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BOOSTXL-PGA460: PGA460: Source of bias in distance measurements

Kartik Sastry
Prodigy 20 points
Part Number: BOOSTXL-PGA460
Other Parts Discussed in Thread: PGA460-Q1, PGA460

Hello,

I am using the PGA460 as a time-of-flight sensor. For now, I am simply converting the time-of-flight reported by PGA460 to distance via the recommended formula on Page 38 of the PGA460-Q1 User Guide:

DISTANCE [m] = (V_SOUND [m/s] * TIME_OF_FLIGHT [us] * 1e-6 [s/us] / 2).

I recently performed a calibration experiment where I collected data of the form (true distance, PGA460 reported distance) for a single planar reflector at normal incidence. I noticed a consistent bias in the results of about 1 inch - the distance computed from the PGA460-reported time of flight is consistently about 1 inch greater than the true distance between the transducer face and the planar reflector.

When I examined Figure 32 of the PGA460-Q1 User Guide, I saw several reasons why the time-of-flight reported by PGA460 could be less than the true time of flight, but not larger than the true time of flight, as I noticed in my experiment.

I am not sure what could be the source of this bias. I would really appreciate any insight you could provide into this!

Thanks,
Kartik

  • 0
    TI__Mastermind 27950 points

    Hi Kartik,

    Have you factored changes in ambient temperature to accurately set the speed of sound? The speed of sound is dependent on temperature:

    vsound= 331m/s + (0.6m/s/°C × Temperature(°C))

    If you are testing in a cooler environment, the speed of sound will be slower, and could result in a positive offset from the true value if your equation always assumes nominal temperature (20°C --> 343m/s). A 1 inch (2.54cm) offset would require an difference of 5°C --> 334m/s from nominal temperature. Are you testing in an cooler ambient environment?

    Also, is your threshold set to cut the echo at the base near the noise floor or at the peak? A base cut is more accurate than a peak cut. Peak cuts add positive offset.