Capacitive RPM Sensor for Data Aqc.

Other Hardware General Electronics
1

Hello, I am college student getting into electronics and the data aqc personnel for the SAE Baja team. I am currently trying to develop the teams first actual working data collecting system so that we can confirm numbers and give good reasoning for the design choices we made. Right now I have accelerometers at each wheel to gather data for the shocks but I'm having problems with measuring RPM.

The way I am going about it is taking the pulses from the spark plug by wrapping wire around the spark plug wire. This is the only way that I know that we can measure RPM because the rules says that we cant modify the engine in anyway, plus it would be nice if we can move it from car to car with little hassle. The problem I am getting is I tried the circuit from
Reading engine RPM - #7 by TomGeorge - Project Guidance - Arduino Forum which is originally from rpm-interfacing microcontroller circuit: engine's spark coil | Forum for Electronics, and it did nothing as far as I could tell.

My questions are:

  1. Are both ends of the wire that wraps around sparkplug wire (L1 in the schematic) suppose to connect to the circuit?
    I ask this because I tried measuring the voltage with my multimeter and with both leads I got nothing but when I only had one it was reading the ac voltage.
  2. What is the recommended ac voltage should L1 be producing? I know the voltage can be increased by wrapping more wire around it. I have it wrapped around 10 times and it gives 3 to 5 volt I believe when I had the one lead connected to the wire.

Other helpful details:
The engine is a single piston, two stroke engine, Model 19 Briggs and Stratton.
UNO is being used to test but eventually it is going to be transferred to a mega.
I have not made code for this but I was thinking to use the pulseIn() function to measure time between pulses.
I have had a class in Circuits 1, which only covered dc basics so I don't know much about ac and other advanced electrical theory.

I appreciate any help that anyone can offer.

Sensore Candela Accensione.jpg

2

Why not try sensing the current at the spark coil feed, a lot lower voltage and a much stronger signal. Good luck to you and the AE Baja team.

3

Hi,

The engine is a single piston, two stroke engine, Model 19 Briggs and Stratton.

Probably means magneto type ignition.
Instead of a high impedance capacitive input that will pick up all sorts of noise.
Try a toroidal current loop around the spark lead.

This may help, using UNO to make a timing light using a current clamp.

Tom... :slight_smile:

4

Are you really sure about the engine? I see: "Manufacturing for model 19 started in Jan 1957.". That was the year I graduated from high school. DO you mix gasoline and oil for the fuel?

If the engine is new, it will be using the flywheel to generate DC and store in a capacitor, then use a CD ignition system to fire the spark plug.

Paul

5

The pulsIn() function is easy to use.

Some programmers are critical of pulsIn() as it blocks the code while waiting for the pulse to end. They prefer the interrupt approach. It all depends on what work your program has to do and can it be done between the pulsIn() calls.

Can you attach a Hall Effect sensor to some rotating part? That has worked for me on a car.

John.