Optimum Seeking of Redundant Actuators for M-RCM 3-UPU Parallel Mechanism

The singularity problem brings troubles to the design and application for the parallel mechanism. Currently, redundant actuation is one of the useful methods to solve this singularity problem. However, faced to the numerous joints in a parallel mechanism, how to make a quantitative criterion of seeking the most efficient joints added actuators for letting the mechanism passes through singularity is a necessarily open issue. This paper focuses on a 2R1T 3-UPU (U for universal joint and P for prismatic joint) parallel mechanism (PM) with two rotational and one translational (2R1T) degrees of freedom (DOFs) and the ability of multiple remote centers of motion (M-RCM). The singularity analysis based on the indexes of motion/force transmissibility and constraint shows that this PM has transmission singularity, constraint singularity, mixed singularity and limb singularity. To solve these singular problems, the quantifiable redundancy transmission index (RTI) and the redundancy constraint index (RCI) are proposed for optimum seeking of redundant actuators for this PM. Then the appropriate redundant actuators are selected and the working scheme for redundant actuators near the corresponding singular configuration are given to help the PM passes through the singularity. This research proposes a quantitative criterion to optimum seeking of redundant actuators for the parallel mechanism to solve its singularity.

Analysis and Evaluation of CDPR Cable Sagging Based on ANFIS

The cable sagging problem of cable-driven parallel robots (CDPRs) is very complicated, because several models for calculating cable sag based on the well-known catenary equation have been studied, but time and computational efficiency are a problem to be solved. There is still no simple mathematical model to calculate cable sag by considering all relevant conditions due to the complexity and nonlinearity of the cable sagging model, which involves many dominant variables and their influence on the position accuracy of CDPRs. In this study, we proposed an ANFIS (adaptive neuro-fuzzy inference system) architecture to estimate cable sag for large-sized CDPRs. The ANFIS model can be used to solve nonlinear functions and detect nonlinear factors online in the control system; this characteristic is consistent with the nonlinear model of cable sag. The trained data for ANFIS models were taken from calculation results by Trust-Region-Dogleg algorithm based on two cable tension calculation algorithms as Dual Simplex Algorithm and Force Distribution in Closed Form. Cable sagging data obtained from ANFIS and Trust-Region-Dogleg algorithm are compared and evaluated by statistical factors of evaluations consisting of root-mean-square error, correlation coefficients, and scatter index. The analytical results show that the ANFIS gave computed results with small errors and can be applied to predict cable sagging for any CDPR configuration, with the advantage of fast calculation time and high precision. The results of these models are also applied on a CDPR that contains two redundant actuators.

Overconstrained Cable-Driven Parallel Manipulators Statics Analysis based on Simplified Static Cable Model

In recent years, cable-driven parallel manipulators (CDPM) become more and more interesting topics of robot researchers due to its outstanding advantages. Unlike traditional parallel robots, CDPMs use many flexible cables in order to connect the robot fixed frame and the moving platform instead of using conventional rigid links. Since cables used in CDPM is very light compared to rigid links, its workspace can be very large. Besides, CDPMs are often enhanced load capacity by adding redundant actuators. They also help to widen the singularity-free workspace of CDPM. On the other hand, the redundant actuators produce the underdetermined system i.e. the system has non-unique solutions. Moreover, the elasticity and bendability of flexible cable caused by self-weight and external forces act on it, resulting in the kinematic problem of CDPMs are no longer related to the geometric problem. Therefore, the system of CDPM become non-linear when the deformation of cable is considered. In this study, we introduce the simplified static cable model and use it to linearize the static model of redundantly actuated CDPM. The algorithm to solve the force distribution problem is proposed in Sect. 4. The static-workspace and the performance of those are analyzed in a numerical test.

Research on Innovative Trim Method for Tiltrotor Aircraft Take-Off Based on Genetic Algorithm

Tiltrotor aircraft possesses redundant actuators in take-off phase and its flight control is more complicated than ordinary aircraft because the structural and dynamic characteristics keep changing due to tilting rotors. One of the fundamental bases for flight control is trim, which provides steady flight states under various conditions and then constructs the reference trajectory. Tiltrotor aircraft trim models are described by multivariate nonlinear equations whose initial values are difficult to determine and bad initials could lead to incorrect solution for flight control. Therefore, an innovative trim method is proposed to solve this issue. Firstly, genetic algorithm (GA), which possesses strong capability in searching global optimum, is adopted to identify a coarse solution. Secondly, the coarse solution is further refined by the Levenberg-Marquardt (LM) method for precise local optimum. The innovative trim method combines the advantages of these two algorithms and is applied to a tiltrotor aircraft’s flight control in the transition process of incline take-off. The limitation of trajectory is discussed, and tilt corridor is constructed. Finally, the incline take-off simulations are conducted and the effectiveness of the proposed trim method is verified through good match with the designed reference trajectory.

Optimum Seeking of Redundant Actuators for M-RCM 3-UPU Parallel Mechanism

This paper focuses on a 2R1T 3-UPU (U for universal joint and P for prismatic joint) parallel mechanism (PM) with two rotational and one translational (2R1T) degrees of freedom (DOFs) and the ability of multiple remote centers of motion (M-RCM). The singularity analysis based on the indexes of motion/force transmissibility and constraint shows that this PM has transmission singularity, constraint singularity, mixed singularity and limb singularity. To solve these singularproblems, the quantifiable redundancy transmission index (RTI) and the redundancy constraint index (RCI) are proposed for optimum seeking of redundant actuators for this PM. Then the appropriate redundant actuators are selected and the working scheme for redundant actuators near the corresponding singular configuration are given to help the PM go through the singularity.

Optimal Force Allocation and Position Control of Hybrid Pneumatic–Electric Linear Actuators

Hybrid pneumatic–electric actuators (HPEAs) are redundant actuators that combine the large force, low bandwidth characteristics of pneumatic actuators with the large bandwidth, small force characteristics of electric actuators. It has been shown that HPEAs can provide both accurate position control and high inherent safety, due to their low mechanical impedance, making them a suitable choice for driving the joints of assistive, collaborative, and service robots. If these characteristics are mathematically modeled, input allocation techniques can improve the HPEA’s performance by distributing the required input (force or torque) between the redundant actuators in accordance with each actuator’s advantages and limitations. In this paper, after developing a model for a HPEA-driven system, three novel model-predictive control (MPC) approaches are designed that solve the position tracking and input allocation problem using convex optimization. MPC is utilized since the input allocation can be embedded within the motion controller design as a single optimization problem. A fourth approach based on conventional linear controllers is included as a comparison benchmark. The first MPC approach uses a model that includes the dynamics of the payload and pneumatics; and performs the motion control using a single loop. The latter methods simplify the MPC law by separating the position and pressure controllers. Although the linear controller was the most computationally efficient, it was inferior to the MPC-based controllers in position tracking and force allocation performance. The third MPC-based controller design demonstrated the best position tracking with RMSE of 46%, 20%, and 55% smaller than the other three approaches. It also demonstrated sufficient speed for real-time operation.

Empirical Quasi-Static and Inverse Kinematics of Cable-Driven Parallel Manipulators Including Presence of Sagging

Cable-driven parallel manipulators (CDPMs) have been of great interest to researchers in recent years because they have many advantages compared to the traditional parallel robot. However, in many studies they lack the cable’s elasticity that leads to flexible cables just being considered as extendable rigid links. Furthermore, an external force acts on the extremities of cable and the self-weight is relevant to the length of it. Experimentally, a small change in length produces a huge change in tension act on the entire cable. By this property, the adjusting length of cable is often added to the traditional inverse kinematic solution in order to reduce the tension force exerted on the cable. This means that the load on the actuator is also reduced. Because of the relationship between tension that acts on the cable and its length, the kinematic problem itself does not make sense without concerning the static or dynamic problems. There is often interest in planning forces for actuators and the length of cables based on a given quasi-static trajectory of the moving platform. The mentioned problem is combined with the quasi-static problem with the inverse kinematic problem of CDPM. In this study, we introduce a novel procedure to produce the quasi-static model and inverse kinematic model for CDPM with the presence of sagging by using both an analytic approach and empirical approach. The produced model is time-efficient and is generalized for spatial CDPM. To illustrate the performance of the proposed model, the numerical and experimental approaches are employed to determine particular solutions in the feasible solutions set produced by our model in order to control the two redundant actuators’ CDPM tracking on a certain desired trajectory. Its results are clearly described in the experimental section.

Accurate physical modeling and synchronization control of dual-linear-motor-driven gantry with dynamic load

To achieve high-accuracy tracking of dual-linear-motor-driven (DLMD) gantry, high-level synchronization between redundant actuators is a nonnegligible factor and also a difficult issue to be solved prior. Especially, when both XY axes are simultaneously operating to accomplish complex tasks efficiently, additional coupling effects will be generated by the dynamic load presented on the crossbeam, which makes the synchronization issue more complicated compared to the case with static load. However, due to the absence of an accurate model to fully reveal the complete coupling characteristics, existing approaches to this issue still have inherent limitations. Therefore, this paper focuses on the systematic physical modeling and synchronization control of DLMD gantry with a dynamic load presented on the crossbeam. A complete coupling mathematical model is established firstly, by fully considering two linear motions (X-axis and Y-axis) and also including the additional rotational motion of the crossbeam. Built upon the effective model information, corresponding solutions by compensating the dynamic load effects and actively controlling the rotational dynamic to regulate the internal forces have been proposed, leading to a novel adaptive robust synchronization control method. Comparative experiments are carried out, and the results show the effectiveness and superiority of the proposed method in dealing with synchronization issue subjected to dynamic load effects.

Introduction of a redundant actuator using planetary gear trains for human centred robotics

Matching motor efficiency and performance with the load demands can significantly improve the overall efficiency of a driveline. Inspired by the automotive sector -with the high interest of hybrid and electric cars currently-, the authors have studied how state of the art technologies can be used in the relatively new field of collaborative and Human centred robotics. Multiple transmission systems have been considered, among others redundant actuators (both static and kinematic) and continuously variable transmissions. Based on these findings and the experience of the research group on customised planetary gear trains for Human Limb Assistance and Replication, an extensive review of existing redundant actuators is presented in combination with an alternative transmission system which does not need any auxiliary gear transmissions and hence can be lighter and more compact than state of the art drivelines for Human centred robotics. A calculation was performed -including the efficiency model presented by Müller- which shows the high potential of this type of dual-motor actuator.