Servo system for positioning and driving a movable member using two reference voltages

This invention relates to a servo system for positioning and driving a movable member such as a pickup head of an optical reading system, for example. A sinewave signal is generated by a position detector in response to movement of the movable member. A first reference signal is used with the sinewave signal to position the movable member. A second reference signal is used with a modified signal which is generated by differentiating and rectifying the sinewave signal, to drive the movable member.

BACKGROUND OF THE INVENTION 
In an optical reading system, for example, an optical pickup head is 
controlled in a radial direction to precisely follw any of a plurality of 
information tracks spirally formed on a rotatable disc in a normal 
playback mode and controlled in the same direction to move from one track 
position to another track position in an access mode. Such a syetem is 
disclosed in U.S. Pat. No. 4,397,009. In the U.S. Patent, a tracking zero 
signal of sinewave is detected in response to the radial movement of the 
pickup head relative to the rotatable disc and utilized to control the 
position of the pickup head. However, such a prior system is required to 
be separately provided with a positioning servo loop to control the pickup 
head so as to maintain the pickup head on an information track and a 
driving servo loop to move the pickup head to another position. Thus, it 
will be noted that the whole servo system is complicated and has many 
parts, which causes the servo system to be expensive. 
SUMMARY OF THE INVENTION 
Accordingly, it is a principal object of the invention to provide a servo 
system for positioning and driving a movable member adapted to be 
simplified by positioning and driving the movable member by means of a 
single servo loop and therefore inexpensively obtained without many parts. 
Another object of the invention is to provide a servo system for 
positioning and driving a movable member adapted to drive the movable 
member at constant velocity. 
In accordance with the present invention, there is provided a servo system 
for positioning and driving a movable member comprising means to generate 
a position signal of sinewave in response to movement of said movable 
member with a frequency of said position signal depending on a velocity of 
movement of said movable member; means to differentitate said position 
signal of sinewave and to rectify the thus differentitated signal to 
generate a modified signal; means to selectively generate one of first and 
second reference signals of different DC voltage; means to selectively 
compare said position signal with said first reference signal to generate 
a first control signal and said modified signal with said second reference 
signal to generate a second control signal; and means to control said 
movable member to position said movable member in response to said first 
control signal and to drive said movable member in response to said second 
control signal.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Referring now to FIG. 1, there is shown a servo system for positioning and 
driving a movable member such as a pickup head for an optical reading 
system, for example. The movable member may be driven by a motor 14 such 
as a linear motor which is shown in FIG. 2. 
The servo system comprises a control 1 and a controlled section 8. The 
motor 14 is controlled by a power amplifier 12 in the controlled section 8 
as shown in FIG. 2. As shown in FIG. 1, an operational amplifier 2 has a 
(+) input terminal connected to a first output terminal 1A and a (-) input 
terminal connected through a resistor R1 to a DC potential +Vcc and also 
through a resistor R2 to a DC potential -Vcc. An output terminal of the 
operational amplifier 2 is connected through a forwarded diode D1 and a 
resistor R3 to a (+) input terminal of an operational amplifier 4. An 
operational amplifier 3 has a (+) input terminal connected through the 
resistor R1 to the DC potential +Vcc and also through the reistsor R2 to 
the DC potential -Vcc and a (-) input terminal connected to a second 
output terminal 1B of the control 1. An output terminal of the operational 
amplifier 3 is connected through a reversed diode D2 and a resistor R4 to 
the (+) input terminal of the operational amplifier 4. 
The (+) input terminal of the operational amplifier 4 is also grounded 
through a resistor R5 to earth. The operational amplifier 4 has an output 
terminal connected through a resistor R10 to a (-) input terminal thereof 
and also connected to an input terminal 8A of the controlled section 8. 
The controlled section 8 has an output terminal 8B connected to a (+) 
input terminal of a comparator 9 and also connected to a movable contact 
10A of a switch 10. The comparator 9 has a (-) input terminal grounded to 
earth. 
An OR gate 5 has input terminals connected to the first and second output 
terminals 1A and 1B of the control 1, respectively. An EXOR gate 7 has 
input terminals connected to the first output terminal 1A of the control 1 
and an output terminal of the comparator 9. The EXOR gate 7 has an output 
terminal connected to an input terminal 1C of the control 1. An AND gate 6 
has input terminals connected to an output terminal of the OR gate 5 and 
the output terminal of the EXOR gate 7, respectively. 
The switch 10 has a fixed contact 10B connected through a resistor R6 to a 
(-) input terminal of an operational amplifier 11 and a fixed contact 10C 
connected directly to a (+) input terminal of the operational amplifier 11 
and grounded through a resistor R7 to earth. The operational amplifier 11 
has an output terminal connected through a resistor R8 to its (-) input 
terminal and through a resistor R9 to the (-) input terminal of the 
operational amplifier 4. The operational amplifier 4 forms a differetial 
circuit together with the resistors R3, R4, R5, R9 and R10. The operation 
of the switch 10 is controlled by an output signal from the AND gate 6. 
In FIG. 2, references Rm and Lm express a resistance and an inductance of a 
motor coil of the motor 14, respectively, while a reference .phi.x 
expresses a magnetic flux from a motor magnet and can be expressed by the 
following formula in which reference x expresses displacement of the 
movable member. 
EQU .phi.x=n.multidot.d.phi./dx 
Furthermore, M expresses mass of the movable member, .sigma. expresses 
stiffness of a spring to support the movable member and .rho. expresses 
mechanical resistance of the spring. 
A position sensor SNS generates a position signal Vs of sinewave voltage in 
response to displacement x of the movable member. A frequency of the 
position signal Vs depends on a velocity of the movable member. The 
position signal Vs can be expressed by the folllowing formula in which P 
expresses pitch between information tracks while Vp expresses a peak 
voltage. 
EQU Vs=Vp.multidot.sin(2.pi..multidot.x/P) 
An operational amplifier 13 forms a differetiation circuit together with 
peripheral components of resistors R12, R13, R14 and a capacitor C2. The 
differentiation circuit has a predetermined frequency response. 
FIG. 3 shows an equivalent circuit of the controlled section 8 of FIG. 2. 
Inductance Ls and capacitance Cs can be determined by resonance frequency 
and resonance sharpness Q in the circuit of FIG. 2. In FIG. 3, Vm 
expresses an input voltage while Vx expresses an imaginary voltage 
proportional to the displacement x of the movable member and can be 
expressed by the following formula. 
EQU Wx=x.multidot..sigma..multidot.Rm/.phi.x 
A SNS' circuit serves to convert an input signal of imaginary voltage Vx 
into an output signal of sinewave voltage Vs. The output signal Vs can be 
generally expressed by A1.multidot.sin B1.multidot.Vx in which A1 and B1 
are constant values, but actually expressed by the following formula in 
view of a sensitivity of the differentiation circuit and when the value of 
d Vs/ d Vx is 1 as Vx is equal to 0. 
EQU Vs=Vp.multidot.sin (Vx/Vp) 
In operation, level conditions of "High" and "Low" of the signal level will 
be reffered to "Hi" and "Lo", respectively hereinafter. In the positioning 
servo mode, the output terminals 1A and 1B of the control 1 have output 
signals of "Lo". At that time, because of the set value of the resistors 
R1 and R2, the operational amplifier 2 has an output voltage of -Vc 
generated while the operational amplifier 3 has an output voltage of +Vc 
generated. Then, the operational amplifier 4 has a first reference signal 
Vr of 0 V received at the (+) input terminal thereof. On the other hand, 
the output terminals of the OR gate 5 and the AND gate 6 are "Lo" and the 
movable contact 10A of the switch 10 is connected to the fixed contact 
10C. Therefore, the operational amplifier 11 serves as a voltage follower 
of gain 1 and the output voltage signal Vr from the controlled section 8 
is introduced through the resistor R9 into the (-) input terminal of the 
operational amplifier 4 as it is. In this case, since the differentiation 
circuit serves only as an amplifier due to the low frequency of the output 
voltage signals Vs from the position sensor SNS, the output voltage signal 
Vr is one which is produced only by amplifying the output voltage signal 
Vs from the position sensor SNS. Thus, it will be noted that when the 
first reference signal Vr of 0 V is applied to the (+) input terminal of 
the operational amplifier 4 and the output voltage signal Vr is applied to 
the (-) input terminal of the operational amplifier 4, the servo loop 
serves as the positioning servo to make the output voltage signal Vr of 0 
V whereby the movable member is normally disposed at a zero cross point of 
positive inclination region of the output voltage signal Vs from the 
position sensor SNS corresponding to the position in which the movable 
member is precisely disposed on the information track. 
A driving servo mode in which the movable member moves in a forward 
direction will be described hereinjustbelow. A forward driving command 
signal "Hi" is generated from the first output terminal 1A of the control 
1 by operation of the operator. At that time, the output signal from the 
output terminal 1B is kept at "Lo". Therefore, a second reference signal 
Vr2 which is produced by dividing the output voltage +Vc from the 
operational amplifier 2 by the resistors R3 and R5 is applied to the (+) 
input terminal of the operational amplifier 4. Thus, a driving signal is 
applied from the operational amplifier 4 to the motor 14 to begin to move 
the movable member in a forward direction. The position sensor SNS 
generates the output signal Vs of sinewave voltage and the controlled 
section 8 generates an alernating output signal Vr in response to the 
output signal Vs from the position sensor SNS. 
In response to a negative polarity half of the alternating output signal 
Vr, the output signal of the comparator 9 becomes "Lo" and the output 
signal of the EXOR gate 7 becomes "Hi". Accordingly, the output signal of 
the AND gate 6 becomes "Hi", which causes the movable contact 10A of the 
switch 10 to change to the fixed contact 10B during the negative polarity 
half of the output signal Vr. While the switch 10 is moved to the fixed 
contact 10B, the operational amplifier 11 serves as an inverting amplifier 
of gain 1 under the condition of R6=R7 and applies a signal produced by 
inverting the output signal Vr to the (-) input terminal of the 
operational amplifier 4. It should be noted that at that time the 
operational amplifier 11 serves as a voltage follower while the switch is 
positioned at the fixed contact 10C in response to a positive polarity 
half of the alternating output signal Vr in the same manner as 
aforementioned. Thus, when the movable member is moved to generate the 
alternating output signal Vr from the controlled section 8, a signal Vq 
which is produced by rectifying the output signal Vr in a full-wave manner 
is applied to the (-) input terminal of the operational amplifier 4. 
In this case, the gain of the differential circuit formed by the 
operational amplifier 4 can be expressed by the following formula. 
EQU Vm=A.multidot.(Vr2-Vq) 
in which A is equl to R3/R5=R4/R5=R9/R10. 
FIG. 4 shows waveforms of the second reference signal Vr2, the alternating 
output signal Vr and the rectified signal Vq. Since the frequnecy of the 
output signal Vs from the position sensor Vs is high, the operational 
amplifier 13 serves as a differenttiating amplifier and as a result, the 
alternating output signal Vr from the controlled section 8 has a waveform 
produced by defferentiating the output signal Vs. 
Thus, it will be noted that when the second reference signal Vr2 is applied 
to the (+) input terminal of the operational amplifier 4 and the rectified 
signal Vq is applied to thw (-) input terminal of the operational 
amplifier 4, the servo loop serves to move the movable member to the point 
where the second reference signal Vr2 crosses the positive inclination 
region of the rectified signal Vq, but the movable member continues to 
move due to its inertia without stopping. At that time, since the servo 
loop serves as the differentiating servo, the alternating output signal Vr 
of voltage from the controlled section 8 is proportional to a velocity of 
movement of the movable member and therefore the rectified signal Vq of 
voltage is also proportional to the velocity of movement of the movable 
member. Thus, if the velocity of forward movement of the movable member is 
lowered, then the rectified signal Vq of voltage becomes lower than the 
second reference signal Vr2, which causes the servo loop to serve to 
increase the velocity of forward movement of the movable member. 
Reversely, if the velocity of forward movement of the movable member 
increases, then the servo loop serves to lower the forward movement of the 
movable member. Thus, it will be noted that the servo loop drives the 
movable member at constant velocity. It will be understood that the 
velocity of movement of the movable member can be set at a predetermined 
value by the value of the second reference signal Vr2. 
The output of the EXOR gate 7 becomes "Hi" at the zero cross point of the 
positive inclination region of the alternating output signal Vr from the 
controlled section 8 and becomes "Lo" at the zero cross point of the 
negative inclination region to provide the input terminal 1C of the 
control 1 with the position information of the movable member. 
The control 1 can move the movable member to the selected position of 
information tracks. For example, when the movable member is required to 
move from the position in which it is disposed at one zero cross point to 
the position in which it is disposed at n-numbered zero cross point (n is 
positive integer), the first output terminal 1A of the control 1 continues 
to be "Hi" and the second output terminal 1B of the control 1 continues to 
be "Lo" until the n-numbered raised signal is applied to the input 
terminal 1C of the control 1. The first and second output terminals 1A and 
1B are made "Lo" in response to the n-numbered raised signal whereby the 
positioning servo is returned. 
Although, in the aforementioned description, the movable member is moved in 
the forward direction, it may be moved in a reverse direction, in which 
the first output signal 1A of the control 1 is made "Lo" while the second 
output signal 1B of the control 1 is made "Hi". The operation in reverse 
movement of the movable member is substantially identical to that in 
forward movement of the movable member except for reverse polarity of the 
signals. 
Although, in the illustrated embodiment, the alternating output signal Vr 
is converted into a full-wave rectified signal, it will be understood that 
it may be converted into a half-wave rectified signal. 
While one preferred embodiment of the invention has been described and 
illustrated with reference to the accompanying drawings, it will be 
understood by those skilled in the art that it is by way of example and 
that various changes and modifications may be made without departing from 
the spirit and scope of the invention, which is intended to be defined 
only to the appended claims.