Help me alter this device

[Thedrive2112]

Wondering if someone who knows more about electronics can help me out here. I purchased this device its purpose is to take electronic spike signals generated by a vehicles alternator and output a square wave to drive a tachometer. I put this device on my old oscilloscope as RPM is increased the square wave frequency increases and duty cycle is DECREASED. What I need the device to do is as RPM is increased the square wave frequency is increased and the duty cycle is also INCREASED.

I have been searching the web and it looks like other people who have added resistors to adjust duty cycle.

just wondering if there is a way to make that happen by altering the device or if I need to make something.

Thanks all

 

[jpanhalt]

First, let's clarify a few definitions.
1) "Duty cycle " is the percent or fraction of time on compared to the total period. It is not the length of time on. Thus, if the frequency increases, and the duty cycle is constant, the length of time on will shorten.
2) Duty cycle cannot be more than 100% (1.00). If you increase frequency and duty cycle, it will be possible that the signal will be constantly on.

Now to your question, if you want toe signal to turn full on at a certain rpm, that is relatively easy to do. If you really want the time on (duty cycle) to increase with rpm (frequency), by what factor do you want the duty cycle to increase, if rpm increases by, say , 1%? Edit: What frequency and duty cycle are produced at some low rpm? What is the highest rpm need to measure?

John

 

[Thedrive2112]

John, thanks a ton for taking the time to look at this with me, and for the clarifications. I dabble in electronics and took some classes in high school so im still a novice when it comes to component level. Really what im trying to accomplish is to emulate a gasoline injector signal. My understanding is as RPM in a engine is increased both frequency of the wave (RPM) and duty cycle (injector on time) is increased and during wide open throttle (WOT) the injector can remain on all the time im guessing this is 100% duty cycle. What I have noticed with the device I posted pictures of is when RPM is increased frequency is increased (RPM gauge rises) but duty cycle (injector on time) decreases. That is all starting to make sense when reading your clarifications on duty cycle, it must be staying constant when frequency increases. What I need is for frequency and duty cycle to both increase with RPM. I probably need to measure the range of 700-3000 rpm. I thought about trying a hall effect sensor and mount some magnets on the harmonic balancer to get a reading but I believe I will have the same issue as frequency will increase but duty cycle will be constant

 

[jpanhalt]

At low idle, what is the actual frequency and on time? Both measurements can be gotten from your scope. Then, we will assume 100% duty cycle at 3000 rpm. It won't be that hard to raise the full-on rpm to 3500.

Finally, I assumed you want an analog solution. But just in case, are you familiar with microcontrollers or do you know someone who is?

John

 

[Thedrive2112]

Just thought of something that might change things. I believe auto injectors are almost always controlled by switching the the ground leg where the 12v is constant. would that mean we are looking at a negative reference duty cycle? If the device im using is sending a positive duty cycle but I need a negative duty cycle that would cause the problem on injector on time (duty cycle) decreasing when RPM is increased?

I know about microcontollers but I have never used/programmed one. I would like to learn and im a quick study but might be better to keep that for another day.

I will pull the scope out and see what I can dig up. its an old analog scope that probably way out of calibration. I normally dust it off only a few times a year to identify a waveform. looking to get one of those nice digital scopes soon.

 

[jpanhalt]

I was thinking along the lines of using something that is called a one-shot. The frequency would be controlled by an edge of the tach signal, and the duration of the one shot (aka pulse) would also be controlled by a voltage that was proportional to the frequency. One could use a sophisticated F--> V converter, such as the LM2907/LM2917 from TI or a simple low-pass filter, which is just a resistor and capacitor. As the freequency increases, the voltage output increases.

Of course, that all sounds like an application for the venerable LM555/NE555. Moreover, I found a nice lab exercise from PennState doing exactly that:http://www.personal.psu.edu/axl17/242l9.pdf

Here's the circuit:


(Attachment worked this time. No idea what the glitch was. Image would not upload before.) The tach signal goes to the trigger pin and the falling edge will trigger the one shot. The width of the pulse will be determined by the voltage on the control pin (pin 5). Some of the component values can be calculated given the frequency information I requested. As for the control signal, the component values can be estimated, but it may be best to do in simulation. I can help do that later, if needed. I do not have time tonight.

This plan is not very sophisticated, but for testing, it might do. The 555 output would, of course, drive the injector driver. If you need to invert it, that can be done easily.

John

 

[Thedrive2112]

Wow thanks! I will dig into it

last question. do you think I could invert the square wave on the device I already have? I believe it has a 555 timer chip.

 

[jpanhalt]

Yes, I suspect that output can be inverted. You can use a logic inverter, which will provide buffering or a simple transistor (look up common emitter). Since logic inverters often come in a single IC with 6 inverters, that is an easy way to go and may result in fewer parts than using transistors.

I was wondering how you were going to synchronize the injector to the crankshaft when using the alternator as your signal.

John

 

[KrisBlueNZ]

Can you clarify what you want to use this signal for. You said you want to emulate an injector signal; does that mean you want to control an injector with this signal? Or are you just using it to monitor engine RPM? Or something else?

 

[Thedrive2112]

Kris thanks for jumping in on this and John for your continued effort. I do natural gas conversions for diesel vehicles(doubles your MPG, adds power and runs cleaner). Im putting together a "injected" type kit that meters the CNG much better than the traditional fogging style that waists CNG. I am re-purposing an old natural gas ECM for petrol vehicles to handle the metering and delivery. This EMC looks at injector pulses and injector duty cycle for RPM and injection timing and then matches it with CNG output (I can adjust the levels by programming). If I connect to the diesel injectors for this input information my injection times fall when RPM rises witch is exactly the reverse effect i want to have. I purchased the RPM signal generator in hopes that I could modify it as i cant modify the diesel injector signal without it effecting the diesel operation. After clarifications from John I believe my problem is as RPM in increased the frequency of the wave gets smaller but duty cycle stays the same so naturally it is decreased the CNG ECM detects this fall in duty cycle and injection times are also decreased with RPM.

RPM and injection time dont really need to match the vehicles true timing I just need it to rise when RPM is increased and fall when it is decreased. I can make adjustments in programming to allow more or less CNG at certain intervals.

Really really appreciate the help. everything is working on my test vehicle except this and im planning to take the vehicle on a long road test this weekend to California as we are meeting family for the holiday.

 

[Thedrive2112]

Kris thanks for jumping in on this and John for your continued effort. I do natural gas conversions for diesel vehicles(doubles your MPG, adds power and runs cleaner). Im putting together a "injected" type kit that meters the CNG much better than the traditional fogging style that waists CNG. I am re-purposing an old natural gas ECM for petrol vehicles to handle the metering and delivery. This EMC looks at injector pulses and injector duty cycle for RPM and injection timing and then matches it with CNG output (I can adjust the levels by programming). If I connect to the diesel injectors for this input information my injection times fall when RPM rises witch is exactly the reverse effect i want to have. I purchased the RPM signal generator in hopes that I could modify it as i cant modify the diesel injector signal without it effecting the diesel operation. After clarifications from John I believe my problem is as RPM in increased the frequency of the wave gets smaller but duty cycle stays the same so naturally it is decreased the CNG ECM detects this fall in duty cycle and injection times are also decreased with RPM.

RPM and injection time dont really need to match the vehicles true timing I just need it to rise when RPM is increased and fall when it is decreased. I can make adjustments in programming to allow more or less CNG at certain intervals.

Really really appreciate the help. everything is working on my test vehicle except this and im planning to take the vehicle on a long road test this weekend to California as we are meeting family for the holiday.

 

[Thedrive2112]

Yes, I suspect that output can be inverted. You can use a logic inverter, which will provide buffering or a simple transistor (look up common emitter). Since logic inverters often come in a single IC with 6 inverters, that is an easy way to go and may result in fewer parts than using transistors.

I was wondering how you were going to synchronize the injector to the crankshaft when using the alternator as your signal.

John

John, Because the nature of how CNG is introduced into the vehicle it doesnt need to be synchronized to original injector timing(although it would be nice!) im hoping to do some beta road tests this weekend and injection times that rise and fall with RPM would be perfect.

 

[Thedrive2112]

What about something like this tied to the factory diesel injector signal? when voltage is applied to diesel injector the device will go high and when voltage is removed from the injector it will go low as long as voltage is higher than reference it will stay high(duty cycle).

http://www.ebay.com/itm/DC12V-Volta...-control-/170976000503?_trksid=p2054897.l4275

 

[jpanhalt]

What about something like this tied to the factory diesel injector signal? when voltage is applied to diesel injector the device will go high and when voltage is removed from the injector it will go low as long as voltage is higher than reference it will stay high(duty cycle).

http://www.ebay.com/itm/DC12V-Volta...-control-/170976000503?_trksid=p2054897.l4275

That seems to describe the function of a comparator. If input is higher than reference, it is on; if lower than reference, it is off.

John