Accelerometer measures non inertial or inertial ref. frame

In summary, the discussion revolves around whether accelerometers should be seen as non-inertial or inertial sensors. Some believe that accelerometers measure non-gravitational acceleration, but others argue that they are self-orienting. The use of 3 component accelerometers in seismic surveys has brought up questions about how they can measure the force acting on them from the Earth and still retain their orientation for correction. The issue of centrifugal acceleration is also raised when mounting an accelerometer on a pendulum.
  • #1
ylim
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Hi there,

I am doing some measurement using an accelerometer. I am rather confused with regards to whether I should be see an accelerometer as a non-inertial or an inertial sensor.

I have with me 2 commercial accelerometers evaluation board (analog to computer interface). One by the analog/crossbow collaboration board tells the static g as a +1g value, which is seen as a non-inertial reference frame whereas the other one by ST microelectronics with some third party tell static g as -1 (the inertial ref. frame).

Both of them tells positive acceleration swing from a positive axis acceleration and vice versa (this is okay).

Say now if I am interested to tell if I should be considering the centrifugal acceleration (I am mounting the accelerometer to a pendulem). Should I be taking the centrifugal acceleration as a non inertial or an inertial one? I am assuming that based on the fact that (Both of them tells positive acceleration swing from a positive axis acceleration and vice versa (this is okay).), I will be seeing a negative centrifugal acceleration (given positive direction to skyward). So I will not be able to see a non inertial centrifugal centrifugal accleration?

Hope to hear from you guys.
 
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  • #2
Accelerometers are inertial-frame sensors. The reason an accelerometer registers 1g upward when it is not moving relative to the Earth can be explained from a classical physics perspective or from a general relativity perspective.

First, a little factoid. No device can be made that directly measures the force due to gravity. A wonderful sci-fi device called a "gravity shield" would be needed to perform such a measurement. An accelerometer is not a sci-fi device. It cannot measure the acceleration due to gravity.

From a classical physics perspective, an accelerometer is a device that measures the acceleration due to all forces acting on the accelerometer case except gravity. Suppose we have an accelerometer mounted on the surface of non-rotating planet with a gravitational force equal to that of the Earth. The downward gravitational force on the accelerometer case is balanced by an upward normal force exerted by the surface of the planet. (Note well: this is not a third law action-reaction pair. It is something different.) The accelerometer cannot sense the downward acceleration due to gravity, but it can sense the upward acceleration due to the normal force. Net result: the accelerometer registers 1g upward.

In general relativity, gravitation is a fictitious force rather than a "real" force. The GR point of view is that an accelerometer is a device that measures the acceleration due to all real forces acting on the accelerometer case, period. An accelerometer mounted on a the surface of non-rotating planet is not in an inertial frame! From the GR perspective, an accelerometer measures acceleration relative to an inertial frame, which in the case of the accelerometer on the surface of a planet is accelerating toward the center of the planet. The accelerometer is accelerating up relative to this frame. Net result: the accelerometer registers 1g upward.
 
  • #3
Hi DH,

Thank you for your reply.

I was hoping that you can answer my doubt though.

>>Say now if I am interested to tell if I should be considering the centrifugal acceleration (I am mounting the accelerometer to a pendulem). Should I be taking the centrifugal acceleration as a non inertial or an inertial one? I am assuming that based on the fact that (Both of them tells positive acceleration swing from a positive axis acceleration and vice versa (this is okay).), I will be seeing a negative centrifugal acceleration (given positive direction to skyward). So I will not be able to see a non inertial centrifugal centrifugal accleration?


Regards,
 
  • #4
If you mount an accelerometer to a pendulum, what real (non-fictitious) forces other than gravity act on the accelerometer?

One thing to keep in mind when mounting an accelerometer to a pendulum is that the accelerometer's case frame is rotating.
 
  • #5
D H said:
If you mount an accelerometer to a pendulum, what real (non-fictitious) forces other than gravity act on the accelerometer?

One thing to keep in mind when mounting an accelerometer to a pendulum is that the accelerometer's case frame is rotating.

1. translational acceleration
and
2. rotation?

Centrifugal can be seen as either a non inertial or an inertial ref. frame, that will account for differing signs. That is my concern.
 
  • #6
Hi DH,
I have this question concerning accelerometers. I believe accelerometers measure "non gravitational" acceleration but I am unclear about the principles behind the belief by some authors that accelerometers are "self orienting". I am particularly interested in this because latest seismic surveys being carried out use 3 component accelerometers to measure P and S waves and no matter the orientation of the accelerometer it can be corrected based on "apparent gravity". I know accelerometers don't measure gravity, it measures the force acting on it wrt to a gravitational field but can't understand how these accelerometers can measure the force acting on it from the earth(as a result of seismic energy being generated by a seismic source) and at the time retain its orientation for onward correction.
 

1. What is an accelerometer?

An accelerometer is an electronic sensor that measures acceleration forces in a specific direction. It is commonly used in scientific and technological applications to measure the movement of objects.

2. What is the difference between non-inertial and inertial reference frames?

A non-inertial reference frame is one in which an object appears to be accelerating due to a force acting on it. An inertial reference frame is one in which an object appears to be at rest or moving at a constant velocity.

3. How does an accelerometer measure in a non-inertial reference frame?

In a non-inertial reference frame, an accelerometer measures the apparent acceleration of an object caused by the force acting on it. It takes into account the acceleration of the frame itself, allowing for more accurate measurements.

4. Can an accelerometer measure in both non-inertial and inertial reference frames?

Yes, an accelerometer can measure in both types of reference frames. However, it is important to calibrate the accelerometer properly for each specific frame in order to obtain accurate measurements.

5. What are some common uses for accelerometers?

Accelerometers are commonly used in the fields of engineering, physics, and sports science to measure the acceleration, vibration, and movement of objects. They are also used in smartphones, gaming devices, and other consumer electronics for features such as screen rotation and step tracking.

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