Research Academics  
 
   
 

Two-Axis Micromachined Accelerometer for
Gesture Recognition
 
Contact Person : Dipl.Ing. B.Schellin

Introduction

In recent years many types of accelerometers have been fabricated. Most of them are one-axis accelerometers with a range of approximately 50g. A wide field of application is the automotive industry. The sensor described here was especially designed for applications in gesture recognition. It is a two-axis-accelerometer (x-z-axis) which can easily be extended to a monolithic three-axis accelerometer. The sensor is mounted on a data glove to detect the characteristic accelerations of human gestures. The occurring accelerations are in the range of ±5g and show a bandwidth of some hundred Hertz.

Figure 1: Structure of the two-axis accelerometer

Structure and Principle

The structure of the accelerometer is shown in Fig.1. The sensor consists of a glass-silicon-glass structure. The silicon is structured using bulk-micromachining technology and consists of two opposite, conventional, one-dimensional, piezoresistive accelerometers. The mass of each one-dimensional accelerometer is suspended by two beams. Due to their inertia, the masses try to stay in their original position when the sensor is accelerated. This leads to a bending of the cantilever beams which causes stress. There are piezoresistors on the surface of the cantilever beams and their resistance is stress dependent. Using two longitudinal and two transverse piezoresistors, which have opposite signs of resistance changes, and connecting them to a wheatstone bridge makes it possible to get a signal voltage which is proportional to the acceleration. Using one wheatstone bridge for each one-dimensional accelerometer it is possible to get the z-axis acceleration by adding the signals of the two bridges. Subtraction of the bridge signals gives the x-axis acceleration. By optimizing the sensor design it is possible to obtain almost equal sensitivities for x-accelerations and z-accelerations. Damping of the sensor is achieved by air gaps between the masses and the upper glass plate. The gaps are filled with air at atmospheric pressure. Critical damping can be adjusted by variation of the gap width. Small deviations of the air pressure from the initial value will hardly effect the damping behavior.
Figure 3 shows pictures of a fabricated sensor chip.

 


Fig 3: Fabricated sensor chip without upper glass plate. Left: SEM-Picture, Right: Photograph of the device

 

Experimental Results

To evaluate the sensor characteristics such as sensitivity, cross-axis-sensitivity and linearity error, the sensor is rotated in the earth's gravity field. Fig. 4 shows the sensor output signals (gain = 1000) for rotating the sensor around the z-axis (left) and the x-axis (right). The measured sensitivities were 1,15 mV/g for the z-signal and 0,41 mV/g for the x-signal (5V power supply).

Figure 4: Sensor signals for rotating the sensor around the z-axis (left) and the x-axis (right)