extra The Project Solutions for Automotive Efficiency Extract Chassis & Steering: DEVELOPMENT PLATFORM FOR FUTURE STEER-BY-WIRE SYSTEMS October 2014

Transcription

1 extra October 214 Extract Chassis & Steering: DEVELOPMENT PLATFORM FOR FUTURE STEER-BY-WIRE SYSTEMS The Project Solutions for Automotive Efficiency

2 CHASSIS AND STEERING DEVELOPMENT PLATFORM FOR FUTURE STEER-BY-WIRE SYSTEMS Steer-by-wire systems allow a variety of steering and support functions to be implemented, and offer the potential to make better use of the existing package and to reduce variant diversity. ThyssenKrupp has designed a flexible R&D vehicle with a modular steering gear and feedback actuator that can represent both a mechanical fall-back level and fault tolerant systems. Particular attention is given to the implementation of a consistent and natural steering feel. This development environment enables the definition of complex requirements concerning the actuator system, the sensor system, fault tolerance and control and allows these to be further developed to form modern, customer-friendly steer-by-wire systems. One of the biggest challenges in developing a steer-by-wire system is to simulate an authentic steering feel. In addition to a test object, ThyssenKrupp therefore also uses a test rig to analyze the feedback actuator system and its components in detail, 1. The integrated sensor system offers very high accuracy and resolution to enable even the smallest torque and angle changes to be detected. The torque sensor is not based on a torsion bar, as is otherwise usual. Instead, a process involving differential magnetic field measurement in a rigid overall system is used to measure torque. All of the influences of individual components and their interaction can be rep- Test rig layout Friction bearing Endlock Metal bellows coupling Rigid coupling Disc motor Brushless DC motor Torque sensor 1 Friction element 4 Friction element 2 Friction element 1 Steering wheel Belt drive Friction element 3 Hysteresis brake Torque sensor 2 Steering Relative 1 Test rig setup for analyzing various feedback concepts 8

3 CHASSIS AND STEERING resented and analyzed on the test rig. The individual components can be variably combined and exchanged. The following components and concepts can be objectively and subjectively compared under identical boundary conditions: friction in various positions stiffnesses sensor concepts and specifications actuator designs and concepts direct drive and other transmission concepts (belt drive, worm gear, etc.) solutions for simulating high torque requirements software functions and controller strategies. R&D vehicle with steer-by-wire system 2 Roding Roadster R&D vehicle R&D VEHICLE WITH EXTENSIVE SCOPE The test rig results can be simulated and tested directly in the R&D vehicle. To test and validate solution concepts and functionalities which cannot be developed in CAE or on the test rig, the R&D vehicle and the steering components are extensively designed in modular form. Based on a Roding Roadster, 2, a small-scale sports car, the R&D vehicle offers extraordinary design opportunities in terms of the steering gear, steering column and feedback actuator due to its mid-engine concept. The feedback actuator, 3, is designed in-line with the test rig. In addition to high-precision angle and torque sensors, it allows two different actuators to be compared directly against each other, as they are connected to the steering wheel via an electronically switchable coupling. To prevent an artificial steering feel around the zero position (on-center feel) during straight-ahead driving, a certain level of friction is required in the feedback system. In the R&D vehicle, this can be set using software and is simulated by a specially developed friction element, 4. The friction element enables the simulation of a homogeneous torque to rotate without stick-slip effects when changing directions. The virtually mechanical friction generated in this way can be infinitely adjusted from.5 to 1 Nm. The steering gear is based on a rack EPS system from ThyssenKrupp. With its centrally located ball screw, it allows the analysis of redundant systems with two power packs. This enables different redundant solution and control approaches to be developed and tested Steer-by-wire system Absolute Relative Electric steering column lock Safety clutch Non-contact torque sensor Brushless DC motor Disc motor Steering actuator 1 Absolute Magnetorheological friction element Steering actuator 2 Relative 3 Steering gear (left) with feedback actuator (right) October

4 CHASSIS AND STEERING Torque to rotate.8 Standardized torque [Nm] Steering angle [ ] 4 Standardized representation of the torque to rotate with varying energization of the magneto-rheological friction element with one piece of hardware. The power packs can be coupled via a definable stiffness up to and including play or can act independently of each other. To guarantee prototype and failure safety, the feedback actuator is connected to the steering gear via an actively open mechanical clutch. In addition to the steer-by-wire-specific sensors, the vehicle is equipped with measurement technology which precisely registers further signals such as the vehicle speed, lateral acceleration and yaw rate. An AutoBox is used as a real-time system for processing this information and transforming it into corresponding Steering gear position [mm] Nonlinear control Desired position Actual position Error Steering gear position [mm] Linear control Desired position Actual position Error Steering gear control 82

5 CHASSIS AND STEERING control commands. The test object is equipped with both a CAN and a FlexRay bus system which ensures communication with the steering gear, feedback actuator and vehicle. The overall configuration enables flexible development and testing of various software functions at vehicle level. STEERING GEAR CONTROL CONCEPTS The steering gear rack movement control is responsible for precisely following the steering movement desired by the driver in terms of position and speed. Two different control concepts have been developed and applied for this. One is based on the conventional, linear state space control, the other on the extended, nonlinear Lyapunov control. Both concepts are based on the mathematical-physical description of the mechanical steering gear by means of differential equations. The majority of naturally occurring systems are nonlinear, but can be described precisely enough using linear models. However, there are limits. Outside of these, the model is no longer sufficient and the theoretical behavior deviates excessively from the real behavior. Initial simulations and tests on the test rig show the influence of the nonlinearities in the higher dynamic range of the nominal position, as can be seen in 5. The described R&D vehicle forms the basis of the methodically oriented procedure for understanding the complex requirements on a steer-by-wire system and developing individual solutions. Only coordinated development steps enable the creation of tailored component requirement specifications and achievement of the maximum cost advantage while simultaneously implementing a natural steering feel. Redundancies and fall-back levels are also being developed independently of the steering as part of InCar plus. One strategy is torque vectoring as a fall-back level for the steering system. In this, specific drive and braking torques at the wheels steer the vehicle. To develop this strategy, ThyssenKrupp has established a simulation environment along with a corresponding vehicle model, and has used these to analyze various strategies and their influence on vehicle response, 6. Initial validation of the simulation results is being undertaken on the ETH Zürich Formula Student Race Car, which is equipped with electric individual wheel drive. FUTURE TASKS The establishment of this research and development platform is an initial step towards the development of modern steer-by-wire systems. ThyssenKrupp will use this to develop and analyze the following systems and concepts: different concepts for redundancy and functional safety sensor and actuator system concepts and requirements linear and nonlinear controller approaches various approaches and functions for designing the steering feel extended driver support functions cost-optimized component selection. This list only contains a few of the possible options. Through its work, the project team has generated extensive knowhow in the field of steer-by-wire and is implementing this together with vehicle manufacturers. Torque vectoring Yaw rate [ /s] 2-2 Lateral acceleration [m/s²] Road wheel angle δ [ ] 2-2 Vehicle slip angle β [ ] Torque vectoring Conventional Reference 6 Comparison of a conventional passenger car with standard axle geometry and a vehicle with torque vectoring October