Dear Lewm: Yes, they did indeed. These are exerpts from a Sony research in 1975:
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Ordinary Motor Servo System
The design object of a turntable motor servo-control system is to
have the least amount of change in rotational speed when the motor
is subjected to changes. Load conditions, such as application of
stylus pressure on the record and the change in fractional resistance
between stylus and record groove. Fig. I shows the static characteristics
of the servo system.
When the motor load torque is to and the supply voltage is Vo, the
angular velocity is Po. If the load increases to tl, the angular
frequency goes down to Pi provided that the supply voltage is kept
at Vo. In this case, th8 amount of change in angular velocity per
unit change of load is as shown below.
6P Po-P_ I
KT- a_- - ?,-7o - Do I-o.
Do=Fluid resistance
In order to bring the angular velocity back as close as possible to
Po, the supply voltage must be increased when the angular frequency
is lowered from Po to Pl' The amount of change in angular velocity
of the motor per unit of supply voltage, Ky, is shown as follows:
81_ KT
Kv=_,V - 0o I-b
KT = Cons'i'anf of 'torque .generefion
The amount of change in voltage per unit angular velocity, K, is expressed
as follows:
aV
K- FC
AW
_p in FiE. 2 is the amount of change in angular velocity after the
servo system has stabilized and is shown as follows:
Ap = I+K Kv
_7 = chanqein Icedtorque
i + KKv in the preceding equation Is called the loop gain. From the
equation 1-a and 1-b the following is obtained:
f
P Do
---- I-C
Ay I+KKV
Thls shows that curve S In Fig. I which indicates the amount of change
In angular velocity after the serve has stabilized, is improved when
multipled by the loop gain over the change in angular velocity before
the serve is stabilized. Above, we discussed the function of the serve
system with a change in load torque under DC conditions. In the case
of a change In load torque under AC conditions, the relation among various
parameters in shown in Fig. 3.
The dotted lines in Fig. 3 show the characteristics of disturbance suppression
by the moment of inertia of the motor rotor and turntable.
The solid line shows the characteristics after the serve is stabilized.
Fig. 3 is also expressed In the following equation:
Po I -.f
glo= (I+KKv) j
I.
z_P(Jl) Do
A7 (,.Pt,) - I,KKv t-9
when J_ ¢ d'_co
Therefore,
aPfJ_)
I-h
/',7'(_'t,) J-d"bo
This indicates that in order to obtain higher suppression of external
disturbance, it is necessary to make the moment of inertia of motor
rotor and turntable as well as the angular velocity response of serve
system larger. For example, under the following conditions:
,.,T=ZOO(§.cTn. seci), J_o --Z/-(X lO (red/acc)
Stylus point from spindle = 15(cra)
Stylus pressure = 3f§)
Coefficienf of frichon, xz =0.4
_Y = I_ x 3 ×O.Z..=I-18 (9 .cra)
The amount of change in angular velocity of the motor spindle dP is,
therefore, expressed as follows:
2
Ap = J_L_ ' AZ = ZOOx Z 7_x IO X 18 = I,z+33XIO -3 (md/sec)
· = O.OI37(rpm)
This change in the standard speed of 33 1/3 rpm in percentage,_, is:
/
aP O.OI37
?_ ---- _P_3__- X IOO --- lOC X IOO = O,Oql (%)
3
Phase-locked Motor Servo System With Quartz Generator
As discussed above, conventional servo system requires detection in change
of angular velocity for compensation in changes of speed. Therefore, unless
the moment of inertia in the mechanical system is infinite or the
angular velocity of servo response is infinite, it is impossible to avoid
changes in speed totally. Increase in the moment of inertia will result
in shorter life of the motor/turntable bearings and slower start-up time.
Increase in angular velocity of servo response has also its limitations.
The phase-locked servo system utilizes the prinipal that if a change in
angular velocity is converted to a change in phase, the conversion constant
becomes infinite at DC. The change in angular velocity P(t) is expressed
as follows:
Pit) =A PcosJ1t &p: arnountchangedin
angular velocity
The change in phase f8 (rD;n this case is-
_(:t)=fPtt)dt- AP sin Zlz / ./ _b
Therefore, the transfer function H (_) is =
Therefore,¢h¢¢rans_r fu_ffion H(_) is:
H(g) = -p-=(t) ..I... 2-_
Pit) jJ_
The preceding equation 2-a indicates that when a change in angular
velocity (_) is zero (or DC), the conversion gain becomes infinite
and the phase is always 90° behind regardless of angular velocity.
The transfer function, HT (_-) is the change between angular velocity,
and voltage is expressed as follows: .
Ka
Hr (Z1,)= K,_ H(_%)=
K_: Cons-Cainntconvertingchange
inphase intochange in voltage
Fig. 4 shows the block diagram of the serve system with HT _ in the
serve loop, and Fig. 5 shows its disturbanee suppression characteristics.
In Fig. 5 the dotted lines show the disturbance suppression
3
characteristics of the moment of inertia in the mechanical system.
The solid line shows suppression of characteristic after the velocity
detection servo is stabilized. The broken line shows the characteristics
of servo system including the phase comparator system. As shown,
compared to the system only with velocity detection, the phase comparator
system improves the disturbance suppression characteristics when
the torque disturbance angle frequency is belowS; and when there is
no disturbance _.=O), the change in turntable angular velocity_p becomes
zero, or in perfect equilibrium. If there is an error in the reference
itself, to which the angular velocity of the motor is compared,
this will of course result in an error in motor speed. In the case of
the Sony PS-8750, a quartz generator is used as the reference source.
Since the speed error of the motor in this case is equal to that of the
generator, it is kept to below 0.003%.
The effect of the stylus pressure and friction between the stylus and
record groove is shown in Figures 6-1 and 6-2. Fig. 6-1 shows that
with "velocity only" servo systems a two-gram stylus pressure on the
outer grooves results in approximately 0.02% slow down inturntable
speed in addition to the speed fluctuations of about 0.014% caused by
the audio signal modulation in the record grooves. Fig. 6-2 shows
speed stability of the phase-locked servo system. As it is shown,
there is little effect from stylus pressure and the change in friction
between the stylus and grooves on the turntable speed. The frequency
of the external disturbance stays mostly below 1Hz. Therefore, as shown
in Fig. 5 it is suppressed very effectively with the phase-locked servo
system, j_In the PS-8750Jcase, the phase-locked servo responds below 1.4
Hz (fc =_-_= 1.4 (Hz). At around 0.07Hz where most of the disturbance is
found the suppression is approximately 20 times greater than that of
the "velocity only" servo system. If the same effect is to be obtained
with the servo system without a phase comparator, 20 times more moment
of inertia is required in the mechanical system.
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Denon use/used that PLL system as other manufacturers.
Regards and enjoy the music,
R.
