In the previous section a general form of the IVCS architecture is introduced assuming the control authority over the vehicle’s 6 rigid-body motions. However, for several technical and/or economic reasons (such as customer requirement, system cost, actuators availability or actuators physical limitations), most integrated vehicle dynamics control systems are designed to control only some of the vehicle’s DoF. The modularity and flexibility of the proposed IVCS system structure enables us to design a customised control system solution based on the required vehicle global features (i.e. system requirements and specifications).
The analysis and design of an integrated control system can be performed based on three different methodologies, namely: Top-down, bottom-up, hybrid approaches (Gordon, Howell, & Brandao, 2003). In a top-down approach, similar to the above described IVCS system, the system requirements are first defined and then the system specification and architecture, including the requisite number and specification of the actuators are designed to meet the requirements. However, in a bottom-up approach the high level requirements and specifications of the system are defined based on the ‘pre-determined’ number, type and specifications of low-level actuators. In both top-down and bottom-up approaches, it is assumed that each layer is only interacting with its neighbouring layers which in reality are not the case. Because of high degree of interaction among different layers, any practical design would involve a combination of a top-down and a bottom-up approach (so called hybrid approach) (Gordon, Howell, & Brandao, 2003).
In this dissertation, it is assumed that the EHB hydraulic value modulation unit (independent wheel braking) and EPAS (torque assisted front steering) are the
only existing vehicle dynamics actuators in the vehicle. It is further assumed that the mechanical components of the sub-systems have already been designed, so that their mechanical specifications are known and fixed. By these assumptions, and adopting the hybrid design approach, the design objective is limited to the control system development of an integrated vehicle dynamics system with the control authority only on vehicle planar motion. Based on the process presented in section 1.3.1, the development starts with the system requirements and specification definition and then conducted by the system architectural design, which are the subject of the following sections.
2.1.2.1 System Requirements
System Requirements are the textual documentation determined from the customer’s needs and requirements. The functional and non-functional requirements of the SUD (in this thesis) are defined as follows:
A: Functional Requirements:
1. The vehicle is intended to be equipped with EPAS and EHB systems. 2. There is no other vehicle dynamic active system that will be added to
the vehicle.
3. The system should provide a means of driver comfort in case of normal driving conditions.
4. The system should provide a means of vehicle safety in case of vehicle instability.
5. The system application should cover all the range of driving conditions.
6. The only available sensors in the vehicle are those belong to EPAS and EHB systems.
7. No other sensor will be added to the vehicle.
B: Non-functional Requirements
8. The system should be implemented with Low Cost.
9. The system should be Robust against environmental, and vehicle parameters changes.
2.1.2.2 System Specifications
System specifications are the translation of requirements into technical terms. By analysing the above-defined system requirements, the technical specifications for the SUD are derived as follows:
1. The desired vehicle linear and angular motions are derived from steady- state response of a bicycle model.
2. The vehicle equipped with EPAS, provides control authority over front tyre self-aligning moment. The vehicle lateral velocity as well as yaw rate should be controlled by altering tyre self-aligning moment through steering system.
3. The vehicle equipped with EHB brake intervention system, provides continuous control authority over four lines hydraulic brake pressure. 4. Each tyre longitudinal forces should be controlled by means of controlling
the corresponding tyre longitudinal slip.
5. In case of brake intervention, the vehicle yaw rate should be controlled by individual wheel braking (ESP functionality).
6. There is no direct control over vertical, roll and pitch motions.
7. In conclusion, the motions for vehicle dynamic control are limited to longitudinal, lateral, and yaw motions, i.e. vehicle planar motion.
8. The EPAS will reduce the driver steering wheel torque in normal driving conditions to provide the driver comfort.
9. For maintaining driver comfort and also reducing tyre wear, the steering base stability has the priority over the brake base stability in mild stability condition.
10. In the situation that the steering based stability is unable to stabilise the vehicle (hazardous stability condition), the brake based stability system (ESP) should be activated and is predominant.
11. In case of oversteering situation, the EPAS has to reduce the steering torque accordingly to recover the vehicle stability.
12. Because of the front tyre saturation, there is no control authority over steering in terminal understeering situation. Therefore, in terminal understeering situation, the EPAS based stability system will not work.
13. The lateral acceleration, longitudinal acceleration, yaw rate, wheel speed, hydraulic brake pressure, vehicle speed, steering column torque and EPAS electric motor current are the available signals provided through the sensor measurements.
14. The other required vehicle parameters such as front tyre self-aligning moment, road-tyre coefficient of friction, and vehicle sideslip should be estimated accurately and robustly.
15. The integrated control system has to be reconfigurable so the EPAS and EHB can work in a redundant manner to provide fault tolerance.
16. The mechanical systems are assumed fixed. In order to reduce the system cost, the control algorithms could be run on inexpensive processors, i.e. need less computational efforts.
17. The control system has to be robust against system structured and unstructured uncertainties and adaptive to external parameters (such as road surface coefficient of friction or vehicle parameters) variations.