Accelerometer sensor with flat pendular structure

An accelerometer sensor comprising a flat pendular structure having a fixed part and a flat test body suspended from the fixed part by two parallel blades flexible in the plane of said test body, so as to be able to move in translation along a sensitive axis. Said test body extends at least partially into the space between said two flexible blades.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an accelerometer sensor using a flat 
pendular structure in which the sensitive axis is situated in the plane of 
the structure. 
It relates more particularly to an accelerometer sensor in which the 
pendular structure may be formed by micro-machining a crystalline wafer 
made for example from silicon or quartz and comprises a flat test body 
suspended by two flexible parallel blades in the plane of said test body. 
2. Description of the Prior Art 
So as to overcome the effects produced by the transverse components of the 
forces exerted on the test body and so as to increase the sensitivity of 
the sensor in its measuring axis, the flexible blades must be designed so 
as to have as little stiffness as possible in the plane of the structure 
and as high a stiffness as possible perpendicularly to this plane. 
However, because of the mechanical properties of the material used and 
because of the manufacturing techniques employed, it has proved impossible 
to totally remove the influence of these transverse components, mainly in 
the case where it is desired to form highly sensitive sensors. 
A first aim of the invention is therefore to provide a pendular structure 
of the type mentioned above, which, because of its geometrical forms, 
reduces the transverse stresses exerted on the flexible blades by the test 
body when this latter is subjected to transverse forces (transverse 
accelerations). 
SUMMARY OF THE INVENTION 
To arrive at this result, the invention provides a pendular structure in 
which the test body extends at least partially into the space between the 
two plates so as to bring the center of gravity of the test body as close 
as possible to the anchorage points (or feet) of the flexible blades on 
the fixed part of the structure. 
Furthermore, in an accelerometer sensor using a structure such as described 
above, determination of the acclerations necessarily involves the 
detection of the movements of the test body. 
The invention therefore also provides capacitive detectors for measuring 
the translational movements of the test body, specifically designed for 
the above described pendular structure. 
In a first embodiment of the invention, these capacitive movement detectors 
comprise at least a first capacitor plate supported by at least one zone 
of the edge of the test body substantially perpendicular to the sensitive 
axis of the sensor and a second capacitor plate carried by at least one 
zone of the edge of the fixed part of the pendular element, parallel to 
said first plate and spaced a small distance therefrom. These two 
capacitor plates may be advantageously obtained by metallizing said zones 
of the edge of the test body and of the fixed part of the pendular 
structure. 
In a second embodiment of the invention, said capacitive detectors comprise 
at least one capacitor plate consisting of at least one strip of an 
electrically conducting material, carried by a face of said test body and 
at least a second strip of an electrically conducting material extending 
parallel to the first strip and at a small distance therefrom, said second 
strip being carried by a frame firmly secured to the fixed part of the 
pendular structure. 
In the above described embodiments, the electrical connections between the 
capacitor plates supported by the test body and the electronic circuit 
supported by the fixed part of the pendular structure are advantageously 
formed by metallizing the thin faces of the flexible blades (parallel to 
the plane of the structure). 
In addition to the means for detecting the translational movements of the 
test body, the sensor may further comprise a return motor counterbalancing 
the external action exerted on the test body. Thus, according to another 
characteristic of the invention, this motor uses, in a conventional way, 
the Laplace force obtained by the action of a magnetic induction on a 
current flowing in a coil printed at the periphery of at least one of the 
two faces of said test body. 
Said coil may be formed by known methods such as metal deposits, chemical 
etching of a uniform deposit, etc. . . . in this case, the connections 
with the outside are provided through conducting areas, possibly formed by 
metallization, supported by the thin faces of the flexible blades, which 
are not used for the electrical connection of the capacitor plates. To 
avoid overlapping of the electrical connections between, on the one hand, 
the capacitor plates and the coil and, on the other, the conducting areas 
supported by the flexible blades, the test body may comprise at least one 
metallized passage between its two faces so that said electrical 
connections may be arranged on one and/or the other face of said test 
body. 
The magnetic circuit for producing the magnetic induction on the coil is 
then formed by a permanent magnet, magnetized crosswise and having two 
pole pieces which extend parallel to one face of the test body and at a 
small distance therefrom. This magnetic circuit is relooped by an armature 
which extends parallel to the other face of the body. 
More precisely, said coils comprise two opposite rectilinear zones, 
perpendicular to the sensitive axis of the sensor and through which a 
current flows in the opposite direction. In this case, said pole pieces 
are adapted so as to cover respectively said two rectilinear zones, so 
that the forces generated at the level of these two rectilinear zones on 
the test body by the current flowing in the coil are exerted in the 
direction of the sensitive axis and in the same direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the basic geometry of a pendular structure in accordance with 
the invention. As mentioned above, this structure is flat and may be 
formed is a single piece by micro-machining a crystalline silicon or 
quartz wafer which may further serve as substrate for an integrated 
electronic circuit. 
It is formed from a fixed part or base 1 on which are anchored the lower 
ends or feet 2, 3 of two parallel flexible blades 4, 5 of the same length, 
supporting at their upper ends a test body 6 of substantially rectangular 
shape which extends for a large part into the volume between the two 
flexible blades 4, 5. 
The flexible blades 4, 5 have, in the plane of the structure, as small a 
width as possible. Their thickness is that of the crystalline wafer. 
Consequently, these blades 4, 5 have very low stiffness in the plane of 
the structure, whereas they have relatively high stiffness perpendicularly 
to this plane. The sensitive axis .tau.X of this structure is therefore 
situated in the plane of the structure and is perpendicular to the 
flexible blades 4, 5. 
In the rest of the description, the walls of the different elements 
situated in the thickness direction of the blade and obtained for the most 
part by electromachining, will be designated by the expression "edges". 
Thus, for example, each of the flexible blades 4, 5 will comprise two 
parallel edges, namely an inner edge 7 orientated towards the test body 6 
and an outer edge 8 orientated towards the outside. Similarly, the test 
body 6 comprises two lateral edges 9, 10 extending parallel to the inner 
edges of the flexible blades 4, 5, a longitudinal inner edge 11 connecting 
together the lateral edges 9, 10 parallel to the corresponding inner edge 
12 of the fixed part 1 of the structure, and a longitudinal outer edge 13 
situated slightly beyond the ends of the flexible blades 4, 5. The 
connection of the flexible blades 4, 5 to the test body 6 is provided by 
means of two lateral projections 14, 14' provided on the test body 6 for 
determining the spacing between the flexible blades 4, 5 and the lateral 
edges 9, 10 of the test body 6. 
As can be seen in this Figure, the pendular structure may be obtained by 
forming in the crystalline wafer a U shaped recess forming the inner edges 
7 of the flexible blades 4, 5 the two lateral edges 9, 10 and the 
longitudinal inner edge 11 of the test body 6 as well as the inner edge 12 
of the fixed part 1 of the structure. 
Similarly, the outer edges 8 of the flexible blades 4, 5 and the 
longitudinal outer edge 13 of the test body 6 may be obtained by a cut-out 
in the form of an inverted U. 
In the example shown in FIG. 1, test body 6 further comprises a rectangular 
recess 15 for lightening the structure. 
The geometry of this pendular structure has the advantage of bringing the 
centre of gravity G of the test body as close as possible to the feet 2, 3 
of the flexible blades 4, 5. 
Of course, the invention is not limited to the form of the pendular 
structure shown in FIG. 1. It may be more particularly adapted so as to 
allow capacitive detectors to be mounted. 
Generally, these detectors necessarily comprise at least two capacitor 
plates mobile with respect to each other, one being fixed to the test body 
6 and the other being secured to the fixed part 1 of the pendular 
structure, these two plates being adapted so that a movement of the test 
body 6 along its measuring axis .tau.X causes a corresponding variation of 
the capacity, the measurement of this capacity variation allowing the 
amplitude of the movement of the test body 6 and, consequently, of the 
component of the acceleration along the measuring axis .tau.X to be 
determined. 
Two types of capacitive detectors may be used for equipping the pendular 
structure described above, namely: 
frontal movement detectors such as those shown in FIGS. 2 to 5 which 
comprise at least two parallel plates, perpendicular to the sensitive axis 
.tau.X of the pendular structure and whose spacing and, consequently, 
capacity vary depending on the movements of the test body 6, and 
lateral movement detectors such as those shown in FIGS. 6 and 7 which 
comprise at least two plates parallel to the measuring axis .tau.X; in 
this case, the spacing between the two plates is constant and the movement 
of the test body 6 causes displacement of one of the plates with respect 
to the other and, consequently, variation of the capacity. 
In the example shown in FIG. 2, the lateral sides of the test body comprise 
two extensions 16, 17 extending outwardly beyond the points at which 
flexible blades 4, 5 are joined to the test body 6. These two lateral 
extensions 16, 17 each comprise a rectilinear edge 18, 19 perpendicular to 
the sensitive axis .tau.X of the structure and extending substantially in 
the plane of the corresponding flexible blade. These edges 18, 19 are 
metallized and form two capacitor plates. 
Furthermore, the fixed part 1 comprises two wings 20, 21 having, parallel 
to the metallized edges 18, 19 and at right angles thereto, two respective 
metallized edges 22, 23 forming two capacitor plates associated with the 
plates formed by edges 18, 19. 
In this case, the electrical connection between the metallized edges 18, 19 
and the circuit carried by the fixed part 1 is provided by metallizing the 
thin faces of the flexible blades 4, 5 defined by edges 7, 8. 
The capacitive detector equipping the pendular structure shown in FIG. 3 
comprises two plates formed by metallizing the two lateral inner edges 9, 
10 of the test body 6, these two plates cooperating with two respective 
plates formed by the metallization of the corresponding parallel edges 25, 
26 of two wings 27, 28 of the fixed part 1 of the pendular structure which 
extend respectively into the space between the two flexible blades 4, 5 
and the inner lateral edges 9, 10 of the test body 6. 
In the example shown in FIG. 4, test body 6 comprises two lateral 
extensions 29, 30 extending towards the outside beyond the points where 
the flexible blades 4, 5 and the test body 6 are joined together, and 
coming back towards the points 2, 3 where said flexible blades 4, 5 are 
anchored to the fixed part 1 of the pendular structure. 
In this case, these two lateral extensions comprise outer lateral edges 32, 
33 perpendicular to the sensitive axis .tau.X of the pendular structure. 
These edges 32, 33 are metallized so as to form capacitor plates which 
cooperate with corresponding parallel metallized edges 34, 35 on the two 
outer lateral wings, 36, 37 of the fixed plate 1 of the pendular 
structure. 
In the example shown in FIG. 5, the inner longitudinal edge 11 of test body 
6 and the corresponding inner edge of the fixed part 1 of the pendular 
structure have crenelated shapes which comprise square projections each 
having a front edge portion and first and second lateral edge portions, 
which extend perpendicularly to said front edge portion, the square 
projections of each of said inner edges being separated from each other by 
square notches, and the square projections on one of these edges engaging 
in the square notches of the other inner edge. 
The lateral edge portions of these two edges 11, 12 are then metallized so 
as to form capacitor plates, which cooperate therebetween, so as to form 
flat capacitors whose capacity varies depending on the movements of the 
test body. 
FIG. 5 shows one embodiment of the connections of the plates of these 
capacitors. In this embodiment, the first lateral edge portions of the 
square projections of the lower longitudinal edge 11 of the test body 6 
have metallizations which are connected to an AC voltage source +E by 
conductors 38, whereas the metallizations of the second lateral edge 
portions of this edge are connected by conductors 39 to an AC voltage 
source -E, in phase opposition with the voltage source +E. The difference 
voltage is collected on the metallizations of the lateral edge portions of 
the inner edge 12 of the fixed part of the pendular structure, which are 
connected in parallel by a conductor 40. 
The advantage of the above described structure is that first of all it 
allows the center of gravity G of the test body to be placed exactly in 
the middle of the length of flexible blades 4, 5 (which, under the 
influence of a transverse acceleration .tau.Y, allows the test body 6 to 
move in a translational movement) or even closer to the feet 2, 3 of the 
flexible blades 4, 5, which reduces the stresses exerted on said blades 4, 
5 when the test body 6 is subjected to a transverse acceleration .tau.Y. 
This solution further allows an appreciable increase in the area of the 
detectors and, consequently, an increase in sensitivity. It further allows 
the distances between the detection zones and the difference signal 
amplification circuit to be reduced and consequently reduces the risks of 
disturbance. 
Of course, the longitudinal edge of the test body, opposite the inner 
longitudinal edge 11, and a corresponding edge of the fixed part may also 
have crenelated shapes with mutually interpenetrating square projections 
and notches. In this case, in a similar way, the lateral edge portions of 
these indentations may then be metallized so as to form capacitor plates. 
This solution, which is shcwn with broken lines in FIG. 5, allows an 
appreciable increase of the detection areas to be obtained and, 
consequently, an increase in sensitivity. 
As mentioned above, the capacitive detectors equipping the pendular 
structure of the invention may be of the lateral movement type. 
FIG. 6 shows one embodiment of such a detector in a pendular structure of 
the type shown in FIG. 1, but in which the test body 6 comprises two 
central recesses 41, 42 separated by a central strip 43 substantially 
parallel to the flexible blades 4, 5. 
This central strip 43 comprises on one side two parallel conducting areas 
44, 45 spaced slightly apart from each other and formable by flat 
metallization. These two conducting areas 44, 45 which form two capacitor 
plates cooperate with a conducting area 46, parallel to the plane of the 
test body 6 and separated therefrom by a distance for example of the order 
of a few hundredths of a millimeter. This conducting area 46 is supported 
by a frame 47 firmly secured to the fixed part 1 of the pendular 
structure. 
At rest, the conducting area 46 is centered in the median plane of symmetry 
of the two conducting areas 44, 45 so that, when the test body 6 moves in 
its plane, the capacity of the flat capacitor formed by the conducting 
area 46 and one of the two conducting areas 44, 45 of the test body will 
increase, whereas the capacity of the flat capacitor formed by the 
conducting area 46 and the other conducting area 44, 45 of the test body 6 
will decrease. 
In this example, areas 44, and 45 are connected respectively to two AC 
voltage sources in phase opposition +E and -E, whereas area 46 is 
connected to a circuit for detecting and amplifying the difference signal. 
In the example shown in FIG. 7, the inner longitudinal edge 12 of the fixed 
part 1 of the pendular structure is below the anchorage point 2, 3 or feet 
of the flexible blades 4, 5. Moreover, the test body 6 comprises, in the 
vicinity of this edge 12, a zone on which is formed a plurality of pairs 
of conducting areas of the same type as areas 44, 45 used in the pendular 
structure shown in FIG. 6. These pairs of conducting areas are connected 
in parallel and form two crenelated circuits 50, 51 whose projections are 
intermingled. One of these circuits 50 is connected to an AC voltage 
source +E, whereas the other circuit 51 is connected to an AC voltage 
source -E in phase opposition with source +E. 
These pairs of conducting areas each cooperate with a corresponding 
conducting area on a frame 47 in a way similar to that described in 
connection with FIG. 6. These conducting areas, which are in the form of a 
comb 53, are connected to a difference signal detection and amplification 
circuit. 
With such an arrangement, the gain of the sensor may be increased and/or 
the height of the pendular structure decreased for a given gain (better 
use of the area of the substrate). 
Furthermore, with such an arrangement the two faces of the test body may be 
equipped with two respective coils for servo-controlling the sensor, as 
will be described hereafter. Of course, as in the case of the embodiment 
shown in FIGS. 5 and 6, it allows the center of gravity G of the test body 
to be positioned as required. 
FIG. 8 illustrates one embodiment of the connections of a sensor of the 
type shown in FIG. 7, whose test body is equipped with two coils (one per 
face 55, 56). 
In this example, the indented circuit 50 of the capacitive detector is 
connected to a connection area 57 situated on the fixed part 1 of the 
pendular structure by means of an electrical connection provided on the 
front face of the pendular structure and comprising a conducting strip 58 
extending flat along the side edge portion of the test body 6 or even if 
required along the edge 10 thereof, this conducting strip being connected 
to a metallized one of the thin faces of the flexible blade 5, itself 
connected, at the level of foot 3, to the connection area 57. Similarly, 
the indented circuit 51 of the capacitive detector is connected to a 
connection area 60 provided on the rear face 56 of the fixed part of the 
pendular structure by means of an electric connection comprising a 
conducting strip 61 extending along the lateral edge portion of the rear 
face 56 of the test body 6 or even along the edge 9 thereof, this 
conducting strip being connected to a metallized one of the thins faces of 
the flexible blade 4, itself connected, at the level of foot 2 to the 
connection area 60. 
Furthermore, the two faces of test body 6 each comprise a return motor coil 
62, 63 formed for example by a flat metallization which extend to the 
periphery of the test body 6 around the central recess 15. 
The outer end of coil 62 which extends over face 55 is connected to a 
connection area 64 provided on the fixed part 1 of the pendular structure 
by an electrical connection formed for example by a metallization on the 
other thin face of the flexible blade 4. 
The inner end of coil 62 is connected to the inner end of coil 63 (situated 
on the other face 56), by a current passage 65 formed on the edge 
surrounding recess 15. 
The outer end of coil 63 is connected to a connection area 66 provided on 
the fixed part 1 of the pendular structure, by an electrical connection 
formed for example by a metallization on the other thin face of the 
flexible blade 5. 
It is clear that the connection areas 57, 60, 64, 66 correspond to the 
input/output terminals of the sensor, terminals 64, 66 serving for the 
return motor and terminals 57, 60 for energizing the capacitive detector. 
Of course, the invention is not limited to the method of connection 
described above. Thus, for example, the test body could comprise only a 
single coil. Furthermore, the current passages from one face to the other 
of the test body could be formed, either by the edge, and/or by metallized 
holes. 
It can be seen that, in the example shown in FIG. 8, the two thin faces of 
each flexible blade 4, 5 are metallized which, from the mechanical point 
of view, forms an appreciable advantage. 
As mentioned above, for servo-controlling the above described accelerometer 
sensors, a motor is required for returning the test body for 
counterbalancing the external action. This return motor may use, in a way 
known per se, the Laplace force obtained by the action of a magnetic 
induction on a current flowing in at least one coil printed on one of the 
two faces of the test body. 
One embodiment of such a return motor is shown schematically in FIGS. 9 and 
10. 
This return motor comprises two coils 62, 63 mounted respectively on the 
two faces of the test body 6 as in the case of FIG. 8. Each of these coils 
62, 63 has a rectangular shape and comprises two lateral zones 70, 71, 72, 
73 in which the turns are rectilinear and have a current flowing 
therethrough in opposite directions. 
The magnetic circuit is formed of a permanent magnet 74, magnetized cross 
wise, two pole pieces 75, 76 associated with the magnet 74 and disposed on 
one side of the test body 6 at right angles to the two zones 70, 72 and 
71, 73 of coils 62, 63 and a flux relooping armature 77 situated parallel 
to the other face of the test body. 
The winding direction of the turns of the two coils 62, 63 is provided so 
that the Laplace forces generated by the magnetic induction B on zones 70, 
72 and 71., 73 of coils 62, 63 are orientated in the same direction (in 
the plane of the test body) and are added to each other. 
It will be noted that, so as not to overload the drawing in FIG. 9, only 
two coil turns have been shown for each face of the test body 6. Of 
course, in practice, the number of turns of these coils may be much 
higher. 
The metal deposited on the pendular structure for forming the electric 
circuits (coils, capacative detectors, connections, etc. . . . ) may be 
gold or a gold alloy such as a chromium-gold alloy for example. However, 
according to an advantageous feature of the invention, these circuits are 
preferably made from silver or aluminium, which metals have more 
especially better electric conductivity and a smaller specific mass. 
The above described accelerometer sensor may be housed in a case comprising 
a base and a cover. In this case, the assembly formed by the permanent 
magnet and the pole pieces may be integral with the base, whereas the flux 
relooping armature may be supported by the cover. Such an arrangement 
considerably simplifies the assembly of the sensor.