The active control of suspension geometry has historically been a challenging perspective for chassis teams of auto makers. The new and revolutionary approach of active geometry control system reinvents the possibilities of rear suspension active toe control.
Despite the increasing interest in chassis electronic systems that use brake circuit and engine injection control to improve vehicle stability at the limit of adherence, the vehicle dynamic properties at mid range of lateral acceleration is normally controlled by passive suspension systems. However, conventional suspension systems are always limited by the different performance trade-offs: handling, ride comfort and vibration isolation, tire wear and stability. This means that by artificially modifying the geometric characteristics of the suspension parameters, there is a potential room for improvement of the vehicle response. The system object of this study modifies rear toe angles, thus offering 4Wheel Steering (4WS) characteristics.
We have to look back in 1938 to find the first 4WS concept developed by Mercedes. This system steered the rear wheels reverse to the front wheels in order to shorten turning radius. The first 4WS mass-production road car was Nissan Skyline in 1985. Unlike the Mercedes, it steered the rear wheels in the same direction as the front wheels with a maximum angle of 0.5 degree.
Honda installed a 4WS system in the Prelude (1987), where the steering angle of rear wheels depended on the front wheels turn. The system was completely mechanical, and the handling performance improvement was well accepted, but additional cost and weight was excessive for the benefits.
Described here are the characteristics of an innovative 4WS system developed by Hyundai Motors Company, and tuned with the help of Applus+ IDIADA.
The system is embraced in the family of chassis active control systems that modifies vehicle handling, by changing rear suspension toe angles during driving manoeuvres. The system uses an electronic control unit to activate an electrical actuator wisely placed on the suspension links.
AGCS (Active Geometry Control System) is an active device whose objective is to improve vehicle transient response by modifying the level of toe angle variation of the rear suspension.
Figure 1
The layout of the system consists of an electrical actuator that is commanded by an Electronic Control Unit (ECU) (Figure 1). The ECU uses only two vehicle parameters (steering wheel angle and speed) to determine the control logic of the actuation times and levels. The principles of functioning of the system have some special characteristics that constitute a novelty in the field:
This results in a more specific and efficient way to improve vehicle response (Figure 2). The system acts on the origin of the response, providing a natural feel, energy optimised actuation and excellent performance within moderate ranges of lateral acceleration.
Figure 2
The main objective of this paper is to describe the development activities undertaken for the definition of the AGCS’ control logic. The potential of the hardware layout is to be maximised with a clever but simple logic definition.
The first steps of the system development were performed by Hyundai Motor Company using different simulation activities. The results of these calculations served to evaluate the potential of the system performance, and to define the driving situations on which the system was more effective. However, a complete reconsideration of the actuation logic was performed when running prototypes were available.
In order to determine the nominal effect of AGCS on the toe variation curves (vertical and roll steer), two different measurement methods were used: kinematics and compliance testing (Figure 3), and road dynamic wheel motion measurement (using Dynawheel, Figure 4). These measurements delivered the effect of the AGCS using different actuation strokes of the electrical motor.
Figure 3
As predicted, the actuation of AGCS induced a severe change on the roll steer characteristic of the suspension, increasing the understeer level of the vehicle in a turning manoeuvre. The range of possible stroke values and its influence on the roll steer permitted to establish different levels to be used in the control logic.
The roll steer tendency will be increased with respect to the actuation stroke of AGCS. The system can easily triple the understeering kinematics characteristics of the passive suspension.
Taking into consideration the results of the kinematics test, it was possible to establish a correct approach to the definition of the control logic of the system. The objective of this set-up philosophy was to maximise the positive effect of the AGCS on the precise situations where this influence was more necessary. The most important aims were:
The definition of all the logic has been done bearing in mind the control parameters available for the set-up which are the steering wheel angle, steering wheel rate and vehicle speed. Only an additional throttle position switch has been used for the consideration of the power-off in turn situation.
Figure 4
The idea to be applied in the control logic is to establish the starting point of the actuation of the AGCS at a given level of lateral acceleration of the vehicle. This level will be determined by the steady-state characteristics of the vehicle at different speeds. Therefore, with a given steering wheel angle and speed the system will determine the corresponding steady-state lateral acceleration and decide the actuation status of the AGCS.
Additionally, different mappings will be defined based on the steering wheel angle rate. This will give information about the level of transient content of each driving condition of the vehicle. The more transient the situation, the sooner and the more aggressively the system will act on the suspension.
Figure 5
The first step is determined by using the results from the steady-state test. This test was performed at constant speed and continuous steer angle increment (one of the possible alternatives of the application of the ISO 4138). The averaged values (from left and right turns) of the steer wheel angle for different levels lateral acceleration will be used in an XY graph representing: steer angle and vehicle speed. This final plot will be used for the definition of the different actuation mappings of the AGCS based on the steer wheel angles.
The tests were performed at 5 different speeds from 80 to 160 km/h, in order to cover all the range of potential AGCS intervention.
The test data is used to plot the Delta/Speed graph for three different levels of lateral acceleration using the points obtained during the tests: 3 m/s2, 4 m/s2 and 5 m/s2
These points are approximated with potential curve fits in order to define the boundaries of the different maps to be used on the control logic. Figure 6 represents the three curves obtained from this process.
Figure 6
The next step was to define the different situations at which the different maps will be activated according to the steering wheel angle rates and steering wheel angle levels. To determine the actuation strokes at the different maps was also a very relevant issue. The logic flow is defined as follows:
The system allocates the control map based on the delta rate level. This will then be transferred to delta maps that are different for different levels of Delta Rates. If the delta-speed mapping is valid, the AGCS will then be activated with the corresponding stroke.
The delta rate maps are based on the level of transient response that induces quick steering inputs. In spite of defining the speed dependent level of delta rate boundary, these maps will be flat here, and only on the delta-speed plots will we find speed dependent curves. The minimum level (in map 1) will correspond to the minimum level found on the previous logic, so no situation is neglected here that it is not considered before.
The following map regions are defined based on steering wheel rate levels:
AGCS OFF - DeltaR: 0 – 80º/s
This region corresponds to a quasi steady-state situation and the AGCS will not be activated in any case.
Map 1 - DeltaR: 80 – 150º/s
This is the first map of activation of AGCS. The situation is slightly transient, and the stroke will be small in order to achieve a progressive initial actuation of the system, and also because the effect needed is not as important as the effect for counteracting heavier transient situations. The lateral acceleration level at which this mapping will be activated is the highest: 5 m/s2. The more stationary the driving situation is the higher the starting point of the AGCS.
Map 2 - DeltaR: 150 – 300º/s
This second area corresponds to a clear transient driving condition. The delta speeds defined here correspond to moderate to rapid steering inputs, typical of lane change manoeuvres. The effect needed is higher than the previous map. The level of lateral acceleration defined for the activation of this map will be 4 m/s2.
Map 3 – DeltaR > 300 º/s
This last map describes heavy transient situations that reproduce limit handling avoidance manoeuvres; that is, very rapid steering inputs of all types with severe overshoot of vehicle response. The effect of the system needed is maximum. However, some stroke travel is kept for correction of power-off reaction. Figure 7 shows the structure of the control logic taking into consideration all the parameters.
Figure 7
Control of power-off reaction
The final tuning of the control logic is based on the power-off situation. The idea is to increase the actuation stroke when a power-off is detected in one of the maps defined before with delta-delta R-speed. Power-off induces a clear increase of the turning parameters of the car. AGCS will counteract this figure by increasing the understeer characteristics of the response of the vehicle at the time the power off situation occurs.
Even considering different engine torque characteristics, the power-off reaction will be somehow present with different levels, the effect of AGCS is at all times positive as it counteracts the oversteer tendency of the vehicle and will increase the active safety level of the vehicle independently from the engine used. The logic will increase the actuation stroke of AGCS when a power-off reaction is detected and always if AGCS is already working.
An extensive test program was conducted in order to validate the improvement in the vehicle dynamics performance of the Hyundai Sonata when using AGCS. This test program contained subjective evaluations in different driving situations and different objective handling tests, mainly focussed on transient response characteristics.
Based on the evaluations from expert drivers, it was noticed that the most important effect was that the vehicle with AGCS improves the transient reaction by increasing the yaw damping. Other points were also improved such as a faster reaction, greater progressiveness and safer response in all transient situations.
Figure 8
Figure 9
The generation of slip angles at the rear axle improved substantially in transient manoeuvres, so all vehicle reactions were smoothed out having less overshoot in the yaw movement. However, the steady state cornering response was equal to the base vehicle, and the manoeuvrability was not affected by the increase of understeer characteristic in transient situations.
The power-off switch logic introduces a very effective and simple way to improve the reaction of the vehicle under this critical situation. The yaw rate increase tendency of the vehicle is better controlled in all cornering situations where AGCS was previously activated.
The objective test program included a long list of handling tests, however, only the most representative test results of AGCS performance will be shown in this paper. The three selected tests for this purpose are:
The step steer test makes it possible to analyse with high accuracy the vehicle response under a given step steer input. So, gain, overshoot and delay of response are measured for different output variables, i.e. yaw rate, lateral acceleration, slip angles, etc.
From Figure 8, it can be observed that the slip evolution is reduced as a direct influencing factor of AGCS. Whilst controlling rear toe, the lateral speed (slip angle related) of the vehicle is reduced.
Concerning the response values of the vehicle, AGCS intervention is principally noticed in the damping of yaw rate evolution. Both the frequency and peak values of the yaw curve are lower with AGCS test runs.
The lane change is, contrary to the step steer test, a closed loop test. The driver corrects the input based on trajectory tracing objectives. Therefore, the different test runs are more difficult to compare.
However, the stability graph that plots slip angles versus slip rates gives a good indication of the performance of the vehicle in such driving situation.
Finally, the frequency response test gives the information of how the rear toe control system damps the yaw gain response in a range of steering input frequencies (Figure 9). The vehicle without AGCS shows a typical yaw response with an increasing gain function until resonance frequency (around 1.1 Hz). When the AGCS is connected the yaw response gain is much flatter, and the increase of gain is very small. This is an indicator of yaw damping, and the effect of the system is very obvious with the results of this test.
This paper has presented the innovative control system developed by Hyundai, and tuned with the support of Applus+ IDIADA. The development of the control logic has been the result of a joint effort between these two companies, combining simulation resources and extensive experimental tools for the characterisation of the performance of the vehicle and the influence of the AGCS in various driving conditions.
The improvement on the transient response of the vehicle is very notable, and the concept of the control system enables a natural and efficient application that delivers an important benefit to the overall active safety level of the vehicle.
Keywords: Active Geometry Control System, Hyundai Sonata, Suspension systems, System Actuator, Toe Variation, Roll Steer, Actuation, Yaw, Step Steer, Transient Response
Namio, I. and Junsuke K. “4WS technology and the prospects for improvement of vehicle dynamics”, SAE No. 901167
Sangho Lee, Hyun Sung, Unkoo Lee (Hyundai Motor Company), “The Development of Active Geometry Control Suspension (AGCS) System”, SAE 2005-01-1927
ADAMS/Car User’s Guides, Mechanical Dynamics, Inc., 2002
IDIADA Technical Report, “Dry handling tuning of AGCS systems with Hyundai NF”, July, 2004
