The 3-axis accelerometer is ideal for detecting bumps and drops, as well as changes in orientation relative to the Earth’s gravitational field. It is always poised to record events, so there is no need to set extremely high recording frequency for fear of missing brief acceleration events: when an acceleration occurs, the sensor immediately begins recording data for Knut at a frequency of 10 samples per second.
Specs for the 3-axis accelerometer
- Units: 1/1000 of the acceleration g due to gravity (milli-g, mg)
- Optimal range: -2g to +2g (that’s -2,000 milli-g to +2,000 milli-g)
- Sampling frequency: 10Hz
- Recording triggered by: movement; user-defined recording intervals
- Safe for use in: Most environments
- Not safe for use in: Environments unfriendly to Knut (i.e. high g-force environments)
Considerations when using the 3-axis accelerometer
The orientation of the three axes–X, Y and Z–is indicated on the accelerometer sensor board. Z is perpendicular to the surface of the largest face of Knut’s case. When Knut is sitting flat on a table, the accelerometer will measure the acceleration due to gravity as X=0mg, Y=0mg, Z=1,000mg.
The accelerometer operates optimally when it is the only external sensor attached to Knut. This is related to the extremely high rate of data sampling for the 3-axis accelerometer and the sizable amount of data that the accelerometer generates in an extremely short amount of time. To optimize resolution of acceleration data, the accelerometer should be the only sensor on Knut. If lower resolution data is sufficient, then the accelerometer is entirely compatible with additional Knut sensors.
By default, Knut will record measurements from the accelerometer until it fills its quota (set by you) for data points per transmission. If you would like to enable recording of acceleration data beyond this limit, you can take advantage of the microSD card reader. When Knut is equipped with a microSD card, the card capacity becomes the new–and much higher–limit for data collection, and is ideal for analyzing sustained or reoccurring changes in acceleration.
When defining rules and alert conditions relating to the accelerometer, individual rules must be made for each axis. Technically, the accelerometer is actually four sensors (it has an onboard temperature sensor in addition to the three axes of acceleration sensors) so you can set individual rules for acceleration along each axis.