Linear actuator

Details Linear actuator Linear actuator

1999-11-02

2001-05-22

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Linear

Reexamination Certificate

active

06236125

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of linear actuators with permanent magnet, which in general is thin. Such linear actuators comprise a stator structure having two poles excited by an electric coil, plus a moving part provided with a yoke and a magnetized portion.
2. Discussion of the Background
The general principle of such actuators is described in French Patent FR 97/10585.
Such actuators employ a magnetic structure comprising three thin magnets magnetized in alternating directions.
A first disadvantage of the prior art actuators is the need for three permanent magnets. High-performance magnets are relatively expensive, and for this reason the prior art structures have a high manufacturing cost.
A second disadvantage is related to the fact that the alternation of polarity of the thin magnets makes it necessary to magnetize the magnets before assembly, and thereafter to bond the three magnets adhesively to the yoke.
A third disadvantage of the prior art actuators results from the fact that the magnetostatic force produced by the architecture of these actuators tends to pull the moving portion backward strongly in the middle of its travel, thus causing problems for certain applications.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an improved actuator which remedies these disadvantages while retaining a satisfactory force per ampere-turn.
In the state of the art there have been suggested actuators which have a moving portion formed by a magnet embedded in a yoke of soft magnetic material. Such an embodiment is suggested in U.S. Pat. No. 5,175,457, or in
FIG. 1
of International Patent WO 86/05928. The Applicant has established that the force is not constant as a function of position when the number of ampere-turns applied to the coil is increased (see FIG.
1
).
Embodiments have also been suggested in which the magnet is fixed on the surface of the yoke. Such an embodiment is suggested in U.S. Pat. No. 4,195,277. For such actuators, the force remains substantially constant along the useful travel. In this case, the Applicant has established that the efficiency is poor, and in any case inferior to the efficiency of actuators of the first type (see FIG.
2
).
The purpose of the invention is to provide an actuator having a substantially constant force along the travel with improved efficiency. The invention which is the object of the present patent results from the Applicant's analysis of the influence of position of the magnet in the interior of the yoke on the characteristics of an actuator, and from the design, on the basis of the said analysis, of a new type of actuator having optimized performance.
To this end, the invention relates to a linear electromagnetic actuator comprising a stator structure having two poles excited by at least one electric coil, plus a moving portion provided with a yoke and a magnetized portion, characterized in that the moving portion is provided with one or two permanent magnets magnetized in a direction perpendicular to the plane of the air gap, seated in a cavity provided in the moving yoke, which is made of ferromagnetic material.
The cavity depth e is chosen judiciously so as to increase the force delivered by the actuator compared with the case without cavity (e=0), while retaining a substantially constant force along the useful travel.
When the actuator is not saturated, the force created along the axis OX of the degree of freedom of the moving portion can be resolved into three components: the magnetostatic force F
0
(without current), which is often negligible over the useful travel Xc, a polarized force component F
nI
proportional to the ampere-turns nI, and a force component F
nI
2
proportional to the square of the ampere-turns, due to the variable reluctance created by the cavity (see FIGS.
1
and
2
):
F≈F
nI
+F
nI
2
For a given value nI of ampere-turns, the force components increase with increase of the thickness e of the cavity in which the magnet is placed (see the notations in FIG.
4
).
Without cavity, e=0, F
nI
2
=0.
The force component F
nI
is almost constant along the useful travel, regardless of the value of e. In contrast, the component F
nI
2
varies linearly with position. To obtain a substantially constant force over the entire travel for a given value of ampere-turns, it is necessary to have a relatively low F
nI
2
/F
nI
ratio, for example lower than 15%. This ratio can be expressed to a first approximation by an equation of the type
F
nI
2
F
nI
=
0.25
·
X
2
I
-
X
·
ni
H
c
·
L
where X=e/E represents a coefficient of embedding and E denotes the air gap between the bottom of the cavity in which the magnet is placed and the plane passing through the surface of the stator poles, without deduction of the thickness of the magnet;
nI is the magnetic potential created in the magnetic circuit by the current passing through the coil or coils;
H
c
·L is the magnetic potential of the magnet, H
c
is its coercive field and L is its thickness in the magnetization direction.
The F
nI
2
/F
nI
ratio strictly increases with cavity depth e.
For a given magnet and value of ampere-turns, the cavity height e is chosen to be as large as possible, in order to increase the force while keeping the F
nI
2
/F
nI
ratio relatively low, for example lower than 0.15, in order to obtain a substantially constant force along the travel.
The cavity depth e is then chosen judiciously (0.1 L<e<0.9 L), by appropriate calculation and/or simulation, in order to optimize the force delivered by the actuator.
For very small values of ampere-turns (ni<100 At), the cavity depth will preferably be greater than 50% of the thickness of the magnet, most preferably on the order of 80%.
For large values of ampere-turns, the cavity depth will preferably be less than 50% of the thickness of the magnet, most preferably on the order of 40%.
The relation defining the F
nI
2
/F
nI
ratio can be determined rigorously and precisely by taking into account the leaks and relative permeability of the iron.
According to a first modified embodiment, there are used two magnets, magnetized in the same direction and partly embedded in two cavities, each situated at one end of the ferromagnetic yoke.
According to a first modification, the stator structure is provided with two legs, each wound by an electric coil.
According to a second modification, the stator structure is of tubular shape and is provided with an internal annular recess in which there is seated an annular coil, the moving portion being formed by a ferromagnetic internal tubular yoke provided with an annular recess of depth e in which there is positioned a radially magnetized annular magnet.
According to a third modification, the actuator according to the invention comprises an internal stator structure of tubular shape and is provided with an annular recess in which there is seated an annular coil, the moving portion being formed by a ferromagnetic external tubular yoke provided with an annular recess of depth e in which there is positioned a radially magnetized annular magnet.
According to another modification, the linear actuator according to the invention comprises two moving portions, the magnets of one moving portion being magnetized in the direction opposite to that of the magnets of the other moving portion under the effect of the current, the two moving portions being displaced in opposite directions.


REFERENCES:
patent: 5175457 (1992-12-01), Vincent
patent: 5386275 (1995-01-01), Kato et al.
patent: 5475277 (1995-12-01), Johnson
patent: 5559378 (1996-09-01), Oudet et al.
patent: 6081052 (2000-06-01), Hosoe et al.
patent: 38 20711 (1989-12-01), None
patent: 9301646 (1995-01-01), None

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