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/s2. Accelerometer measurements can be interpreted in exactly this way.
The 3-Axis Accelerometer contains three acceleration-sensing integrated circuits (ICs), along with the associated electronics. It is functionally equivalent to three of our Low-g Accelerometers (order code LGA-BTA) mounted in a small block at orthogonal angles. Each of the accelerometers measures acceleration along one line and produces a signal on one of the three outputs. These three axes and three outputs are labeled X, Y, and Z. The IC sensors are similar to those originally designed to control the release of air bags in an automobile. This IC is micro-machined with very thin “fingers” carved in silicon. These fingers flex when accelerated. They are arranged and connected like the plates of a capacitor. As the fingers flex, the capacitance changes, and a circuit included in the IC monitors the capacitance, converting it into a voltage. An op-amp circuit amplifies and filters the signal from the IC. The net result is that the voltage varies in a linear way with acceleration.
Each of the outputs is labeled with X, Y, or Z. This corresponds with the directions shown on the label on the sensor. Accelerations are normally measured in either meters per second per second (m/s2) or g. One g is the acceleration due to gravity at the Earth’s surface, or 9.8 m/s2. This accelerometer will measure accelerations in the range of –5 g (–49 m/s2) to +5 g (+49 m/s2) in each direction.
This is a range of accelerations that a human body could experience without damage. Many collisions will produce much larger accelerations. In fact, dropping the accelerometer on a hard surface from even a few centimeters can produce accelerations of 100 g. The 3-Axis Accelerometer will not be damaged by accelerations up to 1000 g.
When properly calibrated, when the arrow representing an axis points upward, that channel reads +9.8 m/s2. When an axis arrow points down, that channel should read –9.8 m/s2. When an axis arrow is held horizontally, that channel will read zero. In most cases, data-collection software can be used to create a new column to calculate the square root of the sum of the squares of the accelerations. It will be equal to 9.8 m/s2 when the 3-Axis Accelerometer has no acceleration and zero when it is in free fall. The orientation of the 3-Axis Accelerometer does not matter. To understand how this works, try holding the 3-Axis Accelerometer in your hand and very slowly rotate it about all three axes. The graphs below show the result. The graphs have all three components of acceleration and the net acceleration (the square root of the sum of the squares of the accelerations). Notice that it stays near 9.8 m/s2 throughout all of this rotation.
Rotating the 3-Axis Accelerometer
The 3-Axis Accelerometer is designed to measure small accelerations with minimal electronic noise. The noise is typically on the order of 0.5 m/s2 peak to peak. The offset voltage (voltage output at 0 m/s2) will drift somewhat with temperature.
Since the 3-Axis Accelerometer is equivalent to three Low-g Accelerometers, you can also do any experiment that uses only one or two axes. Examples include:
- Measure the acceleration of dynamics carts as they roll down inclines or have force applied to them.
- Measure acceleration vs. time on elevators, remote-controlled cars, bicycles, or automobiles.
- Use the accelerometer to measure the tilt of an object. Since each channel of the accelerometer senses the vertical component of gravity, its reading will change as its orientation is changed from horizontal to vertical. You can measure angles to the nearest degree.
The 3-Axis Accelerometer can also be used to collect data in more complex experimental settings such as:
- Amusement park rides such as roller coasters, swings, swinging ships, and Tilt‑a‑Whirl
- Bungee jumps done by dolls or humans
- Tossing a padded box in the air
Compare the three individual accelerations to the net acceleration.
3-Axis Accelerometer tossed in the air but not rotated
In the first graphs, the box was tossed in the air but not rotated. Notice that the x‑and z-axis accelerations before the toss are zero whereas the y-axis acceleration is 9.8 m/s2. During the toss all three accelerations and the net acceleration go to 0 m/s2.
In the next graphs, 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
Using the 3-Axis Accelerometer as a Single Axis Accelerometer
Since the 3-Axis Accelerometer is equivalent to three Low-g Accelerometers, you can use just one channel of it to study acceleration along a single axis. Mount the accelerometer so that a particular axis is in the direction of interest and monitor just that channel. If the motion is linear, it will keep the analysis simple.