PWM injector driver

[Bernd Felsche]

Bruce Bowling tapped away at the keyboard with:

> > From: Ken Bauman <303print at northrim.net>

> > A link I came across for making your own Peak and Hold drivers.
> >
> > http://www.ednmag.com/ednmag/reg/1995/090195/graph/18di5fg1.htm

> This circuit controls average current using a PWM signal, with the
> duty cycle yielding an average current, as explained in the text.
> But, it does not measure the actual current in the power
> transistor circuit - it has a fixed

That's true. For a given injector type, that may well be a cheap
(easy) way out. But as you note, it's not a general solution,
especially in view of the variability of battery voltage; especially
during starting.

> You can do this function directly from a microprocessor port which
> has PWM capability - make the initial PWM duty cycle 100%, and
> after so many milliseconds (like 2 - 3), switch the PWM duty to
> something like 25% or so to invoke the current limit.

> The actual number for the duty cycle can be computed from the
> injector impedance, PWM frequency, and transistor saturation
> voltage drop (or Rds for a MOSFET), or better yet measure the
> current during current-limit mode and adjust the duty cycle to
> achieve the desired holding current - if the micro has a spare ADC
> channel then you are golden. Simply hook up a sense resistor in
> series (like 0.1 ohms)  with the drive portion Emitter circuit),
> measure the current across the resistor, and set up a feedback
> loop.

When you're driving an inductive load that also stores mechanical
energy, you need to be careful about when and how you measure the
current. The peak current is easy enough to measure as it's due to
non-switching voltage. The hold-current is another matter;
expecially when you're switching at about 15kHz. It's perhaps only
feasible to sample and hold the sense voltage immediately prior to
each driver switch-off as the current would tend to rise rapidly to
that peak as determined by the injector's inductance.

Employing a microcontroller does have other benefits: It would be
capable of learning injector response (peak and hold current) and to
identify a malfunctioning injector by measuring the injector's
electrical output when the driver is switched off. In particular,
the mechanical response of the injector can be determined by
observing the current pulses as the injector closes.

With a "slow" ADC, one may only be able to take one sample of such
pulses on each event. There are typically a few nominally-identical
events every second so off-setting the sampling could be used to
determine the response pulse (i.e. a "sliding window").

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia
----------------------------------------------------------------------------
To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org