August 07, 2006
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.