Abstracts:Traditional robots and structures are designed with nearly rigid members and actuators to have a stiff output for better control; however, this method of design typically produces systems which lack the finesse and sensitivity of natural systems with nimble control in both low force and high force applications. Natural systems achieve this function by muscles which work as actuators and possess both tunable force and stiffness control. Structure controlled stiffness (SCS) tunable stiffness mechanisms (TSMs) typically change stiffness by varying the structural parameters within a particular deformation mode, i.e. varying moment of inertia or length of a beam for a spring in bending. A novel low profile TSM is proposed with the purpose of achieving a high range of stiffness by tuning the deformation mode contributions. The e-Spring consists of a non-prismatic, tapered circular arch beam which is connected with one short strut to a fixed center. The radially applied loading and circular shape provide a "zero moment" condition at the fixed center reducing the need for large torque capacity to actively drive the system, and a worm gear interfacing with the drive shaft provides powerless locking of the rotation for improved power savings. The variation of the stiffness is achieved by rotating the e-Spring about its center to change the point of application of the force, thus changing the mode of deformation (from bending to shear to axial). Analytical and finite element (FE) models are developed for e-Spring analysis. The total stiffness range of the system is maximized using design optimization under a set of defined constraints. Based on the optimization results, three designs (stiff, median and soft) are selected, fabricated and tested by using the assembly of a DC motor with encoder and worm gears with feedback control to obtain the desired angle of rotation. Analytical and FE results closely match the experimental validations for different stiffness vs. angle of control conditions. A wide range of stiffness change is experimentally observed with a linear stiffness response of 0.5 N/mm to 1782 N/mm for the optimal design under the set of defined system constraints. By implementing the proper control algorithms, better human robot interactions and structures with new adaptable/tunable features can be achieved with the system developed in this research.

Abstracts:The efficiency of electric vehicles (EVs) can be improved by applying multi-speed transmissions (MSTs), while ensuring that gear-shifting is swift and smooth. This paper establishes a gear-shifting control algorithm for a novel MST, with the advantages of simplicity and modularity, designed for EVs. Firstly, the mathematical model of the proposed MST is derived. Next, the control algorithm developed for gear-shifting is clarified, which guarantees seamlessness and swiftness. The system under study is over-actuated, with end constraints on some control inputs. Therefore, for acceleration and jerk continuity, while satisfying the input terminal constraints, one input is suggested to be changed independently, based on a 2-3 blending polynomial. Then, the new fully-actuated system is controlled using a linear quadratic integral (LQI) controller, which is an extension of the linear quadratic regulator (LQR) for tracking problems. Simulation results indicate the effectiveness of the proposed control algorithm in the presence of unknown external disturbances.

Abstracts:The research work reported here was motivated by a class of two-limb parallel Schönflies-motion ¹ 1 These motions comprise four degrees of freedom: three translations and one rotation about one axis of fixed direction. generators, which offer simplicity, isostaticity, and symmetry. The crucial components required to construct a parallel two-limb robot of this class are a cylindrical drive, namely, a cylindrical motion generator, and a load-carrying link playing the role of the moving platform in parallel robots, with four degrees-of-freedom, i.e. 3D translation and rotation about a vertical axis, which operates based on the same cylindrical motion generator. However, the design requirements of such components call upon screw joints with an unusually large pitch, which are not available off-the-shelf; the authors thus propose a cable-driven virtual screw with an arbitrarily large pitch. This concept is elaborated on with regard to its various alternatives, each suitable for different circumstances and applications. Furthermore, the concept of variable-pitch virtual screw is introduced, which enables the designer to adapt the specification of the robot to any given task. In addition, the novel application of the virtual screw in the context of the architecture of Schönflies-motion generators is studied. Finally, the authors report the design and fabrication of two prototypes to conduct a comparison between the screw joint and the virtual screw.

Abstracts:Generally, the Cartesian stiffness matrix of closed-loop mechanisms can be divided into the passive stiffness and the active stiffness. The passive stiffness was determined by the kinematic parameters and joint stiffness, while the active stiffness is sub-divided into the forced active stiffness K E , related to the external wrench, and active stiffness K IG generated by internal force. The passive stiffness is always positive definite, however, the active stiffness can be positive, semi-positive or negative definite. We exemplified it with a redundantly actuated planar rotational parallel mechanism (RAPR-PM), showing negative stiffness criteria for the active stiffness. Whether the elastic limbs are compressed or stretched, once the preload is known, the sign of the active stiffness can be determined from kinematic parameters. The results can be used to design variable stiffness mechanisms. Since some biological structures of animals, like fish, can be represented by a series of joint-linked RAPR-PMs, the conclusions in this paper can be extended to study the inherent stiffness mechanism of animals.

Abstracts:A nonlinear hybrid dynamic model of a helical gearbox is proposed in this study. The model considers the nonlinear coupling effect of time-varying mesh stiffness (TVMS) and transmission error excitation. The effects of tip relief and lead crowning on the TVMS and dynamic characteristics of the helical gear transmission system are studied. Numerical methods are used to obtain the frequency response curves of the vibration acceleration of the helical gear system. The optimal values of tooth modification parameters are then determined in order to minimize the vibration amplitude. The simulation results indicate that the optimized tooth modification parameters can effectively decrease mesh stiffness fluctuation in the alternating areas of the teeth and reduce the vibration acceleration amplitude at its resonance frequency. Finally, the theoretical model is validated against an experimental platform of a high-speed rail traction gearbox transmission system, and the dynamic responses are compared.

Abstracts:Recently, more and more non-circular gear pairs have been used in gear transmission mechanism. The transmission ratio of some non-circular gear pairs have been presented, however, the relative motions of the two gears are different in different gear pairs. The analytical model and mathematical equations for the classification of the transmission pattern, transmission ratio and relative motions of non-circular gear pair have been established. The classification model illustrates the non-circular gear pairs with both fixed axis and movable axis are existing, presents some other possible new transmission patterns. This paper focuses on the complex governing equations for the compound motion of this curve-face gear pair and shows that the kinematical response of pinion-spring system is continuous intermittent collisions with incomplete vibrations in the meshing process. The limitation conditions for the critical velocity of this gear pair are proposed in this paper. The results indicate that the vibration of this gear pair at low velocity is obvious and the critical velocity should be limited to not more than 700 rpm, which is affected by the parameters of this gear pair. These theoretical results, which have been verified by the experimental rolling, provide the theoretical support for further analysis and application of this compound motion face gear pair.

Abstracts:Computational simulations of a multibody dynamic response are an important tool for the analysis and design of various mechanical systems. While the governing dynamic equations of these systems are well known, the identification of model parameters, especially those associated with joints, can prove difficult and time consuming. Traditionally, experimental methods are used to deduce the physical joint parameters by isolating the joint from the rest of the structure and testing it under static or dynamic loads. An alternative to pure experimental joint-parameter identification is the model-based methods, which rely on finding such parameter values that the predicted dynamic response coincides with that of the real system. As the equations of multibody systems are highly nonlinear, linearization techniques are applied to efficiently deduce the system’s dynamic parameters using modal analysis. Although significant progress has been made in recent years, none of the studies that propose the linearization technique has addressed the effect of multibody system equilibrium-point selection on the accuracy of the parameter-identification procedure. Therefore, here, a new general model-based parameter-estimation method is proposed that minimizes the difference between the experimentally and numerically obtained dynamic system’s natural frequencies. The basic idea of the proposed method relies on the development of an algorithm that identifies the optimal equilibrium point of the linearization for a given multibody system. The equilibrium point is deduced in such a way as to minimize the interplay between the different joint parameters on the system’s natural frequencies. Using the proposed approach it is possible to localize the influence of the individual joint’s stiffness parameters to one particular natural frequency. The presented case study highlights the efficiency of the developed parameter-estimation procedure and with this the importance of a proper linearization equilibrium-point selection for a reliable and accurate parameter-identification process.

Abstracts:An accurate calculation of the maximum tooth root stress (TRS) and critical section location (CSL) provides a basis for predicting and improving gear performance. The irregular profile represented by the implicit function may cause the calculation to be more complex. In current research, finite element methods (FEM model) and experimental test methods (ET model) can obtain accurate results but need large computational resources and time. The results from ISO 6336:2006 (ISO model) and AGMA 2101-D04 (AGMA model) are obtained conveniently but sometimes not reliable. Therefore, a new analytical model based on the mechanics theory with an accurate profile equation is established to calculate the maximum TRS and corresponding CSL quickly and accurately by solving the extreme value. Finally, the results of the spur gear in five cases with different parameters are obtained and compared to those of the FEM, ISO and AGMA models. It is shown that the results of the new model are in agreement with those of the FEM model, even under different parametric conditions.

Abstracts:The purpose of this research is to present a grasping force model for a soft robotic gripper with variable stiffness. The soft robotic gripper was made of shape memory alloys (SMAs) with contraction and variable stiffness properties. A variable stiffness mechanism with embedded sets of SMA fibers was developed; however, the response characteristics of its backbone did not comply with the constant-curvature model when it was subjected to complex forces/torques, such as gravity, grasping forces and driving torques. In this case, the Cosserat theory was used to implement real-time computations of the grasping force of the soft robotic gripper that was subjected to complex forces. Finally, a series of tests were conducted on the grasping force of the soft finger and the gripper. The elicited results showed that the grasping force is related to the stiffness and to the object's offset and friction coefficient. Moreover, experimental results showed that the grasping force of the soft robotic gripper increased by 48.7% when the Young's modulus of the SMA-2 wires increased from 25 GPa to 48 GPa.

Abstracts:This work proposes a novel approach to obtain a nearly constant power output from an offshore wind turbine by incorporating a priority flow divider valve and an accumulator into the power hydraulic system. The priority flow divider valve allows flow in two different ways, i.e. priority line and by-pass line, simultaneously. It controls the fluid (seawater) flow by adjusting its priority port flow as per the user setting to ensure minimum fluctuation of power output, whereas the excess flow is passed through the by-pass port and stored in an accumulator. A control strategy is applied on the control valve to allow the flow from the accumulator to the nozzle as per demand. A mathematical model of wind turbine with hydraulic power transmission is developed by using bond graph technique, which is suitable for this multi-energy domain system. Simulation results show that the output from the wind turbine is stabilized by using the proposed hydraulic power transmission system under fluctuating wind speed.