Flat lamination solenoid

Details Flat lamination solenoid Flat lamination solenoid

2000-12-19

2003-04-22

Shaver, Kevin (Department: 3732)

Valves and valve actuation

Electrically actuated valve

Including solenoid

Reexamination Certificate

active

06550745

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to solenoids using ferromagnetic armatures subdivided into laminations to reduce eddy current losses. It relates more specifically to a lamination stacking geometry that combines good electrical/magnetic properties with high mechanical strength. It further relates to the use of stacks of slotted laminations, to provide an armature with high strength, reduced weight, high flux handling, and low eddy current losses. This invention is applicable especially to actuation solenoids for automotive engine valves.
BACKGROUND OF THE INVENTION
Most solenoids are fabricated from iron or silicon steel alloys, where silicon alloying causes a large increase in electrical resistivity, which is traded off against a small decrease in flux handling capacity. Even with silicon steels, however, eddy current losses present significant performance problems in two broad classes of solenoids.
The first eddy-sensitive class is solenoids that are excited by AC rather than DC currents. AC excitation offers certain advantages, most notably, inductive self-limiting of current, so that an open AC solenoid pulls the high current needed to close, while the closed solenoid pulls a much lower current needed to maintain latching, the current reduction arising from the higher inductance of the closed solenoid. AC solenoids are generally constructed of laminations rather than solid metal, in order to reduce power dissipation by eddy currents and prevent overheating.
The second eddy-sensitive class is high performance solenoids that are excited by DC or pulse width modulated AC or DC and that are designed to move and be energized and de-energized very rapidly, often with a need for tight magnetic control or servo control of motion, and possibly actuated very frequently. Significant in this class are dual-acting solenoids used to open and close cylinder valves in automotive engines. Rapid energization and de-energization induces large eddy currents in unlaminated metal solenoids, with several adverse consequences. First is the matter of heating and power dissipation, which become significant for solenoids that are operated very frequently. Second is the dissipation-related issue of output capacity for the solenoid power supply and switching electronics—capacity that must be increased to overcome eddy current losses. Third is the issue of response speed, which is slowed when eddy currents oppose the magnetomotive force of winding currents. Eddy current phase lag and reduced response bandwidth compromise both the speed and precision achievable with servo control.
While tubular solenoids and open-frame solenoids using a single bent piece of metal are common in DC and low performance applications, stacked laminations in an “E-I” or “U-I” configuration are typical of laminated designs, as illustrated respectively in
FIGS. 1 and 2
by assemblies
101
and
201
. The “E” core yoke of
FIG. 1
includes both E-shaped yoke laminations and a single electrical winding,
120
, drawn with a smooth outer surface (e.g., a paper wrapping) and a circular or spiral pattern visible on the bottom of the winding. The “U” core yoke at
201
of
FIG. 2
includes U-shaped laminations and two electrical windings,
220
and
225
, shown surrounding the two legs of the “U”. These two windings are typically wired either in series or in parallel with reinforcing magnetomotive forces, promoting the flux loop through the “U” and “I” cores and across the gaps of width indicated at
240
. The moving armature element in a laminated solenoid may consist of a stack of “I” laminations forming a flattened rectangle, e.g., armature
130
of
FIG. 1
or armature
230
of FIG.
2
. The typical mechanical solenoid configuration is similar to transformer configurations, except that in a transformer the “I” laminations are placed on alternating sides so that the “E” or “U” laminations interleave with the “I” laminations. In a solenoid, the laminations do not interleave, and the “I” laminations are all stacked on one side as a moveable armature, as shown with
130
and
230
, or else a solid slab of metal substitutes for the “I” lamination stack. Magnetic flux travels in a loop around the box formed by a “U-I” pair of lamination stacks, as through yoke
210
, across air gap
240
, into armature
230
, back across gap
240
on the opposite side, and returning to
210
to complete the circuit. As the armature moves axially to close gap
240
, the reluctance of the magnetic circuit excited by windings
220
and
225
is reduced, reaching a minimum when the armature approaches or contacts the yoke, closing the magnetic circuit with minimal air gaps. In the case of an “E-I” pair, the flux path describes a pair of loops, going through the center of the “E”, e.g., of
110
, across gap
140
to armature
130
, splitting into separate paths to travel to the ends of
130
, back across gap
140
to the outer fingers of
110
, and completing the circuit as the separate flux paths converge back to the middle of
110
. In either the “U-I” or “E-I” configuration, most flux completes a full loop within the plane of individual pairs of laminations of the yoke and armature. Eddy currents induced by such a flow of magnetic flux tend to circulate in a plane perpendicular to the direction of the B-field. Since the B-field itself flows in the parallel and typically flat planes of the laminations, the plane in which eddy current loops tend to circulate is chopped up by the laminations, as is desired so that the laminations inhibit the eddy currents.
The disadvantage of an armature consisting of a relatively deep stack of narrow “I” laminations is that it is inherently weak against bending moments in a direction tending to cause separation of the laminations. In the “E-I” configuration of
FIG. 1
, it may be necessary to reinforce and strengthen the armature in various ways that add weight and, sometimes, introduce undesirable eddy current paths, partially defeating the function of the laminations. In engine valve solenoids, common practice has been to use a solid unlaminated armature, accepting the penalty in eddy current performance in order to achieve strength. Thus, there are inherent difficulties in achieving a mechanically robust armature using laminations to good advantage.
Note that the figures do not show components for coupling solenoid armatures to a mechanical load. Typically, a shaft would connect to, or penetrate through, the center of the armature lamination stack of
FIG. 1
or of FIG.
2
. The figures omit these details to focus attention on the configuration of magnetic lamination material.
The prior art offers examples of armature laminations stacked in a plane perpendicular to the axial direction of motion, but not in solenoids structurally or functionally similar to the present invention. As will be shown, the present invention relates to variable reluctance actuators in which an armature closes an axial magnetic gap with a yoke structure. Magnetic reluctance in such solenoids changes abruptly with the closure or near-closure of that axial gap, producing rapid armature flux changes acting strongly to produce eddy currents. It is characteristic of such solenoids to exert high forces over short ranges near closure, with highly nonlinear characteristics. It is also characteristic of such solenoids to produce high bending stresses in their relatively thin rectangular or disk-shaped armatures. In U.S. Pat. No. 4,395,649, Thome et al. illustrate a solenoid adapted for inducing vibrations, based not on axially disposed armature and yoke with a closing axial gap, but rather on radially-disposed armature and yoke with a non-closing radial gap. The variation of reluctance with armature position is smooth, not abrupt, avoiding the abrupt shifts in magnetic flux that tend strongly to excite eddy currents in Applicant's context. Thome et al. do not discuss the relationship between lamination orientation and eddy currents. The armature taught by Thome et al. is a relatively deep cylinder, not a thin rectangle or disk, so that

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