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Speed Sensor filter or signal conditioner needed

Catwell
I want to clean up the pulses from a gear tooth speed sensor in the presence of incredible amounts of noise.


Currently the device senses 1 pulse per revolution. Maximum RPM is 3000. One main issue I have, is at lower speeds, say 1-100 rpm, the returned results are not accurate. At those speeds the pulses are too long and the sensor is reporting more of them during the contact incident. The PC then translates that as a higher RPM.
 

Filtered, accurate, RPM reports across the range of motion is important. What are some options I could try?
 

One option I was planning to try, is to condition the signal by sending it through a PIC 10F. So every pulse(s) received by the PIC will be changed to a single pulse, if certain conditions are met. And noise would also be filtered. But I think a microcontroller is overkill. I would like a IC solution, like a Schmitt trigger or op-amp.
 

Thoughts?

 
Cabe
  • in reply to jvdberg@ieee.org

    JV,

     

    I just remembered a simple way of controlling/protecting the signal. I used differential line drivers for the encoders on a CNC mill I build. I could use one with this sensor. My main issue was the cost, $30 per driver is a bit much. This solution uses a similar filter effect that you describe for audio and data.

     

    Perhaps mimicking the circuit used in the line drivers is a good option. I will look into this.

     

    Cabe

  • Hi Cabe,

    Magnetic pick-up sensors like this Cherry part use completion of a magnetic circuit to cause a change in magnetic flux which induces an ac voltage in a coil. They consist of a pole piece with a coil of wire wrapped around it and a magnet at the back end. The magnetic circuit is completed when there is a low impedance path for magnetic flux between the pole piece in the middle of the sensor and the casing. In your case with just one metal screw, the head of the screw should be flat and it should be larger than half the diameter of the sensor so it spans the distance from the pole piece to the casing. And it should be made of mild carbon steel to conduct magnetic flux well. The closer the screw passes to the sensor the larger the raw signal will be - it is very important that it passes very close to the sensor, especially at low speeds, because the raw pulse amplitude is proportional to the rate of change of magnetic flux, and this rate of change is lower at slower speeds. This sensor has built-in signal conditioning which converts the raw ac signal to dc with some hysteresis, so the main thing you need to do is focus on the geometry and mechanics to get the best possible raw signal.

    If there are extra transition pulses at low speeds all you need to do to eliminate them is use a one-shot circuit with a period just less than your minimum expected period. At 3000 rpm you would get a pulse every 20 ms, so if your one-shot has a period of 18 ms, it will eliminate any noise pulses that are less than 18 ms without affecting pulses at your maximum speed.

    Doug

  • in reply to dougw

    Douglas,

     

    3 years later, you replied to my post!

     

    Thanks for the thoughts and ideas. I never did fix this issue. I will take what you say into consideration.

     

    Cabe