7
9.8 m/s
2
. During the toss all three accelerations and the net acceleration go to 0 m/s
2
.
In the graph below, the interface and accelerometer were tossed in the air and spun.
During the toss the x-, y-, and z-axis accelerations all change. Notice that the net
acceleration does not quite go to zero due to centripetal acceleration.
3-Axis Accelerometer tossed and rotated in the air.
Frequent Questions on Accelerometer Measurements
Since the accelerometer is sensitive to both acceleration and the Earth’s gravitational
field, interpreting accelerometer measurements is complex. A useful model for
understanding accelerometer measurements is a spring-based scale with a reference
mass (or object) attached to the scale. If the scale is pointing upward (the usual
orientation for such a device) the weight of the mass causes the spring to compress,
and you get a non-zero reading. If you were to turn the scale upside down, the spring
will be extended, instead of compressed, and we get a reading of the opposite sign. If
you turn the scale so it points sideways, and keep it motionless, then the spring will
just be at its relaxed length, and the reading will be zero. If you accelerated the scale
toward the mass, then the spring would compress; away, and the spring would stretch.
In each case the scale is reading a value corresponding to the normal force on the
mass. This reading can be made relative by dividing out the mass, giving units of
N/kg, which is the same as m/s
2
.
The accelerometer measurements can be interpreted in exactly this way.
Q: What does an accelerometer measure?
A: Normal force per unit mass.
Note that it’s not the net force per unit mass (which would be acceleration), but it is
the normal force per unit mass. This unusual quantity corresponds with what a rider
on a roller coaster feels during the turns. This interpretation is useful even for the
scalar total acceleration value, which is 9.8 N/kg for a three-axis accelerometer at
rest, zero for one in free fall, and greater than 9.8 for one making a corner.
8
This normal force interpretation works even for a one-axis accelerometer being
accelerated in a horizontal direction. The reading is non-zero as the test mass inside
the device has to have a force applied to accelerate it. That's just a normal force that
happens to be horizontal.
When discussing the accelerometer reading, we can call it the Normal Force per Unit
Mass, with units of N/kg.
Q: I thought the Accelerometer measured acceleration!
A: Here we are being very careful to not call something an acceleration when it is
not a kinematic acceleration. For example, an "acceleration" of 9.8 m/s
2
for an object
that remains at rest is clearly a problematic interpretation, yet that’s what the
accelerometer reads.
You can correct the Accelerometer reading to get a true acceleration by adding the
component of the gravitational acceleration field along the direction of the sensor
arrow. For example, if the axis of the accelerometer is pointing upward, then the
gravitational component is –9.8 m/s
2
. The Accelerometer reads 9.8 m/s
2
when the
arrow is upward and the device is at rest. By adding –9.8 m/s
2
, we get zero, which is
the correct acceleration. If the arrow is horizontal, then the reading is zero, but the
gravitational component is zero, and we still have zero for the true acceleration. If an
Accelerometer is zeroed to remove the influence of gravity, the zeroing is later
wrong if the Accelerometer is rotated.
Q: What about g-force measurements?
A: We avoid the term g-force because the quantity doesn’t have units of force.
Instead, g-factor can be used as a simplified label for Normal Force per Unit Mass in
axis labels and discussions.
You can see that the
g
-factor is then 1 for an object sitting at rest on a table, zero in
free fall, etc. The
g
-factor is dimensionless. If the Normal Force is a vector, then so is
the
g
-factor.
g
-factor is completely optional–it is just a shortcut to avoid a long name.
Warranty
Vernier warrants this product to be free from defects in materials and workmanship
for a period of five years from the date of shipment to the customer. This warranty
does not cover damage to the product caused by abuse or improper use.
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•
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•
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•
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Rev. 2/16/10
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