Creating closed-loop motion systems using SQUIGGLE motors
New Scale has created a number of tiny, high-resolution
closed-loop systems with SQUIGGLE motors incorporating TRACKER
position sensors and other position feedback devices. We excel
at creating application-specific
closed-loop miniature motion 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.
Read the app note below or
download the PDF (93kb)
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. New Scale has also introduced its
TRACKER position sensors,
a line of miniature position sensors with smallest size and
direct digital output right from the sensor.
Choose an option with resolution and travel
range to fit your application. See the application note on
choosing position sensors.
Determining position resolution
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.
The TRK-1T02-E TRACKER position sensor evaluation pack
includes an 11 mm long magnetic strip for 7 mm of travel, and
provides a resolution of 2 microns. Its on-chip encoder provides
direct digital output, eliminating the need for an A/D
converter.
SQUIGGLE motors in closed-loop systems - some examples
Phone camera modules
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-1.8
SQUIGGLE motor to push lens holders along rails
(Figure 1, top right). TRACKER miniature encoders
are integrated into the device and provide feedback resolution of 2 microns. The
complete system includes two lenses, two motors and two
encoders, in a module measuring only 6.5 x 13 x 23 mm. Both motors are driven
by the NSD-1202 dual piezo motor driver ASIC.
Lock modules
For electronic lock applications, tiny size is prized but the
required motion is simpler: the motor must move from as fast as
possible from its reverse limit (lock open) to its forward limit
(lock closed) and back. For this and similar applications, an
SQL Series SQUIGGLE motor can be integrated with simple limit switches
to indicate end of travel range.
Small stages
New Scale has also created several closed-loop positioning
stages. The model SQ-2115 linear stage (Figure
2, right)
has superior resolution (0.020 microns) and long, 15-mm
travel. We used a linear optical encoder 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.
A custom linear micro stage (Figure
3, bottom right)
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-1100 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 needs for closed-loop miniature motion
systems.
|
