Auto focus for pico projectors
A reference design using a SQUIGGLE RV micro motor, a TRACKER position sensor and an ultrasonic rangefinder
Rev. 0 | May 3, 2010
New Scale has developed a reference design that demonstrates the smallest solution for adding auto focus to a pico projector. The design uses New Scale’s piezoelectric SQUIGGLE motor, NSD-2102 drive ASIC, NSE-5310 position sensor, and a commercial ultrasonic rangefinder.
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.
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.
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).
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.
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).
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
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 NSTsales@newscaletech.com.