Note: Descriptions are shown in the official language in which they were submitted.
~085912
~ACKGROUND OF T~E INVENTION
.
Thc present invention relates generally to systems for
controlling the rotations of direct current (DC) motors and more
particularly to a system for controlling the rotational speed
and rotational phase of a DC motor by means of a simple circuit.
In one system known heretofore for controlling the
rotational speed and rotational phase of a DC motor, a signal
responsive to the rotation of a rotary structure driven in rota-
tion by the DC motor is derived, and a phase error signal and a
rotational speed error signal are obtained from this signal thus
derived and are used to control the rotation of the DC motor.
This known system, however, has been accompanied by
certain problems such as complicated circuit organization and
high cost due to the necessity of employing parts such as fre-
~uency detector circuits in order to obtain the rotational speed
error signal.
SUMMA~Y OF THE INVENTION
Accordingly, it is a general object of the present
invention to provide a novel and useful system for controlling
the rotation of a DC motor in which system the above described
problems have been overcome.
Another object of the invention is to provide a system
of simple and inexpensive circuit organization capable of
controlling the rotational speed and the rotational phase of a
DC motor.
Still another object of the invention is to provide a
rotation control system of a DC motor which system has electri-
cal circuit means operating to detect the rotational speed of
the DC motor from the counter electromotive force thereof and to
compensate for the voltage drop across the armature resistance
due to the armature current 50 that the counter electromotive
force will become constant, and means for detecting the rotational
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phase error of a rotary structure rotated by the DC motor and
applying the resulting detection signal as a speed reference
signal to the above mentioned circuit means.
Other objects and further features of the invention
will be apparent from the following detailed description with
respect to preferred embodiments of the invention when read in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRA~INGS
In the drawings:
Fig. 1 is a basic circuit diagram, partly in block
diagram form and partly in schematic form, of a DC motor rotation
control system according to the invention;
Fig. 2 is a graph indicating torque versus rotational
speed characteristics of a DC motor;
Fig. 3 is a graph indicating a characteristic of an
improvement factor of constant speed characteristic of the DC
motor;
Fig. 4 is a block schematic diagram of a DC motor
rotation control system constituting a first embodiment of the
,:
invention; ~-
Fig. 5 is a circuit diagram of an essential part of ~-
the system shown in Fig. 4;
Fig. 6 is a diagrammatic perspective view showing a
- modification of one part of the system illustrated in Fig. 4; and
Fig. 7 is a block schematic diagram of a DC motor
rotation control system constituting another embodiment of the -~
invention.
DETAILED DESCRIPTION
. .. _
Referring first to Fig. 1 a DC motor 10 is connected
respectively to the inverting input terminal of a differential
amplifier 11 constituting of an operational amplifier and through
a resistor Rl to the output terminal of the differential ampli-
10~5912
fier 11. The noninverting terminal of the differential amplifier11 is connected respectively through a resistor R2 to a terminal
17 and through a resistor R3 to the output terminal of the dif-
ferential amplifier 11.
Here, the counter electromotive force Eo of the DC
motor 10 is expressed by the following Eq.(l).
Eo = E - IoRo .......... (1)
where : Ro is the armature resistance of the DC motor;
Io is the armature current; and
E is the voltage applied to the DC motor.
The counter electromotive force Eo is proportional to the rota-
tional speed of the motor. Furthermore, the armature current Io
is proportional to the rotational torque of the motor. For this
reason, when the torque becomes large, the voltage drop IoRo
across the armature resistance Ro becomes large, and, as is
apparent from the above Eq.(l), the counter electromotive force
Eo drops, while the rotational speed also decreases. Accordingly,
the torque versus rotational speed characteristic of the DC motor
is as indicated by the line I in Fig. 2 in the case of constant
voltage driving. However, when the armature resistance Ro is
low, the torque versus rotational speed characteristic becomes
as indicated by the line II. In the case where Ro is zero, Eo
is equal to E, and the rotational speed becomes constant irres-
pective of the value of the torque as indicated by the line III
in Fig. 2.
In the case where, when the voltage El applied to the
terminal 17 in the circuit shown in Fig. 1 is made constant, the
rotational speed of the motor 10 decreases, the armature current
I increases, and the voltage drop across the resistor Rl becomes
large. On the other hand, the currents flowing through the
resistors R2 and R3 do not change. Therefore, in comparison
with their states prior to the lowering of the rotational speed
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of the motor, the voltage at a junction 18 becomes lower than
that at a junction 19, and the output voltage of the differential
amplifier 11 becomes high.
Here, while the current flowing through the resistor
Rl and the motor 10 does not change, the current flowing through
the resistors R3 and R2 increases by an increment corresponding
to the rise in the output voltage of the differential amplifier
11 since the voltage at a terminal 17 does not change. As a
consequence, the voltage drop across the resistor R3 becomes
large, and the output voltage of the differential amplifier 11
increases until the voltage drop across the resistor R3 and that
across the resistor Rl coincide. When these voltage drops
across the resistors Rl and R2 coincide, the difference between
the two input voltages of the differential amplifier 11 becomes
zero, and the output voltage of the differential amplifier 11
becomes constant at that instant of time. Consequently, the
motor voltage and the voltage at the terminal 17 become equal,
and the motor 10 assumes a state wherein it continually rotates
at a constant rotational speed in accordance with the voltage
applied to the terminal 17.
Accordingly, with the circuit shown in Fig. 1 a charac-
tstic by which the resistor Ro is cancelled by the resistors Rl
and R2 is obtained, whereby the characteristic indicated by the
line IIor III in Fig. 2 is obtained.
When, with the line I in Fig. 2 the characteristic of
the DC motor 10 becomes the characteristic indicated by the line
II, the improvement factor G of the motor constant speed charac-
teristic becomes as follows.
30G - a = 20 log 1 RlR2 ( ......... (2)
RoR3
Where a and b respectively indicate the rotational speed differ-
. . .
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ences between the line III and the lines II and I.
Then, when RlR2/RoR3 is expressed by A, the relation-
ship between this A and the improvement factor G becomes as
indicated in Fig. 3. In the case where A=1, that is, in the
case of the characteristic indicated by the line III in Fig. 2,
G is e~ual to infinity. Then, for A=0.9, G=20(dB); and for
A=0.99, G=40(dB).
Therefore, in the circuit comprising the resistors
Rl, R2 and R3 and the differential amplifier 11, the rotational
speed of the DC motor is detected by the counter electromotive
force thereof, and the voltage drop across the armature resis-
tance due to the armature current is so compensated for that
this counter electromotive force becomes constant. Thus, the
rotational speed of the motor 10 is continually and automatically
controlled and maintained at a constant value.
However, while the rotational speed of the motor is thus
controlled by only the above described circuit, the rotational
phase is not controlled. Accordingly, in the system of the
present invention, means for detecting the rotational phase and
applying the resulting detection signal to the terminal 17 of
the above described circuit is provided.
A rotary structure 12, which is driven in rotation by
the DC motor 10 either directly or through motive power trans-
mission means such as an endless belt, is provided thereon with
a permanent magnet. A magnetic head 13 operates cooperatively
with this permanent magnet to detect the rotation of the rotary
structure 12. It will be apparent that this rotation detection
means need not be limited to a combination of a maynetic head
and a permanent magnet but may be any other suitable rotation
detection means known in the art.
A detection signal is thus produced by the magnetic
head 13 is response to the rotation of the rotary structure 12
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and is fed by way of an amplifier 14 to a phase comparator 15,
which operates to compare the phase of this detection signal with
that of a reference signal from a reference signal generator 16.
The resulting output phase error signal of the phase comparator
15 is applied to the aforementioned terminal 17.
In the case of the preceding circuit description, the
voltage applied to the terminal 17 was considered to be constant.
In actual practice in the system of the invention, however, a
signal responsive to the rotational phase of the rotary structure
12 is applied in this manner to the terminal 17. Thus, the DC
motor 10 is so controlled that its rotational speed is constant
and, r,loreover, the rotational phase of the rotary structure 12 is ~-
constant.
The embodiment of the invention shown in Fig. 4 is an
embodiment wherein the system of the invention is applied to a
so-called head servo for control of the rotation of rotary heads
in a video signal recording and/or reproducing apparatus. The
apparatus has a guide drum 30 around which a magnetic tape is ~-~
wrapped in a helical path over a specific angular extent around
the guide drum. This guide drum 30 comprises an upper rotating
drum 31, video heads 32a and 32b provided on the rotating drum 31
for recording and/or reproducing video signals, and a lower
stationary drum 33. The rotating drum 31 is driven at constant
rotational speed by motive power from the DC motor 10 transmitted
via a small diameter pulley 34, an endless belt 35, and a large
diameter pulley 36.
The rotating drum 31 is provided on its lower surface
at diametrically opposite positions with permanent magnets 37a and
37b. A pickup head 38 is fixed at a position to confront the
orbital path of the permanent magents 37a and 37b and operates '
cooperatively with the permanent magnets 37a and 37b to generate
alternately detection signals of different polarity in accordance
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108S912
with the rotation of the rotating drum 31. These detection sig-
nals from the pickup head 38 are supplied to monostable multi-
vibrators 39 and 40 to trigger the same alternately. These mono-
stable multivibrators 39 and 40 are respectively triggered thus
alternately once per revolution of the rotating drum 31 and pro-
duce respective output signals, which are supplied to a flip-flop
41 to trigger the same. As a result, the flip-flop 41 produces
as output a rectangular wave of one cyclic period per revolution
of the rotating drum 31. This output rectangular wave is formed
into an inclined trapezoidal wave by a trapezoidal wave forming
circuit 42 and is then supplied to a sampling circuit 43.
On the other hand, at the time of recording mode of
operation, a video signal to be recorded is applied through a
terminal 44 to-a vertical synchronizing signal separation circuit
45, where a vertical synchronizing signal is separated from the
input video signal. The frequency of this vertical synchronizing
signal is divided into one half by a monostable multivibrator 46.
The signal thus frequency divided is recorded, on one hand, as a
control signal on an edge of the magnetic tape by a control head
47 and, on the other hand, is supplied by way of an amplifier 48
to a monostable multivibrator 49 to trigger the same. This mono-
stable multivibrator produces output pulses of narrow width,
;~ which are fed to the aforementioned sampling circuit 43, where
they are used to sample the trapezoidal wave from the trapezoidal
wave forming circuit 42.
At the time of reproducing mode of operation, the above
mentioned control signal is reproduced by the control head 47 and
is supplied by way of the amplifier 48 to the monostable multi-
vibrator 49. The resulting output pulses of the monostable multi-
vibrator 49 are used for sampling in the sampling circuit 43similarly as in the recording mode of operation.
In the sampling circuit 43, the phases of the rotational
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phase detection signal of the rot,ating drum 31 and the recording
or reproducing control signal used as a reference signal are com-
pared, As a result, the sampling circuit 43 produces as a
sampling output a low voltage when the rotational phase of the
rotating drum 31 is advanced relative to that of the reference
signal and a high voltage when the rotational phase is retarded
relative to that of the reference signal. This sampling output ~ -
is fed to a rotation control circuit 50 of the DC motor 10.
One embodiment of a specific circuit in concrete form ~ ~
10of the rotation control circuit 50 is shown in Fig. 5. In Fig. '~ , -
5, those parts which the same as corresponding parts in Fig. 1 ;~ ~'
are designated by like reference numerals and characters. In
this circuit, the output terminal of an operational arnplifier 51
is connected to the base of a transistor 52 for current amplifica-
tion. The collector of this transistor 52 is connected to a
power supply terminal 53, and the emitter is connected through a
junction point 54 to resistors Rl and R3. The armature resistance
Ro of the DC motor 10 and the resistors R1, R2, and R3 constitute
a bridge circuit similarly as in the circuit shown in Fig. 1.
The resistor R3 comprises a fixed resistor R3a and a fine-adjust-
ment variable resistor R3b. The output signal of the sampling
circuit 43 is applied to the terminal 17. Here, when the variable
resistor R3b is so adjusted that
Rl x R2 = R3 x Ro ................. (3)
the voltages of thedifferential amplifier ll and the terminal 54
become constant when the voltages at the junction points 18 and
19 are the same.
When the control voltage Ec applied to the terminal
17 becomes low, the voltage at the junction point 19 becomes
lower than that at the junction point 18, and the voltage at
the junction point 54 decreases toward zero from the above
mentioned constant voltage. Similarly, when the voltage at the
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terminal 17 becomes high, the voltage at the junction 19 becomes
higher than that at the junction 18, and the voltage at the junc-
tion 54 increases toward the power supply voltage from the above
mentioned constant voltage in accordance with the difference
between the voltages at the junctions 19 and 18. ~s a result,
the voltage Ec and the counter electromotive force Eo of the DC
motor 10 become continually the same.
Since the counter electromotive force Eo is proportional
to the rotational speed of the DC motor 10 as mentioned herein-
before, the DC motor rotates at a rotational speed determined bythe output voltage Ec of the sampling c,ircuit 43. Also as men-
tioned hereinbefore, this voltage Ec is a low voltage when the
rotating drum 31 is rotating with'a phase which is advanced rela-
tive to the reference phase and is a high voltage when the drum
31 is rotating with a phase which is retarded relative to the ''
reference phase. For this reason, the rotation of the DC motor ~''
10 is so controlled that the rotational phase of the rotating drum
31 is continually equal to the reference phase.
In the above described embodiment of the invention, the
.~.
20 rotating drum 31 is rotated by motive power transmitted thereto
from the DC motor through a power transmission mechanism includ-
ing the endless belt 35. However, as,illustrated by the example
of modification in Fig. 6, the rotating drum 31 may be coupled
directly to and thus driven by the DC motor 10. In this case,
the rotating shaft 60 of the rotating drum 31 is coupled directly
to the rotor shaft 62 of the DC motor 10 by a coupling 61. In
this case, the permanent magnets 37a and 37b may be mounted on
the rotating shaft 60 or the rotor shaft 62. ;
An embodiment of the present invention wherein it is
applied to a so-called capstan servo for control of the rotation
of a capstan is illustrated in Fig. 7. The capstan 70 provided
with a flywheel 71 is driven in rotation by the DC motor 10
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through a pulley 72, an endless belt 73, ana a pulley 74. A `
permanent magnet 75 is embeddedly provided at a peripheral part
of the flywheel 71, and a pickup head 76 is provided to confront
the ~ermanent magnet 75.
The pickup head 76 produces, in accordance with the ~ ~
rotation of the capstan 70, an output signal having one pulse per ~; .
revolution of the capstan 70, which output signal is supplied by
way of an amplifier 77 to a trapezoidal wave forming circuit 78.
The resulting output trapezoidal wave is fed to a phase comparator
79. On the other hand, a reference signal from a reference sig- :
nal generator 80 is counted down by a counter 81 thereby to be
divided so that its period becomes equal to the rotational period
~; of the capstan 70. The resulting output signal of the counter 81
is fed to the phase comparator 79, where its phase is compared
with that of the above mentioned trapezoidal wave.
The phase comparator 79 thus produces as output a phase
error signal of a voltage lower than the reference voltage when
the rotational phase of the capstan 70 is advanced relative to
that of the reference signal and of a voltage higher than the
reference voltage when the rotational phase of the capstan 70 is
retarded relative to that of the reference signal. This output
of the phase comparator 79 is applied to the terminal 17 of the
rotation control circuit 50. The DC motor 10 is thereby controlled
in rotation so that its rotational speed is constant and, more-
over, the rotational phase of the capstan 70 becomes constant.
In accordance with the system of the present invention,
tne rotational speed of a DC motoris detected by utilizing the
counter electromotive force thereof, and, by means of an electronic
circuit, the voltage drop across the armature resistance due to
the armature current of the motor is so compensated for that the
counter electromotive force becomes constant. Accordingly, the
rotational speed of the motor is automatically controlled so that
-- 10 --
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it becomes constant by the above mentioned electronic circuit pro-
vided as an accessory of the motor. Consequently, fluctuations
of short duration in the rotational speed such as wow and flutter
are eliminated by the electronic circuit. Accordingly, the phase
control may operate 50 that the deviation of the rotational speed
over a long period becomes zero. For this reason, a rotational
phase detection of the rotary structure driven in rotation by the
motor of the order of one or two cycles per revolution of the
rotary structure is sufficient, and there is no necessity of
carrying out high-density and hi~h-precision phase detection.
Therefore, the control system as whole çan be made very inexpen-
sive.
Further, this invention is not limited to these embodi-
ments but various variations and modifications may be made without
departing from the scope and spirit of the invention.
