(Also) Measuring Fuel Injector Duty Cycle

[MikahB]

I'm starting the design of a circuit with the goal of generating a 0-2.75V, 20mA output signal proportional to the 0-100% duty cycle of a fuel injector. Injector pulse frequency range for this application is approximately 5Hz-20kHz and injector pulse voltage is 12-14VDC.

I have +5V available to power the circuit, but I would like to use as little driving current as possible. Response time is not critical in my application but if possible, I'd like to try to design the circuit to respond and settle within 2-3 input cycles so that I do not regret things down the road when I need better response.

I've done a lot of searching here and around the web and found a circuit in this document: https://www.edn.com/contents/images/102501di.pdf (on Page 4-5) that I like. I like the simplicity, low component count, and the sample and hold stages are attractive to me as well.

Does this seem like a good starting point for this project? This is basically a slightly fancier R/C filter, so what are my other good options for measuring duty cycle over a wide frequency range? I keep hoping to find an IC with a "PULSE IN" pin that will do this, but no luck so far.

 

[3v0]

I would use a micro controller but given that you are looking for ICs I expect that is not an option.

 

[MikahB]

I would use a micro controller but given that you are looking for ICs I expect that is not an option.

I'm not completely adverse to using a micro controller if that is the best way to tackle it. It feels like something I should be able to accomplish without one, but perhaps not.

 

[alec_t]

Are you sure about the injector pulse voltage? I ask, because other posts concerned with injector drive circuits reveal drive voltage is often raised to e.g. 28V or higher to ensure rapid injector opening.
If you need a response time of only 2-3 injector periods then that would seem to rule out an analogue approach and I think you'll have to go for a digital solution.

 

[MikahB]

The number I posted is based on testing I did on a couple different vehicles, but the nature of this device is to be used on many different ones, so that is a very good point - thanks for bringing it up.

I have been anticipating needing some circuit protection for the incoming pulse anyway and will probably use the pulse to drive the gate of a FET then feed a ~4.5V pulse as the input to the rest of the circuit.

Thanks for your insight.

 

[jpanhalt]

You might want also to consider a frequency to voltage converter such as the LM2907 (https://www.electro-tech-online.com/custompdfs/2012/04/lm2907-n.pdf ). Its datasheet describes a dwell-time to voltage converter. Note the low-pass filter and direct coupling to the signal.

Some other F/V converters, such as the AD650 probably won't work, because they have a one-shot after the comparator that obliterates pulse-width information.

Automotive products will often have higher voltage ratings.

John

 

[MikahB]

Thanks, John. I can't say I completely understand that schematic - it seems like the output voltage would depend on both frequency AND dwell time, but I am likely misreading it.

One option I had considered, though, was using a F/V converter like that with an ADC to actively "pick" the best-fit RC integrator from several combinations. It seemed contrary to my goals of simplicity and minimal parts count, but it might work okay.

 

[alec_t]

Something else to consider (if you haven't already) is that the injector drive pulse duty cycle is probably not the same as the injector duty cycle. Because of mechanical inertia, friction and the finite time it takes for current to ramp up and decay in the injector's solenoid, the injector open time can be less than the pulse width.

 

[MikahB]

Yep - that is definitely true. I'm using this signal as a reliable indicator of engine load under a wide variety of conditions, and the resolution (so far) is not really a big concern, so I should be okay there.

I have LTSpice but have not yet learned to use it. Maybe I can do some simulations of various RC combinations to get an idea if there is some acceptable combination that spans this wide frequency range.

 

[alec_t]

Talking of frequency range, do you know of any injectors which will operate up to 20kHz? I was under the impression they all need at least 0.5 mS to open or close, plus the dwell time, so wouldn't the max frequency be < 1kHz?

 

[MikahB]

When I first read this, Alec, I thought you had caught an epic brain fart as 500-20,000 is my RPM range. But, the 20kHz requirement is real, but the lower frequency should be 5Hz representing 500rpm - I have corrected it in the original post.

Some environments where this unit will be installed already have 0-12V PWM (pump driving) signals at up to 20kHz that tell me exactly the duty cycle that I need to know. Other cases do not, so I turn to injector pulse width and I want this one device to be able to handle both cases (and any in between).

I started working on the of a F to V chip coupled with a 4-bit ADC and even though I think it could work (progressively adding in parallel resistors and capacitors) the component count goes way up. I'm now looking - per 3v0's initial suggestion and jpanhalt's thoughts in another thread - at a PIC. That is probably going to be the way to go for low power requirement, low component count, and flexibility across a wide range of input frequencies.