Closed-loop Multibody Systems Research Articles

Dynamic modelling and singularity-free simulation of closed loop multibody system driven by electric motors

This paper presents the dynamic model and singularity-free simulation of electromechanical systems including closed loop multibody systems, massless gear transmission and electric motors. The dynamic model of these systems is established in matrix form and written in a Differential-Algebraic Equations form by applying the Lagrangian equation with multipliers and substructure method. Moreover, this paper deals with two difficult issues in the simulation of closed-loop multibody systems which are to overcome smoothly the singular configurations and to stabilize the constrained equations due to accumulated errors. The singularity-free simulation is solved by using null-space of Jacobian matrix to eliminate the constraint forces – Lagrangian multipliers in equations of motion. The drift in the constraint equation during simulation is restricted by a combination of Baumgarte’s stabilization and post-adjusting technique. Some numerical experiments are carried out to the planar 3RRR parallel manipulator driven by electric motors. Simulation results confirm the effectiveness of the proposed approach in overcoming the singular configurations and in stabilization of the constraint.

Numerical methods of closed-loop multibody systems with singular configurations based on the geometrical structure of constraints

Numerical methods of closed-loop multibody systems with singular configurations based on the geometrical structure of constraints

Iterative refinement algorithm for efficient velocities and accelerations solutions in closed-loop multibody dynamics

To simulate closed-loop multibody systems in effective and accurate way, the semi-recursive formulations can be used together with optimal numerical integrators. Although numerous contributions have been reported in this field, there still exists a demand to improve the computation efficiency for the faster-than-real-time simulation and hardware-in-the-loop control applications. This paper presents an iterative refinement algorithm to accelerate the numerical integration process of a semi-recursive multibody formulation. By reusing the constraint Jacobian matrix factorization and generalized mass matrix factorization, respectively, the presented algorithm is introduced to enhance the solution of dependent relative velocities and independent relative accelerations. The introduced procedure consists of three steps; initial guess determination, iterative refinement process and termination criteria. The iterative refinement process is designed to find an accurate numerical solution, which contains residual computing, solution increment computing and solution updating. For higher efficiency, the initial guess can be re-determined to repeat the iterative refinement process. Three closed-loop multibody systems with increasing complexity are taken as numerical examples to verify the accuracy and efficiency of the presented iterative refinement algorithm. The results highlight more than 15% efficiency gain. The algorithm also can be implemented in other numerical integrators and multibody formulations.

Transversal vibration analysis of the upper span of nonlinear closed-loop track systems

In order to analyze the effect of track pre-tension on the transversal vibration of the upper track, a multibody dynamics model of a tracked vehicle needs to be established. In view of the complex structure of tracked vehicles, the computational efficiency of conventional methods cannot meet engineering design requirements. In this paper, the Riccati Transfer Matrix Method for nonlinear closed-loop multibody systems is proposed and applied to the modeling and simulation of tracked vehicle systems. It avoids the overall dynamic equations of the system and achieves efficient computation with a smaller matrix scale. By selecting accelerations and forces as elements of the state vectors instead of coordinates and forces, linearization is avoided. The track chain is cut in a revolute joint. Its effect on the closed-loop system is represented by a set of constraint equations and internal constraint forces. An improved Riccati transformation for closed-loop systems is proposed to connect the internal force and acceleration parts in the state vector with the system constraint internal forces. The track pre-tension can be adjusted by moving the position of the idler relative to the chassis. The root mean square of vertical acceleration of mid-span with respect to the chassis is selected to evaluate the transversal vibration of the upper track. Simulation of a tracked vehicle driving on different road conditions is carried out with different idler displacements. Finally, a reasonable adjustment strategy for the track tensioner is proposed corresponding to different running conditions.

A time optimal trajectory planning algorithm for closed-loop multibody systems and its verification by experiments

A time optimal trajectory planning algorithm for closed-loop multibody systems and its verification by experiments

Modeling and simulation of structural components in recursive closed-loop multibody systems

Passenger cars, transit buses, railroad vehicles, off-highway trucks, earth moving equipment and construction machinery contain structural and light-fabrications (SALF) components that are prone to excessive vibration due to rough terrains and work-cycle loads’ excitations. SALF components are typically modeled as flexible components in the multibody system allowing the analysts to predict elastic deformation and hence the stress levels under different loading conditions. Including SALF component in the multibody system typically generates closed-kinematic loops. This paper presents an approach for integrating SALF modeling capabilities as a flexible body in a general-purpose multibody dynamics solver that is based on joint-coordinates formulation with the ability to handle closed-kinematic loops. The spatial algebra notation is employed in deriving the spatial multibody dynamics equations of motion. The system kinematic topology matrix is used to project the Cartesian quantities into the joint subspace, leading to a condensed set of nonlinear equations with minimum number of generalized coordinates. The proposed flexible body formulation utilizes the component mode synthesis approach to reduce the large number of finite element degrees of freedom to a small set of generalized modal coordinates. The resulting reduced flexible body model has two main characteristics: the stiffness matrix is constant while the mass matrix depends on the elastic modal coordinates. A consistent set of pre-computed inertia shape integrals are identified and used to update the modal mass matrix at each time step. The implementation of the component mode synthesis approach in a closed-loop recursive multibody formulation is presented. The kinematic equations are modified to include the effect of the flexible body modal elastic coordinates. Also, modified constraint equations that include the effect of flexibility at the joint connections and the necessary details of the Jacobian matrix are presented. Baumgarte stabilization approach is used to stabilize the constraint equations without using iterative schemes. A sample results for flexible body impeded in a closed system will be presented to demonstrate the above mentioned approach.

A parallel Hamiltonian formulation for forward dynamics of closed-loop multibody systems

This paper presents a novel recursive divide-and-conquer formulation for the simulation of complex constrained multibody system dynamics based on Hamilton’s canonical equations (HDCA). The systems under consideration are subjected to holonomic, independent constraints and may include serial chains, tree chains, or closed-loop topologies. Although Hamilton’s canonical equations exhibit many advantageous features compared to their acceleration based counterparts, it appears that there is a lack of dedicated parallel algorithms for multi-rigid-body system dynamics based on the Hamiltonian formulation. The developed HDCA formulation leads to a two-stage procedure. In the first phase, the approach utilizes the divide and conquer scheme, i.e., a hierarchic assembly–disassembly process to traverse the multibody system topology in a binary tree manner. The purpose of this step is to evaluate the joint velocities and constraint force impulses. The process exhibits linear O(n) (n – number of bodies) and logarithmic O(log_{2}{n}) numerical cost, in serial and parallel implementations, respectively. The time derivatives of the total momenta are directly evaluated in the second parallelizable step of the algorithm. Sample closed-loop test cases indicate very small constraint violation errors at the position and velocity level as well as marginal energy drift without any additional form of constraint stabilization techniques involved in the solution process. The results are comparatively set against more standard acceleration based Featherstone’s DCA approach to indicate the performance of the HDCA algorithm.

Singularity-free simulation of closed loop multibody systems by using null space of Jacobian matrix

Numerical simulation of closed loop multibody systems is associated with numerical solution of equations of motion which are, in general, in the form of DAE’s index-3 systems. For assuring continuous simulation, one should overcome some difficulties such as stabilization of the constraint equations, singular configuration of the system. In this paper, the system equations of motion with the Lagrange multipliers is rewritten by introducing generalized reaction forces. The combination with the condition of ideality of constraints leads to the system of equations which can be solved by numerical techniques smoothly, even over singular positions. Based on the new criterion of ideality of constraints, which relates generalized reaction forces and the null space matrix of Jacobian matrix, it is possible also to remove reaction forces and use only the reduced system of equations with null space matrix for passing singular positions. In order to prevent the constraint equations from the accumulated errors of integral time, the method of position and velocity projection has been exploited. Some numerical experiments are carried out to verify the proposed approach.

Natural Coordinates in the Optimal Control of Multibody Systems

The formulation of multibody dynamics in terms of natural coordinates (NCs) leads to equations of motion in the form of differential-algebraic equations (DAEs). A characteristic feature of the natural coordinates approach is a constant mass matrix. The DAEs make possible (i) the systematic assembly of open-loop and closed-loop multibody systems, (ii) the design of state-of-the-art structure-preserving integrators such as energy-momentum or symplectic-momentum schemes, and (iii) the direct link to nonlinear finite element methods. However, the use of NCs in the optimal control of multibody systems presents two major challenges. First, the consistent application of actuating joint-forces becomes an issue since conjugate joint-coordinates are not directly available. Second, numerical methods for optimal control with index-3 DAEs are still in their infancy. The talk will address the two aforementioned issues. In particular, a new energy-momentum consistent method for the optimal control of multibody systems in terms of NCs will be presented.

Modeling and simulation of closed loop multibody systems with bodies-joints composite modules

Modular approaches are effective in improving the modeling efficiency in multibody system dynamics. A modular method, called bodies-joints composite simulation (BJCS) is presented in this paper. Two types of bodies-joints composite modules, i.e., f modules and 0 modules, are defined according to topology design rules of closed loop mechanisms. By this module partitioning, the differential-algebraic equations of motion of the system can be separated into purely algebraic and differential equations by structure decomposition. The stability criterion for the simulation is derived and a closed-form formulation to solve the algebraic loop problem is proposed. Simulation results of the forward dynamics of a 5R mechanism and a 6-UPS platform are presented to show the feasibility of the method. The CPU times of these two case studies are provided and compared with the generalized coordinate partitioning of Wehage and Haug and the modular simulation method of Kubler and Schiehlen, which indicate that the closed-form algebraic loop solver is more efficient than the numerical ones.

On the derivation of energy consistent time stepping schemes for friction afflicted multibody systems

The work outlines the modeling of dissipative effects in multibody systems while designing an energy consistent time integration scheme. Standard integrators typically lead to numerical growth or decay of energy while solving the equations of motion. Consequently it is almost impossible to distinguish between physically based dissipation and numerical dissipation. The present contribution relies on a thermodynamically consistent modeling of dissipation. This formulation facilitates the design of time integration schemes which preserve mechanical quantities like linear and angular momentum and total energy balance. Examples deal with standard mass point systems as well as applications to complex closed loop multibody systems.

Numerical integration of discrete mechanical systems with mixed holonomic and control constraints

The present work aims to incorporate control (or servo) constraints into finite-dimensional mechanical systems subject to holonomic constraints. In particular, we focus on underactuated systems, defined as systems in which the number of degrees of freedom exceeds the number of inputs. The corresponding equations of motion can be written in the form of differential-algebraic equations (DAEs) with a mixed set of holonomic and control constraints. Apart from closed-loop multibody systems, the present formulation accommodates the so-called rotationless formulation of multibody dynamics. The rotationless formulation has proven to be especially well-suited for the design of energy and momentum conserving schemes, which typically exhibit superior numerical stability properties (see [4, 7, 10]). Subsequent to the incorporation of the servo constraints, we deal with a reformulation of the underlying DAEs, which is amenable to a direct numerical discretization. To this end, we apply a specific projection method to the DAEs in terms of redundant coordinates. A similar projection approach has been previously developed in the framework of generalized coordinates by Blajer & Kolodziejczyk [12]. A numerical example is presented, which deals with a 3D rotary crane.

Numerical integration of differential‐algebraic equations with mixed holonomic and control constraints

AbstractThe present work aims at the incorporation of control (or servo) constraints into finite–dimensional mechanical systems subject to holonomic constraints. In particular, we focus on underactuated systems, defined as systems in which the number of degrees of freedom exceeds the number of inputs. The corresponding equations of motion can be written in the form of differential–algebraic equations (DAEs) with a mixed set of holonomic and control constraints. Apart from closed–loop multibody systems, the present formulation accommodates the so–called rotationless formulation of multibody dynamics. To this end, we apply a specific projection method to the DAEs in terms of redundant coordinates. A similar projection approach has been previously developed in the framework of generalized coordinates by Blajer & Kołodziejczyk [1]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy-momentum conserving integration of multibody dynamics

A rotationless formulation of multibody dynamics is presented, which is especially beneficial to the design of energy-momentum conserving integration schemes. The proposed approach facilitates the stable numerical integration of the differential algebraic equations governing the motion of both open-loop and closed-loop multibody systems. A coordinate augmentation technique for the incorporation of rotational degrees of freedom and associated torques is newly proposed. Subsequent to the discretization, size-reductions are performed to lower the computational costs and improve the numerical conditioning. In this connection, a new approach to the systematic design of discrete null space matrices for closed-loop systems is presented. Two numerical examples are given to evaluate the numerical properties of the proposed algorithms.

The discrete null space method for the energy consistent integration of constrained mechanical systems: Part I: Holonomic constraints

The present work deals with energy consistent time stepping schemes for finite-dimensional mechanical systems with holonomic constraints. The proposed procedure is essentially based upon the following steps: Firstly, the index three differential-algebraic equations corresponding to the constrained mechanical system are directly discretized. Secondly, the discrete Lagrange multipliers are eliminated by using a discrete null space matrix. In many cases it is feasible to further reduce the number of unknowns by employing specific reparametrizations. The proposed method entails a number of advantageous features such as size-reduction and improved conditioning of the resulting system of algebraic equations. It is shown that the newly developed method is well-suited for both open-loop and closed-loop multibody systems.

Contribution to the Optimization of Closed-Loop Multibody Systems: Application to Parallel Manipulators

This paper describes an original and robust method to optimize the design of closed-loop mechanisms, especially parallel manipulators. These mechanisms involve non linear assembling constraints. During optimization, the Newton-Raphson algorithm we use to solve these constraints may fail when the Jacobian matrix of the constraints is ill-conditioned and stops the redesign process. To circumvent the difficulty, the technique we propose takes advantage of numerical conditioning to penalize the objective function. Applications to an academic example and parallel robots demonstrate the capabilities of the methodology.

RECURSIVE KINEMATICS AND DYNAMICS FOR PARALLEL STRUCTURED CLOSED-LOOP MULTIBODY SYSTEMS1*

A kinematic formulation for the parallel structured closed-loop multibody mechanical systems, such as Stewart Platform and Hexapod machine tools, is presented in this paper, which is recursive in nature. This also leads to the minimum order representation of the dynamic equations of motion. The recursive algorithms are known for their efficiency when a system is large. They also provide many physical interpretations. On the other hand, the minimum order dynamic equations of motion are desired in control and simulation. For the latter a minimum set of dynamic equations of motion leads to a numerically stable integration algorithm that does not violate the kinematic constraints. Two recursive algorithms, one for the inverse and another for the forward dynamics, are proposed. The overall complexity of either problem is O(n) + n O(m), where n and m are the number of legs and the total number of rigid bodies in each leg, respectively. Hence the proposed formulation exploits the advantages of both minimum order representation and recursive algorithms, which earlier were available only for the open-loop systems such as serial manipulators. The method is illustrated with three examples: a one-degree-of-freedom (DOF) slider-crank mechanism, four-bar linkage, and a two-DOF five-bar planar parallel manipulator. *Communicated by J. McPhee.

A Poisson-Based Formulation for Frictional Impact Analysis of Multibody Mechanical Systems With Open or Closed Kinematic Chains

Analysis of frictional impact in a multibody mechanical system requires a friction model capable of correct detection of all possible impact modes such as sliding, sticking, and reverse sliding. Conventional methods for frictional impact analysis have either shown energy gain or not developed for jointed mechanical system, and especially not for closed-chain multibody systems. This paper presents a general formulation for the analysis of impact problems with friction in both open- and closed-loop multibody mechanical systems. Poisson’s hypothesis is used for the definition of the coefficient of restitution, and thus the energy gains inherent with the use of Newton’s hypothesis are avoided. A canonical form of the system equations of motion using Cartesian coordinates and Cartesian momenta is utilized. The canonical momentum-balance equations are formulated and solved for the change in the system Cartesian momenta using an extension of Routh’s graphical method for the normal and tangential impulses. The velocity jumps are calculated by balancing the accumulated system momenta during the contact period. The formulation is shown to recognize all modes of impact; i.e., sliding, sticking, and reverse sliding. The impact problems are classified into seven types, and based on the pre-impact system configuration and velocities, expressions for the normal and tangential impulses are derived for each impact type. Examples including the tip of a double pendulum impacting the ground with some experimental verification, and the impact of the rear wheel and suspension system of an automobile executing a very stiff bump are analyzed with the developed formulation. [S1050-0472(00)02304-7]

An effective solver for absolute variable formulation of multibody dynamics

This paper presents an effective and general method for converting the equations of motion of multibody systems expressed in terms of absolute variables and Lagrange multipliers into a convenient set of equations in a canonical form (constraint reaction-free and minimal-order equations). The method is applicable to open-loop and closed-loop multibody systems, and to systems subject to holonomic and/or nonholonomic constraints. Being aware of the system configuration space is a metric space, the Gram-Schmidt ortogonalization process is adopted to generate a genuine orthonormal basis of the tangent (null, free) subspace with respect to the constrained subspace. The minimal-order equations of motion expressed in terms of the corresponding tangent speeds have the virtue of being obtained directly in a “resolved” form, i.e. the related mass matrix is the identity matrix. It is also proved that, in the case of absolute variable formulation, the orthonormal basis is constant, which leads to additional simplifications in the motion equations and fits them perfectly for numerical formulation and integration. Other useful peculiarities of the orthonormal basis method are shown, too. A simple example is provided to illustrate the convertion steps.

Dynamical equations of constrained robotic devices

Different forms of the dynamical equations for constrained chain systems are investigated. First Kane's equations with Lagrange's multipliers are deduced from the standard form of d'Alembert's principle. Next it is proved that, even for constrained robotic devices, Euler's equations are equivalent to Kane's equations provided only that relative coordinates are employed. This equivalence yields an outstanding mechanical interpretation of Kane's equations. The possibility of applying the procedure to closed-loop multibody systems is also examined.