2009-01-0048 TRW - Artifical steering feel

2009-01-0048Artificial Steering FeelDaniel E. Williams and Kenneth A. SherwinTRW Commercial Steering Systems Copyright © 2009 SAE InternationalABSTRACTA computer controlled steering system providing an artificial feel or synthetic torque feedback to the driver has recently been launched into production in the commercial vehicle market. This work compares the artificial feel control strategy with prior electric power steering control strategies and hydraulic power steering. Suitability for integration with other vehicle control systems such as lane sensing and electronic stability enhancement is explored.BACKGROUNDAn advanced steering system that provides a computer-generated synthetic torque feedback to the driver has been available in select heavy vehicles for the past two years. For duty cycles typical of a pick-up and delivery vehicle, distinguished by relatively low vehicle speed and large displacement steering inputs, driver workload is reduced by approximately 75%.i Over-the-road commercial vehicles have a somewhat contrasting duty cycle dominated by high speed lane maintenance. For these vehicles the synthetic torque increases apparent precision to the driver. The combination of unprecedented speed-proportionality and on-center precision allow this system to add significant value to commercial vehicles by enhancing steering system performance.iiCommercial vehicle duty cycles are characterized by low lateral accelerations. In general, the lateral accelerations of commercial vehicles are limited by rollover. Passenger cars do not have this limit, and therefore can generate lateral acceleration until the contact patch of the tires can no longer produce additional lateral forces resulting in increased yawing moments. Regardless of the different duty cycles, the advantages of artificial steering feel in passenger car application are likely to include enhanced ability to interface with other vehicle control systems, e.g. electronic stability enhancement and lane keeping. Enabled by a new control strategy, more precise transmission of haptic signals to the driver is possible than with conventional electric power steering (EPS) control strategies.Conventional hydro-mechanical power steering systems generate an auxiliary assist based on input torque from the driver through the handwheel. This input torque deflects a torsion bar that in turn results in the displacement of a rotary valve. The amount of hydraulic pressure used to generate the steering assist is determined by valve displacement, and is therefore ultimately a function of input torque. Therefore actuatoreffort is a function – specifically a nonlinear amplifier - of input effort commonly referred to as the “bath-tub” curve shown in Figure 1. The “bath-tub” curve is a natural consequence of hydraulic flow through the open center control valve used in the steering gear. For many years it has worked well in power steering applications, producing minimal assist on center, and higher assist levels away from center to minimize the peak parking efforts required from the driver. Such an actuator can be referred to as “open loop” in the frequency range of driver input as actuator output torque is a function of input torque. (Although in a strict control systems sense it is a closed loop position follower with attendant high frequency potential instabilities).For some time an electrically actuated passenger car steering system has been used to directly replace the hydraulic system in some smaller passenger cars. Electrical power steering includes an integrated assembly consisting of an electric motor, ECU, position and torque sensors requiring an input connection including only power and CAN. The upper end of the unit is connected to the handwheel, and the lower end to the pinion and mechanical rack. With the advent of electrical power steering, it is much easier to tune a passenger car steering system to specific vehicle operating conditions, such as speed, while intentionally reproducing the nonlinear behavior presently produced by open-center hydraulics of little assistance near center, and disproportionately large assistance away from center. The on-demand nature of EPS eliminates the parasitic losses associated with open-center hydraulic power steering. Electric power steering also provides packaging and environmental benefits relative to the hydraulic system it replaces.iii, ivThe commercial vehicle application of EPS hardware, shown in Figure 2, is different than the passenger car application. The hydraulic gearbox is retained rather than replaced as in the passenger car application. This hardware configuration can be generically referred to as torque-overlay. The high torques and forces required to steer heavy axles make the power density of hydraulics an attractive actuation technology for commercial vehicles. The conventional steering system has proven to be effective, inexpensive, durable and reliable, however it loses many of these advantages when programmability is incorporated into the hydraulics. Using the control strategy described below, the electric motor is used to modify torque feedback to the driver, and the hydraulic gearbox does the work required to steer the heavy axle. Thus driver feedback and actuation are decoupled. Because torque feedback to the driver is synthetically derived independent of road wheel forces, the proposed strategy is easily portable across vehicle types.In many respects performance characteristics of steering systems have been compromised or constrained by steering system feel. The fundamental compromise in steering system design is that drivers favor a relatively high stiffness around the center position when they are maintaining a lane, and also prefer low efforts when they are parking a vehicle. This is particularly true of commercial vehicles, where high power is required to statically steer heavy axles.v The “bath-tub” curve inherently provides little assistance on center, so that steering feel perceived by the driver is partially determined by mechanical steering and vehicle properties generated at the contact patch and propagated up the mechanical drive train. At higher input torque values increased assistance overwhelms the mechanical effects, and the driver perception is determined by the hydraulic valve and torsion bar.ReservoirPower Steering PumpPower Steering GearPitman ArmSteering ColumnIntermediate ShaftDraglinkColumnDrive Figure 2: Pictorial and Schematic Commercial Vehicle ColumnDrive InstallationsLash or looseness is particularly objectionable to the driver on center while maintaining a lane. If lash is present in the steering system on center, the driver will constantly be uncertain about how much input is required to correct the vehicle position within the lane. Lash is an inevitable effect of wear in interfacing mechanical parts and can sometimes be improved by designing these parts with a bit of preload. This preload causes unavoidable friction, which is as offensive as lash. In both cases, the unique relationship between handwheel torque and handwheel position is sacrificed as there can be a range of handwheel torques for a given displacement, or vice versa. Worse yet, in many cases different parts of the mechanical connection between the handwheel and road wheel contribute lash and friction respectively, resulting in complex nonlinear behavior. Given sufficient on-center stiffness, this uncertainty or unpredictability in the heavy vehicle steering system is most objectionable.Much of the theoretical understanding of the driver comes from the aerospace industry’s attempt to model pilot behavior. The torque exerted by the driver on the handwheel is reacted against by the steering system dynamics, with the net sum of torques producing a handwheel motion. This handwheel motion is sensed by the hands and arms of the driver and proprioceptively fed back to the driver’s brain where it is compared with the intended handwheel displacement.vi,vii Thus the uncorrelated handwheel torque and position are ultimately reconciled through driver control effort. To the extent that torque and position are well correlated, this control task is less intense.Figure 3: Typical EPS Passenger Car Torque Control [iv, viii]C LOSED LOOP TORQUE CONTROLThe various compromises present in steering systems result in sub-optimal torque feedback to the driver. Steering is not stiff enough when maintaining a lane, too heavy when parking, and too degraded by mechanical nonlinearities. Such problems are inevitable consequences of the open loop torque amplification function of conventional steering systems.Since the assist control function of typical EPS controllers is programmed to generally mimic the “bath-tub” curve describing the nonlinear relationship between torque input from the handwheel and the assisting output torque, EPS and the hydraulic system share many of the same functional constraints. More advanced EPS controlalgorithms shown in Figure 3 include damping and return functions that act against the torque amplification of the assist function and also inertia compensation. These modifying functions can be complex, nonlinear and speed dependent, allowing for many degrees of freedom for tuning the vehicle steering.viiiA closed loop is formed when the steering system transforms the EPS assistance torque from the motor to handwheel position and torque, which are inputs to the actuator control. The main function of the EPS control is to reduce driver efforts, therefore the principle input to the assist control is handwheel torque. Hence a control loop is closed on driver torque with a zero command or reference point. As shown in the “bath-tub” curve of Figure 1 there is effectively zero gain on-center. Since the nonlinear assist characteristics of the hydraulic “bath-tub” curve are preserved in the assist control function, the EPS has limited authority to modify torque feedback in on-center lane keeping. Small input torque does not produce appreciable assist torque, and it takes relatively large input torques to produce significant assist. This lack of authority coinciding with the reference command combines to produce a nonlinear system that is effectively open-loop, despite the presence of a feedback loop in the control schematic. Opposed to conventional open loop systems discussed previously are closed loop systems, where actuator effort is a function of the difference between a measurement and the desired value of the measurement. The closed loop actuator always acts to reduce the difference between the desired and actual value, and if the actuator has sufficient authority and bandwidth, it can be assumed that the measurement is the desired value. Knowing the mechanical properties of the handwheel and input drive shaft the actual torque feedback to the driver is controlled, and not merely the measured torque at the shaft sensor. As shown in Figure 4 the commercial vehicle application of torque-overlay, ColumnDrive, uses closed loop control of column torque to achieve a calculated handwheel torque feel.ix Control algorithms for ColumnDrive perform two general functions. The “synthetic torque calculator” first determines the optimal handwheel torque feedback, and the “dynamic compensator” conditions the closed torque loop to provide maximum fidelity to achieve the calculated value.Linear dynamics of the rotating inertia between applied handwheel torque and the torque measurement are accounted for in the synthetic torque calculation so that the desired effect to the driver is achieved. Mechanical nonlinearities between the sensor location andhandwheel, arising from universal joints and slip shafts will significantly degrade closed loop torque-overlay performance as perceived by the driver. Therefore, the optimal location for the electric motor is in the upper column as shown in Figure 2. When assembled in this configuration, nonlinearities associated with the intermediate steering shaft are submerged in the closed torque loop and therefore not perceived by the driver. The torque feedback to the driver is determined completely by the synthetic torque calculator based on sensor inputs. For example it is possible to construct a relationship between torque feedback and handwheel position so that there is a high stiffness region around the center position of the steering system consistent with lane-keeping corrections, and reduced-gradient efforts away from center as shown in Figure 5.In the closed loop torque control system, torque on the mechanical shaft connecting the electric motor to the hydraulic gearbox can be considered a disturbance to the control loop. As such, this disturbance is greatest during a static steering input, when the road wheels on the heavily loaded axle are rotated on dry pavement with a high friction coefficient while the vehicle is motionless. In this demanding situation, the effectiveness of the closed torque loop is evident in Figure 6. When a conventional hydraulic commercial vehicle steering system is steered from one lock to the opposite lock in this condition, a large hysteresis loop is present. Moving away from an initial position at full lock, as handwheel torque is removed the torsional windup of the tire is relaxed, and then handwheel torque is applied in the opposite direction to wind up the tire until the tire begins to skid across the dry pavement. Once this skidding begins, relatively constant torque is required until the opposite lock is reached, and the process is reversed. The hysteresis loop for a conventional hydro-mechanical steered commercial vehicle is shown as “no EPS” in Figure 6.In its traditional passenger car application, the EPS is programmed to behave much like the hydraulic system – an open loop torque amplifier. If the passenger car open loop EPS is installed in series with a conventional commercial steering system, the qualitative behavior is unchanged, however less handwheel input torque is required to static steer as it is twice amplified, once electrically and again hydraulically. This can be seen in Figure 6, as the magnitude of the “open loop” hysteresis plot has been reduced, but the same qualitative effect remains of handwheel torque increasing until the open loop assist level that it generates is able to skid the tires, albeit at a lower handwheel torque. When the electric motor control algorithm is changed to a closed torque loop, the hysteresis plot is qualitatively transformed. If there is no torque input from the driver, the system will return automatically to its exact nominal position. Furthermore, it can be seen that the return path from full lock to center retraces the trajectory to full lock from center, signifying a very well compensated torque loop that has the capability to completely reject the largestFigure 5: High Speed Torque as a Function of Handwheel Positionanticipated disturbance which is encountered in static steering.If the closed loop strategy was employed in a passenger car application, without the hydraulic gearbox, the feedback torque from the mechanical rack would be a disturbance into the closed loop, similar to the commercial vehicle dry park. In both cases, the artificial feel exclusively determines the torque felt by the driver. The precision associated with the lack of hysteresis can be seen in lane maintenance performance data from an on-highway semi-tractor as shown in Figure 7. When maintaining a lane it is desirable to have the steering at its nominal position and torque. With these criteria in mind, the closed torque loop control does a much more effective job of allowing the driver to maintain a lane with minimal correction. This is because nonlinearities of thesteering system are masked, and the torque feedback to the driver is a synthetic product of the operating condition of the vehicle.A very important feature of the ColumnDrive control is its ability to dynamically change the nominal center position. As shown in the high speed stiffness plot of Figure 5, the center position moves left or right in order to ensure that on average the driver inputs zero-mean long term handwheel torque. As steady state steering disturbances are encountered, such as changes in road crown or cross wind, the handwheel must be displaced, causing an immediate increase in torque feedback to the driver. Over time, the required torque input to maintain this displacement offset is decreased, so that eventually no torque is required as seen in Figure 8.Figure 7: On-center Performance of Various Systems (closed loop data is dynamically centered asdescribed below, whereas open loop data contains a sensor bias)has shown that a minute is about right to null a 10 degree handwheel offset. This pull compensation is a valued performance feature, particularly for heavy vehicles with large profiles that are more significantly impacted by such road crown and side wind disturbances. Its real value is more subtle, however. In the past, on-center stiffness was limited by the amount of handwheel torque that could be tolerated to respond to such biased inputs. Higher stiffness requires higher torques for a given handwheel displacement, and the vehicle can never be precisely aligned for all roads and winds. Since the synthetic torque calculation automatically varies the center position to account for such very low frequency disturbances, optimally high on-center stiffness can be achieved without compromise. INTEGRATION WITH OTHER VEHICLE SYSTEMSClosed loop torque-overlay improves on-center handling because the synthetic stiffness can be much higher. This synthetic stiffness feels like a spring pulling the handwheel to a position corresponding to a straight-ahead vehicle trajectory. The center position of the handwheel torque and position relationship shown in Figure 5 is slowly moved horizontally to correlate zero torque with the most common long term average handwheel position. If the vehicle is assumed to be sensed lane position or the dynamic state of the vehicle in a demanding handling maneuver.For example, when the center position is calculated and the vehicle is experiencing a high speed turn on an interstate, handwheel torque is required to offset the displacement from the nominally calculated straight-ahead center. If the vehicle were equipped with a lane-keeping sensor and an algorithm that would calculate a handwheel position corresponding to a desired vehicle lane trajectory back to center, the steering system could receive the external signal defining the desired handwheel position. The synthetic torque feedback would generate a torque encouraging the driver to steer closer to the desired steering position thereby moving the vehicle closer to its desired lane position. This could be easily accommodated with slight modifications in the control software, as the center position previously calculated would now be an input.The synthetic torque feedback minimizes noise transmitted to the driver arising largely from the nonlinearities in the mechanical input driveline, just as before. The synthetic torque feedback architecture shown in Figure 4 has favorable signal to noise properties when transmitting the externally derived signal, as shown in Figure 9. For the particular case measured, the handwheel torque associated with the external signal was roughly 3 Nm. In a similar manner,The adaptation window is tunable, typically experienceIt has been anticipated that the electric power steering system can be used to assist the driver to make the correct steering input in demanding handling maneuvers.x When the handwheel is rigidly held and there is a step change in the desired center position, handwheel torque urging the driver in the newly desired direction is developed quite quickly, as shown in Figure 10. After roughly 65 milliseconds half the desired torque is generated, and after roughly 125 milliseconds the full torque is achieved. This response time to generate a feedback torque to the driver is roughly seven times quicker that a typical commercial vehicle lateral handling response,xi so the steering system can generate a torque signal to the driver that accurately reflects the dynamic state of the vehicle.Passenger car vehicle dynamics are roughly three times quicker than commercial vehicles, so the synthetic torque feedback of an external signal would be less accurate in dynamically transmitting broader bandwidth vehicle state information to the driver. However, it is important to remember that often times the biggest delay in the man-machine closed loop response is the driver, sometimes as much as a 0.65 sec for an alerted driver and up to twice that for normal driving.xii So while the synthetic torque control might not be able to transmit allhigh frequency vehicle dynamic signals to the driver, it can transmit some information up to an order of magnitude more quickly than the driver can formulate his/her own response.The free response of the steering system is slower than when the handwheel is held, as the considerable inertia of the handwheel must be accelerated. The nominal center position of the torque-position relationship of Figure 5 can be offset, and the free response of the steering system measured. This “hands-free” response of the steering system to a change in center position is a function of the synthetic stiffness as shown Figure 11. Alternatively, the electric motor could be controlled in a position loop, driving the hands-free response of the system to the demanded steering displacement. The response of the electric motor when reprogrammed in a position loop is shown for reference. Even though the on-center stiffness as shown in Figure 5 is varied, the maximum torque is limited in current practical applications to around 7-10 Nm. This torque authority level has been found sufficient to provide good steering feel given the presence of the hydraulic gear. When the passenger car EPS is the sole source of power assist in the typical passenger car application the hardware can generate up to 55 Nm. This full torque authority is available in the position loop, which allows a faster response. The faster response of the well conditioned position loop sacrifices all sense of steering feel to the driver. To the extent that the on-center stiffness and maximum allowable torques of Figure 5 are increased, the torque control system described in Figure 4 increasingly resembles a position control system. Thus there is a continuum of possibilities ranging between driver assistance and driver automation.For real autonomous vehicle behavior, particularly for highly precise low speed maneuvers such as trailer docking or parking, a position loop could be required. It is possible to have the electric motor in such a position loop control to maximize vehicle response, but revert to the typical torque loop control shown earlier in Figure 4 when an external torque input from the driver is sensed on the handwheel, e.g. the driver grabs hold of the handwheel. In this manner, for example a transit bus could be autonomously parked, and yet the driver could still easily assume control of the vehicle in the event of an unanticipated hazard.CONCLUSIONThe evolution of steering systems has been discussed, showing the artificial steering feel provided by closed loop torque overlay (ColumnDrive) as an innovation beyond the passenger car application of electric power steering (EPS). This synthetic torque feedback provides demonstrated value in the commercial vehicle application, but also enables integration with othervehicle control systems that provides further increasedvalue in all vehicle segments. This integration is easilyachieved with relatively minor modification to current production software.ACKOWLEDGMENTSThe artificial feel steering work described in this paperwas performed by the advanced engineering group ofTRW Commercial Steering Systems, with enthusiastic support of our management. It is the authors’ privilegeto present the work of our group. Dr. Mark Tucker fromTRW Conekt provided a valued review.CONTACTDaniel E. Williams, Ph. D., P.E., Chief Engineer, TRWCommercial Steering Systems, Lafayette, IN.Telephone:765-429-1691,email:dan.williams@.Kenneth A. Sherwin, Staff Engineer, TRW CommercialSteering Systems, Lafayette, IN. Telephone: 765-429-1890, email: ken.sherwin@REFERENCES[i] Sherwin, Ken, and Dan Williams, SAE paper 2008-01-2702, October 2008.[ii] Williams, Daniel E., “Synthetic Torque Feedback toImprove Heavy Vehicle Drivability,” Journal of AutmotiveEngineering, to appear.[iii] Badawy, Aly A., Farhad Bolourchi, and Steve K.Gaut, “The Design and Benefits of Electric PowerSteering,” SAE 973041, October 1997.[iv] Badawy, Aly, Jeff Zuraski, Farhad Bolourchi andAshok Chandy, “Modeling and Analysis of an ElectricPower Steering System,” SAE 1999-01-0399, March1999.[v] Peppler, S.A., J.R. Johnson, and D.E. 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