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unable to read intermediate frequency (fmax) values from FFT applied on vibration data recovered from LSM6DSL based vehicle RPM Techno-meter

VK Verma
Senior

‎2018-06-28 08:28 AM

Posted on June 28, 2018 at 17:28

Hi to All !

We are working on LSM6DSL based vehicle RPM Techno-meter with STM32L431KCU6 micro controller running at the speed of 80 Mhz. Our target is to read RPM by applying FFT F(max) on the single axis vibration data collected from 

LSM6DSL 

accelerometer from the range of F = 20 Hz to 200 Hz.

The picture of the double layer PCB board is attached herewith for your reference.

The Setting for FFT are 

  • Settings: ODR = 6000 Hz, FFT Samples = 256, Magnitude = 200mg, Full scale = +-2g
  • FFT F(max) values received are 25Hz, 50Hz, 76Hz, 101Hz, 127Hz and not like 25,26,27,28 in continuous but getting with approximate resolution of 25Hz between two consecutive values 
  • whereas we are getting Fmax values in continuous range from one of the commercially available RPM techno-meter based on MEMS accelerometer and due to this we are unable to hold the accelerator of the vehicle at the desired RPM of the vehicle for vehicle testing purposes.

Please suggest the best FFT parameter settings for the Fmax range of 20Hz to 200 Hz if we are making some mistake or suggest where we are wrong.

I hope we have selected right ST MEMS part for such application or suggest new another one.

Thanks in advance!

Thanks and Best Regards,

VK Verma

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2 REPLIES 2
Miroslav BATEK
ST Employee

‎2018-07-02 07:31 AM

Posted on July 02, 2018 at 16:31

Can you please share picture of your FFT results and expected RPM.

I think it is correct if you get multiple of first harmonic frequency (25Hz in your case, which should be 1500 RPM). 

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VK Verma
Senior
In response to Miroslav BATEK

‎2018-07-03 04:49 AM

Posted on July 03, 2018 at 11:49

Hi Miroslav,

In our observation, we are getting RPM which is roughly 30 times of F max and that is why 25Hz is close to 750 RPM and the same is observed in the vehicle tachometer. Now please help us to reduce 25 Hz factor between two consecutive readings so that we will be able to achieve RPM resolution of +/-10 rpm.

Your support will be highly appreciated.

Thanks and Best regards,

VK Verma

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