Apparatus for controlling linear tracking arm in record player

A control system for a linear tracking arm in a record player. A signal representative of a tracking error angle of the arm is, after A/D conversion, supplied to a microprocessor for producing a control signal which controls an arm drive device so that the tracking error angle constantly remains zero. For invalidating an offset quantity possibly produced in the output of a tracking error angle detecting sensor due to changes in the characteristic thereof, a signal produced by the sensor when the arm is lifted in precedence to the playback operation is stored in a memory of the microprocessor. During the playback, the tracking error angle is detected with reference to the stored value. An average moving speed of the arm is measured for discriminating transient increasing in the speed of the arm caused by eccentricity of the disc from the increasing of the speed caused by the arm entering a lead-out groove of the disc.

The present invention relates generally to an apparatus for controlling a 
linear tracking arm in a linear tracking type record player or the like. 
In particular, the present invention concerns a linear tracking arm 
control system in which a microcomputer is made use of for performing 
servo-control in such a manner that a tracking error angle remains 
constantly zero, with a simplified circuit arrangement which nevertheless 
assures stabilized servo-control characteristics. 
The record player of a so-called linear tracking type in which a tone arm 
is linearly displaced in the direction radially of a record disc in 
playback operation has been increasingly used. In the record player of 
this type, it is desirable that a needle mounted at a free front end of 
the tone arm and an arm base adapted to support the tone arm at the rear 
end thereof are located on a line extending tangentially to a circular 
tone groove formed in the record disc. In practice, however, there often 
occur such situations that the needle is caused to follow the leading 
movement of the base arm sometimes with difficulty or inversely the arm 
base portion is caused to follow the tracking movement of the needle under 
the influence of frictional resistances between the needle and the record 
disc on one hand and between the arm base portion and a guide member 
therefor on the other hand. As the consequence, the tone arm is compelled 
to move with a certain angle to the tangential direction of the tone 
groove. This angle is referred to as the tracking error angle. 
FIG. 1 of the accompanying drawings shows schematically a typical example 
of the hitherto known systems for controlling the tracking movement of the 
tone arm. Referring to the figure, a tracking error angle produced upon 
movement of the cartridge 1a mounted at the free front end of a tone arm 1 
is detected by a sensor 2 and, after having been amplified through a 
differential amplifier 3, supplied to a motor drive circuit 5 by way of a 
filter 4 having an appropriate transfer function, whereby the arm base 7 
is caused to move in the direction to cancel out the tracking error angle 
by a motor 6 which is driven by the motor drive circuit 5. A reference 
voltage V.sub.1 applied to the reference input of the differential 
amplifier 3 is so adjusted as to be equal to the voltage derived from the 
output of the sensor 2 when the tracking error angle of the tone arm 1 is 
zero. The serve-control system mentioned above exhibits complicated 
mechanical resonance characteristics ascribable to the inherent mechanical 
arrangement and requires the filter circuit 4 imparted with 
correspondingly complicated filter characteristic in order to attain the 
stabilized servo-control characteristic. Consequently, implementation of 
this serve-control system by electronic components or circuitries will 
require a large number of circuit elements, involving high expenditure. 
Further, it is noted that the output characteristic of the sensor 2 for 
detecting the tracking error angle undergoes variations as time elapses. 
In such case, the output of the sensor 2 produced when the tracking error 
angle is zero becomes different from the reference voltage V.sub.1 applied 
to the reference input of the differential amplifier 3, resulting in that 
an offset error angle is produced to make it impossible or difficult to 
obtain the true tracking error angle, giving rise to another problem. 
In the hitherto known linear tracking arm controlling system, there is 
provided a trace speed detecting circuit 9 for detecting that the tone arm 
has been moved to the end of the record groove. This circuit 9 serves to 
detect the tracing or tracking speed of the tone arm 1 and convert the 
latter into a corresponding voltage which is then supplied to a logic 
circuit 10 to be compared with a reference value set internally in the 
logic circuit 10. Describing in more detail in this connection, the 
tracing speed of the tone arm is relatively low so long as the needle 
point is tracing or tracking the tone groove. However, no sooner the 
needle point has entered a lead-out groove formed in the vicinity of the 
center of the record disc from the tone groove than the trace speed is 
steeply increased. When a voltage representing proportionately the trace 
speed of the tone arm as detected by the trace speed detecting circuit 9 
becomes higher than a preset reference value, a return signal is produced 
for controlling the motor drive circuit 5 to lift and return the tone arm 
to the rest or starting position. In the case of the groove end detecting 
apparatus of the trace speed detection type in which the tracing speed is 
solely compared with the reference value, as mentioned above, there arises 
such a problem that the return signal may be produced to reset the tone 
arm to the rest position even in the cource of tracing the tone groove, 
when the tracing speed is instantaneously increased due to possible 
eccentricity of the record disc. 
It is therefore an object of the present invention to provide a linear 
tracking arm control system in which the problems of the prior art systems 
are solved satisfactorily. 
In view of the above and other objects which will be made more apparent as 
description proceeds, there is provided according to an aspect of the 
present invention a linear tracking arm control system in which the 
tracking error angle of a linear tracking arm is detected to produce a 
corresponding electric signal, which is then supplied to a microprocessor 
to derive a servo-control signal which can cancel out the tracking error 
angle. With such arrangement, a complicated transfer function inclusive of 
non-linear components can be easily realized by making use of inherent 
functions of the microprocessor such as arithmetic operation, storage 
capability, decision making function and the like, whereby stable 
servo-control characteristics can be attained. 
According to another aspect of the present invention, there is provided a 
linear tracking arm control system in which the offset error angle of the 
linear tracking arm is previously written and stored in a memory 
incorporated in the microprocessor every time playback operation is to be 
effected, wherein the adjustment of the offset error angle is rendered 
unnecessary by comparing the angle value stored in the memory with the 
actual tracking error angle produced in the course of playback operation, 
while assuring insusceptibility to the influence of the offset error angle 
due to the changes in the characteristic of the sensor as possibly brought 
about as time elapses. With this arrangement, the offset error angle of 
the tone arm is newly set upon every playback operation to allow the 
preset offset error angle to be compared with the actual tracking error 
angle. Thus, the tone arm can trace the groove with the correct offset 
angle being maintained and without being subjected to the influence of the 
change in the sensor characteristic as brought about as a function of age, 
whereby the tracking or tracking performance of the tone arm is 
significantly improved, to an advantage. 
According to a further aspect of the present invention, there is provided a 
linear tracking arm control system in which position detector means for 
converting the movement or instantaneous position of the arm into a series 
of position pulses to detect that the arm has been moved to the end 
position of a record disc, by counting the position pulses by means of a 
pulse counter for a predetermined time interval. When the contents or the 
pulse number of the pulse counter exceeds a preset reference value, the 
arm return signal is generated. With this arrangement, the return signal 
is produced on the basis of the average displacement of the arm, whereby 
the end position can be detected with an enhanced reliability without 
being likely returned to the rest position on the way under the influence 
of the eccentricity of the record disc, as is the case of the prior art 
control system.

Referring to FIG. 2 which shows a linear tracking arm control system 
according to an embodiment of the present invention in which the transfer 
function of an arm servo-control loop is generated by a microprocessor, a 
tracking error angle produced as a cartridge 11a mounted on a tip end of a 
tone arm is moved is detected by a sensor 12 whose analogue output signal 
is converted into a corresponding digital signal by an analogue-to-digital 
or A/D converter 13 to be inputted to the microprocessor 14. The digital 
signal or data thus inputted is arithmetically processed by the 
microprocessor 14 to produce a driving pulse signal which is then 
converted into a corresponding analogue signal by a D/A converter 15 to be 
utilized for rotating a motor 17 through an arm drive circuit 16 for 
causing an arm base 18 to be displaced. The instantaneous position of the 
tone arm 11 is detected by an arm position detector 19. With the 
arrangement mentioned above, generation of a complicated transfer function 
inclusive of non-linear components can be much facilitated by making use 
of the available functions of the microprocessor 14 such as arithmetic 
operation, storage function and the decision making function. Further, the 
transfer function can be readily modified on the software base. 
Describing in more detail in this connection, when a servo mechanism for 
controlling a given tone arm is constructed, the transfer function of the 
servo system can be previously made known through experiments. The 
transfer function exhibits extremely complicated non-linear characteristic 
ascribable to mechanical elements constituting parts of the servo-control 
mechanism. More specifically, when lead (or lag) of the tone arm with 
reference to the arm position at which the tracking error angle is zero is 
taken along the abscissa of a coordinate system while a servo-control 
output quantity required for cancelling the lead (or lag) quantity is 
taken along the ordinate, the relationship between the lead (or lag) and 
the servo output quantity presents an extremely complicated 
characteristic. In view of the above, it is taught by the present 
invention that the lead (or lag) quantity of the tone arm at a given 
instant is determined on the basis of pulse duration or width of the pulse 
signal supplied from the A/D converter 13 at that instant in accordance 
with a program stored in the microprocessor 14, and that the servo output 
quantity required for cancelling out the lead (or lag) is arithmetically 
determined on the basis of data stored in the memory of the microprocessor 
14 to thereby produce at any instant the correct servo output quantity for 
cancelling out accurately the lead (or lag) of the tone arm. 
A minute change of the transfer function can be finely regulated by 
modifying the program only a bit. 
FIG. 3 shows in more concrete a linear tracking arm control system 
according to a first embodiment of the invention. In this figure, parts or 
components having substantially same functions as those shown in FIGS. 1 
and 2 are denoted by the like reference numerals. Further description of 
these parts will be unnecessary. The sensor 12 is composed of a 
photocoupler 20 constituted by a light emitting element and a light 
receiving element disposed in opposition to each other with a distance 
therebetween and a shutter member 21 swingable into the space defined 
between the paired elements of the photocoupler 20 in compliance with the 
swing movement of the tone arm 11. The A/D converter 13 is composed of 
transistors Q.sub.1 and Q.sub.2, a resistor R.sub.1 and a capacitor 
C.sub.1. Broken line blocks attached with the reference numerals 15/16 
represent motor drive circuits composed of operational amplifiers 22 and 
23, resistors R.sub.2, R.sub.3 and R.sub.4, R.sub.5 and capacitors C.sub.2 
and C.sub.3, respectively. Each of the motor drive circuits exhibits a low 
pass characteristic and serves for the function which corresponds to those 
of the D/A converter 15 and the arm drive circuit 16 shown in FIG. 2. 
With the arrangement described above and shown in FIG. 3, an analogue 
voltage produced by the sensor 12 in proportion to the tracking error 
angle defined hereinbefore is applied to the base of the transistor 
Q.sub.2 to cause the collector current I.sub.1 thereof to be varied. In 
this connection, it should be noted that when the transistor Q.sub.1 is 
periodically turned on and off by a pulse signal produced from the 
microprocessor 14 to thereby charge and discharge the capacitor C.sub.1 
repetitionally, the tracking error angle can then be given in terms of a 
time taken for the collector potential of the transistor Q.sub.2 to attain 
a predetermined reference voltage. In other words, there can be obtained 
from the collector of the transistor Q.sub.2 a pulse signal of which width 
or duration varies in proportion to the tracking error angle. This pulse 
signal is supplied to the microprocessor 14 to be processed in accordance 
with a program stored in the microprocessor for producing the servo output 
signal which can cancel out the tracking error angle. The output signal 
from the microprocessor 14 is of course utilized for correspondingly 
controlling the motor 17 through the D/A converter/arm drive circuits 
5/16, resulting in that the arm base 18 is displaced by the motor 17 to 
compensate the tracking error angle. 
It will be appreciated that the control circuit can be realized 
inexpensively in a much simplified manner by using the A/D converter 13 
which is capable of varying the pulse width of a pulse signal in 
accordance with the voltage signal representative of the tracking error 
angle, as is the case of the embodiment of the invention described above. 
In the linear tracking arm control system described above, the tracking 
error angle of the linearly driven tone arm is detected to thereby produce 
a corresponding electric signal which is then supplied to the 
microprocessor to be so processed as to produce as the output of the 
microprocessor the servo output signal for compensating or cancelling out 
the tracking error angle. It will be appreciated that the complex transfer 
function inclusive of non-linear components can be easily realized by 
making use of the arithmetic function, storage function and the decision 
making function which are inherent to the microprocessor, whereby the 
stabilized servo characteristic can be attained. 
Next, description will be made on an improvement of an initial 
characteristic deterioration brought about by the changes occurring in the 
characteristic of the sensor 12 as time elapses. 
In general, in the linear tracking arm control system of this type, the 
output characteristic of the sensor 12 which can be originally represented 
by a curve A shown in FIG. 4 will be changed to the characteristic 
represented by a curve B shown in the same figure as a function of time 
lapse, whereby the sensor output voltage V.sub.o corresponding to the zero 
offset angle of the tone arm 11 is changed to a sensor output level 
V.sub.o '. As the consequence, the sensor 11 produces the output signal 
(V.sub.o ') even when the offset error angle of the tone arm is zero to 
thereby cause the tone arm to be displaced through the arm driving motor 
17, resulting in that the so-called offset error angle is generated and 
this deteriorates the tracking performance of the tone arm 11. 
With a view to solving the above problem, it is taught according to another 
feature of the present invention to zero constantly the offset error angle 
regardless of changes in the output characteristic of the sensor 12 
possibly brought about as time elapses with the aid of a structure shown 
in FIGS. 5 to 7 and the processings illustrated in flow charts of FIGS. 8 
and 9. 
Referring to FIG. 5 which shows a linear tracking arm control system 
according to another embodiment of the present invention, low pass filters 
each denoted by reference numerals 25 and 25' and having the substantially 
same construction as the circuits 15 and 16 shown in FIG. 3 are connected 
to output terminals O.sub..phi. and O.sub.1, respectively, of the 
microprocessor 14 to linearize the offset error angle signals produced 
from these output terminals. Further, driver circuits denoted by reference 
numerals 26 and 26' and composed of transistors Q.sub.3 and Q.sub.4 and 
resistors R.sub.8 and R.sub.9, respectively, are connected to output 
terminals O.sub.2 and O.sub.3 of the microprocessor 14, respectively, so 
that the output signals making appearance at the output terminals O.sub.2 
and O.sub.3 of the microprocessor 14 are applied to the bases of the 
transistors Q.sub.3 and Q.sub.4, respectively, to thereby control the 
driving voltage applied to the motor 17. 
Referring to FIGS. 6 and 7, the tone arm 11 is provided with the cartridge 
11a and a balancing weight 27 at the front and the rear end thereof, 
respectively, and is supported by an arm supporting mechanism 28 at an 
intermediate portion so as to be rotatable horizontally and vertically. 
The tone arm 11 is provided with a V-like projection 11b at the lower 
surface. The projection 11b is adapted to engage in a V-like groove 30a 
formed in an upper surface of a lift bar 20 which is adapted to be 
reciprocately moved by a cueing plunger 29. 
The microprocessor 14 is adapted to execute a plurality of main routines 
illustrated in FIG. 8 and a subroutine illustrated in FIG. 9. Referring to 
FIG. 8, the main routines include an initializing main routine 31, a 
standby main routine 32, a start main routine 33 and a play main routine 
34. In the initializing main routine 31, initial values are set in a 
random access memory of the microprocessor in response to the start 
command, which are followed by setting of initial states. In the standby 
main routine 32 which follows the initializing main routine 31, reading of 
data inputted by key operation is effected, and it is determined whether 
or not the start key is actuated. Unless the start key is actuated, the 
reading of the key input is again executed. On the other hand, when the 
start key is actuated, the execution proceeds to the start main routine, 
in which a pre-offset angle of the tone arm 11 upon lifting thereof which 
will be defined hereinafter is read in. Subsequently, the size of a record 
to be played are detected and the number of rotations is set. Next, it is 
determined whether or not the tone arm is in the lead-in position of the 
record. Unless the tone arm is in the lead-in position, the tone arm is 
displaced in the forward or inward direction and it is again checked 
whether or not the tone arm is at the lead-in position. This process is 
repeated until it is decided that the tone arm is at the lead-in position 
of the record. In the succeeding play main routine 34, the offset servo 
control is activated and it is determined whether or not the tone arm has 
come to the final position of the record. If the tone arm is not at the 
final end position of the record, activation of the offset servo control 
is continued. On the other hand, when it is detected that the tone arm is 
at the final end position, return is made to the initializing main routine 
31 described above. On the other hand, in the subroutine illustrated in 
FIG. 9, detection of a pre-offset error angle is initiated in response to 
a start command. After lapse of a predetermined standby time, data of the 
pre-offset error angle employed in the preceding playing operation is 
cleared and a new pre-offset error angle is set. Subsequently, it is 
determined whether or not a pre-offset error angle is present or not. If 
the pre-offset error angle is absent, detection of the pre-offset error 
angle is terminated. Otherwise, data of the pre-offset error angle is 
loaded into the pre-offset error angle counter, which is followed by 
determination as to whether the contents of the pre-offset error angle 
counter has attained a predetermined value. If the result of the decision 
is negative, the process of loading the pre-offset error angle data into 
the pre-offset error angle counter is repeated until the pre-determined 
value has been attained. 
In the linear tracking arm control systems described above, the 
microprocessor 14 includes a memory for storing the output value of the 
sensor 12. Data of the offset angle of the tone arm 11 corresponding to 
the zero offset angle of the tone arm, that is, the output value of the 
sensor 12 at the time when the tone arm 11 is lifted up with the V-like 
projection 116 of the tone arm engaging in the V-like groove 30a of the 
lift bar 30, is stored in the memory as the pre-offset angle data 
mentioned hereinbefore. In the course of the playback operation, the 
offset error angle of the tone arm is detected by the sensor 12 and 
converted into a corresponding voltage which is then converted into a 
corresponding time duration or a pulse having a corresponding width by 
means of the A/D converter 13, as described hereinbefore in conjunction 
with FIG. 3. The data of time duration thus obtained is inputted to the 
microprocessor 14 which arithmetically determines magnitude of the offset 
angle from the time duration data and outputs the corresponding digital 
signal. The digital signal is caused to pass through the low pass filter 
25 to be converted into a corresponding DC voltage which is applied to the 
drive circuit 26 for driving the motor 17. The output of the sensor 12 
corresponding to the zero offset error angle of the tone arm 11 (this 
output corresponds to the pre-offset error angle mentioned hereinbefore) 
is converted into a pulse signal having a corresponding time duration or 
pulse width by the A/D converter 13 and stored in the memory of the 
microprocessor 14. The storing operation of the pre-offset angle is always 
effected in precedence to the starting of the playback operation, whereby 
the old data used in the preceding playback operation is replaced by new 
data for the succeeding playback operation. When the record player is put 
into operation with the tone arm 11 tracing the tone groove, the output 
signal from the sensor 12 is converted into the pulse signal having a 
corresponding pulse duration by the A/D converter 13. The offset error 
angle data thus determined by the microprocessor on the basis of the pulse 
duration is compared with the pre-offset error angle data. When it is 
detected through the comparison that the tracking error angle is produced, 
the microprocessor 14 supplies an output signal to the filter 25 to 
control the arm driving motor 17 through the drive circuit 25 so that the 
tracking error angle becomes zero. In this way, by storing the pre-offset 
angle in the memory incorporated in the microprocessor 14, adjustment of 
the pre-offset error angle can be rendered unnecessary. In other words, 
the troublesome adjustment of the pre-offset angle which is required upon 
every playback operation in the hitherto known control system is made 
utterly unnecessary according to the present invention by storing the 
pre-offset error angle in the memory of the microprocessor 14. Further, 
since the pre-offset error angle is newly stored or updated in the memory 
of the processor 14 without fail in precedence to the playback operation 
to be compared with the actual error angle, the tracking performance of 
the tone arm will undergo no influence even when the voltage value 
representative of the pre-offset error angle is varied as brought about by 
the change in the characteristic of the sensor. 
As will be appreciated from the above description, the adjustment of the 
pre-offset error angle is made unnecessary by virtue of the feature of the 
invention that the pre-offset error angle is previously stored and 
compared with the detected error angle in the course of the playback 
operation. Further, since the pre-offset error angle is always newly 
stored or updated in precedence to every playback operation, no 
degradation will occur in the tracing performance of the tone arm during 
the playback operation even if the output of the sensor should be changed 
due to deterioration of the sensor characteristic. 
In the foregoing description, it has been assumed that the pre-offset error 
angle is stored in the memory of the microprocessor after the A/D 
conversion. However, it will be readily appreciated that the pre-offset 
error angle may be directly written in an analogue memory. 
Next, referring to FIG. 3, description will be made about the function for 
detecting that the tone arm 11 has been moved to the final end position of 
the record. In FIG. 3, a reference numeral 35 denotes a photocoupler of 
the structure similar to that of the photocoupler 20 which includes a slit 
disc 36 adapted to be driven by the motor 17 so that at least the 
peripheral portion of the disc having a plurality of radial slits formed 
therein passes through a space defined between the light emitting element 
and the light receiving element of the photocoupler 35. An arm position 
detector 19 for detecting the instantaneous positon of the tone arm 11 
(also refer to FIG. 2) in the form of the pulse signal is constituted by 
the photocoupler 35 and the slit disc 36. The pulse signal output from the 
tone arm position detector 19 is inputted to the microprocessor 14. 
Further provided is a synchronizing signal generator 37 which is adapted 
to generate a synchronizing signal in synchronism with rotation of a 
turntable 38. The synchronizing signal is also supplied to the 
microprocessor 14 to be utilized for determining the time taken for the 
turntable 38 to complete a single rotation. During a time interval 
corresponding to the single complete rotation of the turntable 38, the 
number of pulses produced by the arm positon detector 19 is counted to 
detect the speed at which the tone arm 11 is moved. When the speed exceeds 
a predetermined reference value, it is determined that the tone arm 11 has 
reached the final position of the record, whereupon the tone arm 11 is 
returned to the rest or starting position. 
In this manner, the average moving speed of the tone arm 11 during the 
single complete rotation of the turntable 38 is measured, whereby 
erroneous decision of the turntable having reached the final end position 
of the record which might otherwise be made upon transient increasing of 
the moving speed of the tone arm as brought about by eccentricity of the 
disc or other causes to thereby return unintentionally the tone arm to the 
rest position can be positively excluded. 
The determination of the average moving speed of the tone arm may be 
effected by counting the number of pulses produced during a predetermined 
period set by a timer rather than the period corresponding to the single 
rotation of the turntable 38.