Decouplable Steering System with the Possibility of Same- and Opposite-Direction Wheel Engagement

Abstract

In the automated, driverless, electric vehicle concept U-Shift, a novel type of mobility is created by separating a vehicle into a drive module and a transport capsule. The autonomous driving module, known as a driveboard, is able to change the transport capsules independently and thus serves to transport both people and goods. In order to be able to use the vehicle sensibly, especially in urban areas, the space required for manoeuvring and loading or unloading the capsules must be kept to a minimum. This poses special challenges for the steering system.

In this contribution, a new type of steering system is presented, which enables both same-direction and opposite-direction wheel steering. During normal cornering, there is a mechanical coupling between the wheels. This means that occurring forces and moments are mutually supported by the wheels, which minimises the energy requirement of the steering system. The mechanical coupling of the wheels can be separated for manoeuvring the vehicle. By turning the front wheels in opposite directions towards the centre of the vehicle, the centre of rotation of the vehicle can be shifted to the centre of the rear axle. The vehicle can thus be rotated around the centre of the rear axle which allows to reduce the space required for manoeuvring to a necessary minimum.

1 U-Shift Project

The growing volume of traffic, especially in urban areas, and the increasing scarcity of parking spaces, as well as the demand for the expansion of local public transport, suggest the development of universally usable vehicles. Today, a privately owned vehicle spends on average more than 90% of its lifetime unused in a parking space or garage. With a view to environmental protection and resource-saving mobility, this suggests the separation of the driving module and the usable transportation space in order to enable at least an efficient use of the drive unit. Within the U-Shift project, an automated, driverless, electrically driven vehicle concept is developed and built. This Vehicle implements and demonstrates the highly innovative concept of “On the Road” vehicle modularization. To make this possible, the vehicle is divided into a drive unit, also called a driveboard, and the payload compartment, also called capsules.

Fig. 1.

(Source: german aerospace center (DLR))

Driveboard with passenger capsule.

The driveboard is able to change the capsules independently and without the use of any additional auxiliary equipment or personnel. The same driveboard can thus be used to transport both people (Fig. 1) and goods (Fig. 2).

Fig. 2.

(Source: german aerospace center (DLR))

Driveboard with cargo capsule.

The purpose-designed capsules enable a wide variety of business models, from “on-demand buses” and goods deliveries to shops, mobile packing stations or the collection of waste containers. Thanks to the universally applicable driveboard, mixed fields of application are also possible. The same driveboard can deliver cargo capsules with goods to shops at night, support local public transport at peak times and be used flexibly for transport tasks that arise during the rest of the day.

2 Technical Challenge

The broad spectrum of different applications and the resulting variety of different capsules pose special challenges for the development of the driveboard and especially the chassis. The possibility of using a large number of different capsules results in a wide range of possible load cases that have to be carried by the chassis. The increase in weight between the weight of the driveboard without capsule and the maximum load with a personal capsule is over 270%.

In addition to the pure load absorption, there is also another task of the chassis, namely the lifting function required for changing the capsules. To change the capsules, the body of the driveboard must allow to be lowered. To change the capsule the driveboard lowers so that the capsule is placed firmly on the ground. Afterwards, the driveboard can be moved out from under the capsule in the lowered state and afterwards raised to driving level again. As soon as a new capsule is to be picked up, the driveboard is lowered again and can thus drive under the capsule that has to be transported (Fig. 3).

Fig. 3.

(Source: german aerospace center (DLR))

Taking up a passenger capsule.

When the driveboard is in place below the capsule, the body is raised back to driving level and the capsule is picked up. The driveboard then can move the picked-up capsule to its destination.

In addition to the pure lifting function, the chassis concept also enables level control to facilitate easier loading and unloading when using a cargo capsule and easy boarding and alighting of people when using a passenger capsule.

In order to expand the possible areas of application of the vehicle, especially in urban areas, the space required for the maneuvers to change the capsules must be as small as possible. To optimize the space required for changing the capsules, several maneuvers have been investigated. Figure 4 shows two exemplary changing maneuvers.

Fig. 4.

Maneuvering with front wheel steering, left: half transverse movement, right: full transverse movement

Simulation results in Fig. 5 show that enabling large steering angles at the front axle can effectively reduce the required movement space, even without all wheel steering.

Fig. 5.

Space requirements for changing capsules with a lateral distance of 3 m depending on the turning maneuver.

Even if wheel-individual control of the wheel steering angle seems promising for very large steering angles, a conventional steering linkage should not be completely dispensed with, as it has advantages in terms of reliability and energy consumption during normal driving. It should be noted that with individual wheel steering, the tire self aligning torque resulting from the lateral force as well as the circumferential force must be compensated by the respective actuator. This leads to a correspondingly higher energy consumption. In contrast, in a mechanically coupled system the torques of the two wheels compensate each other within the steering system, so that only the differential moments have to be provided by the actuators.

To further reduce the space required for maneuvering and turning, it is advantageous to turn the wheels in opposite directions. By turning both wheels in the direction of travel, the center of the curve can be moved to the center of the rear axle as shown in Fig. 6. This allows turning on the spot and the turning radius is reduced to the length of the wheelbase.

Fig. 6.

Maneuvering with opposite-direction wheel engagement

The special body shape of the U-Shift driveboard severely limits the choice of possible wheel suspensions, especially on the rear axle. Due to the additionally limited construction space, different wheel suspensions are used on the front and rear axles respectively. The CAD model of the chassis concept is shown in Fig. 7.

Fig. 7.

CAD model of the chassis concept (front axle on the left, rear axle on the right)

3 Front Axle

To fulfill the functionalities of steering with large steering angles on the one hand and height adjustment on the other hand, a double wishbone wheel suspension system is implemented on the front axle.

Since a height adjustment of 200 mm is necessary to change the capsules, special measures are necessary. Realizing such a large change in level with a conventional double-wishbone wheel suspension without additional precautions would result in a significant change in track width. Since the lifting process takes place while the vehicle is stationary, the wheels and thus the tires would be pulled over the ground while the tires are not in motion, which would result in enormous lateral stress in the suspension system and additional wear at the tires.

The steering gear is connected to the body the steering linkage connects the steering gear to the wheel carrier. If the steering gear would be mounted on the body and move relatively to the wheels during the lift, this would result in a change of the wheel steering angle. In order to avoid such unintended steering movements, the steering system must be decoupled from the lifted part of the body during the lifting process. In order to prevent the mentioned change of track width as well as unintended steering, the body has to be separated from the wheel suspension, which is realized by a special subframe as shown in Fig. 8. All components of the wheel suspension, as well as the lifting device and the steering system, are mounted on this subframe. The level of the wheel suspension in relation to the road surface therefore remains unchanged when the level of the driveboard body is changed by the lifting device.

The u-shaped design of the subframe makes a significant contribution to reducing the installation space. Due to the special shape of the subframe, the entire space between the side parts is available for other vehicle components like the battery pack and the electronic controller devices.

As all parts of the suspension system and the steering system as well as all components of the lifting system can be attached to the subframe a simple pre-installation of the assembly is enabled.

Fig. 8.

CAD model of the subframe

In addition, the integration of all components and assemblies required for the implementation of all sub-tasks in the chassis reduces the total required installation space to the minimum. The forces that occur while driving are absorbed by the structure of the subframe. Fig. 9 shows the integration of the relevant subsystems.

Fig. 9.

CAD model of front axle components and subsystems on the subframe

The lifting actuators and linear guide bearings are firmly connected to the subframe on one side and to a mounting base on the vehicle body on the other side. When the body is raised to receive a capsule, the body is lifted by the subframe. The height level of the subframe in relation to the road surface remains unchanged. Since the wheel suspension is not connected to the body, but only to the subframe, changes in track width due to lifting are avoided. By attaching the steering system to the subframe, complicated constructions of the steering system, which would otherwise be necessary due to the lift, are also avoided.

4 Steering System

The operation of the steering system can be divided into two modes. One is normal driving and the other is maneuvering and turning maneuvers. The two operating modes are described below. First the mechanics of the steering linkage and then the functioning of the steering gear are described.

4.1 Mechanical Design

The steering system consists of a left steering gear-side track rod (4), a right steering gear-side track rod (5), a left wheel-side track rod (6), a right wheel-side track rod (7), several joints (8), a left reversing lever (2) and a right reversing lever (3), a steering gear (1) and at least two steerable wheels.

Straight-ahead travel

During straight-ahead travel (Fig. 10), the track rods (6, 7) are connected to the steering arms and the reversing levers (2, 3) via joints. The track rods (4, 5) are coupled to the steering gear and also connected to the bell cranks (2, 3). As with conventional steering systems, the wheels (9) are kept on track. Steering moments that occur are transmitted from the wheels via the steering linkage as forces to the joints and the steering gear, where they are mutually supported. The function of the bell cranks is to transmit the movements and forces of the track rods (4, 5) to the track rods (6, 7). Due to the special position of the steering gear given by the installation space, the bell cranks are also necessary to limit the movement of the track rods (4, 5, 6, 7) to a minimum. The steering levers therefore have the task of transmitting forces as well as complying with the installation space restrictions and preventing the steering linkage from colliding with components of other assemblies.

Fig. 10.

Straight-ahead travel

Steering during normal driving

When turning the wheel in the same direction, i.e. cornering (Fig. 11), the mechanical coupling inside the steering gear remains. During a steering maneuver, the track rods (4, 5) move in the same direction. Through the bell cranks (2, 3) and the joints, the thrust movement of the track rods (4, 5) is transmitted to the track rods (6, 7) and from there via the track levers to the wheels. The displacement of the track rods (4, 5) by the steering gear results in the wheels turning in the same direction.

Fig. 11.

Same-direction wheel engagement

Steering during maneuvering and turning maneuvers

To reduce the space required for parking and maneuvering, the wheels (9) are turned in opposite directions (Fig. 12). The mechanical coupling inside the steering gear (1) is disengaged. The left (4) and right (5) track rods are shifted in opposite directions. Through the bell cranks (2, 3) and the joints, the pushing movement is transmitted to the track rods (6, 7) and through them to the wheels. The displacement of the track rods (4, 5) in opposite directions therefore results in the wheels turning in opposite directions.

Fig. 12.

Opposite-direction wheel engagement

4.2 Operating Modes

The implementation of the different operating modes is realized in the steering gear through the combination of recirculating ball gears and clutches. The different driving modes are shown in Figs. 13 to 15. The gearbox consists of a drive (17), a toothed belt drive with two wheels (18) and a toothed belt drive with three wheels (19) whose output sides are each coupled with a ball screw (23), a toothed gear (20), two clutches (21, 22) and a center take-off with disc spring (24).

Straight-ahead travel

When driving straight ahead, as shown in Fig. 13, the forces acting on the wheels during travel are mutually supported by the coupled steering linkage. No additional forces are applied by the drive (17).

Fig. 13.

Steering gear when driving straight ahead

Steering during normal driving

When cornering, the wheels are turned in the same direction. The necessary gear shift is shown in Fig. 14. The clutch (21) is closed and the clutch (22) is open. The torque of the drive is transmitted to the toothed belt drives (18, 19). On the output side, the torque is transmitted to the ball screw drives (23). Due to the existing connection of the couplings, the same direction of rotation is present at both ball screw drives and the racks are shifted in the same direction, which results in wheel engagement in the same direction. The forces acting on the wheels are mutually supported by the existing mechanical coupling.

Fig. 14.

Steering gear with same-direction wheel engagement

Steering during maneuvering and turning maneuvers

To enable the wheel to turn in the opposite direction, the clutch (22) must be closed and the clutch (21) opened. The direction of rotation coming from the drive (17) is reversed by the toothed gear (20). As a result, the two toothed belt drives (18, 19) have an opposite direction of rotation, which they transmit to the ball screws (23). The opposite directions of rotation of the ball screws result in a displacement of the toothed racks in opposite directions and thus the wheels turn in opposite directions (Fig. 15).

Fig. 15.

Steering gear with opposite-direction wheel engagement

5 Summary

The separation of the U-Shift vehicle into a driveboard and a transport capsule enables high flexibility in use and high utilization times of the driveboard. At the same time, this leads to a very large variance in vehicle weights and axle loads, especially on the rear axle. Therefore, the requirements on the front and rear axles are different. The rear axle is subject to challenges in terms of available installation space, particularly due to the U-shaped body used. By developing the suspension system separately for the front and rear axles, the respective requirements in terms of installation space and functionality are optimally met.

A double wishbone suspension is fitted on the front axle. Due to this, a change in track width must be avoided during the lifting process, which is realized with the help of a subframe. Additionally the steering must be installed and achieve large steering angles to allow high maneuverability for use in urban areas. This is made possible by a steering gear with a detachable mechanical coupling. Due to the mechanical coupling of the wheels, the special steering gear ensures an energy consumption comparable to conventional steering systems when driving straight and cornering. However, it also enables the wheels to turn in the opposite direction when maneuvering and turning, thus limiting the space required to the bare minimum.