System for controlling rotation of motor

A system for controlling the rotation of a motor comprises a detector for detecting the rotation of the motor, frequency discriminator for frequency discriminating the output of the detector, a loop filter comprising resistors and a capacitor which lowers the high-frequency components of the output signal of the frequency discriminator, a switching circuit which is connected to the output side of the loop filter, the switching circuit grounding the output side of the loop filter and discharging the capacitor of the loop filter in a state before starting of the high-speed rotation of the motor, and non-grounding the loop filter in a state in which the motor is undergoing high-speed rotation, and a motor driving circuit which drives the motor in accordance with the output signal of the loop filter through the switching circuit. The capacitor is charged up with a predetermined time constant due to the output of the frequency discriminator upon starting state of the high-speed rotation of the motor and gradually increases the output voltage of the loop filter with the predetermined time constant.

BACKGROUND OF THE INVENTION 
The present invention relates generally to motor rotation control systems, 
and more particularly to a system for controlling rotation of a reel 
driving motor in a magnetic recording and/or reproducing apparatus. 
Generally, in a magnetic recording and/or reproducing apparatus, 
fast-forwarding or rewinding of a magnetic tape is performed by rotating 
the tape winding reel or the tape supplying reel at high speeds. This 
operation is not limited to the application in magnetic recording and/or 
reproducing apparatuses for audio signals, but also applicable in magnetic 
recording and/or reproducing apparatuses which record and/or reproduce 
video signals (hereinafter, referred to as video tape recorders, or simply 
as VTRs). In these VTRs, the fast-forwarding and rewinding operation are 
generally performed in a state in which the tape is not wound around the 
guide drum but accommodated in a tape cassette, in order to minimize the 
load for tape travelling. In the above magnetic recording and/or 
reproducing apparatuses for audio signals and VTRs, the tape travelling 
load is made samll upon fast-forwarding and rewinding of the tape. 
Accordingly, when a predetermined voltage is applied to the reel motor to 
rotate the reel at high speed, the motor starts to rotate rapidly. 
Therefore, at this moment, the tape receives a violent shock due to the 
sudden pulling of the tape by the reel, and in extreme cases, the tape can 
be damaged. Furthermore, acceleration and inertia are introduced in the 
reel due to the rapid start of the motor, and resulted in inconveniences 
such as the unstable travelling of the tape and uneven winding of the tape 
to the reel, due to the slack introduced in the tape. 
SUMMARY OF THE INVENTION 
Accordingly, a general object of the present invention is to provide a 
novel and useful motor rotation control system, in which the above 
described problems have been overcome. 
Another and more specific object of the invention is to provide a motor 
rotation control system which uses a capacitor in a loop filter provided 
within a loop of the control system to gradually allow the rotation of the 
motor, by charging the above capacitor upon fast-speed rotation of the 
motor such as during the fast-forwarding and rewinding of the tape. By the 
system of the present invention, the motor does not start a rapid 
fast-speed rotation, and hence when applied as a reel motor, does not 
apply a sudden force on the tape or damage the tape, and moreover, the 
fast-forwarding of rewinding of the tape can be performed smoothly, 
without introducing any uneven winding of the tape. 
Other objects and further features of the present invention will be 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION 
An example of an automatic tape loading type magnetic recording and/or 
reproducing apparatus for video signals applied with a motor rotation 
control system of the present invention, will first be described in 
conjunction with FIG. 1. 
A tape cassette 10 contains within a tape supply roll 11 and a tape take-up 
roll 12, and is further provided with tape guide pins 13 and 14. A 
magnetic tape 15 drawn out from the tape supply roll 11 is guided by the 
guide pin 13, and as shown by a one-dot chain line of FIG. 1, reaches the 
tape take-up roll 12 through the front side of the cassette, guided by the 
pin 14. The cassette 10 is provided with a cut-out opening 16 at its front 
and bottom surfaces. This opening 16 has an opening and closing cover (not 
shown) which opens upon insertion of the cassette 10 into the loaded 
position within the apparatus, and closed upon non-usage of the cassette 
10 in order to protect the magnetic tape 15. When the cassette 10 is 
mounted on a tape supply reel disc 17 and a tape take-up reel disc 18, in 
a predetermined position as shown in FIG. 1, the cover of the cassette is 
opened and a tension pole 19 and a guide pole 20 are relatively inserted 
within the cassette opening 16. 
The tension pole 19 is provided at the tip end of a tension lever 21, and a 
pushing spring 23 is provided between the tension lever 21 and a lever 22. 
The guide pole 20 is provided at the tip end of a rotating lever 24. 
When a play operation button is pushed upon recording or reproducing mode, 
the levers 21 and 22 rotate in clockwise directions and the lever 24 
rotates in a anti-clockwise direction from positions shown by the one-dot 
chain line of FIG. 1, to reach positions shown by the solid lines. By the 
rotations of the above levers 21 and 22, the magnetic tape 15 is engaged 
to and drawn out from within the cassette 10, upon projection of the poles 
19 and 20 through the opening 16 of the cassette 10. Due to the above 
movements, the magnetic tape 15 forms a loop shown by the solid line of 
FIG. 1. 
Next, a ring 25 rotates in a clockwise direction. In conjunction with this 
movement, a guide pole 26 and a pinch roller 27 provided on the ring 25 
also rotate in a clockwise direction from positions shown by the one-dot 
chain line to positions shown by solid lines of FIG. 1. Here, the guide 
pole 26 engages to and draws the magnetic tape 15 around the guide drum, 
and load the magnetic tape 15 onto a travelling path of a loop shown by 
the two-dot chain line of FIG. 1 upon completion of the rotating movement 
by the ring 25. After the magnetic tape 15 is thus loaded, a capstan 28 
and the pinch roller 27 drive to travel the magnetic tape. 
The magnetic tape drawn out from the tape supply roll 11 is erased of its 
video information by an erasing head 29 upon recording, and makes contact 
with the guide drum 30 provided with rotating video heads through the 
tension pole 19. The video signals are recorded on and/or reproduced from 
the magnetic tape 15 by the rotating video heads. The audio signals are 
recorded and/or reproduced at an audio head 31. Accordingly, the magnetic 
tape is driven by the capstan 28 and the pinch roller 29, guided by the 
guide pole 26, a guide pole 32 and the guide pole 20, and further travels 
through a tension pole 33 and the pin 14, to be wound up by the take-up 
roll 12. 
In a state where the lever 21 is in a position shown by the solid line of 
FIG. 1, its arm part 21a makes contact with a core 35 of a differential 
transformer 34. When the tension of the tape 15 guided by the tension pole 
19 varies, the lever 21 undergoes a rotational displacement together with 
the arm part 21a, and the core 35 accordingly undergoes displacement in 
the differential transformer 34. Similarly, when the tension of the tape 
15 guided by the tension pole 33 varies, a lever 36 undergoes rotational 
displacement, and a core 38 accordingly undergoes displacement in a 
differential transformer 37. 
The reel discs 17 and 18 are respectively rotated by reel motors 39 and 40. 
The motors 39 and 40 are respectively controlled of their rotation by 
motor rotation control circuits 41 and 42. Rotating bodies 43 and 44, each 
provided with teeth, are respectively provided on the rotating axes of the 
motors 39 and 40. Rotation detectors 45 and 46 each of which comprises a 
photo-interrupter, are respectively provided having the teeth of the 
rotating bodies 43 and 44 inserted therein, so that the rotations of the 
rotating bodies 43 and 44 can be detected. The detected output signals of 
the detectors 45 and 46 can be supplied to the motor rotation control 
circuits 41 and 42. 
The fast-forwarding and rewinding of the tape 15 are performed in a state 
where the tape 15 forms a loop shown by the solid line of FIG. 1. The 
operation upon this state will now be described in conjunction with FIGS. 
2 and 3. However, since the motor rotation control circuits 41 and 42 both 
comprise the same circuit system shown in FIG. 2, both the motor rotation 
control circuits 41 and 42 will be described in conjunction with FIG. 2 in 
order to simply the illustration of drawing. 
First, the operation upon rewinding of the tape will be described. In the 
motor rotation control circuit 42 during this state, when a switch 50 is 
closed, a low and constant voltage from a voltage source 51 is applied to 
the motor 40 through the switch 50, a diode D3 and a motor driving circuit 
52. Hence, the motor 40 applies a back tension on the tape being rewound 
to the tape supply roll 11 from the tape take-up roll 12. Furthermore, no 
voltage is supplied to the motor driving circuit 52 through switching 
circuits 55 and 65. 
On the other hand, in the motor rotation control circuit 41, the detection 
output signal from the rotation detector 45 is frequency-discriminated at 
a frequency discriminator 53, and then supplied to a loop filter 54 which 
lowers the gain of the high-frequency components in the loop of the loop 
system so that the loop system does not oscillate. The loop filter 54 
comprises a lag-lead filter having resistors R1 and R2 and a capacitor C1, 
as shown in FIG. 3. 
In a normal state before the rewinding operation, a switching signal is 
applied to the base of a transistor Q1 of the switching circuit 55 from a 
terminal 56, and the transistor Q1 is in an ON state. In this state, 
before the rewinding is started, no signals are obtained from the 
rotational detector 45 since the motor 39 is not rotating. Hence a maximum 
output signal is obtained from the frequency discriminator 53 and supplied 
to the loop filter 54 through a terminal 70. However, because the 
transistor Q1 is in an ON state and an output terminal 71 is therefore 
grounded, the output signal is not obtained from the output terminal 71. 
Furthermore, the capacitor C1 of the loop filter 54 discharges through the 
transistor Q1. 
Next, when the signal supplied to the base of the transistor Q1 is 
interrupted responsive to a start signal just upon starting of the 
rewinding operation, the transistor Q1 becomes OFF. Moreover, the motor 39 
starts rotating responsive to the start signal, an output signal of the 
frequency discriminator 53 responsive to the rotational detection output 
signal from the rotation detector 45 is supplied to the loop filter 54, 
and the capacitor C1 is charged. Accordingly, the terminal voltage of the 
capacitor C1 gradually rises, responsive to the time constant which is 
determined by the values of the resistors R1 and R2 and the capacitor C1. 
Therefore, a voltage which gradually rises is supplied to the motor 
driving circuit 52 from the loop filter 54 through the diode D1, and the 
motor 39 gradually starts to rotate. Hence, the motor 39 does not rapidly 
start a high-speed rotation, and the tape never receives any shock, and 
introduction of unevenness in the winding of the tape is prevented. 
The level of the output signal of the frequency discriminator 53 gradually 
decreases after the motor 39 starts rotating, and the motor 39 is 
controlled of its rotational speed to be constant, after completion of the 
charging of the capacitor C1 and reaching of the rotational speed of the 
motor 39 to a predetermined rotational speed. 
In order to stop the rewinding of the tape, a stop signal is applied to the 
terminal 56 to turn the transistor Q1 ON. The output side of the loop 
filter 54 is thereby grounded, and no voltage is obtained from the 
terminal 71, and thus the motor 39 immediately stops its rotation. 
Furthermore, the switch 50 is opened, and the supply of the constant 
voltage to the motor 40 is stopped. 
In the case of fast-forwarding operation of the tape, the operations 
performed on the motors 39 and 40 are opposite to those of the above. That 
is, a low voltage for applying back tension is applied to the motor 39, 
and the output signal from the above loop filter 54 is applied to the 
motor 40. This operation is similar to that described above, and hence the 
description will be omitted. 
Description on the tape tension control system will now be made. In 
accordance with the loading operation, the switching circuit 65 is opened 
by the supplying of an OFF-signal of the tension servo-system to a 
terminal 66. Furthermore, the switch 50 is closed and constant voltages 
are applied to the motors 39 and 40, introducing torques of predetermined 
values in the motors 39 and 40 so that the tape is subjected to a tension 
of a predetermined value. 
On the other hand, when a signal is applied to a control terminal 62, an 
NPN transistor Q3 in a switching circuit 61 shown in FIG. 4 becomes ON, 
and a PNP transistor Q2 accordingly becomes ON. Hence, a DC voltage from a 
DC voltage source 60 is voltage-divided by resistors R9 and R10, and is 
applied to the base of a transistor Q4 of a switching circuit 59 through 
the switching circuit 61. Accordingly, the transistor Q4 becomes ON, and a 
capacitor C2 of a loop filter 63 is charged. 
An output signal detected from a tape tension detector 57 comprising the 
differential transformer 34 in accordance with the tape tension, is 
converted to a DC signal by a DC converter 58. In the case where the 
impedance between the emitter and collector of a transistor Q2 shown in 
FIG. 4 is substantially smaller than that of a resistor R14, and the 
impedances of the resistors R9 and R10 are respectively substantially 
smaller than those of resistors R5 and R6, the voltage at the base of the 
transistor Q4 becomes a fixed voltage determined by the resistors R9 and 
R10. Here, the voltage from the voltage source 60 applied to the 
transistor Q4, is applied to the loop filter 63, and the output of the DC 
converter 58 is not applied to the loop filter 63. 
When the tape is fed to a position shown by the one-dot chain line of FIG. 
1 due to the rotation of the ring 25, a micro-switch (not shown) closes, 
and the applying of the signal to the terminal 62. Accordingly, the 
transistors Q3 and Q2 are turned OFF, and their impedances become high as 
compared to that of the resistor R14. Hence, the output from the DC 
converter 58 which is higher than the voltage from the voltage source 60, 
is applied to the transistor Q4, and charges the capacitor C2. On the 
other hand, a signal is applied to the terminal 66 to turn the switching 
circuit 65 ON, and the signal in the tension servo loop is provided to the 
motor driving circuit 52 through the diode D2. 
Moreover, since a stable predetermined voltage which has undergone 
voltage-division by the resistors R9 and R10 upon loading, is applied to 
the capacitor C2 beforehand, the capacitor C2 is charged up to a 
predetermined capacity within a short period of time. The output from the 
DC converter 58 is applied to the motor 39 (or 40) through the switching 
circuit 59, loop filter 63, a DC amplifier 64, switching circuit 65, and 
motor driving circuit 52. 
Accordingly, because the capacitor C2 is applied with a predetermined 
voltage beforehand, and charged, the tension servo system operates on the 
motor 39 (or 40) almost simultaneously as the loading is completed, and 
hence a predetermined tape tension can be obtained, allowing for a smooth 
shift from the loading mode to the play mode. 
Next, an embodiment of a concrete circuit of the motor driving circuit 52, 
will be described in conjunction with FIG. 5. 
A current from an AC power source 80 flows through windings 80a and 81b of 
the motor 39 (or 40), a diode bridge rectifier circuit 82, a transistor 
Q5, and a resistor R18. The resistor R18 for detecting the motor operating 
current, and a smoothing circuit comprising a resistor R17 and a capacitor 
C3, are connected in parallel between the emitter of the transistor Q5 and 
the ground. The connection point between the resistor R17 and capacitor C3 
is connected to the gate of a field-effect-transistor (FET) 83. The drain 
of the FET 83 is connected to the base of the transistor Q5 as well as to 
the resistor R15, and its source is connected to a current limiting 
resistor R16. 
When the rotating speed of the motor 39 (or 40) is held constant by 
controlling the conducting state of the transistor Q5 by use of a control 
signal from a terminal 84, the voltage between the terminals of the 
resistor R18 is smoothed by the resistor R17 and capacitor C3, and 
conducts (operates) the FET 83 according to the magnitude of this voltage. 
A voltage corresponding to the voltage between the terminals of the 
resistor R18 is fed back to the base of the transistor Q5. 
In this case, when a voltage V.sub.IN of the control signal from a terminal 
84 is relatively high, the voltage between the terminals of the resistor 
R18 becomes high, and hence the gate voltage of the FET 83 accordingly 
becomes high, putting the FET 83 into a fully conducting state. Hereupon, 
a voltage equal to R16/(R15+R16). V.sub.IN is applied to the base of the 
transistor Q5. If the impedance of the circuit connected to the collector 
of the transistor Q5 is designated by Z, the gain of the circuit is 
(Z/R18).times.[R16/(R15+R16)]. On the other hand, when the voltage 
V.sub.IN is relatively low, the voltage between the terminals of the 
resistor R18 becomes low, and thus the gate voltage of the FET 83 
accordingly becomes low, putting the FET 83 into a nonconducting state. 
Here, the voltage V.sub.IN is applied to the base of the transistor Q5, 
and the gain of the circuit is then Z/R18. 
Accordingly, sufficient varying range of the circuit gain can be obtained, 
namely, in a range varying from (Z/R18).times.[R16/(R15+R16)] to Z/R18. 
Furthermore, when the series impedance of the FET 83 and the resistor R16 
is designated by X, the gain on the input side is X/R15, and the gain as a 
whole becomes (Z/R18).times.(X/R15). 
On the other hand, because the emitter voltage of the transistor Q5 is 
detected by the resistor R18 and fed-back to its base, when the control 
voltage V.sub.IN is constant, the base voltage of the transistor Q5 can be 
held constant, for example, even when there are fluctuations in the output 
of the AC power source of unevenness in the temperature characteristic of 
the transistor Q5, and controllable in a stable manner. 
Further, this invention is not limited to these embodiments but various 
variations and modifications may be made without departing from the scope 
of the invention.