The implementation of smaller, lighter, and more agile military systems requires new actuation technologies that offer high power density in compact form factors. The Compact Hybrid Actuator Program (CHAP) is pursuing active material based, rectifying actuators to create new actuation solutions for these demanding applications. Our actuator approach is based on thin film NiTi membranes operating in parallel (high intrinsic power density, >125 kW/kg) combined with liquid rectification, MEMS passive check valves, and commercially available power electronics. Previous results demonstrated 8 micron thick membrane actuation with 150 Hz forced convection response and force output of 100N. This paper focuses on two developments critical in scaling up previous single membrane results to power levels sufficient for military applications. This first is the development of SOI MEMS fabricated microvalve arrays which exhibit high flow rate at high frequencies. The second focus area is the design, fabrication, and assembly of a form factor compact actuator. The initial prototype demonstration of this concept shows great promise for thin film NiTi based actuation both in military technologies and in other areas which demand extremely compact actuation such as embedded fluid delivery for biomedical applications.
In this paper, a thin film nickel-titanium (NiTi) shape memory alloy (SMA) was used to develop a prototype compact hybrid actuator. SMA was selected as an actuating mechanism because it had the highest work density among active materials. Combining this attribute with high frequency response of thin films resulted in large power output. High drive frequency was also possible in part from manipulating the liquid flow to directly cool the SMA membranes. The actuator reached a drive frequency of 100Hz while producing 2.6Watts. The results indicated that power output is linearly related to the drive frequency since the volume flow rate increased proportional to frequency.
The objective of our Compact Hybrid Actuator Device (CHAD) program is to produce a novel, ultra-compact, high force actuator to meet the aggressive requirements for navigation, guidance and control of a compact missile as well as other military and commercial applications confronted with tight volume constraints. Our approach to this challenge uses the high power density of thin film shape memory alloys coupled with fluid rectification and commercial power electronics. Phase One of our program demonstrated the performance of critical technical elements in a non-compact form factor. NiTi films were reproducibly deposited and then fabricated into bubble actuators that demonstrated ≥ 100 Hz performance when forced convection heat transfer to a liquid was optimized. Increased efficiency in thermal activation was achieved through high Joule heating rates for short duty cycles; this allowed simplification of the power electronics. These technical elements were combined to produce a thin film SMA pump which ultimately demonstrated force outputs on the order of 250 N and average power densities on the order of 50 W/kg when operated at 100 Hz. The demonstrated performance shows great promise for applications requiring ultracompact form factors with high output force.
This paper describes the development of a micro-machined passive check valve for an SMA-based compact hybrid actuator device (CHAD). The overall diameter of the valve is 12 mm and the thickness is 1 mm. The structure houses an array of 56 micro check valves. Each micro valve has a 250 μm diameter orifice covered by 10 mm thick nickel flap. Stoppers on each micro valves limit the displacement of the flaps during an opening. This design allows the Ni flaps to withstand high-pressure gradient created by the actuator while achieving high flow rate. A finite element analysis is used to characterize the static and dynamic behaviors of the valve flap for the prediction on flow rate. The prediction is found to be in good agreement with the experiment on static flow rate. The test results indicate that the flaps are able to withstand pressure difference of 0.28 MPa while achieving flow rate of 20 cc/sec. The valve also has low cracking pressure and reverse leakage.
In this paper, a prototype SMA-based actuator for a compact kinetic energy missile was fabricated. Thin film nickel-titanium was selected as an actuating mechanism because it exhibited high power density compared to other smart materials. This study represents a proof of concept that high drive frequency and high power density can be both achieved with thin film SMA. The thin film reached a drive frequency of 80Hz while achieving a power density of 27900 Watts/kg. As for the pump, the power density was 2.93 Watts/kg, but obtaining higher value can certainly be achieved by reducing the chamber weight through optimization. In any case, CFD analysis revealed that the pump chamber had to be redesigned to change the flow profile because the present design created non-circulating dead zones immediately adjacent to the diaphragm. Therefore by redirecting the liquid flow to directly cool the SMA diaphragm improved the heat transfer and thus improved performance of the actuator can be achieved.