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This article was first published in 2004.
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When using a scope on a car there are two major
dramas to overcome: how to make the scope see what you want it to see, and then
how to interpret the resulting pattern. As we covered last week in
Using Oscilloscopes on Cars, Part 2, if the scope hasn't got good enough specs,
it's possible to see things on the screen that don't match very well with
reality - but a lot of what you see depends on how you adjust the scope. In most
cases, an 'auto set-up' button is just the starting point.
But anyway, how do
you work out what that squiggly line actually means?
The Range of Tasks
At its simplest, you may be using a scope to
confirm that a certain pin on the ECU is in fact the speed signal. You jack up
the drive wheels, connect the scope's earth, backprobe the pin, and then see
what pattern appears on the screen.
If the waveform is a square wave or sine wave, and
if its frequency increases with road speed, you've just found the speed sensor.
On the other hand, if it is a waveform (of any shape) that increases in
frequency with engine speed rather than road speed, it's not the
right pin! Furthermore, if the waveform is an odd shape - it has big spikes on
it, for example - that's a clue that it probably isn't a speed sensor, which
should have a consistent waveform.
At the more complex end of things, you may be
monitoring the action of an interceptor, for example one that alters ignition
timing.
Assuming that the scope has two inputs (which in
other than basic scope designs, will be the case) you might be looking at the
input signal (eg from the crank position sensor) on one trace, and the output
signal of the interceptor on the other trace. In this case you'd be able to
compare the waveform shapes and the phasing (ie the relationship of the peaks
and troughs of each waveform to each other).
In between these tasks there are plenty of other
uses - but the first step is to understand the trace.
What's the Trace Show?
This is a screen dump from a Fluke Scopemeter 123.
It was made with the scope's negative lead earthed, and the input probe
connected to the switched side of an injector of a mid-Eighties BMW 735i.
Let's go through the display step by step.
The Scopemeter - like most digital scopes -
calculates data from the waveform. Here it is showing a frequency of 19.6Hz,
that is, this injector is firing just under 20 times a second, or 1176 times a
minute.
The Fluke can also display duty cycle calculated
from the waveform, and here it's at 6.6 per cent - appropriate for a car at
idle.
The injector waveform is also displayed. Remember,
the waveform is just a graph of injector voltage over time, with time on the
horizontal axis and voltage on the vertical axis. So when the line is
horizontal, there's no change in measured voltage. When it rises quickly, the
injector voltage must be rising quickly.
Like any graph, you need to look at the scales.
Here it is listed at 20 volts/division (and we know that's per vertical division
because voltage is measured on the vertical scale) and 20 milliseconds per
horizontal division. As we said, voltage on the vertical scale and time on the
horizontal scale.
By reading off the scale we can tell the time
between the firings of this injector. Taking the time from the end of the
injector firing to end of next time it fires (as arrowed), we can see it's just
over 2.5 divisions - say 2.6 divisions. Each division is 20 milliseconds long,
so the injector is firing about every (2.6 x 20) = 52 milliseconds, or every
0.052 seconds. We know that frequency is 1/period, so how does that stack up
against the 19.6Hz that the Fluke calculated? 1/0.052 = 19.2Hz - pretty close
for a manual reading of the waveform!
But what about the voltage? The injector gets
switched open every time it is pulled down to ground by the ECU - that is, one side
of the injector is fed 12V all the time and is activated by the ECU connecting
the other side to earth. So we'd expect to see a running car voltage of about
13.8 volts dropping to zero as the injector is switched on. These three arrows
show just this process happening - the voltage graph dropping from battery
voltage to zero.
But what are these huge spikes that occur when the
injector gets turned off? Reading off the vertical scale they're about 3.6
divisions, or 72 volts! Where's this huge voltage come from? It occurs when the
magnetic field in the injector coil collapses as the injector is switched off -
just like the voltage spike generated by the ignition coils when they're turned
off.
So from this frozen screen (and all digital scopes
allow you to freeze the on-screen display for easier reading) we've been able
to:
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Read off the injector duty cycle and frequency
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Calculate the approximate frequency from the
waveform (and we could have calculated the duty cycle too)
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See the shape of the waveform (with the exception
of the spike, it's a square wave - the injectors are either on or off)
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See the collapsing magnetic field voltage spike
that many of us wouldn't have even know occurs
Changing the Timebase
However, some of the above was a bit hard to see
because there were three injector openings all on the one screen. It would be
nice to have only one injector operation shown so it could be examined in more
detail. You don't want to change the vertical scaling (or you might get the
voltage spike going off-screen) but the horizontal scaling can be altered. This
is called changing the time-base.
Here the time base has been expanded from 20
milliseconds per horizontal division to 500 microseconds per division. (To make
this change it's just a case of pressing a time-base button until the image
looks right - no calculations are needed.) The vertical scale has been left at
20 volts/division. In this view you can clearly see how quickly the injectors
switch on and off and how the voltage spike takes a while to die away. (Note
that without multiple injector operations being displayed on the screen, the
Scopemeter can no longer calculate frequency and duty cycle - so the numbers up
the top are now blank.)
However, not all injectors are operated in this
way. This scope image, taken from the excellent Autonerdz.com site, shows an
injector that uses a 'peak and hold' style of injector operation. That is, the
current is reduced after the injector has been pulled open to just sufficient to
keep the injector open. This Nissan uses a pulse-width modulated technique to do
it - that is, during this 'hold' period it pulses the injector current really
quickly to reduce the average amount of current flowing. Note the two collapsing
field spikes - the first when it switches from full current to 'hold' current
and the second when the injector switches right off.
Comparing Traces
All but the simplest of scopes allow you to
display two signals at once. This is idea for comparing the input and output
signals of an interceptor, for example. That's what's been done here - the input
and output signals from an Xede interceptor are displayed. The signal is from
the crank angle sensor on an Impreza WRX - the signal that is intercepted to
alter the ignition timing. In this scope image the timing has not been altered.
Most important is to look at the fact that the input and output waveforms look
identical in shape.
Here three traces are shown, again originating
from an Impreza WRX crank sensor. The top trace shows the original, the middle
trace the intercepted output of the Xede interceptor, and the bottom trace the
output of a competitor interceptor. As you can clearly see, the competitor
interceptor grossly distorts the waveform, however because in this case the ECU
is looking only for the point at which the voltage cross the centreline, it can
still recognise the timing of the signal. However, obviously there will be
specific cars where the distorted lower waveform simply isn't adequate. Without
a scope you'd not have any idea why a problem might occur whenever the
interceptor is in place...
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The
Velleman HPS10
The
Velleman HPS10 is one of the cheapest digital scopes around - in Australia it
costs just AUD$349 from Jaycar Electronics (cat no QC-1916). In addition to
displaying waveforms, the scope can also be used as a simple-to-set-up graphical
data-logger.
As
you would expect from its price, the Velleman HPS10 is not a high speed, wide
bandwidth design. It has a max sampling rate of 10 megasamples/second
(repetitive signals) and only 2 megasamples/second for one-shot events. The
bandwidth is 2MHz.
Using
the Techtronix 'rules of thumb' covered in Part 2 in this series, the highest
frequency waveform that can be accurately displayed by this scope is 200 kHz, or
a period of 5 micro-seconds. That's certainly not good enough for servicing much
radio equipment, but in most applications it's fine for cars. Buffer memory
length is 256 samples. The screen is a non-backlit design 64 x 128 pixels. The
scope uses five AA-alkaline cells or can be powered by a mains adaptor. Battery
life is quoted as "up to" 20 hours.
The
scope has a good range of functions and is ideal for someone wanting a basic
design. However, it has only one input (ie can display only one signal at a
time) and the lack of backlighting of the LCD can be annoying.
But
after only a little time to gain familiarity we were able to:
Display
the waveform of an idle air control valve...
...view
the injector waveform (note the lack of voltage spike shown on the far right
pulse width - the result of the slower sampling speed of this scope
design)...
...and
data-log the airflow meter output voltage. This screen shows 47 seconds of the
airflow meter's output, of the 1 minute 31 seconds that was logged. Over that
period the maximum voltage was 3.277V and the minimum, -0.32V. (This data is
shown on the right of the display.)
We
also lent the Velleman to John Nash, head technician at ChipTorque. John
normally uses a sophisticated Tie Pie Handyscope 3 in his car work, but he
thought the Velleman a very good instrument for its cost.
If
you'd like to to occasionally view automotive waveforms and be able to do some
data-logging, the Velleman is ideal.
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Thanks to ChipTorque for their help in
assembling this article. The Velleman HPS10 was loaned to us by Jaycar
Electronics.
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