GB1592833A

GB1592833A - Rotary drive the rotational speed of which can be regulated

Info

Publication number: GB1592833A
Application number: GB192178A
Authority: GB
Original assignee: Braun GmbH
Current assignee: Braun GmbH
Priority date: 1977-03-22
Filing date: 1978-01-18
Publication date: 1981-07-08
Also published as: FR2385175B3; JPS53121114A; FR2385175A1

Description

(54) A ROTARY DRIVE, THE ROTATIONAL SPEED OF WHICH CAN BE REGULATED (71) We, BRAUN AKTIEN GESELLSCHAFT, a German Body Corporate of 22, Russelsheimer Strasse, D6000 Frankfurt-am-Main, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates to a rotary drive, the rotational speed of which can be regulated, and comprising a drive motor driving an output shaft, a rotationally symmetrical carrier surface carrying a signal bearing means and coupled to the output shaft for rotation in synchronisation therewith, and a detector for detecting the signals carried by the signal bearing means and for producing a pulse sequence from the emitter independence thereon so that the frequency of the pulse sequence is proportional to the speed of rotation of the output shaft. The output shaft may be the turntable shaft of a record player or the capstan shaft of a magnetic tape drive. It should be understood that the signals carried by the signal bearing means may be in the form of optical markings or in the form of magnetised portions of magnetic material.

Record player and magnetic tape drives must fulfil the condition that variations in the speed of rotation of the turntable as well as its running constancy must not exceed standard values. In addition, particularly in the case of a record player, the vibrations emanating from the motor, and also from any gears which may be present, must remain sufficiently small in the lower audio frequency range as against the reproduced information signal.

Today, for the fulfilment of the first requirement, record player drives are used which can be regulated with respect to their speed of rotation and, for this purpose, have a drive motor, as for example a DC motor or an asynchronous motor connected to an electronic arrangement, the speed of rotation of which can be regulated. An essential component of this regulating arrangement is a signal emitter, for the generation of a regulating signal proportional to the speed of rotation, which may also be described as a tachogenerator. The actual value of the rotational speed obtained in this way is compared with the required value fed into the arrangement and the difference used to regulate the speed to the most constant value possible. Regulating arrangements of this kind form the state of the art.

The state of the art also includes socalled direct drives in which the motor acts directly on the turntable or capstan shaft, such as drives with reduction gears between the turntable or capstan shaft and the drive motor. With more superior drives, gears with a drive belt are often employed. In the case of the last-mentioned belt driven drives, the actual value for the rotational speed is generated by a tacho-generator on the motor. The disadvantage of this is that the influences of the belt transmission cannot be assessed. With direct drive this influence is eliminated. Either the opposingly induced voltage in the drive motor is used as the actual value, or a pulse generating signal emitter is fitted directly or indirectly to the turntable or capstan shaft.

Such signal emitters can use either an optical or a magnetic principle. In the case of optical systems, either a disc with a multiplicity of holes or slots, or light and dark visions, is employed to which a source of light and a photodetector are applied. In the case of magnetic systems, localised magnetic markings, so-called magnetic poles, are applied to the turntable or capstan shaft, which magnetic poles can be sensed by a detector in the form of a magnetic head. With both systems, a periodic pulse sequence is received by the detector, the frequency of which pulse sequence is proportional to the rotational speed of the turntable or capstan shaft.

It is known to arrange optical or magnetic markings, for obtaining a rotational speed signal, on the cylindrical surface of a record player turntable (German Patent Specification No.

1,572,442). The disadvantage of this is the tendency to high cost because of the necessary precision of such optical or magnetic markings and the difficulties with respect to fault-free reproduction in mass production. The application of preprepared signal bearing means and/or preprepared divisioning to a cylindrical part is unsatisfactory as it is practically impossible to maintain an equidistant divisioning on application of the signal bearing means to the cylindrical surface. It is also known to arrange a smooth surface with optical markings, for the generation of a signal proportional to rotational speed, on the underside of the turntable (German Offenlegungsschrift No. 1,907,822. Preprepared signal bearing means and/or divisions which are arranged on a disc can only be arranged exactly centrally on the carrier surface with great expense.

"Centrally" here refers to the relationship of the signal bearing means to the rotational axis of the carrier surface, capstan shaft or turntable about which the signal bearing means must run with absolute equidistant divisioning and exact centricity.

The main object of the invention is to provide a rotary drive of the type described in the introductory paragraph with very accurately functioning signal bearing means at low cost and in a manner which permits mass production.

According to the invention, this object is achieved with the rotary drive described in the introductory paragraph in that the signal bearing means comprises a discshaped member having an even surface which has been provided with signals for generating said pulse sequence after application of the member to the carrier surface.

The even surface and the subsequent application of the signals make possible an accuracy not so far known with respect to the equidistant spacing and the centricity of the signals. Initially the disc-shaped member is mounted on the carrier surface without signals, with only limited care needing to be taken with respect to its central location. The rotational axis of the carrier surface and/or the necessary hole arranged in the carrier surface for the capstan or turntable shaft then serves as a centering aid for the device for the application and/or feeding in of signals.

This ensures that the rotational axes of the turntable or capstan shaft and the signals coincide, this common axis being described as the axis of the system.

The invention also relates to a process for providing a rotary drive with signal bearing means whereby, according to a further feature of the invention, a disc-shaped member is applied to the rotationally symmetrical carrier surface, and an even surface of the member is subsequently provided with signals for generating said pulse sequence such that the signals are rotationally symmetrically disposed and concentric with respect to the carrier surface, whereby the frequency of the pulse sequence produced by the detector is proportional to the speed of rotation of the output shaft. The signals can be either an optical divisioning in the form of a network of lines or a magnetic divisioning in the form of a multiplicity of pairs of poles, which are arranged equidistant from and centrally with respect to the system axis.

The invention permits the manufacture, at a low cost, of a record player or magnetic tape drive which has extraordinarily low variation in rotational speed as well as low deviations in running constancy.

Production is also possible with a high degree of reproduceability.

Magnetic signal bearing means most practically takes the form of a disc-shaped formed mass of a magnetisable permanently magnetic material which has been magnetised at periodic intervals in a peripheral direction. The number of poles should be chosen to be as high as possible.

If the size is sufficiently large, the signal bearing means should be magnetised with at least ten pairs of poles. The mass of permanent magnetic material, for example, a paste-like mixture of ferrite powder and a binder can be applied initially as a shapeless mass to the carrier surface which, in use, rotates in strict synchronism with the turntable or capstan shaft. It is, however, of particular advantage if the signal bearing means is a smooth ring of magnetic powder in a binder, which is connected to the carrier surface. Such a ring can for example, be sprayed or stamped out of a sheet of the permanent magnetic material.

Attachment to the carrier surface can then, for example, be carried out by adhesion.

The smooth formation of the signal bearing means offers a simple possibility to produce a highly accurate spacing of the magnetic signals; for this purpose, a rotationally symmetrical magnetisation device is produced made from soft iron with a multiplicity of very accurate milledin radial slots. The accuracy of the angular specing of the slots can, for example, amount to +10 seconds of arc. A coil is inserted in the slots so that individual poles are produced. Furthermore, the magnetisation device has a hole, the centre of which is coincident with the centre about which the poles are spaced, which receives the shaft of the carrier surface for the signal bearing means. The cost of such a magnetisation device has no practical effect on the cost of the product as it is a matter of a one-off manufacturing aid.

After the smooth signal bearing means has been attached to its carrier surface without any attempt at accurate centering, the shaft of the carrier surface is inserted in the hole of the same magnetisation device.

Thereupon, a current pulse is applied to the winding of the magnetisation device, whereby localised pairs of poles are produced in the signal bearing means, which pairs of poles correspond very accurately to the spacing of the poles of the magnetisation device and are absolutely central with respect to the shaft. The thickness of the magnetisable layer and its planarity have only a surbordinate effect on this accuracy.

This simple process could not be used with a magnetisable layer applied to a cylindrical casing because of the tolerance with respect to the diameter of the cylinder.

Furthermore, accurate magnetic divisions on cylindrical surfaces are equally difficult to produce and are as expensive in manufacture as the necessary magnetisation device required for them.

For example, the individual pairs of poles would have to be produced in sequence, the surface of the cylinder being moved forward in steps relative to a magnetisation device.

In the case of a directly driven magnetic tape capstan shaft, which is usually fixed directly to the rotor of the motor, the magnetic signal bearing means is attached to the rotor, for example on its lower side.

For a record player the magnetic signal bearing means can be applied directly to the turntable, for example on its underside, i.e.

the carrier surface for the signal bearing means is part of the turntable itself. It is, however, also possible to arrange the signal bearing means on a component which carries the turntable. In this case also, the carrier surface is connected in strict synchronisation to the turntable. In each case, the pulse sequence created by the magnetised signal bearing means is a true representation of the rotational speed of the turntable. The object of the invention is thus applicable both to direct drives and to drives with a reduction gear.

The invention has additional advantages in the case of a drive which has a reduction gear with driving belt and pinion between the drive motor and the turntable or capstan shaft. In this case, the signal bearing means can be arranged with particular advantage on the underside of the pinion, below which the detector for the signal bearing means can be located. It is particularly advantageous if the pinion has an annular groove on its lower side in which the signal bearing means is inserted. In this manner, a simple precentralising of the initially still unmagnetised signal bearing means is facilitated.

As the turntable or capstan shaft is in strict synchronisation with the pinion, all influences on the speed of rotation of the turntable or capstan shaft which originate in the gear train are picked up by the regulation process and compensated for.

Thus, even expensive magnetic tape drives with driving belts gain, to a great extent, the advantageous properties of a direct drive.

In order to magnetically screen the signal bearing means from interference by magnetic alternating fields, it is desirable to arrange a ferro-magnetic body between the signal bearing means and the source of interference. The casing of the motor can undertake this function advantageously, in so far as it consists, for example, of sheet steel. It is, however possible to provide a special screen plate of ferro-magnetic material above the magnetic signal bearing means. This plate can even be the carrier surface of the signal bearing means itself, thus providing the screening action and simultaneously increasing the magnetic field strength because the plate represents so-called magnetic return path.

A particularly simple and effective design of detector in accordance with the invention comprises a magnetic head with an E-shaped iron core, the longitudinal axis of the "E" lying in the peripheral direction of the signal bearing means and the pole pieces being perpendicular to the signal bearing means. The sensitivity of the receiver thus has a preferential direction directed to the signal bearing means. This is particularly important if, as in the case of one of the embodiments described hereinafter, the signal bearing means is mounted on the rotor of the electric motor.

Without this preferential direction, a permanent interference frequency would be picked up by the magnetic head because of the rotating field of the motor. This is reliably prevented by the solution proposed.

Furthermore, it is also advantageous if an even number of magnetic heads is distributed around the perimeter of the signal bearing means. An arrangement with the magnetic heads located at diametrically facing positions with respect to the system axis is particularly convenient. On the other hand, the reactional sensitivity is thereby increased; on the other hand, the effect of wobbling movements of the signal bearing means are effectively compensated.

Another simple and effective design of detector in accordance with the invention comprises a magnetic head with a so-called multiple air gap. By this, the induced signal voltage is correspondingly amplified. A detector of this type can, according to a preferred feature of the invention, be fitted with pole pieces which consist of two Lshaped plates, mutually confronting coplanar limbs of which are shaped and located in such a manner that alternating projections and recesses engage in each other and enclose a sinuous air gap. The coil of the detector is arranged between the parallel limbs of the plates, with the axis of the coil perpendicular to these limbs.

Furthermore when applied to record players, the invention permits an improvement with respect of the so-called "rumble" noises. The vibrations in the lower audible range proceeding from the motor and gears must be sufficiently small with respect to the scanned information signal. This requirement is, according to a preferred feature of the invention, fulfilled in a particularly simple manner by weighting the drive motor with an additional mass and suspending it on the chassis by means of a resilient member. A further improvement of the conditions is achieved by locating a respective resilient member between the drive motor and the additional mass, and between the additional mass and the chassis. In this manner, there is produced a mechanical damping sequence which dampens the "rumble" noises in an excellent manner.

The particular advantages of a signal bearing means with a smooth surface can also be exploited using an optical technique. In this arrangement, the signal bearing means consists of a material whose smooth surface is furnished with optically detectable signals after its application to the carrier surface, and a source of light and a light sensor are applied to the signal bearing means. The optically detectable signal can, for example, be produced by a printing process or a photographic process.

In both cases, once again, the system axis and/or shaft around which the carrier surface for the signal emitter rotates, serves as a means of centering, during generating of the optically detectable signals.

Further advantages and details of the invention from the following description of two embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is an exploded perspective view of the essential parts of a record player drive, Figure 2 is a radial section through a pinion with a magnetic signal emitter taken along the lines 11-11 in Figure 1, Figure 3 is a section through a detector taken along the lines Ill 111 in Figure 1, Figure 4 is a vertical section through the axis of a magnetic tape drive taken on the line lV-IV in Figure 5, Figure 5 is a plan view of the carrier plate and detector of the drive shown in Figure 4, Figure 6 is an enlarged section through the detector taken on the line Ill-Ill in Figure 5, and Figure 7 is an enlarged section through the detector taken on the line lV-IV in Figure 5.

Figure 1 shows a chassis 10, in which there is a cylindrical depression 11 to receive a turntable 12 having a turntable mat 13. The turntable consists of ferromagnetic material, and the turntable mat 13 of an elastic material. In the bottom of the depression 11 is a coaxial cylindrical depression 14 which serves to accommodate a reduction gear 15, which is described in more detail hereinafter. In the centre of the depression 14 there is a bearing 16 for the lower end of a shaft 17. The bottom wall of the depression 14, which is not described in further detail, contains a recess 18 in which there is arranged a magnetic pulse detector 19.

Laterally adjacent the depression 14, a cylindrical extension 20 is fitted to the chassis 10 and serves to accommodate a drive motor 21. An additional mass 22, consisting of a cylindrical ring of die-cast zinc, is arranged between the drive motor 21 and the extension 20 and/or the chassis 10. The additional mass 22 is inserted in the extension 20 with adequate play and is braced against the chassis 10 by resilient members 23. The resilient members consist of individual rubber bodies, which are placed on the upper and lower end-faces of the additional mass 22. Further resilient members 24 consisting of axially parallel rubber strips, are placed on the inner cylindrical surface of the additional mass 22. In the present case, there are three peripherally distributed resilient members 24 between which the drive motor 21 is inserted. The whole arrangement comprising the drive motor 21 and the additional mass is held in the extension by a lid 25 which is screwed to the chassis 10.

Thus, a mechanical damping assembly is created from two sets of resilient members with additional masses placed between thsm so that the generation of rumble noises is effectively combated. The drive motor 21 has a belt sheave 26 round which a driving belt 27 is led. The belt sheave 26 and the driving belt 27 are part of the reduction gear 15 which also includes a pinion 28 which also acts as a large belt cheave. The pinion 28 consists of plastics material, for example, foamed polystyrol, and contains a central hole 29 which is formed by precision drilling and serves for acceptance of the shaft 17. This precision drilling and the shaft 17 define the system axis "A". The pinion 28 has, on its upper periphery, three projections 30, on which lies the turntable 12 with the turntable mat 13. A support surface for a signal emitter, according to the invention, is formed on the lower side of the pinion 28. These essential elements are shown in detail in Figure 2.

The detector 19 is a magnetic head with a multiple air gap 31 which has a sinuous shape (see Figure 1) and is formed between two pole pieces 32 and 33 which face each other. Further details thereof are shown in Figure 3.

As shown in Figure 2, the pinion 28 consists of an essentially cylindrical disc which has a flat annular groove 35 on its underside 34. This annular groove 35 contains a signal emitter 36 having a smooth surface 37. The base of the annular groove 35 serves as a carrier surface 38 for the signal emitter 36 which is attached thereto by an adhesive. The signal emitter 36 is a preformed, smooth ring 39 of magnetic powder (ferrite) and a binder. The detector 19 is located below the signal emitter 36.

Referring to Figure 3, the pole pieces 32 and 33 of the signal detector 19 are formed on mutually confronting coplanar limbs of two L-shaped plates 40 and 41. As can be seen from Figure 1, these limbs are stamped out in such a manner that alternate projections and recesses engage in each other and enclose the above-mentioned sinuous air gap 31. Because of the shape of the air gap 31 shown in Figure 1, four and a half pairs of poles (i.e. nine individual poles) are scanned simultaneously, whereby the signal voltage is multiplied and the arrangement acts as a filter against interfering frequencies. The two other legs of the L-shaped plates 40 and 41 run parallel to one another and enclose between them a coil 44 whose axis is prependicular to the legs 42 and 43. The coil contains a spool 45 made from insulating material and a ferro-magnetic tube 46 as a core. A fixing screw (not shown) extends through the legs 42 and 43 and the sleeve 46. The pole pieces 32 and 33 are attached to a mounting plate 47 consisting of insulating material.

The arrangement represented in Figures 2 and 3 can be replaced by one with an optical signal emitter. In this case, the smooth ring 39 would consist of a printable material or a photographic material whose smooth surface 37 is subsequently provided with a circularly extending optical pattern centred on the system axis "A". The magnetic head of the detector 19 in accordance with Figure 3 is then replaced by a source of light and a photodetector.

In Figure 4, an encapsulated magnetic tape drive is shown marked 50, which consists of a capstan shaft 51 and a rotor 52, which is secured to the capstan shaft for simultaneous rotation therewith. A multiple rotor magnet 53 is fixed to the rotor 52 and has, on its upper side, a pole piece ring 54 of ferro-magnetic material, which acts as a magnetic return path. Similarly, on the opposite side, there is a lower pole piece ring 55 which also serves as a magnetic return path and which is spaced apart from the rotor magnet 53. The capstan shaft 51, carrying the rotor 52 and the lower pole piece ring 55, is held in a bearing sleeve 56 by means of two bearing bushes 57 and 58.

The capstan shaft engages with a magnetic tape 59, which is driven by an associated pressure roller (not shown).

The bearing sleeve 56 is fixed in an upper casing part 60, which encloses the rotor 52 and is essentially rotationally symmetrical.

A similarly rotationally symmetrical lower housing part 61, which contains an adjustable thrust bearing 62 is connected to the upper casing part 60. The two parts of the casing are made from steel sheeting and support between them a mounting plate 63 made of insulation material, which also extends into the air gap between the rotor magnet 53 and the lower pole piece ring 55.

In the area of this air gap, on the mounting plate 63, there are stator coils 64 for interaction with the rotor magnet 53.

Operation of the stator coils 64 is controlled by a control system (not shown) which contains, among other things, a source of light 65 and a photodetector 66. Between the source of light 65 and the photodetector 66 there runs a circular shutter 67 which consists of a cylindrical flange with cut-outs 68. The control system for the stator coil 64 is, however, not a feature of the invention and consequently there is no need for further explanation.

The rotor 52 includes a carrier surface 69 which is formed by the base of an annular groove. In this annular groove is inserted a signal emitter 70 having a smooth lower surface 71. The signal emitter is fixed to the carrier surface 69 by an adhesive. The signal emitter 70 is a smooth ring of magnetic powder in a binder.

Facing the smooth surface 71 of the signal emitter 70, two detectors 72 are arranged at respective diametrically opposite positions. The details of the detectors 72 are shown more clearly in Figures 6 and 7. Of the two detectors 72, Figure 4 only shows the right hand one because of the selected section line which can be seen from Figure 5. The upper casing part 60 is fastened to a chassis 74 by means of screws 73. The casing parts are held together by three spring clips 75 distributed round the circumference.

In Figure 5 it can be seen that three pairs of light sources 65 and photodetectors 66 are arranged distributed around the periphery. At diametrically opposite positions, two detectors 72 are provided for picking up the magnetic fields of the signal emitter 70, which move past each detector 72 on the rotation of the rotor 52.

From Figure 6 is can be seen that each detector 72 consists of an E-shaped iron core, which possesses two exterior poles 76 which are part of a strip of metal bent into a U-shape. Between the exterior poles 76 there is a yoke 77 to which a bolt-shaped central pole 78 is riveted. A coil 79 is arranged on the central pole 78. The poles 76 and 78 are directed towards the signal emitter 70, whose north and south poles generated by the magnetisation thereof are symbolically represented in Figure 6. From Figure 5, it can be seen that Figure 6 is a tangential section so that the sequence of north and south poles on the rotor 52, which rotates with the signal emitter 70, runs over and past the detector 72 and induces a sequence of voltage pulses in the coil 79.

The detector 72 is inserted between two Ushaped projections 80, which are a part of the mounting plate 63. Below the mounting plate 63 there is a conductor plate 81 which is not of interest here. The longitudinal axis of the E-shape formed by the poles 76 and 78 runs in a peripheral direction, which coincides with the section lines VI--VI.

The sectional view of Figure 7, which is looking towards the right hand side of the section shown in Figure 3, is essentially an enlarged but reversed representation of the outer right hand part in Figure 4. It can be seen from the drawings that the individual poles of the rotor magnet 53 can have no influence on the detector 72, but on the other hand, the magnetised poles of the signal emitter 70 can have, so that a definite separation of signals is produced.

The signal emitter system can also be used with a drive with a reduction gear instead of the direct drive of the capstan shaft as shown in Figures 4 and 5. In this case, tlie rotor 52 has no rotor magnet, but a normal motor with a belt sheave is arranged adjacent the rotor 52 in such a manner that the belt sheave lies parallel to the capstan shaft 51 next to the cylindrical casing surface 82 of the rotor 52. A driving belt (not shown) is laid over the belt sheave and the rotor, which thus takes on the role of a pinion. The housing parts 60 and 61 are suitably modified to permit this arrangement.

The arrangement represented in Figures 4 and 5 can be replaced by one with an optical signal emitter. In this case, the smooth surface 71 would consist of a printable material or a photographic material which is subsequently provided with a circularly extending optical pattern centred on the system axis "A".

In place of the detector 72 formed as a magnetic head as shown in Figures 6 and 7, a source of light and light sensor would be used.

WHAT WE CLAIM IS:- 1. A rotary drive, the rotational speed of which can be regulated, comprising a drive motor driving an output shaft, a rotationally symmetrical carrier surface carrying signal bearing means and coupled to the output shaft for rotation in synchronisation therewith, and a detector for detecting the signals carried by the signal bearing means and for producing a pulse sequence in dependence thereon so that the frequency of the pulse sequence is proportional to the speed of rotation nf the output shaft, said signal bearing means comprising a discshaped member having an even surface which has been provided with signals, for generating said pulse sequence, after application of the member to the carrier surface.

2. A rotary drive according to claim 1, wherein the signal bearing means is a discshaped formed mass of a permanently magnetic material which has been provided with signals by being magnetised at periodic intervals in a peripheral direction.

3. A rotary drive according to claim 2, wherein the signal bearing means has been magnetised so as to have at least ten pairs of poles.

4. A rotary drive according to claim 3, wherein the signal bearing means is a smooth ring made from magnetic powder and a binder, the ring being connected to the carrier surface.

5. A rotary drive according to any preceding

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

casing part 60 is fastened to a chassis 74 by means of screws 73. The casing parts are held together by three spring clips 75 distributed round the circumference.

In Figure 5 it can be seen that three pairs of light sources 65 and photodetectors 66 are arranged distributed around the periphery. At diametrically opposite positions, two detectors 72 are provided for picking up the magnetic fields of the signal emitter 70, which move past each detector 72 on the rotation of the rotor 52.

From Figure 6 is can be seen that each detector 72 consists of an E-shaped iron core, which possesses two exterior poles 76 which are part of a strip of metal bent into a U-shape. Between the exterior poles 76 there is a yoke 77 to which a bolt-shaped central pole 78 is riveted. A coil 79 is arranged on the central pole 78. The poles 76 and 78 are directed towards the signal emitter 70, whose north and south poles generated by the magnetisation thereof are symbolically represented in Figure 6. From Figure 5, it can be seen that Figure 6 is a tangential section so that the sequence of north and south poles on the rotor 52, which rotates with the signal emitter 70, runs over and past the detector 72 and induces a sequence of voltage pulses in the coil 79.

The detector 72 is inserted between two Ushaped projections 80, which are a part of the mounting plate 63. Below the mounting plate 63 there is a conductor plate 81 which is not of interest here. The longitudinal axis of the E-shape formed by the poles 76 and 78 runs in a peripheral direction, which coincides with the section lines VI--VI.

The sectional view of Figure 7, which is looking towards the right hand side of the section shown in Figure 3, is essentially an enlarged but reversed representation of the outer right hand part in Figure 4. It can be seen from the drawings that the individual poles of the rotor magnet 53 can have no influence on the detector 72, but on the other hand, the magnetised poles of the signal emitter 70 can have, so that a definite separation of signals is produced.

The signal emitter system can also be used with a drive with a reduction gear instead of the direct drive of the capstan shaft as shown in Figures 4 and 5. In this case, tlie rotor 52 has no rotor magnet, but a normal motor with a belt sheave is arranged adjacent the rotor 52 in such a manner that the belt sheave lies parallel to the capstan shaft 51 next to the cylindrical casing surface 82 of the rotor 52. A driving belt (not shown) is laid over the belt sheave and the rotor, which thus takes on the role of a pinion. The housing parts 60 and 61 are suitably modified to permit this arrangement.

The arrangement represented in Figures

4 and 5 can be replaced by one with an optical signal emitter. In this case, the smooth surface 71 would consist of a printable material or a photographic material which is subsequently provided with a circularly extending optical pattern centred on the system axis "A".

In place of the detector 72 formed as a magnetic head as shown in Figures 6 and 7, a source of light and light sensor would be used.

WHAT WE CLAIM IS:- 1. A rotary drive, the rotational speed of which can be regulated, comprising a drive motor driving an output shaft, a rotationally symmetrical carrier surface carrying signal bearing means and coupled to the output shaft for rotation in synchronisation therewith, and a detector for detecting the signals carried by the signal bearing means and for producing a pulse sequence in dependence thereon so that the frequency of the pulse sequence is proportional to the speed of rotation nf the output shaft, said signal bearing means comprising a discshaped member having an even surface which has been provided with signals, for generating said pulse sequence, after application of the member to the carrier surface.
2. A rotary drive according to claim 1, wherein the signal bearing means is a discshaped formed mass of a permanently magnetic material which has been provided with signals by being magnetised at periodic intervals in a peripheral direction.
3. A rotary drive according to claim 2, wherein the signal bearing means has been magnetised so as to have at least ten pairs of poles.
4. A rotary drive according to claim 3, wherein the signal bearing means is a smooth ring made from magnetic powder and a binder, the ring being connected to the carrier surface.
5. A rotary drive according to any preceding claim, wherein the signal bearing means is attached to the rotor of the motor.
6. A rotary drive according to any of claims 1 to 4, wherein a reduction gear with a driving belt and pinion is arranged between the drive motor and the output shaft.
7. A rotary drive according to claim 6, wherein the signal bearing means is arranged on the lower side of the pinion.
8. A rotary drive according to claim 5, wherein the signal bearing means is embedded in an annular groove on the lower side of the rotor.
9. A rotary drive according to any preceding claim, wherein the detector consists of a magnetic head with an shaped iron core, the longitudinal axis of

the E running in a peripheral direction of the signal bearing means and the pole pieces of the core being perpendicular to the signal bearing means.
10. A rotary drive according to claim 9, wherein an even number of detectors are spaced around the periphery of the signal bearing means.
11. A rotary drive according to claim 10 wherein two detectors are arranged diametrically opposite to each other with respect to the axis of rotation of the signal bearing means.
12. A rotary drive according to any of claims 1 to 8, wherein the detector consists of a magnetic head with a multiple air gap.
13. A rotary drive according to claim 12, wherein the detector has pole pieces which consist of two L-shaped metal plates, one limb of each plate being coplanar with and confronting the corresponding limb of the other plate, the confronting edges thereof being so shaped that alternate projections and recesses engage in each other and enclose a sinuous air gap, a coil being arranged between the two other limbs which are mutually parallel, the coil axis being perpendicular to these two other limbs.
14. A rotary drive according to claim 1, wherein the signal bearing means has been provided with optically detectable signals by printing or photography after application, and the detector comprises a source of light and a light sensor.
15. A rotary drive according to any preceding claim, wherein the output shaft is the capstan shaft of a magnetic tape drive.
16. A rotary drive according to any of claims 1 to 14, wherein the output shaft is the shaft of the turntable of a record player.
17. A rotary drive according to claim 16, wherein the signal bearing means is attached to the turntable.
18. A rotary drive according to claim 16, wherein the signal bearing means is arranged on a member supporting the turntable.
19. A rotary drive according to any of claims 16 to 18, wherein the turntable consists of a ferromagnetic material.
20. A rotary drive according to any of claims 16 to 19, wherein the drive motor is weighted by an additional mass and suspended on a chassis by means of at least one resilient member.
21. A rotary drive according to claim 20, wherein at least one elastic member is arranged between the drive motor and the additional mass and chassis.
22. A process for providing a rotary drive with signal bearing means, the drive comprising a drive motor driving an output shaft, the rotational speed of which can be regulated, a rotationally symmetrical carrier surface coupled to the output shaft for rotation in synchronisation therewith, and a detector for detecting the signals carried by the signal bearing means and for producing a pulse sequence in dependence thereon, the process comprising applying a disc-shaped member to the carrier surface, and subsequently providing an even surface of the member with signals for generating said pulse sequence such that the signals are rotationally symmetrically disposed and concentric with respect to the carrier surface, whereby the frequency of the pulse sequence produced by the detector is proportional to the speed of rotation of the output shaft.
23. A rotary drive substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.
24. A rotary drive substantially as hereinbefore described with reference to Figures 4 to 7 of the accompanying drawings.
25. A process for providing a rotary drive with signal bearing means substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.
26. A process for providing a rotary drive with signal bearing means substantially as hereinbefore described with reference to Figures 4 to 7 of the accompanying drawings.