Choosing Proper Filters for High-Speed Accelerometer Impact Sensing

Click For Summary

Discussion Overview

The discussion revolves around the selection of appropriate filters for high-speed accelerometer impact sensing, specifically in the context of measuring peak accelerations in a crash test dummy's head during simulated bicycle/car collisions. Participants explore the implications of sampling rates, filter types, and the characteristics of the accelerometers being considered.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant is considering using either the ADXL375 or ADXL1001 accelerometers and seeks guidance on filter choice, specifically whether to use a low or high pass filter and the appropriate cutoff frequencies.
  • Another participant suggests that the choice of filter depends on the measurement goals, emphasizing the need for an anti-aliasing low-pass filter and questioning the necessity of a high-pass filter.
  • A different participant reiterates the importance of measuring acceleration during a brief impact and expresses a desire to minimize noise in the data analysis.
  • Concerns are raised about the chosen sampling rate of 10 kS/s, with one participant questioning its necessity given the specifications of the accelerometer.
  • It is noted that to fully resolve the accelerometer's bandwidth, a sampling rate of at least 40 kHz would be required, leading to a recommendation for an anti-aliasing filter with a cutoff at 5 kHz.
  • Another participant acknowledges the relationship between the sensor's output frequency and the sampling rate, indicating that a low-pass filter could help achieve a smoother digital output.
  • Consideration of the bit depth of the ADC is mentioned as a factor that could influence the clarity of the digital reconstruction.
  • A participant inquires about the expected deflection of the accelerometer upon impact, indicating a need for further understanding of the physical dynamics involved.
  • One participant performs calculations related to the expected impact dynamics, estimating the average acceleration and displacement of the dummy's head during collisions, while questioning the reasonableness of these estimates.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of the sampling rate and the specifics of filter implementation. While there is some agreement on the need for anti-aliasing filters, the exact parameters and additional considerations remain contested.

Contextual Notes

There are unresolved assumptions regarding the impact dynamics and the specific characteristics of the accelerometers, as well as the implications of the chosen sampling rate on data fidelity.

Who May Find This Useful

Individuals interested in high-speed impact sensing, accelerometer technology, signal processing, and those involved in experimental design for crash testing may find this discussion relevant.

ConnorM
Messages
77
Reaction score
1
I’m planning on using the ADXL375 (200g/3200Hz Bandwidth) or ADXL1001 (100g/11,000Hz Bandwidth) to measure the peak accelerations in a crash test dummy’s head during a bicycle/car crash. My goal is to sample at atleast 10,000 S/s using either a Teensy 3.6 or a Rasp Pi3.

The dummy will be mounted on the bike and launched at 20km/hr, then it will be struck by a car from the rear and from the side driving at 30km/hr.

My question is about filter choice after the analog accelerometer that I choose. Should a low or high pass filter be chosen, and where should the corners be?
I would just like some guidance and I’d like to learn how I can go about choosing proper filters for sensors. I have taken a course on electronics that briefly covered Chebychev and other popular filters.
 
Engineering news on Phys.org
What filter you choose depends on what you are trying to resolve. You would certainly want some kind of anti-aliasing low-pass filter, but whether you wanted your cutoff to be lower than Nyquist or a high-pass of any kind depends on your measurement goals, really.
 
So for my situation,

- I am looking to measure the acceleration of a crash test dummy’s head during a very brief impact (15milliseconds).

- I want to be able to measure the accelerometer at 10 kSamples/s

- The current accelerometer that I am looking at measures +/-150 g’s with a bandwidth of 2 Hz —> 20 kHz (https://www.digikey.com/product-detail/en/te-connectivity-measurement-specialties/1-1001220-0/MSP1001-ND/279641)

What from these should I be using to determine my filter? I obviously want to be able to look at my acceleration data after and be able to analyze the head acceleration with as little noise from other sources as possible.
 
Is there any reason you want 10 kS/s? That's extremely overkill for your chosen sensor.
 
10 kS/s is the specified minimum sample rate that sensors in crash test dummy’s should be sampled at. This is a design project through my school and we are trying to get as close to this sample rate as possible.

Also, 10 kS/s will give us some assurance that the peak acceleration was captured.
 
Actually I misread your last post. Still, look up sampling theory and the Nyquist rate. In order to resolve the full 20 kHz of your sensor you would need to sample at 40 kHz. Since you are so far limited to 10 kHz, it means any content above 5 kHz in your signal will be lost. It also means you should use an anti-aliasing filter with a 5 kHz cutoff. You can look up anti-aliasing to check that out.
 
Oh thank you I have been reading about Nyquist Theory and you finally clued me into where the filtering aspect comes into play. This sensor can output 20 kHz but I can cut this to 5 kHz via a LPF, so I can sample at 10 kS/s for a full reconstruction of the analog wave.

So this would this ensure a smoother digital output? Are there any considerations I should make that would give me the clearest possible digital reconstruction?
 
You might also consider the bit depth of your ADC.
 
Do you have an idea of how much deflection you expect the accelerometer to experience on impact?
 
Last edited:
  • #10
It's always good to do a sanity check before you start building so that you know that it's going to work. So let's throw a few numbers around and see what happens.

In the first example with the bike going 20 km/hr, the car 30 km/hr and approaching from behind, the difference in speed is 10 km/hr. You are expecting the impact to ocurr in less than 15 ms with a peak acceleration of less than 150 G's. Converting everything to meters and seconds we get v = 2.78 m/s. Since I can only calculate average acceleration instead of peak, I'm going to use 75 G's instead of 150. v = a*t so t = 0.00378 s or 3.78 ms. (I multiplied 75 G's by 9.8 to get acceleration in m/s^2 of 735.) Using these numbers the bicyclist's head would move 735*0.00378^2/2 or 0.525 cm during impact? Does that seem reasonable?

Using the side collision at 30 km/hr the numbers we get are t = 11.3 ms, a = 75 G's and the distance the head moves during impact is 4.72 cm. Is this what you're expecting?
 
Last edited:

Similar threads

Which Accelerometer is Best for Measuring G-Force in a Car-Bicycle Collision?
  • · Replies 9 ·
Replies
9
Views
3K
Guidance on Sensor Choices for High Speed Impact Sensing
  • · Replies 3 ·
Replies
3
Views
2K