A framework for electromechanical actuator design
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Electromechanical actuators are becoming an increasingly popular alternative to traditional hydraulic actuators for ship, aircraft, vehicle suspension, robotic, and other applications. These actuators generally include an electric motor, gear train, bearings, shafts, sensors, seals, and a controller integrated into a single housing. This integration provides the advantages of a single shaft, fewer bearings, and ultimately, reduced weight and volume. Research has shown that the motor and gear train are the most critical, performance-limiting components in an actuator, and balancing the performance parameters (torque, weight, inertia, torque density, and responsiveness) among them is not trivial. The Robotics Research Group currently addresses this task by using intuitive rules of thumb and the designers’ experience, and this often requires multiple design iterations between the motor and gear train. In this regard, this research will provide preliminary guidelines for choosing the gear ratios and relative sizes of the motor and gear train when integrating a switched reluctance motor (SRM) with three different gear trains (hypocyclic gear train (HGT), star gear train coupled with a parallel eccentric gear train (Star+PEGT), and star compound gear train coupled with a parallel eccentric gear train (Star Compound+PEGT)) in the preliminary design stage. Research has also shown that there are cost benefits to developing actuator product families to meet the needs of a particular application domain. In this regard, scaling rules for the SRM, HGT, PEGT, and integrated actuators built from them (with diameters ranging from 6 to 50 inches and gear ratios from 100 to 450) will be developed. These scaling rules describe how the performance parameters vary as the size (diameter and aspect ratio) is varied and are useful for quickly sizing motor, gear train, and actuator designs. These scaling rules are useful for two purposes: 1) learning the relationships between the performance and dominant design parameters and 2) obtaining intermediate sizes not previously considered. The rules will be simple enough for designers to learn and use to make intelligent design parameter choices (purpose 1) but will also have sufficient accuracy for obtaining intermediate designs (purpose 2). The scaling rules are summarized in a series of three-dimensional design maps, with an emphasis on the development of visual decision-making tools. This research also formulates an actuator design procedure that incorporates the two central concepts of this research, balancing parameters and scaling, and this procedure is embedded within computational (MatLab) and solid modeling (SolidWorks) software programs. In addition to developing rules for scaling and balancing parameters, the procedure was also used for the following purposes. First, direct drive and geared actuators were compared in terms of their torque density and responsiveness to determine which alternative is superior for different gear ratio, diameter, and load inertia combinations. Second, alternative minimum sets of actuators were developed for an illustrative application, and the anticipated performance losses due to using common parameters among the sets were quantified.