A hardware in the loop and virtual reality test environment for steer by wire system evaluations

The efficient development of steer-by-wire hardware and software for both the human-machine haptic interface and the directional control assembly servo-motors require a repeatable test e

A Hardware-in-the-Loop and Virtual Reality Test Environment

for Steer-by-Wire System Evaluations

Pradeep Setlur, Dr John Wagner‡, Dr Darren Dawson, and Lance Powers

Automotive Research Laboratory Departments of Mechanical and Electrical/Computer Engineering Clemson University, Clemson, South Carolina 29634

‡ Corresponding author: jwagner@clemson.edu

Abstract

A high precision, cost effective, experimental

hardware-in-the-loop steer-by-wire test environment is presented and

discussed to support engineering and psychology studies In

this project, a suite of chassis models and nonlinear control

algorithms are developed and validated, as well as various

human-machine interface design issues investigated This

paper provides an overview of the real time steering

simulator which has been created to facilitate the

comparison and evaluation of vehicle and steering system

control strategies in a repeatable manner The insertion of

novel driver input devices, with adjustable force feedback,

permits the study of the human-vehicle interface The

remote operation and/or supervision of semi-autonomous or

autonomous vehicles can be studied using a

human-in-the-loop system to provide insight into the relative importance

to the driver of various vehicle parameters

1 Introduction

The emergence of unmanned/manned ground vehicles

represents an important tool in the manufacturing

environment, intelligent highway system, space exploration,

defense, and security fields to complete repetitious and

hazardous tasks An important vehicle technology is

drive-by-wire [1] which permits local and remote operation in

both semi-autonomous and autonomous scenarios The

efficient development of steer-by-wire hardware and

software for both the human-machine haptic interface and

the directional control assembly servo-motors require a

repeatable test environment, similar to the actual vehicle,

with a variety of prescribed operating profiles One of the

advantages of laboratory testing is the ability to fully

address safety and performance issues prior to extensive

in-vehicle testing

In this paper, an unmanned/manned vehicle steering system

hardware-in-loop testbed with virtual reality capabilities

will be presented The vehicular steering testbed

components include the steering and chassis mathematical

models, driver interface with adjustable force feedback,

rack and pinion directional control assembly, tire/road

interface, and driver With the help of various vehicle

databases, the hardware-in-the-loop experimental station

can facilitate the investigation of different hybrid vehicle

steering strategies Although the initial focus of the project

is on front wheel steer-by-wire technology for ground transportation vehicles, the emerging concept of four wheel steer-by-wire systems can be accommodated with the appropriate mathematical models, control algorithms, and steering hardware The scenario of autonomous vehicle operation requires a tracking controller that can follow a prescribed path or trajectory One possible operating mode

is for the human operator to supervise the vehicle's functionality and intervene, if appropriate, to assume control of the steering actions An important consideration

is the availability of reliable information that can pinpoint the vehicle's orientation and position

The haptic interface utilizes a nonlinear tracking controller

to ensure that the steering mechanism follows the operator commands and simultaneously, a tunable force feedback is provided to the driver The driver's interface features a switched reluctance motor to offer resistive feedback for the steering maneuver based on the desired road "feel" A high torque dc motor in the directional control assembly actuates the front wheels to the desired steer angle To provide the driver with visual feedback during the steering operation, a virtual reality tool has been integrated to display the changing environment as a result of the vehicle's motion Standard steering maneuvers may be completed to evaluate the performance of the control algorithms

The paper is organized as follows In Section 2, the modeling and control algorithms for the steering subsystem are discussed The hardware-in-the-loop steer-by-wire test platform is described in Section 3 Preliminary experimental results are presented in Section 4 to demonstrate the opportunities to impact the drive-by-wire design process Finally, concluding remarks are discussed

in Section 5

2 Modeling and Control of Steering Systems

The foundation of the steer-by-wire test environment begins with the mathematical modeling of the steering system, chassis, tire/road interface, and wheels (refer to Figure 1) Servo-motors exist within the driver interface and (front wheel) directional control assembly to replace the functionality of the traditional hydraulic pump with spool valve and mechanical linkage between the steering column

and rack and pinion The mathematical models are needed

to recreate a realistic steering feel to drivers in the

steer-by-wire system To regulate the motor voltages, real time

control algorithms must be designed and implemented with

experimental validation to be performed on the steering test platform The four wheel steer-by-wire system requires at least one additional servo-motor to regulate the rear rack and pinion directional control assembly

Steer-by-Wire System Powertrain System

Driver

Steering Linkages

Wheel Dynamics

Platform Dynamics

Tire/Road Interface

Suspension System Brakes

Engine Control Unit

Steering Control Unit

Chassis System

Engine/

Transmission

Driver Interface and Directional Control Assembly

Environment

Figure 1: Vehicle dynamics diagram for a steer-by-wire system

2.1 Steer-by-Wire Vehicle Dynamics

The emergence of hybrid vehicles, which may include a

gasoline or diesel engine, electric motor with battery pack,

and/or fuel cell, to achieve fuel efficiency and emissions

gains, mandates alternative power steering systems An

attractive replacement to the traditional hydraulic steering

system assist is steer-by-wire, which replaces the hydraulic

fluid actuation with a high torque servo-motor The direct

link between the driver and wheels has been removed and

replaced by a shortened steering column which is connected

to a low torque servo-motor

The automotive steering system has been investigated for

the last five decades to provide improved vehicle lateral

performance and insight into the underlying dynamics for

enhanced steering system designs Analytical and empirical

dynamic models allow the driver's commanded steering

wheel input to king pin torque output to be studied in terms

of the vehicle's lateral motion Post and Law [2] developed

a hydraulic power steering system model to evaluate a

vehicle’s lateral response to various driving maneuvers

Electrical [3] and steer-by-wire [4] power assist steering

system configurations have been examined to investigate

the handling gains and safety requirements

To facilitate the investigation of the human-machine

interface and vehicle's lateral responsiveness to

steer-by-wire systems, a suite of nonlinear models have been

developed In the paper by Mills et al [5], the governing

dynamics are derived and presented for hydraulic, electric,

and steer-by-wire steering systems Of particular interest

are the steer-by-wire subsystem equations based on the freebody diagram shown in Figure 2 This steering model has been integrated with a fourth-order chassis model (e.g., [6]) to estimate the vehicle behavior for various driving maneuvers in the Matlab/Simulink software environment Initially, a passenger vehicle was modeled and the simulation results undergoing validation with experimental transient response data

The equation of motion developed for the front wheel steer angle [5] is of interest in studying the effects of the vehicle dynamics on the driver-vehicle interface These lumped parameter dynamics are described by









−

−

−

−









−

−

L

rack i L w

r

y K I

where T fr , kp is the nonlinear king pin friction [7], M z is the aligning torque at the road/wheel interface, B kp in the front wheel assembly damping coefficient, and I wω Ω is the gyroscopic torque about the steering axis with precession angular speed Ω = (δ,δ&,γ,γ &, z ) which is a function of wheel camber angle, γ, and wheel bounce, z, respectively

A four wheel steering system introduces a rear steering actuator (e.g., second rack and pinion) or individual distributed actuators at each wheel for enhanced vehicle maneuverability and responsiveness in comparison with the

 Registered trademark of The Mathworks, Natick, MA

conventional front wheel steering system [8] For example,

the rear assembly may be counter-phased at low speeds for

greater maneuverability and same-phased at high speeds for

improved stability Lee et al [9] investigated the vehicle

handling performance available with independent four

wheel steering systems

2.2 Autonomous Vehicle Controller

As explained in Section 2.1, an electric or steer-by-wire

system can satisfy the variable power source cycling

scenarios Although electric steering is an acceptable

alternative, it does not permit autonomous vehicle operation

due to the required driver's steering wheel input In contrast,

steer-by-wire systems can accommodate both

semi-autonomous and semi-autonomous operating modes which

facilitates the deployment of these technologies Reliable

positioning systems (e.g., global positioning systems)

enable accurate trajectory generation and following since

the error between the actual and desired paths may be

continually monitored for corrective actions In the paper

by Setlur et al [10], an exact model knowledge nonlinear

tracking controller has been designed to force the vehicle's

trajectory to follow a given reference path This functionality is required for autonomous vehicle operating modes The subsequent Lyapunov-based stability analysis demonstrated that the position and orientation tracking errors were globally, exponentially forced to a neighborhood about zero, which can be made arbitrarily small

Extensive numerical simulations have been performed in both SIMNON (SSPA, Department of Automatic Control, Lund Institute of Technology, Sweden) and Matlab/Simulink to validate the controller design At this time, various mathematical models from different sources have been selected to generate the vehicle dynamics This modeling constraint may be partially attributed to the real time execution requirements within the experimental test station Thus, the validation efforts take on greater emphasis to ensure that the system performance is acceptable especially in the presence of unmodeled dynamics In Section 3, the hardware-in-the-loop test platform discussion provides an overview of the important (i.e., primary) components of the infrastructure needed in such a testing facility

L

K

2

i

M

B

2

M

I

rack

m

kp

W

F

kp fr

rack fr

2

M

θ

rack y

z

Control motor

Steering linkages

L

2

s

K

1

i

L R

1

s

V

1

M

I

M

B

sc

B

1

K

sw

I

1

M

θ

Driver interface

Figure 2: Steer-by-wire system mechatronics diagram

2.3 Haptic Interface Controller

The natural progression in this research leads to the

requirement of road "feel" provided to the driver in both the

cases of local and remotely operated vehicles The need for

force feedback has been extensively studied and understood

in the field of tele-operated robotics As reported by Liu

and Chang [11], force feedback is the second highest rated

inputs to the driver (note that vision ranks highest) While

force feedback provides the driver with an invaluable

sensory input, excessive feedback will expose the operator

to unnecessary vibrations and road surface disturbances

(e.g., bumps) Thus, it is essential for the control strategy to ensure that the road "feel" provided by the force feedback can be adjusted to provide a comfortable driving environment Many researchers have worked to establish a model for generic systems and performed experiments to identify system parameters with the intention of providing simulated force feedback (e.g., [12] and [13])

Recently, Setlur et al [14] developed a method for

integrating force feedback into the driver interface without compromising closed-loop stability The impedance control formulation [15] uses a target system to generate the

reference signal for the driver interface The controller

adapts for parametric uncertainties in the system while

ensuring global asymptotic tracking for the "driver

experience error'' and the "locked error'' However, torque

measurements are required An extension was presented to

eliminate the torque sensor measurements, albeit knowing

the system parameters To illustrate the strategy, assume

that the simplified steer-by-wire driver interface system

dynamics are

1

1+N θ ,θ =τ +T

θ&& & , θ&&2 +N 2(θ2 ,θ&2)=τ2 +T 2

where )θ1(t and θ2(t) represent the angular position of

the driver input device and vehicle steering system,

respectively The auxiliary nonlinear functions N1(.) and

(.)

2

dynamics The variables τ1 ( t ) and τ2 ( t ) denote the

driver input torque and transmission torque between the

actuator on the steering column and mechanical subsystem

actuated by the steering column Finally, 1 ( t ) and T2(t)

are the control input torque applied to the driver input

device and the steering column, and the required torque at

the wheel/road interface The constant values have been

assumed to be unity without loss of generality Then θ&&1 is

forced to follow reference θ which is generated based ond

2 2 1 1 d d

1

T

d N (θ ,θ ) α τ α τ

θ&& + & = + to ensure appropriate

force feedback Lastly, θ is forced to follow 2 θ such that1

the vehicle is steered as commanded by the driver

It is important to note that pure numerical simulations fall

short in providing complete insight into the efficacy of the

controller performance since "feel" must be experimentally

demonstrated which necessitates experimental testing To

facilitate the investigation of alternative driver feedback

variables, based on the available vehicle dynamics, in the

steer-by-wire system, an index will be constructed that

incorporates scaled torque information For instance, the

index may be composed of the aligning torques, king pin

friction torques, and gyroscopic torques, as well as

additional data such as roadway obstacles, surrounding

vehicle locations, and environmental hazard information

The individual elements may be toggled on/off and scaled

to achieve the most desirable feedback to the specific

driver

3 Hardware-in-the-Loop Testbed

A variety of driving simulators are currently used in the

automotive industry including the motion based VIRTTEX

simulator at Ford [16] and assorted fixed based commercial

units (e.g., DriveSafety by GlobalSim, STISIM Drive by

Systems Technology) In this project, a cost effective test

station has been developed which allows investigators to

focus on the human-machine interface of steer-by-wire

systems A suite of mathematical models, nonlinear control

algorithms, and steering interface designs may be evaluated

through experimental testing A real time hardware-in-the-loop (e.g., [17]) experimental test platform has been constructed to provide a virtual reality interface for operators to drive the "vehicle" and evaluate various steering input devices (refer to Figure 3) The experimental work has been performed in the Automotive Research and Mechatronics Laboratories at Clemson University

Pentium III-based computer workstations, executing real-time LINUX operating system and Matlab/Simulink based models, provide the means to implement the control algorithms Additionally, a graphical user interface, developed in-house, on the QNX operating system using C++ has also been used to demonstrate the flexibility of the testbed Data acquisition and control implementation are performed using a Servo-To-Go input/output board at a frequency of up to 800HZ

The various displacements in the steering system are measured directly from encoders and/or LVDT transducers (refer to Figure 4) Furthermore, torque transducers are used

to measure the user's applied and the tire/road interface torques A switch reluctance motor (SRM) along with its torque driver provides the necessary force feedback to the driver A servo motor attached to the steering column is used to provide force feedback and/or steer the vehicle Additional active and passive devices have been added at the rack to apply different road reaction forces A second computer workstation performs virtual reality (VR) computations which can be performed in Matlab However,

to facilitate the need for creating custom scenes and situations, VRML rendering was considered more fruitful

A 60" X 80" screen, along with a high capacity projector, provide the visual feedback for the driver-in-the-loop experiments

Figure 3: Steer-by-wire test bench with computer control

An important feature of the research is alternative steering input devices [18] that may assist both normal and physically challenged drivers For example, custom designed joysticks which integrate force feedback have been designed to accommodate individual drivers (refer to Figure 5) It is anticipated that the research will yield appropriate feedback metrics and mechanism designs for all

types of drivers working in both local and remote vehicle

operating modes The steering test platform will also

support psychology studies on the operator's decision

making process and ability to handle driving distractions

Motor # 1

Motor # 2

Optical Encoder # 2 Amp #1

Amp #2

Power Supply

(+ 90 VDC )

(+ 24 VDC)

(+/- 15 VDC)

Optical

Encoder # 1

Mobile Bench Hydraulic

Cylinder

Power Manifold

Torque Sensor # 2 Torque Sensor # 1

LVDT

Rack

Servo Amplifier I/O BOX

Torque Sensor # 2 Output Torque Sensor # 1 Output

Optical Encoder # 1 Output

LVDT Output

Optical Encoder # 2 Output

PC

Figure 4: Experimental test station component schematic

To accommodate four wheel steer-by-wire systems, an

additional number of steering actuators are required For

coupled front/rear steering, two rack and pinion assemblies

with high torque dc motors are needed Similarly,

independent four wheel steering requires four dc motors

that are distributed to each corner of the vehicle In each

case, the driver's interface remains the same However, the

steering control algorithm, which regulates the motors,

must be redesigned to realize the desired wheel steer angles

Figure 5: Two prototype force feedback driver interfaces

4 Experimental Results

Preliminary tests are being performed to validate the

experimental steer-by-wire laboratory test platform The

simulated vehicle's response has been compared with a

conventional hydraulic assist steering system passenger

vehicle since test data exists for this configuration

However, additional validation activities are planned using

light and medium-duty vehicle test data The vehicle

tracking controller has been validated numerically; the

haptic interface is undergoing operator-in-the-loop testing

As shown in Figure 6, the Cartesian trajectory of the

simulated vehicle in response to the user's provided input at

the steering wheel in the testbed has been displayed In this experiment, the operator has been requested to perform a standard "J" turn maneuver; haptic force feedback was not initially considered However, various force feedback situations shall be introduced for the haptic interface design studies as previously discussed

Figure 6: Simulated vehicle trajectory in response to user

torque at the steering wheel Several typical driving scenarios will be created and simulated using the test platform Some of the research studies will include i) exploring the steering system's “feel”

as communicated to the driver, ii) developing the haptic interface force feedback index, and iii) semi-autonomous and autonomous vehicle operation from a coupled controls and operator perspective in normal/degraded conditions

5 Conclusions

In this paper, a hardware-in-the-loop vehicle simulator has been presented to support steer-by-wire system development and human-machine interface studies The overall concept of the test station is shown in Figure 7 which features a Honda CRV interior to house the steer-by-wire simulator's hardware/software and create a realistic, yet safe, environment Preliminary test results along with the associated theory have been briefly discussed While the computational power of the PC was sufficient for individual tests, to provide testing facilities for driver-in-the-loop experiments, a dSPACE based system will be used to control the hardware and virtual reality functions

Acknowledgement

The authors gratefully acknowledge the partial support of this work by the U.S Army Tank Automotive Command through the Automotive Research Center

References

Opportunities, Challenges and Trends", SAE paper

no 2003-01-0113, 2003

-180 -160 -140 -120 -100 -80 -60 -40 -20 0 20

x direction (meters)

y direction (meters)

[2] Post, J W., and Law, E H., "Modeling, Simulation

and Testing of Automobile Power Steering Systems

for the Evaluation of On-Center Handling", SAE

Paper No 960178, 1996

to an Automotive Variable Ratio Power Steering

System Using Model Reference Robust Tracking

Control”, SAE paper no 960931, 1996

[4] Harter, W., Pfeiffer, W., Dominke, P., Ruck, G., and

Blessing, P., “Future Electrical Steering Systems

Realizations with Safety Requirements”, SAE paper

no 2000-01-0822, 2000

Modeling and Analysis of Automotive Steering

Systems for Hybrid Vehicles”, proceedings of the

ASME IMECE, Design Engineering, NY, NY, 2001

Closed Loop Driver/Automobile Performance With

Four Wheel Steering Control”, SAE paper no

920055, 1992

[7] Post, J W., "Modeling, Simulation, and Testing of

Automobile Power Steering Systems for the

Evaluation of On-Center Handling", Ph.D

dissertation, Department of Mechanical Engineering,

Clemson University, 1995

Steering Control of 4-Wheel Drive Electric Vehicle”,

proc IEEE Workshop on Power Electronics in

Transportation, pp 159-164, Dearborn, MI, 1996.

Independent Steering System for Vehicle Handling

Improvement by Active Rear Toe Control”, JSME

International Journal, Series C, vol 42, no 4, pp.

947-956, 1999

[10] Setlur, P., Dawson, D., Wagner, J., and Fang, Y.,

"Nonlinear Tracking Controller Design for

Steer-by-Wire Automotive Systems", proc American Control Conference, Anchorage, AK, pp 280-285, 2002.

[11] Liu, A., and Chang, S., "Force Feedback in a

Stationary Driving Simulator'', proceedings of the IEEE International Conference on Systems, Man and Cybernetics, vol 2, pp 711-1716, 1995.

[12] Ryu, J., and Kim, H “Virtual Environment for Developing Electronic Power Steering and

Steer-by-Wire Systems”, proceedings of the IEEE/RSJ International Conference on Intelligent Robots and Systems, pp 1374-1379, Piscataway, NJ, 1999.

[13] Gillespie, B R., Hasser, C., and Tang, P.,

"Cancellation of Feed Through Dynamics Using a

Force-Reflecting Joystick'', proceedings of the ASME IMECE, Nashville, TN, 1999.

[14] Setlur, P., Dawson, D., Chen, J., and Wagner, J., "A Nonlinear Tracking Controller for a Haptic Interface

Steer-by-Wire Systems", proc IEEE Conference on Decision and Control, Las Vegas, NV, 2002.

[15] Lewis, F L., Abdallah, C T., and Dawson, D M.,

Control of Robot Manipulators, Macmillan

Publishing Company, 1993, ISBN 0-02-370501-9

[16] Tilin, A., "Redesigning the Driver - You Are About

to Crash", Wired, vol 10, no 4, April 2002.

[17] Wagner, J., and Keane, J., "A Strategy to Verify Chassis Controller Software - Dynamics, Hardware,

Automation", IEEE Transactions on Systems, Man, and Cybernetics, vol 27, no 4, pp 480-493, 1997.

[18] Andonian, B., Rauch, W., and Bhise, V., "Driver Steering Performance Using Joystick vs Steering Wheel Control", SAE paper no 2003-01-0118, 2003

Figure 7: Steer-by-wire test station with virtual reality, cockpit, and input mechanisms

Mission

Human Operator

Graphical Display Audio

Real Time Virtual Environment

Wheel or Joystick Interface

Directional Control Assembly

SCU Environment

Vehicle Dynamics

Visual Information

Sound Force Feedback

Force/Torque Input