An energy-based modeling framework for the nonlinear dynamics of spatial Cosserat rods undergoing large displacements and rotations is proposed. The mixed formulation features independent displacement, velocity and stress variables and is further objective and locking-free. Finite rotations are represented using a director formulation that avoids singularities and yields a constant mass matrix. This results in an infinite-dimensional nonlinear port-Hamiltonian (PH) system governed by partial differential-algebraic equations with a quadratic energy functional. Using a time-differentiated compliance form of the stress-strain relations allows for the imposition of kinematic constraints, such as inextensibility or shear-rigidity. A structure-preserving finite element discretization leads to a finite-dimensional system with PH structure, thus facilitating the design of an energy-momentum consistent integration scheme. Dissipative material behavior (via the generalized-Maxwell model) and non-standard actuation approaches (via pneumatic chambers or tendons) integrate naturally into the framework. As illustrated by selected numerical examples, the present framework establishes a new approach to energy-momentum consistent formulations in computational mechanics involving finite rotations.

This work introduces a port-Hamiltonian (PH) model for constrained mechanical systems, which is directly derived from the Lagrangian equations of motion. The present PH framework incorporates a singularity-free director representation of rigid body rotations, resulting in constant mass matrices. It is shown that the power-preserving interconnection of PH rigid-body subsystems is mathematically equivalent to the classical description of ideal joints using kinematic pairs. This establishes a PH multibody dynamics framework that is consistent with traditional modeling paradigms. Notably, the PH structure of the governing index-2 differential-algebraic equations enables the application of an implicit, structure preserving midpoint time integration. The proposed scheme is able to satisfy both the balance laws for total energy and angular momentum as well as the position-level constraints. These properties make the proposed method remarkably robust and enable stable long-term simulations. Furthermore, a variationally derived index-reduction strategy is incorporated that enforces velocity-level constraints in addition to position-level constraints while preserving the port-Hamiltonian structure. Numerical examples illustrate the favorable properties of the proposed formulation, which is well-suited for energy-based control design.

We propose a mixed and structure-preserving discretization framework for finite-strain thermoelasticity based on General Equation for Non-Equilibrium Reversible-Irrever\-sible Coupling (GENERIC). GENERIC provides a systematic decomposition of the dynamics into reversible and irreversible contributions while guaranteeing consistency with the first and second laws of thermodynamics. Starting from a Hu--Washizu-type extension of the variational principle of Livens and a polyconvex representation of the internal energy, a mixed thermo-mechanical GENERIC framework is constructed. The resulting formulation introduces additional mixed fields associated with thermodynamic driving forces arising from the internal energy and entropy representation. A central aspect of the proposed approach is that the kinematic compatibility conditions are enforced at the rate level, which naturally opens the framework to broader classes of mixed finite element formulations. The temporal discretization is based on so-called discrete derivatives and yields a time-discrete formulation preserving the total energy for closed systems while guaranteeing consistent entropy production. Furthermore, the spatial discretization of the underlying weak form leads to a mixed finite element formulation preserving the thermodynamic structure of the semi-discrete problem. Numerical examples are presented to investigate the numerical performance, thermodynamic consistency, and stability properties of the proposed framework.

The Reissner-Simo and Hodges models are two equivalent continuous descriptions of finite-strain beam dynamics. The Reissner-Simo formulation uses displacements and rotations, while the Hodges formulation is intrinsic and avoids both variables. Although equivalent in theory, the two approaches behave differently after discretization and offer distinct numerical advantages. In this work, we develop a structure-preserving discretization of the intrinsic formulation. Because the intrinsic equations involve linear differential operators, both kinematic and dynamic boundary conditions can be imposed naturally using mixed finite elements. The resulting formulation also enables multibody systems to be assembled without algebraic constraints, avoiding the stiff differential-algebraic equations typically introduced by kinematic constraints. We demonstrate the approach on different examples, also showing that closed kinematic loops can be modeled without algebraic constraints. The resulting interconnected systems retain a port-Hamiltonian structure,with all nonlinearities confined to the interconnection operator. This structure allows exact energy preservation when combined with implicit midpoint time integration. Furthermore the scheme appear to require less Newton iterations compared to existing energy preserving scheme.

We propose a port-Hamiltonian formulation and structure-preserving discretization of finite elasticity. The energy functional (or Hamiltonian) is based on a polyconvex representation of the stored energy and gives rise to three strain-type fields, which play the role of energy variables in the port-Hamiltonian formulation. We show that a Hu-Washizu-type extension of the variational principle of Livens can be used (i) to derive the continuous port-Hamiltonian formulation and (ii) to perform a structure-preserving spatial discretization. In particular, we show that the spatial finite element discretization of the underlying mixed formulation yields a discrete port-Hamiltonian system. Moreover, the temporal discretization of the underlying continuous formulation yields a new energy-momentum consistent framework, which accommodates alternative finite element formulations. The new framework, in particular, covers mixed finite elements that have been shown to be well suited for handling quasi-incompressible material behavior. Numerical examples are provided to evaluate the numerical performance and stability of the newly devised energy-momentum schemes.

ABSTRACT Numerous mesh‐distortion‐insensitive Petrov‐Galerkin finite element formulations have been developed for the simulation of elastostatic problems. This contribution presents initial results on how these formulations can be extended to linear elastodynamics. In this context, three different approaches are examined, building upon an established 8‐node elastostatic formulation. The first two approaches demonstrate that preexisting strategies reach their limits when applied to such unsymmetric formulations. In contrast, the novel third approach enables stable simulations. Moreover, the resulting formulation appears to be less sensitive to mesh distortion than a standard 8‐node Bubnov‐Galerkin formulation.

A three-cable suspension manipulator is an underactuated mechanical system designed to transport a payload precisely within a workspace. This study addresses its inverse dynamics problem using servo constraints, leading to a high-index system of differential-algebraic equations (DAEs). To overcome the numerical challenges, the index reduction by minimal extension method is applied to the servo constraint problem of the differentially flat system. The proposed approach facilitates precise trajectory tracking and serves as a foundation for implementing feedforward and feedback control strategies in future developments.

This paper deals with the optimal control of constrained mechanical systems, with potential additional kinematic constraints at the final time. Correspondingly, the equations of motion of the underlying mechanical system assume the form of differential-algebraic equations with end constraints. The proposed discretisation of the optimality conditions yields a scheme which is capable of preserving control angular momentum maps resulting from the rotational symmetry of the underlying optimal control problem. The numerical solution of the discretised system is first tested with two solution strategies: Monolithic and staggered approaches, and then also solved with hybrid approaches, which combine salient features of each individual strategy. The monolithic strategy solves all the optimality conditions for all time steps as a single system of non-linear equations and relies on a Newton-Raphson scheme, which guarantees quadratic rates of convergence in the vicinity of the optimal solution trajectory. The staggered strategy is based on the Forward-Backward Sweep Method (FBSM), where state and adjoint equations are solved separately, and the control equations provide an update of the control variables, which we here achieve with also a Newton-Raphson scheme. The proposed hybrid strategies combine the advantages of a conventional gradient-based FBSM with the individual Newton-based solution procedures once the solution is close to the optimal trajectory. The strategies are developed and compared through three representative numerical examples, which show that all schemes yield very similar solutions. However, the hybrid approaches become more advantageous in the computation time when the time-step decreases or the size of the problem increases.

Galerkin-based time integration approaches are investigated for discrete mechanical systems subject to holonomic constraints. In this connection, rigid body rotations are described by using unit quaternions, resulting in a configuration-dependent mass matrix. Two different Galerkin methods are developed: (i) a Petrov-Galerkin approach leading to the common time-stepping format of numerical integrators for initial value problems and (ii) a Bubnov-Galerkin method leading to a global solution procedure. The numerical performance of the alternative schemes is compared, including their algorithmic conservation properties, accuracy, and satisfaction of the constraints on configuration and velocity level. The global Galerkin-based approach is deemed to be advantageous for the solution of inverse dynamics and optimal control problems, in which a flow of information backwards in time is required. Numerical examples involving large rotations are investigated to assess the numerical performance of the alternative Galerkin approaches. In addition to that, a comparison of the quaternion-based rigid body formulation with a director-based formulation is included.

We propose a new, port-Hamiltonian formulation for the highly nonlinear dynamics of planar geometrically exact beams, which are amenable to arbitrary large deformations and rotations. A structure-preserving spatial and temporal discretization procedure - using mixed finite elements and second-order time-stepping methods - is proposed. It is observed that the present approach is objective, locking-free and provides an exact discrete representation of the energy and angular momentum balance. By comparing the approach to a classical displacement-based scheme from the literature it is shown that the port-Hamiltonian formulation paves new ways for the design of energy-momentum schemes in computational mechanics. Numerical examples underline the applicability to flexible multibody systems and beneficial numerical performance.

In the present work the geometrically exact beam model is formulated in terms of unit quaternions. A projection‐based discretization approach is proposed which is based on a normalization of the quaternion approximation. The discretization relies on NURBS shape functions and, alternatively, on Lagrangian interpolation. The redundancy of the quaternions is resolved by applying the method of Lagrange multipliers. In a second step the Lagrange multipliers are eliminated circumventing the need to solve saddle point systems. The resulting finite elements retain the objectivity of the underlying beam formulation. Optimal rates of convergence are observed in representative numerical examples.

A consistent, structure-preserving space-time discretization for coupled nonlinear electro-thermo-elastodynamical problems is presented. The underlying polyconvexity-inspired mixed framework is facilitated by the properties of the tensor cross product. The elastodynamic problem is then extended by the energy balance as well as Gauss's and Faraday's law to integrate the thermodynamic and electrostatic contribution, respectively. A suitable polyconvexityinspired internal energy function is chosen to complete the nonlinear, fully coupled electrothermo-elastodynamical formulation. Additionally, we present a structure-preserving, secondorder accurate time integration scheme, utilizing discrete derivatives in the sense of Gonzalez (1996), ensuring a stable and robust simulation even for large time steps. Finally, we assess the numerical performance of our newly developed method through representative examples also showing the possibilities of the framework in the field of boundary control. Copyright (C) 2024 The Authors.

Mechanical systems with singular and/or configuration-dependent mass matrix can pose difficulties to Hamiltonian formulations, which are the standard choice for the design of energy-momentum conserving time integrators. In this work, we derive a structure-preserving time integrator for constrained mechanical systems based on a mixed variational approach. Livens’ principle (or sometimes called Hamilton–Pontryagin principle) features independent velocity and momentum quantities and circumvents the need to invert the mass matrix. In particular, we take up the description of rigid body rotations using unit quaternions. Using Livens’ principle, a new and comparatively easy approach to the simulation of these problems is presented. The equations of motion are approximated by using (partitioned) midpoint discrete gradients, thus generating a new energy-momentum conserving integration scheme for mechanical systems with singular and/or configuration-dependent mass matrix. The derived method is second-order accurate and algorithmically preserves a generalized energy function as well as the holonomic constraints and momentum maps corresponding to symmetries of the system. We study the numerical performance of the newly devised scheme in representative examples for multibody and rigid body dynamics.

This contribution proposes a nonlinear and dissipative infinite-dimensional port-Hamiltonian (PH) model for the dynamics of geometrically exact strings. The mechanical model provides a description of large deformations including finite elastic and inelastic strains in a generalized Maxwell model. It is shown that the overall system results from a power-preserving interconnection of PH subsystems. By using a structure-preserving mixed finite element approach, a finite-dimensional PH model is derived. Eventually, midpoint discrete derivatives are employed to deduce an energy-consistent time-stepping method, which inherits discrete-time dissipativity for the irreversible system. An example simulation illustrates the numerical properties of the present approach. Copyright (C) 2024 The Authors.

This work deals with optimal control problems for constrained mechanical systems whose motion is governed by differential algebraic equations (DAEs). Both index-3 DAEs and stabilized index-2 DAEs are considered. Two alternative formulations of the optimal control problem are compared to each other. It is shown that symmetries of the optimal control problem lead to the conservation of generalized momentum maps. These generalized momentum maps are related to quadratic invariants of the optimal control problem. A direct discretization approach is newly proposed which is (i) capable to conserve the quadratic invariants, and (ii) equivalent to the indirect approach to the optimal control problem. Numerical examples are presented to access the properties of the newly developed schemes.

Port-Hamiltonian (PH) systems provide a framework for modeling, analysis and control of complex dynamical systems, where the complexity might result from multi-physical couplings, non-trivial domains and diverse nonlinearities. A major benefit of the PH representation is the explicit formulation of power interfaces, so-called ports, which allow for a power-preserving interconnection of subsystems to compose flexible multibody systems in a modular way. In this work, we present a PH representation of geometrically exact strings with nonlinear material behaviour. Furthermore, using structure-preserving discretization techniques a corresponding finite-dimensional PH state space model is developed. Applying mixed finite elements, the semi-discrete model retains the PH structure and the ports (pairs of velocities and forces) on the discrete level. Moreover, discrete derivatives are used in order to obtain an energy-consistent time-stepping method. The numerical properties of the newly devised model are investigated in a representative example. The developed PH state space model can be used for structure-preserving simulation and model order reduction as well as feedforward and feedback control design.

We provide a fully nonlinear port‐Hamiltonian formulation for discrete elastodynamical systems as well as a structure‐preserving time discretization. The governing equations are obtained in a variational manner and represent index‐1 differential algebraic equations. Performing an index reduction, one obtains the port‐Hamiltonian state space model, which features the nonlinear strains as an independent state next to position and velocity. Moreover, hyperelastic material behavior is captured in terms of a nonlinear stored energy function. The model exhibits passivity and losslessness and has an underlying symmetry yielding the conservation of angular momentum. We perform temporal discretization using the midpoint discrete gradient, such that the beneficial properties are inherited by the developed time stepping scheme in a discrete sense. The numerical results obtained in a representative example are demonstrated to validate the findings.

A novel mixed framework and energy‐momentum consistent integration scheme in the field of coupled nonlinear thermo‐electro‐elastodynamics is proposed. The mixed environment is primarily based on a framework for elastodynamics in the case of polyconvex strain energy functions. For this elastodynamic framework, the properties of the so‐called tensor cross product are exploited to derive a mixed formulation via a Hu‐Washizu type extension of the strain energy function. Afterwards, a general path to incorporate nonpotential problems for mixed formulations is demonstrated. To this end, the strong form of the mixed framework is derived and supplemented with the energy balance as well as Maxwell's equations neglecting magnetic and time dependent effects. By additionally choosing an appropriate energy function, this procedure leads to a fully coupled thermo‐electro‐elastodynamic formulation which benefits from the properties of the underlying mixed framework. In addition, the proposed mixed framework facilitates the design of a new energy‐momentum consistent time integration scheme by employing discrete derivatives in the sense of Gonzalez. A one‐step integration scheme of second‐order accuracy is obtained which is shown to be stable even for large time steps. Eventually, the performance of the novel formulation is demonstrated in several numerical examples.

In this work we make use of Livens principle (sometimes also referred to as Hamilton-Pontryagin principle) in order to obtain a novel structure-preserving integrator for mechanical systems. In contrast to the canonical Hamiltonian equations of motion, the Euler-Lagrange equations pertaining to Livens principle circumvent the need to invert the mass matrix. This is an essential advantage with respect to singular mass matrices, which can yield severe difficulties for the modelling and simulation of multibody systems. Moreover, Livens principle unifies both Lagrangian and Hamiltonian viewpoints on mechanics. Additionally, the present framework avoids the need to set up the system's Hamiltonian. The novel scheme algorithmically conserves a general energy function and aims at the preservation of momentum maps corresponding to symmetries of the system. We present an extension to mechanical systems subject to holonomic constraints. The performance of the newly devised method is studied in representative examples.

Energy‐Momentum‐Entropy (EME) time‐stepping schemes are distinguished by their numerical stable and robust behaviour, which stems from their ability to preserve the structure of the underlying system. In the context of closed dissipative thermomechanical systems, they are energy‐ and momentum‐preserving as well as entropy‐producing. In order to illustrate the qualification of the GENERIC framework for the design of EME integrators, a thermoviscoelastic double pendulum is chosen as discrete model problem to which the discrete gradient operator due to Gonzalez [1] is applied. The acronym GENERIC pertains to ‘General Equation for Non‐Equilibrium Reversible Irreversible Coupling’ and provides by design a thermodynamic admissible mathematical framework for the evolution equations of dissipative thermomechanic systems. This contribution enlightens the incorporation of constraints in the GENERIC formalism and the necessity of a Lyapunov function as stability criterion.