The SPEEDBOX contains a multi axis precision MEMs type sensor for measuring acceleration directly, rather than relying on differentiating the speed output from the GPS signal. There are a number of significant advantages of using a dedicated accelerometer; the most important point is that the accelerometer sensor’s measurement bandwidth is much higher than is practical with GPS. In the case of the SPEEDBOX the accelerometer bandwidth is set to 300Hz. A high quality acceleration measurement is essential for the combined accelerometer/GPS processing to be effective.
The noise characteristics of the accelerometer are class leading, with a noise floor of about 2mg. This can be validated by simply placing the accelerometer on a very solid base and logging the acceleration output via either the serial or CAN outputs. The results from this test are shown below:
Typical accelerometer noise
The accelerometers are calibrated at 1g (gravity) in a jig to hold them in a vertical orientation. The linearity of each accelerometer is not individually verified, but is specified by the manufacturers as better than 0.2%. The example below shows X and Y axis accelerometer outputs during the calibration process.
X and Y axis accelerometer outputs during calibration