UNIVERSITY OF TRENTO Doctoral School in Engineering Of Civil And Mechanical Structural Systems Adaptive Brake By Wire From Human Factors to Adaptive Implementation Thesis Tutor Doctoral Candidate Prof. Mauro Da Lio Andrea Spadoni Mechanical and Mechatronic Systems December 2013 - XXV Cycle 1 Adaptive Brake By Wire From Human Factors to Adaptive Implementation 2 Adaptive Brake By Wire From Human Factors to Adaptive Implementation Table of contents TABLE OF CONTENTS .............................................................................................................. 3 LIST OF FIGURES .................................................................................................................... 6 LIST OF TABLES ...................................................................................................................... 8 GENERAL OVERVIEW .............................................................................................................. 9 INTRODUCTION ................................................................................................................... 12 1. BRAKING PROCESS FROM THE HUMAN FACTORS POINT OF VIEW .................................... 15 1.1. THE BRAKING PROCESS AND THE USER-RELATED ASPECTS ......................................................................... 15 1.2. BRAKE ACTUATOR AS USER INTERFACE ................................................................................................. 16 1.3. BRAKE FORCE ACTUATION: GENERAL MOVEMENT-FORCE DESCRIPTION ....................................................... 17 2. BRAKE PEDAL FEELING AND IMPACT FACTORS ................................................................ 21 2.1. INTRODUCTION ON BRAKE FEELING ..................................................................................................... 21 2.2. IMPACT FACTORS ON BRAKE FEELING................................................................................................... 21 2.3. RESEARCH PERFORMED BY RENAULT ABOUT FACTORS IMPACTING ON BRAKE FEELING .................................... 24 3. PEDAL FEELING DESIGN ................................................................................................. 27 3.1. HIGH-LEVEL REQUIREMENTS ............................................................................................................. 27 3.2. GENERAL ASPECTS .......................................................................................................................... 29 3.3. DESIGNING EXAMPLE....................................................................................................................... 30 3.4. SITUATION ADAPTED PROPERTIES - USE CASES AND USERS' EXPECTATIONS .................................................. 34 3.5. SITUATION RECOGNITION CRITERIA..................................................................................................... 37 3.5.1. Adaptation braking speed reduction while driving or to stop .............................................. 37 3.5.2. Target and parking braking................................................................................................ 38 3.5.3. Emergency braking ............................................................................................................ 38 3.5.4. Constant speed braking (e.g. downhill) .............................................................................. 38 3.5.5. Repeated hard braking - Similar to emergency braking (e.g. race scenario) ........................ 38 3.5.6. Fading effect - Holding the vehicle and hard repeated braking actions ............................... 38 3.5.7. Stopping in vehicle manoeuvre........................................................................................... 39 3.5.8. Holding the stopped vehicle for restarting on plane road ................................................... 39 3.5.9. Holding the vehicle and adapt its position on a slope ......................................................... 39 3.5.10. Holding the vehicle stopped for restarting on a slope ......................................................... 39 3.5.11. Stop the vehicle and key off - Parking service brake............................................................ 39 3.5.12. On highway, not perform speed reduction but foot touch the brake pedal ......................... 39 3.5.13. Full loaded vehicle.............................................................................................................. 40 3.6. SCENARIO – USE CASES DESCRIPTION ................................................................................................. 40 4. PEDAL FEELING CURVES ................................................................................................ 43 4.1. ADAPT CURVE ............................................................................................................................... 44 4.2. LOW/PARKING CURVE ..................................................................................................................... 45 4.3. EMERGENCY CURVE ........................................................................................................................ 46 4.4. CONSTANT CURVE .......................................................................................................................... 47 5. LOGICAL ARCHITECTURE AND IMPLEMENTATION ........................................................... 49 5.1. LOGICAL ARCHITECTURE FLOW .......................................................................................................... 49 3 Adaptive Brake By Wire From Human Factors to Adaptive Implementation 5.2. MATLAB/STATEFLOW DIAGRAM ....................................................................................................... 50 5.2.1. Macro State ....................................................................................................................... 50 5.2.2. Observer Module ............................................................................................................... 52 5.2.3. States enter conditions within “Observer” module ............................................................. 52 5.2.4. State Action/Output........................................................................................................... 54 5.2.5. AccCalc Module ................................................................................................................. 54 5.2.6. “PedalControl” Module ...................................................................................................... 55 5.3. MODEL SIMULATION & DEPLOY ........................................................................................................ 57 5.3.1. Model Simulation............................................................................................................... 57 5.3.2. DSPACE MicroAutoBox HW ................................................................................................ 57 5.3.3. Model Deploy on DSPACE ................................................................................................... 58 6. MODEL VALIDATION THROUGH EXPERIMENTAL VEHICLE DATA ....................................... 61 6.1. VEHICLE INTEGRATION ..................................................................................................................... 61 6.2. TRIALS DATA ................................................................................................................................. 63 6.2.1. Adaptation braking ............................................................................................................ 63 6.2.2. Running ............................................................................................................................. 63 6.2.3. Braking action at low speed - Park/Stop ............................................................................. 64 6.2.4. Emergency braking ............................................................................................................ 65 6.2.5. Constant speed braking ..................................................................................................... 65 6.2.6. Repeated hard braking ...................................................................................................... 66 6.2.7. Keep the stopped vehicle on plain road before restart ........................................................ 66 6.2.8. Keep the vehicle and adapt its position on a slope .............................................................. 66 6.2.9. Stop the vehicle, key off and parking brake activation ........................................................ 67 6.2.10. Mixed Scenario .................................................................................................................. 67 6.3. BENCH PREPARATION ...................................................................................................................... 68 6.4. VALIDATION RESULTS ON “OBSERVER” MODULE ................................................................................... 68 6.4.1. Repetition of all the tests ................................................................................................... 70 6.5. VALIDATION RESULTS “ACCCALC” AND “PEDALCONTROL” MODULE .......................................................... 70 7. TESTS ON CLOSED-LOOP MOTOR CONTROLLER ............................................................... 71 7.1. EC MOTOR ................................................................................................................................... 71 7.2. MOTOR CONTROL TEST ENVIRONMENT .............................................................................................. 71 7.3. STEP INPUT ................................................................................................................................ 72 7.4. RAMP INPUT .............................................................................................................................. 73 7.5. STAIR INPUT ............................................................................................................................... 74 7.6. SINE INPUT ................................................................................................................................. 75 7.7. RESULTS ..................................................................................................................................... 76 8. CONCLUSIONS .............................................................................................................. 77 9. NEXT STEPS .................................................................................................................. 81 9.1. DRIVING SIMULATOR TEST & EXPERIMENTAL PLAN ................................................................................ 81 10. ANNEX 1: THE DESIGN OF BRAKE PEDALS - MATERIALS AND RATIO.................................. 83 11. ANNEX 2: BRAKE BY WIRE PEDAL DESIGN HYPOTHESES AND REVIEW OF PATENTED TECHNOLOGIES .................................................................................................................... 87 11.1. GENERAL REMARKS ......................................................................................................................... 87 11.2. REVIEW OF RELEVANT PATENTS.......................................................................................................... 89 11.3. ALTERNATIVE PEDAL DESIGN SOLUTIONS .............................................................................................. 92 4 Adaptive Brake By Wire From Human Factors to Adaptive Implementation 11.3.1. Linear sensor, spring and rubber design solution ................................................................ 92 11.3.2. Rotating motor-driven solution .......................................................................................... 95 11.3.3. Magneto-rheological brake design ..................................................................................... 96 11.3.4. Other design solution ......................................................................................................... 97 12. ANNEX 3: BRAKE BY WIRE PEDAL REACTION TIMES AND BRAKING BEHAVIOR: EXAMPLE FROM AN EXPERIMENTAL RESEARCHES ............................................................................... 101 12.1.1. Relevant research studies about braking forces and deceleration ..................................... 104 12.2. EMERGENCY REACTION AND ACTUATION TIMES................................................................................... 107 13. REFERENCES ............................................................................................................... 109 5 Adaptive Brake By Wire From Human Factors to Adaptive Implementation List of figures FIGURE 1: RESEARCH WORKFLOW ................................................................................................................... 13 FIGURE 2: CONTROL LOOP DRIVER-VEHICLE DURING BRAKING PROCESS (BREUER , BILL, 2008)............................. 15 FIGURE 3: TIME PHASES OF AN EMERGENCY BRAKE ACTION (BREUER , BILL, 2008) ............................................. 16 FIGURE 4: EXAMPLE OF NORMAL BRAKING PHASES (KASSAGI, 2003) ................................................................. 16 FIGURE 5: BRAKING SYSTEM HMI INPUT-OUTPUT ............................................................................................ 17 FIGURE 6: PEDAL AND DECELERATION CHARACTERISTICS (BRAUER , BILL, 2008) .................................................. 18 FIGURE 7: EXAMPLE OF PEDAL FORCE AND PEDAL TRAVEL DESCRIBED BY TYPE OF VEHICLE (BREMBO 2012) ......... 19 FIGURE 8: REGIONS OF THE USUAL BRAKE PEDAL CHARACTERISTIC (BILL ET AL., 1999) ........................................... 21 FIGURE 9: BRAKE PRESSURE, FORCE, JUMP-IN AND PEDAL TRAVEL ..................................................................... 24 FIGURE 10: PARAMETERS IDENTIFIED BY RENAULT TO ADJUST BRAKE FEELING LAWS (DAIROU, 2003). ..................... 25 FIGURE 11: CORRELATION AMONG PREDICTED SENSATION AND SENSORY RATINGS (RENAULT, DAIROU, 2003) ......... 26 FIGURE 12: EXAMPLE OF BRAKE BY WIRE ARCHITECTURE (CHEON ET AL., 2010).................................................... 29 FIGURE 13: EXAMPLE OF FUNCTION-ANALYSIS-SYSTEM-TECHNIQUE DIAGRAM FOR BRAKE DESIGN (PAN, 2007) .... 30 FIGURE 14: EXAMPLE OF QUALITY-FUNCTION-DEPLOYMENT DIAGRAM FOR BRAKE DESIGN (PAN, 2007)............... 31 FIGURE 15: DERIVED CURVES BY JUMP-IN VARIATION 0%, 5%, 10% (BILL ET AL., 1999). ..................................... 33 FIGURE 16: DERIVED CURVES BY IDLE PEDAL TRAVEL VARIATION 10MM, 20MM, 30MM (BILL ET AL., 1999). ........... 33 FIGURE 17: ACTIVE BRAKE PEDAL BEHAVIOUR STRATEGY OUTLINE ..................................................................... 34 FIGURE 18: DRIVING USE CASES CONCERNING BRAKING ................................................................................... 35 FIGURE 19: DESIRED SITUATION-ADAPTED PROPERTIES OF THE ACTUATION DEVICE (BILL ET AL., 1999) .................... 35 FIGURE 20: HYPOTHESES OF SITUATION-ADAPTED BRAKING CURVES ACCORDING TO USERS' EXPECTATIONS ........... 37 FIGURE 21: ADAPT CURVE - FORCE VS PEDAL TRAVEL AND ACCELERATION VS PEDAL TRAVEL ................................ 44 FIGURE 22: LOW/PARKING CURVE - FORCE VS PEDAL TRAVEL AND ACCELERATION VS PEDAL TRAVEL..................... 45 FIGURE 23: EMERGENCY CURVE - FORCE VS PEDAL TRAVEL AND ACCELERATION VS PEDAL TRAVEL ........................ 46 FIGURE 24: CONSTANT CURVE - FORCE VS PEDAL TRAVEL AND ACCELERATION VS PEDAL TRAVEL .......................... 47 FIGURE 25: SW ARCHITECTURE ..................................................................................................................... 49 FIGURE 26: MODEL MAIN STRUCTURE ............................................................................................................ 50 FIGURE 27: MODEL: INIT STATE MACHINE AND CONTROL BLOCK ....................................................................... 51 FIGURE 28: “OBSERVER”, “PEDALCONTROL”, “ACCCALC” ................................................................................ 51 FIGURE 29: "OBSERVER" MODULE ................................................................................................................. 52 FIGURE 30: “ACCCALC” MODULE ................................................................................................................... 54 FIGURE 31: “PEDALCONTROL” MODULE ......................................................................................................... 56 FIGURE 32: PID SETTING ............................................................................................................................... 57 FIGURE 33: DSPACE MICROAUTOBOX HW ................................................................................................... 57 FIGURE 34: SIMULINK® LIBRARY WITH DSPACE COMPONENTS .......................................................................... 58 FIGURE 35: DSPACE CONTROL PANEL ........................................................................................................... 59 FIGURE 36: HW INTEGRATION VIA OBD......................................................................................................... 61 FIGURE 37: DATA COLLECTION VIA SMARTPHONE & SMARTPHONE AXIS ............................................................. 62 FIGURE 38: ADAPTATION BRAKING ................................................................................................................. 63 FIGURE 39: RUNNING ................................................................................................................................... 64 FIGURE 40: BRAKING ACTION AT LOW SPEED .................................................................................................. 64 FIGURE 41: EMERGENCY BRAKING .................................................................................................................. 65 6 Adaptive Brake By Wire From Human Factors to Adaptive Implementation FIGURE 42: CONSTANT SPEED BRAKING DATA COLLECTED ................................................................................. 65 FIGURE 43: PLAIN ROAD AND RESTART ........................................................................................................... 66 FIGURE 44: KEEP THE VEHICLE AND ADAPT ITS POSITION ON A SLOPE................................................................ 67 FIGURE 45: MIXED SCENARIO ........................................................................................................................ 67 FIGURE 46: LOG REPLAY CHAIN...................................................................................................................... 68 FIGURE 47: EC MOTOR AND ENCODER ............................................................................................................ 71 FIGURE 48: MOTOR CONTROL BY PID CONTROLLER ......................................................................................... 71 FIGURE 49: MOTOR POSITION (MAGENTA) VS STEP INPUT (YELLOW) ................................................................. 72 FIGURE 50: MOTOR POSITION (MAGENTA) VS STEP INPUT (YELLOW) - ZOOM-IN ................................................ 72 FIGURE 51: MOTOR POSITION (MAGENTA) VS RAMP INPUT (YELLOW) ............................................................... 73 FIGURE 52: MOTOR POSITION (MAGENTA) VS RAMP INPUT (YELLOW) - ZOOM-IN............................................... 73 FIGURE 53: POSITION (MAGENTA) VS STAIR INPUT (YELLOW)............................................................................ 74 FIGURE 54: MOTOR POSITION (MAGENTA) VS STAIR INPUT (YELLOW) - ZOOM-IN ............................................... 74 FIGURE 55: MOTOR POSITION (MAGENTA) VS SINE INPUT (YELLOW) ................................................................. 75 FIGURE 56: MOTOR POSITION (MAGENTA) VS SINE INPUT (YELLOW) - ZOOM-IN ................................................ 75 FIGURE 57: DRIVING SIMULATOR ................................................................................................................... 81 FIGURE 58: TRADITIONAL BRAKE PEDAL FORCE OUTLINE ................................................................................... 84 FIGURE 59: CALCULATION OF THE PEDAL RATIO .............................................................................................. 84 FIGURE 60: INSTANTANEOUS RATIO VS PEDAL TRAVEL FOR A FIXED RATIO PEDAL (MOK, 2007). .............................. 85 FIGURE 61: 3D MODEL AND FINAL ASSEMBLY OF VARIABLE RATIO CONCEPT BRAKE PEDAL (MOK, 2007) .............. 86 FIGURE 62: EXAMPLE OF MORPHOLOGICAL CHART FOR BRAKE-BY-WIRE PEDAL DESIGN (PAN, 2007) .................. 88 FIGURE 63: EXAMPLE OF PUGH TABLE FOR BRAKE-BY-WIRE TECHNOLOGY EVALUATION (PAN, 2007)................... 89 FIGURE 64: RELEVANT PATENT TIMELINE......................................................................................................... 91 FIGURE 65: PEDAL SIMULATOR : BRAKE FEEL TUNING BY SOFTWARE (CHEON ET AL., 2010) .................................... 92 FIGURE 66: PEDAL SIMULATOR DESIGN - RUBBER TYPES AND EFFORT (CHEON ET AL., 2010) ................................... 93 FIGURE 67: EXAMPLE OF SOFTWARE DESIGN ARCHITECTURE (CHEON ET AL., 2010) .............................................. 93 FIGURE 68: SPRINGS/RUBBER FEELING SIMULATOR CONCEPT (CONTINENTAL PATENT US20060071545) ............. 94 FIGURE 69: TTTTECH AND DELPHI CONCEPT FOR BRAKING PEDAL MODULE ...................................................... 94 FIGURE 70: MOUNT SENSOR TO PEDAL AND BACK BRACKET WITH A FOUR-BAR LINKAGE (PAN, 2007). ..................... 95 FIGURE 71: LINEAR SENSOR BRAKE BY WIRE PEDAL CONCEPT (PAN, 2007). .......................................................... 95 FIGURE 72: MOTOR DRIVEN FORCE FEEDBACK IN HAPTIC ACCELERATOR PEDAL (DE ROSARIO ET AL., 2010) .............. 95 FIGURE 73: BRAKE PEDAL DRIVEN BY MOTOR (STACHOWSKY ET AL. PATENT US20030122418) ............................ 96 FIGURE 74: MAGNETO-RHEOLOGICAL BRAKE PEDAL FEEL EMULATOR (DELPHI PATENT US20020108463) ........... 96 FIGURE 75: BASIC CONFIGURATION OF A MAGNETO-RHEOLOGICAL BRAKE CONCEPT (PARK ET AL., 2006) ................. 97 FIGURE 76: "NARUSE" PEDAL (NARUSE PATENT, US20110107870)................................................................ 97 FIGURE 77: PEDAL EFFORT PLOTTED AGAINST TIME DURING PANIC AND NORMAL BRAKING (FITCH ET AL., 2010B) ... 103 FIGURE 78: CUT-OFF PFG VALUES FOR SATISFACTORY DRIVER-VEHICLE BRAKING PERFORMANCE (DOT, 1970).... 105 FIGURE 79: RECOMMENDED DECELERATION/PEDAL FORCE SPACE (DOT, 1970) ............................................... 105 FIGURE 80: BRAKING PEDAL PRESSURE DURING BRAKING BY PROFESSIONAL AND NON (GREIBE, 2007)............... 106 7 Adaptive Brake By Wire From Human Factors to Adaptive Implementation List of tables TABLE 1: DETAILED VARIABLES INFLUENCING THE BRAKE PEDAL FEEL (BASED ON BILL ET AL, 1999). ......................... 23 TABLE 2: BRAKE BY WIRE POTENTIAL BENEFITS ................................................................................................. 28 TABLE 3: FUTURE BRAKING REQUIREMENTS AND POSSIBLE BENEFITS (SOURCE: BREUER, BILL, 2008) .................... 28 TABLE 4: EXAMPLE OF PEDAL DESIGN ENGINEERING SPECIFICATION: ACTIVE PEDAL FEELING DESIGN PARAMETERS... 32 TABLE 5: DRIVING CONDITIONS ENUM ............................................................................................................ 54 TABLE 6: MAPPING VEHICLE STATE - CURVE SELECTED ...................................................................................... 55 TABLE 7: VALIDATION RESULTS ON "OBSERVER" ............................................................................................... 69 TABLE 8: SIMULATOR TESTS EXPERIMENTAL PLAN ............................................................................................. 81 TABLE 9: LIST OF RELEVANT PATENTS ABOUT BRAKE BY WIRE TECHNOLOGY ....................................................... 89 TABLE 10: REVIEW OF EMERGING BRAKE TECHNOLOGIES (WINKLER, 2005). ........................................................ 98 TABLE 11: DRIVERS' BRAKING PERFORMANCE AT THE SURPRISE CONDITION (FITCH ET AL., 2010A) ........................ 101 TABLE 12: DRIVERS' BRAKING PERFORMANCE AT THE EXPECTED CONDITION (FITCH ET AL., 2010A) ....................... 102 TABLE 13: DRIVERS' BRAKING PERFORMANCE TO AN EXPECTED AUDITORY ALARM (FITCH ET AL., 2010A) ............... 102 TABLE 14: EXAMPLE OF APPLIED BRAKING FORCE IN COACHES AT GRADUAL LEVELS (GRSG-91, 2006)................ 104 TABLE 15: SUMMARY OF PUBLISHED DATA ON HUMAN REACTION TIMES (SOURCE: BREUER , BILL, 2008) .......... 107 8 Adaptive Brake By Wire From Human Factors to Adaptive Implementation General overview Brake systems are undergoing radical change. Decisions to apply (or release) brakes are being made automatically by computer-controlled systems (e.g., ABS and ACC). The type, complexity and shear number of such automatic systems is growing. To enabled automatic control, electro- hydraulic valves acting under computer control have been introduced to the traditional hydraulic systems. Electro-hydraulic brake-by-wire has been introduced, and in the foreseeable future, the brake application system is likely to become predominately electro-mechanical, largely doing away with hydraulic components (NHTSA, 2005). In recent years, the friction brake and the hydraulic actuation system have remained overwhelmingly dominant, but computer-controller “intelligent” systems and are being given increasingly important (and numerous) roles in deciding how and even when to apply brakes. This trend will continue and may lead to the demise of the traditional hydraulic system in favour of the so-called brake-by-wire systems: systems using electrohydraulic actuation and, eventually, electro- mechanical actuation. Recently, regenerative brakes have appeared in small numbers (in hybrid and fully electric vehicles) to supplement the friction brake. The trend is likely to continue, albeit slowly. There may be some very long-term potential for reversal of these roles, i.e., motors acting as regenerative brakes supplying the primary braking effort with smaller friction brakes playing a supporting role. Computer-controlled brake application has become commonplace. Some of these systems modify braking initiated by the driver. Anti-lock braking systems (ABS) and traction control systems apply (or release) hydraulic pressure to prevent excessive (positive or negative) wheel slip. Brake-assist systems accelerate (in time), and may also raise the level of, brake application to improving emergency brake applications. Current brake-assist systems do so by altering the behaviour of the vacuum booster when the driver applies the brake more rapidly than normal. Other automatic systems apply the brake(s) on their own (as apposed to modifying driver- initiated braking). Stability enhancement systems apply individual wheel brakes to prevent pending yaw instability or can apply brakes generally to lower speed for the purpose of avoiding excessive lateral acceleration and, hence, rollover. More recent versions of Adaptive Cruise- Control (i.e. ACC) systems apply service brakes to adjust speed and maintain headway relative to a leading vehicle. The future is likely to see expanded use of similar automatic brake application in the forms of various crash mitigation and crash-avoidance systems. Traditionally, the primary control interface between the driver and the brake system has been the brake pedal: a mechanical link through which the driver energizes the hydraulic master cylinder with forces applied by the foot. A secondary control interface is the parking-brake actuator, usually a hand operated lever or another foot pedal. Dash mounted warning lights provide an interface by which the driver can monitor the health of the brake system and its auxiliary elements. Finally, the rear-mounted brake lights constitute and advisory interface with other drivers regarding braking activity of the host vehicle. The introduction of automatic braking functions has already resulted in some changes to the behaviour of these interfaces. The introduction of ABS effected the driver’s primary interface with the brake system in that pressure pulses induced by the actions of some ABS systems are felt in the form of vibration or pulsation of the brake pedal. This was initially a source of some confusion for drivers but apparently has not been a major problem. Brake-assist systems now on the market 9 Adaptive Brake By Wire From Human Factors to Adaptive Implementation also provide a distinctly different pedal feel when the system is activated as compared to pedal feel during normal applications. The introduction of brake-by-wire systems has the potential for major changes to the primary driver/system interface. In theory, brake-by-wire does not require any forceful actuation by the driver, thereby eliminating the basic reason for actuation via a foot pedal. While the primary actuator could certainly remain as a pedal with either a displacement or pedal-force transducer, it could just as well be a hand-operated lever (joystick) or button as has been demonstrated in some experimental systems. On the other hand, in the foreseeable future, it is apparent that manufacturers have no plans to launch such changes. Indeed, the greater concern seems to be the desire to maintain a traditional pedal feel as brake-by-wire systems evolve. Moreover, manufactures will favour maintaining at least two-wheel hydraulic brake actuation as a fail-safe backup system for some time. Thus, the master cylinder is likely to remain for some time as part of a backup system. The electro-mechanical form of brake-by-wire also offers obvious motivation for altering parking- brake application. In many cases the same electro-mechanical device used to apply the wheel brake for service braking would be appropriate for applying the brake in emergency or parking situations. (Separate electric parking brake applicators have already been developed). With electric application, all of the numerous mechanical components associated with parking brake application could seemingly be replaced with little more than an electric switch and some wiring, implying very attractive weight and cost savings. If we assume that “the brake system” encompasses all of the automated systems that apply brakes, the monitoring interface between the driver and that brake system is becoming far more extensive and complex. The common “ABS” warning light was the first small step in this progression. This warning light provides a simple binary message regarding the general health of the system: either the ABS is properly operational or it is not. On the other hand, ACC systems, for example, have rather elaborate dash-board displays that not only indicate general system health, but provide continuing information on the current operating state on a moment-to-moment basis. These visual displays may indicate various driver-selected settings as well as monitoring system status (e.g., target vehicle recognized/not recognized). Additional visual displays and/or audible warnings indicate that the driver must take over the braking task. As they become available, forward-crash avoidance, lane-departure avoidance and other such systems will presumably have similarly elaborate performance-monitoring interfaces (NHTSA, 2005). Finally, vehicle brake systems interface not only with the driver of the vehicle on which they are mounted, but also with the drivers of following vehicles. Previously, the decision as to when to turn on brake lights was rather straight forward: when the driver applied the brakes. The means to do so was similarly simple: with a switch activated by initial brake-pedal motion or initial brake- pressure rise. Whether or not automatic brake application should be accompanied by brake-light actuation is not always so clear cut. The proliferation of automatic systems for applying brakes may raise this same question again in differing contexts. In any case, the means by which brake lights are actuated cannot remain so simple. Some set of brake actuations must result in brake lights turning on; others must be filtered out so that do not cause the brake lights to illuminate (NHTSA, 2005). The brake itself is also changing. Electric regenerative braking has been introduced as augmentation to the friction brake to improve the energy efficiency of electric and hybrid vehicles. 10