Mahdi Agheli

Study of the Foot Force Stability Margin for multi-legged/wheeled robots under dynamic situations

Stability analysis of multi-legged and wheeled robots is necessary for maintaining control of the robot especially under dynamic situations. This paper studies the use of the Foot Force Stability Margin and a modified extension of the Foot Force Stability Margin under dynamic situations. The Foot Force Stability Margin and its modified variant are applicable to all types of multi-legged and multi-wheeled robots under both static and dynamic situations. To facilitate the numerical validation of the stability margin under dynamic conditions, a foot force distribution technique is presented. The simulation results confirm that under dynamic situations, the Foot Force Stability Margin and its modified variant are accurate, simple in terms of calculation cost, and sensitive to top-heaviness and the geometry of the robot, making it practical for use within on-line controllers.

Hierarchical Kinematic Design of Foldable Hexapedal Locomotion Platforms

Origami-inspired folding enables integrated design and manufacturing of intricate kinematic mechanisms and structures. Here, we present a hierarchical development process of foldable robotic platforms as combinations of fundamental building blocks to achieve arbitrary levels of complexity and functionality. Rooted in theoretical linkage kinematics, designs for static structures and functional units, respectively, offer rigidity and mobility for robotic systems. The proposed approach is demonstrated on the design, fabrication, and experimental verification of three distinct types of hexapedal locomotion platforms covering a broad range of features and use cases.

Closed-Form Solution for Constant-Orientation Workspace and Workspace-Based Design of Radially Symmetric Hexapod Robots

The workspace of hexapod robots is a key performance parameter which has attracted the attention of numerous researchers during the past decades. The selection of the hexapod parameters for a desired workspace generally employs the use of numerical methods. This paper presents a general methodology for solving the closed-form constant orientation workspace of radially symmetric hexapod robots. The closed-form solution facilitates hexapod robot design and minimizes numerical efforts with on-line determination of stability and workspace utilization. The methodology can be used for robots with nonsymmetric and nonidentical kinematic chains. In this paper, the methodology is used to derive the closed-form equations of the boundary of the constant-orientation workspace of axially symmetric hexapod robots. Several applications are provided to demonstrate the capability of the presented closed-form solution in design and optimization. An approach for workspace-based design optimization is presented using the provided analytical solution by applying an iterative optimization algorithm to the find optimized structural parameters and an optimized workspace.

Lateral reachable workspace of axially symmetric mobile machining hexapod robots

Mobile machines allow for remote repair and maintenance within constrictive, hazardous, and inaccessible environments. Hexapod robots are a salient solution to mobile machines due to their maneuverability and controllable orientation. One important aspect of mobile machines is the reachable workspace of the currently loaded tooling. Generally, machining operations are restricted to planar motion operating within the lateral plane of the mobile machines. This paper introduces an analytical methodology for finding the lateral reachable workspace of an axially symmetric hexapod robots used as mobile machining systems. The closed-form solution to the lateral reachable workspace of axially symmetric hexapod robots is facilitated by the correlation of the lateral motion of a hexapod robot with an equivalent 2-RPR planar parallel mechanism. Since the solution is closed-form, it has high accuracy and reliability.

Foot Force Based Reactive Stability of Multi-Legged Robots to External Perturbations

Many environments and scenarios contain rough and irregular terrain and are inaccessible or hazardous for humans. Robotic automation is preferred in lieu of placing humans at risk. Legged locomotion is more advantageous in traversing complex terrain but requires constant monitoring and correction to maintain system stability. This paper presents a multi-legged reactive stability control method for maintaining system stability under external perturbations. Assuming tumbling instability and sufficient friction to prevent slippage, the reactive stability control method is based solely on the measured foot forces normal to the contact surface, reducing computation time and sensor information. Under external perturbations, the reactive stability control method opts to either displace the CG or the foot contacts of the robot based on the measured foot force distribution. Details describing the reactive stability control method are discussed including algorithms and an implementation example. An experimental demonstration of the reactive stability control method is presented. The experiment was conducted on a hexapod robot platform retrofitted with a tiny computer and force sensitive resistors to measure the foot forces. The experimental results show that the presented reactive stability control strategy prevents the robot from tipping over under external perturbation.

Design and fabrication of a foldable hexapod robot towards experimental swarm applications

This paper presents the development of a lightweight origami-inspired foldable hexapod robot. Using a single sheet of polyester and a laser cutter, the hexapod robot can be fabricated and assembled in less than one hour from scratch. No screw or other external tools are required for assembly. The robot has built-in polyester fasteners considered in its crease pattern. The design uses four-bar mechanisms, which makes the robot flexible to be adjusted for different speeds or other task metrics. For a given desired locomotion velocity, various parameters of the four-bar mechanisms in the crease pattern can be modified accordingly. Design flexibility, ease of fabrication, and low cost make the robot suitable as an agent for swarm objectives. This work presents the foldable hexapod design and its kinematic analysis. The robot is fabricated, assembled, and tested for functionality. Experimental results show that the robot prototype runs with a maximum forward speed of 5 body lengths per second and turns in place with a speed of 1 revolution per second. The final robot weighs 42 grams.

Closed-form solution for reachable workspace of axially symmetric hexapod robots

The reachable workspace of hexapod robots is a key performance parameter in robot design. This paper presents an analytical solution for the reachable workspace of axially symmetric hexapod robots. The solution methodology decomposes the hexapod robot into three intersecting modified four-bar mechanisms. The contribution of each modified four-bar mechanism to the overall workspace is found and combined to form the reachable workspace. The presented methodology for finding the analytical solution to the reachable workspace is extendible to other types of workspaces, hexapod robot geometries, and applicable to any n-pod robot. An example is provided to mathematically illustrate the presented methodology.

Force-based stability margin for multi-legged robots

Stability analysis of multi-legged robots is necessary for control especially under dynamic situations. This paper presents the Foot Force Stability Margin, a force-based stability margin that utilizes measured contact normal foot forces as the stability metric, simplifying data and computational requirements. The Foot Force Stability Margin assumes tumbling instability and sufficient friction to prevent slippage. A modified extension of the Foot Force Stability Margin is provided to enhance stability sensitivity to desired robot characteristics. The Foot Force Stability Margin and its modified variant were validated through numerical and physical experiments. The numerical simulations compare the Foot Force Stability Margin with its modified variant, the Force Angle Stability Margin, and the Zero Moment Point. The physical experiments were conducted using a Lynxmotion hexapod robot retrofitted with a Gumstix and force sensitive resistors. The experimental results confirm that the Foot Force Stability Margin and its modified variant are accurate, simple with regard to computational cost, sensitive to robot characteristics, and applicable to flat and uneven terrain, making it practical for use within on-line controllers. FFSM stands for Foot Force Stability Margin, a new force-based stability criterion for legged robots.FFSM has been modified (MFFSM) to adjust stability sensitivity of the robot.FFSM/MFFSM can be used to determine system stability in real-time usable for stability control.FFSM and MFFSM are compared with the Force Angle Stability Margin, and the Zero Moment Point.Both simulation and physical experiments validate the proposed stability margin (FFSM and MFFSM).

Lateral Stable Workspace of Hexapod Walking Machines With Constant Orientation Platform

Due to the importance of the workspace and stability in mobile robot dynamic control, a variety of workspace and stability criteria exist in the field of multi-legged and wheeled robotics. This paper presents a methodology for determining the stable workspace, the subspace of the workspace for which the system is considered stable. The presented derivation utilizes the normal foot force distribution of the system to determine stability and integrates the stability into the lateral workspace of a mobile machining hexapod robot. The analytical inequalities governing the boundary of the stable workspace are derived. A discussion on the effects of physical and geometrical characteristics of the hexapod robot on the stable workspace methodology is given. The stable workspace methodology is validated through a simulation and an application to mobile machining is presented.Copyright

Theoretical Modeling of a Pressure-Operated Soft Snake Robot

This paper addresses the theoretical modeling of the dynamics of a pressure-operated soft snake robot. An accurate dynamic model is a fundamental requirement for optimization, control, navigation, and learning algorithms for a mobile robot that can undergo serpentine locomotion. Such algorithms can be readily implemented for traditional rigid robots, but remain a challenge for nonlinear and low-bandwidth soft robotic systems. A framework to solve the 2-D modeling problem of a soft robotic snake is detailed with a general approach applicable to most pressureoperated soft robots that are developed by a modular kinematic arrangement of bending-type fluidic elastomer actuators. The model is simulated using measured physical parameters of the robot and workspace. The theoretical results are verified through a proof-of-concept comparison to locomotion experiments on a flat surface with measured frictional properties. Experimental results indicate that the proposed model describes the motion of the robot.