MEMS alternatives for miniature auto-focus cameras

November 1, 2011 — In a Solid State Technology webcast, presented by DigitalOptics Corporation, a wholly owned subsidiary of Tessera Technologies, Dr. Giles Humpston, Director of Applications, presented Lens Tilt in Small Auto-Focus Cameras.

What is a small camera? Most would qualify it as about 1cm3, disposable, cheap (<$1/MP) and integrated into digital cameras, cell phones, webcams, etc.

Figure 1. Simplified diagram of a miniature camera.

Small cameras range from the simplest, most rugged and most limited fixed-focus systems to higher-end auto-focus (AF) devices that are the subject of Humpston’s talk. AF cameras have electronic controls that allow them to focus much closer (as on a barcode) and in lower light than other designs.

Auto-focus cameras have evolved from large, highly mechanical, expensive devices. Today the majority of AF cameras are based on voice-coil motor (VCM) technology which works on magnetic attraction/repulsion principles.

Figure 2. A VCM miniature camera set up, with springs and magnets to control lens movement.

VCM pros? The devices are compact, almost silent, and cheap, with good focus range. Cons? They are slow, and not the best choice for video. Power consumption is in the hundreds of milliwatts (mW), and the smaller the VCM, the higher the power consumption. VCMs also suffer from lens tilt (Figure 3).

Figure 3. Lens tilt. Springs can change length, pulling the lens, or a user can tilt the lens through the influence of gravity and cause permanent changes by dropping the device. Tilt control is expensive and increases package form factor.

High-performance next-generation auto-focus cameras will not use VCMs, says Dr. Humpston.

Alternatives are in development– electrostatic silicon actuators manufactured as micro electro mechanical systems (MEMS).  The comb drive actuator — the most common design — is very low power.

Figure 4. Comb drive actuator.

Silicon MEMS can also be formed into a complete auto-focus component, with springs, etc. on one chip. With no mechanical play, MEMS actuator actions are extremely reproducible, operation is faster and optical performance is maintained through a wide range. With only one moving lens, as opposed to the entire optical train, image quality is desensitized to lens tilt. The module form factor becomes thinner as well.

Table. Key features of an auto-focus camera using VCM and MEMS.

 

MEMS

VCM

Actuator dimensions (mm)

7.4 x 7.4 x 1.7

8.5 x 8.5 x 5.0

Peak Power*

0.5mW

250mW

Repeatability

1µm

10µm

Hysteresis

3µm

20µm

Speed+

5ms

30ms 

Reliability cycles

10 million

1 million

Reflow compatible

Yes

No

* MEMS peak power <30µW, remainder is the driver chip

+ Half-stroke settling time

VCM alternatives based on MEMS will start hitting the market commercially in 2012, Humpston says. Get all the design details and in-depth discussion of lens options, reliability, and more in the webcast: Attend now.

Dr. Humpston is a metallurgist with his name on patents and publications, including articles on nanotechnology and semiconductors for ElectroIQ.

Attend the webcast: FREE WEBCAST: Lens Tilt in Small Auto-Focus Cameras

Read Dr. Humpston’s article: Nanotechnology for semiconductors

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