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LCD signals corrupted by electromagnetic field of engine

I made a complete lap timing , telemetry system for my 2 stroke 125cc shifted kart.It consists of 2 boxes, one with the microcontroller and the additional hardware to read the analog sensors and one box with the LCD (hitachi 4 line standard) and other signalisation. Both boxes are shielded with aluminium to mass. and they are connected with a 25 pins RS232 printer cable (also shielded to mass). Now The system works perfect when the engine is not running , but when the engine is running the LCD is doing really strange , the software on the microcontroller is however still running ok as i can see at my RPM indication with leds wich is doing fine. Soo the signals from the microcontroller via the RS232 cable to the LCD box are corrupted by the electromagnetic field of the engine spark plug though i shielded everything. This field is even soo strong that when i put my digital oscciloscope within 2 meters from the running engine it shuts down (safety mode).

Do any of you guys have experience with these kind of strong electromagnetic fields and how to shield against it or how to solve this problem. I am aware that it should be a lot better when the LCD wiring was very short to the microcontroller but this was not possible with respect to place...

thanks for any input on this one

Yannick

Reply to
Yannick
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You gotta cram the microcontroller in the box with the display. That will almost surely stop the noise problem. The lines to the display are effectively 'bus' signals; very susceptable to your ignition noise.

Luhan

Reply to
Luhan

I'll take a swing at this one. There are several sources of interference, including:

1) electromagnetic field due to spark current, which flows in a relatively large loop, through plug wires, plug gap, through engine metal, to magneto/coil ground point. 2) electrostatic field due to high dv/dt on plug wire

3) conducted interference in V+ wires.

You can reduce the magnitude of these sources of interference as follows:

1) * use coaxial plug wires to allow return current to flow in the shield and greatly reduce loop area 2) * use resistor spark plugs or add series resistors to plugs to reduce dv/dt on plug wires * coaxial plug wires can also help 3) * series inductance and shunt capacitance between ignition and your load
  • use a separate battery for your electronics

You can reduce the susceptibility of your electronics to the above interference sources as follows:

1) Reduce the size of all loops in your wiring by means of twisted pairs, adjacent return conductors, shielded wires 2) If you're sharing the vehicle battery for power, ground your electronics only at the battery and not anywhere else 3) see #3 above

The 'sealed box' approach will work, of course, but it shouldn't be necessary. Paul Mathews

Reply to
Paul Mathews

(snip rest)

Magneto ignition? Every consider the magnetic field from these, and the poor attenuation provided by aluminium?

Reply to
budgie

try adding 1 kOhm resistors in series with every INPUT, and change to resistor type spark piug wires

Mark

Reply to
Mark

If it is caused by the magnetic field from the magneto, IMHO that won't make a heap of difference.

Reply to
budgie

Yes indeed, this would only help due to reducing bandwidth andso reduce capacitive crosstalk but the di/dt gives a voltage and this resistance will not change that. I am now busy rebuilding everything in one box with very short tracks on the pcb between microcontroller and LCD display , + making every input from a sensor differential soo that the difference between the two signal lines does nog change with interference of Electromagnetic waves. Then i will shield everything with ferro material , and if this wont work then i just stop and have lots of admiring for the guys who are making these lap time and telemetry systems;)

thanks for the help soo far guys!

Yannick

Reply to
Yannick

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