Using Oscilloscopes on Vehicles

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What types of electrical signals are there and what are their parameters?



DC and AC
Direct current (DC) is the unidirectional flow of electric charge. It can be positive or it can
be negative. Direct current is produced by such sources as batteries and electric machines of the
dynamo type. Also may be obtained from an alternating current supply by use of a current switching
arrangement called a rectifier. The picture below shows the current of a car battery.  





The wave produced by a car battery  

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Another example for DC of a much more complex nature is the rectified current of a generator.
This current is positive, but it’s also pulsating. These pulsations are additionally amplified by the
fact that in the shown example below there is a damaged diode in the rectifier bridge.  







Example of complex DC voltage  






Alternating current (AC) - In AC the movement of electric charge periodically reverses
direction. In direct current (DC), the flow of electric charge is only in one direction. The signal
varies around 0V. Its momentary value can be both positive and negative. Such voltages are
represented by almost all signals from inductive sensors: the CKP sensor, the CMP sensor, the
signal from the ABS sensor etc.

The voltage in the electrical circuit is also AC and has sinusoidal shape. All examples viewed in
the “Periodical and non-periodical signals” section are also examples for variable signals.  

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Periodical and non-periodical signals
A signal is a periodical one if the values of its voltage pulsations and the shape of these
pulsations are identical and are repeated through equal intervals of time.

The time needed for one periodical signal to complete one full cycle is called period. The
number of periods per second is called the frequency of the signal. If the waveform of the
voltage of the periodical signal crosses the ‘zero’ line the signal is called a varying signal. If the
waveform does not cross the ‘zero’ line the signal is a constant one. Example waveforms of
different periodical signals are shown below.

The first signal shown is a sine wave. This signal is characterized by 2 parameters – amplitude
and frequency. In the automobile electronics similar to the sine wave signal are the signals
generated by inductive speed and position sensors. Similar signals are generated by some
crankshaft position sensors (CKP), camshaft position sensors (CMP), vehicle speed sensors
(VSS) and others.





Sine wave periodical signal

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Square periodical signal
The injector control signal is a square one similar to the one shown above. Its impulses repeat
periodically.

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Triangular periodical signalSignals that don’t repeat through equal time intervals are called non-periodical.  

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Non-periodical variable signal


An example for non-periodical signals is the digital data transfer between the cars differednt
controllers. There is another type of non-periodical signals-single signals. This is a kind of signal
that is represented by a single impulse, which may never repeat or may repeat after a long interval
of time.

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There is another type of complex signals, which can be both periodical and non-periodical.
These signals are such that include more than 1 frequency. An example of such a signal is shown
in the following picture:




For such a signal to be synchronized on the screen, the oscilloscope needs a specialized function
called “Trigger hold off”. (* Look up in the Synchronization section)  

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Slow-Variable SignalOne of the biggest advantages of the digital oscilloscope is the ability to show waves of
processes with a large period, meaning signals that change slowly through time. An example for
such a signal is the waveform on the following picture.



Slow-Variable Signal

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Analog and Digital signals

All the signals examined until now are analog signals, they are uninterrupted signals. The values
of their voltage changes through time by some rule or randomly. As an example for a complex
analog signal the lambda sensor (O2 sensor) signal can be mentioned.

A waveform of digital signals switches between two voltage levels representing the two states of
a Boolean value (0 and 1), even though it is an analog voltage waveform, since it is interpreted in
terms of only two levels – high and low, on and off. Such voltage levels are called Logic voltage
levels. In most cases, the logical levels have constant voltage values: +5V and 0V for example.

Digital signals are generated by switches. These switches are represented by transistors
switching between “open/closed” states. Sometimes digital signals are generated by mechanical
switches, electromechanical relays. An example of digital signals in the automobile electronic is
the Hall sensor, the throttle end position sensors; the closed throttle position sensor (CTPS), the
wide open throttle (WOT) sensor and the data transfer signals between the different ECUs. As
the analog signals digital signals can be periodical and non-periodical.  

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Frequency

Frequency is the number of occurrences of a repeating event per unit time. The SI unit of
frequency is the hertz (Hz), defined as one cycle per second. In the automobile electronics the
number of rotations of the engine is measured per minute – (RPM). Using the waveform of the
voltage of a periodical signal we can easily calculate the frequency of the signal. In order to do
this we need to measure the period of the (the duration of a complete cycle of the signal). The
value obtained can be recalculated into frequency using the respective formula.

Let us examine the following example. A sensor generates 1 voltage impulse per crankshaft
rotation. The time lapse between 2 impulses is called a period. In the case given 2 consecutive
impulses are separated by 7.4 divisions of the screen of the oscilloscope. The scale of the screen
used for visualizing this signal is 1 division equal to 100 ms or 1/100 of the second, thus the
period of the signal is 0.74 seconds. Knowing the length of the period of the signal we can
calculate how many cycles per second are there, hence the frequency of the signal in Hz. When
converting period to frequency we need to divide the time period chosen (in our case 1 second)
by the length of the period of the signal (in our case 0.74 seconds):
1/0.074 = 13.5 Hz

If in this case we calculate the number of repeated periods per minute we will get the frequency
of rotation of the crankshaft in RPM. When converting the period in frequency in RPM we need
to divide the time period chosen (60 seconds) by the length of the period of the signal (0.74
seconds)
60/0.74 = 81 RPM

Such calculations can be made using all kinds of waveforms with different scales of
division, but some oscilloscopes can directly show the results in RPM.