A Comprehensive Multibody Model of a Collaborative Robot to Support Model-Based Health Management

Abstract

Digital models of industrial and collaborative manipulators are widely used for several applications, such as power-efficient trajectory definition, human–robot cooperation safety improvement, and prognostics and health management (PHM) algorithm development. Currently, models with simplified joints present in the literature have been used to evaluate robot macroscopic behavior. However, they are not suitable for the in-depth analyses required by those activities, such as PHM, which demand a punctual description of each subcomponent. This paper aims to fill this gap by presenting a high-fidelity multibody model of a UR5 collaborative robot, containing an accurate description of its full dynamics, electric motors, and gearboxes. Harmonic reducers were described through a translational equivalent lumped parameter model, allowing each constitutive element of the reducer to have its decoupled dynamics and mating forces through non-linear penalty contact models. To conclude, both the mathematical model and the real robot on a test rig were tested with a set of different trajectories. The experimental results highlight the ability of the proposed model to accurately replicate joint angular rotation, speed and torques in a wide range of operational scenarios. This research provides the basis for the development of a model-based PHM-oriented framework to carry out detailed and advanced analyses on the effects of manipulator degradations.

Designing Digital Twins of Robots Using Simscape Multibody

“…This quantity is constant and can be derived from the DH table. The inertia matrix and the gravity vector are obtained from the expression of the total kinetic energy (11) and the total potential energy (8) by differentiation as…”

“…As friction depends on lubrication and temperature, these coefficients depend on operating conditions. However, friction can be easily included in the Simscape model by expoiting Simscape Fundation Library, as previously done in [9,11]. In particular, a Rotational Friction block can be added to each joint, as in Figure 12.…”

“…For these reasons, Simscape Multibody is a perfect candidate to build high-fidelity digital twins of robots. In fact, Simscape Multibody has already been successfully used in the field of robotics to build models of articulated [9][10][11][12][13][14][15][16][17], parallel [18,19], mobile [20,21] and legged robots [22,23]. It has also been used to test a robot dynamic parameter identification method in [24], to test robot control strategies in [25][26][27][28], to model and simulate a multi-fingered robot arm grasping [29] and to design a 5 degrees of freedom manipulator for additive manufacturing in [30].…”

“…However, previous studies using Simscape Multibody have mostly focused on the kinematic modeling of robots, neglecting its dynamic modeling and the inclusion of non-idealities, friction, reduction gears, and motor dynamics. Only in [9,11] friction was included and only in [11,30] the motor dynamics was taken into account. Nevertheless, while robot manipulators are typically equipped with Permanent Magnet AC Synchronous Motors (PMSMs), in [11] PMSMs motors were described through their simplified equivalent DC monophase models while in [30] bruslhess DC servomotors were modeled.…”

“…Only in [9,11] friction was included and only in [11,30] the motor dynamics was taken into account. Nevertheless, while robot manipulators are typically equipped with Permanent Magnet AC Synchronous Motors (PMSMs), in [11] PMSMs motors were described through their simplified equivalent DC monophase models while in [30] bruslhess DC servomotors were modeled. This approach is sufficient if the aim of the simulation is to take into account the motor dynamics and their saturation limits, but if the aim of the simulation is to develop a custom robot controller and servo drives, the robot model should have a level of detail to the motor inverters.…”