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SERUICE IUIA]IUAI No.127
Contents
1. Outline of development . . . I
2. Features I
3. Comparisons with other types of motors 2
4.Operatingprinciple .......2
5.Uni-torque.... ...4
6. Servo control circuit . 5
7. Specifications.. 8
* Wow/flutter * Load characteristics
* S/N ratio * Start-up characteristics
* Time characteristics * Temperature characteristics
8. Circuit diagram . .. . l0
Output axis (record guide)
1. Outline of development Cover
The signal-to-noise ratio, wow/flutter and other
Rotor magnet (B-Pole)
important characteristics of turntable which employ
direct drive motor directly rely on the performance detection
of the motor. This is because the platter and the base board
Drive coils (2)
motor are coupled directly.
Conventionally, multi-pole DC servomotors or cup type
AC servomotors have been available for direct drive switching use Ball
turntables, but each type has its merits and demerits.
Hitachi set out to develop a high-performance direct Fig. 1 Construction of main uni-torque motor unit
drive motor which combines the merits of both systems,
and the result of its endeavors was the flat, commutator- the platter does not rotate smoothly), and the output
less DC servomotor. (A total of 15 patents have been torque is constant so that there is very little vibration
applied for). and the platter rotates smoothly. This makes for a
The construction of the new motor is completely superb signal-to-noise ratio and wow/flutter.
different from that of conventional motors, and in (2)High starting torque
theory, the torque is constant, there are few vibrations Unlike the cup type AC motor, the new motor pro-
and it allows the platter to rotate smoothly. Hitachi vides a high starting torque for its small size without slip.
called it the 'Uni-torque motor'. Its construction is (3)Simple construction, high reliability
shown in Fig. l. The motor is composed of two core-less flat, star-shaped
drive coils and a base board for speed detection, and
2. Features of a brush-less switching mechanism which contains
also
(l)Excellent S/N and wow/flutter two Hall elements for an overall simple construction.
Thanks to the new core-less, slot-less flat star-shaped High reliability is yielded by the small number of parts
coil construction, there is no slot phenomenon (whereby used.
September 1977
(4)Very little rise in temperature
At start-up, the motor starts operating with its maxi-
mum torque provided by square waves, but when its
speed has become constant, vibrations are prevented by
sinusoidal waves. Furthermore, when locked together
+ Force
with the platter, the protection circuit is actuated to
automatically reduce the drive current and safeguard
against rises in temperature.
\ \/'.// --'-
3. Comparisons with other types
of motors / irA,.\
/ \
The table below compares the uni-torque motor with
conventional multi-pole DC servomotors and cup type
AC servomotors. /./-----/
ry''* uurrent
Performance required DC motors Uni-torque Fig.2 (a) Fleming's Left Hand Rule
AC motor
for direct drive motor Brush Brush-less motor
Torque fluctuations Poor Poor Good Very good
Vibration Good Good Poor Very good
Temperature rise Good Good Poor Very good
Simple construction Fair Fair Fair Very good
Long service life Poor Good Good Good
Efficiency Very Good Poor Good
good
Ease of control Good Good Fair Good.
Comparison of performance of motors
Fig. 2 (a) shows a graphical representation of Fleming's
Left Hand Rule. In a magnetic field, a conductor will
be subjected to the force of wire motion at right angles
to the direction of the current, and this diagram shows Fie. 2 (b)
the basic operation of a rotating machine. According to
this rule, the coil in Fig. 2 (b) will be subjected to the
upward force. If the coil is fixed, a repelling force will
be generated in the magnets and the direction of this
force is shown by the dotted arrows, i.e. downward.
If the length of the coil across the magnetic field is
'l', then the relationship between this length and the If, as shown in Fig. 4 (a), the magnets are moved to the
force F, to which the coil is subjected, the magnetic left in the direction of the 'x' axis, then the force to
flux density B and current I can be expressed by the which the drive coil is subjected will be canceled out,
following equation. F=BI1 since the forces on edges 'a' and'b' under the north
Let us now consider two magnets which are bonded pole are applied in opposite directions, and force fx
together with their polarities reversed, as shown in Fig. which is generated decreases.
3, with a V-shaped drive coil placed beneath them. If the magnets are moved further to the left, as in
The magnetic flux under the magnets go in opposite Fig. 4 (b), forces falx and fb2x are balanced, and the
directions and so even when the direction of the current force generated is zero. Moving the magnets even
is the same, the forces on edge 'b' under the south pole further to the left means that the value of fb2x will
and on edge 'a' under the north pole will be exerted in increase more than that of falx, and that the direction
opposite directions. If these forces are Fa and Fb, of the force applied to the drive coil will be reversed
and the component forces in the 'x' axis direction are and tend to the left. Fig. 5 shows the relationship
fax and fux, then the size of these component forces between the distance moved by the magnets from the
will be: position in Fig. 3 and the forces which are applied to
fax = fa sin0 the drive coil.
Ibx = fb sin0 Let us now proceed to extend the example by aligning
four pairs of circular magnets, which were used in
1
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{ = ,lorE
,.ca^ro ++1 111 /.\
lau6ey\
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^rou fq pue 'EId
se q3ns IIoc olup pedeqs-erenbs e Eutsn 't
Let us consider the forces in the direction across the
diameter of the magnets and in the direction of the
circumference. These are numbered Frl to Fr8, and
F0 I to F08, respectively. With forces Frl to FrS, the
value of the counterforces are the same, but their di-
rections are reversed, and so they cancel one another out.
This means that the force on the drive coil is the sum
total of the eight forces (F0l to F08) applied in the di-
rection of the circumference. If the torque with the
sum total of forces F0 I to F08 is 'T', then the relation-
ship between T and the angle of the magnets' rotation 0
will be repeated every 90". This is expressed graphically
in Fig. 7. As can be seen from the figure, there are
periods during which a negative torque is generated and
so if the direction of the current which flows to the
drive coil during these periods is reversed, the torque re-
presented by the dotted lines in Fig. 7 will also be re-
versed, and a positive torque will be generated.
F
o
f
o ^
o
t--
o
O (angle of magnets' rotation)
Fig.9
5. Uni-torque 1
We looked into the principle behind the torque gener-
ation in section 4, and the last equation which we came
up with was
T=Kicos40. ...(l)
Fig.6 Now, if the current flowing to the drive coil is based on
sine waves, then:
F
i=I cos 40............(2)
o
f
and so substituting equation (2) for equation (1), we
o arrive at:
o
F T=KI cos240 ...(3) -
Let us now consider shifting the coil 22.5" and adding
another drive coil: we shall call the first drive coil
'coil l' and the second drive coil 'coi,2'. (Fig. l0)
First, the phase angle of the current flowing to both
Angle of rotation of magnets
coil I and coil 2 is shifted 90". The current flowing to
Fig.7 coil 2 will be:
i2 = KI sin 40
In an actual motor, the magnets are shaped cylindrically and the torque generated by coil 2 will be:
in order to enhance the efficiency and the drive coil is T2 = KI sin240. . . . . . . . .(4)
shaped like a star, as is illustrated in Fig. 8. If T6 is the sum of the torques (T1 and T2) generated
When a constant current is passed to the drive coil, the in the two drive coils, then from equations (3) and
relationship between the torque generated by the drive (4), this will be:
coil and the angle of rotation will materialize in the form To = Tr 1 Tz =KI (sin24g + cos240) = KI
of sine waves (see Fig. 9). The relationship between Both K and I are constants, and so To is constant
torque T and 0 will be T =Ki cos40 (K = constant). The regardless of the angle of rotation. (Fig. I l)
dotted lines in Fig. 9 show what happens when the To allow a sine wave current which has been shifted )
current which flows to the drive coil is reversed during 90o to flow to drive coil I and drive cotl 2, the uni-
the negative torque periods only. torque motor employs two Hall elements.
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Conventional 8-pole, 24slot DC servomotor
3oO X.om t
--T-- -
I
5! X.cn
Uni-torque motor
---+ T
! 80" Angle of rotation
Fie.12
and the eight voltages combine to form a single voltage.
In other words, a signal is provided which reverses the
rotation of the magnet 72 times. This signal is used to
detect the voltages at the eight locations and so it is
^
a high-precision and constant output signal at all times
even if
there are fluctuations in the magnet or eccentrici- , 2345678? W tt t2 t3t+titantatf
ty. These conductors are actually made with high-
precision print patterns. The detection frequency for
33-ll3 rpm is 20H2, and2THz for 45 rpm.
(2)Limiter amplifier, waveform shaper
Fig. 13
The speed detection signal has a low level and still
contains AM components, and so it will produce an error V,, (+1v)
a
if it is not amplified. Therefore, it is amplified in an O OOI collector
0
excess saturation by the high-gain amplifier and then by
the waveform shaper, and it becomes a square wave Vrc G1v)
signal. This means that the AM components are limited @ oOZcollector
0
and the error eliminated.
(3)Differentiation circuit, multiplier
The servo loop response frequency is determined by the @ cos, nos Vcc G'lv)
speed detection frequency, and the higher the response d ifferentiators
frequency, the faster the system's response and the
higher the stability with regard to disturbances. This @ co+,aoe
gt
Vcc vl
is why the speed detection frequency is multiplied and d ifferentiators
the response is accelerated. The signals which are
differentiated by C03, R08, C04 and R09 resemble
6r*gicnoz, cnos Vec (+1v)
those illustrated in Fig. 14, although only the downward output
Vcc G 1v)
pulse is added by diodes CR02 and 03, the pulse inter- Actually, O03
vals are halved and the frequency doubled. ON/OFF
goes
VeeF'tv) 1'
Fig. 14
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7. Specifications
a
I
I I
rl ll
lt l II lt
to t0 20
Frequency (Hzi Frequency (Hz)
w&F;0.025lowRMS W&F: 0.O40loWRFlS
Uni-torque motor Conventional 8-pole, 24-slot
DC servomotor
^\
Wow/flutter (frequency analysis)
a
Meter read-out
rll
Record : lacquer disc 1o. ttzz-4|
-
Wow/flutter meter: Meguro Electronics _
-\
Wow/flutter characteristics
II nrl, 3 xHz
+ 0.10
;
.o
+ 0.05
o
!0
!o
o
o-
a _ 0.05
- 0.10
a,
0 2.0 3,0 4.0 (3)
Tracking force
Load characteristics
-8-
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8. Circuit diagram
1
It
ic z'fls
rxs
lo6
o
It
U'
.j r P
:f
C'
t,
o
c
o
(o o
F@
o F o
AI
IN
I
()
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o o
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o d
o ^c lj 'ed
t
o
G.
o "Q i
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