as well, when an optional tip-force sensor is incorporated.
Piezoelectric actuators represent a large segment of
linear actuators. They can achieve extremely fine positioning
resolution and come in various types:
Piezo Stack Actuators are layered structures of specialized
ceramic interleaved with metallic electrodes. The piezoceramic
can expand in a controllable manner with the application of an
electrical field. These actuators provide short travel ranges
(about 1 percent of their length), sub-nanometer precisions,
high forces and sub-millisecond response. These are the
mainstay of today’s advanced nanotech applications, both
in laboratory research and in industrial applications such as
semiconductor manufacturing and genomic sequencing. Piezo
stack actuators are inherently non-magnetic, solid state, and
vacuum-friendly, with no wear processes.
Ultrasonic Piezomotors are monolithic piezoceramic
structures that are stimulated at their resonant frequency,
typically above 100 kHz, causing them to flutter on a submicron
scale. A friction tip formed or bonded at a resonant node
conveys this fluttering oscillation to a work piece that rides in
bearings. The work piece thereby experiences a force that
drives it one direction or the other. These motors can achieve
many millimeters of travel and extraordinary speeds in a very
Inertia Drives are another type of piezomotor that uses tiny
piezoceramic elements are actuated in a saw tooth pattern,
driving a shaft or other actuated element via a friction coupling.
The sloped portion of the saw tooth actuation is what provides
the motion; the rapid retraction breaks the stiction of the
coupling and the actuated element does not retract with the
piezo ceramic element. A well-design inertia drive can achieve
silent, virtually stepless operation and long travels, together with
nanoscale precision, and self-locking for high stability when
Walking Piezomotors use four or more piezoceramic fingers
which actuate in a stepping sequence to drive a workpiece
in a desired direction (see Figure 2). Between steps, sub-
nanoscale actuation can be achieved. High power-off
holding forces and essentially unlimited travel characterize
these designs. The usual non-magnetic and vacuum-friendly
attributes apply. These have proven to be enablers in sensitive
optical positioning applications where carefully established
positions must be maintained with nanometer stability..
Electromechanical Actuators represent another category
of actuators. These are typically based on linear shafts driven
by rotational electromagnetic motors via lead nuts or ball nuts.
Rotary motion of the motor is converted to linear displacement.
The actuators have a generally cylindrical format. Small
versions are used to replace micrometers or precision screws,
conferring automated actuation.
Electromechanical actuators typically employ either stepper
motors or DC servomotors. Stepper motors actuate a toothed
rotor within a toothed, surrounding stator. The most common
type – the permanent magnet stepper motor – uses a rotor
composed of a magnetized material. By configuring the
magnetic windings of the stator so that groups of its teeth can
be specifically magnetized, the rotor rotates in steps by partially
energizing the windings. Consequently, a driving mode that
yields mini- or microsteps can be implemented, multiplying the
stepping resolution of the motor.
DC servomotors employ a magnetized rotor within a
magnetized stator, both of which have a north and a south
pole. The poles of each are attracted or repelled to each
other, causing rotation to an equilibrium orientation. By
varying the magnetization of the rotor or stator, or both
electromagnetically, such as by switching their polarities using
a brushed or electronic commutation approach, the motor
can be made to spin freely and, with the addition of a position
feedback encoder, provide precise positioning with exceptional
responsiveness. Brushless DC motors, with electronic
commutation rather than brush commutation, provide enhanced
lifetime, especially in high-dynamic applications.
In both cases, the motors can be operated open- or closed-loop.. A stepper motor can be actuated through any specified
number of steps in either direction and offers a high probability
of achieving them, though certainty can only be achieved by
adding a position encoder. Rotary encoders track the position
of the rotating motor; linear encoders directly encode the
output position of the driven linear shaft, eliminating backlash
and other errors that might otherwise accumulate in the
drivetrain where a rotary encoder cannot observe them.
A linear or translation stage builds on the principles of a
linear actuator, but adds a platform or workpiece for attaching
an application load, or for stacking additional stages to form a
multi-axis configuration. The stage’s workpiece is a precision
component with a linear bearing for guidance.
A linear translation stage restricts the application load
to a linear single degree of freedom. An ideal linear stage
completely restricts three axes of rotation and two axes of
translation, thus allowing for motion on only one translational
axis. In reality, there is no perfect guiding system, and every
Figure 2. Walking piezomotors have four or more ceramic fingers that actuate
in a stepping sequence to drive a work piece in the desired direction.