The present invention relates to shape memory actuators, i.e. actuators in which the actuating member consists of an element (for example a wire element) made from a shape memory alloy (indicated in the following as "SMA"), and in particular to an actuator used for adjusting the position of a lens in an auto-focus device. Although specific reference is made in the following to the use of a wire as actuating member, it should be noted that what is being said also applies to other similar shapes with a dimension much greater than the other two dimensions which are generally very small, e.g. strips and the like.

It is known that the shape memory phenomenon consists in the fact that a mechanical piece made of an alloy that exhibits said phenomenon is capable of transitioning, upon a temperature change, between two shapes that are preset at the time of manufacturing, in a very short time and without intermediate equilibrium positions. A first mode in which the phenomenon may occur is called "one-way" in that the mechanical piece can change shape in a single direction upon the temperature change, e.g. passing from shape A to shape B, whereas the reverse transition from shape B to shape A requires the application of a mechanical force.

On the contrary, in the so-called "two-way" mode both transitions can be caused by temperature changes, this being the case of the application of the present invention. This occurs thanks to the transformation of the micro-crystalline structure of the piece that passes from a type called martensitic (M), stable at lower temperatures, to a type called austenitic (A), stable at higher temperatures, and vice versa (M/A and A/M transition).

A SMA wire has to be trained so that it can exhibit its features of shape memory element, and the training process of a SMA wire usually allows to induce in a highly repeatable manner a martensite/austenite (M/A) phase transition when the wire is heated and to induce an austenite/martensite (A/M) phase transition when the wire is cooled. In the M/A transition the wire undergoes a shortening by 3-5% which is recovered when the wire cools down and through the A/M transition returns to its original length.

In particular, this characteristic of SMA wires to contract upon heating and then to re-extend upon cooling can be advantageously exploited to obtain an actuator that is very simple, compact, reliable, silent and inexpensive with respect to the piezoelectric actuators and motors that are conventionally employed in auto-focus devices. Also the configurations of the present invention provides further advantages with respect to other SMA based autofocus devices, such as the one described in the international patent application WO 2011/122438 as will be illustrated later on.

Therefore the object of the present invention is to provide an auto-focus device with a shape memory actuator which achieves the above-mentioned advantages. This object is achieved by means of an auto-focus device comprising a lens carrier slidably received for a reciprocating motion within a stationary housing frame, said reciprocating motion being provided by an actuator that includes at least one V-shaped SMA actuating member, the lens carrier being provided with a guide pin embedded in a corner projection of the lens carrier and the ends of the guide pin being received within bushing holes formed respectively in the housing frame and in a bottom plate. Other advantageous features are disclosed in the dependent claims.

The advantages and characteristics of the auto-focus device with shape memory actuator according to the present invention will be clear to those skilled in the art from the following detailed description of an embodiment thereof, with reference to the annexed drawings wherein:

Fig. l is a perspective front view of the device;

Fig.2 is a top plan view of the device;

Fig.3 is a top plan view of the lens carrier;

Fig.4 is a side view of the lens carrier;

Fig.5 is a partially see-through side view of the device showing in particular the position of the guide pin and relevant bushing holes;

Fig.6 is a partially see-through side view of the device showing in particular the position of the return spring;

Fig.7 is a bottom view of the device, with the bottom plate removed;

Fig.8 is a partially see-through perspective view of the device showing in particular the position of an additional flexure spring;

Fig.9 is a partially cut-away perspective view of the device showing in particular the position of the SMA wire terminals and of the SMA wire;

Fig.10 is a partially cut-away perspective view of the device showing in particular the position of the control electronics of the device mounted on a flex printed circuit;

Fig.11 is a partially cut-away perspective view of the device showing in particular the position of a position sensor for detecting the position of the lens carrier;

Fig.12 is a partially cut-away perspective view of the device showing in particular the electrical connection between the position sensor and the control electronics; and

Fig.13 is a partially cut-away perspective view of the device showing in particular the electrical connection between the SMA wire terminals and the control electronics.

With reference to figures 1 to 6, there is seen that an auto-focus device according to the present invention includes a lens carrier LC carrying a lens L and slidably received in a stationary housing frame FIF so as to perform a reciprocating motion for achieving the function of auto-focusing for a camera module or the like. This motion is preferably guided by a metal guide pin GP embedded into the lens carrier LC at a corner projection CP thereof. The guide pin GP can be made integral with the lens carrier LC either by over moulding (also called insert moulding, i.e. pre-inserting the guide pin GP into the plastic lens carrier mould) or by post-assembling the two parts.

The ends of the guide pin GP are received in a pair of concentric bushing holes BH respectively formed in a corner of the housing frame FIF and of a bottom plate BP that can be either mounted on the former or made integral therewith.

The autofocus device according to the present invention is structurally and functionally different with respect to that disclosed in the aforementioned international patent application WO 2011/122438 due to the presence of a guide pin extending over the whole height of the lens carrier and confined by bushing holes, said guide pin in the present invention controlling the tilting performance of the module itself, while in WO 2011/122438 there is just a holding element for the return spring.

The clearance between the guide pin GP and the bushing holes BH will be the key factor to control the dynamic tilt performance of the actuator and to align the movement of the lens carrier LC and guide pin GP. For the sake of simplicity the clearance of the guide pin can be exemplified as the difference between the diameter of the bushing holes BH, and the diameter of the guide pin GP. The inventors found that in order to have enhanced performances, it is preferable for the clearance to be equal to or less than 0,75% of the height of the lens carrier LC. So the preferred clearance results comprised between a minimum of 0, 1 %, to ensure freedom of movement, and a maximum of 0,75%.

As previously mentioned, the device according to the present invention includes a shape memory actuator in which a SMA actuating member, preferably a SMA wire SW, engages the lens carrier LC such that when the SMA wire SW in contracted by heating it this contraction results in a sliding motion of the lens carrier LC. The return run of the lens carrier LC is preferably provided by at least one return spring, but it could also be provided by a second SMA wire that acts on the lens carrier LC in the opposite direction. The return spring is preferably a coil spring CS arranged over the guide pin GP between the housing frame HF and the lens carrier LC in a seat formed in the corner projection CP of the latter, so as to provide a return force when the SMA wire SW cools down and returns to its uncontracted state. To this purpose, the coil spring CS is designed with a compressed length to provide a preload force to the lens carrier LC.

The SMA wire SW is pre-crimped on terminals Tl, T2 at its ends, said terminals being used both for assembling alignment and for electrical connection. The wire SW is then hooked on the lens carrier LC for actuation preferably in a V-shaped configuration by passing it over a recessed portion of the corner projection CP located closer to the bottom plate BP than the crimped connections to the terminals Tl, T2.

A flexure spring FS, illustrated in figures 7 and 8, is preferably used to prevent the rotation of the lens carrier LC during its motion by fixing one end of the flexure spring FS on the bottom plate BP at point PI and fixing another end of the flexure spring FS on the lens carrier LC at point P2. The constraint provided by the flexure spring FS limits the movement of the lens carrier LC to only axial components without tilting components, so as to achieve the alignment of the lens carrier LC to the optical center after the device is integrated into a camera or the like.

Referring also to figures 9 to 13, it is noted that the present device also typically includes a position sensing mechanism such as a Hall sensor HS mounted on the housing frame HF which detects the position of a magnet M mounted on the lens carrier LC at a facing position (other types of sensors, e.g. an optical sensor of the reflective type, can be used). The sensing integrated circuit is mounted on a flex printed circuit FPC with the position corresponding to magnet polarization, and magnet M is attached to the lens carrier LC so that the Hall sensor HS can easily provide a feedback of the movement of the lens carrier LC to the control electronics for compensation.

A laser direct structuring (LDS) process is preferably applied to the device for providing the electrical connections EC between the crimp terminals Tl, T2 (at solder areas SA) and solder pads SP on the flex printed circuit FPC, so that the driver integrated circuit DIC which is mounted on the FPC can apply the current to the SMA wire SW.

It is clear that the above-described and illustrated embodiment of the auto-focus device with shape memory actuator according to the invention is just an example susceptible of various modifications. In particular, in addition to the above-mentioned variants, it should be noted that the SMA actuating member(s) and the return spring(s) can be arranged in many different ways that are within the knowledge of a person skilled in the art.

In order to appreciate the performance of an autofocus device according to the preferred embodiment of the present invention reference will be made to the module tilting, with the angle expressed in min (1/60 of a degree).

AF modules M1-M5, with different guide pin clearance and lens carrier height, are evaluated and the corresponding tilting reported; it is to be underlined that tilting is a detrimental phenomenon since it is a spurious side effect of the AF module movement, so the lowest the better.

possible to observe, AF devices according to the preferred embodiment of the present invention, i.e. with a clearance/lens carrier height ratio comprised between 0, 1 and 0,75% (M1-M3), provide enhanced performance with a tilting angle lower than 10 min that is considered as a limit for high end AF modules. Since this tilt represents the dynamic tilt (the inclination during the actuator movement), the smaller the dynamic tilt the better since it will less affect the optics. The actuator for camera moves the lens along with the image/optical axis to the image sensor, so less tilt leads to more uniform image formation/sharpness (or optical projection).