GB2257820A - Position control system for webs - Google Patents

Abstract

A web position control system, e.g. for controlling the position of a moving tape 16 past an optical or magnetic recording head 30, has a pair of outer bearings 32, 34 which constrain the web to follow a linear path which passes symmetrically through gaps between two pairs of air bearings 38, 38. Thus variations in tension do not cause lateral displacement. An air bearing may have a guide surface whose distance from the tape increases in the width direction of the tape, so that there is a wedge-shaped gap 28: Fig 2b with a pressure differential. This may urge the tape so that an edge 26 abuts a guide flange 24. This can provide edge guidance which is more delicate and less damaging than the known use of a resiliently urged opposing flange. <IMAGE>

Description

POSITION CONTROL SYSTEM FOR WEBS The present invention concerns position control systems for webs, particularly webs which serve as recording media, e.g. tapes, foils, films and discs.

These are examples of webs that have to be displaced at high speed and with high positional accuracy past specific locations, notably reading and/or recording heads. Generally it is necessary to ensure that there is limited or no movement other than in a single plane and in a single direction.

Recording data on tape has up until recently been done almost exclusively by magnetic methods. For recording, write heads produce magnetic flux changes along narrow tracks within the magnetic coating of tapes.

For reading, these flux changes induce very small currents in electrical coils within read heads. Because of the very small amounts of current being generated by the magnetised tape passing over the head coils, control of vertical movement of the tape relevant to the head is critical. If the tape is allowed to drift up or down, the track along which the flux changes occur is then not directly opposite the read head coil, so that a loss of signal is experienced.

A loss of signal also occurs if the tape is allowed to move away from the head due to the reducing flux field as the tape moves further away from the head coils.

To control the vertical (i.e. in-plane, transverse to the direction of travel) movement of the tape relative to the head, sophisticated tape guidance systems have been developed. These mainly rely on flanged rollers and bollards, but occasionally air bearings are used for the more advanced recorders.

Horizontal (i.e. out-of-plane) guidance is usually provided by allowing the tape to touch the head and rub across its surface while reading or writing. Some advanced recorders have especially shaped head profiles which produce a self generating air bearing between the tape and the head during tape movement. This bearing lifts the tape from the head by a very small amount with only very small signal losses.

A more advanced technique that is now being developed is optical recording. For this, a special optical tape has to be produced which changes its character when hit by powerful light. The light is normally produced by laser diodes and focused onto the tape using a lens system. For recording, one or more diodes is pulsed, and a pattern of extremely small round dots is made on the tape. These dots may have a diameter of only O.OOlmm, and be spaced at 0.0016 mm along a particular track, with a track spacing of 0.0016mm.

Because of the very small dimensions involved with this type of recording, special systems have had to be developed to control the lens system, and the tape. The diodes and lens system are generally known as the head.

Within the head there are two servo-ed movements: 1. Autofocus: this action servo-es the lens system relative to the tape's position in a horizontal plane thereby keeping the focal point of the laser always on the active surface of the tape. The depth of focus is nominally O.OOlmm.

2. Track on data servo: this system follows a previously written track to enable it to write the next at the correct track spacing.

To enable the system to be able to record or read at a sufficiently high speed, the tape has to pass the head at many metres/second, depending on the number of tracks written or read at any one time.

Because of the mass of the lens system, and the restricted power of the servo systems, a tape speed limit is reached, when the servo cannot accelerate the lens system fast enough to keep up with the combination of frequency and amplitude of the tape movement. The more accurately the tape is held in its correct plane while reading or writing, the faster the tape can be run and consequently the higher the throughput of data.

Many bearing systems exist for controlling the vertical tape movement, usually employing a pair of flanges between which there is a cylindrical surface which the tape wraps around and which guides the tape around an arcuate path. One of the flanges is a sprung flange which loads the tape against the opposite, fixed flange. Between the cylindrical bearing surface and the tape, high pressure air is usually fed to provide a very low friction air bearing. The sprung flange may cause undue or premature tape edge damage.

Thus it will be appreciated that optical tape systems require position control systems of very high accuracy and reliability. Of course such systems may also be applied to other forms of recording, including disc systems, and, indeed to other systems involving web displacement.

According to the invention in a first aspect there is provided a web position control system comprising: spaced first and second web guide means, said guide means defining a linear web path between themselves; first and second bearing assemblies located between said guide means at spaced locations along said path, each bearing assembly comprising an opposed pair of air bearings confronting one another across said path.

In a second aspect the invention provides a data recording and/or reading system employing a web-form data carrier and including a web position control system according to the first aspect.

In a third aspect the invention provides an air bearing for use in guiding a tape wherein the bearing has a flange extending in a plane for guiding an edge of said tape, and a guide surface extending away from the flange, so in use a tapering air gap is being defined between the guide surface and the tape; the arrangement being such that, in use, differential pressures within the gap tend to urge the tape to abut the flange.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: figure 1 shows a side elevation of an optical or magnetic recording head with a tape shown in vertical section; Figure 2a is a plan view showing the tape at an air bearing.

Figure 2b is a vertical section through the bearing on A-A; and Figure 3 shows a bearing system for controlling a tape's planar movement.

Figure 1 shows a recording head 10 mounted to a body 12 of a recording device. The head 20 has a front face 15. Means (not shown) within the head 10 are operable to record a track of data on a tape 16 as it is passed by the front face 14. (For convenience of description, it is assumed that the face 15 rises vertically from the body 12 but the designations 'horizontal' and 'vertical' are not of absolute significance.) The tape 16 is driven past the head, travelling into (or out of) the plane of the paper. In the plane of the paper, the section of the tape is desirably wholly immobile, since either horizontal (arrow 18) or vertical (arrow 20) displacement will seriously impair the recording process. Therefore the tape passes one or more bearings adjacent the head 10. A preferred form of bearing is shown in Fig. 2.

This is an air bearing 22. It has, at the bottom, a fixed flange 24 which projects outwardly of the bottom of an arcuate guide surface 25. There are means for providing an air cushion so that the tape 16 can be guided adjacent the surface 26, without substantial contact with it. The air cushion may be self generating and/or assisted by high pressure feed air.

The tape 16 moves across the head in either direction, causing a pressure build-up under the tape.

If the air bearing is fed with high pressure air this air pressure will be present under the tape at all times and without tape movement.

As shown in Fig. 2b, the guide surface is shaped so that a vertically extending tape 16 having a side edge 26 at or adjacent the flange 24 defines a wedge-shaped air space 28 between itself and the surface 25, widening towards the flange 24. Thus air flow within the space creates a pressure differential.

As the front face of the bearing slopes towards the flange a larger volume of air will be trapped beneath the tape nearest this fixed flange 24. As a result of this larger volume of air, a slight decrease in air pressure will be experienced at this point resulting in a relatively low air pressure, with respect to the higher air pressure where the air volume is least.

Owing to this difference in air pressure the tape will tend towards the fixed flange 24. Thus any loading spring pressure as applied by an upper flange may be reduced or deleted, thereby increasing tape life.

Fig. 3 shows a recording apparatus having a magnetic or optical recording head 30 and a tape transport system employing upstream and downstream rollers 32,34. Between the rollers the tape 16 follows a straight path. At each side of the head 30 there is a nip defined between a pair of self generating or pressurised air bearings 36.

As the tape 16 streams through the gap between each pair a pressure is developed each side of the tape. The bearings of each pair confront each other across the nip with arcuate surfaces 38. Thus whichever direction the tape moves, there is initial narrowing. Thus in plan, there are wedge-shaped air gaps, with higher pressure generated at the narrower portions.

The bearings 36 may be substantially as shown in Fig. 2 with flanges as described, and air cushions that are wedge-shaped in vertical section, to provide vertical control also.

If each of the four bearings is of equal shape, size and surface finish the pressure produced on each side of the tape will be equal. As the tape begins to move, a pressure is built up in all four bearings. The tape will, unless disturbed by other influences, take up a position whereby the air pressure on each side of it will be equal. This position is nominally the centre of the air gap.

Preferably, a line drawn between the rollers 32, 34 exactly bisects the gaps between both pairs of bearings.

Therefore any tension variation will not introduce a horizontal component and therefore will not urge horizontal movement.

With the above arrangement the tape will at all times during movement, and under all tape tension conditions, take up a position central to the bearing air gap. Fig. 3 shows an arrangement particularly appropriate to optical recording, in which the tape path is slightly spaced from the recording head 30 so as to be in the focal plane thereof. Of course the spacing also leads to very low wear of tape and head, which would also be advantageous for magnetic recording. This system could be force fed with air which would overcome problems with tape sticking to the bearings while stationary. It may be fed with air during operation, or only while stationary.

As the bearings are firmly held in position, the tape will take up a true position that will not vary.

This will allow a much wider band-width for the head servo-ing devices and therefore a much faster tape speed and data transfer rate.

The rollers 32,34 shown in figure 3 could also be of an air bearing design.

Claims (13)

1. A web position control system comprising: spaced first and second web guide means, said guide means defining a linear web path between themselves; first and second bearing assemblies located between said guide means at spaced locations along said path, each bearing assembly comprising an opposed pair of air bearings confronting one another across said path.

2. A position control system according to claim 1 wherein said first and second web guide means each comprise an air bearing.

3. A position control system according to claim 1 or 2 wherein an air bearing has a guide face constructed and arranged to define a tapering air gap between itself and a web extending through the system.

4. A position control system according to claim 3 wherein the air gap tapers in the direction of extension of the web along the path.

5. A position control system according to claim 3 or 4 wherein the air gap tapers in a direction which is perpendicular to the direction of extension of the web along the path and parallel to the plane of the web adjacent the bearing.

6. A process according to claim 5 for use with a web which is a tape, and wherein the bearing has a flange extending in a plane for guiding an edge of said tape, and a guide surface extending away from the flange, the tapering air gap being defined between the guide surface and the tape; the arrangement being such that, in use, differential pressures within the gap tend to urge the tape to abut the flange.

7. A web position control system substantially as herein described with reference to and as illustrated in the accompanying drawings.

8. A data recording and/or reading system employing a web-form data carrier and including a web position control system according to any preceding claim.

9. A data system according to claim 8 having a reading and/or recording head located between said first and second bearing assemblies.

10. An air bearing for use in guiding a tape wherein the bearing has a flange extending in a plane for guiding an edge. of said tape, and a guide surface extending away from the flange, so that in use a tapering air gap is defined between the guide surface and the tape; the arrangement being such that, in use, differential pressures within the gap tend to urge the tape to abut the flange.

11. An air bearing according to claim 10 having means for feeding pressurised gas into the air gap.

12. An air bearing according to claim 10 constructed and arranged so that the differential pressures are generated by relative displacement of the tape.

13. An air bearing substantially as herein described with reference to and as illustrated in the accompanying drawings.