Last week in
Part 2
we covered the basics of variable duty
cycle and variable frequency control of solenoid valves. Solenoids controlled in
this way include the injectors, boost control solenoid, and solenoids in the
auto trans line and power steering. As we saw in that article, most of these
valves are pulsed at a fixed frequency and the duty cycle is varied, but in the
case of the injectors, the duty cycle and frequency both change – the latter
getting faster as the engine speeds up.
We also touched on Silicon Chip magazine’s Digital Pulse Adjuster and
Independent Electronic Boost Control electronic projects that let the user take control of the action of all duty cycle controlled valves, allowing the
easy modification of car systems never before able to be easily altered.
This week we’ll explore duty cycle control in more detail.
Variable Duty Cycle Control
One of the kits that Silicon Chip developed is called the Nitrous Fuel
Controller. (And because retailer Jaycar Electronics
was worried this sounded illegal, it’s also called the Motor Speed Controller!)
It’s relevant to this series because the kit is simply a variable duty
cycle generator designed to directly control heavy loads.
The way the project works is summarised in this diagram. A variable duty
cycle generator is controlled by two user-adjustable inputs. A potentiometer
("pot") can be twiddled with a screwdriver, changing the duty cycle output over
the range from 0-100 per cent. The frequency of the output is also able to be
adjusted, but more clumsily by altering the value of a small capacitor on the
board. The variable duty cycle generator drives a switching transistor called a
mosfet, which is able to operate loads taking up to 10 amps.
As built, the output has a frequency of 60 Hertz. This frequency is slow
enough that when the output is being used to drive a fuel injector, the injector
turns fully on and off without the pintle hovering at mid-positions. (If you get
confused by the use all these terms, refer back to last week’s article at Part 2
). Connect up the injector and you can manually control its
duty cycle to be anywhere from 0-100 per cent, allowing tuning of the nitrous
fuel enrichment. And of course, if you use a stainless steel injector (like one
designed for use with methanol) you can also run water through it to make a
water injection system.
However, it’s important to remember that this module outputs a fixed
frequency, so the amount of water or fuel doesn’t go up with revs. In a nitrous
application that’s fine, because the amount of nitrous doesn’t vary with revs
either.
So with this module we have a simple controller for operating an injector.
But here’s where it becomes interesting. Instead of an injector, wire a
light-bulb across the output. The light-bulb is now being turned on and off 60
times per second. If you set the duty cycle to 5 per cent, each time the light
turns on it will be with only a tiny burst of current. So the light is dim. Set
the duty cycle to 85 percent and the burst of currents are much longer – so the
light is brighter.
At this frequency the light looks like it’s glowing continuously – you can’t
see the 60Hz flashing. If the bursts of power are at 12V and the duty cycle is
set to 50 per cent, the bulb glows as if it’s being powered by 6 volts.
At 75 per cent duty cycle, the light bulb ‘sees’ 9 volts. In this way, the
average current flowing through the light bulb is able to be varied.
Exactly the same control approach can be taken with an electric motor – say
the fuel pump, or the pump in a water/air intercooling system or. The speed of
the motor can be varied by operating it with different duty cycles, preferably
at a higher frequency. In fact, variable duty cycle control is the best
way of varying the speed of a DC electric motor.
OK, so far we’ve been able to use this variable duty cycle controller to
operate an injector or other solenoid valve, and by working at higher
frequencies, act as a dimmer for a light bulb or as a motor speed control. But
what if we slow the frequency right down – say to 2Hz? In that case, the output
is being switched on 2 times per second – and so it can be used as a lamp
flasher or horn pulser. The duty cycle can still be changed to be high or low,
giving great versatility to the finished effect.
So the one module can work as:
- Nitrous injector controller
- "Hovering pintle" solenoid flow controller
- Light dimmer
- Motor speed controller
- Light flasher
- Horn pulser
Think through all the various effects the controller has, all by just
changing the frequency and duty cycle of its output.
Measuring the Signal
If you have even a half-decent multimeter, you can directly measure frequency
and duty cycle. (The exception is in trying to measure the duty cycle of
injectors which use a peak/hold design approach.) It takes away a lot of the
mystery if you can see these figures coming up on the screen!
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Information Rich Signals
The Nitrous Fuel / Motor Speed Controller described above doesn’t change in
either frequency or duty cycle unless the user alters a capacitor value or turns
a pot, respectively. As mentioned, that makes it unsuited to controlling an
extra injector, one that for example might be required because the standard
injectors are running flat-out. In fact, while the module is very versatile in
what it can do, in other respects it’s pretty dumb – once set, its output is
constant.
In contrast, factory car systems that use variable duty cycle control are
very seldom fixed at one value. Instead, the duty cycle with which the valve is
being controlled varies a lot – that’s why manufacturers choose that type of
control. Take a boost control solenoid valve, for example. The valve may be able
to be operated at all duty cycles from 5 to 85 percent - so you sure as hell
wouldn’t expect the manufacturer to use only 40 per cent and 60 per cent duty
cycles. Instead, there’s going to be plenty of different duty cycles used at
different times to suit different situations.
Or even easier to understand, the injectors in a car aren’t going to run just
20, 30, 40, 50, 60 and 70 per cent duty cycles, are they? Instead, they’ll
increment up and down in 1 per cent (or even finer) steps, allowing as precise
matching of the fuel supply to the engine’s requirements as possible.
In short, factory control systems have duty cycles that change in small
steps.
To put this another way, if we tap into this factory duty cycle signals, we
have a very rich source of information on how that system is being run. So
instead of thinking of duty cycle signals as just being used to power a
solenoid, we can think of them as potential input signals into a modification
module – an interceptor. Let’s take a closer look at this idea.
Intercepting Duty Cycle Signals – the Digital Pulse Adjuster
The maximum duty cycle in any system is 100 per cent (always on) and the
minimum is 0 per cent (always off). That means if we’re monitoring the duty
cycle signal going to a factory solenoid, we already know the full range that
we’ll have to cope with. (Compare that with say the voltage output of an airflow
meter – it might have an output of 1-5V, 1-5.5V, 2-12V, etc.)
But just monitoring the signal isn’t enough - if we want to be able to change
these signals to suit our modification outcome, we need a way of altering these
duty cycles. And even more important, altering each of the factory duty
cycle points. After all, there’s no point in just making all the duty cycles
‘60’, or another single figure like that!
The Silicon Chip Digital Pulse Adjuster (DPA) is designed to modify
factory variable duty cycle control signals. It intercepts the signal that the
factory computer is sending to the solenoid and then takes over the driving of
the solenoid. All the possible duty cycles that the factory computer is using to
drive the solenoid are spread across a range of 128 ‘load sites’ that are
continuously monitored by the DPA. This means that if the factory computer is
sending out a duty cycle of 50 per cent, the DPA assigns the load site of 64 to
it. If the duty cycle being sent by the factory ECU is 75 per cent, the DPA sees
this as load site 96. In other words, the DPA can measure the input duty cycle
down to a resolution of 0.78 per cent.
The DPA allows the adjustment up or down in the output duty cycle at each of
these load points. So in order that a modification to a system works well, each
of the 100 factory duty cycle values can be altered. For example, every time the
factory ECU spits out a 54 per cent duty cycle, the DPA can make it so that the
signal that actually gets to the solenoid is 64 per cent. The frequency with
which the valve is being pulsed remains unchanged – the DPA always follows the
input frequency.
So let’s take a look at a real example. This graph shows the adjustments made
to the duty cycles going to the power steering assistance control valve in a ’98
Lexus LS400. After it was connected up, the DPA recognised load sites 33 – 113,
which means the output duty cycle in the Lexus control system varies between 26
and 88 per cent. At low duty cycles, the duty cycles being received by the
solenoid were increased (up to a maximum of 19 units of adjustment). At medium
and higher duty cycles, the duty cycles being received by the valve were
decreased (to a maximum of -75 adjustment units). Apart from at one load site
(#45, which corresponds to 35 percent valve duty cycle), this graph shows that
every single duty cycle coming from the factory ECU has been altered up or down
in value.
These changes lightened the steering at low road speeds (in this steering
system, duty cycle proved to increase with road speed) while massively improving
steering feel and weight at high speeds. It was also very easy to tune the
changes by driving the car on the road and feeling what was going on.
Producing Duty Cycle Signals – the IEBC
As described above, the DPA follows the frequency of the original signal
source. This allows it to drive the original solenoid valve at the same
frequency as the factory ECU – only duty cycles can be changed.
On the other hand, the IEBC runs a fixed frequency – the default is 10Hz.
This allows it to monitor a duty cycle signal but drive a new valve at a fixed
frequency. The other difference over the DPA is that the IEBC user sets whatever
output duty cycle he or she wants for each input duty cycle. That is, at one
input level they specify (say) 42 per cent output duty cycle, rather than a
plus/minus adjustment of the original input duty cycle.
So why would you want to pulse a valve at a new frequency, rather than
following any changes in input frequency? The answer is that you might be
controlling a valve where a variation in frequency would be unsuitable. For
example, in its most common use, the IEBC is using as its input signal the
injector duty cycle and pulses a boost control valve at 10Hz. (10Hz is a
frequency that suits nearly all 12V solenoid valves.) At 6000 rpm, the injectors
are typically operating at 50Hz – a frequency the boost control valve is
unlikely to work at. So by using the IEBC, we lose the frequency component of
the original signal and make use of just its duty cycle information.
Here is an example of one of the two output duty cycle maps for the IEBC (the
IEBC has two switchable maps, each with 64 load points). As can be seen, all 64
load points are available, showing that in this modified car, the injector duty
cycle is working across the full range from 0-100 per cent. The output duty
cycle being sent to the boost control valve is shown on the graph – it can be
seen that it varies from 0 (where the wastegate is shut) through to about 44 per
cent. The decreasing duty cycle at higher load points prevents the boost level
dropping away, which on this car it has a tendency to do.
Conclusion
The kits mentioned in this story allow you to generate variable duty cycle
signals, and intercept and re-map variable duty cycle signals, with or without
retaining the original pulsing frequency. Those functions are enormously
powerful – you can modify the operation of any variable duty cycle signal
in the car.
Independent Electronic Boost Control
Digital Pulse
Adjuster
High Performance Electronics for Cars
Next week: airflow meter and MAP sensor signals and their
modification
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