9.5: SAI by manufacturer
|Sudden Acceleration NEWS|
by Dr Antony Anderson C.Eng FIEE
3. Cruise Control : Vehicle Speed Control : principles of operation and implementation
The block diagram below shows the main elements of a
typical cruise control system (vehicle speed control
system).With a manual throttle, the cruise control uses a
stand alone speed control amplifier and a servo that
operates on the main throttle. With an electronic
throttle, the cruise control electronics reduces to the
input switches and logic, the electronic control function
becomes part of the Engine Control ECU software and
operates on the main throttle. From a functional point of
view, the cruise control system remains the same with
either a manual or electronic throttle.
A signal proportional to road speed is fed back and compared with a set speed reference to give a speed error signal that is used to control throttle position, and hence engine power, so as to change the speed to reduce the speed error signal to zero. In some analogue systems, the speed reference voltage is held in a sample and hold amplifier that uses a low loss capacitor. [ Note 1]. In other systems, the speed reference voltage is stored as a binary number in a digital counter.
Cruise control systems are well described in "Understanding
Automotive Electronics" Edition 5 by William B.
Ribbens Newnes 1998 ISBN 0-7506-7008-8. Further references
are given in Section 9.
Analog, mixed analog/digital implementations and
fully digital implementations of cruise control systems
are functionally similar, although designs may may look
Method of throttle actuation:
Don't forget to bookmark Section 9 Links and References before leaving this site.
Note 1: Sample and hold circuit for speed reference purposes
The figure below shows a representation of a typical sample and hold circuit feeding the speed error amplifier, as used in an analog cruise control circuit. Voltage holding capability depends upon the storage capacitor not losing or gaining charge via a leakage path. This may be difficult to guarantee under all circumstances in an automobile environment. In order to keep down the leakage current from the capacitor, Rs must be kept very high.
Note 2: [August 2005]
Multiplexed systems. Increasingly, automobile
electronic systems are multiplexed and typically use one
or more CAN-Bus systems to interconnect the electronic
control elements. Multiplexing does not
functionally change systems such as cruise control.
However, multiplexed systems have to be designed very
carefully if they are to avoid introducing additional
failure modes caused by transiently overloading
the bus - sometimes called the "babbling idiot
Note 3: [May 2009]
Measurement of road speed. There are two kinds
of speed measuring systems used. One is analog and
produces a signal whose frequency and amplitude is
proportional to road speed. The other is digital and
produces a train of pulses of constant amplitude at a
frequency proportional to speed. At low speeds, either
type of speed measuring system is sensitive to EMI -
especially mechanically induced EMI caused by an
intermittent electrical contact in the sensor circuit -
which can result in the generation of a false speed
signal that is of much higher frequency than the road
speed signal. See
Reference for significance
Jan 31st 2003, August 2005, May 15th 2009, August 26th 2009,July 28 2011, March 8th 2012, January 8th 2013