for moving the magnetic head slider 5 to a recording position corresponding to the command. In this event, the control portion 14 obtains information of an environmental temperature in accordance with a signal output from the temperature sensor 9. Then, corresponding to a temperature zone of the environmental temperature indicated in the obtained information, the control portion 14 obtains a control parameter (control parameter corresponding to the recording event) stored in the storage portion 15. The control portion 14 outputs to the preamplifier 7 a specification for setting the electricity supply amount for the heater 5c to the above-described specified value. In response, the preamplifier 7 supplies the heater 5c with a current as an electricity amount corresponding to the specified value. Then, the heater 5c heats the vicinity of the recording/playback elements of the magnetic head slider 5.
Concurrently, the read/write channel 11 outputs to the preamplifier 7 a signal formed by modulation of the recording-targeted data, and the preamplifier 7 amplifies and outputs the signal to the recording element 5a of the magnetic head slider 5. Thereby, the recording-targeted data is recorded onto the magnetic disk medium 3.
Similarly, upon receipt of a data playback command and playback data from the host 20, the HDC 13 outputs a playback specification in accordance with the playback command to the read/write channel 11, and outputs to the motor driver 12 a specification for moving the magnetic head slider 5 to a playback position corresponding to the command. In this event, the control portion 14 obtains information of an environmental temperature in accordance with a signal output from the temperature sensor 9. Then, corresponding to a temperature zone of the environmental temperature indicated in the obtained information, the control portion 14 obtains a control parameter (control parameter corresponding to the playback event) stored in the storage portion 15. The control portion 14 then outputs to the preamplifier 7 a specification for setting the electricity supply amount for the heater 5c to the above-described specified value. In response, the preamplifier 7 provides to the heater 5c a current supply as an amount of electricity corresponding to the specified value. Then, the heater 5c heats the vicinity of the recording/playback elements of the magnetic head slider 5.
Concurrently, the preamplifier 7 amplifies and outputs a playback signal output from the playback element 5b of the magnetic head slider 5 to the read/write channel 11. The read/write channel 11 generates playback data by demodulation of the signal amplified by the preamplifier 7, and outputs the playback data to the HDC 13. The HDC 13 outputs the playback data to the host 20.
A second embodiment of the present invention will be described hereinbelow with reference to the drawings.
The value of the magnetic playback signal amplitude before contact varies greater than in that after contact, so that the standard deviation of the items of data significantly varies. First, a value obtained by performing a multiplication of the standard deviation by a fixed multiplication factor, such as 3 (times) or 5 (times), of a standard deviation in an obvious noncontact state is set as a threshold value. Then, the electricity supply amount for the heater 5c is gradually increased, in which a state where the standard deviation of the items of data has exceeded, the state is determined to be a contact state. As a parameter representing the magnetic playback signal amplitude, either the servo VGA gain or data VGA gain can be used for simplification.
According to the present embodiment, the measurement position can be at only one portion. In consideration of deflection of the magnetic disk medium 3, however, it is desirable that the playback signal amplitude be measured at a plurality of circumferential positions. By way of example,
As determination manners in the event of measurement at a plurality of circumferential positions, three methods described below are available.
A first method is such that a single threshold value is set, in which, an instance in which an average of standard deviations at a plurality of positions exceeds the threshold value, the instance is determined to be a contact instance. This method is advantageous in that, since the standard deviation in a noncontact instance is small and stable, in an instance where the standard deviation is transiently increased for a causative factor than contact, the risk of making an erroneous determination of the instance to be a contact instance is low. On the other hand, however, the method is disadvantageous in that the sensitivity to the contact is low.
A second method is such that a single threshold value is set, in which, in an instance where a maximum one of standard deviations at a plurality of positions or some percent thereof exceeds the threshold value, the instance is determined to be a contact instance. In contrast to the first method, the second method is advantageous in that the sensitivity to the contact is high, but is, on the other hand, disadvantageous in that, in an instance where the standard deviation is transiently increased for a causative factor other than contact, the risk of making an erroneous determination of the instance to be a contact instance is high.
A third method is such that threshold values are set respectively corresponding to a plurality of positions, in which, in an instance where any one or some percent of standard deviations at a plurality of positions exceeds the respective threshold values, the instance is determined to be a contact instance. The third method is illustrated in
Technical measures regarding electricity-supply time periods are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-013733 | Jan 2007 | JP | national |
