|
This article was first published in 2004.
|
It wasn't all that long ago that people modifying
- or otherwise working on - cars were complaining about the need for a digital
multimeter. Previously they'd used just a 12V test light - the measurement meant
that power was either there, or it wasn't. But with the very real likelihood of
the test light dragging down the voltage of the measurements being made and the
requirement that voltages be accurately quantified, the need for high input
impedance digital multimeters came about. These devices could accurately measure
the signal without at the same time changing them - a pretty important
requirement, you'd have to agree!
But now the time has arrived where to know what's
going on requires an oscilloscope - normally abbreviated to a 'scope'. These
instruments show not only the voltage level, but also the real-time variation in
that signal. The simplest example is the output of the oxygen sensor - in closed
loop, it's a voltage that rapidly rises and falls. Your trusty old test light
would have killed the oxy sensor output entirely, and your digital multimeter is
going to get very confused when the signal keeps varying - the meter needs a
steady voltage so that it can get a reading.
Enter the scope - it pictorially shows the
changing voltage over time, drawing a trace that accurately depicts the
pattern of voltage variation. In fact a scope is the only
way that you're going to be able to look at signals coming out of camshaft
and crankshaft position sensors, speed sensors and ABS sensors, amongst others.
And it's also the only way that you're going to be able to see the signals
going to injectors, idle air control valves, boost control solenoids,
auto trans pressure control solenoids, and so on.
Traditionally, scopes have been used by mechanics
to look at primary (low voltage) and secondary (high voltage) ignition signals.
And that's a valuable use for a scope. But these days a scope is far more likely
to be used to look at inputs and outputs of the ECU. In fact, most good factory
workshop manuals now show sample scope traces, so that you can use a scope to
quickly find if the output signal from the sensor or ECU looks as it should.
|
How Ugly?
Think
about this: if in a modified car you're changing those signals by using an
interceptor, just how good a job is the interceptor doing? Does
the waveform going into the interceptor from the crankshaft position
sensor look like the intercepted waveform coming out of the modifier
(except of course for a change in timing)? Or is the output waveform horribly
distorted - perhaps the cause of that Check Engine light problem that always
rears its ugly head when you have the interceptor working?
|
Or what if you want to make a modification in an
area that few other people have played with - for example, change the auto trans
pressure for firmer shifts? These days it will be controlled by a pulsed
solenoid - but actually seeing how the solenoid is pulsed in different
conditions will require a scope.
It's important to realise that scopes have a major
role to play in car modification, not just in diagnosing obscure ignition system
problems....
In this three part series we'll start with the
basics of scopes. Getting your head around a signal that's constantly varying
can be quite hard to do, especially when most of us are used to reading
relatively unchanging values on a multimeter. After that we'll look at what
scopes are suitable for automotive work, then we'll get stuck into using a scope
on modified and standard cars.
And hey, hang onto your seats. This is a
revolution every much as big as the change from the test light to the
multimeter...
The Oscilloscope
|
Acknowledgements:
Much of the following section is closely based on XYZs of Oscilloscopes
by Tektronix. Tektronix
[http://www.tek.com]
is a well-respected
manufacturer of high quality digital and analog scopes. The automotive scope
waveforms are from Pico Technology. Pico Technology
[http://www.picotech.com/auto/]
manufactures automotive scope adaptors that work with laptop PCs.
|
An oscilloscope is basically a graph-displaying
device - it draws a graph of an electrical signal. In all automotive
applications, the graph shows how signals change over time: the vertical (Y)
axis represents voltage, and the horizontal (X) axis represents time.
But don't be fooled - this simple graph can tell
you many things about a signal, such as:
- The time and voltage values of a signal (how many
volts and when it changes)
- The frequency of an oscillating signal (how fast
the voltage is rising and falling)
- The frequency with which a particular portion of
the signal is occurring relative to other portions (is there a part of the
signal that varies more rapidly up and down than other parts?)
- Whether or not a malfunctioning component is
distorting the signal (do the sine waves look more like square waves?)
- How much of the signal is noise and whether the
noise is changing with time ('noise' is normally seen as a superimposed signal -
jagged edges on a sine wave, for example)
The generic term for a pattern that repeats over
time is a 'wave' - sound waves, brain waves, ocean waves, and voltage waves are
all repetitive patterns. An oscilloscope measures voltage waves. One cycle of a
wave is the portion of the wave that repeats. A waveform is a graphic
representation of a wave. A voltage waveform shows time on the horizontal axis
and voltage on the vertical axis.
Waveform shapes reveal a great deal about a
signal. Any time you see a change in the height of the waveform, you know the
voltage has changed. Any time there is a flat horizontal line, you know that
there is no change for that length of time. Straight, diagonal lines mean a
linear change - rise or fall of voltage at a steady rate. Sharp angles on a
waveform indicate sudden change.
You can classify most waves into these types:
- Sine waves
- Square and rectangular waves
- Triangle and saw-tooth waves
- Complex waves
In automotive applications, sine and square waves
dominate.
The sine wave is the fundamental wave shape. It
has harmonious mathematical properties - it is the same sine shape you may have
studied in high school trigonometry class. Mains AC voltage varies as a sine
wave. ('AC' signifies alternating current, although the voltage alternates too.
'DC' stands for direct current, which means a steady current and voltage, such
as a car battery produces.) Many speed sensors produce sine wave outputs - this
waveform is from an ABS inductive speed sensor.
-
Square and Rectangular Waves
The square wave is another common wave shape.
Basically, a square wave is a voltage that turns on and off (ie goes high and
low) at regular intervals. An injector waveform is fundamentally a square wave -
the injector is either on or off. A rectangular wave is like the square wave,
except that the high and low time intervals are not of equal length. That is,
the 'on' and 'off' times are not equal. Again, this is often the case with an
injector, where at low loads the 'off' time will be much longer than the 'on'
time. The waveform shown here is from a Hall Effect road speed
sensor.
Waveform Measurements
Many terms are used to describe the types of
measurements made with an oscilloscope.
If a signal repeats, it has a frequency. Frequency
is measured in Hertz (Hz) and equals the number of times the signal repeats
itself in one second. Hertz can also be referred to as 'cycles per second'. A
repetitive signal also has a period - this is the amount of time it takes the
signal to complete one cycle. Period and frequency are reciprocals of each
other, so that 1/period equals the frequency and 1/frequency equals the period.
For example, the sine wave here has a frequency of
3 Hz and a period of 1/3 second. Some scopes can calculate frequency and display
it as a standalone number, while in other cases the period needs to be read off
the scope screen and the frequency then calculated from this.
Voltage is the amount of electric potential - or
signal strength - between two points in a circuit. Usually, one of these points
is ground, or zero volts. DC signals are measured on a scope as you would with a
multimeter - from ground to the amplitude (height) of the signal.
Automotive AC signals are often measured from the
maximum peak to the minimum peak of a waveform, which is referred to as the
peak-to-peak voltage. The peak-to-peak voltage of this inductive crank sensor is
just under 16 volts.
Types of Scopes
Oscilloscopes can be classified as analog and
digital types.
An analog oscilloscope works by applying the
measured signal voltage directly to the vertical axis of an electron beam that
moves from left to right across the oscilloscope screen - usually a cathode-ray
tube (CRT). The back side of the screen is treated with luminous phosphor that
glows wherever the electron beam hits it. The signal voltage deflects the beam
up and down proportionally as it moves horizontally across the display, tracing
the waveform on the screen.
Analog oscilloscopes are characterised by the
large screens used in traditional 'tune-up' machines and the smaller scopes with
the glowing green screens used in electronics. They are excellent tools, however
in automotive use they suffer from major drawbacks - the need for mains power,
the greater difficulty in set-up and the absence of a storage mode that allows
the freezing of the on-screen image.
A digital oscilloscope uses an analog-to-digital
converter (ADC) to convert the measured voltage into digital information. It
acquires the waveform as a series of samples, and stores these samples until it
accumulates enough samples to describe a waveform. It then re-assembles the
waveform for display on the screen.
The digital approach means that the oscilloscope
can display any frequency within its range with stability, brightness, and
clarity. It can also easily freeze the waveform, allowing it to be studied at
leisure. Digital scopes can usually be powered by batteries and use an LCD
screen. All scope adaptors that are used with laptop PCs are digital.
Scope Systems and Controls
An oscilloscope has three main controls, labelled
Vertical, Horizontal, and Trigger. You need to adjust these three basic settings
to accommodate an incoming signal:
a) The
attenuation (reduction) or amplification (increasing) of the signal - use the
volts/div (volts per on-screen division) control to adjust the height of the
signal to the desired measurement range.
b) The time base - use the sec/div (seconds per
on-screen division) control to set the amount of time per division represented
horizontally across the screen.
c)The triggering of the oscilloscope - use the
trigger level to stabilize a repeating signal, or to trigger on a single event.
These adjustments sound more complex than they
actually are: what you want to see is a steady waveform that fits on the screen.
The first point (a) simply fits the waveform on the screen vertically, (b) sets
the bottom axis so that the waveform repeats sufficiently that you can recognise
it, and (c) makes sure that the waveform is clearly depicted.
And as we said, some digital scopes have an 'auto'
button that do all of these things for you!
Scope Measurement
The oscilloscope is primarily a voltage-measuring
device. The most basic method of taking voltage measurements is to count the
number of divisions a waveform spans up the oscilloscope's vertical scale.
Adjusting the volts/div control signal to allow the signal to cover most of the
screen vertically makes for the best voltage measurements. The more screen area
you use, the more accurately you can read from the screen. Then it's as simple
as reading off how many divisions per volt the scope is set to, and estimating
on-screen how many divisions the waveform covers.
With AC signals (eg a sinewave from a speed
sensor), you would normally look at the peak to peak voltage. With a DC voltage,
the whole line will be elevated from the zero point. Some scopes will do these
calculations for you.
-
Time and Frequency Measurements
You can make time measurements using the
horizontal scale of the oscilloscope. Time measurements include measuring the
period and pulse width of pulses. Remember that frequency is the reciprocal of
the period, so once you know the period, the frequency is one divided by the
period. Like voltage measurements, time measurements are more accurate when you
adjust the portion of the signal to be measured to cover a large area of the
screen. Again, some scopes will do these calculations for you.
-
Pulse Width and Rise Time Measurements
In many applications, the details of a pulse's
shape are important. Pulses can become distorted and cause a circuit to
malfunction, and the timing of pulses in a pulse train is often significant.
Standard pulse measurements are pulse width and pulse rise time. Rise time is
the amount of time a pulse takes to go from a low to high voltage. By
convention, the rise time is measured from 10% to 90% of the full voltage of the
pulse. Pulse width is the amount of time the pulse stays high. Some scopes will
calculate and display pulse width (measured in seconds) and also duty cycle (the
proportion of time that a pulsetrain is high.)
Conclusion
Using a scope gives you a window into a new world.
No longer do you just see a static (or more often, flickering around!) voltages
coming out of a sensor or the ECU. Now you can see the shape of that
signal - which is a whole lot more illuminating...
|