Friday, July 01, 2011

Tessera on Drawbacks of VCM AF Actuators

Tessera published a nice whitepaper "Auto-Focus Mechanisms for Miniature Cell Phone Cameras". VCM actuators are thoroughly analysed and their drawbacks are summarized:


  1. VCM typically draws around 250mW continuously during focusing. Although new designs claim to reduce this drain to around 80mW, this is still too high to permit the VCM to be used for anything other than momentary operation during photography. Attempting to capture a video clip with auto-focus operating the entire time would rapidly drain the handset battery. Having such high power dissipation in close proximity to the image sensor can also degrade image quality.
  2. The power requirements of a VCM actually increase as the mechanism gets smaller. This effectively places a limit on the minimum size of an auto-focus camera module. The problem is that as the electro- and permanent magnets get smaller, the available magnetic force decreases. For the electromagnet, the field is dictated by the Ampere-turns, while for the permanent magnet it is the volume. Higher power magnets cannot be used due to their cost, so the only method to compensate for a reduction in dimensions is to increase the amperes through the coil, which is highly undesirable due to battery power drain and heating of the image sensor.
  3. Another problem with auto-focus mechanisms based on VCMs is speed of actuation. More accurately, the problem is time of settling. A half-stroke jump of 125ìm may only take the actuator a few milliseconds to achieve, but it then bounces about the new set position as the oscillatory magnetic and spring forces diminish to balance. It can easily take 50ms before the oscillations reduce to less than 5ìm and the lens position is considered stable enough for image capture.
  4. Lens tilt:


Tilt of VCMs can be controlled. Some of the options are a longer barrel, more springs, larger springs and more complex springs. As might be expected, all of these in some way impact the power consumption, size or cost so the scope for their implementation is limited.

8 comments:

  1. This comment has been removed by the author.

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  2. I think Fig 10 is reversed.
    I sure would like to see Tessera (aka Siimpel) regain the traction with this technology lost in 2007 after the big post-Razor Moto implosion. In 2007 MEMS AF was designed into 7 or more new phones - all of which Moto cancelled when the stuff hit the fan.
    (re-posted due to typo)

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  3. We have seen several successful companies from the poster above and we should all know how well it was going before he left the last one. Unfortunately overhiring and never delivering is not well.

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  4. I suppose you are referring to Tessera since Siimpel remained pretty lean and made all deliveries on time to Flex, our alpha customer and supplier to Moto.

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  5. Wasn't the first generation MEMS AF just a silicon spring, never sure how this was an improvement over a standard spring apart from the marketing pitch.

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  6. I would not characterize it as "just a spring" but I think I understand your question. So, yes, and there is a paper on it from 2007 IISW.
    It is a sort of magic spring, due to mechanical qualities of single-crystal silicon, and the precise dimensions/aspect ratios involved. So it had flexibility along the motion axis and very high stiffness in other motion axes so that motion was highly confined to be one dimensional and without the hysteresis found in spring made of other material. The result was very repeatable precise movement. This is important in AF because you always overshoot the optimum focus point and you want to go back to the optimum position. If there is hysteresis, it is difficult to go back to the same spot so you have to an iterative search mechanism in that case so AF is slower and the algorithm more complicated.

    Best to read the paper. The electromagnetic part was also fairly complex in field shaping for the given size and current etc.

    The thing about VM AF is that it is just a spring and so it is less expensive to make than micromachining a part from silicon.

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  7. "Attempting to capture a video clip with auto-focus operating the entire time would rapidly drain the handset battery."

    This is a very surprising statement. Most high end phones are using CAF in video mode ...

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  8. I don't know if these numbers are current, but say 1000 mAh 3.6V battery, 200 mW VCM AF, I get 55 mA draw and 18 hrs of battery life. That is a lot of video, if AF was the only power draw.

    This does not support "rapidly", really, but nevertheless 55 mA is still a big draw on the battery.

    At 1300 mAh and 80 mW AF, the numbers look even better for battery life.

    I recall in ~2006 that continuous AF power drain was of significant concern to handset makers so while not a lot has changed, perhaps the perspective of manufacturers on customer priorities has migrated to support continuous AF.

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