Data processing: measuring – calibrating – or testing – Calibration or correction system – Position measurement
Reexamination Certificate
2001-11-13
2003-06-10
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Position measurement
C310S0400MM
Reexamination Certificate
active
06577975
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention regards a device and a method for automatic calibration of a microelectromechanical structure included in a control loop.
In particular, the present invention finds an advantageous, but not exclusive, application in the compensation of the position offset of an inertial sensor, to which the ensuing treatment will explicitly refer, without this entailing any loss of generality.
2. Description of the Related Art
As is known, owing to their reduced size, excellent technical characteristics, high reliability and low cost, integrated inertial sensors manufactured using the micromachining technique are progressively laying claim to market segments up to now occupied by conventional inertial sensors.
One of the main applications of the above inertial sensors is in the field of airbag systems for motor vehicles as a means for measuring the deceleration to which a motor vehicle is subjected upon impact.
An inertial sensor, also known as accelerometric sensor or accelerometer, is a particular transducer device capable of measuring and converting an acceleration into an electrical signal, and is basically formed by two distinct elements: a sensor proper and an electrical interface.
The sensor proper is the element that is able to carry out conversion between the quantity (acceleration), the value of which is to be determined, and a quantity that may be measured by means of circuitry of an electrical nature, whilst the second element of the transducer device is a capacitive reading interface, i.e., a charge integrator, capable of determining the capacitance variation due to the presence of an acceleration.
An integrated rotary inertial sensor, i.e., the only movement of which is of a rotational nature, is described in the European Patent No. 99830568.4 filed on Sep. 10, 1999, in the name of the present applicant and is shown in FIG.
1
.
The inertial sensor, designated as a whole by 1, is made of semiconductor material, has a circular structure, and comprises an inner stator
2
integral with the die
3
in which the inertial sensor
1
is formed, and an outer rotor
4
electrostatically coupled to the stator
2
.
The rotor
4
comprises a suspended mass
6
having an annular shape, a plurality of mobile arms
8
extending radially towards the stator
2
from the suspended mass
6
, identical to each other and angularly equispaced, and elastic-suspension and anchorage elements
10
(represented schematically as springs) elastically connecting the suspended mass
6
to fixed anchoring and biasing regions
12
, through which the suspended mass
6
and the mobile arms
8
are biased (typically at a potential of 1.5 V).
The stator
2
comprises a plurality of pairs of fixed arms
14
,
16
, one pair for each mobile arm
8
of the rotor
4
, which extend radially with respect to the suspended mass
6
towards the suspended mass
6
itself, are arranged in such a way that between each pair of fixed arms
14
,
16
a corresponding mobile arm
8
of the rotor
4
is arranged, and are connected to respective fixed anchoring and biasing regions
18
,
20
, through which the fixed arms
14
,
16
are biased (typically at a potential ranging between 1.5 and 2.2 V).
The fixed arms
14
,
16
are connected, via the fixed anchoring and biasing regions
18
,
20
, to a sensing circuit having the purpose of picking up, from the fixed arms
14
,
16
, information regarding the relative position of the rotor
4
with respect to the stator
2
.
The inertial sensor
1
can be electrically modeled as shown in
FIG. 2
, i.e., by means of two capacitive elements
21
,
22
having a half-bridge configuration, wherein the two outer plates are defined by the fixed arms
14
and
16
, respectively, of the stator
2
, and the two inner plates are defined by the mobile arms
8
of the rotor
4
.
When the suspended mass
6
is subjected to an angular acceleration, it undergoes a rotation such as to determine a modulation in phase opposition of the capacitances, indicated in
FIGS. 2
as C
S1
and C
S2
, of the capacitive elements
21
and
22
, respectively, which, in the absence of angular acceleration or deceleration applied to the inertial sensor
1
, should assume the same value. Consequently, by measuring the capacitances C
S1
and C
S2
it is possible to measure the magnitude of the unknown inertial quantity, i.e., the acceleration or deceleration to which the inertial sensor
1
is subjected.
On account of the imperfect configuration of the elastic-suspension and anchoring elements
10
and on account of the residual mechanical stress of the material of which the inertial sensor
1
is made, the rotor
4
is generally affected by a position offset, i.e., the effective zero position of the rotor
4
does not coincide with the nominal zero position, centered with respect to the stator, envisaged in the design phase.
The position offset consequently gives rise to a corresponding capacitive offset, defined as the difference between the capacitances of the capacitive elements
21
,
22
in the absence of acceleration or deceleration, which has an adverse effect on the overall performance of the system comprising the inertial sensor
1
and the corresponding driving and measuring circuitry.
To carry out compensation of the aforesaid capacitive offset, the inertial sensor
1
is provided with an integrated microactuator
24
made of semiconductor material, coupled to the rotor
4
and having the purpose of rotating the rotor
4
by an amount equal to the position offset to bring it back into the nominal zero position.
In particular, the microactuator
24
comprises four distinct actuator groups
26
, each of which is arranged in a respective quadrant of the inertial sensor
1
and is formed by a plurality of actuator elements
28
, numbering four in the example illustrated in
FIG. 1
, identical to one another and angularly equispaced.
In detail, each actuator element
28
is defined on the silicon wafer together with the suspended mass
6
of the rotor
4
, and comprises a mobile arm
30
integral with the suspended mass
6
(and consequently biased at the same potential as that of the suspended mass
6
), extending radially outwards from the suspended mass
6
, and carrying a plurality of mobile electrodes
32
extending from either side of the respective mobile arm
30
in a substantially circumferential direction, arranged parallel to one another, and equispaced along the respective mobile arm
30
.
Each actuator element
28
further comprises a pair of fixed arms
34
,
36
which extend radially with respect to the suspended mass
6
, arranged on opposite sides of the corresponding mobile arm
30
and facing the latter, and connected to respective fixed anchoring and biasing regions
38
,
40
, through which the fixed arms
34
,
36
are biased (typically at a potential ranging between 1.5 and 5 V). Each of the fixed arms
34
,
36
carries a plurality of fixed electrodes
42
,
43
extending in a substantially circumferential direction towards the corresponding mobile arm
30
and interleaved, or “comb-fingered,” with the mobile electrodes
32
of the corresponding mobile arm
30
.
The fixed arms
34
,
36
of the actuator elements
28
are connected, through the fixed anchoring and biasing regions
38
,
40
, to a driving circuit (not shown) having the purpose of applying a biasing voltage to either one or the other of the two fixed arms
34
,
36
of each actuator element
28
in such a way that the potential difference between the fixed arm
34
,
36
thus biased and the corresponding mobile arm
30
causes a rotation of the rotor
4
in one direction or the other, sufficient for bringing the rotor
4
back into the nominal zero position.
In particular, as a result of the electrostatic coupling existing between each mobile arm
30
and the corresponding fixed arms
34
,
36
, the rotor
4
is subjected to a transverse force proportional to the number of pairs of fixed arms and mobile arms
30
,
34
,
36
. This force tends to move the
Barlow John
Dougherty Anthony T.
Jorgenson Lisa K.
Seed IP Law Group
STMicroelectronics S.r.l.
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