Prof. Bill F. Spencer, Jr.
Prof. Gul Agha
Calibration Guide for Wireless Sensors
Determining the Calibration Constants for Each Sensor
Last updated on February 9, 2011 for use with Imote2s with the ISM400 (formerly SHM-A) Sensor Board produced by MEMSIC.
This guide describes both static and dynamic methods to determine calibration constants for wireless sensors. The raw outputs of the sensor board are the ADC values from the QF4A512. To convert the raw data to units of acceleration, the scale and offset values must be determined for each sensor as follows:
Acceleration = (ADC values – offset)/scale
The scale and offset are known as the calibration constants, which can be utilized to obtain the physical acceleration. The scale allows conversion from the raw ADC values to acceleration. The offset is the mean of the ADC values. Nominal calibration constants are usually given in a sensor’s data sheet; however, inherent differences in electronic components and manufacturing dictate that the calibration constants should be determined for each channel of a sensor to achieve a high quality of measured data.
Because the ISM400 sensor board can measure DC signals, the earth’s gravitation can be used to perform straightforward sensor board calibration. This static approach is described in the first section of this guide. Subsequently, the guide describes how to compare wireless sensor data to data from a reference sensor for the purpose of dynamic calibration.