Bi-stable actuator based on electromagnetic attraction

1. An electromagnetic actuator comprising:

a moving armature comprising a pin movable in a guided manner along a longitudinal axis thereof, and a permanent magnet arrangement arranged coaxially with the pin and fixed thereto, and

an excitation coil arrangement arranged coaxially with the pin, said coil arrangement being suppliable with current to cause the moving armature to move,

characterized by further comprising a surface of ferromagnetic material, said permanent magnet arrangement being axially interposed between the surface of ferromagnetic material and the excitation coil arrangement,

wherein said permanent magnet arrangement comprises a plurality of permanent magnet regions arranged around the pin of the moving armature, in which each permanent magnet region has a polarity in the direction of the longitudinal axis of the pin which is opposite to the polarities of the permanent magnet regions angularly adjacent thereto, and

wherein said excitation coil arrangement comprises at least one coil having a lobed shape, and comprising a number of coil lobes equal to half the number of permanent magnet regions of the magnet arrangement.

2. The actuator according to claim 1 , wherein said excitation coil arrangement is planar.

3. The actuator according to claim 1 , wherein each of the coil lobes has a pair of radially outwardly extending side lobe parts which are arranged axially aligned with boundary areas between permanent magnet areas having opposite polarities.

4. The actuator according to claim 1 , further comprising elastic means configured to prevent the moving armature from moving away from the excitation coil arrangement by action of gravity.

5. The actuator according to claim 4 , wherein said elastic means comprise an elastic membrane fixed to the moving armature.

6. The actuator according to claim 1 , wherein the moving armature is capable of assuming a stable off position adjacent to the excitation coil arrangement, and a stable on position adjacent to the surface of ferromagnetic material, and wherein a current pulse supplied to the excitation coil arrangement is capable of moving the moving armature from the off position to the on position, the moving armature staying in the on position by action of magnetic attraction between the surface of ferromagnetic material and the permanent magnet arrangement.

7. The actuator according to claim 6 , further comprising a sliding guide within which the pin of the moving armature is partially arranged, and a reference surface at which the sliding guide debouches, wherein when the moving armature is in on position a free end of the pin protrudes with respect to the reference surface by a length such that a pressure exerted on the free end of the pin and capable of taking the free end of the pin to the level of the reference surface is not capable of bringing the moving armature back to the off position.

8. The actuator according to claim 6 , further comprising mechanical restoring means axially interposed between the surface of magnetic material and the permanent magnet arrangement, said restoring means being controllable to bring the moving armature back to the off position.

9. A tactile display comprising a plurality of actuators according to claim 1 .

10. The display according to claim 9 , comprising

a display plate through which a plurality of sliding guides are formed, within each of which the pin of the moving armature of a respective actuator is partially arranged, and having a reference surface at which the sliding guides debouch, and

a plurality of support plates arranged parallel to the display plate, each support plate supporting a respective array of actuators, wherein the actuators supported by one support plate are arranged laterally offset with respect to the actuators supported by the other support plates.