ROTARY TRANSFORMER

FIELD OF THE INVENTION

The present invention relates to a rotary transformer, in particular a rotary transformer suitable for coupling data signals to a spinning display. The invention also relates to display systems incorporating such a transformer.

BACKGROUND OF THE INVENTION

Display systems using a spinning array of LEDs or other light sources are used for display systems, and have the advantages of high efficiency and a much reduced number of active elements compared to an equivalent two dimensional matrix display.

To operate, either drives for the individual elements, or power and data need to be transferred to the rotating part. This can be accomplished by use of slip rings, or non contact means. Slip rings are typically unreliable or expensive, so non contact means are preferred.

Options include magnetic, capacitive or optical. Magnetic is the natural choice for power transfer, using a conventional split transformer of appropriate size. For the data transfer, data rates from 10Ombit/sec to 1 Gbit/sec are needed. This can be done by a free space optical link, requiring a laser at the higher data rates, giving cost and lifetime issues.

Various types of broad band transformers suitable for data transfer are known in the art. Ruthroff Proc IRE, Aug 1959: "Some broad band transformers", describes transmission line transformers with various wire wound toroidal arrangements. C W Allen and H L Krauss: "A wideband rotating coupler for VHF use", IEEE transactions on microwave theory and techniques, May 1976 describes an extension to this arrangement for use as a rotary coupling having concentric wire windings to allow wide band signals in the VHF range to transfer from a stator to a rotor. US 5,587,859 develops such rotary transformer for use in a recording apparatus and has an arrangement of wire windings within grooves to form a transformer.

SUMMARY OF THE INVENTION

We have appreciated that improvements may be made to rotary transformers, in particular for applications requiring high data rates such as rotary displays. We have further appreciated an improvement, as described here, is to use a rotary transmission line transformer in the form of a planar transmission line rotary data transformer for a spinning high definition display system

The invention is defined in the claims to which reference is now directed.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a cross section through the transformer; Figure 2 shows a plan view of one half of the transformer;

Figure 3 shows the transformer in a simple unidirectional link configuration;

Figure 4 shows the transformer coupled to single ended cables; and

Figure 5 shows a display system of the type that may embody the invention.

DETAILED DESCRIPTION

Overview

Figure 3 shows the transformer in simple link, transferring run length limited digital data (12) from the stationary side to the rotating side (13). The data is connected via balanced transmission lines (10), (11) of impedance typically 100 Ohm, terminated at both ends with appropriate resistors (7), (8).

The signal levels, transmitter (6) and receiver (9) can be standard PECL devices. The run length limiting and data framing can be done by a pair of serialiser/deserialiser parts using e.g. 8b/10b standard.

The rotating transformer (5), detailed in Fig 1 and Fig 2 is connected to the balanced lines. The two halves of the transformer (22) and (23) are separated by a gap (21 ) and rotate relative to each other about axis (20).

Each half of the transformer consists of a coil (3) formed on circuit board (1), set in a ferrite core (2). The front of the coil (1 ) is flush with the surface of ferrite (2). The dimensions of the transformer halves are typically 10 to 20 mm diameter and 3 to 6 mm thick. The operating air gap is typically between 0.25 and 3 mm.

It is important for good broad band operation than the turns of the coils facing each other go in opposite directions - which will be obtained if the two halves made as shown are identical. This ensures that the outer and inner coil connections share the same polarity on either side of the transformer, ensuring that the capacitance between the coils acts to support the magnetic signal transfer rather than against it. If this is not done, there will be a notch and subsequent polarity reversal as the frequency is increased, limiting the useable bandwidth of the system. This issue is equivalent to the techniques described in Ruthroff Proc IRE, Aug 1959: "Some broad band transformers". Use of balanced differential drive on both sides of the transformer is also important to obtaining the widest bandwidth. This is not done in C W Allen and H L Krauss: "A wideband rotating coupler for VHF use", IEEE transactions on microwave theory and techniques, May 1976, resulting in significant loss in bandwidth. In addition the planar approach described here is easier to construct, and has wider alignment tolerances on the two halves of the rotating transformer. If single sided (e.g. coax) connections are needed to the transformer, baluns (15), (16) should be placed on either side of the rotating transformer (Fig 4).

Description of preferred features in the figures

Some of the preferred features in the figures will now be described. It is first noted that each coil is formed by a circuit board track having a greater width than depth. The capacitance between opposing coils is a function of the width of the circuit tracks and the separation of the tracks from one another. Accordingly, by using a circuit board track of greater width than typical wire arrangements, a greater capacitance between opposing coils can be achieved for a given separation gap (21 ). This enables a more appropriate choice of capacitance between opposing coils to be made so as to achieve the desired impedance of the transformer.

As can be seen in Figures 1 and 2, the front of each coil (1 ) is flush with the surface of the ferrite core (2). This means that the separation of the coils is

defined by the gap 21. For a given capacitance between coils, a greater separation between the two halves of the transformers (22, 23) is thereby achieved.

As previously noted, the coil connections are such that the opposing coils have turns in opposite directions. The capacitance between the two halves of the transformer is thereby such that a similar equivalent circuit is capacitance between the upper connection of the transmission line 10 and the upper connection of the transmission line 11 , and also a capacitance between the lower connection of the transmisson line 10 and the lower connection of the transmission line 11.

The arrangement of capacitance between the coils is such that the transformer has behaviour similar to a transmission line. The impedance of a transmission line is given by the know formula

where Z is the impedance, L the equivalent inductance and C the equivalent capcitance.

The use of planar printed circuit tracks for the coils to increase the surface area of the coils and the flush mounting of the coils with respect to the ferrite allows the capacitance C to be increased for a given gap (21). This allows a greater choice of values so as to match the impedance of the transformer to the impedance of the transmission lines (10, 11), such as the 100 Ohm value noted above.

As explained above, the printed circuit tracks are such that the turns of the coils facing each other go in opposite directions because the two halves made as shown are identical. At the frequencies of operation discussed herein, the effect of this arrangement, due to phase shifts, is that coils have the same polarity as noted above.

Preferred Values

Some preferred values for the embodiment are now described. Each coils is arranged to have 4 turns, comprising a 0.3 mm track having a -.3mm gap between the tracks. The pot cores are 15mm diameter and 4 mm thick. The transformer has a 100 ohm differential source and load.