AVEC ’04Handwheel Force Feedback for Lanekeeping Assistance:Combined Dynamics and StabilityJoshua P. SwitkesStanford UniversityEric J. RossetterStanford UniversityIan A. CoeStanford UniversityJ. Christian GerdesStanford UniversityMechanical EngineeringStanford, CA 94305-4021, USAPhone: 650-724-4058Fax: [email protected] assistance holds the promise to save thousands of lives per year by maintaininglane position in the absence of driver steering commands. Here we combine force feedbackwith a lanekeeping controller based on lateral and heading error, and analyze the effectof various sources of force feedback on the stability of the vehicle. In addition to forcefeedback based on the aligning moment or artificial damping and inertia, the force canbe based on the level of assistance being given. This coupling of the force feedback andassistance systems can destabilize the vehicle if not designed properly. Linear modelingverified by experiments shows the effect of varying the gains on both the force feedback andthe lanekeeping assistance itself. This analysis shows that within a range of values that feelreasonable to the driver, changes to the lanekeeping controller or force feedback can havemarked affects on the response of the vehicle.Topics / L ane D eparture Prevention, Driver Assist Systems, Steering Assistance and Control1. INTRODUCTIONAccording to the U.S. National Highway Ad-ministration, 55 % of vehicle fatalities are the r esultof unintended lane departure. By maintaining laneposition in the absence of adequate driver steeringcommands a large number of lives can be saved. Thelanekeeping system analyzed here [4] applies a cor-rective force to the vehicle based on its lateral andheading deviation from lane center. Intuitively, thesystem attaches a spring between the car and theroad center line, and this force attracts the vehicleto lane center. The driver can still steer the vehi-cle with the handwheel, and the driver commandis simply added to the lanekeeping command. Thislanekeeping controller has been shown both analyt-ically and experimentally to keep the vehicle in thelane in the absence of driver steering commands.A lanekeeping controller of this type requiressteer-by-wire to allow computer control of the roadwheel steering angle. When the mechanical connec-tion between the hand wheel and road wheels is bro-ken in steer-by-wire, there is no longer a naturalsource of force feedback. Here we explore a varietyof force feedback sources in conjunction with thelanekeeping system described above:• Added inertia• Added damping• Aligning moment• Force based on assistance being givenMost work on handwheel force feedback has fo-cused on transmitting the mechanical aligning mo-ment resulting from the steering geometry in a com-fortable way to the driver. This is largely a result ofthe fact that the only source of torque available to betransmitted on a conventional vehicle is the aligningmoment. To date, researchers have mainly exploredways to recreate the feel of a conventional vehicle insteer-by-wire. Many researchers have investigatednonlinear modeling of steering feel [10],[7],[11],[5] aswell as ways to ensure the road wheels track thehandwheel adequately [8]. Other work has devel-oped force feedback for alternative input sourcessuch as a joystick [12], or schemes to cancel out ve-hicle induced accelerations on the input device [1].From a different perspective, attempts havebeen made to gauge the acceptability of various as-sistance systems to drivers. Some of these systemsare passive, using noise or torque to warn the driverof an imminent lane departure. Sato et al.[6] foundthat a torque warning to the driver of imminentlane departure is more effective than sounds whileSuzuki[9] found that a torque warning can causethe driver to steer in the wrong direction if not de-signed properly. Some researchers have developeddriver models to allow analysis of human responseAVEC ’04to assistance systems [3], while others have designedsystems that take control authority away from thedriver when necessary [2].The goal of the lanekeeping system analyzedhere is to keep the vehicle in the lane in the absenceof driver inputs. Thus when combined with forcefeedback we seek to meet two basic objectives:1. Present easily controllable feel to driver: Thesystem should generate forces on the handwheelto enable the dr iver to control the vehicle.2. Remain stable with hands off handwheel: Theoverall system consisting of the handwheel andthe vehicle itself must remain s table and in thelane.In a mechanically steered vehicle, stability of theoverall system is assured by designing the steeringsystem such that the aligning moment acts to returnthe handwheel to its center position. With the forcefeedback now based on vehicle position states, theforces on the handwheel may not always be in astabilizing direction.To analyze the combined system of force feed-back and lanekeeping assistance, linear models ofthe vehicle and force feedback subsystems are pre-sented. Next these models are combined to form alinear model of the entire system. The specific typesof force feedback of interest are added to this modelas functions of the states. A linear root-locus analy-sis is then performed on this linear system to exam-ine the effects of modifying the force feedback. Thisanalysis shows that within a range of values thatfeel reasonable to the driver, changes to the lane-keeping controller or force feedback can have markedaffects on the response of the vehicle. Experimentson a steer-by-wire vehicle allow validation of the lin-ear modeling and qualitative evaluation of the forcefeedback.2. LANEKEEPING CONTROLLERThis work builds on the lateral and heading er-ror based lanekeeping controller of [4]. This con-troller seeks to keep the vehicle in the lane throughthe application of forces derived from an artificialpotential energy. The potential is shown conceptu-ally in Figure 1, where it provides zero force on lanecenter, and increasing force as the vehicle deviatesfrom the specified path. This force is derived fromthe lateral and heading errors of the vehicle, andcan be thought of as a spring connecting the vehicleto the lane center. The controller has two parame-ters: the potential field gain k, which represents theeffective spring constant, and a lookahead distancexla. This lookahead distance is a gain on the head-ing error of the vehicle, and is necessary for stabilityat high speeds.