Fault Detection for Automotive Shock Absorber

Fault detection for automotive semi-active shock absorbers is a challenge due to the non-linear dynamics and the strong influence of the disturbances such as the road profile. First obstacle for this task, is the modeling of the fault, which has been shown to be of multiplicative nature. Many of the most widespread fault detection schemes consider additive faults. Two model-based fault algorithms for semiactive shock absorber are compared: an observer-based approach and a parameter identification approach. The performance of these schemes is validated and compared using a commercial vehicle model that was experimentally validated. Early results shows that a parameter identification approach is more accurate, whereas an observer-based approach is less sensible to parametric uncertainty. 1. Motivation Safety processes and technologies typically improve the overall equipment effectiveness and plant efficiency, with benefits that drop to the bottom line. One study also showed the following benefits from embedded safety systems: 5% productivity increase, 3% reduction in production expenses, 5% reduction in maintenance costs, 1% savings in capital expenses and 20% reduction in insurance costs, [4]. Considering that an automobile is integrated by many systems, such as, transmission, steering, exhaust, cooling, electrical, gasoline engine, fuel supply, electrical, braking and suspension system; where each system is a set of interdependent parts fulfilling a specific function and capable of functioning on its own. These systems can fail due to any number of possible failures that may occur in the sensors, actuators and devices that integrate them. Although the automobile is not a safety-critical system, several driving conditions can lead to lose the vehicle stability with fatal consequences. The World Health Organization [19] estimates that 2.2 % of the global mortality (around 1.24 million deaths) is related to vehicle collisions caused by multiple factors, e.g. human factors, road quality, vehicle design and its maintenance, etc. The economic cost of motor vehicle collisions is estimated at over US $ 100 billion per year in middle-income countries (i.e. 1 2 % of their gross national product). Therefore, the preservation of the safety and comfort of passengers are the most fundamental goals for vehicle manufacturers and automotive control engineers. This research will focus on the controlled shock absorbers of the Semi-Active (SA) suspension system. 12th European Workshop on Advanced Control and Diagnosis (ACD 2015) IOP Publishing Journal of Physics: Conference Series 659 (2015) 012037 doi:10.1088/1742-6596/659/1/012037 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 The primary functions of an automotive suspension system are to [9]: (1) isolate the chassis motion from road irregularities, (2) keep the tire-road contact with minimal load variations, (3) resist roll of the chassis, (4) react to the control forces (longitudinal and lateral) produced by the tires and braking and driving torques, and (5) maintain the wheels in the proper steer and camber attitudes to the road. The last two objectives are related to the kinematic behavior of the suspension geometry which can be solid axle or independent suspension; while, the first three objectives directly depend on the vertical force that it must transmit from the tires to the chassis. According to the capability to adjust the damping force, the automobile suspensions can be classified as: passive, active or SA. Passive suspensions are only able to dissipate the energy and their damping characteristics are time invariant, while active ones are able to store, dissipate and generate energy through a variable damping coefficient but they are very expensive to apply because require an external power supply. SA suspensions represent a good solution to control the vertical vehicle dynamics using lower energy consumption and lower cost. A suspension is integrated by 3 main elements: (1) an elastic element (i.e. a coil spring) which delivers a force proportional and opposite to the suspension elongation, this part carries all the static load; (2) a damping element (i.e. shock absorber), which provides a dissipative force proportional and opposite to the elongation speed; this part delivers a negligible force at steady-state, but plays a crucial role in the dynamic behavior of the suspension and (3) a set of mechanical elements which links the suspended (sprung) body to the unsprung mass. From the dynamic point of view, the spring and the damper are the two key elements, [21]. A SA suspension has an adaptable shock absorber where the continuous variable damping coefficient adjusted by external control signals along a road surface offers much better comfort and road holding performance with respect to Passive suspensions. The most fundamental advantage is its cost-benefit in comparison with Active suspensions. Although, the variable absorbing force may only affect transient behavior of the suspension deflection and does not affect its steady state as in an Active damper, SA shock absorbers are efficient to manage the balance between comfort and road holding. During past decades, different SA damper technologies have been examined, whose bandwidth oscillates from 0 to 30 Hz. There are four main technologies of SA shock absorbers in the market: Electro-Hydraulic, Pneumatic, Magneto-Rheological (MR), and Electro-Rheological. An extensive work on SA suspension control systems emphasizes that Magneto-Rheological shock absorbers are one of the best solutions to manage the compromise between comfort and road holding. The transient response time of MR shock absorbers is around 12 ms, the power requirement is lower, they are robust to extreme temperature variations, and they are easier to manufacture and implement [5]. However, the use of these MR shock absorbers increases the necessity of fault detection. It has been noted that SA shock absorbers are more susceptible to faults. This is specially relevant since the effect of damper faults could vary from the increase of chassis vibrations to the loss of stability. This paper, an extended version of [12], is organized as follows: Section 2 briefly reviews previous works on Fault Detection and Isolation for SA suspensions Section 3 defines the vehicle model. Section 4 describes the proposal. Section 5 discusses the results. Section 6 concludes the research project. 2. State of the art Failures are inevitable and unpredictable in vehicle suspensions. The considered fault as case of study is represented as a loss of oil in an automobile shock absorber. There are different aspects that cause a damper leakage [20], which are related to a bad installation, an excessive use, a damage by external devices or a bad design of control system that saturates the manipulation. The direct consequence of a faulty shock absorber is the loss of damping force that causes a performance reduction in the suspension capabilities (comfort and safety for passengers) at any driving situation. There are few papers dealing with Fault Detection and Isolation (FDI) for SA shock absorbers, and most of them are based on methods that inherently handle additive faults. These methods have the advantage of low computational requirements and do not need permanent excitation. 12th European Workshop on Advanced Control and Diagnosis (ACD 2015) IOP Publishing Journal of Physics: Conference Series 659 (2015) 012037 doi:10.1088/1742-6596/659/1/012037