Longitudinal Wheel Slip Estimation

As has become clear in the previous chapters of this book, the problem of estimating the longitudinal wheel slip is crucial for an effective design of ABS, TCS and ESC control systems, especially in the case when the control problem is formulated as the regulation of the wheel slip itself.

To obtain an accurate estimate of the longitudinal wheel slip one first of all needs to correctly process the wheel speed sensor measurements in order to obtain an estimate of the wheel speed. This topic is discussed in detail in Appendix B, where the two main algorithms employed for this task are discussed and their advantages and disadvantages outlined.

Further, according to the wheel slip definition, see Equation 2.3, one needs to estimate the longitudinal vehicle speed. In fact, this variable can be directly measured only by means of optical sensors, which are expensive and fragile and hence used only for prototyping purposes.

The crucial difference in setting up a speed estimation algorithm for ABS and TCS control systems is that in the former case all wheels are in general subject to braking torque (and thus to a non-zero wheel slip), whereas in the latter only the driving wheels transmit the traction torque to the ground, and thus two of the wheel speeds in fact evolve in almost free rolling (thus with no – or with negligible – wheel slip) and can hence be used estimate the vehicle speed.