Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – With magneto-mechanical motive device
Reexamination Certificate
2001-06-21
2003-04-29
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
With magneto-mechanical motive device
C335S258000
Reexamination Certificate
active
06556114
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of electromagnetic actuators and in particular those forming part of accelerometers intended for geophone applications. Geophones allow very detailed analysis of the nature of the Earth's strata with a view to detecting any sources of oil. Excitation sources send mechanical waves into the ground and these act on the accelerometer, its response providing information about the nature of the Earth's strata encountered by the mechanical waves. In such an application the actuator operates with a travel of very small amplitude and must have very high accuracy.
2. Discussion of the Background
An electromagnetic actuator comprises a stationary stator element coupled to a magnet, a coil and a movable polar element. The actuator is intended to displace an external member secured to the movable polar element.
In an accelerometer, the movable external member is a movable mass coupled to a slaving system with means for detecting its position. The slaving system delivers a slaving current to the coil so as to compensate, through electromagnetic induction phenomena, for a displacement of the mass under the effect of external conditions, for example seismic movements.
The value of the acceleration experienced by the mass is deduced from the value of the slaving current flowing through the coil which is necessary to prevent the displacement of the mass.
The electromagnetic actuator can be employed in other applications in which it operates in a similar manner, for example, the external member to be displaced can be a valve needle which the movable polar element must displace from an initial position to a final, working position. Means for detecting the position of the needle or of the movable polar element are provided so as to control the supply to the coil through a slaving current which allows a displacement of the movable polar element through electromagnetic induction. In this application, very high accuracy in the displacements is also required.
FIG. 1
diagrammatically illustrates an exemplary accelerometer incorporating an electromagnetic actuator of known type.
The electromagnetic actuator referenced
10
comprises a stationary stator element
12
comprising a radially magnetized permanent magnet
5
. The magnet
5
surrounds a coil
6
inside which there is a movable polar element
14
secured to a mass
2
to be displaced. The stationary stator element
12
and the movable polar element
14
define between them at least two gaps
51
and form a magnetic circuit. The magnet
5
creates magnetic field lines in the magnetic circuit across the gaps. In the example, the stationary stator element
12
is in the form of an enclosure represented as a cylinder of revolution and the magnet is represented as an annulus. In the accelerometer application, the mass
2
is suspended from a peripheral frame
3
by arms
80
,
81
. The peripheral frame
3
is fixed to the stationary stator element
12
. A system
7
for slaving the position of the mass
2
to be displaced and hence the position of the movable polar element
14
generates a slaving current in the coil
6
as long as the mass to be displaced is not in a reference position. This slaving system has the effect of preventing any displacement of the mass
2
under the effect of external conditions, for example seismic movements. More precisely, the slaving system
7
comprises means
82
,
83
for detecting the position of the movable mass
2
and hence of the movable polar element
14
and means
70
for causing a slaving current to flow through the coil
6
when the mass leaves a so-called reference position, in such a way as to displace the movable polar element
14
through electromagnetic induction phenomena until the mass
2
reverts to its reference position. The movable polar element
12
compensates for the displacement which the mass
2
undergoes under the effect of the external conditions.
The value of the displacement which the movable mass
2
would have undergone, under the effect of the external conditions, is deduced from the value of the slaving current required to prevent the displacement of the movable mass
2
.
In the electromagnetic accelerometer application, the means for detecting the position of the mass may be piezoelectric strain gauges
82
,
83
placed on the arms
80
,
81
for suspending the mass
2
.
When the supply to the coil
6
is off, the movable polar element
14
is subject to various magnetic fields represented by arrows in FIG.
1
. These arrows relate to the magnetic fields created by the permanent magnet
5
. The polar element
14
is in unstable equilibrium and tends to move along an axis YY′ perpendicular to the mid plane of the magnet
5
and leaves a middle position represented by the axis AA′ and thereby displaces the movable mass
2
. In this middle position, the movable polar element
14
is placed symmetrically with respect to the permanent magnet
5
.
When the slaving system
7
comes into play and a slaving current flows through the coil
6
, it creates a compensating magnetic field (not represented) which tends to return the movable polar element
14
to the mid plane AA′ and hence to return the movable mass
2
to its reference position.
SUMMARY OF THE INVENTION
Given the great sensitivity demanded of such accelerometers, which have to detect very small accelerations, the components of the actuator must be perfectly dimensioned and also positioned with respect to the mass, to the peripheral frame and to the strain gauges before commencing a series of measurements.
It is desired that, when the accelerometer is switched on, in the absence of motion, the mass
2
should be in its reference position, this corresponding to a strainfree state for the piezoelectric strain gauges
82
,
83
. This state corresponds to an “electrical zero”. This condition implies that the base of the stationary stator element
12
, which is secured to the peripheral frame
3
, and the face of the non-magnetic element which is fixed to the mass
2
should be in one and the same plane to within around a micrometer. This non-magnetic element forms the interface between the movable polar element
14
and the mass
2
since the movable polar element
14
is generally not connected directly to the mass
2
. This positioning between the stationary stator element
12
and the non-magnetic element corresponds to a “mechanical zero”. This condition also implies that the movable polar element
14
be in a position of “magnetic zero”. In this position the vector sum of the forces applied to the movable polar element
14
is zero. If the components of the actuator had ideal dimensions, the magnetic field lines in the actuator would be distributed in a substantially equal manner between the two gaps
51
.
In point of fact, the succession of dimensions involved in the construction of the components of the actuator can only ensure an accuracy of 10 micrometers if one wishes to avoid an unintended increase in the cost of construction of these components. It is then appreciated that, even if the components of the actuator are assembled with care, on turning on the accelerometer, in the absence of motion, the mass
2
is not in the “electrical zero” position and that the actuator is not in the “mechanical zero” position even if the movable polar element
14
is in the “magnetic zero” position. The quality of the measurements performed suffers.
To overcome the problems of dimensioning the components of the actuator and to facilitate their mounting and their positioning, the present invention proposes that a polar element for adjusting the position of the movable polar element with respect to that of the stationary stator element be included in the actuator. An adjustment can then be performed before the electromagnetic actuator is operated.
More precisely, the subject of the invention is an electromagnetic actuator intended to displace an external member, comprising a stationary stator element associated with a mova
Giroud Pierre
Guillemin Pierre
Donovan Lincoln
Thales Avionics S.A.
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