Shape Memory Alloy (SMA) wire ratchet actuators overcome SMA wire strain limitations by accumulating actuation stroke over multiple cycles. The underlying architecture is effective for producing large strokes from a small package, creating continuous rotation or extended displacement, and precise. It also provides discrete positioning with zero-power hold. While there have been several successful implementations of SMA ratchet actuators, most are designed ad-hoc since limited models exist to predict the stroke and force interaction during actuation cycles. Since the SMA wire actuation is highly dependent on the forces experienced through the ratchet mechanism, a model requires the prediction of the force interaction between the rack and pawl teeth along with friction in the device, and of the external force variation over actuation cycles due to the relative position change between the external system and the SMA wire. This paper presents a model-based systematic design methodology for SMA ratchet actuator which actuates position-dependent external systems. A generalized ratchet mechanism and operation sequence is introduced along with a force balance model for both austenite and martensite equilibrium to address the mechanical coupling changes. Analytical kinematic and kineto-static rack and pawl interaction models are reviewed, which feed into the force balance models. The effective stroke is evaluated by subtracting backlash from the SMA wire stroke, found through equilibrium with the mechanism and external system. This effective stroke accumulates to produce the overall actuator motion. A design methodology is suggested along with visualization methods to aid design decisions. Parametric studies expose the effects of design parameters on the SMA ratchet actuator to gain further design insight. This model-based design foundation and parametric understanding enable the synthesis of SMA wire ratchet actuators.
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