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Joint European Project
МАЪРУЗА №1
Оscilloscope –автомобилларни диагностикалашнинг универсал қурилмаси
The Oscilloscope – a Powerful Tool for Automotive Diagnosis
Made by JPI and based recommendation prof.Gabriel Anghelache
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Topics
• Oscilloscope presentation
Analog/Digital oscilloscope
Oscilloscopes for Automotive Diagnosis
Digital Oscilloscope Control Panel
Signal Synchronization (Trigger)
• Application: Gasoline Fuel Injectors Diagnosis
Construction and Functioning of Fuel Injectors
Voltage and Current Signals
Diagnosis Issues
• Conclusions
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Oscilloscope
• The oscilloscope is an instrument that
permits the observation of signals evolution in time and measurement of some
characteristics of the signal (period, frequency, amplitude).
• Electrical components have characteristic
signals, which can offer important
information about their condition.
3
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Oscilloscope
• Analog
• Digital
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Analog Oscilloscope
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Digital Oscilloscope
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Oscilloscopes for Automotive Diagnosis – compact sizes
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Oscilloscopes for Automotive Diagnosis
– computer interface
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Oscilloscopes for Automotive Diagnosis
– professional service solutions
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Oscilloscope interface Bosch Type FSA 720
Temperature
Primary ignition Voltage
Stroboscope
probes probes
probe
Battery
clamps
Ignition
trigger Current Channel 1 clamp
• To enable the oscilloscope to connect to the
required points, test probes are
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required.
• Voltage probes:
▫ Are the most widespread
▫ Passive elements included
▫ 1:1 (straight through connectivity) or a
defined degree of attenuation (e.g. 1:10)
Current scale
selector The current probe
•
▫ A current probe generates a voltage
proportional to a current in the circuit
▫ It has two jaws which open to allow
clamping around an electrical conductor
without having to make physical contact
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12
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Digital Oscilloscope Control Panel (stopped)
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Measuring Option for Digital Oscilloscope (cursors)
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Effects of Magnitude and Time Scale Variation on the Same Signal • The amplitude of the signal is less than 5 V and the period is
20 ms, that means a frequency of 50 Hz. •
•
•
•
• We can measure one period or more than one for a better
accuracy 15
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Measuring Option for Digital Oscilloscope Signal analysing: peak voltage, duty cycle, frequency
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Measuring Option for Digital Oscilloscope (magnifying) ZOOM
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Measuring Option for Digital Oscilloscope
– signal analysing between cursors
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Signal Synchronization (Trigger)
• The oscilloscope trigger function is
one of the key elements of any
oscilloscope, enabling a stable image
to be viewed on the screen
• This is achieved by starting the time-base sweep at a selected point on the signal. As a result, triggering allows the display of periodic signals such as sine waves and square waves, as well as non-periodic signals such as single pulses, or pulses that do not recur at a fixed rate.
The level setting only is notFor a good synchronisation • enough to see a single signalboth. trigger level and slope This can conduct at a "mirror"mustbe set.
viewing of an armonic signal. well synchronized unsynchronised level synchronized
Without a trigger, or other form of synchronisation, it would not be possible to display a steady signal on the screen.
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Trigger Level and Trigger Slope
• The trigger level detects when a certain voltage
level has been reached and at this point sets the
time-base in operation to sweep across the screen.
• The trigger slope determines whether the time-base
sweep is triggered on a positive or negative going
edge or slope.
• The setting of both the oscilloscope trigger levels
and slope determine the that image is seen on the
screen.
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Trigger Settings
Trigger level
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Choosing the Right Trigger
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Gasoline Fuel Injectors Diagnosis Gasoline Fuel Injector - Construction and Operation
• The fuel injectors are solenoid operated valves,
controlled by the ECU
• The opening of the injector is determined by
the magnetic field developed by the current
flow in solenoid, which attracts and lifts the
injector’s needle.
• The gasoline flows through the fuel injector (cooling the coil) and is sprayed into the intake port.
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The Gasoline Fuel Injector Connections
• The fuel injector is
permanently connected at
(+)
• The injector command is achieved by
closing the circuit to the ground using
a transistor.
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Voltage Signal from a Gasoline Fuel Injector
• Due to the permanent connection of the
injector to the ignition plus, a connection to
the (+) terminal of the injector will provide a
constant voltage of 14 V, with no information
about injector working.
• Therefore in this case the oscilloscope will be
connected between the minus connection of
the injector and ground.
• Thus, a voltage fall from 14V to 0V results in
the moment of circuit closing and it returns to
the initial value of 14 V when the circuit is
open.
• The period of time in which the voltage
remains null (5 ms in picture) represents the
power supply time of the fuel injector
(command time).
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Voltage Signal from a Gasoline Fuel Injector
• For a better view of the fuel injector
voltage signal (the start injection point
fixed, instead of the final point), a
manual setting of the trigger is necessary.
• The trigger has to be set at a level
between 0 V and 14 V, descending (6.47
V in this example).
• The intersection between signal and
trigger level will determine the “zero”
time point.
• The voltage spike which appears at the end of injection is due to the presence of the
injector coil.
• When the transistor is turned off, the
magnetic field of the coil collapses
causing a back “voltage spike” that can
damage the ECU.
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• To prevent this fact, a Zener diode limits
the voltage at 70 V. Voltage Signal from a Gasoline Fuel Injector
[1] • At the time scale corresponding to a single injection view, the spike is very sharp and it doesn’t appear to be “cut”, but the maximum voltage is constant.
• For a better observing of the voltage limiting,
the oscilloscope time scale has to be changed to
a smaller one.
• In order to keep the spike in the same position
on the screen, the trigger has to be changed to
the positive slope.
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• It can be observed that an increasing
of time scale alters the signal • At the first step, not all the spikes will
have the same level.
• At a further increasing of the time scale
it can be observed that the spikes almost disappear and the square part of signal gets a spike appearance.
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Voltage Signal on Different Time Scales • For the time bases less than 1 s, the image is
displayed by screen refreshing. • For synchronising reason, the next image is not
displayed until the trigger condition is true. • That means that the last image can remain on
the screen for seconds if the trigger condition is
not satisfied. • This is the case of engine overrunning when
ECU cut the injection. • So, it is possible for the injector "signal" to
remain on the screen even when this signal
doesn't exist anymore. • This is not the case for the time scales larger
than 1 s, when the signal displaying is cursive
and every modification can be easily view. • For example the injection cutting is visualised
like a continuous line, without injection pulses.
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Evolution of Current in a Coil
• From electrical point of view, the injector is a
coil.
• A coil reacts when current changes, in the
sense that it is going to resist to the change of
its magnetic field and generates an electro-
motive force which opposes to the field
variation.
• This delay depends on the time constant of the
coil T = L / R, in other words it depends on the
character of coil: less inductive or less
resistive.
• As the current through the coil grows, it accumulates magnetic energy. When opening the electric circuit all the energy is released, generating self-induced voltage that has been seen previously.
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Evolution of Current in Fuel Injector Coil • Due to the coil characteristic, a period of
time is needed until the current through
the coil reaches the minimum value
necessary for injector opening. • Thus results a delay between the
moment of circuit closing (command of
the injector) and effectively opening of
the injector. • At the moment of injector opening the
inductance of the coil is modified, due
to the change of the needle position. • The consequence is that the time
constant (T=L/R) of the coil changes as
well as its characteristic.
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Importance of the Inflection Point for Diagnosis
• The opening time (t0) represents the time elapsed from the injector command until its opening.
• It depends only on the injector constructive
characteristics (coil inductance, spring elasticity, needle mass, etc.) and it is invariable with the engine functioning parameters (speed, load etc.)
• Consequently, identical injectors must
have identical opening time.
• If the opening time changes, for the same command time (t ECU), the effective injection time (tinj) will be altered.
• An increasing of the opening time
denotes a partial grip of the injector needle, causing the cylinder to run lean.
• A shorter time of opening can reflect an altering of spring elasticity, and determine a mixture enrichment for cylinder.
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• The absence of the inflexion point indicates
that the injector does not open (is blocked), despite of voltage supply.
Informativity* of the Injector Signals
• When the injector needle is blocked it doesn't move and the coil inductance is unchanged. The increasing of electric current through the coil has no inflection point.
• If the current pulse is rectangular instead of
curved, the injector is shorted.
• No current means the injector coil is probably open-circuit.
• The inductive spike represents the general health of the injector coil windings.
• A spike voltage less than Zener diode limit
indicates a partial short-circuit of the coil windings, which determines a reduction of the coil inductance and energy, that conducts to a lower self-induced voltage.
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* Informativity= the capacity of a diagnostic parameter to offer informations about the real state of a device. Changing the Injection Time
• The ECU adjusts the pulse width to
control the engine power.
• This can be easily tested by observing
the widening of the injector pulse
when blipping the throttle or
increasing the idle speed.
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Visualisation of the Injector Opening Time on Voltage Signals
• The existence of a small electrical
resistance on the command circuit
of the injector determines a small
voltage drop on the command
circuit as the current increase.s
• This voltage is very small compared
to injector working voltage, but with
an appropriate scale of oscilloscope it
can be visualised.
• Thus, what apparently was a “zero
voltage line” became s a voltage
variation proportional to the
current.
•
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Conclusions
• The oscilloscope is a powerful diagnostic tool.
• For obtaining optimal results, a very good understanding of construction and functioning of
automotive systems is required.
• The oscilloscope is an universal diagnostic tool and it can be used for any type of vehicle,
regardless of manufacturer.
• The conclusions of diagnosis are dependent on experience and knowledge of the staff.