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Focus: Industrial strength, smart phone sensibilities

Lens motion technologies developed for phone cameras make their way into small embedded imaging systems for biometric, medical, machine vision and security applications

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Published by Electronic Products
May 2012
By David Henderson

Smart phone cameras are fast becoming “the only camera you need” for consumer photo and video. It’s not just about megapixels: Much of the quality improvement stems from advanced optics and tiny new motion systems for precision autofocus.

Autofocus is more than a convenience. It allows larger apertures (for more light on the sensor), compensating for the resulting lower f-number and reduced depth of field by ensuring that the image plane is precisely aligned on the sensor’s surface.

It’s no surprise that engineers want to put tiny smart phone cameras in the non-consumer products they’re designing. The trends are the same – we want smaller devices with higher-resolution images and more powerful features.

Unfortunately, even the most impressive phone cameras are not sufficient for most non-consumer applications. While image quality does matter in a phone, it yields to demands such as a super-thin form factor, low cost and high-volume manufacturability. In non-consumer applications, the quality of the image and the detail of the information it contains are not so negotiable. While an iPhone app can perform facial recognition, you may not want to rely on it for access to your bank account.

Nonetheless, much of the technology developed for today’s smart phone cameras is being extended for use in non-consumer applications.

From VCMs to piezos for focus

Compact and cheap, VCMs were widely used for lens motion in early phone cameras. With image sensors up to 5 megapixels, VCM motion precision is good enough. However, as phone camera manufacturers push to improve image quality – employing higher-resolution sensors and larger apertures – the need for greater precision in lens motion becomes clear.

Precision lens motion has two parts:

1. Positioning of the lens along the z-axis, perpendicular to the sensor. The need for precision increases with decreasing f-number, which lets in more light for better sensor performance but decreases the depth of focus.

2. Lens motion without tilt. Lens tilt decreases depth of focus and leads to blurring on the edges of the image (figure 1).

Fig. 1: The depth at which the entire image is in focus is reduced with increasing tilt: When the lens is positioned so that the center of the image is focused on the sensor, one edge of the lens will be further away from the sensor and the opposite edge of the lens will be closer to it.

VCM focus for phone cameras

Fig. 2: In a VCM, low spring stiffness leads to lens tilt, overshoot, oscillations, and long settling times (as long as 100 ms).

In a VCM-based focus system, a spring or flexure supports a lens assembly that “floats” in a lens barrel. An electric coil around the lens assembly and permanent magnets in the housing create a closed magnetic flux path. Applying current to the coil generates a proportional Lorentz force along the z-axis. The spring bends and the lensmoves along the z-axis with position resolution of about 5 microns. Low spring stiffness leads to overshoot, oscillations and settling times up to 100 ms. Constant current and power are needed to maintain a position.

The reliance on springs also leads to significant lens tilt; greater than 0.3 degrees. Gravity induces tilt if the camera is used in any non-vertical position. Tilt control can be added at increased cost and size -- negating the two key advantages of the VCM.

The use of springs also limits lens mass to about 0.2 grams, and lens travel to about 0.3 mm.

Piezoelectric focus for phone cameras

Fig. 3: UTAF piezo lens motion system for phone cameras is thinner than a VCM system and offers better position resolution and lower lens tilt.

Phone camera designers have begun using new piezoelectric focus systems in place of VCMs, to improve both z-axis position resolution and tilt. These enable higher-resolution photo and video in the same tiny form factors.

In a piezoelectric system, electrical signals produce minute bending motions in a piezoelectric element. Various mechanisms are employed to convert these tiny motions into long-range lens travel.

For example, for phone camera applications, New Scale Technologies has licensed its Ultra-Thin Auto-Focus (UTAF) modules in which an ultrasonically vibrating piezoelectric beam is placed in direct contact with the lens assembly holder (figure 2). >The minute motions of the piezo element push the lens smoothly along the z-axis with 1 micron position resolution, a 5x improvement over VCMs.

A pin bushing guides the lens and limits tilt to less than 0.1 degree, a greater than 3x improvement over VCMs.

The UTAF was designed to phone camera specifications: light lenses (up to 0.25 grams), very thin form factors, very low power consumption, and high-volume manufacturability at low cost. It has a thinner profile than a VCM and an equivalent x-y footprint.

A non-consumer focus

While optical requirements vary, non-consumer imaging systems almost always require lens assemblies with greater mass than the 0.2 grams that VCMs can support. Reasons include the need for larger lens diameters, larger apertures, and heavier materials such as glass for high clarity or transmission at specific wavelengths. They usually require longer lens travel (stroke) as well.

Unlike VCMs, piezo solutions can be scaled to accommodate greater lens mass and longer travel. At the same time, piezo technology keeps overall camera size to a minimum – usually only millimeters larger than the lens assembly itself.

New Scale’s M3-F focus module, for example, supports lens assemblies up to 5 grams – 25 times heavier than VCMs can support. It offers lens travel of 1.5 mm or more, compared to 0.3 mm for a VCM.

Fig. 4: M3-F piezo lens motion system maintains the benefits of high-position resolution, low lens tilt, and small size, while supporting the longer travel and greater lens mass requirements of most nonconsumer imaging applications.

The M3-F incorporates a long-travel piezoelectric SQUIGGLE motor to move the lens barrel in a housing (figure 3). Ultrasonic vibrations of the piezoelectric elements rotate a screw and cause it to translate along the axis of the motor, parallel to the z-axis of the lens barrel. The screw tip is directly coupled to the lens assembly, moving it along the lens barrel with a resolution of 0.5 microns. As with the UTAF, a pin bushing limits tilt to less than 0.1 degree.

For non-consumer applications, there is additional value in higher position resolution and low tilt offered by piezo systems. For example:

Hysteresis vs. repeatability

Photo: New Scale's M3-F focus system shown mounted on a board camera. The module extends the benefits of piezo focus systems found in phone cameras, to meet the more stringent demands of non-consumer imaging and video analytics applications including biometric detection, medical systems and security/surveillance.

VCMs are open-loop systems, with significant hysteresis that is a function of travel and therefore worse with longer stroke.

Both the UTAF and M3-F piezo lens motion systems are closed-loop systems, with magnetic position sensor and motor drive ASIC integrated in the module. There is no external controller board, so total system size remains small.

As closed-loop systems they do not have hysteresis. Instead, they have bi-directional repeatability that is fixed over any travel length. For the M3-F, this is better than +/- 20 microns (+/- 5 microns unidirectional) over the full travel range. Unidirectional repeatability is +/- 5 microns.

Additional benefits include fast response and fast settling time, and stable high-stiffness holding force without oscillation. A comparison of the systems discussed in this article is shown in Table 1.

Just as these compact focus systems helped drive advances in phone cameras, they are set to add new capability in non-consumer imaging applications.