GB2069222A

GB2069222A - Linear tracking arm assembly

Info

Publication number: GB2069222A
Application number: GB8107934A
Authority: GB
Original assignee: Pioneer Electronic Corp
Current assignee: Pioneer Corp
Priority date: 1978-04-28
Filing date: 1981-03-13
Publication date: 1981-08-19
Also published as: DE2917154C2; GB2020082A; GB2020082B; JPS54143806A; GB2069222B; DE2917154A1

Abstract

A linear tracking arm assembly for causing a pick-up arm (22) to be tracked radially over an audio or video disc includes a guide rail (10) carrying through the medium of a roller (20), a conveyor (14) which carries the pick-up arm (22). A linear electromagnetic drive (30, 32, 34) controls movement of the conveyor linearly along the guide rail (10) in accordance with a servo feedback system arranged to keep the arm (22) tangential to the information track on the disc. The damping factor in the servo loop can be varied by choice of a viscous fluid for damping movement of the roller in relation to the conveyor. <IMAGE>

Description

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GB 2 069 222 A

1

SPECIFICATION

Linear tracking arm assembly

" 5 The present invention relates to a tracking arm assembly for example for use with an audio disc player, a video disc player, or the like, for controlling a pick-up arm to move linearly to trace a track formed on a recording medium such as an audio or video disc placed on a turntable or the like. The linear movement will generally be radially of the disc.

A conventional linear tracking arm assembly usually has a rotary electric motor with reduction gearing for 10 providing the linear movement. A worm gear has been used.

However, a rotary motor produces unwanted shocks and vibrations which give a poor signal/noise ratio in reproduction. Furthermore, a large capacity electric source is required for energising the rotary motor and produces ripple components in the associated circuits. In addition, a servo mechanism must be provided to maintain constant rotational speed of the rotary motor. Such servo mechanism causes the overall assembly 15 to be large, and complex and expensive.

Accordingly, it is a primary object of the present invention to provide a linear tracking arm assembly which is simple in construction and yet gives good reproduction.

According to the invention a lineartracking assembly for pick-up for tracking a recording medium carrying audio, video or other information comprises support and guide means, a conveyor carrying a pick-up arm 20 and supported by and movable along the support and guide means, a linear electro-magnetic drive for the conveyor along the support and guide means, and damping means arranged to provide viscous resistance to movement of the conveyor along the support and guide means.

The invention may be carried into practice in various ways, and certain embodiments will now be described by way of example with reference to the accompanying drawings, in which;

25 Figure 1 is a fragmentary perspective view of a lineartracking arm assembly;

Figure 2 is a fragmentary perspective view of another lineartracking arm assembly;

Figure 3 is a sectional view of a detail of the assembly shown in Figure 2; and

Figure 4 is a block diagram showing a feed-back control circuit for a lineartracking arm assembly.

The linear tracking arm assemblies described comprise a conveying mechanism driven by an electric 30 linear motor assembly for linearly moving a pick-up arm for playing a recording.

Referring to Figure 1, there is shown a preferred embodiment of the present invention which comprises a guide bar or rail 10 which is supported by a support block 12 so that the guide rail 10 extends along or is coextensive with a disc or other recording medium drive means such as a turntable (not shown).

A support body 14 is carried on the guide rail 10 so that the support body 14 can smoothly move on the 35 guide rail 10. The support body 14 is, in this case, constituted by a pair of side portions 14a and 14b respectively having bores 16a and 166 loosely coupled with the rail 10 and a central portion 14c connecting the side portions 14a and 146. An L-shaped rod 18 stands from the upper surface of the central portion 14c. On a top end of the L-shaped rod 18 is rotatably mounted a roller 20 which is engaged on the guide rail 10 so that the support body 14 is movable along the guide rail 10. A pick-up arm 22 is mounted on a mechanical 40 arrangement such as a rod 24 provided at the lower portion of the support body 14 in such a mannerthatthe pick-arm is swingable or rotatable in at least both planes respectively perpendicular to, and containing, the moving direction of the support body 14. Atone end of the pick-up arm 22 is mounted a cartridge shell 26 for carrying thereon a transducer which is to face the recording medium (not shown) on the recording drive means (not shown), for picking up information recorded on the recording medium. At the other end of the 45 pick-up arm 22 is mounted a weight 28 which is preferably adjusted so as to establish a desired gravity balance in the pick-up arm 22. A permanent magnet 30 is mounted on the upper ends of the support body 14. The magnet 30 has a flat upper surface of a magnet pole of N or S, the opposite lower surface thereof being the opposite magnetic pole S or N. An elongated ferro-magnetic member 32 is supported by the support block 12 so that the elongated magnetic member 32 extends parallel with the guide rail 10 while providing a 50 gap of preselected size between it and the magnet 30. On the elongated magnetic member 32 is wound a coil 34 which has a terminal 34a and anotherterminal (not shown).

The elongated magnetic member 32 and the coil 34 constitute a linear armature and the magnet 30 forms a field magnet which co-operates with the linear armature to form a linear motor. By application of a voltage across the terminals of the coil 34 to cause a current to flow through the coil 34, the magnet 30 is urged in a 5*5 direction so that the support body 14 moves along the guide rail 10 whereby the support body 14functions as a conveyor for the pick-up arm 22.

When, for example, a current flows from the terminal 34a to the other terminals and the supper surface of " the magnet 30 has S type of magnetic pole, the magnet 30 is urged toward the right-hand direction in this figure. When the direction of the current through the coil 34 is reversed, the magnet 30 is urged in the 60 left-hand direction. The magnitude of the force urging the magnet 30 is proportional to the magnitude of the current through the coil 34, so that the speed and direction of the movement of the support body 14 can be controlled by regulating the magnitude and direction of the current through the coil 34. Accordingly, the support body 14 may be moved by an increased current through the coil 34 when a return or lead-in motion of the pick-up arm 22 is desired and be slowly moved by a decreased current when the play motion of the 65 pick-up arm 22 is required.

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GB 2 069 222 A

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In order to desirably move the conveyer means for conveying the pick-up arm 22, a displacement detector (not shown) is fitted on the support body 14. The displacement detector produces a voltage representative of an angular displacement of the pick-up arm 22 in the plane parallel with the guide rail 10 and turn-table from a home or reference direction. A control circuit (not shown) is provided for supplying a voltage across the 5 terminals of the coil 34 so as to make a desired current flow through the coil 34 until the detected 5 1

displacement of the pick-up arm 22 becomes zero or close to it. The control circuit thus completes a closed loop feedback system which tends to keep the arm 22 parallel with the reference direction, which will in general be tangential to the track on the disc being followed. -

It is now to be understood that an appreciable intensity of attractive force is produced between the magnet 10 30 and the coil 34. The attractive force is effective for compensating the total weight of the support body 14 10 and the pick-up arm 22, thereby to reduce frictional force generated between guide rail 10 and the roller 20. The attractive force therefore reduces the electric power necessary in the coil 34 for movement of the support body 14. The intensity of the attractive force may be desirably regulated by changing the magnitude of the magnet 30 and the length of the gap between the magnet 30 and the magnetic member or rod 32. 15 The lineartracking assembly of Figure 1 is simple in construction and easy to control. There is no rotary 15 motor which produces noise in reproduction and there is no complicated servo mechanism and large capacity drive circuit for such a motor. The lineartracking assembly includes air pressure means for floating the pick-up arm.

It is further to be understood that because of the compensation of the pick-up arm weight by the attractive 20 force of the magnet system, the pick-up arm can be heavy and so rigid enough to reduce vibrations. 20

In order to make effective the co-operation between the magnet 30 and the coil 34, the support block 12 and support body 14 are preferably of ferro-magnetic material and the gap between the lower end of the support body 14and the support block 12 is so small as possible in completing the magnetic circuit comprising the magnet 30, the magnetic member 32, the support block 12 and the support body 14. Other 25 forms of magnetic circuit are possible. 25

In Figure 2, there is shown another embodiment of the present invention which issimilartothat of Figure 1 except that the support body 14 comprises a rectangular block provided with two pairs of roller assemblies 40a, 406, and 40c, 40c/which are respectively rotatably mounted on the side surfaces of the support body 14.

A pair of spaced, parallel, co-extensive guide rails 10a and 106 are supported on the support block 12 and 30 carry the roller assemblies 40a, 406 and 40c, 40d, 30

The pick-up arm 22 is hung from the support body 14 by means of an arm holder 24 so that the pick up arm 22 can swing in at least the plane perpendicular to direction of movement of the support block 14, and horizontal plane.

One of the roller assemblies 40a, 406,40c, and 40tf could be omitted if desired.

35 An example of a roller assembly 40 suitable for the present invention is shown in Figure 3. The roller 35

assembly 40 comprises an axle 42 screwed atone end into the support body 14, a roller 44 rotatable on the axle 42 and a stop member 46. A oil sump 48 is preferably provided on the periphery of the axle 42 so that a viscous fluid 50 such as silicone oil can be kept between the roller 44 and the axle 42 to provide a desired degree of viscous resistance between the axle 42 and roller 44. The viscous resistance provided between the 40 roller 44 and axle 42 can provide adequate damping of movement of the support body 14, and radial 40

overshoot of support body movement can be avoided. The magnitude of the viscous resistance can be readily changed by selecting a viscous fluid with appropriate characteristics.

It may be sufficient to provide for such viscous resistance of one or a selected one or more of the roller assemblies, while the remaining roller assemblies are free from such viscous resistance.

45 Furthermore, such viscous resistance may be provided with the roller assembly in the embodiment of 45

Figure 1.

Figure 4 shows how the viscous damping of the roller assemblies improves the response of the overall feedback control system forthe linear motor assembly for positioning the pick-up arm 22.

The displacement detector with a coefficient KV produces a voltage representative of the angular 50 displacement/i of the pick-up arm 22 in the plane of the turntable. The voltage from the displacement 50

detector is converted into a current by a control circuit with a coefficient gm that current is supplied to the coil 34 in the linear motor assembly constituted by the coil 34 and the magnet 30 which converts the current into a drive force for moving the support body 14. The conversion coefficient of the linear motor assembly is represented by Kf. The drive force multiplied by 1/s (sM+D) corresponds to a displacement (o of the body 14 55 caused by the drive force, where s represents the Laplace coefficient, M represents the total mass to be 55

moved and D represents the viscous damping coefficient in the conveyor means and the linear motor assembly. The damping coefficient D is mainly governed by the viscous resistance provided in the roller assemblies connected to the support body 14 as explained in conjunction with Figure 3.

The loop gain o/ i of the above-mentioned feed-back control system will be expressed by: 60 60

Kv.gm.Kf t o, M

(i 2 D Kv.gm.Kf

.(1)

65 s +MS + M 65

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GB 2 069 222 A

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If now;

/Kv.gm.Kf n con = / ,T=V2U/

M

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M.Kv.gm.Kf

Then, the above equation (1) can be rewritten as follows;

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€o ~\

co n s2+2£cons+con

•(2)

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As is well known in the art, such a feed-back control system as expressed by the equation (2) is of the second-order type and £ represents a damping ratio. The loop gain expressed byfo/fi takes various values

15 in accordance with variation of the damping ratio £ Usually, the value of the damping ratio is close to a 15

critical damping level, and is in a range from 1 t\[2 to I.The above-mentioned coefficients or factors M, Kv,

gm, Kf are settled in the overall design of the displacement detector, the control circuit, the linear motor assembly, the conveyor means, and the pick-up arm 22.

Therefore the coefficient and factors M,K,£, and Kf cannot be readily changed without a significant change

20 in the mechanical design. However, the viscous damping coefficient D can be relatively easily changed by 20 selecting a preferred kind of viscous fluid or by adjusting merely the dimensions of the roller assembly, and so the characteristics of the complete lineartracking assembly can be fairly easily set.

Claims

CLAIMS 25 25

1. A linear tracking assembly for a pick-up for tracking a recording medium carrying audio, video or other information, which comprises support and guide means, a conveyor carrying a pick-up arm and supported by and movable along the support and guide means, a linear electro-magnetic drive for the conveyor along the support and guide means, and damping means arranged to provide viscous resistance to movement of

30 the conveyor along the support and guide means. 30

2. An assembly as claimed in Claim 1 in which said damping means comprises a viscous fluid associated with the or each roller to damp rotational movement of the roller in relation to said conveyor.

3. An assembly as claimed in Claim 2, in which the viscous fluid is silicone oil.

4. A lineartracking assembly for a pick-up for tracking a recording medium carrying audio, video or other

35 information including damping means constructed and arranged substantially as herein specifically 35

described with reference to Figure 3 of the accompanying drawings.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.