Actuators for miniature cameras (exemplarily as in smart phones) are used to shift and/or tilt optical elements in the camera to achieve focus, image stabilization or shutter movement. There are several methods to achieve the force needed for actuation. EM actuators use ferromagnetic materials, discussed herein, to provide a magnetic actuation force. Actuator structures are disclosed for example in co-invented and co-assigned PCT patent application PCT/IB2014/062836 filed Jul. 3, 2014 and titled “Electromagnetic actuators for digital cameras”, include at least one stationary ferromagnetic member and at least one moving ferromagnetic member, the stationary and moving members having operating surfaces, the operating surfaces facing each other across a small air gap. Each actuator structure has at least one large air gap in the magnetic circuit that includes all the stationary ferromagnetic members, which do not move during the actuation. Each large air gap is either by-passed or bridged through smaller air gaps by the moving ferromagnetic member. Such actuator structures are designed to pull the moving ferromagnetic member and reduce the magnetic reluctance of the actuator and thereby provide a large magnetic force. In most embodiments the reluctance changes significantly with movement in a first (“force/actuation”) direction, while in the other two (“indifferent”) directions, in-plane and orthogonal to the first direction, the reluctance is hardly changed or not changed at all with movement. As described in detail in PCT/IB2014/062836, the magnetic circuit always acts toward closing the large air gap, thus pulling the moving ferromagnetic member in one direction, e.g. +X. Pulling in the opposite direction (e.g. −X) can be achieved using one of two methods: forced-back-actuation (“method 1”), which uses a spring to retract the optical element to zero position, or dual-actuation (“method 2”), which uses an opposite actuator to create an opposite force.
A great improvement in control would be achieved if one knew the exact position of the moving member relative to the stationary member during the actuation process. For example, “A miniaturized low-power VCM actuator for auto-focusing applications”, Optics Express, 16 (4), p. 2533, 2008 describes how an actuator with a known relative position of a stationary member vs. a moving permanent magnet, reduces the power consumption during actuation. In another example, US patent application 2013/0215511A1 describes position sensing (measurement) performed in conjunction with actuators to improve the accuracy of an optical image stabilization (OIS) system. Position sensing in these two cases is done using an additional sensing circuit, such as a Hall bar, which measures a magnetic field in space.
Position sensing based on a change in the inductance of a coil is known. Various schemes using inductance-based position sensing have been applied to cameras. However, in known designs, special coils are added to the camera just for the purpose of position sensing. This adds at least one component to the apparatus and complicates the design and operation. For example, U.S. Pat. No. 8,180,211 B2 describes a method for position and motion sensing in a miniature camera, using a magnetic circuit that includes a ferromagnetic part and a coil, while motion is done using a permanent magnet and a coil.
There is therefore a need for and it would be advantageous to have position measurement in a digital camera that does not require use of additional sensing circuits, such as additional coils, permanent magnets or Hall bar sensors.