Background to the invention This invention arises from development of a micro relay. One application of micro relays is in the controllable connection between a plurality of telephone subscriber lines and a plurality of lines to an exchange.

Such connections are typically executed using labor intensive, manually fitted wire pair jumpers typically located at at least two points between the exchange and the customer. The first jumpering point is the main distribution frame (MDF) typically located in the same building as the exchange. The second jumpering point is typically located in an outdoor cabinet or pillar near the customers. In some cases further jumpering may occur near the customer, eg. in sealed underground canisters located in small pits.

The jumpers are used to connect or disconnect customer services as required, while maintaining efficient use of the cable pairs between the customers and the exchange, i. e. , only using the valuable exchange pairs for active services.

In the case of the use of digital loop concentrators (DLC, an outdoor cabinet remote multiplexer) the MDF function is replaced by a cross connect field typically located within the DLC.

Relays are still used extensively in the telecommunication industry, for example, in telephone exchanges for line testing and application of ring voltage.

Usually, these devices have been discrete devices, not manufactured in large arrays, and assembled from discrete components.

In the past, the main means of altering remote jumper connections was by physically changing the copper connections at a cabinet/pillar to which several customer copper connections were connected. This required a service person to travel from a depot to the location of the cabinet/pillar, identify the connections to

be changed, and physically make the change before returning to the depot. This sequence of events is referred to colloquially as a truck-roll.

The advent of services such as ADSL creates an increased need for the ability to rapidly and efficiently change the customer connections.

In order to provide remotely controllable links between groups of lines, typically large matrices of relays are required. Conventional relays are not cost or space effective in this application.

Micro relays have been proposed in USA patent 5778513 and WO 01/80258 but these have not addressed the problems of stable switching.

Australian patent application 28005/01 addressed this problem by providing a miniaturized relay formed integrally in a substrate such as a printed circuit board.

In this arrangement, the magnetic circuit is formed by depositing upper and lower sections of the magnetic path on the upper and lower surfaces of the substrate, and then connecting the upper and lower sections by holes through the substrate which are plated or filled with magnetizable (ferro-magnetic) material. The upper path also includes a cantilevered, resilient armature which is spaced above upper surface of the substrate so as to be able to make electrical contact with a contact pad. Preferably the contact pad and the armature are of material which has good ferro-magnetic and electrical conduction characteristics so they form parts of both the magnetic actuation mechanism and the electrical contact path. Such a relay may be used in an array in which current is applied in column and row in the array and a single relay can be switched only by the combined current of a column and row. In the design of such a relay there is a trade-off between the thickness of the armature and the force needed to operate the armature.

USA patent 6040748 proposes a magnetic microswitch in which the median section of the cantilever has a smaller cross-section to have a lower bending resistance It is an object of this invention to improve reliability of magnetic relays or actuators particularly when used in arrays.

Brief description of the invention To this end the present invention provides a magnetic actuator in which a cantilever armature is displaced by a magnetic force wherein a permanent magnet acts on the armature and the magnetic field between the armature and the permanent magnet is modified so that the armature can assume two stable positions without the need for electric current one being adjacent to the magnet and the other being displaced therefrom.

This invention is predicated on the realization that of the two forces acting on the armature the magnetic force is non linear with regard to the displacement while the mechanical restoring force is linear. In arrays where the energizing currents are directed along rows and columns the adjacent actuators or relays must be stable even though they are subject to half the energizing current of the unit to be actuated. This means that the forces generated by the additional half current must be sufficient to overcome the resistance force which is either the mechanical restoring force or the permanent magnet force field.

Studies by the inventors have suggested occurrence of significant amount of leakage flow through the surrounding free space. This has been attributed mainly to the poor permeability of electroformed Nickel of which the relay structure is usually made. While simulation studies have established that proper operation of the relay is possible, the conclusions are that even small variations in the parameter values (due to manufacturing tolerances, temperature changes, etc.) would severely compromise proper operation of the relay, and the use of a reloaded cantilever would improve the robustness. Experimental studies by the inventors have demonstrated the difficulty in achieving reliable latching/unlatching operation of individual relays in the array under uniform levels of electrical excitation.

It is to be noted that a significant number of magnetic and mechanical parameters affect the operation of the relay. It has not been easy, to isolate and understand individual parametric influences on the operation of the relay.

These difficulties have been overcome by modifying the magnetic field. The invention uses a bypass block or blocks between the top pole pieces for reducing the magnetic force when the armature displacement towards the pole pieces is

small, and for increasing the magnetic force when the armature displacement is large.

In a second aspect the present invention provides a magnetic actuator including a) a first pole b) a second pole of opposite polarity c) an armature mounted on one pole for deflection toward the other of said poles d) means to generate a magnetic flux between the first and second poles e) a magnetic flux by pass located between the said poles to provide an alternative path for said flux and alter the magnetic force acting on the armature.

The by pass material is ferromagnetic and acts to increase the force on the armature when it is in close proximity to the pole. Optionally the bypass material also at least partially saturates when the armature is displaced from the pole. The armature is moved from the position displaced from said first pole by electromagnetic force produced by an electric current in one direction and is displaced from a position in close proximity to said first pole by reversing the current.

The actuator of this invention is suitable for use in minituarised relays and for relay arrays in telecommunication systems. The relays may be used in all jumpering points between the telephone exchange and the customer.

Detailed description of the invention Figure 1 is a view of a prior art micro relay design; Figure 2 illustrates the operating principle of micro relay of figure 1; Figure 3 illustrates an array of actuators or relays energized by a combination of column and row current; Figure 4 illustrates the force considerations in designing actuators or relays according to this invention; Figure 5,5A and 5B illustrate possible geometries for the actuator; Figure 6 illustrates the arrangement of this invention using one by pass block between the poles ;

Figures 7, illustrates an embodiment of the invention utilizing two by pass blocks; Figures 8 & 8A illustrate an embodiment of the invention utilizing one bypass block and full or partial slots in the armature ; Figure 9 shows simulated magnetic forces for the pole geometry of figure 8A; Figure 10 &10A illustrate an embodiment of the invention utilizing two bypass blocks and two full or partial slots in the armature.

Figure 1 shows an original PSA switch cell design (101) which is the subject Of patent application 28005/01. With reference to Figure 1, this micro switch includes a permanent magnet sheet 113, a column-addressing coil 111, a row- addressing coil 112, an armature 102, a beam 103, top pole pieces 104 and 105, bottom pieces 109 and 110, and two columns 107 and 108. The switch size is approximately 3000umX2000umX500um.

To form a magnetic circuit, ferromagnetic material is to be used to fabricate components 102,103, 104,105, 107, 108,109, and 110. The maximum air-gap length between the armature (102) and the top pole pieces (104 and 105) is in an order of 200 to 300 um, while the gap between the top pole pieces (104 and 105) is around 400um, to ensure good electrical isolation required in telecommunication applications.

In this micro switch, two forces are acting on the armature: magnetic force and mechanical restoring force. Both of these two forces are a function of the armature displacement towards the top pole pieces. However, the mechanical restoring force is a linear function of the armature displacement, while the magnetic force is a nonlinear function. As a result, it is possible to make the mechanical restoring force larger (smaller) than the magnetic force generated by the permanent magnet when the armature displacement is small (large).

Consequently, the armature can stay in"close"and"open"states without requiring power supply, but depending on the magnetic force generated by permanent magnet (113) or mechanical restoring force.

To switch the armature from"open"to"close"state, the column and row addressing coils (111 and 112) are energised with proper polarity so that the magnetic force generated by permanent magnet and electro-magnet is large

enough to overcome the mechanical restoring force. As a result, the armature will be switched to"close"state and will stay there even the addressing currents are later reduced to zero. Similarly, the armature can be switched from"close"to "open"state by properly energising the addressing currents to reduce the resultant magnetic force, and it will stay at"open"state due to larger mechanical force than magnetic force.

The operating principle of the relay of figure 1 is shown in Figure 2 in terms of forces as a function of armature displacement, where FPM is the magnetic force generated by Permanent magnet only, FPM+EM is the force generated by Permanent as well as electro-magnet with same polarities, FPM-EM IS the force generated by Permanent magnet and electro-magnet having different polarities, and Fmec is the mechanical restoring force.

It should be noted that bi-stable latching is achieved in this micro switch, but, to improve reliability, it is preferred that, for given mechanical force line Oi in Figure 2, the magnetic force generated by permanent magnet (FPM in Figure 2) be as small as possible at point H, and as large as possible at E. This will increase the shaded area Ashade and the contact force Fcont, and, as a result, the operational reliability will be improved. This preferred magnetic force is achieved by the method illustrated in the further embodiments of the invention illustrated in figures 3 to 6.

The principal method is to use a bypass block or blocks between the top pole pieces for reducing the magnetic force when the armature displacement towards the pole pieces is small, and for increasing the magnetic force when the armature displacement is large.

Figure 3 illustrates an array of switches which are energized by a combination of column and row currents.

When a current of sufficient amplitude is passed through the appropriate wires (addressed by a row and a column), the induced magnetic field (PM+EM) in the relay (and specifically in the air gap between the cantilever beam and the left column) will create a force on the cantilever section, pulling the relay closed. This will stay closed due to the field from the permanent magnet (PM), which is not

high enough to pull the relay closed, but will hold it there when the air gap becomes small and the flux density becomes higher.

To reset the relay, a current pulse in the opposite direction is applied, which will reduce the resultant flux (due to PM-EM) in the air gap so that the beam can spring back to the open position.

While addressing a particular relay by energising a row and a column (100% energisation), the remaining relays in the row or column would also be exposed to the row or column current and therefore would be partially excited (50% energisation only, PM 0.5EM). However, this partial excitation should not cause any change in the state of these remaining relays.

Figure 4 illustrates the various Force-airgap characteristics showing the correct latching/unlatching operations of the relay which can be used to develop design rules for the actuator/relays.

The following preferred design rules apply (refer to Fig 4): Design Rule 1 The entire magnetic force characteristics for (PM+EM) should be below the mechanical force line.

Design Rule 2 The switch should not close for the case (PM+0. 5EM). In this instance the mechanical force line must be above the magnetic force line in some range of values of g.

Design Rule 3 Minimum contact force requirement should be satisfied for the case (PM-0. 5 E. That is, the magnitude at zero airgap of the spring force should be less than the magnitude at zero airgap of the magnetic force for the case (PM-0. 5EM) by the minimum contact force required, as shown in Fig 6.

Design Rule 4 For the switch to open for (PM-EM), the magnitude at zero airgap of the spring force should be more than the magnitude at zero airgap of the magnetic force for the case (PM-EM), as shown in Fig 6.

The relay may be fabricated from nickel, with the exception of the contacts at the end of the beam and top of the left column, which may be thin gold or silver plating.

According to this invention magnetic flux in the main flow from the region near the end of the left pole piece to the armature and then from the armature to the region near the end of the right pole piece. It is this flux in the two airgaps that produce the force of attraction between the armature and the two pole pieces.

The force-stroke characteristics of this magnetic actuation device depends on how the reluctance of the two airgaps and hence the flux in the airgap varies as the relative position of the armature, with respect to the pole pieces, changes during the stroke.

The invention proposes an additional leakage path for the flux flow from the left pole piece to the right pole piece. In this way the force stroke characteristic of the magnetic relay can be tailored in a desirable manner that would greatly improve the robustness of operation of the micro relay. This leakage path may be of such cross section or material type that it acts as a good conduit for flux to flow at moderate values of magnetomotive force, but saturates at higher values of magnetomotive force.

Figures 5,5A and 5B illustrate some possible geometries of the pole pieces and the cantilevered armature. A nonlinear magnetic bypass 120 lies between the left pole piece107 and right pole piece 108. The nonlinear magnetic bypass may be made of the same material (for example by electroforming) as the pole pieces (Nickel or permalloy).

Figure 6 shows a first embodiment of the invention 201 having a bypass block (207) between top pole pieces (204,205). With reference to Figure 3, when the armature (102) displacement is small, there is a large air-gap between the armature and top pole pieces (204,205), compared with the gaps (206,208) between the top pole pieces and the bypass block (207). As a result, the magnetic flux in 204 is mainly passing through the gap 206, bypass block 207, gap 208, and top pole piece 205. Consequently, the magnetic force will be reduced than in the case where no bypass block is used.

When the armature (102) displacement is large, the air-gap length between the armature (102) and the top pole pieces (204,205) is quite smaller than the gap lengths between the top pole pieces (204,205) and the bypass block (207). Now the magnetic flux in pole piece 204 is mainly passing through the armature 102.

Due to the presence of the bypass block, the magnetic flux in the armature will be bypassed to the bypass block, which is useful for increasing the magnetic force acting on the armature.

Figure 7 shows an alternative embodiment of the bypass idea using two bypass blocks.

Figures 8 and 8A show another two alternative embodiments using a bypass blocks and a partial or full slot on the armature for better performance.

Figure9 shows the simulated magnetic forces for the pole geometry shown in Figure 8A.

The thickness of top pole pieces is fixed at 50um, the overlap between the pole pieces and armature is fixed at 50um, and the gap between the top pole pieces and the bypass block is fixed at 200+200um, as have been shown in Figure 9.

The thickness of the bypass block has the same thickness as the top pole pieces.

Hence, by using a modified mask, the bypass block could be fabricated together with the top pole pieces.

The slot on the armature in figure 8A separates the armature into two parts. As the beam thickness used in the simulations is 5um, and the armature thickness 20um, the slot thickness is fixed at 15um. The idea behind this selection is that the 5um thick beam can be first plated, then the two pieces of the armature be plated upon the 5um thick beam. It will be noted that various slot length Oslot has been simulated, from 0 to 500um.

For comparison, the magnetic force curve without bypass block and slot is also included in Figure 9.

It is noted from Figure 9 that there is an optimal value for the slot length. Too short (curve 3) or too long (curve 2) a slot length will reduce the performance robustness, compared to the case where there is no slot in the armature (curve 4).

Using a bypass block and a slot will greatly increase the magnetic force when the air-gap between the armature and the pole pieces is small. In this case the reluctances between the armature and the first pole is much smaller than that between the first pole and the bypass block, and the magnetic flux passing through the first pole will mainly pass from the armature to the pole. As the bypass block provides additional air-gaps and paths for the magnetic fluxes passing through the tip of the armature, and the slot acts as a stopper to the magnetic fluxes, a larger magnetic force can be expected.

On the other hand, when the air-gap is large, the reluctance between the armature and the first pole is larger than between the first pole and the bypass block, and the flux from the first pole will mainly pass between the first pole and the bypass block. As the bypass block will bypass the flux from the first pole, the magnetic force acting on the armature will be reduced compared with case where no bypass block is used. Of course, if the bypass effect is too strong, very large ampere-turns will be required in the addressing coils to switch the armature from open to close.

The robustness of the micro actuator or relay of this invention may also be improved by changing the shape of the armature so that it is curved away from the first pole. This increases contact area between the first pole and the armature.

Another suggestion is to preload the armature so that the force acting on the armature is non zero at minimum armature displacement. This may be achieved using upwardly curved armatures and a flat insulating plate placed at a fixed height above the PCB so that it partly depresses the armature.

Those skilled in the art will appreciate that the present invention can be implemented in a range of embodiments apart from those illustrated and also in a range of applications apart from micro relays and arrays of micro relays.