The Flimsy AcceleroMeter: Calibration

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch Winter 2004

The calibration of the accelerometer is simple: when the device stands upright - with the pin axle A being horizontal - we read 1.0 g, the Earth's gravity of 9.806 m/s^2. When we tilt it backwards until the pin becomes vertical, the weight of the mass acts only parallel to the pin axle, and thus provides no force component to make it rotate. In consequence, the springs go back to their rest positions: we read 0.0 g. When the base plate is tilted by an angle alpha against the horizontal, the component of the weight responsible for rotating the pointer is cos(alpha) its value under 1 g. E.g. at alpha = 45 degrees we read 0.7 g, as in the picture above.

A mathematical analysis of the accelerometer yields that as long as the pointer's deflection stays within 30 degrees against the vertical, one may assume a linear relationship between deceleration and acceleration (within 5 percent). So we get a linear dial.

The Role of Friction

When you tilt the device, the pointer should move across the dial easily, following the tilting steadily and without hesitation or backlash. Friction in the bearings and the link will imprede the pointer to react promptly, as the force necessary for a movement must be large enough to overcome any friction. Tilting up and down very slowly will give you a good indication how large the friction is. Try to make it as small as you possibly can!