Numerical analysis of undercut anchor effect on rock

The paper presents the results of a numerical analysis using the Finite Element Method (FEM) of the friction issue in the contact between the undercut anchor head and rock during anchor pull-out. Formation of failure zone of rock medium was analysed assuming different Coulomb friction coefficients in the contact zone of conical anchor head with a rock. The problem is interesting as regards practical aspects of rock mass loosening during anchor pull-out. The analysis revealed a significant effect of the friction coefficient on the propagation and extent of the failure zone. Increasing the friction factor significantly decreases the extent of the failure zone measured on a free rock surface.

Experimental and analytical investigation of impact dampers in free vibration reduction with coulomb friction

The presence of vibration in structures and machines can establish failures and reduce the efficiency. Passive dampers have been used extensively for vibration control because of their simple concept. One of these dampers is impact damper (ID). Impact dampers are suitable to operate in harsh environments and effective over a wide range of frequencies. In this paper, the experimental and theoretical investigation on the effect of the ID in free vibration of a cantilever beam is done. The lateral motion of the free end of the cantilever beam is modeled as one degree of freedom (1-DOF) system. The motion of the 1-DOF system equipped with the ID with Coulomb friction is studied analytically. Free vibration of cantilever beam investigated experimentally. The experimental results are presented and discussed. The damping inclination of the ID in various experimental and theoretical studies is proposed. The results show the validation between the experimental and the theoretical studies.

Kinematic compatibility of a wrist robot with cable differential actuation: effects of misalignment compensation via passive joints

<div><div><div><p>We present the UDiffWrist (UDW), a low-impedance 2-DOF wrist exoskeleton featuring a cable-differential transmission. To investigate the effect of different design strategies for achieving kinematic compatibility, we developed two versions of this robot: One version (UDW-C) achieves kinematic compatibility only in the case of perfect alignment between human and robot joints. The second version (UDW-NC) connects the human and robot via passive joints to achieve kinematic compatibility regardless of alignment between human and robot joints. Through characterization experiments, we found that the UDW-NC was more robust to misalignments than the UDW-C: the increase in maximum interaction torque associated with misalignments was greater for the UDW-C than the UDW-NC robot (p = 0.003). However, the UDW-NC displayed greater Coulomb friction (p < 0.001). Further, Coulomb friction increased more for the UDW-NC than the UDW-C in the presence of misalignments between the human and robot axes (p < 0.001). We also found that torque transfer was more accurate in the UDW-C than in the UDW-NC. These results suggest that for the small (10 deg) 2-DOF wrist movements considered, the advantages of the UDW-NC in terms of kinematic compatibility are likely overshadowed by the negative effects in friction and torque transfer accuracy.</p></div></div></div>

Kinematic compatibility of a wrist robot with cable differential actuation: effects of misalignment compensation via passive joints

<div><div><div><p>We present the UDiffWrist (UDW), a low-impedance 2-DOF wrist exoskeleton featuring a cable-differential transmission. To investigate the effect of different design strategies for achieving kinematic compatibility, we developed two versions of this robot: One version (UDW-C) achieves kinematic compatibility only in the case of perfect alignment between human and robot joints. The second version (UDW-NC) connects the human and robot via passive joints to achieve kinematic compatibility regardless of alignment between human and robot joints. Through characterization experiments, we found that the UDW-NC was more robust to misalignments than the UDW-C: the increase in maximum interaction torque associated with misalignments was greater for the UDW-C than the UDW-NC robot (p = 0.003). However, the UDW-NC displayed greater Coulomb friction (p < 0.001). Further, Coulomb friction increased more for the UDW-NC than the UDW-C in the presence of misalignments between the human and robot axes (p < 0.001). We also found that torque transfer was more accurate in the UDW-C than in the UDW-NC. These results suggest that for the small (10 deg) 2-DOF wrist movements considered, the advantages of the UDW-NC in terms of kinematic compatibility are likely overshadowed by the negative effects in friction and torque transfer accuracy.</p></div></div></div>

Analysis of friction and cutting parameters when milling honeycomb composite structures

In machining, tool/workpiece interface parameters are complicated to estimate by experimental means alone. Numerical methods can then give critical solutions to predict and analyze the parameters influencing the machining. The friction between the tool and the cutter has a direct influence on the milling parameters. Therefore, it is necessary to understand the friction mechanism between the tool and the workpiece to estimate the milling parameters of Nomex honeycomb structures correctly. This work aims to present a 3D Finite Element numerical model allowing the prediction of the cutting forces correctly, the morphology of the chips, and the surface quality generated during the milling of this type of structure. These studies were obtained using the commercial software ABAQUS/Explicit. It has been demonstrated that the coupling between the isotropic elastoplastic approach and the Coulomb friction law can easily simulate the milling of Nomex honeycomb structures and gives excellent results in comparison with those obtained experimentally.

Adaptive model predictive control for an omnidirectional mobile robot with friction compensation and incremental input constraints

In this paper, an adaptive model predictive control (MPC) scheme with friction compensation, subject to incremental control input constraints and parameter uncertainties, is proposed for a three-wheeled omnidirectional mobile robot (OMR). The proposed control framework is in a cascaded structure, wherein the outer-loop is kinematic-based control and the inner-loop is designed based on adaptive linear MPC. First, a complex nonlinear dynamic model of the OMR in the world coordinate frame is transformed and partially linearized into a reduced nonlinear model in the moving coordinate system. The nonlinearity of the reduced model only arises from Coulomb friction. Then an estimated system is established for the reduced nonlinear system, with an adaptive update law estimating the system uncertain parameters. To facilitate the linear MPC design, part of the control efforts is derived by feedback compensation of the Coulomb friction forces, resulting in a linear estimated system. The other part is designed by a constrained linear MPC. Feasibility and stability analyses are given for the proposed adaptive MPC scheme. Finally, experimental comparisons with model-based MPC are carried out to verify the effectiveness of the proposed control scheme.