MEMS ACCELEROMETER

Autocalibration

yunuz1007 — Sun, 06 Sep 2009 17:15:37 +0000

The autocalibration procedure is based on the fact that the

modulus of the acceleration, in static conditions, is equal to that

of the gravity acceleration

the model parameters, we placed the sensor

g2 (4)

Vj,k is the jth MEMS output for the kth orientation.e2

By adding

k

error

’s over all the measured orientations, a cumulative

E is obtained as

E

=

E(Ox,Oy,Oz, Sxx, Syy, Szz, Sxy, Sxz, Syz) =

in

evaluated the sensor output while maintaining it in a strictly

static condition. We define an error

difference between the modulus of the acceleration output by

the MEMS and

Sensor Model

yunuz1007 — Sun, 06 Sep 2009 17:13:04 +0000

Since the adopted accelerometer is designed to provide ratiometric

output , we consider the normalized accelerometer

output

output voltage by the power supply voltage

VT = [vx, vy, vz], which is obtained by dividing theVCC (that is, vi =

V

vector

reference system

the axes

i/VCC, for i = x, y, z). Let us also define the accelerationAT = [ax, ay, az], which is expressed in the sensor local[O,XMEMS, YMEMS, ZMEMS) (Fig. 1), withXMEMS, YMEMS lying on the sensor surface and

Z

A mathematical model describing the accelerometer output

can be described in matrix form as

MEMS orthogonal to the surface.

A

where

= S(V − O) (1)

S

=

⎡

⎣

S

xx Sxy Sxz

S

yx Syy Syz

S

zx Szy Szz

⎤

⎦

, O =

⎡

⎣

O

x

O

y

O

z

⎤

⎦

are the scale factor matrix and the bias vector, respectively. The

diagonal elements of

three axes, whereas the other elements of

factors. These terms allow describing both the axes’ misalignment

and the crosstalk effect between different channels caused

by the sensor electronics . For an ideal accelerometer, the

cross-axis factors should all be equal to zero, whereas for a

real one, they can be as large as 2% of the sensor sensitivity,

as reported in Table I. The highest accuracy is achieved by

considering the cross-axis factors in

Imposing the symmetry constraint on the scale factor matrix

(2)S represent the scale factors along theS are called cross-axisS, as shown in Section IV.S

(that is,

model has nine independent parameters. Calibration consists of

a procedure capable to determine these parameter.

INTRODUCTION

yunuz1007 — Sun, 06 Sep 2009 16:46:14 +0000

The

(MEMS) technology has allowed miniaturized, highperformance,

and cheap sensors to be built. These have found

applications in many fields, ranging from pressure measurement

to hard disk actuators and virtual reality . MEMS

accelerometers have recently been introduced to track the

motion of both humans and machines as they allow

accurate low-budget motion capture systems to be built. This

provides an alternative to costly traditional systems, which are

generally based on optoelectronic technology.

Different from classical MEMS applications (for example,

airbag control), where impulsive accelerations are measured,

motion analysis applications require position and orientation

data. To the scope, MEMS accelerometers are integrated with

gyroscopes or detailed models of the moving body,

and their output has to be integrated over time. The accuracy

of the accelerometer output is, therefore, fundamental, and a

good calibration is needed to profitably use them. The technical

data reported by manufacturers are not accurate enough for this

kind of application ; moreover, the sensor output depends on

temperature and, in general, on environmental conditions . Therefore, they have to be calibrated in the field to obtain

high accuracy.