Transportation Research Part C 33 (2013) 22–36

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Transportation Research Part C

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New driving control system with haptic feedback: Design andpreliminary validation tests

0968-090X/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.trc.2013.04.004

⇑ Corresponding author at: CEIT, Paseo Manuel Lardizábal, 15, E-20018 San Sebastián, Spain. Tel.: +34 943 212800; fax: +34 943 213076.E-mail addresses: jjgil@ceit.es (J.J. Gil), idiaz@ceit.es (I. Díaz), pciaurriz@ceit.es (P. Ciáurriz), mecheverria@ceit.es (M. Echeverría).

Jorge Juan Gil a,b,⇑, Iñaki Díaz a, Pablo Ciáurriz a, Mikel Echeverría a

a CEIT, Paseo Manuel Lardizábal, 15, E-20018 San Sebastián, Spainb TECNUN, University of Navarra, Paseo Manuel Lardizábal, 13, E-20018 San Sebastián, Spain

a r t i c l e i n f o

Article history:Received 26 January 2012Received in revised form 5 March 2013Accepted 3 April 2013

Keywords:Haptic feedbackDrive-by-wireAutomobile technology

a b s t r a c t

This paper presents a new mechatronic system that combines the capabilities of the steer-ing wheel, the throttle and brake pedals in a single all-encompassing device. A two degree-of-freedom mechanism allows controlling all driving functionalities together in a veryergonomic and original way. The system uses drive-by-wire technology with haptic feed-back for an outstanding driving experience. The device has been tested in a simulation plat-form, showing similar performance to conventional set of steering wheel and pedals, andvery good acceptance among users. This work also surveys current drive-by-wire systemsin the automotive industry and the use of haptic technology to assist drivers.

� 2013 Published by Elsevier Ltd.

1. Introduction

Drive-by-wire technology in the automotive industry replaces the traditional mechanical and hydraulic control systems(e.g., steering column) by specific electronics to control a wide range of vehicle operations such as accelerating, braking orsteering. There are several different types of drive-by-wire systems depending on the mechanical component it is replacing:throttle-by-wire, brake-by-wire and steer-by-wire. In any type of these by-wire systems, typical hydraulic and mechanicalcomponents are replaced by sensors that record information and pass data to a computer, which converts the electrical en-ergy into mechanical motion (Chiappero and Back, 2002).

This technology has multiple advantages: safety, for example, can be improved by providing computer controlled inter-vention of vehicle controls (automatic brake in dangerous situations, electronic stability control, etc.). The elimination of thesteering column contributes to safety since, in case of frontal collision, this mechanism may force the hand wheel into thedriver, causing injuries and fatalities. Ergonomics can also be improved by controlling the amount of force and range ofmovement necessary to control the systems. In fact, the driver can simply use a game-like joystick or controller to drivethe vehicle. Furthermore, the number of moving parts in the vehicle are also significantly reduced, thus simplifying vehicledesign and reducing weight and mechanical maintenance costs.

Regarding the disadvantages of drive-by-wire systems, the development costs of these components are higher than con-ventional ones due to their greater complexity. Nevertheless, the greatest disadvantage is the fear traditional drivers have ofsystem failures that may cause a runaway vehicle (similar to computer crashes at home). Because of the complexity of thesesystems, people worry about potential electronic malfunctions in sensors and computers, leading to unknown vehicle behav-ior. To that respect, manufacturers believe that new generations of drivers will be more accustomed to the use of gaming androbotic technology, thereby solving this drawback.

This paper surveys different drive-by-wire solutions in literature, and proposes a new vehicle-control paradigm using amechatronic system that is designed to control steering, throttle and braking functions with a unique hand-manipulated

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 23

device. The proposed mechanism additionally incorporates haptic technology that allows taking advantage of the humansense of touch (Srinivasan, 1995). Haptic systems can apply forces, vibrations, and/or motions upon the user with a high de-gree of control over parameters such as the frequency, duration and amplitude of the signals. These interfaces have proven tobe very efficient for high performance human–machine interactions, exploring the capabilities of the human sense of touchas an advanced communication channel.

Haptic feedback can be used both to improve the driving experience and to warn the driver about dangerous situations.Drive-by-wire systems remove tactile feedback felt by drivers in steering wheels and pedals due to the direct mechanicaltransmission between the wheels and the system. Although this results in a reduction of undesired noise and vibrations,many users claim that with these systems they can not ‘‘feel’’ the road anymore. To overcome this effect, haptic feedbackcan be incorporated into drive-by-wire systems. Regarding safety issues, in certain cases the visual and auditory senses ofthe driver may be overburdened due to the high amount of information displayed (radio, navigation-system, etc.), and there-fore these channels might be inadequate to process warning signals. As an alternative, haptic feedback can be used to alertthe user about warning situations. This feedback usually has a small number of competing demands and can be perceivedsimultaneously with visual and auditory signals (Sklar and Sarter, 1999).

2. Drive-by-wire technology applied to the automotive industry

In this section, a chronological overview of several concept cars that feature drive-by-wire technology is given. Focus is oninnovative vehicles, but alternative drive-by-wire technologies that are considered relevant are also described.

2.1. Joystick-driven concept cars

The replacement of the steering wheel with a joystick is not new. In 1959 General Motors presented a Chevrolet Impalaequipped with a 2-DoF (degrees of freedom) joystick that was located on the side of the driver’s door. More than 30 yearslater, in 1991, Saab developed a joystick steered concept car, based on a Saab 9000. In this case, the joystick was locatedat the site of the shifter. Turning the joystick yields a change in wheel angles and this change is also fed back to the userby adjusting the response of the joystick. The steering ratio is speed dependent. The car is equipped with a conventionalaccelerator and an automatic gearbox.

The first fully drive-by-wire concept car using side stick controllers was developed by Daimler-Chrysler. Not only steer-ing, but also accelerating and braking were commanded by the joystick. A Mercedes–Benz SL-based demonstrator (some-times called Daimler-Chrysler R129 in the literature) was built to test the technology and the feasibility of abandoningconventional steering wheels and pedals (Huang, 2004). Moving the joystick forward makes the car accelerate, movingthe joystick backward activates the brakes or—after stopping—activates the reverse gear. This concept is the forerunner ofseveral vehicles with side sticks: the Mercedes–Benz F200 Imagination (1996) and the Mercedes–Benz SL 500 (1998).

The SL 500 is equipped with two side sticks (from Fokker Control Systems) for accelerating, braking and steering. Theexistence of two joysticks allows the driver to choose which hand to use, or to use both hands. The joysticks in early versionshad two axes. In a second version, acceleration and braking were commanded by applying pressure to the joystick—thus notmoving it forward or backward—in order to have a clearer distinction between steering and speed control, thereby increasingsteering accuracy. If no pressure is applied, the car maintains the current speed. The sideward deflection of the joystick isrelated to the actual wheel angle.

Within the ‘‘Project Z’’ framework, the Bolduc Technology Group and related companies Electronic Mobility Controls(EMC) and AEVIT Services Company (ASC) equipped, in 2005, a Nissan 350Z with a centrally placed drive-by-wire joystickfor steering and with another one for acceleration and braking (placed left). The steering joystick also enables switchinggears and accessing secondary control functions (lights, etc.).

ItalDesign Vadho Concept (2007). This hydrogen powered concept car has joysticks for steering, an electrical throttle andan electrical brake pedal. The position of the joysticks and pedals is adjustable, whereas the driver’s seat is fixed.

The Honda Puyo fuel cell concept car (2007) is equipped with joystick steering and pedals for acceleration and braking.The car can turn 360� in place. Honda earlier performed tests with an Accord with full drive-by-wire joystick control.

Revolution Motors Dagne EV (2008). This is an electric vehicle with three wheels in which all of them are driven by elec-tromotors. The vehicle leans into the corner like a motorbike. Steering, acceleration and braking take place via a singlejoystick.

The DLR FASCar was developed by the German Aerospace Center (DLR), based on a Volkswagen Passat. The developmentof FASCar is part of a project called HAVEit (2008–2011), in which automated driving technologies and new HMI’s have beenresearched. Both a passenger car (FASCar) and a truck (EWBtruck) are presently being developed. In the current version ofFASCar, the driver can always override the assistance systems. The driver can also use a side stick instead of the steeringwheel and pedals.

Control of the Mercedes–Benz F-Cell Roadster concept car (2009) is not through a conventional steering wheel and pedals,instead a joystick mounted in between the seats provides all the vital control functions like steering, acceleration andbraking.

24 J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36

In the Toyota FT-EV II ultra-compact electric vehicle (2009), steering wheel and brake and gas pedals have been replacedby a pair of retro-styled drive-by-wire joystick controls. These joysticks duplicate one another’s movements so it does notmatter which one is pushed, pulled or turned to control the car.

The Rinspeed UC? [sic] is an electric vehicle developed in 2010 by the Swiss entrepreneur Frank Rinderknecht. The UC?uses SpaceDrive technology from the German company Paravan. SpaceDrive is a vehicle control system which uses elec-tronic and digital input devices, one example of which is a joystick. The UC? is driven by means of its four-way joystick con-trol system which works in a similar way to that operated by a jet pilot. In order to perfect the driver’s handling of the vehicleand to provide feedback on the driving characteristics, the joystick gives the operator haptic information about the road anddriving performance.

2.2. Control yokes for drive-by-wire cars

Although steering-by-wire technology can be implemented on conventional hand wheels, this section presents only thoseinnovative cars that use alternative control wheels, similar to flight yokes, that include the possibility of accelerating andbraking.

The Bertone/SKF Filo car (2001) features drive-by-wire steering, accelerating, braking and gear shifts using a steering podthat is placed on a bar coming from the center console. The steering pod contains motorcycle-type twist-grip throttles andbars for braking. The feedback to the driver is provided as a function of the loads acting on the steering rack, by an appro-priate high-torque motor. The same SKF drive-by-wire device is used in the Bertone/SKF Novanta concept car (2002).

The GM Hy-wire hybrid car (2002)—combustion engine and fuel cell—with drive-by-wire technology features a steeringwheel similar to the ones found in aircraft. In order to accelerate, the driver should twist (either of) the handgrips on thesteering wheel. The brakes are activated by squeezing the handgrips. Changing direction is done by pulling the handgripsup or down.

Citroën has also developed a special steering wheel that incorporates control of direction, acceleration and braking usingby-wire technology. It was first presented on the Citroën C-Crosser (2001) and Citroën C-Airdream (2002) concept cars, whilean enhanced device is included on the Citroën C5 by Wire (2005). Acceleration is realized by pressing either of the pads usingthe thumbs. Braking is achieved by applying pressure to the braking pads on the steering wheel.

Fig. 1 shows the main drive-by-wire features of the cars described in the last two sections.

Fig. 1. Overview of drive-by-wire concept cars.

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2.3. Applications for disabled people

Joystick-driven cars have already been available for disabled people for several years. In general, they are customizedvehicles, adapted to fit the specific needs of the driver. Various joystick systems can be connected to the existing steeringsystem in the car. Usually, the joysticks do not provide the user with force feedback.

Several companies offer the possibility to equip a car with joysticks for steering, or even for both steering, accelerationand braking. Examples of companies that install and develop such steering systems are Paravan GmbH, Electronic MobilityControls LLC (also used in the Nissan 350Z described in Section 2.1) and Ahnafield Corp. The joysticks send signals to elec-trical motors that turn the steering wheel. Depending on personal preference either a single two-axis joystick is used or twoseparate one-axis joysticks. Some of the joysticks provide custom control of the response of the joystick, i.e. its rotationalstiffness in all four directions can be adjusted. However, none of them provides controllable force feedback.

A newly developed system, Joysteer, contains two joysticks—both of them for steering, accelerating and braking—and doesprovide force feedback with respect to the steering. It has been developed at Bern University of Applied Sciences, Switzer-land, but commercialized by Bozzio AG. The Joysteer contains two control systems to provide redundancy in case of technicalfailure.

Another joystick-driven car for handicapped people is presented in Wada and Kameda (2009). The joystick operation inback and forward direction controls acceleration and deceleration of a car, while left and right direction controls the steeringof the vehicle. The prototype includes three different driving modes. The first one uses the steering wheel and the foot pedalsin the same way as a normal car. The second method uses the steering wheel and a mechanical knob linked to the pedals foraccelerating and braking. In the latter, all movements are controlled by a joystick. The lateral movement of the joystick iscoupled to the steering wheel, so that the steering commands given to the joystick are reproduced in the steering wheel.

Regarding research about joystick driving by disabled people, the Swedish National Road and Transport Research Institute(VTI) has performed a number of research projects on the use of drive-by-wire joysticks for steering, acceleration and brak-ing in automobiles (Peters and Östlund, 2005). The focus of this research is on the use of joysticks by disabled people, buttests have also been performed on people with normal hand/arm functionality. Joysticks with two rotational degrees-of-free-dom have been researched as well as joysticks which allow for one rotation and one translation. Such research has found thatin driving experiments time delay occurs in the steering system, since the joystick can be moved faster than the steeringsystem can react. Moreover, it is difficult to learn how to brake using angle-controlled braking without feedback. Also—with-out information feedback to the joystick—interference between the primary driving tasks easily occurs and lack of tactilefeedback limits the speed at which lateral movements can be performed.

Lastly, when driving using a joystick, external disturbances (e.g., wind or road surface) can easily influence movements ofthe joystick, especially in the case of disabled people sitting in a wheel chair that is placed in the car. In the experimentscarried out in a driving simulator it was found that decoupling longitudinal and lateral control (i.e. steering and accelerat-ing/braking) has a positive influence on driving behavior and performance. Adding active feedback improves performing lanechange maneuvers, but when driving on rural roads it does not improve control. Moreover, disabled people with limitedarm/hand strength found active feedback less pleasant to use.

2.4. Similar technologies in other applications

Besides passenger cars, other types of vehicles are equipped with joystick control. Moreover, joystick motion control isalso used in applications that have no relation with the automotive sector at all. In this section, some examples of these cat-egories will be given.

Aircrafts. From the 1980s on, modern aircrafts use digital fly-by-wire systems (starting with the Airbus 320 series and fol-lowed by Boeing with the 777 series). Analog fly-by-wire systems were already introduced thirty years before. Advanta-ges of fly-by-wire systems are that pilots can get used to flying new aircrafts relatively easily and the systems only allowcontrol within the safety limits of the aircraft. Aircrafts usually have threefold or fourfold redundancy regarding the fly-by-wire system.Military vehicles. The U.S. Army is investigating the influence of non-standard control devices (e.g., joysticks) on humandriving behavior. Tests have been performed using a simulator in order to get more insight into nonlinear steering ratios.According to the website of the Bolduc Technology Group, EMC drive-by-wire technology has been used on several mil-itary vehicles (as well as on boats).Forklifts. Current developments have led to research on drive-by-wire force feedback to the steering wheel, plus a so-called ‘‘observe-by-wire’’ feedback. The feedback to the steering wheel is thus not only based on the side forces on thetire, aligning torques, friction and inertia, but also incorporate a force based on the distance between the fork and theobject to be lifted. Initial simulations indicate an increase in productivity, since it becomes easier to position the forkliftcorrectly with respect to an object.Wheel loaders. The Komatsu WA800-3 has a so-called ‘‘Advanced Joystick Steering System’’. This fully hydraulic systemenables the driver to perform precise steering movements with the over 100.000 kg vehicle. The single joystick alsoenables changing gears and switching some secondary functions.

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3. Haptic feedback in drive-by-wire devices

Beyond the state of the art in drive-by-wire technology many studies have been conducted to determine the optimal hap-tic interaction methods for active driving joysticks. This section overviews the most relevant conclusions found, which, to-gether with related work presented in the previous section, have served as a basis to the development of the new drive-by-wire system.

3.1. General remarks

According to Kember and Staddon (1989), systems with high-order dynamics, that is, with the capacity of rapid change,can be controlled with considerable advantage with a rigid, force-controlled joystick. A more extensive investigation of suit-able interface properties dependent on the control objective can be found in Rühmann (1978), where it is remarked that if aprocess is being controlled by prescribing its position, a viscously damped joystick is desired; however, if a process is con-trolled by prescribing its velocity, a spring-centered or viscously damped joystick is desired, and if the acceleration is pre-scribed, an isometric (rigid) joystick is desired.

These conclusions are valid for so-called ‘‘compensation control’’ and passive joysticks. Driving a car is usually consideredas compensation control (as opposed to so-called ‘‘following control’’) (Bubb, 1993). In compensation control, people appearto be better in prescribing the velocity. In following control people appear to be better in prescribing the position (Bubb,1978).

The translation of these conclusions to active joysticks in cars (i.e. with haptic feedback) is made in Huang (2004). Forseveral kinds of feedback (force/torque/angle/position and their derivatives) the stability, apparent stiffness and suitabilityfor application in cars are researched. The choice for a specific combination has a very extensive impact on the interactionbetween driver and vehicle and on vehicle handling in general, so enough attention must be paid to this election. The coher-ence between optic and haptic feedback is also an issue that has to be taken into account.

Very related to the choice of the controlled quantities, is the choice for either measuring the joystick deflection and feed-ing back a force to the joystick, or measuring the force/torque exerted by the driver and feeding back a joystick deflection.Each strategy has its benefits and drawbacks (Huang, 2004). Other general remarks in Huang (2004) are: (i) the inertia of thejoystick should be as low as possible, and (ii) when controlling a two-axis joystick, errors in longitudinal direction are usuallylarger than errors in lateral direction due to the anatomy of the hand.

3.2. Steering

The maximum angle of a joystick is generally in the order of 20� to both sides, compared to about 720� to both sides in thecase of a steering wheel. This large difference suggests the need for a speed dependent steering ratio (Östlund, 1999). If thefront wheel deflection is made dependent on the ratio of the joystick deflection to the square of the speed, then a constantratio between joystick deflection and the lateral acceleration felt by the driver is obtained. This change in steering ratio canbe applied, for example, for speeds higher than 30 km/h.

The use of speed-dependent low-pass filtering plus a speed-dependent steering ratio is called ‘‘progressive control’’ (Östl-und, 1999). However, low-pass filtering might be dangerous in a case of rapid (or emergency) maneuvers at higher speed,since the desired movement could be canceled by the filter (Östlund, 1999).

The steering ratio of the Daimler-Chrysler side sticks depends on the force applied in the sideward direction of the joy-stick and on the driving speed (Huang, 2004). The applied force is filtered using a low-pass filter with a 0.16–0.64 Hz cut-offfrequency, depending on the current amount of feedback. The curvature of the direction in which the car drives is fed back tothe joystick (i.e. converted into a joystick deflection). As a result, the stick becomes stiffer at higher driving speeds.

In Huang (2004), it is argued that at low speeds the curvature of the direction in which the car drives should be fed backand at higher speeds the yaw velocity of the car. Systems like ESP and DSC do not only control the yaw velocity of the car, butalso take the slip angle into account, since only controlling the yaw velocity may lead to large slip angles. This is also impor-tant in the case of joystick steering. Moreover, it is said that the steering ratio can be dependent on, e.g. driving-speed, joy-stick deflection, joystick deflection speed, yaw velocity of the car and vehicle load. By controlling the yaw velocity (instead ofthe wheel angle) it is possible to automatically cancel side wind and road disturbances. It is proposed, however, to cancelonly high-frequency disturbances but not low-frequency disturbances, so that the driver still receives information aboutthe current driving circumstances.

3.3. Accelerating and braking

In general, feedback on driving speed has been shown to be stable whereas feedback on acceleration can yield instabilityin the driver-vehicle-control loop (Huang, 2004). In the Daimler-Chrysler side sticks, a linear relationship is established be-tween the force applied by the driver and the acceleration of the car. This linear profile is speed-dependent. A low-pass filteris added (cut-off frequency at 2.4 Hz) and a threshold of 2 N or 0.3 N m has to be overcome. A control system cancels theinfluence of the slope of the road and of air resistance, i.e. it makes sure that the acceleration remains zero, that is, constant

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 27

velocity, as long as the driver does not touch the joysticks. Isometric (rigid) sticks are used, since feedback of the drivingspeed would not be noticeable at low acceleration.

For angle-controlled braking without haptic feedback, the aim is to have a linear transmission between joystick deflectionand ‘‘braking effect’’. Since braking is a power-controlled process, the dynamics of the braking system have to be taken intoaccount (Östlund, 1999,). When the driver brakes very hard, his/her body tends to move forward due to inertia. In case ofjoystick systems for disabled people, usually the braking function of a joystick is directed forward, so that inertia doesnot yield reduced braking (Östlund, 1999).

3.4. Reverse driving

In principle there are two possibilities for reverse driving: a direction compatible system or a functional assignment. Inthe direction compatible case, the joystick is moved forward to drive forward and moved backward to brake. After stopping,moving the joystick backward will make the car drive backward and moving the joystick forward will make the car brake.

In the functional case, moving the joystick forward makes the car accelerate (forward or backward dependent on the gearselection). Moving the joystick backward always activates the brakes. Note that the other way around can also be programed,as is the case for most systems for disabled people (Östlund, 1999): moving the joystick backward makes the car accelerate(forward or backward) and moving the joystick forward always activates the brakes.

Tests in static simulators showed that the direction compatible assignment is preferred by most people. However, realdriving tests with the Daimler-Chrysler side sticks showed that the functional assignment is better. Moreover, it is presumedthat the direction compatible assignment has a higher rate of accident risk (Huang, 2004).

3.5. Assisting the driver

The improvement of safety and pleasure on the road are driving forces of technological advances in the automotive indus-try. In particular, new Advanced Driver Assistance Systems (ADAS) have considerable potential for making the experience ofdriving more relaxed and safer, by means of mitigating human errors (Amditis et al., 2007). One of the critical elements ofADAS is communication with the user, which has to be clear and efficient, but at the same time it must not overload thedriver’s attention and perception resources. In fact, the second and third design goals of the European Statement of Principleson the Design of HMI (ESoP) indicate that the allocation of driver attention while interacting with displays and controls mustbe compatible with the attentional demand of the driving situation, and specifically that the system must not distract orvisually entertain the driver (2007/78/EC,). In order to lessen the visual load, many concepts of ADAS use haptic displays.

One feature of haptic systems is that the part of the body that receives the information is usually the same one that manip-ulates the interface, and thus action and feedback can be coupled. This advantage has been applied to manual interaction in thesteering wheel, through active steering systems that automatically modify the wheel angle or torque resistance, in order toattenuate yaw disturbances, or for ‘‘shared control’’ between vehicle and driver in path following tasks (Ackermann et al.,1999; Steele and Gillespie, 2001). A similar approach for the lower limb has led to the development of active haptic pedals,which exert a variable counterforce depending on vehicle dynamics or surrounding traffic, in order to manage congestions(van Driel et al., 2007; Brookhuis et al., 2009) or control speed (Adell and Várhelyi, 2008,). However, while continuous hapticgas-pedal feedback is effective for car-following, it can be insufficient in more dangerous situations, when the distance to theleading vehicle is small, and quick corrective control actions must be taken to prevent collision (Mulder et al., 2008).

Warning signals in the form of tactile vibrations or pulses have been largely tested in pedals (Lloyd et al., 1999; Tijerinaet al., 2000; Martens and van Winsum, 2001; de Rosario et al., 2010), steering wheel (Tijerina et al., 2000; Suzuki and Jansson,2003; Jordan et al., 2007), and driver seat (Lee et al., 2004; Stanley, 2006; Jordan et al., 2007). What part the vibration is asso-ciated with normally depends on the type of warning and the expected action. It is exerted on pedals to compel brakingevents, in speed or collision warnings, and on the steering wheel for warnings related to lateral events that need steeringactions, such as a risk of lane departure. Seat vibration is used for both types of warnings, and also for other purposes, likeproviding orientational cues in a navigation system (van Erp and van Veen, 2004). This type of directional haptic feedbackmay also be applied to other functions, such as calling attention to the windscreen or the rear-view mirror, by a vibrating beltwith tactors on the driver’s abdomen and back (Ho et al., 2005).

4. New haptic drive-by-wire device

Based on previous literature, this section presents a new haptic drive-by-wire interface that has been designed, built, pro-gramed and preliminarily tested. Its main purpose is allowing the driver to give steering, accelerating and braking com-mands. The system does not include buttons for gear changing (it is assumed that the vehicle includes an automatic gearselection system). Although the technical concept was chiefly designed for people with lower-extremity impairment(Dominguis et al., 2011), the device is intended for providing any user with an enhanced driving experience.

The designed interface couples the steering, accelerating and braking functions in a very different style to that found inthe related work, so it could neither be properly classified as a joystick nor as a control yoke. The design paradigm overcomesmany limitations of common joysticks for a proper driving experience. Additionally, the new device is perfectly suited forhaptic feedback capabilities.

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Following subsections describe the former proof-of-concept prototype designed for lab testing. First, the original designsolution to couple all driving functionalities into a unique hand-held device is presented. Afterwards, the control algorithmsfor its maneuverability and the haptic laws are described. The validation tests into a driving simulator are reported inSection 5.

4.1. Hardware description

The design of the mechanical interface meets several functional and ergonomic requirements: all-in-one solution (steer-ing, accelerating and braking commands) using a single hand (right hand by default), reversible design (adaptable to left-handed people), attractive appearance and user-friendly solution. The sketch of the final design is shown in Fig. 2. An arrowpoints to the forward direction of vehicle motion. The selected material (aluminum) and the use of a nerved structure meetone of the mechanical aspects mentioned in Section 3.1, that is, low inertia (approximately 6.94 g m2 in the steeringmovement).

The haptic drive-by-wire device has two active degrees of freedom. The steering command is correlated with the angle ofthe lever hs (see Fig. 2). The acceleration command is introduced by rotating the handle towards a positive angle ha. This solu-tion decouples better the lateral and longitudinal control (i.e. steering and accelerating) than conventional joysticks. Addi-tionally, user’s inertia during accelerating or braking does not interfere a priori with the driving command. The maincharacteristics of the two active joints are summarized in Table 1.

The lateral deflection of the lever doubles conventional joysticks range in order to provide more versatility to the proto-type. However, the final workspace is reduced by the software (this is a general parameter that can be tuned). The workspaceof the handle is also restricted based on maximum comfortable wrist rotation (Diffrient et al., 1981).

Each joint is commanded by an electric actuator. The first one is a 150 W DC motor (Maxon RE40-148877) connected to thelever by means of a cable transmission with a reduction stage of 10. This type of mechanical transmission presents high revers-ibility, high stiffness, no backslash and near zero friction, allowing the system to reproduce realistic haptic sensations. A highresolution incremental encoder (Quantum devices QD145-05/05-5000) is attached to the motor for measuring joint rotation.

The second actuator is enclosed within the handle itself. It is an 11 W DC motor (Maxon RE-max 24-222053) attached tothe handle through a planetary gearhead with two stages and a reduction of approximately 29. The resulting apparent inertiaof the rotor attached to the gearhead, 0.39 g m2, is still moderate and does not degrade the haptic feedback.

Steering

Accelerating

Forward directionof vehicle motion

Fig. 2. Haptic drive-by-wire device (left) and sketch with active joints definition (right).

Table 1Technical specifications of the active joints.

Parameter Lever Handle

Max. rotation angle (hardware) ±40� ContinuousMax. rotation angle (software) ±30� From 0� to 45�Angular resolution 0�0002600 0�0102700

Max. continuous torque 2.008 N m 0.373 N mPeak torque 4.016 N m 0.467 N mReduction ratio 10:1 729:25

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 29

Notice that the selected motors and sensor have been chosen for the testing prototype and not for its direct use in a realcar; that is, the device is dimensioned to reach a very wide range of torques, enough to test different control possibilities andhaptic feedbacks.

Regarding the braking command, two different strategies have been analyzed with this concept prototype. The first oneuses the opposite direction of the accelerating axis (Fig. 3 left), while the second one uses a passive brake lever (Fig. 3 right).Since it is quite difficult to determine a priori which is the best braking configuration, the validation tests have also beenused for discussing this issue (Section 5.2).

4.2. Control algorithms and haptic feedback

For each active joint two main variables are controlled: (i) the command signal to the vehicle and (ii) the haptic torque fedback to the user. The first one is a normalized value that ranges from �1 to 1 (emulating a conventional joystick signal) andthe second one is a torque definition. This last choice (measuring deflection and feeding back torque) is the common archi-tecture in impedance-type haptic systems. However, the torque definition does not only depend on the position of the de-vice. Some other parameters of the vehicle are also considered.

4.2.1. SteeringThe angle of the steering lever hs and the angle of the wheels hw are not linearly mapped. The desired value for the wheels

hdw is weighted by two sensitivity ratios (r1 and r2):

hdw ¼ r1r2

hs

hs maxð1Þ

The first ratio r1 introduces a dependency of the sensitivity on the angle of the steering lever (less sensitive around the centerposition). The second ratio r2 introduces a dependency of the sensitivity on the vehicle speed v (less sensitive as the vehiclespeed increases) based on Östlund (1999), but tuned differently. Fig. 4 shows these two sensitivity ratios.

Although r1 is a linear function of the normalized steering angle, for a given vehicle speed the commanded value hdw, Eq.

(1), is a quadratic function. This kind of nonlinear behavior is highly recommended for levers with a reduced angular rangecompared to conventional steering wheels (Andonian et al., 2003).

The torque ss restored to the user by the steering lever has three components: one proportional to the angle of the lever(felt as if the lever is attached to the center position by means of a spring with stiffness coefficient Ks), a viscous componentwith damping coefficient Bs, and a third term based on the difference between the desired wheel angle and the true wheelangle.

ss ¼ �Kshs � Bsdhs

dtþ jw hd

w � hw

� �ð2Þ

These two definitions, Eqs. (1) and (2), imply a bilateral communication between the haptic device and the vehicle: the hap-tic device sends the desired angle for the wheels hd

w to the vehicle, and the vehicle reports the actual orientation hw.

Fig. 3. Braking strategies: using the handle (left) and using a passive lever (right).

Fig. 4. Sensitive ratios depending on the angle of the lever hs and vehicle speed v.

30 J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36

4.2.2. AcceleratingRegarding the accelerating command, the opening of the throttle valve hd

v is a normalized value from 0 to 1 directly con-trolled by the rotation of the handle ha. This is comparable to using a conventional gas pedal. The opening of the throttlevalve is linearly increased up to a certain maximum angle hamax. This limit is set to 45� based on the maximum comfortablewrist rotation (Diffrient et al., 1981).

1 http

hdv ¼

ha

ha maxð3Þ

The torque exerted to the driver consists of two resistive terms. The first one is proportional to vehicle speed v and the sec-ond one is a viscous force:

sa ¼ �jvv � Badha

dtð4Þ

Again, Eqs. (3) and (4) imply bilateral communication: hdv is sent to the vehicle and v is received from it. Notice that some

constants (Ks, Bs and Ba) have physical meaning—they are stiffness and viscous coefficients—and can be easily tuned. Otherconstants (jw and jv) have a less intuitive meaning and should be experimentally tuned.

4.2.3. BrakingThe passive brake lever (Fig. 3 right) does not require any haptic feedback (it has a real spring) and the normalized brak-

ing command (from 0 to 1) is proportionally mapped from the rest position to the maximum deflection of the lever. If thehandle is used to brake (Fig. 3 left) the braking command is proportionally mapped to the handle’s turn in the braking direc-tion from 0� up to a maximum angle hb max which is 18�. Within this range, a virtual spring with stiffness Kb is simulated:

sb ¼ �Kbhb ð5Þ

5. Driving simulation tests

The functionalities of the haptic device have been tested by multiple users in a static part-task driving simulation plat-form. The system is validated by comparing its performance against a conventional set of steering wheel and pedals. Futurework will validate its functionalities in a real vehicle.

5.1. Simulation platform

The driving simulator consists of several programs that are running simultaneously. Firstly, the rFactor,1 a computer rac-ing simulator developed by Image Space Incorporated. rFactor has the ability to run any type of four-wheeled vehicle throughmultiple scenarios, but more importantly, it allows a high degree of control and monitoring over many car parameters, and arealistic dynamic response to such variables.

Secondly, the haptic control loop that governs the new driving device runs at 1 kHz in a dSPACE DS1104 board. This dataacquisition board records the encoders information from the device, and applies a proper actuation to its motors.

Data communication between both applications is performed by means of an rFactor plug-in, which allows reading thetelemetry data from rFactor and using it in other applications. An example of the plug-in is available from the developers ofrFactor and it has been extended and compiled with dSPACE commands in order to enable communication from rFactor tothe dSPACE board. As explained in Section 4.2, to provide real haptic feedback, and not to display simple passive springs anddampers, the haptic loop requires information from the vehicle (in this case from the rFactor program), and so does the rFac-tor to display the user’s interaction with the new driving system.

Table 2 reports the values of all the parameters defined in Section 4.2 that have been used in the control and haptic algo-rithms. These parameters have been selected to obtain comfortable driving. Although one of the advantages of the proposedsystem is that the driver can easily modify all the parameters depending on his/her preferences (for example ‘‘softer’’ or‘‘harder’’ for a more sportive driving), they remain constant during these preliminary tests. Later, in Section 5.5, an additionalexperiment is carried out to analyze the influence of these parameters on the controllability of the new driving command.

5.2. Tests design and hypothesis

A set of driving tests have been performed using the simulation platform (Fig. 5) to analyze the usability of the device.Three different driving configurations have been explored: the first one uses a gaming steering wheel with pedals from Log-itech� (G25 Racing Wheel), and the last two employ the drive-by-wire system, but with a different braking configuration(Fig. 3). The following abbreviations are used to identify the three different driving configurations:

://www.rfactorcentral.com/.

Table 2Parameter values for the haptic algorithms.

Parameter Variable Value

Steering stiffness Ks 0.55 N m/radSteering damping Bs 0.1 N m s/radWheel feedback jw 4.5 � 10�4 N mSpeed feedback jv 0.005 N m s/mAccelerating damping Ba 0.015 N m s/radBraking stiffness Kb 0.7 N m/rad

Fig. 5. Simulation platform using the haptic device (left) and using a set of steering wheel and pedals (right).

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 31

– W: Conventional steering wheel with pedals.– B1: Drive-by-wire system, braking with the accelerating handle.– B2: Drive-by-wire system, braking with the passive brake lever.

The objective variable to measure the performance of the driving commands was the time to complete a specific circuit ofrFactor. The selected track was the ‘‘Essington Park GP Circuit’’ (4.012 km and 12 turns), which was considered long enoughto detect significant discrepancies in terms of completion times when using efficient vs. inefficient driving configurations.Thus, the hypothesis is that the conventional steering wheel with pedals should achieve the best experimental results (fasterdriving and therefore shorter completion times), while the drive-by-wire system may differ significantly (or not) with re-spect to this conventional driving (longer completion times are expected).

A group of 16 participants took part in these experiments, 5 women and 11 men, with ages varying from 23 to 50 yearsold (average of 31). All of them were right handed and reported normal tactile and visual functions. All subjects had a driver’slicense and driving experience. Most of them also had prior experience with haptic applications, but none of them with thenew drive-by-wire device.

All the participants drove a specific rally car (Renault Clio Sport) in automatic shift mode, and with the stability, braking,and traction controls of rFactor activated. In order to avoid the influence of non-desirable effects such as the learning effectand the ‘‘well traveled road effect’’ on the results, the tests with each driving command were conducted in different days andalso in different order for each user.

Before each experiment, the users had two warm-up laps to get used to the device and the circuit. Afterwards, they wereasked to complete three laps to the circuit as fast as they were confident. During these three laps, several data were recorded,including velocity, jerk, lap times, partial times in each sector, average and top speeds, average use of the throttle (in per-centage to total time) and any remarkable incident or event, such as collisions, off-track excursions, etc. In case of collisionor off-track excursion, those laps were not taken into account for the results. The key variables for comparing the drivingcommands were the best lap time (no matter if it was the first, the second or the third one) and the jerk measures.

5.3. Measured results

Fig. 6 shows the box plot of the best lap times (min:sec) of all the users for each configuration (W, B1 and B2). The boxescontain the middle half of the data points. The lines inside the boxes are the median values. The vertical lines cover the rangeof all values, except outliers (asterisks). A solid line connects the mean values (crossed circles). It can be seen that, as ex-pected, W obtained the best mean lap time (02:01.32), but only 2.17 s faster than B1 and 2.75 s faster than B2. In general,all results among different configurations were very similar.

Fig. 6. Best lap times for the three different configurations.

32 J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36

The statistical significance of the two drive-by-wire configurations (B1 and B2) compared to the conventional set of steer-ing wheel with pedals (W) for the measure under analysis—the best lap time—can be investigated by using a linear modelthat includes as factors the users and the drive-by-wire configurations. This way, the effect of the user—which can performthe test with very different performance—is removed and it is possible to state whether the experiments reflect the samepattern (in this case that the best lap time is always faster using W despite of the driver from the statistical point of view).The results of this study show that the effects of B1 and B2 on the best lap time are significant, t(30) = 2.293, p = 0.029 andt(30) = 2.901, p = 0.007 respectively, but their overall effect is quite small (an increase of 1.55 ± 0.68% and 1.97 ± 0.68%respectively).

A similar performance of the different driving commands can also be seen in Fig. 7, which shows a comparison of the topand average speeds among the three configurations. Using hand wheel and pedals (W) the speeds were slightly higher, andtheir corresponding standard deviation smaller, but again very similar to those obtained with the drive-by-wire system.

As expected, the average use of the throttle (triangles connected by a solid line in Fig. 7) was higher for B2. This is due tothe fact that both acceleration and braking are uncoupled for B2, and therefore it was possible to accelerate and brake at thesame time in this configuration.

Smoothness is another interesting driving parameter that can be analyzed. A quantitative measure of smoothness can beobtained by computing the rate of change of the vehicle’s acceleration, which is the third derivative of position and alsoknown as ‘‘jerk’’ (Schot, 1978). This parameter can be examined over time by calculating the root of the mean of the squares(RMSs) of instantaneous jerk measurements. A small RMS jerk means a smooth maneuver, and therefore, it can be used as ameasure of the ride comfort (Post, 2011). Two different RMS jerk values were obtained, one for the longitudinal direction ofthe vehicle (Fig. 8 left) and another for the lateral one (Fig. 8 right) recorded during the best lap. The box plots in Fig. 8 showquite similar performance in the jerk measures for the three driving configurations.

The statistical significance of the two new driving configurations (B1 and B2) with the conventional set of steering wheelwith pedals (W) is also investigated for the jerk values by using a linear model that includes as factors the users and the citednew driving configurations. In the longitudinal jerk, no significant differences were found: t(30) = 1.693, p = 0.101 for B1 andt(30) = 0.233, p = 0.818 for B2. However, in the lateral jerk, both B1 and B2 do exhibit a significant influence: t(30) = �2.298,p = 0.029 and t(30) = �3.265, p = 0.003 respectively. Therefore, from these preliminary experiments, it can be stated that thenew driving configurations achieve smaller values in the lateral jerk. However, it is important to note, that the resulting

Fig. 7. Top speed, average speed and % throttle.

Fig. 8. Values for the longitudinal and lateral jerk.

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 33

smoothness is not perceived by the participants in this driving simulation platform. Thus, these results in ride comfortshould be confirmed with in-vehicle tests.

5.4. Subjective evaluation of the new mechatronic device

After the last driving test, the subjects were asked to assess the drive-by-wire prototype regarding five different aspects,including overall rate, satisfaction, ease of use, comfortableness and security. The subjective rating covers a 7-point Likertscale, from 1 (worst) to 7 (best) for each of the adjectives proposed. The results can be observed in Fig. 9.

In general, the new device received good scores in all aspects. Participants were very enthusiastic with the new approach,and enjoyed the new driving paradigm. Only the score obtained for comfortableness was slightly lower than the average.Questioned about this fact, participants stated that the only drawback they found was that by the end of the race they feltthe wrist a little bit tired since they had been continuously accelerating with their hand. Except for motorbike riders, usersare more relaxed accelerating with their feet rather than with their hand. Notice also that the driving task proposed for theexperiments required users to complete the race as fast as possible, thus demanding high accelerations, which would notoccur so significantly in real routes.

Regarding the two design solutions adopted for the braking command, while results presented in Fig. 6 showed no sig-nificant difference in performance, the majority of the users (62.5%) preferred braking configuration B1 (using the acceler-ating handle in the opposite direction).

5.5. Influence of haptic parameters on the controllability of the vehicle

The previous experiments were carried out with certain predefined haptic parameters (Table 2) specified by the authorsfor the new driving command. These parameters were subjectively selected to create a comfortable driving sensation. How-ever, their contribution to vehicle controllability has not yet been discussed.

Fig. 9. Subjective evaluation (mean and standard deviation).

34 J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36

International standard ISO 7401:1988 provides some test methods for measuring the lateral transient response of roadvehicles. Since the overall handling behavior is difficult to assess, the lateral transient response can be used to analyzethe controllability of the vehicle relative to the steering command. One of the methods proposed by ISO 7401:1988 consistsof introducing a random steering input to obtain the frequency response of the vehicle (lateral acceleration or yaw velocityvs. steering command angle). Among other requirements, the input signal is generated by a test driver for 900 s, duringwhich time the vehicle response is measured. Lateral acceleration during the test should remain below 4 m/s2 and vehiclespeed should be constant at 80 km/h.

Fig. 10. Lateral frequency response of the vehicle (yaw velocity vs. steering command angle) for haptic parameters Ks (N m/rad), Bs (N m s/rad) and jw

(N m).

0.2 1 4−12

−10

−8

−6

−4

−2

0

2

4

6

Gai

n (d

B)

−80

−60

−40

−20

0

Phas

e (d

eg)

Frequency (Hz)

Drive−by−wire deviceSteering wheel

412.0

Fig. 11. Lateral frequency response of the vehicle (yaw velocity vs. steering command angle) using the new drive-by-wire device and the steering wheel.

J.J. Gil et al. / Transportation Research Part C 33 (2013) 22–36 35

Following this procedure, several experiments were carried out in the driving simulator to analyze the influence of thehaptic parameters on vehicle controllability. A different driving scenario was selected in order to allow driving in a straightlane. The same driver performed all the tests. In each test, the parameters for the haptic feedback were modified by takingthe following values: Ks = 0.2, 0.55, 1.1 N m/rad, Bs = 0.05, 0.1, 0.25 N m s/rad and jw = 2.4 � 10�4, 4.5 � 10�4, 9 � 10�4 N m.Middle values were equal to those in Table 2 and they were considered default values for the experiments. A total number of9 tests were performed, 3 per parameter (keeping the other parameters at their default values). In addition, another test wasperformed with the steering wheel.

Fig. 10 shows the frequency response of the vehicle’s yaw rate to the steering input. It can be observed that haptic param-eters can be changed within a wide range of values to meet drivers’ preferences without compromising vehiclecontrollability.

It is interesting to note that the dynamic response of the vehicle using the steering wheel from Logitech� is much moredifferent than any recorded response using the drive-by-wire device. For sake of simplicity, Fig. 11 only presents the lateralresponses using the steering wheel (dashed line) and the new device using the default haptic parameters (solid line). In bothcases the input of the experiment was the steering command angle in radians. The reduced range of movement of the drive-by-wire device, ±30�, compared with the steering wheel, ±90�, is the main cause for the different starting gain levels. More-over, the gain of the dynamic response using the new device is higher for all frequencies. As a consequence, with the newcommand, a smaller change of steering angle hs will produce a larger displacement of the wheels, no matter how fast themovement is.

6. Conclusion

Manufacturers are constantly striving to improve existing technology to develop cutting-edge transmission systems thatcan offer drivers an enjoyable driving experience. Drive-by-wire systems are a very promising technology in the automotivefield. We must bear in mind that the absence of a mechanical connection between the command devices and the drivingelements should not be taken as a risk. Moreover, this leaves room for further vehicle improvement in terms of weight, con-trollability, safety, etc. The use of drive-by-wire controls with haptic feedback also allows the system to restore the user tac-tile information as if she/he were manipulating an interface mechanically connected to other parts of the vehicle. Thus, thedriving experience remains without confusion, but new assistance features could be implemented by using this new hapticchannel (e.g., warning signals).

This paper firstly presents a review of existing drive-by-wire systems, as well as the design guidelines proposed in theliterature. Afterwards, a new drive-by-wire command with haptic feedback and its control algorithms are described. Theconcept design takes advantage of previous research and proposes an innovative configuration to decouple the driver’s lat-eral movement for steering with wrist turn for accelerating/braking. The mechatronic solution was tested on a driving sim-ulation platform. The resulting performance was good and similar to what was obtained with a conventional hand wheelwith pedals. Objective and subjective evaluations were carried out as well as a vehicle controllability analysis. In the nearfuture, this command will be installed in an electric vehicle for its final validation in real driving tests.

Acknowledgements

This work has been supported by the project MARTA, led by Ficosa International S.A. and funded by the Centro para elDesarrollo Tecnológico Industrial (CDTI) for the 3rd CENIT Programme, as a part of the INGENIO 2010 Programme of theSpanish Government. The authors would like to acknowledge the fruitful discussions with Dr. Ángel Rubio.

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