Non standard values on RPM sensor

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ptitgros
Posts: 14
Joined: Fri May 15, 2020 9:30 am
Country: France, Metropolitan

Non standard values on RPM sensor

Post by ptitgros » Fri Mar 26, 2021 4:46 pm

Hello every boby,
I built a RPM sensor with oXs, arduino and Infrared Obstacle Avoidance Sensor Module.
It almost always works well, except for a few overspeed spikes
I read eccentric measurements at start-up, or at a sudden go-around (excuse my poor english, i use google translation), sometimes more than 100,000rpm, sometimes more than 15,000rpm on an old gas machinesaw engine runnig usualy arount 5 or 6000RPM.
I have same result with RPM sensor for brusless with motor running around 7 or 8000RPM.
Does somebody have an idea ? What can we do ?
Regards
Gilles


ptitgros
Posts: 14
Joined: Fri May 15, 2020 9:30 am
Country: France, Metropolitan

Re: Non standard values on RPM sensor

Post by ptitgros » Tue Mar 30, 2021 8:42 pm

Hi,
Does somebody have an idea ?
Does somebody have any experience about this kind of sensor ?
Regards
Gilles

ChrisOhara
Posts: 12
Joined: Mon Feb 01, 2021 11:56 am
Country: Germany

Re: Non standard values on RPM sensor

Post by ChrisOhara » Mon Apr 05, 2021 7:10 pm

Hello
I have made a RPM sensor for my gas engine and found a similar issue.
I used the signal of the tacho interface (RCexcl) with an optocoppler etc.
The input pin 8 is used as a Schmitt Trigger.
There is the need of a pull down resistor to make sure that the state of the pin is going from high to low in a reasonable time.

In my case I could resolve the problem by reducing the value of this resistor, so the voltage does not vary around the range where the Schmitt Trigger works.
Hope it helps
regards ChrisOHara

ptitgros
Posts: 14
Joined: Fri May 15, 2020 9:30 am
Country: France, Metropolitan

Re: Non standard values on RPM sensor

Post by ptitgros » Tue Apr 06, 2021 4:22 pm

Hello ChrisOHara,
thanks for this very interesting answer.
witch value or resistor did you use (aproximatively) ?
placed on the line of pin 8 i suppose ?
sorry for these questions, i do not speak currently eletronics :-)
regards
Gilles

ptitgros
Posts: 14
Joined: Fri May 15, 2020 9:30 am
Country: France, Metropolitan

Re: Non standard values on RPM sensor

Post by ptitgros » Tue Apr 06, 2021 4:25 pm

ChrisOHara,
witch kind of sensor did you use on Rcexcl signal ?


ChrisOhara
Posts: 12
Joined: Mon Feb 01, 2021 11:56 am
Country: Germany

Re: Non standard values on RPM sensor

Post by ChrisOhara » Wed Apr 07, 2021 6:59 am

Hello
I did two version:
The first own was to connect a Frsky brushless sensor.
To avoid any noise from the ignition I separated by opto coppler. See first plan attached.

In the second version I replaced the Frsky sensor by an openXsensor where I had the problem with unrealistic rpm values, till I found out that I had to adapt the pulldown resistor. See secondplan, it does only show he RC side because Ignition side is the same as in the first plan.

I made this in december last year. So I have to admit that I haven't flown my plan with it yet.
But I made some test on the bench with an Ignition tester and the original CDI and especially the OpenXsensor does really match the values of the tacho. (value range 400 to 15000 rpm - what is what the tester did supply)
Keep in mind that I am not a professional. So if you copy do some good testing cause I do not guarantee.
Schaltplan-en.pdf
(213.87 KiB) Downloaded 12 times
plan-openXsensor-en.pdf
(243.41 KiB) Downloaded 16 times

ptitgros
Posts: 14
Joined: Fri May 15, 2020 9:30 am
Country: France, Metropolitan

Re: Non standard values on RPM sensor

Post by ptitgros » Wed Apr 07, 2021 9:49 pm

Great, ChrisOhara,
i'll probably try these solutions.
thanks wery much,
i'll keep you posted
regards
Gilles


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