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/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 –5g (–49 m/s2) to +5g (+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 100g. The 3-Axis Accelerometer will not be damaged by accelerations up to 1000g.
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.
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.