Minimizing of Kinetosis during Autonomous Driving

Experts believe that autonomous driving is the key technology for our future transport system. A major advantage of this technology is that the passengers can perform other activities while driving. However, studies have shown that the incidence of kinetosis (motion sickness) is significantly higher in autonomous driving than in conventional vehicles. An active air spring was developed at the Technische Universität Darmstadt to improve active suspension comfort and driving safety.

1 Motivation

Self-driving vehicles are either inexpensive utility vehicles with low driving comfort or vehicles that serve as rolling offices or living rooms and have to fulfil high comfort requirements. To meet these requirements, controlled active spring-damper systems are ideally suited for decoupling the chassis from road excitation. Since 2008, Technische Universität Darmstadt has been developing an active air spring as part of the Collaborative Research Center 805 “Control of Uncertainties in Load-Carrying Structures in Mechanical Engineering.” The active system combines the advantages of air suspension with those of an active system.

2 Active Air Spring

Semi-active air springs with switchable additional volumes are state of the art, but unlike active systems, they do not decouple body and excitation. The required actuating frequency to actively control the movement of the car body is 5 Hz. The resulting compression force of the air spring is (description of the symbols used see Table 1):

There are two ways to design an active air spring: the air spring pressure can be adjusted independently of the air spring deflection, or the load-carrying area can be adjusted. A design evaluation shows that a controlled adjustment of the pressure, for example by increasing or decreasing the air spring volume, is too slow and that an adjustment of the load-carrying area is better suited for manipulating the axial force. For this purpose, an air spring piston was developed at Technische Universität Darmstadt, whose radius rP can be adjusted hydraulically with four segments evenly distributed around the circumference [1, 2], Figure 1 (left). In order to maximize the actuating force and minimize the power requirement, a double-bellows air spring with a circular load-carrying area and two adjustable pistons was used, Figure 1 (right). To increase the total load-carrying area, the upper actuator extends and the lower one simultaneously retracts. The two actuators are hydraulically connected to a double-acting cylinder. By moving the cylinder, one pressure chamber is enlarged and one chamber is reduced in size. The hydraulic system is dry, as the chambers are closed and leakage-free. The cylinder is driven hydraulically or electromechanically. The concept enables regeneration of the energy released when an actuator is automatically retracted. This energy is fed directly to the extending actuator; only differential forces have to be applied by the drive. In experiments, the total energy consumption of the system was reduced by more than 30 %. This is not possible with a conventional hydraulic drive. Either the energy released during tensile loading has to be stored temporarily in an accumulator (recuperation) or it flows into the tank, unused, as heat energy. The actuating force of the functional prototype of the active air spring is ± 1000 N at a static load of 2850 N. The system is partially supporting and the load is adjusted by varying the air spring pressure and individualizing the controller according to the resilience principle “one size fits all.”

Tabe 1 Description of the symbols used (© Technische Universität Darmstadt)

Figure 1

Active rolling piston for adjusting the-load-carrying area of the air spring (left) and.functional prototype of the active air spring with two adjustable rolling pistons (right) (© Technische Universität Darmstadt)

3 Model-based Vertical Dynamics Controller Design

In order to quantify the effect of different parameters of the actuator, such as actuating force or actuating frequency, as well as the influence of the controller set-up on the two performance outputs driving comfort and driving safety, we carried out investigations on the quarter car model, Figure 2. Thus, the ideal design of the actively controlled chassis can be described as a mathematical optimization problem in the following form

subject to

The maximization of driving comfort and safety means minimizing the H2-norm of body acceleration and wheel load fluctuation transfer functions. When the excitation is modelled properly, they correspond to the equivalent standard deviations. Since this is a Pareto optimization, the weighting between the two target variables can be shifted using the parameter ?F. K gives a suitable structural restriction on the controller, for example the restrictions on static feedback. The deflection z and the control input uActuator are limited to one third of the maximum values with regard to their standard deviation. If the constraints are formulated by using additional terms in the objective function, the optimum controller can be calculated directly. Otherwise, the optimization problem, Eq. 2, must be solved numerically. The result of the optimization is a set of Pareto lines. They represent boundary lines, which cannot be undercut under the given conditions according to the motto “It doesn’t get better than this.”

Figure 2

Quarter car model and the model for the design of the vehicle vertical dynamics control (© Technische Universität Darmstadt)

In order to investigate the influence of actuator dynamics, the transfer function of the actuator was modelled as a PT1-element with an edge frequency of 5 Hz for a typical slow-active system and of 30 Hz for a fast-active system. As shown in the conflict diagram, Figure 3 (left), almost no compromises have to be made with the fast-active system and a maximum actuating force of 1 kN compared to an ideal actuator. Both driving comfort and safety are significantly improved. The slow-active system will be used primarily to increase driving comfort. The transfer function of the actuator of the active air spring was identified experimentally and taken into account when the controller was designed. The result is an optimal tenth-order dynamic controller, which, however, is not used in practice for reasons of robustness. For the experiments on the test rig, a simple Skyhook controller with a static feedback of the body and deflection speed was implemented. To compensate the loss of performance due to the simple control, a preview controller was implemented by feeding forward the road excitation [3], Figure 3 (right). In this case, the active air spring improves ride comfort by 35 % with a minimum improvement in driving safety. Similar results could be obtained for other excitations and system parameters [3].

Figure 3

Boundary lines for different actuator configurations with a maximum actuating force of 1 kN (left) and boundary lines for the prototype of the active air spring with different controller configurations (right, grey: Skyhook controller, red: Skyhook controller with preview control) (© Technische Universität Darmstadt)

4 Actuator Validation in Hardware-in-the-Loop Experiments

To experimentally examine the active air spring in the quarter car, Hardware-in-the-Loop (HiL) tests were carried out. In these tests, the air spring is coupled with a virtual quarter car, which is simulated in parallel in a real-time simulation environment by dSpace, Figure 4. The deflection of the air spring calculated in the real-time simulation is transmitted to the test damper system, which adjusts it using an internal controller. The measured axial force response F is fed back into the simulation (closed loop simulation). The controller is also mapped in the simulation model. HiL tests provide the advantage that the interaction of the active air spring with the overall system can be investigated at an early stage of development. It is also possible to easily vary parameters of the virtual system and thus examine the active air spring in different test scenarios (different loads, excitations, etc.).

Figure 4

Basic setup for the HiL simulations with the active air spring (© Technische Universität Darmstadt)

Figure 5 shows the frequency responses for body acceleration and wheel load fluctuation, calculated from the HiL test results for driving on the federal highway in different configurations. In addition to the designed controller with preview, Figure 3 (right), a second controller configuration was used to reduce low- frequency oscillations causing kinetosis. This frequency-specific weighting of the body acceleration was already taken into account in the controller design in the form of weightings according to VDI standard 2057. In comparison to the passively operated air spring, it is clear that the acceleration of the body in the natural frequency is reduced in active operation. This mitigates the incidence of kinetosis. The average power requirement for the configuration with preview is approximately 100 W.

Figure 5

Frequency responses of the body acceleration and wheel load of the quarter car with.the active air spring measured in HiL tests when driving on a typical federal highway at 100 km/h (©.Technische Universität Darmstadt)

In addition to the HiL tests with stochastic road excitations, we performed tests with single obstacles. Figure 6 shows the measured time records for driving over a cosine-shaped bump of 50 mm height at 10 km/h. The active air spring with preview control minimizes the maximum body acceleration by 53 % and wheel load fluctuation by 23 % compared to the passive air spring.

Figure 6

HiL test results for driving over a cosine-shaped bump at 10 km/h (© Technische Universität Darmstadt)

5 Summary and Outlook

At Technische Universität Darmstadt an active air spring with an adjustable rolling piston was developed. HiL tests have shown that body oscillations that cause kinetosis are reduced and driving comfort is increased with this system. Basic investigations on the optimum control of an active chassis and the influence of different actuator configurations on the achievable driving comfort and safety of the quarter car were presented.

In the future, the hydraulic damping will be replaced by integrated pneumatic damping. In addition, research is being conducted into alternative designs of the active air spring — a low and disintegrated design and a semi-active air spring with an adjustable rolling piston. These concepts are suitable for use in electric vehicles and vans.