Horn noise affects RPM data

Question

I am working on a Data Acquisition project using Arduino Due for petrol scooters where I am trying to collect 6 types of data - acceleration, location, brake, throttle, horn and RPM of the wheel. I'm using the BNO055 9 axes IMU shield to collect acceleration data, GPS shield to get location, and a 44E hall effect sensor + magnet to collect RPM data. I am pulling the hall effect sensor using INPUT_PULLUP command in the IDE and not using an external resistor (I hope that's okay). The throttle data is directly given to an analog pin on the Due (I'm using an electronic throttle which has a hall effect sensor inside it). For the brake and horn (it's a mechanical horn), I am using a potential divider that gives an output of 3.3V, to give it to the digital pins (I am recording the instances at which the brake and horn is pressed). The Due is being powered by the 12V lead acid battery on the scooter. The throttle and the hall effect sensor that I'm using for RPM is powered by the 3.3V onboard regulator on the Due. All the data is logged on the SD card at a frequency of 5Hz. I am facing a certain issue with the horn and RPM data, and here it is -

Every time I press the horn, the RPM data goes boink. Even pressing the indicators corrupts the RPM data which is weird, as I am not even collecting indicator data. The more I press the horn, the more crazy the RPM data goes; sometimes it touches 1000-2000 RPM which is absolutely wrong. But when I dont press the horn, I get the correct RPM data. Why is this happening? Why does the horn affect only RPM data? Is it because, the hall effect sensor works on magnetic field and the horn is an inductor which generates a huge amount of magnetic field? Or, because the Arduino and the horn are powered from the same source and hence have a common ground? Is there a way to isolate or filter the noise from the horn?

Answer 1

Is it because, the hall effect sensor works on magnetic field and the horn is an inductor which generates a huge amount of magnetic field?

Possible. However you say the problem also occurs with the indicators, which should not produce large magnetic fields.

First suspect would be the wiring to your RPM hall effect sensor. Make sure it is shielded, use a stronger pullup, and of course a small filter capacitor at the receiving end to kill HF noise spikes.

Since your hall sensor is powered, a small decoupling capacitor right next to it could help.

It would be a good idea to post scope waveforms from the output of the sensor when the problem occurs.

I presume you don't have any ground loops in your wiring, which might force currents through your pcb?

Answer 2

It could be a microphonic effect from ceramic components (capacitors), picking up the vibration produced by the horn's sound pressure level and converting it into voltage variations.

Try to mechanically/acoustically isolate your circuit from the horn and see whether the effect is mitigated or not.

EDIT:

How to mitigate microphonics?

1. First, try to isolate the horn as the culprit of your interferences before anything else. Detach the horn from the bike and mute it (padding it with foam, clothes, whatever). This way you will be able to horn in order to crosscheck whether its sound pressure level and vibrations are the cause of the interference. However, keep it at a similar distance of your PCB just to be able to see if it's a EM interference.

2. Acoustically/mechanically isolate/decouple the PCB from the horn. Don't attach the PCB to a stiff surface that can couple to it the vibrations caused by the horn. Use mechanical mounts (both for the horn and your circuit that damp vibrations). Also, use acoustic foam around PCB to reduce SPL levels, put it inside a box, use potting, etc.

3. Replace ceramics with equivalent electrolytic and film capacitor when possible and convenient.