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Sudden Acceleration NEWS
A Note on Automobile Cruise Control Faults and Sudden Acceleration  [or Unintended Acceleration]

 by Dr Antony Anderson C.Eng FIEE

4. Cruise Control : functional aspects and possible modes of failure

Most cruise control systems are functionally very similar and appear to be of the "proportional + integral" (PI) type of closed loop control system. Cruise control systems are likely to exhibit  similar  failure modes and have the same potential for exhibiting suboptimal performance as other industrial and domestic PI control systems because they use the same electronic technology.

Block diagram cruise
                    control system showing vulnerabilities
Block diagram of cruise control system showing some areas of vulnerability

The automobile engine compartment is a particularly  unfavourable environment in which to expect sensitive electronics to operate reliably:

  • It is hot, dirty, humid and vibration levels are high;
  • Electromagnetic Interference (EMI) levels can be high;
  • There can be appreciable shock loadings that may damage PCB wiring or affect electrical contacts;
  • Electrical connectors can become dislodged or distorted
  • Electrical contacts be subject to fretting fatigue;
  • Wiring, if not properly restrained, can be rapidly damaged by vibration-induced fretting;
  • Servicing of the vehicle, involving the removal and replacement of components, may result in displacement or damage to wiring or unwitting damage to connectors or sometimes the wrong connections being made..
In such a hostile environment,  intermittent electrical, mechanical and electronic faults in the cruise control system would scarcely be surprising. Such malfunctions might be permanent once-only events,  requiring the replacement of a "dead" module, or they might  be intermittent, occuring randomly, once in a blue moon. Typical failures might be caused by :
  • Sensors, switches, connectors and wiring :
    • a speed sensor or its wiring may fail;
    • electrical switches may fail to open or fail to close;
    • electrical switches may become mispositioned, loose or fall off;
    • multiplexed switches, where several switches communicate with a control module over a single wire, may not switch correctly in the presence of a high-resistance joint or earth contact
    • electrical connectors may fail open circuit or short circuit;
    • electrical switches or wiring may overheat causing damage;
    • slip ring connections (to steering wheel switches, for example) may become intermittent or fail;
    • earthing connections may become intermittent or fail;
    • presence or absence of extreme cold/ heat, moisture, pollution, road salt etc. may play a significant role here.
    • presence of monocrystalline tin whiskers resulting from the use of lead free solder that may cause intermittent shorts. See NASA tin whiskers page for information on this subject.
  • The Electronic Control Unit (ECU):
    • electronic components may fail;
    • vibration, shock or thermal cycling; may cause intermittent open or short circuits on PCBs;
    • moisture and surface contamination  may cause electrical tracking across insulating surfaces, in turn causing :
      • the speed reference signal to drift, either up or down;
      • a high gain amplifier or integrator, or digital equivalent, to drift into saturation;
    • logic may be affected by transient signals/noise;
    • logic may lock on or lock off.
    • a microprocessor may get into an endless processing loop
  • The throttle actuator
    • There are various different kinds of actuator used ( electro-pneumatic and electro-mechanical) but they are all essentially power amplifiers, converting a small control signal into throttle movement. The result of an input signal, whatever its source, will be movement of the throttle. Spurious control signals may derive from many sources including :
      • RF noise at the input;
      • false signals from a malfunctioning cruise control module;
      • stray potentials resulting from perhaps poorly earthed components elsewhere in the engine compartment;
      • wiring faults.
    • The actuator has mechanical elements that have the potential to jam in any position from fully closed to fully open.
This list of  possible root causes of failure is lengthy, but  is by no means complete. It illustrates however the importance of not jumping to premature conclusions as to the likeliest cause of any particular event. Broadly speaking, fault mechanisms may be divided into two kinds:
  1. external to the cruise control/actuator modules (external fault mechanisms)
  2. internal to the cruise control/actuator modules (internal fault mechanisms)
A fault may arise from the interplay of a wide variety of combinations of external and internal fault mechanisms.

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0:Cruise Control Home
2:What isCruise Control?
4:Functional aspects
5:What is Sudden Acceleration?
6:Incidence & Examples
failure mechanisms
8:Discussion of Failure Mechanisms
9:Links & References
Sudden Acceleration NEWS

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Section 4 : Version 1.1 Last updated July 9th 2001, July 30th 2010, June 10th 2011,January 8 2013
©Antony Anderson Version 1.0 February 2001 and Version 1.1 July 2001

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