Creating closed-loop positioning systems using SQUIGGLE motors
New Scale has created a number of tiny, high-resolution
closed-loop systems with SQUIGGLE motors. We have expertise in
specifying position sensors and creating application-specific
closed-loop systems for our OEM customers.
This application note provides an overview of SQUIGGLE motors
in closed-loop systems. Please contact us to discuss your
specific application.
SQUIGGLE motors have excellent position resolution: users can
signal the motor to move very small distances, measured in
microns or nanometers. However, the motor speed varies with
applied load and device friction. Therefore you need a
closed-loop control system if you need to achieve exact
positioning, repeatable positioning, or precise speed.
The basics of closed-loop control
In a closed-loop positioning system, a sensor detects the
actual position and feeds the information to the motor
controller. The controller compares actual position to desired
position, and moves the motor to correct any error. This allows
the motor to reach a precisely controlled position. Similarly,
controlled speed is achieved by adjusting the driver gain to
minimize the difference between the required position and the
actual position at regular time intervals.
SQUIGGLE motors are prized for their tiny size, and most
users want tiny position sensors to match. Many choices are
available, ranging from miniature optical encoders to simple
limit switches. Choose an option with resolution and travel
range to fit your application. See the application note on
choosing position sensors.
Position resolution is determined by three factors: the
resolution of the position sensor, the resolution of the motor,
and the A/D converter in the controller. If you need to know the
position within 10 microns, your position sensor must have a
resolution of 5 microns or better, your motor must have a
position resolution of 2.5 microns, and your A/D converter must
be capable of resolving the feedback signal into small enough
increments to allow signals to the motor at its best resolution.
Here’s a design example: say you want to have 10 microns
resolution over a travel range of 2 mm. A possible solution is
to use a Hall Effect sensor with a magnetic strip 4 mm long
(using the more linear center of the magnet and avoiding the
last mm on each end). A 10-bit A/D converter will supply
resolution of 0.001 of the 4 mm, or 4 microns. The motor
resolution is 0.5 microns, so the limiting factor is the A/D
converter. Assuming some background noise, this will still be
enough sensor resolution to achieve 10 microns repeatability.
With a higher resolution A/D chip, the position resolution would
be even better.
SQUIGGLE motors in closed-loop systems - some examples
Some of New Scale’s smallest closed-loop systems to date have
been created for the phone camera market, to move lenses for
focus and zoom. This market requires the smallest sensor
possible, with resolution on the order of tens of microns or
better.
We have created several such systems. One uses an SQL-1.5-6
SQUIGGLE motor to push a lens holder riding on two rails
(top illustration at right). This
supplies a stable, three-point contact on the lens holder as it
moves. A miniature optical encoder (Avago) is integrated into
the device and supplies feedback resolution of 20 microns. The
complete system includes two lenses, two motors and two
encoders, in a module measuring only 22 x 12 x 8 mm (0.87 x 0.47
x 0.31 inches). Both motors are driven with full closed-loop
control by a single board. This board can be used as a reference
design for custom ASIC development.
Small stages
New Scale has also created several closed-loop positioning
stages. The model SQ-2115 linear stage (middle illustration)
has superior resolution
(0.020 microns) and long, 15-mm travel. We used a linear optical
encoder (MicroE Systems) and an SQ-100 series SQUIGGLE motor
inside the body of a 63.5 x 63.5 x 17 mm (2.5 x 2.5 x 0.66 inch)
stage. The incremental encoder counts pulses and has a zero
reference built in to give a repeatable position on startup.
By centering the optical encoder in the body of the moving
stage we achieve the most accurate position feedback possible.
In comparison, some motors use a rotary encoder on the motor
shaft as a sensor. This does not see any errors from the gear
box, coupling or backlash of the slide, resulting in lower
precision.
New Scale’s model SQL-3405 linear stage (bottom illustration)
is a smaller example,
at only 25 x 25 x 10mm (0.98 x 0.98 x 0.39 inches). This stage
uses a Hall Effect magnetic position sensor inside the stage
body. A benefit of the Hall Effect sensor is that it acts as an
absolute position sensor: working by measuring the field along a
magnetic strip, it retains position information even after
power-down cycles. An SQL-3.4-10 SQUIGGLE motor is mounted onto
the bottom plate and the screw pressed against the end of the
top plate, which is spring loaded against the screw. Used with
the MC-1000 controller, this closed-loop stage has 10 microns
resolution over 2 mm of travel. New scale is developing a
high-resolution controller option with 16-bit A/D converter to
achieve a resolution of less than one micron.
Controllers
New Scale’s SQUIGGLE motor controllers
can accept an analog or digital quadrature (RS-422) signal from a
position sensor to provide closed-loop operation. The software
provided allows the user to define the range and resolution,
which are then used in the positioning commands.
The electronics use PID (Proportional, Integral and
Derivative) controls to actively tune the response of the motor
to the feedback signal. These coefficients are interactive and
work to keep the motor speed from leading, lagging or
oscillating with respect to the sensor feedback signal.
Standard New Scale controllers include a 10-bit A/D
converter. A new high-resolution OEM control board features a
16-bit A/D converter for higher resolution of the analog
feedback signal.
In summary, closed-loop control systems provide repeatable
and accurate positioning in tiny modules that take full
advantage of the SQUIGGLE motor’s small size and high precision.
New Scale has demonstrated a number of miniature closed-loop
systems, and works with OEM customers to design complete
closed-loop systems that meet unique requirements.
As position sensors and control electronics continue to
improve, we expect to see even greater resolution and precision
in tiny packages. Contact us today
to discuss your closed-loop micropositioning needs.
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