Kristoffer K D Tagesson

Real-time performance of control allocation for actuator coordination in heavy vehicles

This paper shows how real-time optimisation for actuator coordination, known as control allocation, can be a viable choice for heavy vehicle motion control systems. For this purpose, a basic stability control system implementing the method is presented. The real-time performance of two different control allocation solvers is evaluated and the use of dynamic weighting is analysed. Results show that sufficient vehicle stability can be achieved when using control allocation for actuator coordination in heavy vehicle stability control. Furthermore, real-time simulations indicate that the optimisation can be performed with the computational capacity of todays standard electronic control units.

Implementation of Trailer Steering Control on a Multi-Unit Vehicle at High Speeds

A high-speed path-following controller for long combination vehicles (LCVs) was designed and implemented on a test vehicle consisting of a rigid truck towing a dolly and a semitrailer. The vehicle was driven through a 3.5 m wide lane change maneuver at 80 km/h. The axles of the dolly and trailer were steered actively by electrically-controlled hydraulic actuators. Substantial performance benefits were recorded compared with the unsteered vehicle. For the best controller weightings, performance improvements relative to unsteered case were: lateral tracking error 75% reduction, rearward amplification (RA) of lateral acceleration 18% reduction, and RA of yaw rate 37% reduction. This represents a substantial improvement in stability margins. The system was found to work well in conjunction with the braking-based stability control system of the towing vehicle with no negative interaction effects being observed. In all cases, the stability control system and the steering system improved the yaw stability of the combination.

Combining Coordination of Motion Actuators with Driver Steering Interaction

Objective: A new method is suggested for coordination of vehicle motion actuators; where driver feedback and capabilities become natural elements in the prioritization. Methods: The method is using a weighted least squares control allocation formulation, where driver characteristics can be added as virtual force constraints. The approach is in particular suitable for heavy commercial vehicles that in general are over actuated. The method is applied, in a specific use case, by running a simulation of a truck applying automatic braking on a split friction surface. Here the required driver steering angle, to maintain the intended direction, is limited by a constant threshold. This constant is automatically accounted for when balancing actuator usage in the method. Results: Simulation results show that the actual required driver steering angle can be expected to match the set constant well. Furthermore, the stopping distance is very much affected by this set capability of the driver to handle the lateral disturbance, as expected. Conclusion: In general the capability of the driver to handle disturbances should be estimated in real-time, considering driver mental state. By using the method it will then be possible to estimate e.g. stopping distance implied from this. The setup has the potential of even shortening the stopping distance, when the driver is estimated as active, this compared to currently available systems. The approach is feasible for real-time applications and requires only measurable vehicle quantities for parameterization. Examples of other suitable applications in scope of the method would be electronic stability control, lateral stability control at launch and optimal cornering arbitration.

Advanced emergency braking under split friction conditions and the influence of a destabilising steering wheel torque

ABSTRACT The steering system in most heavy trucks is such that it causes a destabilising steering wheel torque when braking on split friction, that is, different friction levels on the two sides of the vehicle. Moreover, advanced emergency braking systems are now mandatory in most heavy trucks, making vehicle-induced split friction braking possible. This imposes higher demands on understanding how the destabilising steering wheel torque affects the driver, which is the focus here. Firstly, an experiment has been carried out involving 24 subjects all driving a truck where automatic split friction braking was emulated. Secondly, an existing driver–vehicle model has been adapted and implemented to improve understanding of the observed outcome. A common conclusion drawn, after analysing results, is that the destabilising steering wheel torque only has a small effect on the motion of the vehicle. The underlying reason is a relatively slow ramp up of the disturbance in comparison to the observed cognitive delay amongst subjects; also the magnitude is low and initially suppressed by passive driver properties.

The influence of steering wheel size when tuning power assistance

This paper describes how steering assistance should scale with steering wheel size. A method has been developed to scale complete torque felt by the driver, both for continuous and discontinuous feedback. This was used in an experiment with 17 subjects all driving a truck with three differently sized steering wheels. The test took place on a handling track at 45–90 km/h. Continuous feedback was evaluated subjectively; discontinuous feedback by measuring angular response. Results show that torque feedback should decrease as steering wheel size decreases. A rule of thumb is to keep driver force level constant to maintain perceived handling and comfort. This also maintained the average steering wheel angle change response to discontinuous assistance. Furthermore large variance in angular response was observed. The direction, measured 0.25 s after start of a pulse, was the same as that of the pulse applied in 88% of the recordings.

Coordination of motion actuators in heavy vehicles using Model Predictive Control Allocation

The paper presents a Model Predictive Control Allocation (MPCA) method in order to coordinate the motion actuators of a heavy vehicle. The presented method merges the strong points of two different control theories: Model Predictive Control (MPC) and Control Allocation (CA); MPC explicitly considers the motion actuators dynamics before deciding on a suitable input for the actuators while CA dynamically decides how to use the motion actuators in order to modify the vehicle behaviour. The designed MPCA formulation belongs to the class of Quadratic Programming (QP) problems so that the solution is optimization based, i.e. at every step a quadratic cost function has to be minimized while fulfilling a set of linear constraints. Three scenarios were set up to evaluate the effectiveness of the controller: split-μ braking, split-μ acceleration and brake blending. Split-μ means that the wheels on one side of the vehicle are in contact with a slippery surface (e.g. ice) while the wheels of the other side lay on a normal surface (e.g. dry asphalt). The split-μ scenarios aim to combine three different types of motion actuators, disc brakes, powertrain and rear active steering (RAS), in order to brake/accelerate the vehicle while keeping it on course. The third scenario is a mild braking event on a normal road and its purpose is to combine the use of the engine brake with the disc brakes. Simulation results of the scenarios have shown promising vehicle performance when using MPCA to coordinate the motion actuators. Tests on a real vehicle have then confirmed the expected vehicle behaviour in a slit-μ braking scenario. MPCA has also been compared to a simpler CA formulation, in all scenarios. The performance of the two is comparable in steady state, but MPCA shows advantages in transients, whereas CA is less computationally demanding.

Driver response at tyre blow-out in heavy vehicles & the importance of scrub radius

Front tyre blow-outs lead to several fatal accidents involving heavy vehicles. Common for most heavy vehicles is a positive scrub radius. This can result in a destabilizing steering wheel torque at front tyre blow-out. In this study the safety improvement achieved when reducing scrub radius is quantified. By using a heavy truck equipped with a modified electric power steering system it was possible to change the scrub radius virtually. Brakes were configured to emulate front tyre blow-out which appeared as a sudden disturbance on one of the front tyres. In total 20 drivers took part in the study which was run on a test track at 50 km/h. Results show that the produced average lateral deviation from the original direction was 23 cm, when scrub radius was 12 cm, compared to 16 cm, when scrub radius was 0 cm. The main cause of the observed difference was a small, yet significant, initial overshoot in steering wheel angle which can be derived from the destabilizing steering wheel torque.