Choosing position sensors for use with SQUIGGLE motors
SQUIGGLE motors are open-loop devices. While the motor can
move in very precise increments (as small as 500 nm for
SQL
Series motors), the distance moved in response to a single drive
signal pulse depends on the load and the motor’s internal
friction. Any variation in these parameters along the length of
travel will affect the motor speed. If you need to know the precise speed or position of the motor, you need to create a closed-loop system incorporating feedback from a position sensor.
There are many types of position sensors that are suitable
for use with a miniature SQUIGGLE motor. To begin
the selection process you will need to know your requirements
for resolution, travel range, accuracy and physical size.
Linear position sensors include glass scale encoders, magnetic
(Hall Effect) sensors, potentiometers, capacitive gages, LVDTs
(linear variable displacement transducers) and simple limit
switches. Considering the resolution, size and travel range for
your application
quickly narrows the field. For example, a
capacitive gage offers sub-nanometer resolution. However, the
working range is very limited, and resolution decreases as the
travel range increases. This limits its use to select
applications.
Three of the most practical sensors for SQUIGGLE motor
applications are optical encoders, magnetic sensors and
potentiometers.
Optical Encoders
Optical encoders use a light source passing through or
reflecting off of a glass scale with precision etched lines, and
landing on a light detector. The position is measured by
counting the lines. Miniature optical encoders are available
with resolutions as good as 2 μm to 20 μm.
Advantages are the non-contact design; the fact that you don’t
need an A-D converter; and that no EMI or cross coupling is
generated. The main disadvantage is that precision alignment
between the glass scale and detector head is required.
Magnetic Sensors
Magnetic or Hall Effect sensors provide a voltage output that is
directly proportional to an applied magnetic field. The
resolution is related to the length of the magnet and the size
of the A-D converter. Resolution over a 2 mm range can be as low
as 3 μm Since these sensors use permanent magnets, the
position measurement is absolute, and is not lost after a power
interruption. New ASIC sensors are
small enough to integrate with SQUIGGLE motor applications.
Advantages are a non-contact design and the ability to provide
an absolute position measurement. Alignment is simple and
robust, and position resolution is determined by the resolution
of the A-D converter. Disadvantages include the high magnetic
field, and potential cross-coupling of multiple sensors. New
developments such as the AS5305 from austriamicrosystems
overcome these issues.
Potentiometers
Potentiometers use electrical resistance to measure position.
They are similar in both resolution and price to the magnetic
sensors. Resolution of 2 μm has been obtained over a 2
mm distance. Like magnetic sensors, potentiometers offer an
absolute measurement and retain position upon power-up. However,
they require a direct contact connection and create some
additional friction to overcome. There are many suppliers of
such devices.
Advantages include small size and the ability to provide an
absolute measurement with no EMI or cross coupling. The main
disadvantage is the contact measurement technique, which results
in extra friction and wear. The A-D must be at least 12 bits.
Conclusion
Several types of miniature position sensors are readily
available for use with the SQUIGGLE motor. By choosing the right
sensor for you application, you can create a powerful
closed-loop system for precise, repeatable, automated
positioning in miniature devices.
New Scale Technologies’ engineering team works with OEM
customers to select position sensors and integrate SQUIGGLE motor
systems into your next-generation product designs.
Contact us today.
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