Оscilloscope –автомобилларни …ad.highvec.net/files/2_lectures/lec-1.pdf• the...

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.