Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
2002-08-14
2004-10-26
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
Reexamination Certificate
active
06807872
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a force transducer intended to constitute the sensitive member of a force, pressure or acceleration sensor.
The invention relates more particularly to a transducer vibrating flexionally whose frequency varies as a function of the intensity of the force applied to it. The intensity of the force is determined by measuring the vibration frequency.
2. Description of the Prior Art
Generally speaking, the transducer comprises a beam with an elongate shape along a longitudinal axis and means for causing flexional vibration of this beam, one end of the transducer is fastened to a fixed part, and the force to be measured is applied to the other end of the transducer, parallel to the axis of the beam. The beam is therefore subjected to an axial tensile or compression stress, as a function of the direction of the force. The flexional resonant frequency of the beam is highly sensitive to this axial stress, and increases in the case of a tensile stress and decreases in the case of a compression stress. These phenomena are explained by the axial stresses modifying the inherent flexional stiffness of the beam; also, the very slight lengthening or shortening of the beam that results from these axial stresses has only a virtually negligible influence on the frequency variation. These phenomena are analogous to the tuning of a musical instrument, which entails modifying the tension in each string until the required resonant frequency is obtained; the principal difference between a string and a beam is that, because of its practically zero inherent stiffness, a string has no intrinsic natural vibration mode and cannot be subjected to axial compression stresses. A vibrating string force transducer therefore requires the string to be prestressed tensionally, which is the source of string vibration frequency instabilities over time or in response to variations of temperature. This is why vibrating beam force transducers are generally preferred. The reader will also have understood that the sensitivity of a vibrating beam force transducer increases as the inherent flexural stiffness of the beam decreases, and therefore in particular as the dimensions of the cross section, i.e. a section taken in a plane perpendicular to the longitudinal axis of the beam, decrease. However, this incitement to reduce the cross section dimensions of the beam has to be tempered by another consideration regarding the quality of the flexional vibration of the beam, which increases as the cross section dimension parallel to the plane of vibration increases. Measures must also be taken to prevent the quality of vibration being degraded by leakages of vibratory mechanical energy out of the device; this is why the transducer generally includes a vibratory mechanical filtering means. The quality of the vibration, as reflected in a quality factor, has an important influence on the measurement noise of the vibration frequency, and therefore of the intensity of the force to be measured. This is why the dimensions of the beam of a force transducer are generally the result of seeking a good compromise between sensitivity to forces and quality of vibration, in order to obtain a good “signal to noise” ratio in the required measurement range.
The flexional vibration of the beam is generally maintained by means of electrodes and an oscillator electronic circuit. For example, in the case of a quartz beam, the vibration is advantageously excited piezoelectrically using electrodes adhering to the crystal. Quartz is also appreciated for its mechanical qualities and for its low cost.
As a general rule, vibrating beam force transducers are appreciated for the excellent stability of their sensitivity to forces and also because, their output magnitude being a frequency, processing the information delivered by the transducer requires no analog-to-digital converter.
FIG. 1
shows a force transducer of the above kind, with two vibrating beams, which is the subject matter of U.S. Pat. No. 4,215,570. The body of the transducer
100
is obtained by machining a flat plate of material. The body comprises two end blocks
11
and two parallelepiped-shaped beams
3
separated by a slot
12
. The two beams
3
vibrate flexionally parallel to the plane of the plate and in phase opposition, as shown in an exaggeratedly enlarged manner by the dashed lines in FIG.
1
. If two beams are sufficiently similar, the vibratory loads that they exert at their ends balance out and the two end blocks
11
therefore hardly vibrate at all, which means that said end blocks can be fixed to the body (not shown) of a force, pressure or acceleration sensor without degrading the quality of vibration of the beams
3
. The transducer
100
therefore provides a function mechanically filtering the vibrations of the beams
3
. This transducer is suitable for measuring forces of ordinary intensities. On the other hand, this transducer has a drawback in terms of effectiveness of the mechanical filtering of the vibrations if the transducer must be sufficiently sensitive to measure forces of low intensity correctly; in this case, which in particular requires significant reduction in the dimensions of the cross section of the two beams
3
, it becomes difficult to conform adequately to the condition requiring identical dimensions of said beams, which in practice degrades the effectiveness of the mechanical filtering and therefore degrades the quality of the vibrations.
To improve this aspect of measuring forces of low intensity, force transducers have been proposed with a single vibrating beam, two examples of which are described hereinafter.
FIG. 2
shows a first force transducer with a single vibrating beam, which is the subject matter of French patent 2,574,209 in the name of the applicant. The body of the transducer
200
is obtained by machining a flat plate of material. The body comprises two end blocks
11
, two rotationally flexible elements
14
, two rigid inertial masses
15
and a parallelepiped-shaped beam
3
. The beam
3
is subject to flexional vibration parallel to the plane of the plate, as shown in an exaggeratedly enlarged manner by the dashed line in FIG.
2
. The role of the inertial masses
15
and the rotationally flexible members
14
is to provide mechanical filtering of the vibrations of the beam
3
so that the two end blocks
11
vibrate hardly at all, which enables said blocks to be fixed to the body (not shown) of a sensor without degrading the quality of the vibrations of the beam
3
. The alternating displacements in rotation of the inertial masses
15
are not visible on the scale of the dashed outline depiction of the deformation in vibration of the beam
3
, because they are much smaller than the amplitude of vibration of the beam
3
, the deformation of which corresponds practically to that of a beam that is built in at its ends; this is due to the fact that the inertial masses
15
are significantly larger than the mass of the vibrating beam
3
. It will be noted that the vibrating beam
3
and the end blocks
11
of this force transducer
200
are respectively analogous to the vibrating beams
3
and the end blocks
11
of the force transducer
11
shown in FIG.
1
.
A second force transducer with a single vibrating beam was proposed for the monolithic acceleration sensor that is the subject matter of French patent 2,739,190 (now U.S. Pat. No. 5,962,786) in the name of the present Assignee as shown in FIG.
3
. The body of this acceleration sensor CA is obtained by machining a flat quartz plate. This body comprises a fixed part
1
constituted of two segments with a disk-like shape, a flexible frame
5
, a test mass
2
, two articulation blades
81
and
82
for the test mass, a second mass part
4
and a force transducer
3
in the form of a single parallelepiped-shaped beam. This beam
3
flexionally vibrates parallel to the plane of the plate, shown in an exaggeratedly enlarged manner by the dashed line in FIG.
3
. If the sensor CA is subjected to a
Janiaud Denis
Le Traon Olivier
Lecorre Bernard
Muller Serge
Laubscher, Sr. Lawrence E.
Noori Max
ONERA (Office National d'Etudes et de Recherches Aerospatia
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