by Dr Antony Anderson C.Eng FIEE
An automobile cruise control system is an outer speed control loop that "takes over" control of the throttle - normally exercised by the driver through the accelerator pedal - and holds the vehicle speed steady at a set value. The driver controls the state of the cruise or speed control system (typically : ON, OFF, RESUME, SET/ACCEL, COAST) by means of a set of switches usually mounted on the the steering wheel.
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©Antony Anderson Version 1.0 February 2001 and Version 1.1 July 2001
A Summary of :
Hans Mauser (Siemens AG, ZT PP 2) and Erwin Thurner (Siemens AG, ICN WN ES HW 42).
A Dependability Case Study
on Electronic Throttle Control published by Siemens AG
examines the situation in which the link between the
accelerator pedal and the throttle is electronic. It
scrutinises the safety and availability of a variety of
electronic control architectures and their capability to
ensure that under fault conditions built-in system
redundancy will either maintain system
performance, or ensure that it degrades gracefully to
allow safe limp home or a safety stop. It
considers that about 1% of single faults may cause
a runaway condition. The faults that might lead to
runaway include: faulty torque requests induced by
faults in various car components (gear switch signals,
deceleration slip control, faulty pedal reference signal
etc.), faults in analogue
to digital converters, faults in the torque computing
process, processor faults. It concludes that a
dual processor system is required with each
processor checking the other processor, as a minimum, to
ensure an adequate degree of safety and that
the best solution would be a full dual electronic
system, with each system checking the other, and with
one processor checking the process calculations.
Although cruise control - i.e. an outer speed control
loop - is not specifically mentioned in the study,
similar arguments probably apply, since it would hardly
be logical to apply different design criteria to the
engine control system, with in effect a dual control
system, and less stringent design criteria to the cruise
control (single control system with no redundancy).
Volume 5, Issue 10