Page 279 - Understanding Automotive Electronics

2735 | CH 8 Page 266 Tuesday, March 10, 1998 1:19 PM

8 VEHICLE MOTION CONTROL

below the desired speed until it eventually decays to the desired 60 mph.
This curve has a relatively low damping ratio as determined by the
controller parameters K and K and takes more time to come to the final
P
I
steady value.
Next, consider the curve that drops initially to about 40 mph, then
increases with a small overshoot and decays to the desired speed. The
numerical value for this damping ratio (see Chapter 2) is about .7, whereas
the first curve had a damping ratio of about .4. Finally, consider the solid
curve of Figure 8.3c. This curve corresponds to critical damping. This
situation involves the most rapid response of the car to a disturbance, with no
overshoot.
The importance of these performance curves is that they demonstrate
how the performance of a cruise control system is affected by the controller
gains. These gains are simply parameters that are contained in the control
system. They determine the relationship between the error, the integral of the
error, and the actuator control signal.
Usually a control system designer attempts to balance the proportional
and integral control gains so that the system is optimally damped. However,
because of system characteristics, in many cases it is impossible, impractical, or
inefficient to achieve the optimal time response and therefore another response
is chosen. The control system should make the engine drive force react quickly
and accurately to the command speed, but should not overtax the engine in the
process. Therefore, the system designer chooses the control electronics that
provide the following system qualities:
1. Quick response
2. Relative stability
3. Small steady-state error
4. Optimization of the control effort required

Digital Cruise Control
The explanation of the operation of cruise control thus far has been
based on a continuous-time formulation of the problem. This formulation
correctly describes the concept for cruise control regardless of whether the
implementation is by analog or digital electronics. Cruise control is now
mostly implemented digitally using a microprocessor-based computer. For
such a system, proportional and integral control computations are performed
numerically in the computer. A block diagram for a typical digital cruise
control is shown in Figure 8.4. The vehicle speed sensor (described later in
this chapter) is digital. When the car reaches the desired speed, S , the driver
d
activates the speed set switch. At this time, the output of the vehicle speed
sensor is transferred to a storage register.

266 UNDERSTANDING AUTOMOTIVE ELECTRONICS