9.5: SAI by manufacturer
|Sudden Acceleration NEWS|
A Note on Automobile Cruise Control Faults and Sudden Acceleration [ or Unintended Acceleration]
by Dr Antony Anderson C.Eng FIEE
Surface leakage affecting high impedance circuits - two research investigations - (1) single-point-faults causing operational amplifier malfunction - (2) intermittent open and short circuit effects reproduced - Water ingress - Effects of EMI - Summary
In the discussion of sudden acceleration incidents, the focus seems mainly to have been on the external sensors, wiring and interlock arrangements and on the likelihood/unlikelihood that faults in these could have serious consequences. Surprisingly little attention seems to have been paid to the possibility of an internal cruise control module fault causing a throttle actuator malfunction.
Two lines of argument discounting potential faults within the cruise control module seem to be deployed :
Patents relating to cruise control systems go back over 30 years. (US Patent 3,455,411 of July 15 1969.) A feature of a number of early cruise control designs is that the speed reference voltage is stored on a low-leakage capacitor forming part of a high impedance sample and hold amplifier. Such high impedance circuits are very sensitive to surface leakage currents and noise. Click for typical arrangement of sample and hold speed reference circuit
US patent 3,937,980 February 10, 1976 suggests that the development of electronic cruise control systems has not been trouble free, especially in connection with the design of high impedance sample and hold amplifiers. The problem relates to minimising the effects of surface leakage on printed circuit boards. If, for some reason, an unintended leakage path appears to a source of voltage that is higher than the capacitor voltage, it will charge up to the higher voltage. If, on the other hand, the leakage path is to a source of voltage that is lower than the capacitor, it will discharge to the lower voltage In either case, the result of leakage will be that the speed reference voltage will drift away from its intended value, with potentially serious results as far as the control of vehicle speed is concerned.
This particular patent relates to potential guard rings designed to overcome leakage problems found in earlier cruise control modules. It describes very clearly some of the problems that electronics engineers have come across when using printed circuit boards in harsh environments, such as the engine compartment of automobiles. For example, the abstract says :
" When a conventional printed wiring board is subjected to a harsh environment, leakage can develop between adjacent conductors over a period of time. If high impedance circuits are employed on the board, such leakage can adversely affect circuit performance. The effect of such leakage can be minimised if the circuit layout is so arranged that potentially troublesome leakage occurs only to elements that operate at comparable operating potentials. In one embodiment of the invention a circuit is shown having a high impedance point on a printed wiring board intended for an automotive application. The high impedance wiring conductor is completely surrounded by a metal conductor that is connected to a potential point in the circuit that approximates the potential at which the high impedance point will operate. When the automotive environment results in surface leakage, there will be very little change in the operational character of the high impedance circuit."Broadly speaking the picture the patent presents of a harsh environment is comparable to that presented by Kimseng below. The patent continues :
"The combination of moisture with corrosive fumes constitutes a particularly severe problem because it can create conductive paths on insulating surfaces. This means that conductive paths exist where none are desired. This is most troublesome in high impedance circuits. Ordinarily solid state circuits operate at low impedance levels and are relatively tolerant of moderate leakage. However in some applications high impedance circuits are called for and these constitute a particular problem."The particular components that the patent highlights as being vulnerable to leakage are the high quality capacitors [Note 3] used to store speed reference voltages, in which very little leakage can be tolerated because the reference voltage must be kept constant for extended periods of time. To overcome the leakage problem the high impedance points of a printed circuit board are individually surrounded with a closed ring of conductive material.
"This ring can be a separate conductor or in the interest of economy it can be part of the regular wiring. The ring is connected to a source of d-c potential that approximates the operating potential of the high impedance point. Thus even though some surface leakage develops the leakage will not alter the potential at the protected point to the degree that uncontrolled leakage would produce."The patent then continues:
"In a typical embodiment of the invention in an automatic speed control circuit, a high impedance point is surrounded with a conductor ring that is connected to the reference source potential which represents the d-c voltage level that the high impedance point will normally achieve. This action in practice constitutes a safety feature. If the high impedance point were to develop uncontrolled leakage to a point of substantially different potential, the speed control circuit could malfunction to result in excessive accelerations of the vehicle."This paragraph implies : (a) that if surface leakage develops on a cruise control module printed circuit board it may result in charging or discharging the memory capacitor causing the speed control circuit to malfunction and possibly cause excessive accelerations of the vehicle; (b) that that the guard ring constitutes a safety feature that protects against excessive accelerations. By implication, if the guardring should become disconnected, wrongly connected or omitted, it will cease to fulfil its safety function. Without an effective guard ring, moisture and corrosive fumes may cause leakage currents on insulating surfaces which in turn may cause the memory capacitor to overcharge or to discharge, thereby causing a risk of an excessive acceleration.
The originators of the patent have focussed on an innovative way of reducing the effect of leakage in a high impedance circuit in a hostile environment. They do not appear to have considered the alternative of using a low impedance drift-free speed reference circuit, such as might be the practice with a P +I controller used in an industrial application.
The message from this patent is that high impedance
electronic circuits in cruise control systems may suffer
from leakage problems because they operate in a hostile
environment, where it is very difficult to maintain the
integrity of insulation over long periods of time.
Two independently executed research investigations, neither of which appear to have been brought to the attention of the courts, have examined cruise control modules and have sought to establish what failure modes in might arise in these and whether any of these modes might cause sudden unexplained acceleration. They suggest some possible fault mechanisms that may have occurred in particular types of cruise control systems:
Kimseng et al report that over a five year period owners of a particular type of cruise control unit have complained of a variety of faults : from not engaging on command, to the cruise unexpectedly accelerating past the desired speed. They observe that a common thread to the complaints was the intermittent nature of the problems. More than 96% of the reported failed modules removed from vehicles and returned to the unnamed automobile company passed bench tests. They report that up till the end of Oct 1997 the company in question had spent more than $600,000 on parts alone for cruise control warranty replacements for its 1996 vehicles and that the 1997 fleet was showing the same pattern. Note 5
Kimseng et al tested 9 cruise control modules in all : five which had been returned two years previously under warranty with intermittent faults that checked out OK on bench tests and four new units straight from the manufacturer. Their investigation did not address vibration, but focussed on laboratory simulation of heat and humidity conditions in the engine compartment. In accelerated tests they demonstrated that intermittent open and short circuits developed in a manner that appeared to be consistent with the service reports of intermittent faults. Both returned and new units demonstrated two similar failure mechanisms : (1) high resistance fretting between separable interconnects and (2) PCB shorting from humidity and contaminants. Both failure mechanisms vanished after a while once the cruise control module was removed from the test oven and had been allowed to 'sit' for an extended period. The authors expressed the opinion that : "It is possible that some combination of these failure mechanisms is causing the CCM failures and could explain the runaway acceleration especially if the power line shorts with the motor line."
From a control system point of view, intermittent open or short circuits on the control module PCB suggest the following possibilities:
The picture is now becoming clearer. (1) Some of the electronic circuits used in automobile cruise control are high impedance, sensitive circuits that can potentially suffer from leakage because of the harsh environment in which they work. (2) the electronic circuits are vulnerable not only to leakage but to intermittent faults (open and short circuits) developing as a result of either the ingress of moisture and pollution or vibration, separately or in combination (3) cracks in PCB tracks may cause the characteristics of certain functional elements of the circuit to change intermittently from one state to another (4) the results of (1), (2), or (3) may change the characteristics of the cruise control module sufficiently to qualify as an intermittent fault condition.
Measures can be taken to minimise the likelihood of
internal intermittent faults, but they cannot be
eliminated altogether. When intermittent internal faults
occur, there is a possibility that they may cause
the throttle actuator to operate, without there
necessarily being a fault in the external control and
interlock logic. There is no guarantee that the
fault diagnosis systems currently in use will detect
internal intermittent faults.
Since the RoHS Directive was adopted by the EU in
February 2003, lead free solders have now become the
norm. However, lead free solders are sensitive to tin
whisker formation. These very fine whiskers, much finer
than the finest human hair, can form intermittent fault
paths between printed ciruit tracks that can momentarily
carry high currents. They rarely leave a significant
trace behind them afterwards. For further reading on
this subject see the NASA website at http://nepp.nasa.gov/whisker/background/index.htm
There can be little doubt that moisture ingress has
been the cause of some sudden accelerations, the
incidents in a Proton Waja in Malaysia mentioned earlier
being but one example. Sudden accelerations in car
washes seems to have affected Cherokee and Grand
Cherokee Jeeps in particular. The International
Car Wash Association has alerted its members to the
danger of vehicles suddenly accelerating during or after
the car washing process on a number of occasions. As
long ago as September 1988 the ICA advised vehicle
handlers: "Car wash employees should be constantly
reminded to turn off the ignition key when handling a
runaway automobile, when the natural tendency is to put
both feet on the brakes and push as hard as
possible. Simply reaching down and turning off the
ignition will minimize the damage in collisions and
runaways." In 2003 the ICA issued a reminder reminder
to members to report sudden acceleration
incidents so that they could continue to monitor the
situation and be prepared for future actions. In 2006
the ICA issued a revised advisory
notice in an attempt to minimise the risk
of sudden accelerations with Cherokee and Grand Cherokee
Jeeps. On June 13th 2006 Connecticut's Attorney General
Richard Blumenthal and State Representative Patricia
Widlitz called upon NHTSA to investigate sudden
acceleration problems in late model Jeep Grand
Cherokees, saying that NHTSA should require Daimler-Chrysler to
provide all information concerning sudden or
unexpected accelerations, and request information from
car wash owners, auto repairers and insurance
"The rate and severity of these sudden acceleration incidents suggest a severe structural flaw - certainly more than simple coincidence," Blumenthal said. "These incidents - in one case killing a Connecticut man - call for aggressive and vigorous action to prevent another needless, preventable tragedy. No safety official can be neutral: a full-gear federal investigation is vital."
Widlitz said, "It is imperative that the National Highway Transportation Safety Administration immediately launch an investigation into the Jeep Grand Cherokee sudden acceleration incidents. How many deaths and injuries must the public endure before this issue gets serious attention?"
Two days later On June15 2006 , a
52-year-old employee, who had worked at Thunderbird
Car Wash for the past 12 years, got into a 2006 Jeep —
with less than 4,000 miles on it — to drive it off the
conveyor. When he put the car in "drive," it
accelerated out of the tunnel, smashed the open door
of a Cadillac, squeezed between two other cars, jumped
a curb and was about 75 feet down a street before he
brought it under control. Report
in Carwash News For further
reading, Doug Newman a New England car wash owner has
had several sudden acceleration incidents in car washes
that he owns and has gathered a considerable body of
information together on his website at http://jeepsua.googlepages.com/home
Sayler, Bizzak and Nocivelli in their Formal
1991-1995 Jeep Cherokee and Grand Cherokees
(2002) identified the 60 pin connector to the
Power Control Module (PCM), as a point where water
ingress could potentially cause the cruise control to
malfunction. Although this work relates to a specific
PCM and cruise control configuration, nevertheless it
demonstrates that multipin connectors have the potential
for developing sneak circuits that may cause
malfunctions. Multiple parallel faults become a distinct
possibility at the connector interface. This study puts
the incidence of sudden accelerations in Jeep Cherokee
and Grand Cherokees in the context of lesser rates with
other vehicles - they make the case that this shows that
sudden accelerations are vehicle-related rather than
The effects of electromagnetic interference (EMI) on sensitive electronic circuitry should always be considered as a possible contributory factor to malfunction. For example, the 1995 NASA Report on Electronic Systems Failures and Anomalies Attributed to Electromagnetic Interference refers to the following:
There is anecdotal evidence to suggest that, for example, on-board mobile amateur radio equipment, if not properly installed, may upset automobile cruise control circuits. However, generally speaking, for external RFI, the vehicle engine compartment will act as a Faraday cage and should provide a fairly high degree of screening.
When it comes to consider EMI generated within the automobile engine compartment the situation is very different. The most potent sources are probably :
Many throttle actuators now use small stepper motor drives, which are driven by means a series of control pulses originating in the cruise control module or the engine control module (PCM). In the event of stray RF derived pulses appearing at the input of the stepper motor driver circuit, particularly if a logic hold-down resistor should go open-circuit, these may cause a false movement that will either open or close the throttle. Note 6
Increasingly, automobile electronics are adopting CAN-Bus or similar multiplexing technology to enable communication between sensors, electronic control units and actuators. [Note 7] Whilst Bus systems have significant advantages in terms of reduced wiring harness weight and complexity, they may also introduce new potential system failure modes - for example, the "babbling idiot syndrome" where, for some reason, the system momentarily overloads and can no longer handle the message transfer requirements. What is evident is that once data is transmitted over a common network, rather than point to point, the system design becomes critical. It becomes necessary to consider the potential interactions of sub-systems that, although not coupled from a functional point of view, and although hopefully protected from undesirable EMI interaction, may be disrupted by communications bus overloads and other kinds of bus malfunction should these arise.
With regard to sudden acceleration problems that may
arise with electronic throttle controls, a
presentation by Professor Todd Hubing to the National
Academy of Sciences Sudden Acceleration
Investigation Committee in July 2010 is of particular
interest and relevance. See Hubing: 'Analyzing
Unintended Acceleration and Electronic Controls' http://onlinepubs.trb.org/onlinepubs/ua/100701hubing.pdf
Particular implementations of cruise control might be more, or less susceptible to the same kinds of problem. The incidence of failure will be largely dependent on how well the particular manufacturer anticipates the whole gamut of possible failure modes, internal and external, and incorporates measures to prevent their occurrence in his designs.
None of these three sources specifically mention the
possible effects of RFI. However, in my view, depending
on the particular type of cruise control unit used, RFI
from the ignition and/or starter motor, or mechanically
induced RFI caused by intermittent speed
sensor connections, might well be factors in causing
In my opinion, Hubing's work on analysing unintended acceleration and electronic controls (July 2010) mentioned above represents a significant advance in the state of the art regarding the understanding the phenomenon. I would suggest that anyone wanting to improve their own understanding of the underlying issues should read his presentation to the NAS carefully.
9.5: SAI by manufacturer
|Sudden Acceleration NEWS|
|Don't forget to bookmark Section 9 Links and References before leaving this site.|
January 31st 2003, June 17th 2003, July 2nd 2003, June 4th 2007 (mechanically induced RFI reference), August 26th 2008, July 29th 2010, June 10th 2011July 28th 2011.
©Antony Anderson Version 1.0 February 2001 and Version 1.1 July 2001
Note 1:Automobiles fault diagnostic systems will detect many types of fault, but not necessarily all types of fault, and especially not those faults internal to the control unit. As a generalisation, the kind of faults that result in saturation of an amplifier, or a logic circuit locking on, or a microprocessor program getting into an endless loop disappear if the system is switched off. On switching on and testing, the system will behave normally until the next incident. Checking the diagnostic system fault codes and testing the cruise control module will not necessarily bring an intermittent fault to light. Therefore it is important not to draw the conclusion that if the cruise control module tests OK today it did not have an intermittent fault yesterday or may not develop one tomorrow.
Note 2 : RFI Testing : RFI testing seems to have been mainly conceived of in terms of opening the bonnet and firing a Police Radar Gun at the Cruise Control Module. Why this should be supposed to be a rigorous demonstration of the invulnerability of the electronics to RFI is hard to imagine. Equally surprisingly, the internal RFI generated within the engine compartment seems to have received little attention. This is strange, considering that the signal strength is likely to be much higher than any externally generated RF sources and also that the pulse repetition frequencies are low ( related to engine RPM) and comparable to the pulse rates that might arise in the cruise control module during normal operation.
Note 3 : Voltage Reference Capacitors : By way of
example, US patent 3,437,980 of Feb 10 1976
suggests that if the reference voltage is stored in
a 0.01 microfarad capacitor and if the voltage may not
vary over more than 1% during an hour, a time constant of
over 6,000 seconds is required. The capacitor should
therefore have a resistance of the order of 6 x1011
ohms which is normally achieved by using an insulated gate
transistor as the comparator and special wiring and
Note 4: High Gain Amplifiers: In analog cruise control systems, high gain operational amplifiers are used for several different functions such as summing, amplification, inversion, voltage sample and hold, integration etc. If the impedance of either the amplifier input or feedback circuitry should change, the transfer function will change. For example, a faulty joint in a feedback circuit may alter the impedance of the feedback circuit and therefore alter the behaviour of the whole cruise control module.
Note 6 : Simulation of effects of RFI on a stepper motor : The possible effect of RF from the ignition system can be simulated quite readily with a small stepper motor placed in the engine compartment if the pulse train input circuit is allowed to float. The RFI from the ignition circuit is quite sufficient to provide a false pulse train that will cause the motor to rotate in an irregular manner.
Note 7: CAN bus (Controller Area Network). First introduced to a motor vehicle in mass production in 1991. First started appearing to any extent in vehicles of 2003 model year. From 2008 model year on all vehicles sold in the USA had to use the CAN Bus. See http://www.auterraweb.com/aboutcan.html