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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
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
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
Oscilloscope
• Analog
• Digital
Analog Oscilloscope
Digital Oscilloscope
Oscilloscopes for Automotive Diagnosis – compact sizes
Oscilloscopes for Automotive Diagnosis
– computer interface
Oscilloscopes for Automotive Diagnosis
– professional service solutions
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
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
12
Digital Oscilloscope Control Panel (stopped)
Measuring Option for Digital Oscilloscope (cursors)
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
Measuring Option for Digital Oscilloscope Signal analysing: peak voltage, duty cycle, frequency
Measuring Option for Digital Oscilloscope (magnifying) ZOOM
Measuring Option for Digital Oscilloscope
– signal analysing between cursors
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.
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.
Trigger Settings
Trigger level
Choosing the Right Trigger
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.
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.
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).
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.
• 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.
• 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.
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.
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.
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.
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.
• 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.
* 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.
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.
•
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.