(15
`United States Patent
`5,955,799
`Amayaetal.
`Sep. 21, 1999
`[45] Date of Patent:
`
`[11] Patent Number:
`
`US005955799A
`
`[54] LINEAR VIBRATION MOTOR AND
`METHOD FOR CONTROLLING VIBRATION
`THEREOF
`
`[75]
`
`Inventors: Hidetoshi Amaya; Takio Maekawa;
`Toyokatsu Okamoto; Yasuo Ibuki, all
`of Hikone, Japan
`
`[73] Assignee: Matsushita Electric Works, Ltd.,
`Osaka-fu, Japan
`
`[21] Appl. No.: 09/030,233
`
`[22]
`
`[30]
`
`Filed:
`
`Feb. 25, 1998
`
`Foreign Application Priority Data
`
`Feb, 25, 1997
`
`[JP]
`
`Japan vsccssssssscsssssssesesssssseeeeses 9-041238
`
`Tint. C0.occ ceeeeeenseeeccecsernsnmmseneeees H02K 33/02
`[51]
`[52] U.S. Che eee 310/36; 15/22.2; 388/937;
`388/902; 318/119; 310/50
`[58] Field of Search oo... eee 310/36, 50, 12;
`318/119, 126, 127, 128, 135, 686; 15/22.2;
`388/900, 902, 903, 904, 937
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4/1986 Parker et al. oe eeeeeeees 318/128
`4,583,027
`3/1997 Craft et al.ee 310/50 XK
`§,613,259
`5/1997 Motohashi etal. .
`5,632,087
`4/1998 Motohashi el al. oe 310/36
`5,736,797
`FOREIGN PATENT DOCUMENTS
`
`Primary Examiner—Nestor Ramirez
`Assistant Examiner—Judson H. Jones
`
`Attorney, Agent, or Firm—Griflin, Butler, Whisenhunt &
`Szipl, LLP
`
`[57]
`
`ABSTRACT
`
`A linear vibration motor whichincludes a vibration system,
`an electromagnetic driver, a detection system, and a control
`system. The vibration system includes a driven memberor
`reciprocator which is mounted to a magnetic member for
`reciprocating movement. The amplitude of the reciprocating
`movementvaries as an inverse function of load on the motor.
`
`The electromagnetic driver includes a coil and drives the
`vibration system by applying an electromagnetic force to the
`magnetic member, the electromagnetic force being produced
`by a driving current flowing through the coil. The detection
`system detects at least one characteristic of the vibration
`system that is related to a characteristic frequency of the
`vibration system, and produces a feedback signal indicative
`of a value of the at least one characteristic. The control
`
`system is responsive to the feedback signal, during a primary
`mode of operation, to controllably vary the driving current
`flowing through the coil in such a manneras to drive the
`vibration system in resonance with the characteristic
`frequency, and determines the existence of a condition that
`renders the detection system non-functional, and drives the
`vibration system at a prescribed frequency when it is deter-
`mined that the condition exists, during a back-up mode of
`operation.
`
`0 349077
`
`6/1989
`
`European Pat. Off.
`
`.
`
`20 Claims, 9 Drawing Sheets
`
`39
`
`
`SENSOR
`
`
`DETECTING MEANS
`(RESONANT
`
`
` DETECTION)
`FREQUENCY
`
`
`
`90
`
`CONTROL OUTPUT
`FIXED FREQUENCY
`SECTION
`
`SETTING SECTION
`
`
`
`
`11
`
`APPLE 1025
`
`1
`
`APPLE 1025
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 1 of 9
`
`5,955,799
`
`39
`
`CONTROL OUTPUT
`SECTION
`
`
`
`DETECTING MEANS
`(RESONANT
`FREQUENCY
`DETECTION)
`
`
`
`
`
`SETTING SECTION
`
`FIXED FREQUENCY
`
`90
`
`FIG. 1
`
`2
`
`

`

`U.S. Patent
`
`RESONANT
`FREQUENCYf
`
`5,955,799
`
`CURRENTI
`
`Sep. 21, 1999
`
`Sheet 2 of 9
`
`
`
`LOAD N ——=-
`
`|
`SENSOR CAPABLE
`OF DETECTION
`FIG.2
`
`39
`
`
`SECTION
`
`DETECTING MEANS
`(RESONANT
`FREQUENCY
`DETECTION)
`
`CONTROL OUTPUT
`
`
`
`RESONANT FREQUENCY
`f2 STORING SECTION
`
`FIXED FREQUENCY
`SETTING SECTION
`
`55
`
`530
`
`
`
`
`3
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 3 of 9
`
`5,955,799
`
`39
`
`DETECTING MEANS
`(RESONANT
`FREQUENCY
`
`RESONANT FREQUENCY
`f9 STORING SECTION
`
`CONTROL OUTPUT
`
`FIXED FREQUENCY
`
`55
`
`30
`
`DETECTION)
`
`
`SECTION _ SETTING SECTION
`
`
`DRIVING CURRENT
`DETECTING SECTION
`
`56
`
`
`
`FIG.4
`
`39
`
`
`DETECTING MEANS
`(RESONANT
`
`FREQUENCY
`DETECTION)
`
`
`
`FIXED FREQUENCY
`SETTING SECTION
`
`
`
`4
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 4 of 9
`
`5,955,799
`
`
`
`
`
`
`
`
`ININFOVIdSIG
`
`
`
`
`
`
`
`431}
`
`LHO|
`
`JONAVuaiN39NINOLLVYOOT
`
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`INdLNOYOSNIS
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`JONOLOINIG
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`INdLNOONIAG
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`JOWYOISAVM
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`INSYYNDONIATYC
`
`5
`
`

`

`U.S. Patent
`
`Sheet 5 of 9
`
`Sep. 21, 1999
`
`5,955,799
`
`6
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 6 of 9
`
`5,955,799
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`SENSOR
`
` 39
`
`
`
`
`DISPLACEMENT
`
`
`
`DEAD POINT
`MOVEMENT
`VELOCITY
`
`REACHING TIME
`ACCELERATION
`DIRECTION
`
`
`
`DETECTING
`DETECTING
`DETECTING
`
`
`
`SECTION
`SECTION
`SECTION
`
`
`
`
`
`
`CONTROL
`OUTPUT
`SECTION
`
`
`
`F1G.8
`
`7
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 7 of 9
`
`5,955,799
`
`
`
`FIG. 9
`
`8
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 8 of 9
`
`5,955,799
`
`
`
`9
`
`

`

`U.S. Patent
`
`Sep. 21, 1999
`
`Sheet 9 of 9
`
`5,955,799
`
`02
`
`ol
`
` AD
`
`AMPLIFYING
`
`
`
`CONTROL
`OUTPUT
`SECTION
`
`Loe
`
`53
`}
`
`:
`
`:
`=
`=
`
`10
`
`

`

`5,955,799
`
`1
`LINEAR VIBRATION MOTOR AND
`METHOD FOR CONTROLLING VIBRATION
`THEREOF
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to linear motors,
`and more particularly, to a linear vibration motor that is
`suitable for use in an electric shaver, and a method for
`controlling vibration of the lincar vibration motor. This
`application claims priority under 35 U.S.C. § 119 of Japa-
`nese Patent Application No. 09-041238, filed Feb. 25, 1997,
`the disclosure of which is incorporated herein by reference.
`Japanese Unexamined Patent Publication No. 2-52692
`discloses a linear motor that
`is utilized as a source of
`
`reciprocating vibration for a reciprocating type electric
`shaver. ‘This linear vibration motor is a single-phase syn-
`chronous motor that
`includes a reciprocator (or needle)
`comprised of a rod-like permanent magnet, and a stator
`having U-shaped iron cores around which coils are wound.
`A dc voltage having a frequency two times greater than ac
`frequency is supplicd to the coils by a full-wave rectifying
`circuit
`to thereby induce reciprocating movement
`(oscillation) of the reciprocator, and thereby generate vibra-
`tion.
`
`The electromagnetic force that is required to induce the
`reciprocating movement of the reciprocator (or needle) is
`greater than desired.If the reciprocator were supported with
`a spring to thereby form a spring vibration system (including
`the reciprocator), and the spring vibration system driven at
`a frequency equal to a characteristic (resonant) frequency
`thereof, it would be possible to reduce the amount of energy
`necessary to drive the spring vibration system. However,the
`amplitude of the reciprocating vibration could not be kept
`stable when the spring vibration system is loaded.
`As a solution to this problem, there has been proposed a
`linear motor including a stator comprised of an electromag-
`net or permanent magnet, a reciprocator comprised of a
`permanent magnet or electromagnet and supported with a
`spring, a detection system that detects the displacement,
`velocity, or acceleration of the reciprocator and produces a
`feedback signal indicative thereof, and a control system that
`controls the amount of electric power supplied to a coil of
`the electromagnet in response to the feedback signal. With
`this linear motor, even when a characteristic (resonant)
`frequency of the vibration system varics, c.g., duc to
`changesin load, the control system automatically varies the
`amount of electric power supplied to the coil of the elec-
`tromagnet in such a mannerasto drive the vibration system
`at the current resonantfrequency (i.e., to drive the vibration
`system in resonant condition). This is possible because the
`variation of the resonant frequency is related in a known
`mannerto the displacement, velocity and acceleration of the
`reciprocator, at least one of which is detected by the detec-
`tion system.
`However, when the vibration system is heavily loaded,
`with the result
`that
`the amplitude of the reciprocating
`vibration is considerably reduced, the detection system is
`rendered inoperative, thereby making it impossible to drive
`the vibration system in resonant condition. In addition,it
`takes a considerable amountoftime for the vibration system
`to return to resonant condition even after it is unloaded,
`thereby resulting in a considerable reduction in the effi-
`ciency of the linear motor.
`there
`Based on the above,
`it can be appreciated that
`presently exists a need in the art for a linear vibration motor
`which overcomes the above-described drawbacks and short-
`
`10
`
`15
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`comings of the presently available technology. More
`particularly, there is presently a need for a linear vibration
`motor which has the capability of appropriately driving the
`vibration system when the detection system becomes inca-
`pable of carrying out its detection function, and the further
`capability of rapidly returning the vibration system to reso-
`nant condition when the detection system becomes capable
`of carrying out its detection function. The present invention
`fulfills this need in the art.
`
`SUMMARY OF THE INVENTION
`
`invention encompasses a linear vibration
`The present
`motor whichincludesa vibration system, an electromagnetic
`driver, a detection system, and a control system. The vibra-
`tion system includes a driven member which is mounted to
`a magnetic memberfor reciprocating movement. The ampli-
`tude of the reciprocating movement varies as an inverse
`function of load on the motor. The electromagnetic driver
`includes a coil and drives the vibration system by applying
`an electromagnetic force to the magnetic member, the elec-
`tromagnetic force being produced by a driving current
`flowing through the coil. The detection system detects at
`least one characteristic of the vibration system that is related
`to a characteristic frequency of the vibration system, and
`produces a feedback signal indicative of a value of the at
`least one characteristic. The control system is responsive to
`the feedback signal, during a primary modeof operation, to
`controllably vary the driving current flowing through the
`coil in such a manneras to drive the vibration system in
`resonance with the characteristic frequency, and determines
`the existence of a condition that renders the detection system
`non-functional, and drives the vibration system at a pre-
`scribed frequency when it is determined that the condition
`exists, during a back-up mode of operation.
`In a presently preferred embodiment,
`the at least one
`characteristic of the vibration system is the displacement,
`velocity and/or acceleration of the driven member, and the
`condition is the start-up mode of the motor and/or the
`amplitude of the reciprocating movement of the driven
`member falling below a prescribed minimum threshold
`level. Also,
`in the presently preferred embodiment,
`the
`driven memberis a reciprocator assembly, and the vibration
`system further includes a spring system which supports the
`driven member. Further,
`in the presently preferred
`embodiment, the control system pulse-width modulates the
`driving current in response to the feedback signal.
`The control system can drive the vibration system at a
`prescribed frequency which is a single, predetermined fixed
`frequency,or, alternatively, can either controllably varythe
`prescribed frequency in accordance with variations in the
`driving current, or successively use a plurality of different
`frequencies each of which constitutes the prescribed fre-
`quency at the time it is produced. In particularly preferred
`embodiment, the control system includes a portion which
`determines the instantaneous value of the characteristic
`
`frequencyof the vibration system and stores the last instan-
`tancous valuc of the characteristic frequency before the
`detection system becomes non-functional for use as the
`prescribed frequency.
`The present invention also encompasses, in another of its
`aspects, a corresponding method of controlling a linear
`vibration motor of a type which is suitable for use in an
`electric shaver. Such a linear vibration motor includes a
`stator comprised of an electromagnet or permanent magnet,
`a reciprocator comprised of a permanent magnetorelectro-
`magnet and supported with a spring, a detection system
`
`11
`
`11
`
`

`

`5,955,799
`
`3
`which detects at least one of displacement, velocity and
`acceleration of the reciprocator, and a control system which
`controls a driving current supplied to a coil of the electro-
`magnet in accordance with a feedback output of the detec-
`tion system to thereby drive a vibration system including the
`reciprocator and the spring in resonance with a characteristic
`frequency of the vibration system. The method includesthe
`steps of determining when a condition exists which prevents
`the detection system from detecting at
`least one of
`displacement, velocity and acceleration of the reciprocator,
`and using the control system to drive the vibration system at
`a prescribed frequency when it
`is determined that
`the
`condition exists.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and various other objects, features, and advantages
`of the present invention will be clearly understood with
`reference to the following Detailed Description of the Pre-
`ferred Embodiments read in conjunction with the accompa-
`nying drawings, in which:
`FIG. 1 is a block diagram of an exemplary embodiment of
`the linear vibration motor of the present invention;
`FIG. 2 is a diagram which illustrates the relationship of
`resonant frequency, current, and load, in the operation of the
`linear vibration motor of the present invention;
`FIG. 3 is a block diagram of another exemplary embodi-
`mentofthe linear vibration motor of the present invention;
`FIG. 4 is a block diagram of another exemplary embodi-
`mentof the linear vibration motor of the present invention;
`FIG. 5 is a block diagram of another exemplary embodi-
`mentof the linear vibration motor of the present invention;
`FIG. 6 is a timing diagram which illustrates the wave-
`forms and timing relationship of various operational char-
`acteristics of and signals produced by the linear vibration
`motor during operation thereof;
`FIG. 7 is a partial block, partial schematic diagram of a
`linear vibration motor constructed in accordance with a
`preferred embodimentof the present invention;
`FIG. 8 is a block diagram of the detection and control
`systemsof the linear vibration motorof the linear vibration
`motorof the preferred embodimentof the present invention;
`FIG. 9 is an exploded, perspective view of the linear
`vibration motor of the preferred embodimentof the present
`invention;
`FIG. 10 is an exploded, perspective view of the recipro-
`cator assembly of the linear vibration motorof the preferred
`embodiment depicted in FIG. 9; and
`FIG. 11 is a partial block, partial circuit schematic dia-
`gram of the linear vibration motor of the preferred embodi-
`ment of the present invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`With reference now to FIGS. 9 and 10, there can be seen
`a linear vibration motor constructed in accordance with a
`
`preferred embodimentof the present invention. Although the
`linear vibration motorof the present invention hasparticular
`utility in a reciprocating type electric shaver, it should be
`clearly understoodthat the present invention is not limited to
`this or any other particular application. The linear vibration
`motor includes a stator 1, a reciprocator assembly 2 (see
`FIG. 7,) comprised of two reciprocators 21 and 22, and a
`frame 3.
`
`The stator 1 is comprised of an E-shaped yoke 10 con-
`stituted of sintered magnetic material or iron plates, and a
`
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`4
`coil 11 wound around a central projection of the yoke 10.
`Pins 12 stand at opposite end surfaces of the yoke 10.
`The frame 3 has a pair of sidewalls 30, 30 connected at
`longitudinal lower endsthereof to each other through bottom
`plates 31, 31, and hence has a U-shaped cross-section. The
`pins 12 are fixed into grooves 32 of the sidewalls 30 by
`welding or caulking to thereby fix the stator 1 to the frame
`3.
`
`Each of the reciprocators 21 and 22 includes a driven
`body 23 composedof synthetic resin, a reinforcing plate 25
`and a back yoke 26 both composed of a non-magnetic metal,
`and a permanent magnet 20 fixed to a lower surface of the
`driven body23 throughthe reinforcing plate 25 and the back
`yoke 26. The driven body 23 of the reciprocator 21 is a
`rectangular frame, and includes a reinforcing plate 25, a
`back yoke 26 and a permanent magnet 20 at lower ends of
`side walls thereof. The reinforcing plates at the opposite
`sides of the driven body 23 are integrally formed to thereby
`form the single reinforcing plate 25. The reinforcing plate 25
`is integrally formed with the driven body 23 by insert (or
`outscrt) molding. A coupling 24 that is integrally formed
`with the driven body 23 is designed to accept internal blades
`of a reciprocating type electric shaver.
`The reciprocators 21 and 22 are connected to the frame 3
`through leaf springs 4, 4. The leaf spring 4 is formed by
`punching a metal plate, and includes a support plate 40
`attached to the frame 3, and connecting plates atlachedto the
`reciprocators 21 and 22. A central leaf spring 41 connected
`to the reciprocator 22 and a pair of leaf springs 42, 42
`connected to the reciprocator 21 are integral with each other
`through the support plate 40. By virtue of the support plate
`40 being fixed to the opposite ends of the frame 3, e.g., by
`weldingor the like, and further, by virtue of the connecting
`plates 43 being fixed to the opposite ends of the reinforcing
`plates 25 of the reciprocators 21 and 22, the reciprocators 21
`and 22 are suspended from the frame 3, and the coupling 24
`of the reciprocator 22 is located in the reciprocator 21. A pair
`of compression coil springs 28, 28 are supported in a
`direction of reciprocating movementofthe reciprocators 21
`and 22 between spring receivers 26, 26 formed at an inner
`surface of the reciprocator 21 and spring receivers 27, 27
`formed at the coupling 24 of the reciprocator 22.
`The permanent magnet 20 attached to the reciprocator
`assembly 2 vertically faces the stator 1 with a predetermined
`gap therebetween, and is magnetized in a direction of the
`reciprocating movement (oscillation) of the reciprocator
`assembly 2. As illustrated in FIG. 7, the permanent magnet
`20 movesto the right or left in accordance with the direction
`in which current flows through the coil 11 of the stator 1,
`with the leaf springs 4 being deformed. Reciprocating move-
`mentand vibration of the reciprocator assembly2 is effected
`by switching the direction in which the current flows
`through the coil 11 at the characteristic (resonant) frequency
`of the vibration system which includes the reciprocator
`assembly 2,
`the leaf springs 4, and the compression coil
`springs 28 (a spring constant component caused. by mag-
`netic attractive force is further added to the system, to be
`precise).
`Since the magnetic poles of the permanent magnet 20 of
`the reciprocator 21 are arranged oppositely to the magnetic
`poles of the permanent magnet 20 of the reciprocator 22, the
`reciprocating movements of the reciprocators 21 and 22 are
`out of phase by 180 degrees. Whenthe reciprocators 21 and
`22 are urged to move in the outward direction under the
`influence of the current flowing through the coil 11 of the
`stator 1, the springs 28, 28 are compressed.
`
`12
`
`12
`
`

`

`5,955,799
`
`6
`5
`flows through the coil 11 in opposite directions depending
`It is preferable to vibrate the vibration systemin synchro-
`nization with a characteristic (resonant) frequency of the
`upon the direction of movementof the reciprocator assem-
`bly 2, it is possible to detect the direction of movement(i.e.,
`vibration system,i.¢., to put the vibration system in resonant
`positive or negative stroke) of the reciprocator assembly by
`condition with the dual goals of stable vibration and reduc-
`detecting, the polarity of the output voltage of the sensor 39.
`tion in vibration energy. In order to achieve these dual goals,
`the linear vibration motor of the present invention is pro-
`The control system includes the control output section 5
`vided with a detection system which detects a characteristic
`and driver section 53 coupled to the coil 11. In operation, the
`of the vibration system indicative of the resonant frequency
`control output section 5 receives the digital output of the
`A/D conversion circuit 52, which constitutes a feedback
`thereof under current load conditions, and a control system
`which is responsive to feedback signals generated by the
`signal
`indicative of at
`least one of the displacement,
`detection system to control the magnitude and direction of
`acceleration, and velocity of the reciprocator assembly 2.
`the current flowing through the coil 11 in such a manner as
`The control Output section 5 and driver 53 then adjusts the
`to maintain resonant operation of the vibration system.
`magnitude and/or direction of the driving current flowing
`through the coil 11 in accordance with the value of the
`With reference to FIG. 7, the detection system includes a
`feedback signal, to thereby maintain operation of the vibra-
`sensing magnet 29 mounted on the reciprocator assembly 2,
`tion system of the linear vibration motor at the resonant
`the sensing magnet 29 having magnetic poles arrangedin the
`frequency, i.e., to maintain the resonant condition of the
`direction of reciprocating movement of the reciprocators,
`vibration system.
`and a sensor 39 comprised of sensing windings, and sup-
`ported in an opening 34 of the frame 3. The control system
`For example, if the detection system detects a reduction in
`includes a control output section 5 that controls the magni-
`the amplitude of the reciprocating vibration (i.e., a reduction
`tude and direction of the current flowing through the coil 11
`in the range of reciprocating movementof the reciprocator
`in accordance with a current (voltage) induced in the sensor
`assembly 2), e.g., due to an increased load,
`the control
`39 when the reciprocator assembly 2 is in vibration.
`output section 5 will, in response to the feedback signal from
`the detection system, increase the driving current (e.g., by
`More particularly, as is illustrated in FIG. 6, a current
`induced in the sensor 39 varies in accordance with the 2
`increasing a period of time T during which the current is
`supplied and/or by increasing the maximum current) by an
`amplitude of the reciprocating vibration, the location of the
`amountsufficient to increase the velocity of the reciprocator
`reciprocator assembly 2, and/or the velocity and direction of
`the movement of the reciprocator assembly 2. (The termi-
`assembly 2 in such a manner as to maintain a substantially
`constant amplitude of the reciprocating vibration. In an
`nology amplitude of the reciprocating vibration as used
`exemplary embodiment,
`the detection system detects the
`herein is intended to mean the magnitude of the range of
`reciprocating movementofthe reciprocator assembly 2.) For
`velocity of the reciprocator assembly 2 and produces a
`
`example, when the reciprocator assembly 2 reachesafirst digital feedback signalindicative thereof. The control output
`section 5 includes a memory (e.g., a ROM) whichstores a
`limit of its range of reciprocating movement, the magnet 29
`different PWM value for each detected velocity value. In
`stops (due to zero velocity of the attached reciprocator
`assembly 2), and accordingly, the magnetic flux produced
`operation, the control output section 5 reads the PWM value
`therebyis not changed. Thus,at this time, the voltage output
`from its memory that corresponds to the detected velocity
`of the sensor 39 is zero. Whenthe reciprocator assembly 2
`value, and then controls the pulse width of the driving
`reaches a midpoint of its range of reciprocating movement,
`current supplied to the coil 11 in accordance with the
`the reciprocator assembly 2 is moving at
`its maximum
`read-out PWM value. Since velocity is correlated with
`velocity, and thus, at this time, the voltage output of the
`displacementand acceleration, displacementor acceleration
`sensor 39 is at its maximum. Accordingly, the maximum
`may be detected in place of velocity, as is diagrammatically
`illustrated in FIG. 8. The driver 53 includes four FET
`velocity of the reciprocator assembly 2 can be detected by
`detecting the maximum voltage output by the sensor 39, and
`the time at which the reciprocator assembly 2 reaches a dead
`point (i.e., a limit of its range of reciprocating movement)
`can be detected by detecting the zero outputof the sensor 39.
`In addition, the direction in which the reciprocator assembly
`2 moves can be detected based on the polarity of the output
`of the sensor 39.
`
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`switching devices Q1—-Q4. By turning on either the switch-
`ing devices Q1 and Q3or the switching devices Q2 and 04,
`the direction of the current flowing through the coil 11 can
`be switched, to thereby facilitate reciprocating movement of
`the reciprocator assembly 2.
`The control output section 5 only permits the driving
`current to flow in a direction determined in accordance with
`
`the detected direction in which the reciprocator assembly 2
`moves, in order to thereby prevent the driving current from
`braking the reciprocator assembly 2. In addition, the control
`output section 5 minimizes any required increases of the
`driving current by utilizing the biasing movementof the
`spring system, specifically, by allowing the driving current
`to flow when a prescribed period of time t has passed after
`the time t, at which the direction of movement of the
`reciprocator assembly 2 reverses.If the driving current were
`allowed to flow through the coil 11 in a reverse direction
`before the time t,, the vibration of the reciprocator assembly
`2 would be braked. If the driving current were allowed to
`flow through the coil 11 in the direction of movementof the
`reciprocator assembly 2 after the reciprocator assembly 2
`has passed the midpoint of its range of reciprocating
`movement, it would be impossible to realize a multiplied
`force of the driving (electromagnetic) force exerted by the
`coil 11 and the driving (repulsion) force exerted by the
`
`An exemplary implementation of the detection system
`and control system employed in the preferred embodiment
`of the present invention is illustrated in FIG. 11. As can be
`seen, the detection system includes the sensor 39, an ampli-
`fying circuit 51, and an A/D conversion circuit 52. In
`operation, the output voltage transmitted by the sensor 39
`(which has a sinusoidal waveform,as can be seen in FIG. 6)
`is amplified by the amplifying circuit 51. The amplified
`output voltage produced by the amplifying circuit 51 is then
`converted into a digital value by the A/D conversioncircuit
`52. The maximum velocity of the reciprocator assembly 2
`can be detected by detecting the vollaye level of the sensor
`output a prescribed periodoftime(t) after the time when the
`sensor output is zero, or by detecting a maximum voltage
`level of the sensor output between successive zero outputs.
`The time at which the direction of movement of the recip-
`rocator assembly 2 reverses can also be detected by detect-
`ing whenthe sensor outputis zero. In addition, since current
`
`50
`
`55
`
`60
`
`65
`
`13
`
`13
`
`

`

`5,955,799
`
`10
`
`15
`
`5
`
`30
`
`35
`
`7
`spring system, since the driving force caused by the repul-
`sion force of the spring system compressed bythe recipro-
`cator assembly 2 will have already diminished. Hence, the
`time at which the driving current starts to be applied to the
`coil 11 is prescribed to be in a range betweenthe timeat the
`direction of movement of the reciprocator assembly 2 is
`reversed and the time at which the reciprocator assembly 2
`reaches the midpoint of its range of reciprocating move-
`ment. The time at which the reciprocator assembly 2 reaches
`the midpoint of its range of reciprocating movementcan be
`easily detected as it corresponds to the time at which the
`sensor 39 transmits its maximum output. The time t may be
`dynamically adjusted in accordance with the detected veloc-
`ity or acceleration of the reciprocator assembly 2.
`With the above-described drive control system,it is not
`necessary to detect the direction of movementof the recip-
`rocator assembly 2 since the direction of movementof the
`reciprocator assembly 2 is related in a known mannerto the
`polarity of the applied driving current, thereby making it
`possible to simply successively switch the directionin which 5,
`the driving current flows through the coil 11, e.g., by
`appropriately controlling the switching action of the driver
`53. Further, with the above-described drive control system,
`il is not necessary to detect the time al which the direction
`of movement of the reciprocator assembly 2 is reversed,
`since the frequency at which the direction of current flow
`through the coil 11 is switched by the driver 53 is synchro-
`nized with the characteristic or resonant frequency of the
`vibration system, thereby makingit possible to simply apply
`the driving current in each direction for a prescribed time
`interval which can be dynamically adjusted in accordance
`with the detected resonant frequency of the vibration sys-
`tem. With this technique, the driving current can be applicd
`to flow through the coil 11 in the appropriate direction, even
`if the rectprocator assembly 2 is so heavily loaded as to be
`temporarily stopped, and the driving current can be applied
`to flow through the coil 11, even if there is dispersion in the
`characteristic frequency of the vibration system,e.g., due to
`a difference between the masses of the recrprocators 21, 22,
`or between the spring constants of the compression coil
`springs 28, 28. Thus,
`it can be ensured that the vibration
`system converges to the instantaneous characteristic fre-
`quencyofthe vibration system and that the vibration system
`is vibrated at a substantially constant amplitude of recipro-
`cating vibration.
`As previously discussed, the velocity of the reciprocator
`assembly 2 can be detected on the basis of either the
`maximum voltage value output by the sensor 39 or on the
`basis of the time interval between successive times at which
`
`40
`
`45
`
`the output of the sensor 39 is zero. The latter approach
`ensures detection of the time at which the direction of
`
`movement of the reciprocator assembly 2 reverses without
`being influenced by any dispersion or variation in the
`magnetic force of the magnet 29, and any dispersion or
`variation in the gap between the magnet 29 and the sensor
`39. Accordingly, the velocity of the reciprocator assembly 2
`can be detected more reliably and precisely on the basis of
`the time interval between successive times at which the
`output of the sensor 39 is zero.
`As an alternative to the detection system which includes
`the magnet 29 and the sensor 39, it is possible to use a
`detection system which includesa slit plate 60 attached to
`the reciprocator assembly 2 (see FIGS. 9 and 10) and a
`photosensor (not shown)aligned withthe slit in the slit plate
`60, whereby the output of the photosensor would vary in
`dependenceon the displacement, acceleration, and/or veloc-
`ity of the reciprocator assembly 2. In general, a non-contact
`
`50
`
`55
`
`60
`
`65
`
`14
`
`8
`type detection system is preferred in order not to prevent
`vibration of the reciprocator assembly 2.
`With the above-described linear vibration motor and
`control system therefor, if the detection system is rendered
`incapable of performing its detection function due to the
`amplitude of the reciprocating vibration of the vibration
`system falling below a minimum (threshold) level (e.g.,
`when the motor is heavily loaded),
`the control system
`supplies the necessary driving current to the coil 11 because
`the control system no longer receives the feedback signal
`from the detection system which is necessary for its opera-
`tion (i.c., the servo feedback loop is openedor interrupted).
`The present invention is designed to overcomethis short-
`coming.
`Moteparticularly, the linear vibration motorof the present
`invention includes a fixed frequency setting section 50, as
`depicted in FIG. 1, which stores one (or a plurality of) fixed
`frequency values which is (are) utilized by the control
`system whenit is determined that a condition exists (e.g., the
`amplitude of the reciprocating vibration of the vibration
`system falling below the minimum threshold level) which
`renders the detection system temporarily incapable of per-
`forming its detection function (i.e., non-functional).
`Specifically, when it is determined that the detection system
`is temporarily nonfunctional, the control output section 5
`reads a fixed frequency value from the fixed frequency
`setting section 50, and then switches the driver 53 at that
`fixed frequency until the operation of the detection system is
`restored. In this manner, the direction in which the current
`flows through the coil 11 is switched by the control system
`at
`the fixed frequency during times when the detection
`system is non-functional.
`As is depicted in FIG. 2, when a load N on the linear
`vibration motor increases from N,
`to N.,
`the resonant
`frequencyof the vibration system changes from f, to f,. If
`it is assumed that the detection system is rendered non-
`functional when the load N exceeds N., any one or more
`frequencies between f, and f, may be advantageously
`employed a