Page 388 - Automotive Engineering Powertrain Chassis System and Vehicle Body

13.1
Chapter 13.1

Vehicle motion control

William Ribbens

13.1.1 Introduction 13.1.2 Typical cruise control system

The term vehicle motion refers to its translation along and Automotive cruise control is an excellent example of the
rotation about all three axes (i.e., longitudinal, lateral, type of electronic feedback control system. Recall that
and vertical). By the term longitudinal axis, we mean the the components of a control system include the plant, or
axis that is parallel to the ground (vehicle at rest) along system being controlled, and a sensor for measuring the
the length of the car. The lateral axis is orthogonal to plant variable being regulated. It also includes an elec-
the longitudinal axis and is also parallel to the ground tronic control system that receives inputs in the form of
(vehicle at rest). The vertical axis is orthogonal to both the desired value of the regulated variable and the mea-
the longitudinal and lateral axes. sured value of that variable from the sensor. The control
Rotations of the vehicle around these three axes cor- system generates an error signal constituting the differ-
respond to angular displacement of the car body in roll, ence between the desired and actual values of this vari-
yaw, and pitch. Roll refers to angular displacement about able. It then generates an output from this error signal
the longitudinal axis; yaw refers to angular displacement that drives an electromechanical actuator. The actuator
about the vertical axis; and pitch refers to angular dis- controls the input to the plant in such a way that the
placement about the lateral axis. regulated plant variable is moved toward the desired
Electronic controls have been recently developed with value.
the capability to regulate the motion along and about In the case of a cruise control, the variable being reg-
all three axes. Individual car models employ various ulated is the vehicle speed. The driver manually sets the
selected combinations of these controls. This chapter car speed at the desired value via the accelerator pedal.
discusses motion control electronics beginning with Upon reaching the desired speed the driver activates
control of motion along the longitudinal axis in the form a momentary contact switch that sets that speed as the
of a cruise control system. command input to the control system. From that point
The forces that influence vehicle motion along the on, the cruise control system maintains the desired speed
longitudinal axis include the powertrain (including, in automatically by operating the throttle via a throttle
selected models, traction control), the brakes, the aero- actuator.
dynamic drag, and tire-rolling resistance, as well as the Under normal driving circumstances, the total drag
influence of gravity when the car is moving on a road with forces acting on the vehicle are such that a net positive
a nonzero inclination (or grade). In a traditional cruise traction force (from the powertrain) is required to
control system, the tractive force due to the powertrain maintain a constant vehicle speed. However, when the
is balanced against the total drag forces to maintain car is on a downward sloping road of sufficient grade,
a constant speed. In an ACC system, brakes are also constant vehicle speed requires a negative tractive force
automatically applied as required to maintain speed that the powertrain cannot deliver. In this case, the car
when going down a hill of sufficiently steep grade. will accelerate unless brakes are applied. For our initial

Understanding Automotive Electronics; ISBN: 9780750675994
Copyright Ó 2003 Elsevier Ltd; All rights of reproduction, in any form, reserved.