How many g-cell’s does an accelerometer have?

[Bobli]

Hi,

May I ask a question on MMA7660 accelerometer sensor?

How many “MEMS capacitive sensing g-cell”s does a 3-axis accelerometer have? For MMA7660, “Principle of Operation” of its datasheet gave an analogy that the sensing g-cell works like a “movable beam between two beams” (see attached picture).

In Figure 19 (see attached picture), it is shown that the accelerometer has 3 directions. Therefore, is there three “movable beam”s (g-cell’s) as shown in the figure above?

Does the number of g-cell’s determine the axis number, such that 1-axis accelerometer has one g-cell, 2-axis accelerometer has two g-cell, and 3-axis accelerometer has three?

Bob

[bigmac · Accepted solution]

Hello Bob,

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Bobli wrote:

Does the number of g-cell’s determine the axis number, such that 1-axis accelerometer has one g-cell, 2-axis accelerometer has two g-cell, and 3-axis accelerometer has three?

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Yes.

Regards,

Mac

[Bobli]

mac,

quote http://en.wikipedia.org/wiki/Accelerometer

Most micromechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.

Is this the principle?

[bigmac]

Hello Bob,

I do not have specific knowledge about the internal arrangements of the die.  However, the simplified functional block diagram, shown in the datasheet, indicates the presence of three separate transducers for the MMA7660 device you are contemplating.

Regards,

Mac

[Bobli]

mac,

Is there any Freescale employee monitoring this forum? I believe they should have the knowledge to answer this question.

Bob

[bigmac]

Hello,

I cannot see how the internal construction has any relevance to the use of the device within an application.  The performance characteristics and limitations should be adequately stated within the datasheet.

If you feel that the datasheet contains insufficient or incorrect information, it is possible to submit a Service Request to Freescale, to have the matter clarified.

Regards,

Mac

[Bobli]

mac,

I am just curious about it. The answer is already good enough for use.

Bob