NO144501B - DEVICE FOR READING A PLATFORM RECORDING BEAR - Google Patents

Description

The invention relates to an apparatus -for<p> reading of a - the plate qppdrawing carrier on which are stored • signals in tangential"tracks, comprising an optical 'reading unit with a radiation source, an optical system and a reading detector, which radiation source delivers a beam which. is projected onto the recording medium as a reading spot and which via the optical system transmits information in the reading point on the recording medium to the reading detector, a control device for controlling the radial position of the reading point on the desired track, which control device includes a drive device for the optical system and a first measuring device for measuring the reading point radial position and delivering a corresponding first control signal to the drive device, which first measuring device uses a first pattern of beam spots projected onto the recording medium, which pattern is imaged on

a first measuring detector for delivery of the first control signal which, during the reading point's radial movement over a number of track distances, comprises a periodic alternating current component whose period is equal to track distance h, and the control device is stable in one half period of the alternating current component and unstable in the other half period.

Such a device is known from a Norwegian patent application

No. 1238/73. The radial control system used there serves to ensure that when the information recorded on the recording medium is read, the scanning point will always accurately follow the information track on the recording medium. The recording medium may be provided with a number of concentric tracks, but usually the information track is helical. The modulation method for the recorded signal and the manner in which the signal is recorded on the recording medium are not essential for the present invention, so that this shall not be dealt with in more detail. As an example of the recording method, reference should be made to Norwegian patent no. 139,905.

When the recording medium is provided with a spiral-shaped information track, the scanning point must be shifted radially at an approximately uniform speed. This smooth movement is usually achieved by moving the reading unit in a radial direction. In addition to this, it must be possible to make small but rapid radial movements of the scanning point because, as a result of eccentricity of the center of the recording medium, radial deviations from the desired position of the recording track can occur.

This radial displacement of the scanning point takes place with the help of the control device, which consists of the drive device for the optical system and the first measuring device. The optical system usually consists of a reflective element which, under the influence of the drive device, can be swung and thus reflect an incident radiation within a varying angle. Such an optical system according to Norwegian patent application no. 1238/73 can influence the direction of the beam before the beam hits the recording medium or after the beam has hit the recording medium and has already been modulated with the information in the track. The recording medium itself can be both radiation-permeable and radiation-reflective.

The first measuring device for measuring the radial

position of the scan point can have a different design. E.g.

a first pattern of radiation spots can be used which is projected onto the recording medium from the radiation source, which pattern consists of two radiation spots located in the radial direction on opposite sides of the scanning point, each of the radiation spots being imaged on separate parts of the first measuring detector. The relative strength of the signals from the images of the two beam spots on

the separate parts of the measuring detector will change in accordance with the position of the scanning point in relation to the information track.

By subtracting the signals from the separated parts of the first measuring detector, a first control signal is derived which represents the position of the scanning point in relation to the information track. One. another possibility for imaging a number of traces using a relatively large beam spot on a grid-shaped measuring detector is described in Norwegian patent application no. 741/72.

With all these measurement methods, a control signal is produced which, after a radial displacement of the scanning point over a number. track distances, comprises a periodically alternating, component whose period

is equal to the track distance. In that respect must. the track distance is the middle distance measured in the radial direction between the center lines of two adjacent tracks. The aforementioned alternating component is used as a control signal for the drive device for the optical system.

It has been shown that in the event of a -radial displacement a<y> the scanning point.over a number of track distances- the control device is in a stable state for half the period, of the periodic, alternating component, and in an unstable state during the second half term of

the exchange component. This is because the AC component which is used as a control signal for the drive device for the optical system has a positive slope during one half-period and a negative slope during the other half-period, so that in the event of a change in the radial<*> position -of the scanning point has the opposite sign of the variation of the control signal supplied to the drive device.

Normally this value will cause few problems for the controller because if the controller is first in a stable range it will always remain in the stable range under non-normal conditions. If, however, the control device comes out of the stable area as a result of a disturbance, e.g. as a result of an impact or a crack in the record carrier, this size can cause a relatively large, mainly undamped oscillation, so that the scanning point can cause one oscillation over a number of track distances in the radial direction, and correct reading of the information is then impossible until a certain, sometimes a relatively long time has passed since the occurrence of the disturbance.

The purpose of the invention is to provide an apparatus where this problem is eliminated, and this is achieved according to the invention by, in order to ensure the stability of the control device, a second measuring device is used which uses a second pattern of radiation spots which are projected onto the recording medium and imaged on a second measuring detector which supplies a second control signal which by radial movement of. the deflection spot over a number of track distances contains a periodic alternating current component whose period is equal to the track distance and which, in the case of a deviating radial position of the second radiation pattern in relation to the first radiation pattern, has a phase shift of essentially a quarter of a period in relation to the first control signal's alternating current component, and that the control device has a variable transfer function which is controlled by a control unit which receives a third control signal which is derived from the second control signal, and by a movement of the reading point over a number of track distances, the control unit controls the transfer function so that the control device is stabilized.

With the help of the second control device, a second control signal is obtained which gives an indication of whether the control signal is in the stable range or not. Since the alternating component of the second control signal has a phase shift of at least a quarter period in relation to the alternating component of the first control signal, the half period with one polarity of the alternating component will correspond to a half period of the alternating component of the first control signal with a slope with the first sign, and the other the half-period of the opposite polarity of the alternating component of the second control signal will correspond to the second half-period of the alternating component of the first control signal with a slope of the opposite sign.

The second control signal can be used to deliver a control signal that affects the transfer function of the control device when the device enters an unstable region in such a way that when the control device enters an unstable region in such a way that the control device passes a stable and an unstable region, kinetic energy will which is exchanged during the passage of the unstable region be less than the energy exchange during the passage of the stable

car area. To achieve this, several options are available. Firstly, the gain in the control device can be reduced when the control device passes the unstable region. Secondly, it is possible to reduce the degree of damping for the control system, which gives rise to a negative damping in the unstable area. Finally, the sign of the damping factor can be reversed in the unstable area, so that a positive damping is achieved in this area.

Furthermore, the second control signal can be used to derive a control signal which gives an indication of the direction in which the scan point is moved in the radial direction. Such a control signal can be obtained by differentiating the second control signal. In this way, a control signal is obtained which is in phase or in opposite phase in relation to the first signal depending on the direction of the scanning point's movement. When controlling the transfer function by means of the said signal, a reliable stabilization is achieved because the transfer function is varied depending on the direction of the movement either during a positive or negative period of the first signal.

It is also possible to multiply the differentiated second control signal with the first control signal, so that a ■ signal is obtained whose polarity is a direct indication of the direction of the scanning point's movement. This information can then be used to influence the transfer function either during the positive or the negative period of the first control signal.

When combining the two possibilities, some must be taken,

precautions to be explained in more detail in the drawing description.

The second measuring device can have a different design in accordance with the desired accuracy. In a preferred embodiment, the second pattern consists of two radiation spots which are radially offset relative to each other by a distance equal to half the track distance and which are each imaged on a separate part of the second measuring detector, and the second control signal is obtained by subtraction of the signals delivered from the separated parts of the second measuring detector.

This embodiment has the advantage that the second control signal obtained in this way only contains an alternating current component, while the direct current component which is normally present in the signal is eliminated by the subtraction. The direct current kefmponent is both time dependent as a result of noise in the radiation energy supplied by the radiation source and of the position as a result of the difference in the radiation absorption and/or reflection as a function of the position on the recording medium, and as a function of the position in which the beam in the reading unit is transferred, particularly with regard to the lens system used.

A second preferred embodiment of the second measurement signal is obtained by measuring signals received in the reading detector itself, in particular low-frequency variations in this signal. This design has the advantage that no additional measuring detector is necessary, but on the other hand it has the disadvantage that the supplied control signal has a direct current component which can cause less accuracy.

The invention will be described in more detail below with reference to the drawings. Fig. 1 schematically shows an apparatus according to the invention. Fig. 2 and 3 show the shape of the curve for the control signals for the device in fig. 1. Fig. 4 and 5 schematically show two ways to obtain the desired control signals. Fig. 6 shows the curve shape of the characteristic signals in the manner of fig. 5. Figures 7 and 8 show block diagrams for influencing the transfer characteristic of the servo unit supplied with control signals. Fig. 9 shows a block diagram of an alternative device for achieving the desired stabilization. Fig. 10 shows curve shapes in connection with the device in fig. 9. Fig. 10 and 11 show connection diagrams for two versions of the differentiation circuit to achieve the desired effect. Fig. 1 shows a plate-shaped recording carrier 1, which is provided on the underside with a number of concentric or quasi-concentric recording tracks. In the tracks, information is recorded in a known way, e.g. as stated in Norwegian patent no. 139,905|

where the information tracks contain areas whose length represents the stored information. These areas have different effects on a beam that is projected onto the information track, so that the beam is modulated in accordance with the recorded information. E.g. the passage or reflection from the areas can vary so that the reading beam is amplitude-modulated. In the case of a reflective recording medium, it is also possible to place the areas at different levels so that the difference in depth is a quarter of the wavelength

for the radiation used for the reading. In this case, the reading beam is phase modulated. Since the modulation method and the recording method for the signals on the recording medium are only of secondary importance for the present invention, this shall not be dealt with in more detail here.

The drawing carrier 1 is rotated by a motor M, by means of a shaft 2 which extends through a central opening in the recording carrier. The information stored in the recording medium is read using a reading beam which, after cooperating with the recording medium, is detected using an optical reading device which is arranged in a housing 3. The optical reading device which mainly corresponds to Norwegian patent application no. 1238/73 comprises a light source 6, which delivers a reading beam a. Via a partially transparent mirror 7, the beam a is projected onto a mirror 8 so that the beam 1 is reflected in the direction of the recording medium 7. The reflected radiation a is focused to a reading spot S on the underside of the recording medium 1 by means of a lens 11. - The ray which', is reflected by the recording medium. again hits the mirror 8 and is reflected to the partially permeable mirror 7, and from there to a reading detector 12, on which the reading point S' on the recording medium is imaged so that the information. which is contained in the beam is detected'. The detected information is then available for further applications on the output terminal 14.

In order to ensure continuous reading of the information stored on the recording medium, the reading spot S, which is imaged on the reading detector 12, must continuously follow the information track on the recording medium. If the information track is spiral-shaped, this means that the reading spot S must first of all be moved in the radial direction at a speed corresponding to the pitch of the spiral-shaped information track. Furthermore, the reading spot S must be able to follow any radial movements of the information track, e.g. as a result of eccentricity of the central opening in the recording medium.

This necessary control of the radial position of the derivation spot S. is achieved by cooperation between two control devices, namely a coarse control device which only makes a slow radial displacement of the reading spot and a fine control which only makes a relatively small and rapid radial displacement of the reading spot.

In the present design example, the rough control takes place at the tip of a

. motor Mp which via a servo amplifier 16 receives a control signal and which

by means of an exchange e.g. a screw 4 and a rack 5 can move the housing 3 in a radial direction. The fine control takes place with the help of the mirror 8, which can be rotated about an axis 9.

The oscillation of the mirror 8 takes place by means of a drive element 10, which can have different designs, some of which are specified in Norwegian patent application no. 1238/73. The drive element receives the control signal from the servo unit 15.

The information concerning the radial position of the scanning spot S in relation to the desired track is obtained by means of a beam b which is delivered by a radiation source 6, and which also hits the underside of the recording medium 1 through the partially penetrating mirror 7, the mirror 8 and the lens 11 After reflection of the beam b via the mirror 8 and the partially penetrating mirror 7, the beam b reaches a control detector 13 whose output signal is supplied to the servo unit 15. As a control signal for coarse control supplied to the servo amplifier 16, a signal can be used which is a measure of the average oscillation of the mirror -8 relative to a central position. Such a signal can be obtained in different ways, e.g. by means of capacitive or inductive converters as shown only schematically in fig. 1, remove the steering seal from the drive element 10.

Depending on the composition of the radiation b, the control detector 13 can have different designs, e.g. as stated in Norwegian patent application no. 7^1/72. The control detector consists there of a grid-shaped radiation source and radiation-absorbing strips on which a number of tracks on the recording medium are imaged with the help of the beam b. The position of the grid-shaped image of the track pattern in relation to the grid-shaped detector provides a control signal representing the position of the scanning spot S in relation to the desired track by means of suitable converter elements.

Another embodiment is described in Norwegian patent

application no. 1898/73. In this embodiment, two reading spots are projected on either side of the reading spot depicted on the control detector which consists of two separate partial detectors, one for each of the radiation spots. The strength of the two imaged radiation spots varies in accordance with the radial position of the scanning spot, and subtraction of the output signals from the two partial deflectors gives a suitable control signal.

Quite generally, the control signal can be derived by means of a control detector 13 whose signal as a function of the radial position of the scanning spot is mainly shown in fig. 2. Apart from a possible direct current component, the control signal contains an alternating current component whose period is equal to the track distance, i.e. the distance between the center lines of two adjacent tracks. Fig. 2 shows a pure sinusoid. It is clear, however, that depending on the detector system used, it can deviate from the sinusoidal form. For the sake of simplicity, a sinusoidal control signal will be considered below.

Fig. 2 shows the control signal which is obtained by shifting the reading spot over three track distances. The control system is assumed to be stable at the points where the "control signal is 0 and the slope is positive, i.e. the points ?;^, s . and s^ which consequently correspond to the center of the track in question. If, for example, the reading spot.moves from the track s.^ towards a larger value for the radius r, a positive control signal is produced depending on which control signal is used to move the reading spot back to the position s^ For a displacement of the scanning spot towards a smaller value of'r, it is Jclart ', " that the opposite happens.. ■• i

If, however, the scan spot • enters area Q, which corresponds to the negative slope of the control signal, the control system suddenly becomes unstable. When passing through the area Q, the system is supplied with energy, which means that if the deviator covers an entire track distance as a result of a disturbance, it will. automatic backtracking: more track distances.

As a result of a major disturbance, the increase spot S can be moved over a number of track distances, which number is primarily determined by the oscillation of the mirror at the moment of disturbance. As the coarse control is also affected, for oscillation of the mirror 8 corresponding to the displacement, there will be an essentially undamped oscillation of the reading spot over a number of track distances, as a result of the mutual effect of the fine control and the coarse control, so that finally the reading of the information stored on the record carrier becomes impossible for a certain time.

In order to counteract this, the radiation source 6 in the device in fig. 1 a third beam c, which via the partially penetrating mirror 7, the mirror 8 and the lens 11 hits the recording medium. The part of the radiation c that is reflected by the recording medium hits an additional detector 17 via the mirror 8 and the partially penetrating mirror 7. This additional detector 17 delivers an additional control signal whose form mainly corresponds to the form of the control signal delivered by the control detector 13. As a result of a deviation of the radial position of the pattern of radiation spots projected on the recording carrier by the radiation source c in relation to the pattern of radiation spots projected on the recording carrier by the beam b, the additional control signal receives a phase shift of a quarter period in relation to the control signal "supplied by the control detector 13.

For clarity, the two control signals are shown

on fig. 3. The control signal 1^ delivered by the control detector 13 corresponds to the control signal shown in fig. 2. The control signal 1^ delivered by the additional detector 17 has the same form as the control signal 1^ is a function of r, but is phase-shifted by a quarter of a period. From the control signal 1^, a control signal I2' can be derived in a simple way and has a first positive value when the control signal 1^ is positive and a second negative value when the control signal I ? is negative. From fig. 3 it is clear that the positive value of the control signal I^' corresponds to the stable area P of the control signal 1^ and the negative value corresponds to the unstable area Q. It is assumed that a positive value of the first control signal 1^ causes a displacement of the reading spot towards smaller values of r and a negative shift towards larger values. The control signal 'gives an indication of whether the control system is in the stable area P or in the unstable area Q. This information is used to influence the control system during at least part of the time the control system is in the unstable area.

The control signal derived from the additional control signal from the detector 17 by means of a converter 18, which in this case is a square wave generator, is supplied to the servo unit 15

which forms part of the control system and which depending on the value of the control signal 1^ can have two possible transfer functions. The first transfer function that operates in the stable control range P serves to achieve a maximum control within the stable range. The second transfer function which acts in the unstable region Q has such a deviant transfer function that when the scanning spot passes a number of stable and unstable regions P resp. Q, the energy exchange by means of the servo unit 15 during the passage of unstable areas Q will be less than the energy exchange during the passage of the stable areas P. This ensures that after the occurrence of an external disturbance, a damped oscillation is produced in any case, so that the scanning spot in any case is brought

back to steady state. How quickly this happens naturally depends on the degree and manner in which the transfer function of the servo unit 15 is varied. If desired, the signal 1^ can of course also be used directly as a control signal and continuously vary the transfer function at least during the unstable periods.

First, the possibilities for achieving the desired control signals I1 and I2 must be processed. A first possibility is shown in fig. 5, where s^, s2 and s^ show a part of three adjacent recording tracks which have a mutual distance X. The scanning spot S

is the area of the recording medium which is imaged on the readout detector 12. The pattern of radiation spots belonging to the radial control system mainly consists of two radiation spots R^ and R2 located on opposite sides of the scanning spot S at a radial distance 1/4 X. The pattern of additional radiation spots is provided by means of radiation c, which comprises two radiation spots T., and l^. The radiation spot T is radially displaced in relation to the radiation spot R^ 1/4 X ofc the radiation spot T2 is displaced the same distance in relation to the radiation spot R^. Each of the radiation spots is imaged on a separate detector.

Usually the signal - I^e^..from such a detector.-' is delivered as a result of the radiation spot' which is imaged on it, and this signal can be expressed:

where I is the energy delivered by the beam in the radiation spot, m is the amplification of the alternating current component by a radial displacement of the radiation spot, r is the radial position of the radiation spot, and X is the track distance.

It is assumed that the imaging of the beam spot R2 results in the signal:. the image of the radiation field R^. will give a signal:

fed to subtract the signal IR from the signal IR the following is obtained

<«>x *2

control signal:

which is completely 1, correspondence with waveform 1^ in fig. 3.

In the same way, the image of the beam spot T will give the signal:

and the image of the beam spot T2 will give the signal: Subtraction of the signal I„ from the signal I„ gives the additional control-■ > , <x>2 Ll ' • ■ signal:

sop completely is in accordance with the waveform lp in fig. 3.

If desired, the number of beam spots to achieve the desired control signal can be increased. Fig. 4 shows e.g. two additional radiation spots R, and R^, where the radiation spot R^ is located at a distance X from the radiation spot R^ and the radiation spot R^ is located at a distance X from the radiation spot R0. The signals IR and IR originating from the radiation spot R^ resp. R^ is therefore completely in phase with the signals IR and IR originating from the imaging of the radiation spots R, ^ and R„^ If

(Id + ID ) ~ (It, + !q ) is a control signal, the resulting will

2 4 1 3

signal be completely in phase with the control signal 1^, but have the doubled amplitude.

The use of additional radiation spots has some significant advantages. Firstly, as mentioned, the amplitude of the control signal is doubled. Secondly, the effect of the deviation from the track spacing is reduced, because the additional radiation spots result in a certain mean value effect. Furthermore, the impact of the information in the recording tracks on the control signal will be reduced. Normally. the high-frequency information is removed using the filter.• As the extra radiation spots also result in a certain mean value effect in relation to this parameter, the impact of the information JÉomponent will automatically be reduced. It is of course also possible to use more than two extra radiation spots, so that the effect is even more marked. Furthermore, the radiation spot R^ can be located at a track distance X from the radiation spot, i.e. between the tracks and s^, and the radiation spot R^ at a track distance X from the radiation source R^, i.e. between the tracks s^ and Sg. The desired control signal then becomes:

A second possibility for obtaining the desired control signals is shown in fig. 5 and 6. Fig. 5 shows a track s, on which a scan spot S is projected. On either side of the scan spot, two radiation spots R^ and B.^ are projected which are offset radially by a distance 1/ 4 X in relation to the scanning spot S. With the help of the radiation spots R, and R2, the control signal 1^ can be derived in the same way as described with reference to fig. 4, which signal is again in accordance with the expression (4) and shown in fig. 6a.

In this case, however, the additional control signal is not obtained by means of additional beam spots, but is derived from the signal obtained by the imaging of the scanning spot S on the image. the reading detector. In general, the information in the recording track will have a higher frequency than the maximum control frequency. As a result, the low frequency signal component can be derived from the signal provided by the readout detector by means of the filter. This signal component is contained, with the exception of phase shift, in the general expression (1) and can be written:

This signal IT which is shown in fig. 6b contains an alternating current component which is suitable for deriving the desired control signal. By means of a separator, the direct current component I can be extracted, after which the desired control signal I,p, (o), as shown in fig. 6c, can be derived from the remaining alternating current component, whose periods again exactly correspond to stable and unstable states of the control system,

A problem associated with this embodiment is the presence of the direct current component in the control signal 1^. This direct current component I is not completely constant, but depends on both time and position. The time dependence is mainly due to variations in the radiation intensity delivered by the radiation source. The position dependence is due to differences in absorption and/or reflection with regard to the position of the recording medium, and the position where the beam passes the optical system, especially the lenses.

In order to be able to see the consequences of these two variations of the direct current component I in the control signal IT, variations of the direct current level derived from the control signal IT can be observed. However, this gives rise to further variations. If it is assumed that instead of the correct direct current level IQ a direct current level (1) lower than I is derived, a control signal IT' (1) is derived from the residual alternating current component, which control signal is shown in fig. 6b. This shows that only part of the unstable area is in reality identified as an unstable area by the control signal 1^' (1). This can be accepted because, in any case, stabilization functions can be performed during part of the unstable areas, so that a possible oscillation is dampened in all cases. If, on the other hand, it is assumed that instead of the direct current level I a higher direct current level D.C. ( 2) is derived, a control signal 1^,, (2) will be derived from the residual alternating current component as shown in Fig. 6e. This control signal IT, (2) also contains parts of the stable areas P as unstable areas, so that during a part of the stable areas, the overfunction for the unstable areas will apply. This transfer function can then cause an unstable behavior in the relevant part of the stable area, which is naturally very undesirable. With the help of a suitable choice of the transfer function, however, it can be achieved that even with this incorrect•identification of stable and unstable areas, a stable is maintained

system in any case in the stable region. This will be explained in more detail during the description of designs of the servo unit as shown in fig. 7 and 8.

One way to eliminate the aforementioned unwanted direct current component of the control signal 1^,, is to form the sum of the signals produced by the radiation spots R and R2, i.e. the sum of the signal end which is represented by the expressions (2) and (3). This signal sum is equal to 21 and therefore only contains the direct current component. This means that the signal sum can be used to compensate the direct current component I in the control signal IT, because these two components are always exposed to essentially the same variations.

The embodiment shown in fig. 7, comprises a servo unit 15 which receives the signal from the control detector 13, and delivers a control signal for the drive device 10 for the mirror. The control signal delivered by the control detector 13 is fed to both a summation circuit 21 and a differentiation circuit 19. Via an amplifier 20, the output signal from the differentiation circuit 19 is inverted and fed to the summation circuit 21. The output signal from the summation circuit is finally amplified in the amplifier 22, and serves as the output signal from the servo unit. The differentiation circuit 19 together with the amplifier 20 provides the desired degree of attenuation in the transfer function for the servo unit 15.

The transfer function for the servo unit is affected by a control signal which via a square wave shaper 18 is derived from the control signal 1g supplied by the detector 17, and which consequently corresponds to the control signal I^' as shown in fig. 3. This control signal is supplied to either one or both amplifiers 20 and 22. If the control signal is supplied to amplifier 22, the amplification factor can be reduced for the transfer function of the servo amplifier in the unstable areas of the control system. If the control signal is supplied to the amplifier 20, the absolute value of the degree of attenuation in the unstable areas of the control system can be reduced. In an unstable region, the degree of attenuation as a result of the opposite slope of the control signal delivered by the control detector 13" as a function of the radius R can cause a negative attenuation, the impact of which is reduced by the reduction of the amplification factor in the amplifier 20.

In the ways to obtain the control signal as described with reference to fig. 5 and 6, impact on the transfer function of the servo unit is preferable. A reduction of the gain factor for the amplifiers can then be chosen so that the resulting reduced transfer function remains stable even if the switching takes place in the stable region ,P of the servo system,

Fig. 8 shows another embodiment where the servo unit 15 is essentially the same as shown in Fig. 7. However, the output signal from the amplifier 20 is here supplied to an inverter 23, which in a first position directly supplies the output signal to the summing circuit 21, and in the second position supplies the output signal to the summing circuit 21 via an inverting amplifier 24. The inverter 23 is operated by a control device 25, which receives the control signal I2' from the square wave shaper 18. When the control system enters an unstable region, the inverter 23 is brought into the second position, so that the degree of damping produced is additionally inverted. As a result, the degree of damping will also cause a positive damping in the unstable region.

This way of affecting the transfer function for the servo unit 15 is not particularly suitable for use when the control signal is derived as described with reference to fig. 5 and 6." This is because the control signal changes during the stable range, causing the sign of the degree of damping to change, so that the control in the stable

bile area will be seriously disturbed.

Fig. 9 shows a third embodiment where the servo amplifier

15 is identical to the servo amplifier in fig. 7. In this case, however, the control signal delivered by the detector 17 is not

foot a square wave shaper, but a differentiation circuit 26. In the case of a first control signal 1^ (see fig. 10a) which satisfies the expression (H) and a second control signal 1^ (see fig. 10b) which satisfies the expression (7), the differentiation circuit 26 delivers a signal (j, I2) which satisfies the following expression: dt

It follows that the differentiated signal 2 is in phase or out of phase with the first control signal 1^, depending on the sign of, i.e. the direction of, the radial movement. This is used to stabilize the control system.

It is assumed e.g. that the differentiated signal <dI>2

date

has a course according to: as shown in fig. 10c, which signal is riøyak-- '_1 (1) dt ... dr tig in opposite phase to the first control signal with a positive value, i.e. a movement of the scanning point towards a greater value of r.

From this signal dI2^ a square wave signal 1^ (1) can be derived as shown in fig. plumb with the help of a threshold value detector 27 which detects when the differentiated signal exceeds a positive threshold value D. The gain factor for the amplifier 22 is controlled with this square wave signal 1^ (1) in such a way that the gain factor is reduced if the control signal is positive.

This means that the amplification is reduced during the first part of the negative half-period of the first control signal 1^. -If it is again assumed that a positive value of the control signal 1^ will cause a movement of the scanning point towards a smaller value of r, and a negative value of the movement towards a larger value, this reduction of the gain during the negative half-period will result in a stabilization of the control system. When passing an equal number of positive and negative half-periods of the control signal 1^, the energy exchanged during the positive half-period and which counteracts the movement towards a greater value of r which still occurs at this moment, is greater than the energy exchanged during the negative motion. As a result, the movement will be dampened.

If the movement of the scan point is exactly counter-

set, i.e. <0, the signal dI2 ^\ is produced as shown P& fig. 10c

dt as a differentiated second control signal from which the square wave signal 1^ (2) as shown in fig. 10d. The gain in the transfer function is then reduced during part of the positive half-periods of the first control signal 1^, so that the desired stabilization effect is achieved again.

If the threshold value D in fig. 10c for the threshold value detector 27 is chosen equal to 0, the amplification factor will be reduced to unity during the entire positive or the entire negative half-period of the first, control signal 1^ regardless of the speed of the radial movement. This is less desirable because in this case the gain factor will also be changed continuously if the scan moves in the stable region around the desired stable setting (s^, Sg, Sj etc).

This is prevented by choosing a threshold value D which is different from 0. The amplitude of the differentiated second control signal dJ' 2 is proportional to the magnitude of the speed of movement for the scanning point. With a larger amplitude of the differentiated second control signal, e.g. 2 /7l on fig. loe, a square wave signal is obtained such as 1^ (3) in fig. 10e using the threshold value D which is not 0, and whose positive pulse width increases approximately proportionally with the amplitude. This means that - the stabilization effect is proportional to the magnitude of the speed.

If the speed decreases as a result of the stabilizing effect, the pulse width of the square wave signal 1^ will also decrease, which means that the switching points of the gain factor will go further back from the desired setting (s^, s^, s^ etc), so that the desired range of fixed transfer function automatically achieved around this setting. In the extreme case that speed-

heat is so small that the amplitude of signal 2 is less than ter-

date

threshold value D, no square wave signal ly is produced anymore This situation, however, is only hypothetical because before such a situation can occur, the control system must already be locked in a stable Idle state.

In a modification of the embodiment described above, the second control signal dI2 is differentiated from the expression

date

(9) again multiplied by the first control signal, which gives:

The polarity of this signal is a direct indication of the direction

of the radial movement. By reducing the amplification depending on this polarity, either during the positive or during the negative half-period of the first control signal 1^, the desired stabilization effect is again achieved. In this case, a threshold value detector is therefore required which is supplied to the first control signal 1^ and which, in accordance with the polarity of the signal in the expression (10) either during the positive or the negative half-period of the control signal, delivers a control pulse to the amplifier 22 in the servo unit. By giving the threshold value detector a threshold value that is not 0, an area around the desired settings (s^ s2, s^ etc.) is excluded, so that a fixed transfer function occurs in this area under all conditions.

In a second modification, the differentiation circuit 26 is designed in a special way, e.g. as shown in fig. 11. This comprises an operational amplifier V, whose inverting input is connected to ground via a capacitor C and a resistor R to the output. The second control signal 1^ supplied by the detector 17 is supplied to the non-inverting input of the operational amplifier V.

If it is assumed that the signal 1^ satisfies the expression (7), the output signal 1^ from the operation prefetcher V will be:

For rapid radial movement of the scanning point, the second term is dominant, so that in that case the system will work mainly in accordance with the stabilization method described with reference to fig. 9 and 10, because the signal 1^ then mainly corresponds to the signal dI2 in the expression (9). If the velocity ~ of the radial be-dF" at

weight decreases, the impact of the first part of the expression (11) will increase until it finally becomes predominant. In this last limit case, the signal 1^ completely corresponds to the control signal i2, so that the system can work in accordance with the stabilization method indicated in connection with fig. 7 and 8 in combination with fig. 3. Depending on the speed, the stabilization method will gradually change to the other.

An additional advantage of this method of stabilization is that for fast movements the threshold value of the system can be 0 without causing problems, so that maximum stabilization is possible. At low speed, the second stabilization method is therefore used.

The same can also be achieved with the help of a poor differentiation circuit, e.g. as shown in fig. 12. The differentiation network here consists of a parallel connection of a resistor R1 and a capacitor in series with a resistor R^. The transfer function for this network is:

where p is the imaginary angular frequency. If

R + R

1 < p< 1 1 2 the expression (12) can be approximated:

CR^ CR^ Rg

1 which means pure differentiation. However, for p < CR^ the transfer function can be reduced to:

which is a normal voltage division. At relatively high frequencies, i.e. high speed, the signal I2 is differentiated, so that the stabilization method according to fig. 9 is achieved, while at lower frequencies, i.e.

at low radial speed, the stabilization method in fig. 7 and 8 are achieved with a continuous transition between these two ways.

It is clear that the embodiment of the servo unit is not limited to the embodiment shown in FIG. 7,8 and 9. E.g. it is also possible to achieve a damping effect by means of suitable converters in the drive element 10 or the mirror 8. There are many design possibilities and the way in which the amplifiers 20 and/or 22 can be influenced by the control signal, because this type of amplifier is known in many designs and the selection of a suitable amplifier in accordance with the applicable conditions and the influence of the gain factor.properly will not present any problem to the person skilled in the art.

Claims (12)

1. Apparatus for reading a plate-shaped recording medium on which signals are stored in tangential tracks, comprising an optical reading unit with a radiation source, an optical system and a reading detector, which radiation source delivers a beam which is projected onto the recording medium as a reading spot and which via it optical system transfers the information in the reading point on the record carrier to the reading detector, a control device for controlling the radial position of the reading point on the desired track, which control device comprises a drive device for the optical system and a first measuring device for measuring the radial position of the reading point and delivering a corresponding first control signal to the drive device, which first measuring device applies a first pattern of radiation spots projected onto the recording medium, which pattern is imaged on a first measuring detector for delivery of the first control signal as during the radial movement of the reading point over a number of track distances comprises a periodic alternating current component whose period is equal to the track distance, and the control device is stable in one half-period of the alternating current component and unstable in the other half-period, characterized in that to ensure the stability of the control device a second measuring device is used which uses a second pattern (c) of radiation spots which are projected onto the recording medium (1) and imaged thereon a second measuring detector (17) which delivers a second control signal (l^) which, upon radial movement of the reading spot (S) over a number of track distances, contains a periodic alternating current component whose period is equal to the track distance and which, upon deviating radial position of the second radiation pattern (c) in relation to the first radiation pattern (b), has a phase shift of essentially a quarter period in relation to the alternating current component of the first control signal (1^), and that the control device has a variable transfer function which is controlled by a control unit (10 ,15) which receives a third control signal (IJ,) which is derived from the second control signal (I2), and by a movement of the reading point over a number of track distances, . the control unit controls the transfer function so that the control device is stabilized. 2..Apparatus according to claim 1, characterized in that the third control signal (I^) is a square wave^ which ,. < is mainly in phase with the second control signal (l^)• 3. Apparatus according to claim 1, characterized in that the third control signal (I3) is derived by means of a differentiating circuit (26) which is supplied to the second control signal (<I>2). 4. Apparatus according to claim 3, characterized in that the third control signal (I,) is a square wave which is derived from the differentiated second control signal /2" using dt by a threshold value detector (27) which detects the period in which the differentiated second control signal exceeds a certain absolute threshold value and which during the said period delivers a square pulse whose polarity at all times depends on the polarity of the differentiated second control signal in dl-, signal / 2.. dt~' 5. Apparatus according to claim 1, characterized in that the third control signal is a square wave that is delivered by a threshold value detector which detects the periods in which the first control signal has a specific polarity and at the same time exceeds a specific threshold value and during these periods a square pulse is delivered, in that the choice between the two possible polarities of the first control signal that is detected is dependent on the polarity of a product signal which is the product of the first control signal and the differentiated second control signal. 6. Apparatus according to one of claims 1-5, characterized in that the amplification factor of the transfer function is varied in accordance with the third control signal. 7. Apparatus according to claim 1 or 2, characterized in that the damping factor of the transfer function is varied in accordance with the third control signal. 8. Apparatus according to claim 2, characterized in that the transfer function can comprise two mutually inverse damping factors which are dependent on the third control signal. 9. Apparatus according to one of claims 1-8, characterized in that the second pattern of radiation spots consists of two radiation spots which are offset from each other in a radial direction by a distance equal to half the track distance (a) and which are each imaged on a separate part of the second measuring detector (17), the second control signal (I2) being derived by subtraction of the signals supplied by the individual parts of the second measuring detector. 10. Apparatus according to one of claims 1-8, characterized in that the second control signal is derived by measuring the low-frequency component in the signal delivered by the reading detector (14). 11. Apparatus according to claim 3, characterized in that the differentiation circuit (26) comprises an operational amplifier (V) whose non-inverting input is fed to the second control signal (I2), and whose inverting input is connected to a point of constant potential via a capacitor (C ) and via a resistor (R) with the operational amplifier's output. 12. Apparatus according to claim 3, characterized in that a network (C^, R1, R2) is used as differentiation circuit (26) which at relatively low frequencies acts mainly proportionally and at relatively high frequencies acts mainly differentiating.