Determining the Cut-off Frequency of a MEMS Accelerometer

In summary, you need to analyse the signal to determine the maximum frequency at which the noise is an issue, and then design a LPF to filter noise above that frequency.
  • #1
rahlk
4
0
Greetings Forum,
I just bought an Invensense MPU-6050, I need to design an LPF to filter the noise. How do I analyse the signal and how do I determine the frequency beyond which the signal is altered by noise.
Thanks in advance for your help.

Cheers
 
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  • #2
rahlk said:
Greetings Forum,
I just bought an Invensense MPU-6050, I need to design an LPF to filter the noise. How do I analyse the signal and how do I determine the frequency beyond which the signal is altered by noise.
Thanks in advance for your help.

Cheers

Can you post a link to the datasheet? Are there any application notes at the manufacturer's website? What are you going to be connecting this accelerometer to?
 
  • #3
I will be using it to estimate the tilt angle. I am going to interface it to a microcontroller. In fact, I need to submit a term paper on this, so I'll have to describe my filter quite clearly.
The Datasheet - http://www.cdiweb.com/datasheets/invensense/PS-MPU-6000A.pdf
 
  • #4
rahlk said:
I will be using it to estimate the tilt angle. I am going to interface it to a microcontroller. In fact, I need to submit a term paper on this, so I'll have to describe my filter quite clearly.
The Datasheet - http://www.cdiweb.com/datasheets/invensense/PS-MPU-6000A.pdf

Since this is for a school project, you will need to describe the filter quite clearly to *us*! We don't do your schoolwork projects for you here. We can offer some hints and tutorial help, but you must do the bulk of the work on your schoolwork projects.

So tell us what you see in the datasheet. Are there any app notes? What kind of noise do you expect in your data acquisition setup? What order and polynomial do you think you will want to use in your project, and *why*?
 
  • #5
Well, accelerometer is subject to several sources of high frequency noise, including thermal, electrical and mechanical vibrations. I thought of developing a simple first order, single-pole infinite impulse response LPF, given by,

y(n) = α.y(n-1) + (1-α).x(n), where,

x(n) = current accelerometer reading,
y(n) = current estimate; y(n-1) = previous estimate.

My issue is with determination of alpha. If sampling frequency is Fs then,

α = [itex]\frac{\tau Fs}{1+\tau Fs}[/itex].
and,
[itex]\tau[/itex] = [itex]\frac{1}{2\pi Fc}[/itex].

Once I determine Fc, I can justify my choice of α. Now, If the device were analog in nature, I could have designed an RC LPF, got the state equations, taken a laplace transform and applied Bilinear Z Transform to get the digital equivalent. But this device gives acceleration in digital format, a 16 bit number.
I need to be able to somehow use Fourier analysis or some such technique to work on this digital data directly. I would be delighted to get some help on clarifying this conundrum.
 
  • #6
rahlk said:
Once I determine Fc, I can justify my choice of α. Now, If the device were analog in nature, I could have designed an RC LPF, got the state equations, taken a laplace transform and applied Bilinear Z Transform to get the digital equivalent. But this device gives acceleration in digital format, a 16 bit number.

Why would that be a problem? You've gone through the process of designing a digital LPF, starting with an analogue prototype. If you were applying your filter to an analogue signal you would have to digitise the signal then filter it. But here the signal has already been digitised for you so you just need to read the signal out of the MEMS sensor (remembering to keep up with the Nyquist sampling rate requirements) then run it through your digital LPF implementation.
I need to be able to somehow use Fourier analysis or some such technique to work on this digital data directly.
What makes you think you need the complexity of using Fourier analysis? It might be worth trying out a simpler digital LPF first and moving up to more complex designs later if you find the filtering isn't adequate.

From your initial post:
I need to design an LPF to filter the noise. How do I analyse the signal and how do I determine the frequency beyond which the signal is altered by noise.
Try considering what maximum frequency you need for your accelerometer signal to make your application work. You can then choose a LPF to filter off noise above this frequency. If you find it hard to get good estimates for your filter requirements it could be worthwhile building a rough prototype and experimenting with it to see how your estimates work in practice.
 

1. What is a MEMS accelerometer?

A MEMS (Micro-Electro-Mechanical System) accelerometer is a small device that measures acceleration, which is the rate of change of velocity. It is commonly used in electronic devices such as smartphones, tablets, and gaming controllers to detect motion and orientation.

2. How does a MEMS accelerometer work?

A MEMS accelerometer consists of a small mass suspended by tiny springs inside a sealed chamber. When the device experiences acceleration, the mass moves relative to the chamber, causing the springs to stretch or compress. This change in position is measured using tiny electrical components, such as capacitors, which convert the movement into an electrical signal.

3. Why is it important to determine the cut-off frequency of a MEMS accelerometer?

The cut-off frequency is the point at which the accelerometer can no longer accurately measure high-frequency vibrations. It is important to determine this frequency because it can affect the accuracy and reliability of the device's measurements. Knowing the cut-off frequency can also help in selecting the appropriate accelerometer for a specific application.

4. How is the cut-off frequency of a MEMS accelerometer determined?

The cut-off frequency of a MEMS accelerometer can be determined by conducting a frequency response test. This involves applying a known vibration frequency to the device and recording the output signal. The cut-off frequency is then identified as the point where the output signal starts to deviate from the input signal. This test can be performed using specialized equipment or through simulation software.

5. Can the cut-off frequency of a MEMS accelerometer be adjusted?

The cut-off frequency of a MEMS accelerometer is primarily determined by its design and cannot be adjusted once the device is manufactured. However, it is possible to choose an accelerometer with a different cut-off frequency to better suit the needs of a particular application. Some accelerometers also have built-in filters that can be used to adjust the cut-off frequency within a certain range.

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