In this study, mechanical design and control of a novel parallel elastically actuated (PEA) legged robot are presented. Motion under analysis is limited to vertical apex to apex hopping. Robot is composed of a symmetric four link mechanism as the leg, a brushless direct-drive DC motor and a wrapping cam with extension spring. Controller is based on templates (the simplest model) and anchors (more realistic model) scheme, where the template is the Spring Loaded Inverted Pendulum (SLIP) including a viscous damper which is virtually tunable. For a desired apex, required damping constant is calculated to provide necessary energy to SLIP from an approximate analytical map. Template motion is realized in the anchor model by equating its dynamics to the template dynamics through torque control to equate energy inputs and a wrapping cam to equate potential energies. During the motion, a string is wrapped around a cam by relative motion between two links of the four link mechanism. The string pulls the spring and creates a nonlinear elongation function. Desired elongation is obtained from the required template potential energy and the necessary cam profile is calculated analytically. Thus, a linear compression spring is realized with a tension spring with cam. Static force experiments are performed to show that cam works as desired. Overall simulations and details of mechanical design are presented. This novel PEA robot architecture provides an accurate and energy efficient solution with a simple mechanical design.

Spring Loaded Inverted Pendulum (SLIP) is a simple, descriptive and accurate model to study dynamic legged locomotion. Critical design decisions to realize SLIP based legged robots with high energy efficiency and control accuracy are actuation topology and controller. Recent studies converge on series elastic actuation (SEA) and parallel elastic actuation (PEA) regarding actuation whereas, a recently introduced control method, virtual tuning of damping (VTD) have proven to be superior for SEA over other control techniques. However, actuation topology is still under discussion and it is a highly coupled problem with the control approach. In this study, vertical hoppers with PEA and SEA configurations are compared under VTD controller to determine the one results in better energy efficiency and accuracy. PEA and SEA models are extended with drive-train details to provide more realistic results. Models are simulated with various gearboxes and motors to understand their effects. Comparisons among optimum topologies showed that VTD-PEA achieves 0.02% percent apex error where VTD-SEA achieves 0.5% apex-to-apex accuracy (25 times). VTD-PEA also achieved 40% better energy efficiency and 38% higher cost of transport than VTD-SEA.

Store separation analyses are carried out in order to estimate the safe separation envelope for external stores carried on military aircrafts by varying the flight/ejection conditions. Typical separation analysis considers either only the store motion beyond the end-of-stroke (EoS), or solves the EoS conditions with predefined ejection forcing that are assumed to be unaltered with respect to flight/ejection conditions. There exists a gap in the literature in modeling the EoS conditions and precise ejection loads. Data from ground and flight tests show that, dynamic responses of a store differ beyond acceptable limits where aerodynamics and aircraft elasticity are the two main sources of this miscorrelation. In order to have a reliable mathematical model that modifies the ground test data to have a reasonable correlation with the real ejection case, not only ejector and store dynamics but also store aerodynamics, aircraft aerodynamics, aircraft rigid body dynamics, aircraft elasticity should be considered. To show the effects of aircraft deformation on the EoS store conditions, first of all, a methodology that can be used to estimate the ejection loads and EoS conditions of the store more precisely is presented. For the purpose of visualizing the aeroelastic effects on store ejection dynamics, various virtual test cases are handled by changing wing torsional stiffness values and store mounting station positions along aircraft longitudinal axis. The acceleration responses of the store, obtained with and without the inclusion of aeroelastic effects, are used to emphasize the effects of aeroelasticity on ejection.

Certification process is one of the crucial procedures for safety in the design of a new aerial platform. Flight flutter testing is the most critical component for the certification process. Usually a flutter analysis is performed beforehand for the planning of flight flutter testing of an aircraft which mostly requires the Finite Element Model (FEM) together with Ground Vibration Testing (GVT) to construct the structural dynamic model of the complete aircraft for the flutter analyses. GVT is not only required for new aircraft design but also when considerable changes are made to an existing aircraft or when new external load configurations are introduced. Experimental methods require high effort, high budget, long time, and much repetition. Therefore, the computational and theoretical studies seem more applicable in the early phase. However, GVT of an available fighter aircraft in defense projects becomes an issue for the designers if a detailed FEM of the aircraft is not available prior to test. Hence, planning of the GVT in early stage is vital for project leaders. In this study, a rough FEM of a fighter aircraft is developed and correlated to available GVT data for planning purpose. The representative mode shapes are evaluated by estimation of the several sections of the aircraft. It is also shown that a rough FEM of the aircraft can be utilized for determination of the measurement and excitation points on the aircraft in planning stage. The geometrical properties, physical limitations and basic requirements of GVT are also taken into account for an efficient planning.

A dynamically dexterous legged robot has the distinct property that the legs are continuously interacting with the environment. During walking and running, this interaction generates acoustic signals that carry considerable information about the surface being traversed, state of the robot legs and joint motors as well as the stability of the locomotion. Extracting a particular piece of information from this convolved acoustic signal however is an interesting and challenging area of research which we believe may have fundamental benefits for legged robotics research. For example, the identification of the surface that the robot travels on gives us the ability to dynamically adapt gait parameters hence improve dynamic stability. In the present paper, we investigate this particular sub-problem of surface identification using naturally occurring acoustic signals and present our results. We show that a spectral energy based feature set augmented by time derivatives and an average zero crossing rate carries enough information to accurately classify a number of commonly occurring indoor and outdoor surfaces using a popular higher dimensional vector quantizer classifier. Our experiments also suggest that VQ surface models may be velocity dependent. These initial results with a carefully collected but relatively limited dataset indicate a promising direction for our future research on improving outdoor mobility for dynamic legged robots.

This paper discusses the application of Castigliano’s Theorem to a half circular beam intended for use as a shaped, tunable, passively compliant robot leg. We present closed-form equations characterizing the deflection behavior of the beam (whose compliance properties vary along the leg) under appropriate loads. We compare the accuracy of this analytical representation to that of a Pseudo Rigid Body (PRB) approximation in predicting the data obtained by measuring the deflection of a physical half-circular beam under the application of known static loads. We briefly discuss the further application of the new model for solving the dynamic equations of a hexapod robot with a C-shaped leg.