Applications
Board cameras are used for compact imaging systems in security,
medical and industrial applications. They typically consist of a CCD
or CMOS sensor mounted on a small printed circuit board, with a
fixed lens mounted over the sensor.
Replacing the fixed lens with a focus system provides significant
benefits. It enables better image quality over a wider depth of
field, increasing system accuracy. It enables macro imaging as well
as wide field imaging with the same camera, allowing greater
distance (and greater variation in distance) between the camera and
the object being imaged. Applications include:
Advantages over other focus modules
Alternative focusing systems such as liquid lenses or liquid
crystal lenses rely on the ability to control changes in composite
materials: e.g., the curvature of a liquid-oil interface in a
container, or the refractive properties of a liquid crystal
composite. These composite systems are highly dependent on material
consistency and environmental factors such as temperature. In
addition, their transmission and clarity is far inferior to that of
traditional glass or polymer lenses.
The M3-F focus module allows you to move traditional,
high-quality lenses with high resolution, repeatability and
accuracy. As a closed-loop system, it can be easily calibrated
and controlled for high accuracy and repeatability independent
of temperature. It does not exhibit the hysteresis
characteristic of liquid crystal lenses.
The M3-F is a cost-effective system solution with embedded
closed-loop drive circuit for high repeatability with low
external processing requirements. It operates on 3.3 V with low
power requirements and holds focus with the power off.
M3-F advantages over liquid lens solutions:
- Easier integration - no external control board, lower
external processing requirements
- Uses a single glass or polymer lens for superior
transmission and clarity
- No hysteresis
- Less temperature dependency
- Configurable product platform vs. unique custom design for
each camera
M3-F advantages over stepper solutions:
- Smaller size
- Superior tip/tilt specification
- Higher resolution
- Hgher repeatability
M3-F customizable module description
New Scale's M3-F customizable modules can be quickly adapted to fit
a wide range of image
sensor, board and lens combinations. The
M3 platform offers flexibility to accommodate specific board mounting, lens travel (macro to infinity), lens thread and motion
control needs. This enables OEMs to create high-performance board camera
solutions in less time and at lower cost.
Each module incorporates a piezoelectric SQUIGGLE micro motor, magnetic TRACKER position sensor,
motor drive ASIC and microprocessor with a moving lens holder in a
robust polymer housing (Figure 3). The customer defines the PCB
mechanical mounting interface, dimensions of the threaded lens
assembly, and the macro and infinity image plane locations. Housing
dimensions are approximately 20 x 22 x 16 mm depending on OEM
specifications.
Figure 3: M3-F customizable autofocus module design
platform.
System description
A block diagram for a motorized focus module in a board camera system
is shown in Figure 4. The focus module mounts on the OEM's
camera PCB, over the image sensor. The SQUIGGLE motor moves the lens
barrel. The NSE-5310 position sensor detects the movement of a
multi-pole magnetic strip attached to the lens barrel, and provides
the direct digital position information to the microprocessor. The NSD motor controller ASIC
accepts commands from the processor via I2C or SPI interface, and drives the motor. The system
operates on standard 3.3 V battery power.
Additional detail about the components follows.
Micro motor
A patented SQUIGGLE micro motor provides high-precision linear
motion, with no gears and no need for lubrication. It holds its
position without drawing any power. Its high speed enables rapid
focus, further reducing power use and improving the user experience.
This robust piezoelectric device offers quiet, ultrasonic operation.
It is smaller, more power-efficient and more precise than stepper
motors.
Position
sensor
The NSE-5310 is a magnetic sensor array with integrated
on-chip digital encoding. The sensor measures the spatially-varying
magnetic field produced by a moving magnetic strip mounted on the
lens holder. It provides direct digital output of the absolute
linear position within a 2 mm range. It does not require a light source or a separate
zero-reference sensor. There is no need to hunt for zero position on
power-up, resulting in lower power use and less wait time to capture
images. Standard I2C protocol enables simple integration into the
system processor.
Motor driver ASIC
An NSD piezo motor driver ASIC is powered by a single 3.3 V battery. Like the
sensor, the motor driver ASIC uses
a standard I2C interface for integration with the system processor.
It creates pairs of phase-shifted square waves to drive the motor.
On-chip registers store driver instructions. A power-down mode
provides for minimal power consumption in stand-by.
The drive electronics are integrated into the module.
Figure 4: Simplified block diagram of the
customizable focus module design platform.
Resolution,
repeatability and accuracy
A variety of algorithms are used for autofocus. One involves
moving the lens through its entire range and identifying the
lens location which delivers the best image quality. The system
may capture images at multiple points in the process and keep
the best image; in this case the motion step and settle time are
primary concerns. Alternatively, the system may identify the
optimum position for focus and then move the lens back to that
position for final image capture. In this case, the primary
motion concerns are resolution, repeatability and accuracy.
These are discussed below.
Resolution
Position resolution determines the minimum resolvable
distance the lens can move. In the M3-F this is 0.5 µm,
providing excellent fine control over the lens motion.
Repeatability
Repeatability is a measure of the system’s ability to return the
lens to a previously defined position. In many algorithms,
better repeatability means better focusing.
The direction in which the target position is approached impacts
repeatability (Fig. 5). Unidirectional positioning, in which the
target position is approached from the same direction every
time, provides better repeatability.
Figure 5: uni-directional vs. bi-directional positioning.
Figure 6 shows data from an M3-F error and repeatability
measurement. For this measurement the M3-F’s integrated motion
system is used to drive the lens to a series of target points,
forward and reverse, over its full travel range. The process was
performed twice.
The X axis shows the recorded lens position after each commanded
move, as measured by a reference position sensor (Keyence).
The Y axis shows position error for each target point, that is,
the difference in lens location measured by the reference sensor
vs. that measured by the M3-F’s integrated position sensor.
The pink curve shows lens position error while traveling in the
forward direction.
The gold curve shows the error for each of the same positions
while traveling in the reverse direction.
Repeatability is the deviation of actual position measured by
the reference sensor at each of the target positions.
Figure 6: M3-F module repeatability and error
measurement results
The graph illustrates repeatability by the vertical difference
between the curves. The difference of the curves while traveling
in the same direction is less than the difference while
traveling opposite directions.
Consider an imaging application where the image processor
collects a series of evaluation images along the full travel of
the lens while moving in the forward direction, from end to end
(in an image resolution hill climbing algorithm). At the end of
that series of image captures, the lens location is on the pink
curve at position 1500µm. If the processor determined that the
optimal focus position is at 850µm (on the X axis) we need to
move the lens back to the 850µm position to capture the final
image. We have two scenarios:
1. We can simply reverse direction and stop at the 850µm
position. The actual variance from the optimal position could be
up to 6-7µm (bidirectional repeatability).
2. We can move in the reverse direction, travel past the 850µm
position, and then move in the forward direction to stop at the
850 µm position. The variance from the target position would be
reduced to a maximum of approximately 1µm (uni-directional
repeatability).
Scenario 2, with better repeatability, will result in images
with better contrast than scenario 1. The product designer would
need to evaluate this higher contrast vs. the impact of the
additional lens motion on focusing speed.
Accuracy
Accuracy is measured by the total error in the system. For the
test data in Figure 6, that would be the range from the peak of
15 µm on the forward direction curve, to the peak of -4 on the
reverse direction curve, or a total of about 19µm. The module
specification is +/- 30 µm. However, the M3-F module is
calibrated and includes control software in the module that
compensate for the error. Accuracy is therefore not a concern
with the M3-F in an autofocus system
Mechanical integration
The standard focus module in an M3-F developer’s kit can accept a
broad range of lenses and accommodate image sensors up to 17 x 17 mm
including 1/2" and 1/1.8" formats. Please refer to the design note,
Checking
Optical and Imager Geometry in DK-M3-F, for lens
recommendations and guidelines for determining your image sensor and
lens compatibility.
These standard modules can be used in volume in OEM systems, or
New Scale can quickly customize M3-F modules to accommodate OEM
requirements not met by the standard design. See the on-line custom
specification worksheet at
http://www.newscaletech.com/contact_AF-RFP.aspx.
Electrical integration
M3-F modules require input voltage of 3.1 to 3.6 V. They are
configured for standard SPI or I2C serial interface. Details on
communication formats and the command set are found in the
M3-F Command and Control
Reference Guide (registration required).
Specifications
Specifications for standard M3-F modules and developer’s kits are
found at
www.newscaletech.com/M3-F.html. New Scale offers custom motion
systems to meet other OEM specifications as needed.
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