Developing the Next Generation NATO Reference Mobility Model

The NATO Reference Mobility Model (NRMM) is a simulation tool aimed at predicting the capability of a vehicle to move over specified terrain conditions. NRMM was developed and validated by the U.S. Army Tank Automotive Research, Development, and Engineering Center (TARDEC) and Engineer Research and Development Center (ERDC) in the 1960s and ‘70s, and has been revised and updated through the years, resulting in the most recent version, NRMM v2.8.2b. It was originally used to facilitate comparison between vehicle design candidates by assessing the mobility of existing vehicles under specific terrain scenarios, but has subsequently and most recently found expanded use in support of complex decision analyses associated with vehicle acquisition and operational planning support. This paper summarizes recent efforts initiated under a NATO Exploratory Team (ET) and its follow-on Research Technical Group (RTG) to upgrade this key modeling and simulation tool and the planned path forward toward implementing the recommendations of that team. INTRODUCTION Although NRMM has proven to be of great practical utility to the NATO forces, it has several inherent limitations, particularly when compared to modern multibody dynamic (MBD) modeling and simulation (M&S) capabilities. Many of the off-road mobility algorithms are based on empirical observations, and therefore extrapolation outside of test conditions is impossible. It is heavily dependent on in-situ soil measurements and uses onedimensional steady state analysis of powertrain performance. Vehicle dynamic effects are limited to pitch plane for ride quality and all obstacle crossing models were forced to conform to an equivalent walking beam formulation for tracked vehicle suspensions systems. Due to its age and intermittent ad hoc development history and reliance on empirical performance data collected at the vehicle level, NRMM’s software and data architectures do not easily support evolutionary development in vehicle design, terramechanics or vehicle terrain interaction (VTI) models such as the fundamental extension to 3D models that support vehicle turning dynamics and more complete mobility metrics. The means for expansion of the analysis techniques to include intelligent vehicles, custom mobility metrics, stochastic knowledge of terrain and terrain data sets for urban areas are additional pressing needs for a Next Generation NRMM (NG-NRMM). NATO EFFORTS While an effort to update NRMM was initiated in 2002 [1] resulting in some specific advances summarized in a 2011 report [2], this effort did not lay the organizational and architectural foundations required for sustained growth and evolution of the model in a way that opens the model architecture up to multi-scale mechanics solutions, continuous future improvement, non-preferential use of commercial software capabilities while also promoting inclusion of all NATO nations preferred mobility modeling solutions. Thus in 2014, a NATO Applied Vehicle Technology (AVT) Exploratory Team 148 (ET-148) [3] was formed to consider the development of a truly NextGeneration NRMM (NG-NRMM). ET-148 identified seven themes with the following goals: Proceedings of the 2016 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) Developing the Next Generation NATO Reference Mobility Model, McCullough, et al. UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited.(#27849) Page 2 of 20 1) Requirements: Capture, consolidate, and summarize desired capabilities [4]. 2) Methodologies: Develop a plan for deriving a ground vehicle mobility modeling and simulation (M&S) architectural specification for the NG-NRMM [5]. 3) Stochastics: Describe a framework for a stochastic approach for vehicle mobility prediction over large regions for integration into a NG-NRMM [6]. 4) Intelligent Vehicles: Define a NG-NRMM approach and requirements for mobility assessment for intelligent vehicles [7]. 5) Tool choices: Identify the state of the art for NGNRMM enabling simulation technologies as claimed by the technical community of software developers, suppliers, and user nations [8]. 6) Input Data and Output Metrics: To define the input/output data requirements that will inform the NextGeneration NRMM tool development/selection processes and tool recommendations for advanced mapping tools including the means for analysis of remotely sensed Geographical Information System (GIS) data [9]. 7) Verification and Validation (V&V): Develop a plan to provide benchmarks for conducting a successful simulation tool V&V with respect to the NG-NRMM specification [10]. The NATO ET-148 committee consisted of 38 persons from 13 nations (Canada, Czech Republic, Denmark, Estonia, Germany, Italy, Poland, Romania, Slovakia, Spain, Turkey, United Kingdom, and United States) each of whom participated in the detailed research and development goals through membership on one or more of the teams formed to focus on each of the seven goals. NRMM OVERVIEW NRMM is one of the first and few enduring models that comprehensively and realistically quantifies ground vehicle mobility based on terrain accessibility and maximum attainable speeds for comparative force projection assessments of military vehicles via rational consideration of the vehicle's mission, design characteristics, and actual terrain characteristics around the globe [11]. Architecturally, NRMM is a modeling suite comprised of closed form equations for a range of mobility metrics plus numerical models of obstacle crossing and ride dynamics (executed through pre-processors) combined into a main operational prediction module, as shown in Fig. 1. The obstacle and ride dynamics numerical models summarize their respective metrics in well-defined parametric performance curves, but their physics are limited to the vehicle pitch plane. These models require terrain, vehicle and environmental scenario (e.g., dry, wet, snow, sand) data at varying levels of resolution. The operational level performance over a mapped areal terrain is summarized as trafficable percent area (GO/NOGO) and speed made good on the “GO” portions of terrain. Terrain data sets characterizing particular regions of the world are part of the operational model. Fig. 1. NRMM Methodology [4] The operational module combines and considers specific aspects of mobility performance. These include: obstacle override and avoidance, vegetation override and performance, powertrain performance, vehicle/surface interface (soils and hard surfaces), slope effects (grades and side slopes), ride dynamics, visibility, tire constraints, road curvature and braking. Note that in the latest release, version 2.8.2b, VEHDYN II and OBSDP represent differing analysis run streams for ride dynamics and obstacle crossing performance, but use the same vehicle dynamics module. The physics calculations are accomplished with the latest upgrade to the pitch plane vehicle dynamics modeling code, VEHDYN 4.3, which includes a significant list of vehicle suspension and vehicle-terrain interaction modeling enhancements [12] that permit it to cover both the ride dynamics and obstacle crossing analysis run streams. The ride dynamics run stream determines two separate ride quality metrics as lookup tables for the main operational module: 6 watt ride limiting speed vs terrain roughness (using random terrain profiles) and peak acceleration limiting speed vs half round obstacle size. By virtue of this well-defined Proceedings of the 2016 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) Developing the Next Generation NATO Reference Mobility Model, McCullough, et al. UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited.(#27849) Page 3 of 20 mobility metric, this analysis run stream is readily substituted with results from other vehicle dynamics models and ride dynamic metrics. For example, as shown in Fig. 2, highly detailed 3D vehicle models developed in commercial MBD codes have frequently been used because they have been separately validated and/or calibrated with experimental test data [13, 14]. Other ride quality metrics such as ISO 2631 as well as those based on 3D metrics such as longitudinal and lateral acceleration have been proposed as ride limiting speed criteria and could easily be used by substitution of results tables into the higher level operational module. This OBSDP analysis run stream presents a vehicle with a standard set of obstacle trapezoidal shapes as terrain profiles, determining the minimum clearance and the tractive effort required to overcome the obstacle, including the possibility of failure to pass. The output of the model is a lookup table, usually based on 72 standard obstacles, providing minimum clearance, maximum and average tractive effort. This lookup table forms part of the vehicle performance input data set for the main operation module and is the primary means for predicting obstacle override performance over the larger areal terrain data set of mapped obstacles distributions. Mobility failures such as high centering, gap crossing, V-ditch, near vertical step climb, and angles of approach and departure hang-ups are all approximated and predicted in this step. Fig. 2: Examples of MBD model validation and calibration for offroad vehicle dynamic simulations that are already being used in NRMM by substitution of performance tables: a) ride dynamics; b) complex mechanical linkages with flexible bodies (mine plow); c) full vehicle system model of a mine plow with automatic depth control, and calibrated soil cutting [15], flow, bearing [16] and drawbar pull empirical traction models, all based on a height field deformable terrain profile model[14] Proceedings of the 2016 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) Developing the Next Generation NATO Reference Mobility Model, McCullough, et al. UNCLASSIFIED: Distribution Statement A. Approved for pub