1. Technical Field
The present invention relates to a yaw stability system for a vehicle and, more particularly, to a yaw stability system having a control unit using a sliding mode control technique to determine a control yaw moment.
2. Description
Yaw Stability Control (YSC) systems have been in use in the automotive industry for a number of years to increase the stability of the vehicle and to enhance vehicle performance. In general, YSC corrects the under-steering and over-steering of the vehicle in a handling maneuver (e.g. lane change, slalom, etc.), particularly on a low friction surface. It also helps the driver maintain yaw stability of the vehicle in a severe handling maneuver.
The effectiveness of YSC systems varies widely depending on the system design. YSC systems commonly seek to minimize tracking error between a desired vehicle yaw rate and an actual vehicle yaw rate by selectively actuating braking mechanisms associated with the wheels of the vehicle. When a braking mechanism is actuated, the brake exerts a torque on the wheel which in turn induces a vehicle yaw moment. Most YSC systems are based on control techniques that rely on empirical data and are heavily dependent on testing. Systems based on “on-off” control techniques commonly fail to consider the magnitude of the tracking error other than to determine the desired braking torque. For example, the systems do not “over-actuate” a braking device if the magnitude of the tracking error exceeds a predetermined threshold or boundary layer. This deficiency in existing YSC systems often leads to undesirable braking device chatter as the devices are repeatedly actuated.
Moreover, conventional YSC systems commonly use hydraulically actuated friction based braking devices to induce control yaw moments. While these systems are generally suitable, they suffer from undesirably long response times and lack of smoothness during operation. The harsh operation of the braking systems induce undesirable noise, vibration, and harshness (NVH) during operation.
As is discussed in detail in this application, one feature of the present invention is the use of electromagnetic retarders, preferably eddy current machines, as YSC braking devices. While electromagnetic retarders have been used in braking systems for commercial trucks for many years, these retarders are generally not used in YSC systems for a number of reasons, including difficulty in accurately modeling the torque characteristics of the retarder. One modeling consideration of particular interest in YSC systems is the ability to obtain an accurate estimation of the retarding torque generated by an electromagnetic retarder. Accurate torque estimation is important for providing consistent performance. One conventional estimation technique requires an initial estimation of armature temperature which is then used in the torque calculation. Others have estimated electromagnetic retarder braking torques using predetermined look up tables of torque versus peak voltage between the retarder poles at various rotor speeds. Yet others have modeled eddy current brakes as a function of excitation current and rotor speed. However, each of the aforementioned techniques suffers from inaccuracies, assumptions that are not appropriate for many operating conditions, and/or computational intensity.
Thus, a need exists for an accurate and systematic YSC approach that minimizes or eliminates undesirable chattering, reduces dependence on empirical data and testing, improves response time, and minimizes NVH.