Overview
Pico projectors that incorporate a focusing lens typically
provide only a manual dial to adjust the focus of the projected
image. An opportunity exists to automate the lens focus using a
small microcontroller, a rangefinder, a lookup table or polynomial
equation to convert projection distance to lens focus position, and
a closed-loop motion system consisting of a piezoelectric SQUIGGLE
micro motor and TRACKER non-contact position sensor (figure 1). If
the pico projector also includes a camera, the auto focus feature of
the camera, instead of a separate rangefinder, could be used to
establish focus.
Figure 1: Pico projector with rangefinder
Any auto focus system for a pico projector must be very small, and
draw very low power. New Scale has demonstrated the smallest
possible auto focus system using the
SQL-RV-1.8 Squiggle
motor with NSD-2101 drive ASIC and the TRACKER position
sensor (figure 2). The complete closed-loop system is very small,
very precise, and offers extremely low power consumption, achieved
through features such as off-power hold and proprietary motor
control algorithms.
System description
The reference design features an SQL-RV-1.8 Reduced Voltage
SQUIGGLE motor precisely controlled through a closed-loop feedback
system that includes an ultrasonic rangefinder and our NSE-5310
TRACKER position sensor.
We used a developer’s kit model DK-RV-1.8-TRK-33, which includes the
SQUIGGLE motor, driver and TRACKER position sensor, and an
off-the-shelf ultrasonic rangefinder (LV-MaxSonar-EZ0 from MaxBotix)
to provide position information to the controller (figure 2).
Figure 2: New Scale’s closed-loop Developer’s Kit, shown with
the LV-MaxSonar-EZ0 ultrasonic rangefinder, is used to demonstrate a
pico projector auto focus system. The developer’s kit contains the
SQUIGGLE RV micro motor, NSD-2101 drive ASIC, TRACKER position
sensor and a PC interface board with development software.
Custom firmware on the MC-3300-RV controller (included in the kit)
monitors the pulse width output of the rangefinder to determine the
projection distance (figure 3). It then commands a move of the
SQL-RV-1.8 motor to that position. Note: Lens, lens mount and
optical or electro-optical components are not included in this
reference design.
Figure 3: Connection between MC-3300-RV
and MAXSonar-EZ0
A custom PC application was developed to configure a polynomial that
establishes the relationship between the measured projection
distance and the required lens position to focus the image. The
polynomial coefficients are downloaded to the MC-3300-RV controller.
The controller queries the rangefinder and adjusts the motor to
maintain that relationship. The PC program shows auto focus in
progress by displaying the actual lens position vs. desired position
for focus, based on the current projection distance (figure 4).
Figure 4: PC demonstration program showing projector auto
focus in progress
Pico Auto Focus Implementation Scenarios
Figure 5 illustrates system configuration options. The
microcontroller performs the following tasks:
- Queries projection distance from rangefinder (or ISP, if
available). In New Scale’s reference design, the
rangefinder generates a pulse every 50 msec. The width of
the pulse width is proportional to the distance.
- Uses distance either to index the lookup table, or as a
coefficient in a polynomial, to get appropriate lens focus
position.
- Initiates move-to-target process within its PID control
loop.
- Schedules the next distance query.
For more information
Purchase the
DK-RV-1.8-TRK-33 developer’s kit referenced in this design note
online. For access to the custom PC application or for more
information about auto focus applications, contact New Scale at +1
585 924-4450 or email
sales@newscaletech.com.
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