Development and validation of an end-effector for mitigation of collisions

In robotics the risk of collisions is present both in industrial applications and in remote handling. If a collision occurs, the impact may damage both the robot and external equipment, which may result in successive imprecise robot tasks or line stops, reducing robot efficiency. As a result, appropriate collision avoidance algorithms should be used or, if it is not possible, the robot must be able to react to impacts reducing the contact forces. For this purpose, this paper focuses on the development of a special end-effector that can withstand impacts. It is able to protect the robot from impulsive forces caused by collisions of the end-effector, but it has no effect on possible collisions between the links and obstacles. The novel end-effector is based on a bi-stable mechanism that decouples the dynamics of the end-effector from the dynamics of the robot. The intrinsically non-linear behavior of the endeffector is investigated with the aid of numerical simulations. The effect of design parameters and operating conditions are analyzed and the interaction between the functioning of the bi-stable mechanism and the control system is studied. In particular, the effect of the mechanism in different scenarios characterized by different robot velocities is shown. Results of numerical simulations assess the validity of the proposed end-effector, which can lead to large reductions in impact forces. Numerical results are validated by means of specific laboratory tests.

Development of an End-Effector for Mitigation of Collisions

In robotics the risk of collisions is present both in industrial applications and in remote handling. If a collision occurs, the impact may damage both the robot and external equipment, which may result in successive imprecise robot tasks or line stops, reducing robot efficiency. As a result, appropriate collision avoidance algorithms should be used or, if it is not possible, the robot must be able to react to impacts reducing the contact forces. For this purpose, this paper focuses on the development of a special end-effector that can withstand impacts and is able to protect the robot from impulsive forces. The novel end-effector is based on a bi-stable mechanism that decouples the dynamics of the end-effector from the dynamics of the robot. The intrinsically non-linear behavior of the end-effector is investigated with the aid of numerical simulations. The effect of design parameters and the operating conditions are analyzed and the interaction between the functioning of the bi-stable mechanism and the control system is studied. In particular, the effect of the mechanism in different scenarios characterized by different robot velocities is shown. Results of numerical simulations assess the validity of the proposed end-effector, which can lead to large reductions in impact forces.

Design and Prototyping of Rotational Bi-stable Mechanism Using Permanent Magnets

Diverse applications including switches, deployable structures, and reconfigurable robots can benefit from bi-stability characteristics. However, the complexity of implementation and the limitation of structure configuration make it difficult to apply conventional bi-stable mechanisms to the structures that require rotational bi-stability. In this paper, an implementation method using cylindrical magnets for the rotational bi-stable mechanism is proposed. The proposed bi-stable mechanism consists of a revolute joint with two links. It has rotational bi-stability through the magnetic force relationship between the array of magnets on each link. To identify the characteristics of the proposed bi-stable mechanism, a cylindrical permanent magnet is considered as an electromagnet model that consists of one ring with a virtual electric current. The magnetic field of the cylindrical permanent magnet can be calculated using Biot-Savart law. Similarly, the magnetic force between two cylindrical permanent magnets is calculated using Lorentz force law. The criteria of the magnet array for symmetric bi-stability are described and the potential energy diagram of the rotation link is considered as the performance criterion to identify the stable state. The proposed bi-stable mechanism was applied to the prototype of a deployable structure consisting of two links. The load testing of the structure against external torque was performed and it was obtained that the rotation link can stay within 5deg angle to the maximum load applied and was experimentally verified with good agreement.

Effect of End-Effector Compliance on Collisions in Robotic Teleoperation

In robotic teleoperation, hard impacts between a tool and the manipulated object may impair the success of a task. In order to develop a robotic system that is able to minimize the final velocity of an object after impact, a comprehensive approach is adopted in this work, and the effect on the impact of the parameters of the tool and of the robot is studied. Mass, contact stiffness and damping, robot compliance and control and tool compliance are taken into account. A mathematical model including the tool and the robot moving along the approach direction shows that, in most conditions, robot compliance is not enough to mitigate the impact. A mechanical decoupling between the inertia of the tool and the inertia of the robot is needed. An elastic system based on a bi-stable mechanism is developed and its validity is shown by means of numerical simulations.

Strategic Issues in One-to-One Matching with Externalities

We consider strategic issues in one-to-one matching with externalities. We show that no core (stable) mechanism is strategy-proof, extending an impossibility result of [Roth, A. E. [1982] The economics of matching: Stability and incentives, Math. Oper. Res. 7(4), 617–628] obtained in the absence of externalities. Moreover, we show that there are no limits on successful manipulation of preferences by coalitions of men and women, in contrast with the result of [Demange, G., Gale, D. and Sotomayor, M. [1987] A further note on the stable matching problem, Discrete Appl. Math. 16(3), 217–222] obtained in the absence of externalities.

Two-sided strategy-proofness in many-to-many matching markets

We study the existence of group strategy-proof stable rules in many-to-many matching markets under responsiveness of agents’ preferences. We show that when firms have acyclical preferences over workers the set of stable matchings is a singleton, and the worker-optimal stable mechanism is a stable and group strategy-proof rule for firms and workers. Furthermore, acyclicity is the minimal condition guaranteeing the existence of stable and strategy-proof mechanisms in many-to-many matching markets.

Design of Revolute Joint With Bi-Stability Using Permanent Magnets

Bi-stable mechanisms are systems with two distinct stable equilibrium positions within their range of operation. They are capable of steadily staying in positions without external power input and require less energy to move to the next stable state because of their snap-through behavior. Diverse applications including switches, deployable structures, and reconfigurable robots can benefit from bi-stability characteristics. However, the complexity of implementation and the limitation of structure configuration have made it difficult to apply conventional bi-stable mechanisms to the structures that require rotational bi-stability. Thus, in this paper, we proposed an implementation method using cylindrical magnets for the rotational bi-stable mechanism. The proposed bi-stable mechanism consists of a revolute joint with two links; one is the rotational link and the other is the fixed link. It has rotational bi-stability through the magnetic force relationship between the array of magnets on each link. To identify the characteristics of the proposed bi-stable mechanism, a cylindrical permanent magnet is considered as an electromagnet model that consists of one ring with a virtual electric current. Consequently, the magnetic field of the cylindrical permanent magnet can be calculated using Biot-Savart law. Similarly, the magnetic force between two cylindrical permanent magnets of the electromagnet model is calculated using Lorentz force law. The criteria of the magnet array for symmetric bi-stability are proposed and the potential energy diagram of the rotation link is considered as the performance criterion to identify the stable state.