Dan Negrut

University of Wisconsin–Madison | UW · Department of Mechanical Engineering

Recently, there has been a surge of international interest in extraterrestrial exploration targeting the Moon, Mars, the moons of Mars, and various asteroids. This contribution discusses how current state-of-the-art Earth-based testing for designing rovers and landers for these missions currently leads to overly optimistic conclusions about the beh...

Recently, there has been a surge of international interest in extraterrestrial exploration targeting the Moon, Mars, the moons of Mars, and various asteroids. This contribution discusses how current state-of-the-art Earth-based testing for designing rovers and landers for these missions currently leads to overly optimistic conclusions about the beh...

div class="section abstract"> The SAE AutoDrive Challenge II is a four-year collegiate competition dedicated to developing a Level 4 autonomous vehicle by 2025. In January 2023, the participating teams each received a Chevy Bolt EUV. Within a span of five months, the second phase of the competition took place in Ann Arbor, MI. The authors of this c...

In multibody dynamics, formulating the equations of motion in absolute Cartesian coordinates results in a set of index-3 differential algebraic equations (DAEs). In this work, we present an approach that bypasses the DAE problem by partitioning the velocities in the system into dependent and independent coordinates, thereby reducing the task of pro...

This paper introduces DEM-Engine, a new submodule of Project Chrono, that is designed to carry out Discrete Element Method (DEM) simulations. Based on spherical primitive shapes, DEM-Engine can simulate polydisperse granular materials and handle complex shapes generated as assemblies of primitives, referred to as clumps. DEM-Engine has a multi-tier...

In order to enhance the overall comprehension of floating offshore wind turbine (FOWT) performance, a detailed investigation into the dynamic response of a commonly used floater type, namely the tension-leg platform tension-leg platform (TLP), has been undertaken. The objective is to utilize high-fidelity numerical tools to analyze and characterize...

This paper discusses the use of the discrete element method (DEM) to simulate the dynamics of granular systems made up of elements with nontrivial geometries. The DEM simulator is GPU accelerated and can handle elements whose shape is defined as the union with overlap of diverse sets of spheres with user-specified radii. The simulator can also hand...

We present a Vehicle Model (VM) that has 17 degrees of freedom and includes nonlinear tire and powertrain subsystems. Implemented as a relatively small piece of C++ code, the model runs vehicle dynamics 2000 times faster than real time at a simulation time step of 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3...

The soil contact model (SCM) is widely used in practice for off‐road wheeled vehicle mobility studies when simulation speed is important and highly accurate results are not a main concern. In practice, the SCM parameters are obtained via a bevameter test, which requires a complex apparatus and experimental procedure. Here, we advance the idea of ru...

In mechanical systems, formulating the equations of motion in absolute coordinates results in a set of index-3 differential algebraic equations (DAEs). In this work, we present an approach to solving the DAE problem by partitioning the velocities in the system into dependent and independent coordinates, thereby reducing the problem to an ordinary d...

In this contribution, we discuss the pros and cons of using the Smoothed Particle Hydrodynamics (SPH) method in providing support to multiphysics simulations. The Lagrangian SPH method is proposed to address a fluid-structure interaction (FSI) problem in which the response of a flexible baffle with elastic behavior is solved by using three modeling...

This paper introduces DEM-Engine, a new submodule of Project Chrono, that is designed to carry out Discrete Element Method (DEM) simulations. Based on spherical primitive shapes, DEM-Engine can simulate polydisperse granular materials and handle complex shapes generated as assemblies of primitives, referred to as clumps. DEM-Engine has a multi-tier...

Modeling cameras for the simulation of autonomous robotics is critical for generating synthetic images with appropriate realism to effectively evaluate a perception algorithm in simulation. In many cases though, simulated images are produced by traditional rendering techniques that exclude or superficially handle processing steps and aspects encoun...

We discuss the use of the Discrete Element Method (DEM) to simulate the dynamics of granular systems made up of elements with nontrivial geometries. The DEM simulator is GPU accelerated and can handle elements whose shape is defined as the union with overlap of diverse sets of spheres with user-specified radii. The simulator can also handle complex...

This contribution outlines a high-fidelity simulation framework for Human-in-the-loop (HIL) traffic simulation built upon Project Chrono, an open-source physics simulation engine. The framework is designed to provide the software infrastructure for human factors, traffic, and human-automation interplay research. We conclude this platform overview p...

In robotics, simulation has the potential to reduce design time and costs, and lead to a more robust engineered solution and a safer development process. However, the use of simulators is predicated on the availability of good models. This contribution is concerned with improving the quality of these models via calibration, which is cast herein in...

Modeling a robust control system with a precise GPS-based state estimation capability in simulation can be useful in field navigation applications as it allows for testing and validation in a controlled environment. This testing process would enable navigation systems to be developed and optimized in simulation with direct transferability to real-w...

In robotics, simulation has the potential to reduce design time and costs, and lead to a more robust engineered solution and a safer development process. However, the use of simulators is predicated on the availability of good models. This contribution is concerned with improving the quality of these models via calibration, which is cast herein in...

The soil contact model (SCM) is widely used in practice for off-road wheeled vehicle mobility studies when simulation speed is important and highly accurate results are not a main concern. In practice, the SCM parameters are obtained via a bevameter test, which requires a complex apparatus and experimental procedure. Here, we advance the idea of ru...

This work highlights the use of half-implicit numerical integration in the context of the index three differential algebraic equations (DAEs) of multibody dynamics. Although half-implicit numerical integration is well established for ordinary differential equations problems, to the best of our knowledge, no formal discussion covers its use in the c...

Modeling cameras for the simulation of autonomous robotics is critical for generating synthetic images with appropriate realism to effectively evaluate a perception algorithm in simulation. In many cases though, simulated images are produced by traditional rendering techniques that exclude or superficially handle processing steps and aspects encoun...

We discuss a platform that has both software and hardware components, and whose purpose is to support research into characterizing and mitigating the sim-to-real gap in robotics and vehicle autonomy engineering. The software is operating-system independent and has three main components: a simulation engine called Chrono, which supports high-fidelit...

For multibody dynamics simulation using the Absolute Nodal Coordinate Formulation, multiple strategies are reported in the linear elastic material literature for calculating the generalized internal force and its Jacobian matrix. When examining the presentation of these strategies, which are all sound, it is difficult to assess which method is more...

Given the versatility of generative adversarial networks (GANs), we seek to understand the benefits gained from using an existing GAN to enhance simulated images and reduce the sim-to-real gap. We conduct an analysis in the context of simulating robot performance and image-based perception. Specifically, we quantify the GAN's ability to reduce the...

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 matri...

The focus of this contribution is on camera simulation as it comes into play in simulating autonomous robots for their virtual prototyping. We propose a camera model validation methodology based on the performance of a perception algorithm and the context in which the performance is measured. This approach is different than traditional validation o...

We describe a software framework and a hardware platform used in tandem for the design and analysis of robot autonomy algorithms in simulation and reality. The software, which is open source, containerized, and operating system (OS) independent, has three main components: a ROS 2 interface to a C++ vehicle simulation framework (Chrono), which provi...

The Discrete Element Method (DEM) is a suitable approach for modeling and simulating arable soil and tillage processes. In this work, we focus on modeling soil fragmentation which is a vital element of many soil treatments. We present a method for computing the indirect tensile strength of soil aggregates through simulation. DEM aggregates are cons...

This contribution (i) describes an open-source, physics-based simulation infrastructure that can be used to learn and test control policies in off-road navigation; and (ii) demonstrates the use of the simulation platform in an end-to-end learning exercise that relies on simulated sensor data fusion (camera, GPS and IMU). For (i), the 0.5 million li...

We describe an analytical framework implemented in a general-purpose mobility simulation platform for enabling the design of control policies for improved rover mobility in granular terrain environments. We employ a homogenization of the granular material and use an elasto-plastic continuum model to capture the dynamics of the deformable terrain. T...

We seek to develop control policies for optimized rover mobility on deformable granular material terrains using a co-simulation framework implemented in an open-source simulation platform. An elasto-plastic continuum model is employed for the purpose of increasing the simulation speed in capturing the dynamics of the deformable granular material te...

We describe a field-robotics relevant, general-purpose framework to characterize machine-ground interaction and demonstrate it for bulldozing and mobility studies in conjunction with a replica of NASA's VIPER rover operating on deformable, granular material-type terrain. The contribution of this work is twofold: the approach proposed for capturing...

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 matri...

This contribution outlines a multiphase flow solution method developed in conjunction with the incompressible smoothed particle hydrodynamics (SPH) discretization. We present (a) a generalized multiphase particle shifting technique that maintains a regular particle distribution and prevents interface particle mixture; and (b) a color-function-based...

gym-chrono is a set of simulated environments for Deep Reinforcement Learning (DRL) extending OpenAI Gym (Brockman et al., Openai gym, 2016) with robotics and autonomous driving tasks. The physics of these environments is simulated thanks to Project Chrono (Tasora et al., Chrono: An open source multi-physics dynamics engine, 2016), an open-source m...

General-purpose GPU applications increasingly use synchronization to enforce ordering between many threads accessing shared data. Accordingly, recently there has been a push to establish a common set of GPU synchronization primitives. However, the expressiveness of existing GPU synchronization primitives is limited. In particular the expensive GPU...

We describe a simulation environment that enables the design and testing of control policies for off-road mobility of autonomous agents. The environment is demonstrated in conjunction with the training and assessment of a reinforcement learning policy that uses sensor fusion and inter-agent communication to enable the movement of mixed convoys of h...

As a step towards addressing a scarcity of references on this topic, we compared the Eulerian and Lagrangian Computational Fluid Dynamics (CFD) approaches for the solution of free-surface and Fluid–Solid Interaction (FSI) problems. The Eulerian approach uses the Finite Element Method (FEM) to spatially discretize the Navier–Stokes equations. The fr...

Simulation is increasingly important in the development and testing of robots and autonomous vehicles as it opens the door for candidate navigation, perception, and sensor fusion algorithms to be expeditiously probed in complex and safety-critical scenarios. As most robots and autonomous vehicles make heavy use of cameras to perceive their surround...

We outline a continuum approach for treating discrete granular flows that holds across multiple scales: from experiments that focus on centimeter-size control volumes, to tests that involve landslides and large buildings. The time evolution of the continuum used to capture the granular dynamics is resolved in space via the smoothed particle hydrody...

We report on an open-source, publicly available C++ software module called Chrono::GPU, which uses the Discrete Element Method (DEM) to simulate large granular systems on Graphics Processing Unit (GPU) cards. The solver supports the integration of granular material with geometries defined by triangle meshes, as well as co-simulation with the multi-...

We describe an analytical framework implemented in an open-source simulation platform for the purpose of enabling the design of control policies for improved rover mobility in granular terrain environments. In order to speed up the simulation, we employ an elastoplastic continuum model to capture the dynamics of the deformable terrain. The solution...

We propose a framework to investigate the mobility of a replica of NASA's VIPER rover while operating on de-formable granular material terrains under different scenarios. In order to speed up the simulation, we employ a continuum model to capture the dynamics of the deformable terrain. Thus, the granular material is modeled as an elasto-plastic con...

A multibody dynamics-based solution to the fluid dynamics problem is compared herein to two established Lagrangian-based techniques used by the computational fluid dynamics (CFD) community. The multibody dynamics-based solution has two salient attributes: it enforces the incompressibility condition through bilateral kinematic constraints, and it tr...

We use the Smoothed Particle Hydrodynamics (SPH) method to determine the dynamics of granular material in its interaction with a four-wheel rover. The goal of the simulation is to investigate the mobility of the rover while operating on granular terrains. In order to speed up the simulation, we employ a continuum model to capture the dynamics of th...

We describe a simulation environment that enables the development and testing of control policies for off-road mobility of autonomous agents. The environment is demonstrated in conjunction with the design and assessment of a reinforcement learning policy that uses sensor fusion and inter-agent communication to enable the movement of mixed convoys o...

In rigid multibody dynamics simulation using absolute coordinates, a choice must be made in relation to how to keep track of the attitude of a body in 3D motion. The commonly used choices of Euler angles and Euler parameters each have drawbacks, e.g., singularities, and carrying along extra normalization constraint equations, respectively. This con...

Most controllers concerning lateral stability and rollover prevention for autonomous vehicles are designed separately and used simultaneously. However, roll motion influences lateral stability in cornering maneuvers, especially at high speed. Typical rollover prevention control stabilizes the vehicle with differential braking to create an understee...

We compare three solution approaches that use the index 3 set of differential algebraic equations (DAEs) to solve the constrained multibody dynamics problem through straight discretization via an implicit time integrator. The first approach is described in a companion paper and dwells on the connection between the orientation matrix and the SO(3) g...

Book Series Editorial Board John Michopoulos, Naval Research Laboratory David Rosen, Georgia Institute of Technology Chris Paredis, Georgia Institute of Technology Judy Vance, Iowa State University This is the second volume in this book series that aims to capture advances in computers and information in engineering research, especially by research...

Physics-based simulation provides an accelerated and safe avenue for developing, verifying, and testing robotic control algorithms and prototype designs. In the quest to leverage machine learning for developing AI-enabled robots, physics-based simulation can generate a wealth of labeled training data in a short amount of time. Physics-based simulat...

Computer simulation can be a useful tool when designing robots expected to operate independently in unstructured environments. In this context, one needs to simulate the dynamics of the robot's mechanical system, the environment in which the robot operates, and the sensors which facilitate the robot's perception of the environment. Herein, we focus...

Fully homomorphic encryption (FHE) allows computations over encrypted data. This technique makes privacy-preserving cloud computing a reality. Users can send their encrypted sensitive data to a cloud server, get encrypted results returned and decrypt them, without worrying about data breaches. This project report presents a homomorphic instruction...

The last five years marked a surge in interest for and use of smart robots, which operate in dynamic and unstructured environments and might interact with humans. We posit that well-validated computer simulation can provide a virtual proving ground that in many cases is instrumental in understanding safely, faster, at lower costs, and more thorough...

We outline a phenomenological model to assess friction at the interface between two bodies in mutual contact. Although the approach is general, the application inspiring the approach is the discrete element method. The kinematics of the friction process is expressed in terms of the relative 3D motion of the contact point on the two surfaces in mutu...

We use the SPH method to determine the dynamics of granular material in its interaction with the eight wheels of a rover. The rover wheel geometry is defined through a mesh. The granular material is modeled as an elasto-plastic continuum that dynamically interacts with the rigid wheels of the rover in a Chrono [1] co-simulation setup. Several param...

We discuss modeling, algorithmic, and software aspects that allow a simulation tool called Chrono::Granular to run billion-degree-of-freedom dynamics problems on commodity hardware, i.e., a workstation with one GPU. The ability to scale the solution to large problem sizes is traced back to an adimensionalization process combined with the use of mix...

In order to track the desired path as fast as possible, a novel autonomous vehicle path tracking based on model predictive control (MPC) and PID speed control was proposed for high-speed automated vehicles considering the constraints of vehicle physical limits, in which a forward-backward integration scheme was introduced to generate a time-optimal...

Model predictive control (MPC) algorithm is established based on a mathematical model of a plant to forecast the system behavior and optimize the current control move, thus producing the best future performance. Hence, models are core to every form of MPC. An MPC-based controller for path tracking is implemented using a lower-fidelity vehicle model...

In order to investigate how model fidelity in the formulation of model predictive control(MPC) algorithm affects the path tracking performance, a bicycle model and an 8 degrees of freedom(DOF) vehicle model, as well as a 14-DOF vehicle model were employed to implement the MPC-based path tracking controller considering the constraints of input limit...

We outline a phenomenological model to assess friction at the interface between two bodies in mutual contact. Although the approach is general, the application inspiring the approach is the Discrete Element Method. The kinematics of the friction process is expressed in terms of the relative 3D motion of the contact point on the two surfaces in mutu...

Simulation can be an important tool in evaluating autonomous vehicle performance. In a virtual proving ground, challenging and highly unlikely scenarios can be created and novel control algorithms can be tested with no safety concerns and relatively little cost. A critical component of any virtual proving ground is sensor simulation; i.e., the abil...

This is a foreword to a special issue that brings together select papers from the ASME International Design Engineering Technical Conferences & Computers and Information in Engineering (IDETC-CIE) Conferences. The 2019 IDETC-CIE conferences took place in Anaheim, California, from August 18-21.

Contact detection in SPH-based fluid dynamics engines can be efficiently done if a sorted neighbor list of each SPH marker is constructed. In fluid-solid interaction simulations an SPH marker can be one of three different types: fluid, boundary or solid. Solid SPH markers represent a solid body and can be think of as a point cloud representations o...

This contribution looks at how multibody dynamics and fluid dynamics can be used to understand the dynamics of granular material. The study is motivated by two observations: granular dynamics is an ubiquitous problem, given that granular material is second only to water in how often it is handled in practical applications; and, studying the motion...

In large, rigid multibody dynamics problems with friction and contact, encountered for instance in granular flows, one can witness distinctly different system-level dynamics. This contribution concentrates on the case of fluid-like behavior of large multibody dynamics systems such as granular materials, when the system experiences large strains. Th...

This study describes the implementation of a granular dynamics solver designed to run on Graphics Processing Units (GPUs). The discussion concentrates on how the Discrete Element Method (DEM) has been mapped onto the GPU architecture, the software design decisions involved in the process, and the optimizations allowed by those decisions. This solve...

We compare two surface tension models to solve two-phase fluid interaction problems in the context of the mesh-free Smoothed Particles Hydrodynamics (SPH) method. The Continuum Surface Force (CSF) model (later extended to Continuum Surface Stress, CSS), originally derived from grid-based numerical methods, requires an accurate estimation of the int...

A new consistent, spatially adaptive, smoothed particle hydrodynamics (SPH) method for Fluid-Structure Interactions (FSI) is presented. The method combines several attributes that have not been simultaneously satisfied by other SPH methods. Specifically, it is second-order convergent; it allows for resolutions spatially adapted with moving (transla...

Natural and artificial self-propelled systems must manage environmental interactions during movement. In complex environments, these interactions include active collisions, in which propulsive forces create persistent contacts with heterogeneities. Due to the driven and dissipative nature of these systems, such collisions are fundamentally differen...

Chrono::Vehicle is a module of the open-source multi-physics simulation package Chrono, aimed at modeling, simulation, and visualization of wheeled and tracked ground vehicle multi-body systems. Its software architecture and design was dictated by the desire to provide an expeditious and user-friendly mechanism for assembling complex vehicle models...

This paper describes a model for deformable soil based on triangular meshes with vertical deformation. Such soil model can be used in a multi-body simulation environment to study the performance of wheeled or tracked vehicles. The formulation is inspired by the soil contact model (SCM), but unlike the original idea, our implementation uses triangul...

A Smoothed Particles Hydrodynamics (SPH) method for fluid dynamics is coupled with a rigid and deformable body dynamics solution to yield a framework for solving fluid–solid interaction (FSI) problems. The two-way, force–displacement, coupling of the fluid and solid phases is captured via boundary condition enforcing (BCE) markers. The partial diff...

The premise of this work is the observation that real-life mechanical systems limit the use of high order integration formulas due to the presence in the associated models of friction and contact/impact elements. High-order numerical integration methods are impractical in these cases, and producing a numerical solution relies on the use of low orde...

The purpose of this work was to investigate the utility of implicit integration methods for molecular dynamics (MD) simulation. Implicit methods were investigated in an attempt to address the bottleneck associated with short, stability-limited step size lengths available to the explicit methods currently in widespread use. Two implicit methods were...

This technical report outlines a methodology for predicting the time evolution of complex nonlinear dynamic multi-body system in the presence of uncertain inputs. The focus is on ground vehicle systems, and the associated sources of uncertainty are multiple: road topology, friction coefficient at the road/tire interface, and aerodynamic force loadi...

Advances in microprocessor performance in recent years have led to wider use of Computational Multi-Body Dynamic Simulations to reduce production costs, decrease product delivery time, and reproduce scenarios difficult or expensive to study experimentally. With the introduction of massively parallel Graphics Processing Units (GPUs), the general pub...

The validation of a DVI approach for the dynamics of granular material project focuses on comparing the experimental and simulation results of granular flow for two tests in the Chrono::Engine simulation environment. An aluminum rig was designed and fabricated to measure the mass flow rate of a given amount of granular material flowing due to gravi...

This paper addresses some aspects of an ongoing multiyear research project of GP Technologies in collaboration with University of Wisconsin-Madison for US Army TARDEC. The focus of this research project is to enhance the overall vehicle reliability prediction process. A combination of stochastic models for both the vehicle and operational environme...

NASA is interested in designing a spacecraft capable of visiting a Near Earth Object (NEO), performing experiments, and then returning safely. Certain periods of this mission will require the spacecraft to remain stationary relative to the NEO. Such situations require an anchoring mechanism that is compact, easy to deploy and upon mission completio...

This document highlights aspects related to the support and use of unified, or managed, memory in CUDA 6. The discussion provides an opportunity to revisit two other CUDA memory transaction topics: zero-copy memory and unified virtual addressing. The latter two can be regarded as intermediary milestones in a process that has led in CUDA 6 to the re...

The numerical results of standard triaxial test were presented and validated against experimental ones. Two different approaches for modeling the dynamics of granular matter were used. The first one, discrete element method with penalty contact mod-eling (DEM-P) [1], is approach used in e. g. granular flows or powder mechanics simulations. The seco...

Chrono::Vehicle is a module of the open-source multi-physics simulation package Chrono, aimed at modeling, simulation, and visualization of wheeled and tracked ground vehicle multi- body systems. Its software architecture and design was dictated by the desire to provide an expeditious and user-friendly mechanism for assembling complex vehicle model...

In this technical report an algorithms based on DEM via penalty and complemen-tarity were validated against direct shear tests. It turned out that those two methods can be successfully used in such simulations. This outcome is quite remarkable, since methods handle friction and contact using vastly different ways, either from physical or numerical...