Power consumption issues, however, show a clear bias in favor of the use of discrete actuator concept,  when possible, over any continuously deflectable device. When comparing power required to hold a given  actuator position, the discrete actuation concept requires none, while a conventional servo-based (motor  driven) actuator will require steady currents of roughly comparable levels. This is in large part due to the  fact that static servo holding stiffness operates the underlying electric motor at near stall torque levels, a  very inefficient regime for these electromechanical components. Protracted operation holding a given  deflection would impose a significant power drain on the HALE aircraft, limiting its endurance.  Two design variants were investigated, where the wire was electrically actuated at each end with a  potential applied across the entire wire length, and a second where the wire was grounded on both ends  and an additional electrical connection was made at the center of the SMA wire. Initial work with the center  actuated design revealed that adjustment of wire tension through the height change of the center  actuation point was difficult to achieve repeatedly. Subsequent work with the end-point actuated tab  system showed that it suffered in being able to repeatedly actuate the system without shearing off the ball  end fittings. Thus, the final configuration realized in this design process was a hybrid design that  incorporated features from both the center point actuation design and the end point actuation system.  Shown in Figure 11 is the dual tab assembly, where two SMA wire-actuated discrete tabs are mounted to  a “sandwich” of printed circuit board, with the two tensioning straps having a triangular shape added to  promote the snap-through effect of the base tab when the wires are tensioned. This tab design also  incorporated customized screw-adjustable crimps at the inner edge of the tab assembly, permitting rapid  wire tensioning for optimum tab performance.  Figure 11. Final Phase I configuration for dual discrete tab system.  Electronic lntewation Desiannestinq  Electronics and software design for controlling the groups of individual tab actuators has been investigated  as part of the integration work, since the collection of multiple tabs would be interfacing to a single  standard servo connector for both the flap and the aileron assemblies for applications to both CDl’s R/C  airplane and the APV-3 wing. While having multiple tab actuators provides a means of mechanical  redundancy for providing trailing edge control, both signals and power must be supplied to all to make  them function as a group. Brute force approaches for directly wiring each and every tab actuator to a  central controller would be unacceptably heavy and complex, due to the massive amount of wire required  for providing current to each individual SMA wire. Instead, electronics incorporating extremely small  microcontrollers is being used for a serial-bus network that determines which actuator to energize and for  how long. The target microcontroller is the Microchip PIC1 2F675 family, an 8-pin SO-8 surface-mounted  device that has an internal clock and is reprogrammable in-circuit via FLASH memory.  Two schemes were investigated for localized control and communication on this serial bus network for  interconnected tab actuators. The first involves the direct device addressing from a “master”  microcontroller for actuation, whereby a master/slave(s) network is constructed with the master requesting  382