Tag Archives: Comb Drive

How does a smartphone camera autofocus?

How does the camera of a super slim smartphone autofocus?

As long as cell phones were over 10 mm thick, manufacturers had no problem of getting the camera to autofocus. Of the 2 billion cameras manufactured for the phone and tablet market, nearly half of them autofocus. Usually, one of more of the lenses in the camera are moved in or out using a linear actuator, while an algorithm calculates a figure of merit for the sharpness of image for that location of the lens. The best focus for the scene is achieved by repeating this procedure.

This was going fine, until form factors started to get thinner. Manufacturers made thinner phones, and people took this as a paramount design consideration. As the 5 mm form factor was approached, compressing an 8-13 M Pixel auto focus camera that would still produce high fidelity images became a challenge. In addition, the requirement of speed, power and performance also changed, and altogether, forced manufacturers to abandon the old method of Voice Coil Motor in favor of a MEMS linear actuator.

The Voice Coil Motor (VCM) operated using the principle of electromagnetism. This is the same technology used in loudspeakers to produce sound from electricity. When electricity passes through a coil, it produces a magnetic field that reacts with a permanent magnet to either repel or attract the coil. The movement of the coil is restricted such that it can only move along its axis. Springs attached to the coil help to bring it back to its rest position once the electricity in the coil stops flowing.

The main disadvantage of the VCM is the hysteresis of its stroke. Usually, the coil does not return to its original position after a displacement and this prevents rapid tracking of focal distances in a VCM controlled camera lens. Other disadvantages are the high requirement of power for operating the VCM and de-centering and lens-tilting while operating. All these problems became increasingly acute with increasing image sensor resolution, decreasing pixel dimensions and f-numbers. Moreover, with the VCM technology now over 100 years old, the opportunities for further cost reduction are virtually nil.

This paved the way for a competing technology with a commercial opportunity that can deliver improved performance at a reasonable cost. This is the MEMS or Micro-Electro-Mechanical-System that uses components from one to 100 micrometers in size.

The MEMS technology for autofocus integrates the three functions of a linear actuator. It provides a linear vertical movement, has a spring to provide the restoring force and uses an electrostatic comb as a drive to displace the lens. The MEMS technology saves on power since it does not use electromagnetism.

The comb drive is more like interlocking fingers, only the fingers never touch. The electrostatic charge developed when a DC voltage is applied, develops an attractive force causing the combs to be drawn to each other. The lens, which is attached in the center, completes the silicon MEMS autofocus actuator.

The MEMS technology allows only one lens to move very precisely, while the other lenses are locked in the most optimal position. This approach offers an excellent image quality over the entire focal range within the 5 mm allowed in a thin smartphone.