Page 394 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Vehicle motion control C HAPTER 13.1

motor. The stepper motor implementation operates like that discussed for the fuel injector actuator. A high
similarly to the idle speed control actuator described in V c signal energizes the electromagnet; a low V c signal
Chapter 4.1. The throttle opening is either increased or deenergizes the electromagnet. Switching back and forth
decreased by the stepper motor in response to the se- between the two pressure sources causes the average
quences of pulses sent to the two windings depending on pressure in the chamber to be somewhere between the
the relative phase of the two sets of pulses. low manifold pressure and outside atmospheric pressure.
The throttle actuator that is operated by manifold This average pressure and, consequently, the piston force
vacuum through a solenoid valve is similar to that used are proportional to the duty cycle of the valve control
for the EGR valve described in Chapter 4.1 and further signal V c . The duty cycle is in turn proportional to the
explained later in this chapter. During cruise control control signal d (explained above) that is computed from
operation the throttle position is set automatically by the the sampled error signal e n .
throttle actuator in response to the actuator signal gen- This type of duty-cycle-controlled throttle actuator is
erated in the control system. This type of manifold- ideally suited for use in digital control systems. If used in
vacuum-operated actuator is illustrated in Fig. 13.1-7. an analog control system, the analog control signal must
A pneumatic piston arrangement is driven from the first be converted to a duty-cycle control signal. The same
intake manifold vacuum. The piston-connecting rod as- frequency response considerations apply to the throttle
sembly is attached to the throttle lever. There is also actuator as to the speed sensor. In fact, with both in the
a spring attached to the lever. If there is no force applied closed-loop control system, each contributes to the total
by the piston, the spring pulls the throttle closed. When system phase shift and gain.
an actuator input signal energizes the electromagnet in
the control solenoid, the pressure control valve is pulled
down and changes the actuator cylinder pressure by 13.1.3 Cruise control electronics
providing a path to manifold pressure. Manifold pressure
is lower than atmospheric pressure, so the actuator cyl- Cruise control can be implemented electronically in
inder pressure quickly drops, causing the piston to pull various ways, including with a microcontroller with
against the throttle lever to open the throttle. special-purpose digital electronics or with analog elec-
The force exerted by the piston is varied by changing tronics. It can also be implemented (in proportional
the average pressure in the cylinder chamber. This is control strategy alone) with an electromechanical speed
done by rapidly switching the pressure control valve governor.
between the outside air port, which provides atmo- The physical configuration for a digital, microproces-
spheric pressure, and the manifold pressure port, the sor-based cruise control is depicted in Fig. 13.1-8.A
pressure of which is lower than atmospheric pressure. In system such as is depicted in Fig. 13.1-8 is often called
one implementation of a throttle actuator, the actuator a microcontroller since it is implemented with a micro-
control signal V c is a variable-duty-cycle type of signal processor operating under program control. The actual

Fig. 13.1-7 Vacuum-operated throttle actuator.

399