upper SMA wire (stretched)  I trailing edge  snapped-through overhang  configuration  lower SMA wire (contracted)  Figure 13. One-dimensional example of a “snap-through” SMA-driven actuator, showing pinned  constraints and optional overhang.  The actual work performed by this type of actuator as prime mover can come from several sources:  0  0  0  0 the linear motion of the center of the buckled beam may be used to drive a linkage between two  different locations;  the change in slope of the end points of the beam may be incorporated as a form of angular  orientation or camber control;  the snap-through in one direction may influence the convex or concave shape of a doubly-curved  planar surface (such as on the trailing edge tab device above); or  the buckled structure may induce relative forces and moments between two disparate locations  on a flexible base structure (particularly if the base material has cantilever end constraints).  A simple comparison with a notional miniature linear actuator for aerospace use [I] reveals this concept’s  considerable advantages over motor-driven mechanisms. If one matches the end-point stroke and  displacement of the device of [l] using the linear motion of the snapping arch shown above, one realizes a  weight reduction of 91%, since the pre-stressed snap-through element performs the same useful work as  a stepping motor/ball screw combination. This weight savings is effectively due to the replacement of a  motor-driven system with a “reversible actuated spring” unit. Of course, similar devices may be combined  in series or in parallel to provide complex, large deformations in multiple directions as a complete  mechanism system. Several examples will be sketched below to better visualize these configurations.  Figure 14 shows a method for forming a bi-stable actuator from a flat plate that produces a change in  slope between its two ends. The center portion of the stamping comprises the buckled member, while the  two side strips, when joined as shown, provide the geometric constraint that keeps the center piece pre-  stressed. SMA wires mounted on the upper and lower surface of the buckled member are used to “pop”  the device between the two possible stable positions. Figure 15 shows a three-position actuator, similar to  that in Figure 14, but uses two nesting pre-stressed members that are cantilever-mounted on either side  of a cut-out rectangular base support. By snapping one or the other pre-stressed arches through the cut-  out area, one can generate three different slopes between the ends of the base support, or equivalently,  an up-neutral-down type of output for the end displacement (Figure 16). Simple extensions to that  mechanism, ones that include other nested buckled members above and below these two, provide even  higher “resolution” of the relative angles between the two end points of the buckled members.  Combinations of these three-or-more-position actuators distributed across a planar surface could provide  complex warping of the base plate or planar surface to provide localized camber and shape control.  Several actuators ganged in series would produce additive changes in base material slopes, or, very large  end displacements (Figure 16). This large motion response is possible even while limiting the strain of the  SMA wires to less than 3%, a level that is suitable for millions of cycles of operation.  384