WO1982001956A1

WO1982001956A1 - Improvements in phonograph pickup arms

Info

Publication number: WO1982001956A1
Application number: PCT/AU1981/000173
Authority: WO
Inventors: William J Chidzey; John A Chidzey
Original assignee: William J Chidzey; John A Chidzey
Priority date: 1980-11-27
Filing date: 1981-11-27
Publication date: 1982-06-10

Abstract

A pickup arm assembly for gramophone disc records comprises a pickup arm (6) pivotally mounted to a driven arm (8) movable about an axis substantially parallel to the axis of rotation of the record disc (A). The pickup arm pivot (7) carries a target (11) which coacts with a position sensor (12) mounted on the driven arm (8) to sense changes in the angle subtended between the pickup arm (6) and the driven arm (8). The position sensor (12) provides a signal corresponding to any deviation of the angle from a desired predetermined angle to operate drive means (9) for the driven arm (8) to thereby move the arm in a direction to reduce such deviation. The relationship of the position sensor (12) to the driven arm (8) may be varied in accordance with variations in the position of the driven arm (8) to reduce or illuminate tracking errors.

Description

IMPROVEMENTS IN PHONOGRAPH PICKUP ARMS

Field of the Invention

This invention relates to improvements in the construction of phonograph pickup arms and relates particularly to improvements in pivotal pickup arms as distinct from linear tracking arms. Background of the Invention

Pivoted phonograph pickup arms are well known and in common use. Such arms are generally pivoted on a substantially vertical axis adjacent the turntable of a record player so that the stylus carried on the free end ' of the pickup arm tracks in the groove of a record on the turntable. The pickup arm is also pivoted about a horizontal axis, usually intersecting the vertical pivot axis, to enable the stylus to be lowered onto and raised from the record surface^* and to allow the pickup arm to pivot in response to record warp or imperfections in the plane of the record surface.

A simple straight pickup arm which rotates about a fixed vertical pivot, unless the arm is inconveniently long, suffers from excessive errors of stylus alignment with the record groove. Such errors are reduced in modern pickup arm constructions wherein the stylus centre line is offset from the line joining the stylus tip to the vertical pivot and that distance between the stylus tip and the vertical pivot is greater than the distance between the vertical pivot and the centre of the turntable.

Although the errors referred to above are minimized in modern pickup arms, such errors still exist and, in addition, the stylus offset angle results in a side thrust on the record groove. Background Art

Various forms of phonograph pickup arm have been proposed to minimize the errors of stylus alignment and, at the same time, reduce any tendency of the stylus tip producing a side thrust. Such constructions as are previously proposed include weighted arms which are designed to balance the side thrust at any given position of the phonograph pickup arm.

Various other devices have been added to the construction of offset phonograph pickup arms to ^"counter- balance the inherent side thrust, but because of the variability of friction and the variable drag arising from groove modulation on a phonograph record, none of the devices known wholly eliminates side thrust on the stylus. More elaborate constructions of phonograph include linear or parallel motion pickup arm arrangements but these arrangements are, as yet, relatively expensive.

United States Patent No. 3,059,934 to Urmenyi discloses a pick-up arm arrangement wherein the pivot of the pickup arm is at the end of a movable arm and moves on a predetermined path which is related to the distance between the stylus and turntable axis. This patent suggests using switch contacts which open or close dependin on variations of movement of the pickup arm, to control operation of a drive motor which drives the movable arm. I one embodiment, the switch contacts are located to maintain a right angle between the pickup arm and the movable arm as the stylus moves across the record disc. In another arrangement a slide contact switch is used so that the angle between the pickup arm and the movable arm is varied as the stylus moves across the record disc. The use of switch contacts as described in this specification introduces a positive restriction or impediment to the free movement of the pickup arm. The arm is unable to freely move past the position at which the contacts close thereby giving rise to undesirable side pressure on the stylus, particularly if the record groove is not concentric. The stop-start action produced by switch contacts is also highly undesirable.

U.S. Patent No. 3,826,505 discloses a pickup guidance mechanism which is designed to both eliminate side pressure and tracking error. This specification proposes to pivotally mount the pickup arm on a driven arm which is controlled by servo circuitry to move the driven arm to a

Cfi mean position in which the angle between the pickup arm an the driven arm is of a desired value related to the distan of the stylus from the record centre. A system of photo¬ cells and a lamp are mounted on an optical arm pivoted for rotation about the same axis as the pickup arm. The optica arm is biased against a cam to vary the position of the photocells and lamp in relation to the driven arm and pick up arm during movement of the stylus in the record plane. An optical shutter is attached to the pickup arm pivot ste to lie in the path of light from the lamp to the photocells Thus, any deviation from the mean position of the pickup a relative to the driven arm at a given distance of the styl from the record centre is sensed by unbalanced illuminatio of the photocells, which unbalance gives rise to an electrical signal which causes rotation of the driven arm in a direction to restore balance.- The balance position is varied by movement of the optical arm along the cam surface

The apparatus of this specification is relatively complex and requires a complicated pivotal arrangement for the pickup arm. An additional, pivoted, arm is also required to enable the desired angie between the pickup arm and the driven arm to be varied dependent on the distance between the stylus carried on the pickup arm and the record centre. U.S. Patent No. 3,920,249, discloses a further structure incorporating a pickup arm pivoted to a driven arm. The guidance mechanism described includes a profiled shutter mounted on the pickup arm and edge sensing means which co-operates with the profiled shutter and controls movement of the driven arm so that the pickup arm and the driven arm are maintained at a desired angular relationship dependant on the relative position of the stylus carried by the pickup arm.

The structure disclosed in this specification also requires a relatively complicated pivotal arrangment for the pickup arm as well as the provision of a relatively large cam member movable with the pickup arm and a pair of photocells and associated lamps for cooperating with the edges of the cam member.

It is an object of the present invention to provide a pivoted phonograph pickup arm which does not suffer from excessive errors of stylus alignment with the record groove and which substantially reduces or eliminates side thrust. It is a further object of the present invention to provide an improved construction of a pivoted phonograph pickup arm which is relatively compact and simple in construction and relatively economic to produce. A still further object of the present invention is to provide a pivoted phonograph pickup arm wherein the stylus offset angle is optimized at any given position of the stylus tip during movement across a phonograph record. Summary of the Invention According to the present invention there is provided a phonograph pickup arm assembly comprising a first arm, driving means for moving the first arm about a first axis, a second arm freely pivoted to the first arm for movement about a second axis, the second arm being freely pivoted about a third axis substantially at right angles to said second axis, the second arm being adapted to carry a pickup stylus at one end thereof and counterbalance means at the other end thereof, and positional feedback control means controlling said driving means so that, in use, a pre- determined angular relationship is maintained between said first and second arms.

With the pickup arm assembly of the present invention, the angle between the second arm and the first

OMH arm may be maintained substantially constant to provide a desired offset of the stylus centreline to thereby minimize errors of stylus alignment with a record groove. However, since the second arm is freely pivoted to the first arm, no undesirable side thrust will arise.

The positional feedback control means monitors the angular relationship between the first and second arms so that, on any variation of the angle between those arms, the driving means moves the first arm to a position which maintains the desired angular relationship.

Various forms of positional feedback control means may be utilized in performing the present invention. One preferred form of such control means comprises a target which is fixed for movement with the second arm about the second axis and a position sensor which is fixed to the first arm. The position sensor detects any relative movement of the target and provides a feedback signal to control an electric motor driving the first arm about the first axis. In a modified form of the invention, residual ' errors of stylus alignment may be eliminated by maintaining a predetermined angular variation between the first and second arms during movement of the stylus across a record. Thus, the positional feedback control means controls movement of the first arm to maintain a desired angular relationship between the first and second arms for any given angular position of the first arm.

The invention will be more fully understood by reference to the accompanying drawings. , Description of the Drawings

Figure 1 is a schematic plan view of a known pickup arm assembly,

Figure 2 is a schematic plan view of a pickup arm assembly of the present invention, Figure 3 is an elevational view of the assembly of

Figure 2 taken along the lines 3-3 of Figure 2,

Figure 4 is a plan view of a modified form of an assembly according to the invention, and Figure 5 is a block schematic of positional feedbac control means for use with the pickup arm assembly of the present invention.

Referring to Figure 1 there is illustrated an example from prior art of a pickup arm assembly in which a stylus 1 is mounted in a cartridge 2 fitted to a bent arm 3 having a counterweight 4 at its other extremity. The'arm is mounted in fixed pivots 5 about which it is free to rotate both horizontally and vertically. The pickup arm construction and its relation to a turntable A are such that the stylus centreline is offset by an angle θ to the line joining the stylus tip to the centre of the pivots 5, and the latter is located in relation to the distance between the pivots 5 and the stylus tip so that the stylus tip clears the centre of the turntable by an overhang e_. The object of this prior art construction is that by optimising the angle θ and overhang e_ for any given effective radius R from stylus tip to pivot centre 5 it is possible to minimise the departure of stylus centreline from tangency to record groove as the stylus traverses the record. For example, it is noted that for an effective pickup arm radius of 230 mm with an offset angle θ of 24 and an overhang of i8 mm, the tangency error of the stylus centreline will not exceed 2 during a traverse of a standard 12 inch record disc.

As stated above, a disadvantage arising from this prior art construction is that the stylus offset angle θ required results in a side thrust on an unmodulated record groove. This side thrust is approximately equal to Vμ sin θ where V is the downward load on the stylus and μ is the effective coefficient of friction between stylus and record Various compensating devices have been added to the structure of Figure 1 to counterbalance the inherent side thrust, but owing to the variability of friction and to the variable drag arising from groove modulation none wholly eliminates side thrust on the stylus. Description of Preferred Embodiments

Referring to Figure 2 of the drawings there is illustrated a phonograph pickup arm assembly in which a stylus 1 is mounted in a cartridge 2 fitted to a straight arm 6. The -arm 6 is mounted in bearings in a trunnion 7 whereby the arm 6 is able to freely pivot about a substantially horizontal axis. The arm 6 is provided with a counterweight 4 at its other end, in a manner known well in the art.

The trunnion 7 is pivotally mounted on one end of a driven arm 8 so as to be rotatable about a substantially vertical axis. The stylus 1 is therefore able to move both horizontally and vertically relative to the end of the arm 8.

The arm 8 is fixed to the vertical shaft 9A of an electric motor 9 mounted adjacent the turntable A. The electric motor 9 is preferably a stepping motor controlled, as hereinafter described, to control the relative position of the arm 8 relative to the turntable A.

A control arm 10 is'fixed rigidly to the trunnion 7 and carries at its free end a target 11 adjacent to which a position sensor 12 is fixed to arm 8. The target 11 and position sensor 12, which are known in the art, are arrange to sense any mis-alignment between the arm 10 and the position sensor 12 resulting from movement of arm 6 and trunnion 7 about the vertical pivot axis of the trunnion 7. The position sensor 12 provides a signal of magnitude and sense resulting from any such mis-alignment. The signal is used to give a feedback control to the electric motor 9 to move the driven arm.8 to a position which maintains the desired alignment. By this means, the angular relationship between the arm 6 and the arm 8 is maintained constant thereby maintaining constant the angle θ as the stylus 1 traverses the record disc under the influence of the record grooves.

It will be seen that if the offset angle θ and other material parameters are chosen as shown in the prior art system of Figure 1, the pickup arm assembly of the present invention as shown in Figure 2 will maintain a similar degree of pickup stylus alignment with the record groove and, since the arm 6 is freely pivoted about the vertical axis of the trunnion 7 while being substantially tangential to the record groove, no undesirable side thrust will arise. In Figure 4 there is illustrated a modification of the pickup arm assembly of the present invention whereby residual errors in the stylus to record groove tangency resulting from the geometry of the construction may be eliminated. In this modified form of the invention, the position sensor 12 is pivotally mounted to the arm 8 on a pivot pin 15. A cam 17 is mounted in a fixed position relative to the turntable A, such as on the body of the electric motor 9. A cam follower 14 extends from the ^: position sensor 12 to the cam 17 and a biasing spring 16 biases the follower 14 against the cam 17.

The cam 17 is profiled to vary the alignment position between the position sensor 12 and the target 11 in relation to the position of the arm 8 with respect to th turntable A. In other words, the angle between the arm 6 and the arm 8, which is controlled by movement of the arm 8 as the stylus 1 traverses a record disc, is varied as the stylus 1 moves across the record disc The changing angle results in the stylus being maintained accurately tangentia to the record groove during its transverse across the recor disc. Thus, any residual error in tangency of the stylus 1 at any point in its travel is eliminated.

The profile of the cam 17 may be determined either by measurement-of tangency errors at discrete points in the stylus travel with a constant radius cam followed by simple calculation of cam radius corrections, or alternatively by extending the existing published mathematical expressions for tangency errors to give cam radius corrections directly without experiment. Such cam profiling can be made simply by one skilled in the art and familiar with the residual tangency errors of existing phonograph pickup arms.

Referrring to Figure 5, the position sensor 12 may comprise a transducer which generates an electrical signal depending on the relative position of the target 11 in relation to the transducer 12. The electrical signal may vary in frequency and/or amplitude as a result of changes in the relative position between the target 11 and the position sensor 12 and such^* electrical signal is fed to a motor control 13 which provides a control current for driving the electric motor 9.

In one form of the invention the target 11 may comprise a permanent magnet and the position sensor 12 may consist of a linear Haul Effect device, with such a sensor the motor control may consist of a d.c. amplifier to drive a d.c. motor. The motor may be direct coupled to the arm 8 or may be coupled through an appropriate speed reduction. In another embodiment of the invention, the target 11 may consist of an opaque vane with a slot window. The position sensor 12 comprises a slotted photo emitter/dual sensor whereby relative mis-alignment between the sensor and the target results in a voltage variation across the sensor. This voltage variation is fed to the motor control which consists of a voltage control phased pulse generator which, in turn, is fed to the electric motor 9 which comprises a stepping motor. Thus, the mis-alignment detected by the position sensor 12 is corrected by incremental movements of the stepping motor. Many other forms of mis-alignment sensor and feed¬ back control for the electric motor may be used in the performance of the present invention.

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Claims

Claims .

1. A phonograph pickup arm assembly comprising a first arm, driving means for moving the first arm about a first axis, a second arm freely pivoted to the first arm for movement about a second axis, the second arm being freely pivoted about a third axis sxibstantially at right angles to said second axis, the second arm being adapted to carry a pickup stylus at one end thereof and counterbalance means at the other end thereof, and positional feedback control means controlling said driving means so that, in use, a predetermined angular relationship is maintained between said first and second arms.

2. An assembly according to claim 1 characterized in that said driving means comprises an electric motor and said first arm is mounted on a shaft driven by said motor. . An assembly according to claim 1 o ^"claim 2 characterized in that said second arm is mounted in a trunnion pi otally mounted on said first arm.

4. An assembly according to any one of claims 1 to 3 characterized in that said positional feedback control means includes a target and a position sensor, one of said target and position sensor being mounted for movement with the said second arm during its pivotal movement about said second axis and the other of said target and position sensor being either fixed relative to said first arm or located at a predetermined position related to the position of the first arm relative to said drive means.

5. An assembly according to any one of claims 1 to 4 characterized in that said predetermined angular relation- ship between said first and second arms is a predetermined angle.

6. An assembly according to any one of claims 1 to 4 characterized in that said positional feedback control means includes cam means profiled to cause a predetermined variation in the angle between the first and second arms during movement of the first arm.

7. An assembly according to any one of the preceding claims wherein said positional feedback control means

OMP includes a voltage controlled phased pulse generator selectively operable to drive a stepping motor. 8. A phonograph pickup arm assembly comprising a first arm driven by an electric motor for movement about a first axis, a pickup arm pivotally mounted in a trunnion which is pivoted to said first arm so that the pickup arm can freely pivot about axes at right angles to each other, the pickup arm being adapted to carry a record tracking stylus at one end and counterbalance means at the other end, target means mounted on said trunnion to move with movement of the picku arm about one of said axes, a position sensor mounted to sense movement of said target relative to said first arm, and control means responsive to said position sensor to actuate said electric motor and control operation thereof to maintain the target and position sensor in a predeter¬ mined relationship.

9. An assembly according to claim 8 characteized in that said position sensor is mounted on an arm pivoted to said first arm and which carries a cam follower, and a profiled cam is mounted in fixed relationship to said electric motor whereby movement of said first arm relative to said motor results in a predetermined variation in said predetermined relationship.

10. A phonograph pickup arm assembly having a pickup arm freely pivoted to a control arm, and control means maintaining a predetermined angular relationship between the pickup arm and the control arm during record tracking movement of said pickup arm.

11. An assembly according to claim 10 characterized in that a cam and follower is provided on said control arm to vary the angular relationship between the arms during record tracking movement of the pickup arm thereby reducing residual errors of tangency between a stylus carried by the pickup arm and record grooves. 12. A phonograph pickup arm assembly substantially as hereinbefore described with reference to Figures 2 to 5 of the accompanying drawings.

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