Reworking of a playback or recording member guide body

In order to effect repeated reworking of a body, whose tip, during use, engages a record carrier track to guide a playback or recording member along the track, the reworking involving subjecting the body to successive reworking operations after successive periods of use, the tip of the body being worn down to a certain extent during each period of use and material being removed from the tip to restore its original shape during each reworking operation, the useful life of the body is effectively increased by adjusting the maximum removal depth occurring during each successive reworking operation as a function of the depth to which the tip is worn during the corresponding preceding period of use.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and control apparatus for the 
repeated reworking of a body which is provided to guide a playback or 
recording member along a track on a record carrier, and particularly for a 
reworking operation in which part of the body is removed after a period of 
wear. 
Such a track may, for example, be a groove in a rotating disc or foil, 
preferably in a video record. The body to be reworked may be a diamond, in 
particular, which slides along such groove in order to guide a playback 
scanning or recording member. The scanning member preferably is a pressure 
transducer of a playback system operating according to the pressure 
scanning technique, the transducer being rigidly connected with the 
diamond body. 
The record carrier for such a system has a narrow groove width of the order 
of only a few microns and rotates at a high playback speed of, for 
example, 1500 rpm while having a diameter of 21 cm, resulting in 
relatively rapid wear of the guid body due to the high relative speed 
developed during operation between the body and the record carrier and 
despite the low contact pressure of the body against the groove walls. It 
has therefore become necessary to include in video record players designed 
to operate according to the pressure scanning technique a reworking or 
regrinding device which takes care that the diamond body is resurfaced in 
a profiled grinding groove after a certain period of use, for example 
after playing a video record, whereby part of the diamond body is removed 
to such an extent that the profile of the body again coincides with that 
of the grinding groove. 
In a known device of this type, after the playing of a video record for a 
maximum of 10 minutes, with a certain amount of resulting wear, reworking 
by way of repolishing or grinding takes place for a period of 6 seconds. 
This causes removal of such a large amount of material that the 
repolishing period is sufficient even if the contact surface of the body, 
which may have a skid shape for example, has, as an incident of previous 
repolishing operations, been enlarged to a great extent. 
With this type of reworking by repolishing, the usefulness of the body is 
maintained for a longer period than without repolishing because the 
desired profile of the body is reestablished by repolishing after each 
period of wear. Otherwise, the profile of the body would become more and 
more like the profile of the groove in the record carrier and would thus 
lose its suitability for scanning signals or for guiding a pickup member. 
This would particularly be the case if, for a conical or pyramid-shaped 
body, the period of wear would have continued to such an extent that the 
surface of the body resting on the record carrier becomes wider than the 
groove, when seen from the top, producing so-called straddling by the body 
on the groove edges. 
Repeated reworking of the body also has a drawback, however, because 
repolishing produces loss of material in addition to the loss of material 
occurring during use, which shortens the useful life of the body. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to extend the lifetime of the body 
as much as possible while simultaneously causing the profile of the body 
to continue to approximate the ideal shape for as long a period as 
possible. 
These and other objects are accomplished according to the present invention 
by performing the successive reworking operations which are carried out 
during the lifetime of the body in such a manner that the maximum removal 
depth occurring during a given reworking operation is a function of the 
depth to which the tip of the body has been worn during the corresponding 
preceding period of use. 
The invention is based on the realization that in the prior art repolishing 
procedures in which a reworking process of constant duration is provided 
after each period of wear, for example after the playing of one video 
record, much more material is removed, particularly when the body is still 
new, than is necessary to keep the body in working order. It therefore 
results that the body prematurely reaches a state in which removal during 
the fixed polishing period will no longer be sufficient. This state is 
reached at the latest when the body has been ground down to its point of 
fastening or, in the case of a conical or pyramid-shaped body, when the 
body has been ground down to a point at which its contact surface to be 
reworked is broader, when seen from the top, than the polishing or 
grinding groove so that the body is seated on that surface of the 
polishing disc in which the polishing grooves are stamped, without the 
body being able to still touch the bottom of the polishing groove, this 
corresponding to straddling by the body on the edges of the polishing 
groove. 
A body which has been polished until it straddles the polishing groove can 
no longer be worked with the same groove, i.e., it has substantially lost 
the capability of being reworked. Due to the above-mentioned excess 
material removal when the prior art repolishing procedures are performed 
on a body which is still new, the end of the lifetime of such a body, 
determined by its ability to be reworked, is reached earlier than 
necessary. 
This drawback is overcome by the process of the present invention, and the 
lifetime of the body is thus substantially extended, by adjusting the 
removal depth of the body during the repolishing process so that it is a 
function of the polishing state of the body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 shows part of a record carrier 1 in cross section with respect to a 
plane perpendicular to the longitudinal direction of an 
information-bearing groove 2. As is the custom for video records which are 
played back according to the pressure scanning technique, groove 2 is 
rather shallow so that the angle between the groove sides is much greater 
than 90.degree.. A diamond body 3, which is only shown in part, slides in 
groove 2 and constitutes part of a pressure scanning pickup. Body 3 is 
defined, inter alia, by two octahedral surfaces 4 and 5 and preferably has 
the shape of an octahedral pyramid whose tip 6 is rounded into the form of 
a cylindrical section. The cylindrical form of this rounded tip 6 can be 
produced by grinding in a profile grinding groove. Once the shape of the 
tip has been changed, due to wear, in the direction toward an adaptation 
to the shape of the profile in groove 2, it can be reestablished by 
repolishing in an appropriately profiled polishing groove. 
FIG. 2 shows such a polishing groove 7 in a polishing disc 8 which is shown 
partially in section. Body 3, which here is shown to be polished off to a 
greater degree than in FIG. 1, slides in this groove. The dashed lines 
show the same body 3 at a later stage of its useful life, when it has been 
polished off to such an extent that it is straddling the edges 9 and 10 of 
the polishing groove and the polishing disc surface 11. 
The direction of the contact force K of the polishing disc 8 on the body 3 
which is to be reworked is indicated by an arrow. This force is equal in 
magnitude and opposite in direction to the contact force of body 3 on the 
polishing disc 8. 
FIG. 3 is a detail view, to an enlarged scale, of the central portion of 
FIG. 1. The solid line 13 represents the relative position of the record 
carrier surface when the body 3 is in its ideal, or initial, state, while 
the broken line 12 shows the relative position of the record carrier 
surface when body 3 has been subjected to such an amount of wear that 
material has been removed to correspond to the maximum wear depth N. The 
maximum wear depth is the wear depth at the points of tip 6 at which the 
greatest amount of wear occurs, starting from the initial cylindrical 
shape. Thus, wear depth N is measured from the point of contact of a 
groove wall with the not yet worn tip 6. The minimum wear depth n is 
produced at the portion of the tip which is aligned with the center of the 
groove. The amount of wear reduces the distance between a point on each 
surface 4 and 5 of body 3 and the record carrier 1 from a value A to a 
value B, i.e., by the maximum wear depth N. 
FIG. 4 shows the corresponding conditions with respect to polishing groove 
7 in polishing disc 8, the solid lines indicating a state which 
corresponds to the beginning of a reworking process. In this state the 
underside of body 3 still possesses parts of its previously rounded tip 6 
which correspond with the profile of the polishing groove 7, as well as 
parts, or facets, 14 and 15 which have assumed the profile of the record 
carrier groove 2 (FIG. 1) due to wear. Upon completion of reworking, body 
3 and the surface of polishing disc 8 have a position relative to one 
another which is indicated by the broken line 16 representing the 
polishing disc surface. Compared to the starting state, there results a 
displacement equal to the maximum removal depth T. The maximum removal 
depth is the removal depth at the point or points of the tip from which 
the greatest amount of material is removed during reworking. The minimum 
removal depth t occurs at those points where the least amount of material 
is removed during reworking, which correspond to those points where the 
record disc groove walls had worn most deeply into body 3 during playback 
or recording. 
The above-described figures, particularly FIG. 2, show that the surface of 
contact between body 3 and polishing groove 7 becomes wider the more body 
3 is polished, a state which exists long before the state of straddling 
has been reached. If it is assumed that in the repolishing technique 
employed, the contact pressure K and repolishing duration are constant, 
the maximum removal depth T will decrease, from one reworking process 
operation to another, from a relatively high value since the contact 
pressure, i.e., the ratio of contact force K to contact surface, 
decreases. In order for the maximum removal depth to remain sufficient, 
even after many reworking processes, so that wear-induced facets 14 and 15 
will disappear during repolishing, the maximum removal depth T has 
previously been selected to be so deep that repeated reworking of body 3 
would result in relatively rapid removal of material from the body until 
the state shown in broken lines in FIG. 2 had been reached. 
The present invention overcomes this drawback in that, for example, the 
repolishing period for each successive reworking process in polishing 
groove 7 is increased, in dependence on the size of the contact surface of 
body 3, beginning with low values, either continuously or in steps, during 
the course of a plurality of reworking processes. In this way the entire 
useful life of body 3 can be significantly increased, for example by a 
factor of 2. 
In a favorable practical procedure according to the invention, the first 
repolishing process period has a duration of 2.5 seconds and the 
repolishing period per each successive repolishing operation is linearly 
increased by an amount corresponding to an increase of 0.4 seconds for 
each 10 hours of total wear. This applies for the conditions described in 
connection with FIGS. 1 through 4. 
The process can also be adapted to other conditions, for example other 
shapes of body 3 or other angles between surfaces 4 and 5 (FIG. 1). If, 
for example, during the lifetime of body 3 the maximum wear depth N per 
period of wear remains approximately constant, the process of the present 
invention is carried out so that the maximum removal depth T obtained in a 
reworking process is also kept approximately constant. For a body 3 whose 
cross-sectional surface area in planes parallel to the surface of record 
carrier 1 increases at progressively greater distances from the surface of 
record carrier 1, even in those parts of body 3 which will be removed by 
wear, i.e., even the area which, although above the rounded portion, will 
be polished away until the straddle state is reached, the removal 
quantity, measured, for example, in milligrams produced per reworking 
process should be increased during the lifetime of body 3 so that, 
assuming the wear depth N is constant the maximum removal depth T remains 
constant. In other words: with a body 3 which in the area to be removed 
tapers toward the contact surface, the removal quantity per reworking 
process must be increased under the above-mentioned conditions during its 
lifetime. 
Instead of, or in addition to, prolonging the duration of successive 
reworking processes over the lifetime of body 3, it is also possible to 
increase the contact force K directed toward body 3 over its lifetime. 
This is possible, however, only within limits, due to the high compliance 
of the system and the requirement for defined angular disposition of the 
contact surface of the skid-shaped body with respect to the centerline of 
the groove. 
The lengthening of the reworking period or the increase of the contact 
force, respectively, can be controlled in dependence on the cumulative 
total of the lengths of the periods of wear experienced during the use of 
the particular body, or in dependence on the total number of wear 
processes, i.e., for example in dependence on the number of video record 
playings. 
The process of the present invention permits in an advantageous manner, for 
example, the polishing duration to be adapted to the corresponding 
changing degree of efficiency, or decreasing "sharpness," of the polishing 
groove. In order to compensate for a decrease in efficiency of the 
polishing action, it is recommended to increase the polishing period 
and/or contact pressure more than proportionally to time. 
FIG. 5 shows one embodiment of an apparatus for controlling repeated 
reworkng processes according to the present invention and including such 
control means. The duration of reworking per reworking process is here not 
controlled directly in dependence on the cumulative periods of wear of the 
body, however, but rather in dependence on the cumulative periods of use 
of the entire apparatus as long as the body in question is installed in 
this apparatus. If the body is replaced by a new one, the indication of 
the cumulative period of use must be reset to zero. 
The apparatus shown in FIG. 5 includes a drive motor M1, which causes a 
record carrier 1 to rotate. Along a rail 17 a carriage 18 can be guided 
parallel to a radius of record carrier 1. A pickup member including body 3 
is resiliently fastened to the carriage 18 in accordance with the usual 
techniques employed in pressure scanning. 
In order to play the record carrier 1, carriage 18 is brought to the edge 
of record carrier 1 and lowered so that body 3 can slide in a groove 
corresponding to groove 2 of FIG. 1. During playing, carriage 18 slides 
toward the left until it reaches the last groove turn, at which time it is 
raised and moved toward the right above the polishing disc 8 driven by 
polishing motor M2. Carriage 18 is then lowered to bring body 3 into 
contact with disc 8 in order to carry out a repolishing process. After the 
repolishing process a new playing process, with resulting wear of body 3, 
may follow. In the prior art the polishing period was fixed at a constant 
value by a timed switching mechanism. 
In the illustrated device, however, different control means are provided. 
To start the device, switch S1 is closed so as to connect, inter alia, 
drive motor M1 to alternating voltage source 19. At the same time, a 
rectifier 20, and a synchronous motor M3, having a drive mechanism G 
coupled thereto are supplied with alternating current. 
Rectifier 20 supplies a d.c. voltage U+ to a voltage divider composed of a 
potentiometer P and in series with a resistor R. As long as switch S1 is 
closed, drive mechanism G drives the tap of potentiometer P in the 
direction of arrow 30 so that the tapped voltage U between the 
potentiometer tap and the ground terminal M progressively increases. This 
voltage U is fed to a discriminator, or comparator, D which also receives 
the constant supply voltage U+ from rectifier 20. Discriminator D controls 
a switch S2 which remains closed if no supply voltage U+ is present. 
During its back and forth movement, i.e., when switch S1 is closed, 
carriage 18 actuates two switches S3 and S4. The one switch, S3, is kept 
closed by carriage 18 as long as body 3 is lowered on the polishing disc 8 
by carriage 18. Switch S4 is actuated for a short time when carriage 18 is 
raised upon completion of playing of a record carrier 1, carriage 18 then 
being returned in its raised position back to polishing disc 8. 
Closing of switch S4 closes the circuit of a relay winding 21 which is fed 
by rectifier 20 so that the self-holding contact S5 and the operating 
contact S6 of the relay are closed and remain closed, due to the 
self-holding action of contact S5, even after switch S4 opens. When 
carriage 18 returns to polishing disc 8 and there is lowered, switch S3 is 
closed so that alternating voltage source 19 supplies current, via the 
relay operating contact S6, switch S3 and switch S2, to polishing motor 
M2. This is the beginning of a reworking process for body 3. At the same 
time, a second rectifier 22 receives alternating voltage via the same path 
to produce a d.c. output which charges capacitor C via a biasing resistor 
V for a period of time varying between a few seconds and several tens of 
seconds. 
The capacitor voltage Uc is fed to the discriminator D, which is designed 
to open switch S2 when capacitor voltage Uc has reached potentiometer 
voltage U. S2 also remains open if Uc becomes greater than U. Opening of 
switch S2 switches off polishing motor M2, thus terminating the 
repolishing process. 
The next succeeding repolishing process will have a slightly longer 
duration because the potentiometer tap will have been displaced upwardly 
somewhat to increase voltage U and this will slightly increase the time 
which elapses from the start of a repolishing process until voltage Uc 
reaches potentiometer tap voltage U. 
If body 3 is replaced by a new one, the potentiometer tap must be returned 
to its starting position by being moved opposite to the direction of arrow 
30. 
As mentioned above, the repolishing process taking place after each playing 
process is controlled, in the embodiment according to FIG. 5, by the total 
period of operation of the device which is determined by summing of the 
periods of operation of synchronous motor M3 and potentiometer P and which 
is equal to the cumulative total of the time periods during which switch 
S1 is closed. This period of operation of course differs somewhat from the 
period of use, or total wear time, of body 3. However, this difference is 
so slight that it can be neglected. 
If the repolishing process is not to be controlled in dependence on the 
total time of use of body 3 or of the device itself, but in dependence on 
the number of wear processes, a stepping switch mechanism S or a counter 
can be connected to switch S4 to advance potentiometer P, as shown in FIG. 
6. 
If, instead of the reworking period, the contact force K is to be 
controlled, the potentiometer voltage U could be fed to a magnet which 
moves carriage 18 toward disc 8 against the restraining force of a spring 
so as to increase the magnitude of contact force K with increasing 
potentiometer voltage U. 
FIG. 7 is an example for controlling the duration of each repolishing 
process only by the total wear time of the body 3. This kind of 
controlling is achieved by supplying motor M3 via switch S7, thus avoiding 
to take the duration of the repolishing processes in the summing executed 
by motor M3. 
The potentiometer P in FIGS. 5 to 7 may be non-linear for example to 
additionally pay regard to the decreasing sharpness of the polishing 
groove. This decreasing sharpness also may be respected by not completely 
resetting the tap of the potentiometer P to its initial position when a 
new body 3 is inserted. 
The body 3 in its initial shape may be a tip of a diamond octahedron as 
shown in FIGS. 8 to 10. The tip is completed by broken lines to the 
original octahedron in the side view of FIG. 9 for purposes of 
explanation. The faces 31 to 34 are parts of natural octahedron faces, 
while the faces 35 to 38 have come into being by grinding with profile 
grinding means. The tip may be glued to a transducer 39 a part of which is 
shown in dash-dotted lines. The length of the body shown is about 150 
.mu.um and its hight is about 70 to 80 .mu.um. The shape of the surface 
between the legs forming an angle of 115.degree. in FIG. 10 and having the 
radius r of about 1 .mu.um correspond to the profile of the polishing 
groove in FIG. 2. 
The composition of a suitable polishing disc is known from the German 
Offenlegungsschrift No. 23, 50, 732. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.