Using Oscilloscopes on Vehicles

WisdomAugust

Timebase Controls


The oscilloscope plots a graph of the voltage left-to-right, starting from the left side of the
screen. These select the horizontal speed of the spot as it creates the trace; this process is
commonly referred to as the sweep. In all oscilloscopes the sweep speed is selectable and
calibrated in units of time per major graticule division. Quite a wide range of sweep speeds is
generally provided, from seconds to as fast as picoseconds (in the fastest 'scopes) per division.
As mentioned above the sweep is measured in seconds. In automobile measurements
milliseconds are more often used (ms) – 1ms=1/1000 s. The value of the sweep can be changed
with the time switch. The same signal is visualized differently depending on the sweep setting
selected. A shorter period of time is selected when a detail observation of the shape and time
parameters of separate sections of the signal is needed. In such cases a very short time fragment
of the signal is shown on the screen. If we need to observe a larger time fragment of the signal
(for example when showing individual impulses with irregular shape of the signal or skipping
impulses) a bigger sweep is used.  

WisdomAugust

Vertical Sensitivity


The graph on the screen of the oscilloscope shows the relation between the values of the voltage
and time. If the amplitude of the input voltage is greater, a higher range of the vertical amplifier
must be set. Depending on the amplitude of the signal, for a better visual result an appropriate
vertical amplifier is used. The ability to change the scale of amplification of the signal enables
the oscilloscope to show signals with very high amplitudes as well as signals with very low
amplitudes. The appropriate value of the amplification depends on the amplitude parameters of
the signal observed. The same signal would be shown differently depending on the degree of
amplification. A larger range is used when the amplitude of the whole signal needs to be shown.
A smaller range is used when, is need to make a detailed observation of the shape and the
amplitude parameters of separate sections of the signal. In such cases, when the signal is of
higher voltage, only a part of the signal is visible on the screen.

WisdomAugust

The position of the zero line can be displaced vertically according to the geometrical center of
the screen. The need for moving the zero line vertically depends on the type and shape of the
signal, as in the cases when using multi-channeled oscilloscope for a better visualization of
signals from more than one channel.


When we connect a direct current (DC) supply to the oscilloscope, the waveform we would
observe would be a straight line. The line itself would be vertically displaced from the zero line.
The difference between the observed waveform and the zero line is proportional to the value of
the electric current.  


The majority of waveforms have a shape different from a straight line.  

WisdomAugust

Basic terminology when working with oscilloscopes



“Zero” line
If no electrical current source is plugged into to the oscilloscope, the waveform is represented by
a straight line. This line is called “zero” line because it represents a level corresponding to a 0V
voltage at the input of the oscilloscope.  







Waveform – there is no signal at the input of the oscilloscope
(The yellow line is the zero line)  

WisdomAugust

There are divisions on the screen; they allow the signals parameters to be visually measured.
With the divisions on the horizontal axis the time parameters of the signal are measured. The
vertical divisions are used to measure the strength of the voltage.
The “time” scale can vary from parts of a second to several seconds. The “voltage” scale can
vary from several mV to several kV.

The graphics which are visualized on the screen are called waveforms. An oscilloscope can only
observe waveforms produced by an electrical voltage. The screen of the oscilloscope shows the
stretched through time image of the electrical fluctuation allowing for the shape and the
amplitude of the voltage to be rendered into account, as well as making phase and frequency
measurements.

For most of the measurements only 2 probes, such as Multimeter are required. The ground cable
of the probe should be connected to the negative side of the car battery or the chassis, and the
other cable should be connected to the cable whose signal we want to check.  

WisdomAugust

How does one work with an oscilloscope?

All oscilloscopes have a display on which the waveform is shown. The screen can be a cathoderay tube,
a LCD panel; a PC monitor can also be used. Here a typical oscilloscope screen is shown.  

WisdomAugust

Where can scope waveforms used for comparison be found?
Most programs that contain automobile technical information have sections whose headlines
have “waveform”, “pattern” and “trace” in them. In other words these sections contain sets of
standard waveforms. Such programs are Autodata, Vivid Workshop, and many others that
contain technical information about automobiles. When looking at a “waveform” on the screen
you must not forget that this is just an ordinary coordinate system like the one everyone has
learned about at school. Like any coordinate system it has a horizontal (x) axis and a vertical (y)
axis. The vertical axis (height) represents voltage, and the horizontal (width) represents time.
The scale of both axes can be changed.  

WisdomAugust

How many input channels are needed when conducting measurements in auto diagnostics?

When observing signals from sensors, valves, primary ignition chain, secondary ignition chain
etc. no more than one channel is needed. The first kinds of motortesters, which were analog,
needed more than one channel in order to show uniform signals simultaneously on the screen, so
they could be compared with each other. But whit PS based DSO’s this became unnecessary
because a standard waveform can be saved on the PC’s hard drive and opened at any time for
comparison with the currently observed.

Two channels are needed when the sequence in time between 2 signals has to be measured and
how many milliseconds are between them. In other words the second channel is used when the
phase difference between the 2 signals has to be observed and measured. An example for such a
measurement is when simultaneously observing signals from the Crankshaft Position Sensor and
the Camshaft Position Sensor.

Using more than 2 channels is more convenient in some cases but pointless from a functional
point of view.  

WisdomAugust

Features that allow PC-based oscilloscopes to be used in auto diagnostics.

Lower cost than a stand-alone oscilloscope, assuming the user already owns a PC.
Easy exporting of data to standard PC software such as spreadsheets and word processors
The software of the device can be directly installed on a PC and upgraded via CD or directly
download from the Internet without having to send the device back to its manufacturer.
Use of the PC's disc storage functions, which cost a lot extra when added to a self-contained
oscilloscope.

PCs typically have large high-resolution color displays which can be easier to read than the
smaller displays found on conventional scopes. Color can be utilized to differentiate waveforms.
PC-based USB oscilloscope get their power supply from a USB port, so no external source is
required.

The USB oscilloscope as any other USB device can be turned on/off without having to turn
off/on the computer.  

Rose

What is the difference between a motortester and a lab scope?

The motortester is one kind of specialized oscilloscope used for auto diagnostics.
The main difference between a motortester and a universal oscilloscope is that the motortester is
capable of visualizing short-timed processes like the ignition spark process. This process is
exceptionally fast, and the period of repeat of the ignition of sparks in time is many times greater
than the time the spark itself exists. This is easily observable when testing the engine in idle
speed, when the majority of the measurements are conducted.

For example: if we observe the ignition cycle of a 4 cylinder gasoline engine, and an ignition
spark that lasts around 2ms, at 800 RPM, the time period between sparks on a single cylinder
would be 150ms. What this means is that the ‘length’ of the spark would account for around 2%
of the actual work cycle, and therefore the burning of the sparks will be seen as very thin lines on
the screen of the oscilloscope, and no information about the phases of the ignition would be seen.
Because of this many diagnostics are forced to increase the RPM of the engine thus shortening
the ignition cycle thus ‘saturating’ the waveform of the cycle.

The motortester shows all the cylinders simultaneously, and allows for detailed observation of
the time period that includes: dwell period, drilling voltage, burn time and turbulence of voltage.
Most motortesters can show the cylinders graphs next to each other, or under one another,
excluding the long time periods between sparks, this method is also known as “parade”.

Another distinctive feature of the motortester is that it can show its time divisions on its
horizontal (x) axis in milliseconds as well as in degrees – up to 720 degrees.