System for detecting the tracking error in optical readers and recorders

The invention relates to the detection of the tracking error of a track using a process of periodic excursion of a reading beam in a direction which forms an angle with the direction of exploration of the track. The detection system comprises a mirror deflection by a bar vibrating in a flexural mode which is clamped to a base. Two plates of piezoelectric elements mechanically coupled with said vibrating bar are provided to excite the vibrating plate and to supply a reference signal in constant phase relation with the movement of the vibrating bar.

This invention relates to vibrating mirror deflection systems which are 
particularly intended to ensure the periodic excursion of a read-out beam 
in an optical reader or in a recorder comprising a supplementary beam for 
writing in the information. 
During the recording of information along a track on a rotating support of 
the videodisc type or even during the reading of a previously recorded 
support, imperfections in centring or rotation necessitate the use of a 
tracking servomechanism so as to keep the reading spot in correct 
concordance with the track. In the case of a reader, the point in question 
is the focussing point on the support of the reading beam. In the case of 
a recording, the point in question is the focussing point on the support 
of a reading beam which is associated with the recording beam and which is 
particularly intended to following the arrangement of the track outlined 
by the recording beam. 
It has been found that the sensitivity of the servocontrol loop to faults 
in optical alignment or to electronic variations is greatly reduced and 
the adjustment of the optical system simplified when the error signal 
ensuring radial tracking is obtained not by differential comparison of 
output signals of several photodetectors associated with one or more 
tracking beams focussed at one or more points of the support, but instead 
by the synchronous detection from a reference signal of frequency f.sub.o 
of an excursion frequency component modulated in amplitude at the 
frequency f.sub.o. To this end, the reading spot is periodically subjected 
to an excursion at the frequency f.sub.o in a radial direction relative to 
the track. Thus, the same photodetector delivers a signal characterising 
the information stored in the track modulated in amplitude both by the 
non-concordance of tracking and by the periodic excursion at the frequency 
f.sub.o. The periodic modulation detected reflects the degree of 
non-concordance of tracking and the phase detection gives an indication of 
the direction of the non-concordance of the reading spot relative to the 
track. 
The periodic excursion of the reading beam is normally obtained by means of 
a deflecting element which is positioned in the path of the reading beam 
and which is capable of vibrating at a given frequency under the action of 
an external excitation so that the vibration produces a slight alternate 
deviation of the beam. 
Various deflecting elements may be used. The tracking error signal acts on 
an element of the optical reading system which produces a displacement of 
the reading spot to maintain the concordance of tracking. This element is 
generally a pivoting mirror. In some cases, this mirror may be structured 
to vibrate in a natural vibratory mode which not only enables the tracking 
error to be corrected, but also produces the periodic deviation of the 
beam, the two movements being combined, the first being at low frequency 
and the second at very high frequency. The two operations of radial 
tracking and periodic excursion are thus obtained by a single element. 
Unfortunately, this process lacks flexibility because it implies by 
principle a limitation in the gain of the servocontrol loop on account of 
the resonance at very high frequency. 
In another process, the deflecting element is formed by a refractive plate 
vibrating in a flexion mode of which the optical transmission properties 
vary with the stresses which it undergoes. The fixing of this element in 
the optical reading system produces a lowering of the Q factor as a result 
of undesirable mechanical couplings and, hence, a considerable reduction 
in the amplitude of vibration when it is compared with that of the 
non-fixed element. 
The deflecting element of the system in accordance with the invention has 
such a structure that it may be rigidly fixed to the reader which provides 
for correct adjustments of alignment. It co-operates with the pivoting 
mirror which ensures correction of the radial tracking errors. It is a 
reflecting element so that only the reduced surface of the reflecting part 
has to be made with precision to avoid aberrations. Reflection also has 
the advantage over transmission that, for one and the same deviation of 
the deflector, the deviation of the light is distinctly greater. 
Accordingly, this element combines the advantages of conventional systems 
whilst obviating their disadvantages. Its sensitivity may be favourably 
compared with that of the devices mentioned above. The device according to 
the invention essentially comprises an elastic element vibrating in a 
flexural mode which is anchored on a substrate. Finally, electrical means 
are associated with this elastic element to make the excitation frequency 
coincide with the natural frequency of mechanical resonance so as to 
obtain maximum sensitivity and a constant phase relation between the 
excitation signal which also serves as reference for the synchronous 
detection and the movement producing the periodic excursion of the reading 
beam. 
In accordance with the present invention, there is provided a system for 
detecting the tracking error of a beam of radiant energy following a track 
along a read out direction, said system comprising mirror deflection means 
for imparting to said beam a vibratory displacement intersecting said 
direction, excitation means causing said mirror deflection means to 
oscillate said beam at a frequency f.sub.o and detection means collecting 
said beam for supplying a signal representative of said tracking error; 
said mirror deflection means comprising a substrate, an elastic body 
having at least one flexible bar clamped at one end onto said substrate 
and a mirror carried by said bar for reflecting said beam; said mirror 
being supported by said bar at a position where rotational displacements 
are generated in response to a flexural mode of free vibrations occuring 
at said frequency f.sub.o ; said excitation means comprising a 
piezoelectric transducer element carried by said bar and coupled to an 
electrical oscillator circuit for freely setting up said flexural mode of 
free vibration.

FIG. 1 diagrammatically illustrates one example of embodiment of an optical 
reader intended for reading information previously recorded along a spiral 
track of a carrier formed by a transparent videodisc 1. The signal is 
recorded in the form of a string of micropits of uniform width distributed 
along the track. The periodic excursion process is particularly 
advantageous when the depth of the micropits enables a path difference 
equal to half the wavelength of the reading beam to be established in the 
material of the disc 1. The following description may also apply to a 
recorder formed by elements of which some are used for reading the track 
as it is outlined and, in particular, for ensuring radial tracking. 
The disc 1 is read by means of a beam coming from a light source 2 which 
may be a helium-neon laser. The beam is directed and focussed onto the 
surface of the disc 1 at a point I by an optical system formed by a mirror 
3, a convergent lens 4, a mirror 5 pivoting about a point M in the 
direction indicated by the arrow 50 and a lens 6. The precise construction 
of this optical system lends itself to numerous variants and only one of 
the possible variants has been described here. 
It will be assumed that the disc is transmissive. The light beam 
transmitted by the disc 1 is collected by a reading device 7 which 
delivers, on the one hand, an output signal S representing the stored 
information (this signal is not essential in the case of a recorder) and, 
on the other hand, a signal E which is dependent upon the interval e 
between the reading spot I and the nearest turn. 
The periodic excursion process necessitates the provision of a vibrating 
deflecting element. According to the invention, this element comprises a 
supporting member carrying mirror 3. When it is excited, it is capable of 
vibrating, causing the mirror to pivot periodically about a fixed point in 
a direction represented in the plane of the Fig. by the arrow 30. Thus, 
the signal E is an error signal modulated in amplitude to the frequency 
f.sub.o. Electronic means 9 which will be defined hereinafter supply two 
voltages with the same frequency f.sub.o, one of these voltages, X, 
serving to excite the deflector and the other voltage, Y, serving as 
reference to its synchronous detector 11 which extracts from the signal E 
an error signal E' of which the amplitude and polarity respectively 
characterise the degree and the direction of non-concordance of tracking. 
This error signal is applied by means of an amplifier 10 to a drive 
mechanism 8 which controls the pivoting of the mirror 5 about the point M 
and, hence, the displacement of the dot I in a radial direction relative 
to the track so as to correct the mistracking. 
One embodiment of the deflector according to the invention is shown in FIG. 
2. It is formed by a metallic bar 31 which is anchored in a substrate 25 
and which vibrates in a flexural mode. The resonance frequency of the 
fundamental mode has to be very high in relation to the frequency band 
which it is desired to pass in the radial servocontrol which is of the 
order of 2 kc/s. This bar is excited at a frequency equal to the resonance 
frequency of the vibration mode selected by means of a ceramic 
piezoelectric plate 34 bonded to one lateral face of the bar 31. In order 
to obtain maximum sensitivity, it is preferable for the position of this 
plate to coincide with that region of the bar 31 where the strain is 
maximal for the mode selected. The light beam is intercepted by a mirror 
32 which is also bonded to one face of the bar 31, preferably in the 
region where the angular deviation is greatest, generally to the free end 
of the bar 31. In many cases, it is desirable to obtain a signal which is 
in phase with the vibration of the plate. The signal may be the voltage 
collected by a second piezoelectric ceramic plate 33 bonded to the bar 31, 
for example on that face opposite the first piezoelectric plate 34. It 
would also be possible to collect the current flowing through the first 
piezoelectric plate 34. 
It is important for the resonance frequency of the bar 31 to be independent 
of the manner in which the support is fixed to the frame of the reader. On 
the other hand, fixing is required to be rigid. One way of satisfying 
these conditions is shown in FIG. 3 which is a section through the 
deflector. The support 35 of the bar is fixed to the frame of the reader 
37 by means of a screw 36 so that the fixing surface S is small. 
Experience has shown that, in this way, the resonance frequency of the bar 
is invariable. 
The choice of the mode used is dictated by the need for maximum 
sensitivity. FIGS. 4a, 4b and 4c show in highly exaggerated form the 
extreme positions and the mean position of the bar 31 in its flexion. The 
lowest mode of vibration is shown in FIG. 4a and the next mode in FIG. 4b. 
It can easily be verified that, for the same deflection of the end of the 
bar, a greater angular deviation is obtained and it is only this angular 
deviation which is useful for the deflection of the light beam where the 
second mode is used. 
It will be assumed that the mirror is positioned at the end of the bar 31. 
The movement of this mirror is the result of two effects: an angular 
deviation .alpha. about a fixed point 0 which is the node of the 
vibration. This effect is the desired effect because it constitutes the 
pivoting in the direction 30. The second effect is a lateral displacement 
1 which produces a parasitic displacement of the reading spot in the 
direction of the tracks. In practice, this parasitic displacement is 
generally not troublesome and may be disregarded. However, if it is 
desired to suppress this parasitic displacement, it is possible to 
position the mirror at a vibration node, as shown in the FIG. 4c. 
The configuration described above is simple and effective. The vibrator 
fits readily into conventional optical readers because it replaces an 
element of the optical system. Numerous other configurations are of course 
possible. Various methods of anchoring may be contemplated. The shape of 
the vibrating member may vary. For example, it may comprise two arms like 
a tuning fork. It has also been seen that the positions of the 
piezo-electric plates and the mirror may vary without changing the 
principle of operation. 
The simplest way of obtaining the signals X and Y would be to use a source 
delivering a signal of fixed frequency f.sub.o as close as possible to the 
mechanical resonance frequency f.sub.R of the mode selected. The signal, 
amplified and shaped, would supply the reference signal Y intended to 
synchronise the synchronous detector and the signal X for controlling the 
piezoelectric plate 34. 
It is generally preferred to control the frequency f.sub.o by means of an 
oscillation loop and thus to eliminate the effects of any frequency drifts 
of the oscillator and the vibrating bar. 
According to the diagram in FIG. 5, the electrical voltage Z coming from 
the piezoelectric plate 33 is phase shifted by a phase shifter 13. The 
signal V obtained is amplified by an amplifier 12 which supplies the 
signal X. The signal Z is shaped by the device 14 which supplies the 
signal Y. The loop oscillates at a frequency f.sub.o which depends upon 
the phase-frequency response of the bar 31. If .phi..sub.R is the phase 
corresponding to the frequency f.sub.R, the condition of oscillation is 
that the sum of the phase shifts introduced by the elements of the loop 
other than the bar is equal to 2.pi.-.phi..sub.R. The result is obtained 
by the adjustment of the phase shifter 13. 
According to the diagram in FIG. 6, the signal V emanates from a 
voltage-controlled oscillator 15. The frequency f.sub.o of the signal V is 
controlled by a signal F coming from a phase comparator 16 which receives 
on the one hand the signal V and on the other hand the signal emitted by 
the piezoelectric plate 33. 
The two loops described above give good results and the possible phase 
variations of the elements of the loop are generally negligible. The 
frequency f.sub.o is controlled to coincide narrowly with f.sub.R and does 
not vary to any significant extent. 
However, where it is desired to obtain a more perfect coincidence between 
f.sub.o and f.sub.R, it would be possible to control the oscillation 
frequency in dependence upon the resonance frequency of the deflector. One 
example of embodiment is shown in FIG. 7. 
The signal of frequency f.sub.o is frequency-modulated by a signal with a 
frequency f.sub.1 lower than f.sub.o coming from an oscillator 17. The 
response curve of the deflector is thus explored and an amplitude 
modulation is superimposed upon the frequency modulation. A device 18 
effects an envelope demodulation and the error signal F serving to adjust 
the mean frequency f.sub.o of the oscillator 15 emanates from the 
synchronous detection of the signal coming from the amplitude demodulator 
18 by means of a synchronous detector synchronised by the frequency 
f.sub.1. 
Naturally, the preceding three diagrams have been given purely by way of 
example and do not preclude other embodiments of the electronic circuit to 
be associated with the deflector according to the invention.