Using Oscilloscopes on Vehicles

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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.  

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Pulse width

The width of the impulse – this is the time lapse, during which the signal is in active state. The
active state is the level of voltage that triggers the executive mechanism. Depending on how the
actuator is connected the active state can have different voltage levels: 0V, +5V, +12V.
Practically the level can vary around these values. For example: the active state for the injector
control signal in most engine control systems has a voltage of 0V, but can practically vary in
range from 0V to +2.5V and more.  

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Duty cycle

The duty cycle is the fraction of time that a system is in an "active" state. For example, in an
ideal pulse train (one having rectangular pulses), the duty cycle is the pulse duration divided by
the pulse period. For a pulse train in which the pulse duration is 1 μs and the pulse period is 4 μs,
the duty cycle is 0.25. The duty cycle of a square wave is 0.5, or 50%. This period is one of the
PWM signal parameters (Pulse Width Modulation).

The PWM signal is used to control some executive mechanisms. For example in some engine
control systems the PWM signal operates the electromagnetic idle speed valve. Furthermore
PWM signal is also generated by some sensors that transform the measured physical parameter
into direct correlation with the period of ignition.

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What is self-induction?

This concept is not directly connected to the work principle of the oscilloscope, but it is
important to understand why when activating the inductive executive sensors on a 12V car
voltage we get voltages ranging from 60V to 200V and in the primary ignition chain up to 400V-
500V.

Self-induction - This occurs when the current in an inductive circuit changes and the magnetic
field cuts the wires; this induced electromotive force opposes the change in current, restricting it
if the current is increasing and enhancing it if the current is decreasing.  

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Self-induction back-voltage - This is back-voltage produced by self-induction. This induced
electromotive force opposes the change in current, restricting it if the current is increasing and
enhancing it if the current is decreasing.
If the rate of change of the magnetic field in a solenoid (relay solenoid, solenoid injector,
ignition coil, inductive sensor for rotation detection) the self-induction back-voltage can reach up
to thousands of Volts. The magnitude of the back-voltage mainly depends on the inductiveness
of the coil and the rate at which the value of the magnetic field is changing. In electromagnetic
executive mechanisms the value of the magnetic field changes the fastest when the field fades
away after a quick shutdown of the powering voltage. In some cases the self-induction effect is
undesirable and precautions are being made in order to reduce it or to remove it. But some
electric circuits are designed to produce the maximum self-induction back-voltage, for example
the ignition system of gasoline engine. Some ignition systems can produce a back-voltage of
self-induction up to 40kV– 50kV. Such voltages can be easily measured with an automobile
oscilloscope by using capacitive pick-up.