Piezoelectric Actuators For Motion Control in Non-Magnetic and Vacuum Environments

Dynamic Structures and Materials (DSM) has expanded its portfolio of customizable FlexFrame PiezoActuators™.

DSM's FlexFrame PiezoActuator™ product family can be custom-configured for unique geometric and weight constraints as well as for demanding environments including extreme temperature, non-magnetic and vacuum requirements. The lack of sliding between internal components eliminates wear and particulate generation. Flexure guidance provides smooth, stiction-free and backlash-free motion for critical positioning applications with no need for lubrication.

Piezo actuators are used over other methods of motion control when any of the following requirements are important: high-precision and/or high-force motion control, travel ranges up to 10 mm, millisecond response rates, zero-wear and friction-free operation, extreme temperature operation, vacuum or non-magnetic environmental compatibility.

Typical applications include precision positioning and scanning applications in the semiconductor, medical device/instrumentation, switching, biotechnology, aerospace and fluid flow control industries.

"DSM sets itself apart by providing clients with a great experience. We know clients want innovative and skillfully engineered solutions. As important, we understand they want ongoing communication throughout the process of creating solutions that meet project objectives within their schedule and budget. Our standard products provide a cost effective way to satisfy many motion control needs and our custom engineering services can be used to assist with integration or used to create entirely new systems," says owner Dr. Jeff Paine.

DSM's full line of standard FlexFrame PiezoActuator™ products can be seen at http://www.dynamic-structures.com/piezo_actuators.html.

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Good Things Come in Ever Smaller Packages

A team of engineers at Dynamic Structures and Materials, LLC (DSM: Franklin, TN) has used MDA SBIR Phase II funding to squeeze an actuator system - a piezoelectric actuator, sensors and associated electronics - into a small package that provides improved control for missile actuation systems relative to baseline electromagnetic actuators. The novel piezoelectric actuator system's features include the use of low-voltage piezo material that is capable of operating in more extreme temperatures than electromagnetic systems.

If incorporated into missiles valve systems, DSM's technology would be used to control the flow of hot gases in miniature kill vehicles. This type of actuator system could also improve the performance of cold flow propulsion systems like those that are used in an astronaut's Manned Maneuvering Unit (MMU) for Extravehicular Activities (EVAs) or "space walks."

A piezoelectric material changes shape when an electrical field is applied. The resulting electric charge in the piezo element causes it to extend in sub-nanometer increments at a minimum and by approximately 70 to 80 microns at a maximum (less than the width of a human hair) in DSM's valve actuator.

Stacking piezo elements adds incrementally to the displacement range, but to achieve significantly more displacement, the team designed a multi-hinged (flexured) metal composite housing - call it an "exoskeleton" - to bind the piezo elements together and mechanically amplify the piezo element's output. DSM has produced a range of valve actuators with mechanical amplification ratios of 5 to 100 times - producing strokes from 100 microns to 10 millimeters. In the MDA valve application, the stroke is proportionally controlled to a fine degree over the range of zero to one and one-half millimeters (0 to 1.5 mm), which is the amount necessary for proportional control of many miniature missile valve applications.

Because of the choice of piezo material, the actuator system doesn't require much voltage: just 60 to 200 volts. In contrast, typical single-crystal piezo materials, which are considered to be "super" types of ceramics, generally require a substantially higher operational voltage. In addition, the lower voltage range used in DSM's actuator systems enables the use of a much broader selection of associated drive electronic components for miniaturization objectives.

But how does this stoked-up piezo stack up against the baseline electromechanical actuators already in use in missile systems? Compared to electromechanical actuators, DSM's product also has a power advantage.

"We've learned from users in the field that electromechanical actuators have a couple of drawbacks such as backlash and overshoot which can lead to slower move and settle times," Murray Johns, DSM's vice president, explained. "Traditional electromechanical systems require up to 10 to 20 milliseconds to move and settle into position, while we've shown our piezo systems require less than 5 milliseconds," according to Johns. "Moreover, electromechanical systems use a significant amount of power during hold maneuvers to maintain an electric field and, thereby, to hold position. The capacitive nature of the piezoelectric load means that our actuators do not use any power to hold position," Johns said.

Its composition and features also ensure that the actuator can withstand extremely low temperatures. At the opposite end of the temperature scale, the actuator is equally impressive. For example, although a standard piezo material starts to lose its piezoelectric properties above 100 C, the material and electrical connections used in DSM's design help it to operate reliably at up to 250 C.

The most difficult part of building the technology was reducing the size of the system - the actuator, drive electronics and sensor packaging size - to offer higher power density (power per unit mass) relative to electromagnetic systems. Simultaneously, DSM's efforts have focused on increasing the technology's "Technology Readiness Level" for future insertion into MDA platforms. The team has yet to perform hot gas testing but has performed cold flow testing to simulate the application and achieved very stable results, Johns said.

DSM is continuing to pursue its unique miniaturization process on several fronts including material selection and system stiffness/control analysis. "We would also like to conduct hot-flow high-fidelity testing on a missile platform test bed in Phase III of our development process," Johns added.

The company, which is focusing largely on the piezo system's military and space-related applications, has also obtained additional funding from commercial sources to continue developing these technologies for related applications.

DSM Releases New Miniature Piezoelectric Actuators for Weight-Sensitive and Space-Sensitive Applications

New product announcement: Dynamic Structures and Materials, LLC (DSM) is pleased to introduce two ultra-compact piezoelectric actuators or stages for weight-sensitive and space-sensitive applications. These flexure-guided mechanisms are new additions to DSM's line of piezoelectric positioning actuators, which currently offers a range of motion from 10 microns to 10 mm. The use of flexure guidance enables motion that is free of backlash and friction.

The FPA-180E piezo actuator provides 180 microns of displacement range in a 6 gram titanium package of less than 8 x 9 x 26 mm in size. The stiffness of the FPA-180E is 0.39 N/micron, and the device offers a fast dynamic response through an unloaded natural frequency of 1325 Hz.

The FPA-80E is a slightly different stage construction of stainless steel that measures 8 x 10 x 17 mm in size. The corresponding stroke, stiffness, and unloaded natural frequency are 80 microns, 0.5 N/micron, and 1700 Hz, respectively.

Similar to DSM's other piezoelectric actuators, the FPA-180E and the FPA-80E's construction materials are compatible with clean room and vacuum environments. This type of preloaded, flexure-guided actuator design is readily adaptable into a variety of applications including miniature valves, optical switching and scanning, and nanopositioning. DSM's actuators are typically supplied with piezo material rated for -30/150V operation, but other materials and custom designs are available upon request.

Brochures are available upon request, and DSM's full line of standard piezo actuators can be seen at www.dynamic-structures.com/piezo_actuators.html.

About DSM:
DSM specializes in the design, development, integration, and support of micropositioning, nanopositioning, and robotic positioning devices and systems for precision motion control applications in the semiconductor, aerospace, medical, and research industries. DSM's core competencies in the design and integration of specialty piezoelectric actuators and stages, piezo amplifiers and drivers, and robotic platforms enable innovative positioning solutions for unique applications.

Custom Multi-Axis Optomechanical Positioning Mounts

Design service announcement: Dynamic Structures and Materials, LLC (DSM) offers custom design services for single- and multi-axis positioning hardware used in optomechanical assemblies. DSM's expertise in flexure-guided mechanisms for nanopositioning motion stages establishes a foundation for the design of positioning hardware in either manually adjustable or motorized forms. The use of solid-stage flexures ensures smooth, continuous motion in either linear or rotary applications that can carry large loads. For example, the system pictured here provides 2mm of linear motion in one axis and +/- 2 degrees of rotational motion about two additional axes. The construction materials used in this particular system are aluminum and titanium alloys. DSM welcomes custom design requests, and examples of DSM's mechanism design work can be seen at www.dynamic-structures.com/systems.html.

Miniature Titanium Piezoelectric Actuator

New product announcement: Dynamic Structures and Materials, LLC (DSM) is pleased to introduce an ultra-compact piezoelectric actuator for weight-sensitive and space-sensitive applications. The FPA-125 piezo actuator provides 125 microns of displacement range in a package geometry of less than 8mm x 8mm x 26mm that weighs less than 6g. Similar to many of DSM's other flexure-based piezoelectric actuators, this model's construction materials are compatible with clean room and vacuum environments. The FPA-125 features a stiffness of 0.4 N/micron and offers fast dynamic response through an unloaded resonant frequency of 1300 Hz. Designed for aerospace requirements, this piezo actuator is readily adaptable into a variety of applications including miniature valves, optical switching and scanning, and nanopositioning.

A brochure is available upon request, and DSM's line of piezo actuators can be seen at www.dynamic-structures.com/piezo_actuators.html.

Flexure-guided nanopositioning z-stage

Piezo positioning system maker Dynamic Structures and Materials, LLC (DSM) has introduced the flexure-guided CZSA-1000 nanopositioning z-stage featuring a full 1 mm of closed-loop servo motion control.

The stage's compact 25 mm height and 62.5 x 125 mm footprint facilitates integration of the stage into scanning, metrology, inspection, and other nanopositioning applications. The parallel flexure design ensures minimal roll and tilt of the output stage despite the industry-record 1000 µm travel range, and the mechanism's stiff kinematic design ensures stable dynamic responsiveness and position control.

Together with an integrated capacitive probe displacement sensor, the CZSA-1000 stage and DSM's servo piezo controllers provide full control of the system's motion profile for fast acceleration, deceleration and constant velocity scans. RS-232 communication and LabVIEW drivers enable ease of use and integration of the piezo controllers into laboratory test set-ups and OEM systems.

STOP! What's that light at the end of the tunnel?

The safety of after-dark driving depends partly on the retroreflective property of road signs, pavement markings, and other traffic control devices. However, measurements of this property, which is the materials' ability to bounce light back from a vehicle's headlights, have varied by as much as 20 to 40 percent between available instruments and from laboratory to laboratory. These variations and the lack of national calibration standards for retroreflectivity can lead to difficulties between signage manufactures and buyers representing state departments of transportation.

In order to resolve these variations and to establish industry standards, the National Institute of Standards and Technology (NIST) has established the Center for High-Accuracy Retroreflection Measurements. The heart of the new center is a dedicated reference instrument - a high-precision, six-axis goniometer designed by Dynamic Structures and Materials, LLC ("DSM") and installed at NIST's Gaithersburg, MD facility. The goniometer provides closed loop control of six independent degrees of freedom and travels 30m on a precision rail assembly. The system's robust design is large enough to accommodate actual road signs such as the octagonal STOP sign used in the U.S. (approximately 1 m diameter).

A 50m black tunnel houses the goniometer to eliminate stray illumination that could interfere with researchers' measurements. During experimentation, light is projected from a source (representing a headlight) onto a test object such as a road sign or pavement marking. The light is retroreflected to a photometer, which measures the amount of returning light (simulating what a vehicle's driver might see). Measurements in the facility are traceable to the candela, a SI unit maintained by NIST.

The specifications for high angular resolution and accuracy in the goniometer's operation resulted from the desire to use the equipment as a research instrument as well as a calibration device. Vertical deviation of the goniometer system's center over the 30m rail length is within ±0.75mm, and horizontal deviation of the center is within ±1.0mm. Final system specifications are tabulated below:

As part of the system's post-installation characterization, NIST determined that for any one axis move, the system's sphere of confusion is an ellipsoid of dimension 0.35 mm in the vertical direction and 0.12 mm in the horizontal direction.

Researchers at a remote PC control the motion of the system's axes through custom software written under National Instruments' LabWindows. MXI-3 technology, a PCI master/slave system, is used to couple the remote PC via a fiber optic data link running the length of the hallway to a National Instruments PXI-1002 chassis incorporated into the goniometer's structure. The PXI chassis also houses two NI PXI-7334 stepper motor motion cards and a NI PXI-8421/2 card to provide an interface for RS-485 communication with the system's 30m linear encoder.

DSM also incorporated an enclosure in the system's structure to protect the stepper motor drives and two NI UMI-7764 Universal Motion Interfaces. The UMI boxes provide connections for step and direction signals from the motion controllers to the stepper motor drives as well as connections for the majority of the position encoders. E-stop switches installed on the goniometer frame in easy reach of any bystander are routed to relays that disable power to the system's motors.

The accuracy of the goniometer's motion control system over such large motion ranges is made possible through the use of high-end motion components and sensors. Five-phase Vexta Nanostep® CFKII 569 stepping motors were chosen to produce precision motion for the three rotational axis of the goniometer, while two-phase Vexta CSK 268MAT stepping motors drive the linear axes of motion. When set at the smallest step angle, the five-phase Vexta stepper motors have 125,000 steps per revolution. DSM successfully coupled the stepper motors to high accuracy harmonic drives with a 160:1 gear reduction that yielded a potential resolution of greater than 20 million steps per revolution. Using rotary encoders to provide position feedback, actual "closed-loop" minimum step size for the three rotational axes was less than 0.0002 degrees or 1.8 million steps per revolution.

DSM selected HD Systems harmonic drive gear reducers to couple with the stepping motors. The harmonic drives provide a 160:1 single stage gear reduction in a very small package. The HD systems CSF-2UH gearheads have virtually zero backlash and come with built in roller bearings to support the output shaft. The HD harmonic drives provided dramatic increases in stepper motor holding torque to control the rotation of the large goniometer support frame with authority.

Each axis of the goniometer is monitored by an encoder and limit switches. The limit switches were incorporated into the frame to protect each axis against overtravel by disabling signals to the respective axis' motor. The encoders selected for the three rotary axes are from the Mercury 2000 family of high precision encoders from MicroE Systems, Inc. The optical encoders use glass-scales with interpolator electronics that enable up to 4.19 million counts per revolution. MicroE Systems precisely mounted the glass scales to DSM's custom-designed encoder hubs. The encoders' small read heads were easily incorporated into the goniometer's structural design, and their robust tolerance to misalignment made adjustments during installation fast and simple.

With the aid of the new goniometer system's capabilities, NIST researchers hope to achieve agreement among retroreflectivity instruments and to make federally-mandated standards more precise.