Chassis concept of the individually steerable five-link suspension: a novel approach to maximize the road wheel angle to improve vehicle agility

doi:10.21203/rs.3.rs-3996491/v1

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Chassis concept of the individually steerable five-link suspension: a novel approach to maximize the road wheel angle to improve vehicle agility

https://doi.org/10.21203/rs.3.rs-3996491/v1

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published 09 Jun, 2024

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The Institute of Vehicle Systems Engineering at Ulm University of Applied Sciences is currently developing the autonomous concept vehicle Nimbulus-e in the strategic field "Intelligent Commercial Vehicles" with the aim of maneuvering as agilely as possible in confined spaces. To achieve high agility under the conditions mentioned, large road wheel steering angles are necessary. As part of the basic vehicle concept, the first step is to select a suitable chassis for this purpose. Conventional suspensions cannot be applied due to the mechanical connection of the tie rod to the steering knuckle limiting the road wheel angles. Therefore, the approaches published so far for individual chassis concepts with large steering angles are analyzed and evaluated for use. In this paper, the concept of a novel individually steerable five-link suspension is described. The concept includes a vehicle body mounted steering actuator connected to the chassis via a self-locking worm gear. Due to the body mounted connection of the steering actuator, it does not contribute to the unsprung mass. An analysis of the kinematic and elastokinematic properties and the achievable road wheel steering angle is presented. In the Nimbulus-e concept vehicle, the individually steerable corner module is used on both the front and the rear axle. The system is driven by four wheel hub motors. This means that eight control variables are available for the vehicle.

Automotive Suspension

Individual Corner Module

Steer by Wire

Kinematics and Elastokinematics - Autonomous Shuttle Busses

Agricultural Robots

Agility

4 Wheel Steering

Wheel Hub Motor

Against the background of advancing urbanization many research activities on the topic of automated shuttle buses have emerged worldwide in the last ten years. The goals of these projects are manifold and range from innovative vehicle concepts to complex control algorithms for fully autonomous driving functions (Backhaus 2020). One main field of research is the development of innovative mobility concepts for passenger and freight transport in confined spaces. Transport tasks in confined spaces require a high degree of maneuverability, which in turn requires a small turning circle. To meet this requirement, special vehicle/chassis concepts are necessary. Individual corner modules allow wheel steering angles to be significantly increased compared with conventional suspensions, thus minimizing turning circles. Applications of this technology can be found both in the field of autonomous shuttle buses, such as the Schaeffler Mover (Fig. 1), which was shown at the Frankfurt Motor Show 2019, and in innovative agricultural machinery, such as the Yanmar Smash modular agricultural robot shown in Fig. 2, which is used, for example, to investigate plant diseases (Yanmar Europe B.V. 2020). The boundaries between vehicles and robots are merging in these applications. In this context, the Institute of Vehicle Systems Engineering at Ulm University of Applied Sciences is currently developing the autonomous concept vehicle Nimbulus-e in the strategic field "Intelligent Commercial Vehicles". In order to achieve the desired high maneuverability, a novel chassis concept with large steering angles has been developed. This suspension concept is presented in this article.

Degrees of freedom in the context of mechanical systems are defined as independent possibilities of movement. From a kinematic point of view, conventional suspensions have two degrees of freedom at each vehicle corner, which can be described by

the vertical movement of the wheel center relative to the vehicle body (jounce/rebound travel) and
the rotation of the wheel relative to the knuckle.

In the case of a steered axle the left and right road wheel angles represent additional degrees of freedom, usually mechanically coupled by a central steering actuator. The drive torque is generated centrally and distributed to the wheels via drive shafts and differentials. This means that the entire vehicle can be controlled by two actuation channels, the steering angle and the drive or braking torque.

For individual corner modules there is no kinematic coupling between the individual wheel steering angles or between the individual wheel speeds. Each corner module can be steered individually via a steer-by-wire system by an independent actuator and driven by wheel hub motors. A total of eight degrees of freedom are thus available for controlling driving maneuvers if the corner modules are used on both the front and rear axle. Depending on the application, the vertical wheel movement can be controlled by a passive, semi-active or active spring-damper system. Individual corner modules can significantly improve the maneuverability of a vehicle, especially if the road wheel angle operating range is maximized and steerable modules are used both on the front and rear axle. In this case, the large road wheel steering angles occur only at low speeds. The installation space restrictions known from conventional suspension concepts, which are caused primarily by the tie rods contacting the rim flanges and result in a relatively small road wheel angle operation range of about +/- 40°, are eliminated.

Literature describes several approaches for individual corner modules with modified topologies compared with conventional suspensions with the objective to maximize the road wheel angles (Fig. 3). In some concepts, the wheel suspension is mounted at only a single interface relative to the vehicle body, resulting in very large reaction moments at the bearing point. Other approaches attach the steering actuator to the knuckle, resulting in high unsprung masses. Selected chassis concepts are subsequently presented and analyzed.

3.1 Schaeffler intelligent corner module based on a trailing arm suspension

At Frankfurt Motor Show 2019, Schaeffler presented the autonomous shuttle bus Mover, which uses an innovative compact wheel module with wheel hub motor and rotatable fork to which a trailing arm is mounted (Fig. 3a). A strut unit connects to both components. The steering motion is initiated via an electromechanical steering actuator rotating around the vertical axis which is fixed to the body structure and located on top of the corner module. In maneuver-mode wheel steering angles of +/- 90° can be realized. Designed as a modular system identical modules can be used on all four wheels. The corner module is connected to the body structure only at one location. Due to this flying mounting very high reaction moments from the longitudinal and lateral tire forces are introduced into the body structure. In addition, there are the known kinematic and elastokinematic disadvantages of a trailing arm suspension (no roll compensation, lateral force oversteer). Hyundai Mobis presented a similar concept in 2021 with their E-Corner module but realized as a double swing arm providing more lateral stiffness than a single trailing arm (Doll 2021).

3.2 Protean corner module based on a double wishbone axle

In 2019, Protean introduced the 360 + chassis concept based on a rotating double wishbone suspension, a way to steer each wheel 360° and beyond without restriction (Fig. 3b). This gives the vehicle unprecedented maneuverability and means that the vehicle does not have to stop during complex maneuvers, allowing passengers to move forward smoothly. The well-known kinematic and elastokinematic advantages of a double wishbone axle (roll compensation, lateral force understeer, selectable roll center height) can be transferred to this concept. However, the high reaction moments introduced into the body due to the flying mounting of the steering actuator, the heavy suspension carrier and the large amount of space required in the wheel house are conceptual disadvantages.

3.3 The chassis of the SpeedE

The chassis of the RWTH Aachen University SpeedE research vehicle is also based on a double wishbone suspension, but with a knuckle fixed connection of the steering actuator to the kingpin axis (Fig. 3c). The chassis is only used on the front axle of the prototype vehicle. The steering angle range is asymmetrical. To the front, the wheel can be steered by 90°, to the rear by 60°. Here again, the advantageous kinematic properties of the double wishbone axle apply. A disadvantage of this concept is the high unsprung mass, as the steering actuator also moves when the suspension is actuated.

3.4 Schaeffler i-corner

At Frankfurt Motor Show 2021, Schaeffler presented the "i-corner" concept (Fig. 3d) as a technical evolution to their original Mover suspension from 2019 (Schaeffler 2023). From a kinematic point of view, this system represents a double semi-trailing arm suspension with a c-shaped, non-rotating intermediate knuckle relative to which the steering knuckle can rotate about a fixed steering axis - as in a revo-knuckle type suspension. Transverse and longitudinal forces can be effectively supported via the lower and upper control arms. The disadvantages of the original Schaeffler wheel module associated with the flying mounting can be eliminated with this concept. The kingpin axis is further away from the wheel than in the original concept. A steering angle of 90° is possible in one direction of rotation. However, the steering actuator which travels with the suspension has a significant contribution to the unsprung mass.

3.5 Other chassis concepts

In addition to the explicitly described variants, other innovative concepts exist that are not presented in detail here. These include the axle module vTtrack from Reycon (Reybrouck Consulting & Innovation BV 2023), the EasyTurn strut axle concept from ZF (ZF 2023), the Steering-Wishbones from Schaeffler/KIT (Nees et al. 2020) and the UTM trailing arm axle (Rasul et al. 2015).

The Systems Engineering Process has been adopted for target setting starting with the vehicle level customer requirements which are subsequently cascaded down to suspension system and finally to component level. The functional targets that were used as the basis for conceptualizing the Nimbulus-e complete vehicle are:

high agility and small turning circle,
effective design, i.e. an identical module should be usable on all four wheels,
intuitive handling as well as good ride comfort,
effective installation space concept for passenger and freight transport,
lightweight body structure enabled by optimized force transmission.

The following system level targets for the chassis system can be derived from the vehicle targets:

maximization of wheel steering angles,
effective support of the lateral and longitudinal tire forces by avoiding a large lever arm,
small unsprung masses enabled by a steering actuator mounted on the body,
compact wheelhouse package,
kinematic and elastokinematic properties of a good passive conventional suspension.

The following targets were set for the kinematic and elastokinematic properties:

negative camber during jounce (roll compensation),
neutral roll understeer, since the chassis is used on the front axle and rear axle,
(It is also possible to generate a superimposed toe angle by the steering actuator),
selectable roll center height,
backwards movement of the wheel during jounce, i.e. "kinematic wheel recession" to reduce bumps on uneven road surfaces,
defined longitudinal compliance for better rolling comfort,
small scrub radius for small reaction moments,
toe-in during braking forces to improve driving stability,
small track change under lateral forces,
selectable position of the longitudinal poles on the front and the rear axle.

The approach taken in finding a suitable suspension concept is to identify a kinematic mechanism in which a stably mounted wheel steering axle can be controlled by a steering actuator which is fixed to the vehicle body. The basis for the concept is the conventional non-steerable 5-link suspension introduced by Mercedes in the W201 series (Model 190) in the year 1982. In this concept the inner attachment points of all control arms are connected to the vehicle body/subframe by ball joints or bushings.

The idea behind the concept presented in this paper is to move the inner front and rear attachment points in opposite directions in the upper and lower steering plane, respectively, and thus initiate the steering movement. This can be achieved by means of an inner knuckle as an additional interface part. This inner knuckle can rotate about a primarily vertical axis relative to the vehicle body or subframe. The kinematic function of the system can be explained with the planar mechanism shown in Fig. 4. This mechanism is kinematically overconstrained in the general case, but has \(f=1\) degree of freedom in the case of symmetrical control arms. In the planar case, the wheel rotates about the instantaneous pole M. In the transition to a three-dimensional model the instantaneous pole becomes an instantaneous axis of rotation.

Figure 5a shows the spatial model of the suspension as a multibody system. A steering actuator generates a rotation \({\delta }_{L}\) about the inner steering axis and initiates the steering movement. The steering actuator is fixed to the body structure and transmits its drive torque to the inner knuckle. It therefore does not contribute to the unsprung mass. Kinematic transmission of motion to the wheel-side (outer knuckle) takes place via five control arms, four of which are connected to the inner knuckle via ball joints. These control arms (blue) are primarily used to support the lateral forces. The control arms each have a ball joint on both sides. A further control arm (red) connects the outer knuckle to the vehicle body structure. This trailing arm primarily supports the longitudinal forces in the center of the wheel. The trailing arm is connected to the outer knuckle by a ball joint positioned close to the instantaneous axis of rotation of the knuckle.

The trailing arm is structurally connected to the vehicle body by a rubber bearing whose axial direction points in the transverse direction of the vehicle. This control arm must have an arcuate shape to provide the required package space for the movement of the wheel during steering (Fig. 6). Due to the slight inclination of the lateral control arms in the plan view, longitudinal forces are supported in the upper and lower control arm planes. The point \({P}_{11}\) serves as the point of application for the spring-damper system. This point is located on the instantaneous axis of motion to decouple vertical wheel travel and steering motion. Figure 5b shows the kinematic topology of the system in the joint-body representation. The degrees of freedom shown here are the steering angle \({\delta }_{L}\) of the inner knuckle and the vertical movement of the wheel center \({z}_{R}\). The identical wheel module can be used on both the front and rear axle. All that is required is to rotate the trailing arm by 180°.

In the following, essential kinematic and elastokinematic properties of the wheel suspension are described and it is shown by which hardpoint coordinates these can be specifically influenced. One advantage of the system described is that the properties can essentially be defined independently of one another without conflicting targets. The baseline configuration for the following explanations is a suspension system in which all lateral control arms are of equal length, symmetrical to the wheel center and horizontally aligned (variant 0). The inclination angle of all control arms in plan view is 3° in the baseline configuration.

6.1 Roll compensation

To increase the transmissible lateral forces during cornering, a negative camber during jounce motion is desirable. This can be achieved - as with a conventional multi-link axle - by lowering \({P}_{7}\), \({P}_{9}\) or by raising \({P}_{3}\), \({P}_{5}\), respectively. The behavior of the camber angle with respect to wheel travel is shown in Fig. 7. In addition, the roll center height is determined by the inclination angles of the control arms in the lateral plane.

6.2 Kingpin inclination and camber angle

In the case of a general spatial suspension concept, the steering axis cannot be determined geometrically, but is defined as the instantaneous axis of motion of the wheel carrier during a steering movement. For any point on the steering axis, the velocity vector and angular velocity vector of the wheel carrier are parallel to each other. The inclination of the steering axis in the front view defines the kingpin angle. For the symmetrical baseline configuration, the steering axis runs in vertical direction through the centers of the distances \({P}_{8}\)-\({P}_{10}\), resp. \({P}_{4}\)-\({P}_{6}\) (variant 0: kingpin angle \(\sigma =0\)°). By changing the relative length of the distance \({P}_{8}\)-\({P}_{10}\) (upper link plane) to the distance \({P}_{4}\)-\({P}_{6}\) (lower link plane) in the side view, the instantaneous steering axis and thus the kingpin angle can be specifically influenced. If the distance \({P}_{8}\)-\({{P}_{10}}_{ }\) is smaller than the distance \({P}_{4}\)-\({P}_{6}\) the result is an inwardly inclined kingpin axis (variant 1: \(\sigma >0^\circ\)), in the opposite case an outward inclined spread axis (variant 2: \(\sigma <0^\circ\)).

Kingpin angle and the camber angle as functions of the wheel steering angle are shown in Fig. 8 for the configurations mentioned. A positive kingpin angle causes the wheel to turn into positive camber during steering. This behavior is typical for conventional suspensions, such as a double wishbone axle.

6.3 Scrub radius

In conventional drive trains with sideshafts, unlike the brake torque, the drive torque does not act on the suspension but is transmitted to the body via the engine mount system. For wheel hub motors, both drive and braking torque act on the wheel suspension. For this reason, the scrub radius is the determining lever arm for the reaction moments. Since a wheel hub motor occupies significant installation space inside the rim, the outer knuckle connection points to the control arms are positioned relatively far from the center of the wheel. This represents a conceptual disadvantage of this drive concept. A positive kingpin angle reduces the scrub radius. In addition to the option already mentioned in Chap. 6.2 for defining the kingpin angle, the scrub radius can be reduced further without influencing the kingpin offset (disturbance force lever arm at wheel center height) by moving the upper outer attachment points (\({P}_{8}\), \({P}_{10}\)) inboard and the lower attachment points (\({P}_{4}\), \({P}_{6}\)) outboard. With these measures, the scrub radius can be reduced from 120 mm in the base configuration to about 0 mm in the optimized variant.

6.4 Caster angle and trail

With the general suspension concept any desired caster angle/trail can be implemented. Using a common suspension system with identical components for all four corners is desirable to reduce piece cost and tooling. In order to fulfill this requirement the caster angle has been set to 0° in this specific case. Therefore, aligning torques result exclusively from the tire pneumatic trail. Due to the self-locking steering gear, the aligning torques are not transmitted to the rotor of the steering motor but are supported by the body structure via the gear housing.

6.5 Longitudinal poles

The longitudinal poles can be defined by the vertical position of the trailing arm attachment \({P}_{1}\) on the body structure. The position of the longitudinal pole defines on the one hand the brake/drive pitch angles (anti-dive/anti-lift) and on the other hand the kinematic wheel recession during jounce movements (Fig. 9). Since reduced pitch angles and increased wheel recession on the front axle are conflicting requirements, the longitudinal control arm attachment \({P}_{1}\) is positioned lower on the front axle than on the rear axle.

6.6 Elastokinematics

Due to the stiff connection of the control arms with ball joints and the low caster trail only very small elastokinematic track width and toe angle changes occur under lateral forces. The dynamic driving behavior of the vehicle can in addition be actively influenced by individually controlling the four wheel steering angles (Abe 2015). The targeted toe angle change under longitudinal forces can be adjusted by the lateral position of the trailing arm connection point \({P}_{2}\) on the outer knuckle. If this geometry point is located on the instantaneous steering axis the effect is minimal, if point \({P}_{2}\) is moved inward, toe-out under braking force results. Shifting it outward produces toe-in, which is desirable for braking stability under \(\mu\)-split conditions (Fig. 10). In the longitudinal direction of the vehicle, the trailing arm bearing can be designed softly according to the requirements for rolling comfort and road noise.

6.7 Maximum road wheel steering angle

With the selected suspension geometry, a usable road wheel steering angle range of up to +/-70° can be achieved (Fig. 11). The transfer function between the steering actuator angle and the road wheel angle is linear in the range up to about +/-65°. As the links approach the stretch position, which is defined by a toggle angle of 180° between control arm and knuckle, the steering torques increase progressively. The stretch position, which will create a singularity in the underlying mathematical equations, is reached at a wheel steering angle of about +/- 75°. For many practical vehicle driving maneuvers, the wheel steering angle range of +/-70° that can be realized with the concept is sufficient, since turning the vehicle on the spot is possible and maneuverability is significantly improved compared with a conventional vehicle concept. Which compromises relevant to practice are to be accepted in comparison to a system with a +/- 90° road wheel angle will be addressed in the course of further research.

The special boundary condition which has to be considered in the design of the spring-damper system is that all suspension components are subject to substantial spatial movements when a steering angle is applied. A reaction of the steering movement to the spring-damper forces must be minimized for reasons of ride comfort and handling. For this reason, the force application point \({P}_{11}\) of the spring/damper system is positioned on the instantaneous steering axis of the outer knuckle. Considering the installation space requirements, a mechanical connection to the lower control arm plane is not possible as the spring/damper system would collide with the control arms during steering movement. This leads to a relatively high positioned spring/damper unit. Alternatively, the spring/damper system can be actuated via a reversing kinematic system (push- or pull rod system) to make more effective use of the existing package space above the chassis.

An early design state of the system is shown in Fig. 12. Particularly when parking at standing still, very high steering torques occur due to the pure rotation of the tire patch and the weight effect arising from the kingpin angle. For the steering actuator, therefore, a two-stage self-locking gear unit consisting of a worm gear stage and a spur gear stage is used. The worm is connected to the rotor of the steering motor, while the output spur gear transmits the input torque to the inner knuckle. For installation space reasons, the steering actuator is positioned above the chassis, aligned in the longitudinal direction. The trailing arm is connected to the body by a rubber bushing aligned in the transverse direction with high cardanic stiffness to react the static moments resulting from the trailing arm weight. Figure 13 finally shows the 3D-animation of the steering motion based on a Siemens/NX CAD model.

This paper describes the concept of a novel individual corner module for vehicle applications where agility in confined spaces is a priority. The concept is based on a five-link suspension supplemented by an inner pivot bearing (inner knuckle) to initiate the steering motion of the wheels. The advantages of the concept lie in the extensive possibilities for designing the wheel suspension kinematics and elastokinematics, the effective support of the longitudinal and lateral wheel forces and the low unsprung mass. With a maximum wheel steering angle of +/- 70°, maneuverability can be significantly improved compared with a conventional suspension concept. By actively controlling the four steering angles and four wheel speeds, special driving maneuvers can be realized, such as turning the vehicle while standing still or effectively avoiding obstacles. There are many possible applications for this type of chassis concept in the field of transport, commercial and land vehicles. An invention disclosure was submitted for the concept.

Conflict of interest

The author declares no competing interests

Funding

Open access funding provided by Ulm Technical University of Applied Sciences

Author Contribution

TS: Conceptualization, Methodology, Formal analysis and investigation, Visualization, Writing, Review and Editing

Abe M (2015) Vehicle handling dynamics: Theory and application. Elsevier, Amsterdam
Backhaus R (2020) Automatisierte Shuttlebusse - Vom Testfeld zum Linienbetrieb. Motortechnische Zeitschrift 81:8–15. https://doi.org/10.1007/s35146-020-0334-5
Doll S (2021) Hyundai Mobis unveils successful ‘e-corner’ wheel module with crab driving and 0º turns. https://electrek.co/author/scooterdoll/. Accessed 3 July 2023
Hesse L, Schwarz B, Klein M, Eckstein L (2013) The wheel-individually steerable front axle of the research vehicle SpeedE. Fahrzeug- und Motorentechnik ; 1: 8. Oktober 2013
Kraus M (2018) Verschmelzung von Antrieb und Fahrwerk für einen People Mover. Automobiltechnische Zeitschrift 120:48–53. https://doi.org/10.1007/s35148-018-0074-8
Nees D, Altherr J, Mayer MP, Frey M, Buchwald S, Kautzmann P (2020) OmniSteer - multidirectional chassis system based on wheel-individual steering. In: 10th International Munich Chassis Symposium 2019. Springer Vieweg, 2020, Heidelberg
Protean (2019) Protean360+: Advanced electric-drive corner module designed for next-generation urban mobility pods. https://www.proteanelectric.com/technology/#protean360plus. Accessed 29 June 2023
Rasul MH, Zamzuri H, Mustafa AMA, Ariff MHM (2015) Development of 4WIS SBW in-wheel drive compact electric vehicle platform:1–6. https://doi.org/10.1109/ASCC.2015.7360339
Reybrouck Consulting & Innovation BV (2023) vTRACK: change your track. https://reycon.be/home/vtrack/. Accessed 29 June 2023
Schaeffler (2023) BAUKASTEN FÜR DIE NEUE MOBILITÄT | Eine Plattform für alle Fahrfunktionen. https://www.schaeffler.de/remotemedien/media/_shared_media_rwd/06_press/press_release/00195D81.jpg. Accessed 29 June 2023
Yanmar Europe B.V. (2020) Agrarroboter „Smash“ läuft schon im Testbetrieb: Autonom und vollelektrisch in Sonderkulturen. https://www.eilbote-online.com/artikel/yanmar-agrarroboter-smash-laeuft-schon-im-testbetrieb-37261. Accessed 29 June 2023
ZF (2023) EasyTurn – Federbein Achskonzept: Innovatives Federbein-Vorderachssystem für maximalen Lenkwinkel. https://www.zf.com/products/de/cars/products_64199.html. Accessed 5 September 2023

No competing interests reported.

Fig13.avi
Fig. 13 3D-animation of the steering motion

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Editorial decision: Revision requested
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Reviews received at journal
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You are reading this latest preprint version

Chassis concept of the individually steerable five-link suspension: a novel approach to maximize the road wheel angle to improve vehicle agility

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Individual corner modules

3 Implemented corner modules

3.1 Schaeffler intelligent corner module based on a trailing arm suspension

3.2 Protean corner module based on a double wishbone axle

3.3 The chassis of the SpeedE

3.4 Schaeffler i-corner

3.5 Other chassis concepts

4 Target setting on vehicle and system level

5 System design

6 Kinematic analysis

6.1 Roll compensation

6.2 Kingpin inclination and camber angle

6.3 Scrub radius

6.4 Caster angle and trail

6.5 Longitudinal poles

6.6 Elastokinematics

6.7 Maximum road wheel steering angle

7 Spring damper system

8 Design execution

9 Summary

Declarations

Funding

Author Contribution

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1