Constrained multibody kinematics and dynamics in absolute coordinates: a discussion of three approaches to representing rigid body rotation

TL;DR

This paper compares three approaches to solving constrained multibody dynamics in absolute coordinates, highlighting their efficiency, with the matrix approach being notably faster due to simpler equations and Jacobians.

Contribution

It introduces a detailed comparison of three rotation representation methods in multibody dynamics, including new sensitivity analyses and efficiency benchmarks.

Findings

The matrix-based approach is roughly twice as fast as the Euler angles method.

Simpler equations and Jacobians lead to improved simulation speed.

The paper provides the first head-to-head efficiency comparison of these methods in absolute coordinates.

Abstract

We compare three approaches to posing the index 3 set of differential algebraic equations (DAEs) associated with the constrained multibody dynamics problem formulated in absolute coordinates. The first approach works directly with the orientation matrix and therefore eschews the need for generalized coordinates used to produce the orientation matrix $ \mathbf{A} $. The approach is informed by the fact that rotation matrices belong to the SO(3) Lie matrix group. The second approach employs Euler parameters, while the third uses Euler angles. In all cases, the index 3 DAE problem is solved via a first order implicit numerical integrator. We note a roughly twofold speedup of $ \mathbf{rA} $ over $ \mathbf{r\epsilon} $, and a 1.2 -- 1.3 times speedup of $ \mathbf{r\epsilon} $ over $ \mathbf{rp} $. The tests were carried out in conjunction with four 3D mechanisms. The improvements in simulation speed of the $ \mathbf{rA} $ approach are traced back to a simpler form of the equations of motion and more concise Jacobians that enter the numerical solution. The contributions made herein are twofold. First, we provide first order variations of all the quantities that enter the $ \mathbf{rA} $ formulation when used in the context of implicit integration; i.e., sensitivity of the kinematic constraints for all lower pair joints, as well as the sensitivity of the constraint reaction forces. Second, to the best of our knowledge, there is no other contribution that compares head to head the solution efficiency of $ \mathbf{rA} $, $ \mathbf{rp} $, and $ \mathbf{r\epsilon} $ in the context of the multibody dynamics problem posed in absolute coordinates.