Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – With magneto-mechanical motive device
Reexamination Certificate
2001-02-21
2004-05-18
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
With magneto-mechanical motive device
C335S281000, C335S220000, C251S129150
Reexamination Certificate
active
06737946
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to solenoids. More particularly, it relates to solenoids whose pole faces are shaped for controlled force characteristics. Most particularly, it relates to solenoids for direct actuation of automotive valves, achieving efficient pull-in from a distance, rapid deceleration on approach to closure, and a rapid increase to a latching force, with controllable electromechanical damping associated with latching and unlatching.
BACKGROUND OF THE INVENTION
The following discussion, and the invention to be described, relate particularly to solenoids that close magnetically with high speed, high magnetic force, and a large area of mating magnetic pole faces. Solenoids generating large magnetic forces are especially prone to close with an impact, generating noise and damage to the solenoid. The motions in solenoids and magnetic bearings have been successfully controlled by servo feedback control under limited conditions. For gradual motions and where the magnetic gap does not change by large fractional amounts, stable servo control has been demonstrated. When the magnetic gap changes from open to closed with mechanical poleface contact, and when the motion toward closure is very rapid, servo control is very difficult to achieve, especially for obtaining quick closure to a low-impact landing. This difficult control problem needs a solution if there are to be commercially viable automotive electric valve actuation systems, for example.
The invention to be disclosed in the following specification addresses the solenoid control problem not by way of a new servo controller, but by a fundamental redesign of the solenoid itself. The broad objective of the invention is to achieve electromagnetic characteristics that simplify the task of servo control. To understand the criteria behind this solenoid redesign, it is necessary to gain some understanding of the electrical servo control problem itself. The remainder of this background section examines the control problems that, in the prior art, have been addressed almost entirely from the controller side. This examination provides more detail than would normally be devoted to prior art, including issues that may not be widely understood. The intent of this close examination is to define the context of the current invention in sufficient detail that the design criteria will be understood.
A difficulty with servo control in a magnetic solenoid is that, in order for a controller to change electromagnetic force rapidly, the entire energy residing in the solenoid magnetic field must be altered in roughly the same proportion that the force is altered. To close a solenoid, electrical energy applied to windings is converted to magnetic field energy, which can be considered to reside mostly in the air gaps between the ferromagnetic armature and the magnetic yoke, which is part of the stator structure. For a given flux density, the magnetic energy in high-permeability core material is much lower than in air, in proportion to the relative permeability of the core material (typically in excess of 1000). If the total magnetic flux linking the yoke windings is held constant, then the magnetic field energy will diminish as the armature-to-yoke gap closes, eliminating the high-energy field passing through air. In geometries where armature motion is predominantly parallel to the magnetic flux crossing an air gap, the magnetic force of attraction between the armature and yoke is approximately the partial derivative of magnetic field energy with respect to armature position, evaluated under the constraint that total flux is held constant. As a flux-carrying solenoid gap closes, one can say that the field energy in the gap is transformed into mechanical energy. Conversely, when a narrow flux-carrying solenoid gap is mechanically forced to open, mechanical energy is transformed into magnetic field energy as flux-filled magnetic gap volume is created. In the idealized case of zero applied winding voltage and zero winding resistance, e.g., the case of a shorted superconductive solenoid winding with no source or sink of electrical energy, then pulling an armature away from a magnetized yoke will result in a growing armature-yoke gap filled with magnetic energy, that energy coming entirely from the mechanical work that pulls the armature. A difficulty with servo control of solenoid motion through control of magnetic force is that, in order to change force rapidly, a reservoir of magnetic energy must be filled or depleted rapidly. The magnitude of the energy in that reservoir is on the order of the total energy that would be delivered to the armature if the magnetic gap were closed completely, starting from the gap width for which the energy transfer problem is defined.
While the perspective of energy conversion sheds light on the bounds of possibility in a problem of this sort, viewing the problem in terms of momentum is sometimes more helpful for the control details. “Magnetic momentum” may be understood by direct analogy to mechanical momentum. The expression for mechanical kinetic energy, ½ MV
2
for mass M and velocity V, has its magnetic energy counterpart in ½ LI
2
for inductance L analogous to mass and current I analogous to velocity. Differentiating energy with respect to velocity or its analog, current, yields mechanical and magnetic momentum expressions: MV for mechanical momentum, and LI for magnetic momentum. Inductance L expresses the electrical “inertia” that resists change in current I, and when this inertia is overcome by the continued application of voltage, magnetic momentum LI is altered. A basic equation states that “LI=n&PHgr;,” where “n” is the winding turns count and “&PHgr;” is the total magnetic flux linking the n windings. It is common to refer to the product “n&PHgr;” as the flux linkage. One sees that “flux linkage” is an alternative expression for “magnetic momentum”—the different terms emphasize different aspects of the same quantity. There is thus an “inertia” associated with magnetic flux, making flux difficult to change rapidly. The scaling of this magnetic inertia depends on the winding count, n. By extension of the mechanical analogy, a source of mechanical force and motion may transfer energy to a moving mass with a selectable mechanical advantage or disadvantage, established by a fulcrum or gear ratio. The mechanical advantage is selected for an impedance match between the characteristics of the energy source and the load. In a solenoid, by analogy to a mechanical gear ratio, the winding count n establishes the “electrical advantage” exerted by an electronic driver on the magnetic system. If n is low, the electronic driver has a high “advantage” analogous to “low gear” in a mechanical transmission, so that the driver can alter magnetic flux rapidly for a given voltage output. The problem associated with a low winding count is a high current draw, analogous to a motor having to rev at high speed when driving a load in low gear. Raising the winding count reduces the current draw for a given combination of magnetic flux and field gap, and it also reduces the ability of the driver to alter magnetic flux rapidly. As will be shown, an objective of the present invention is to cause needed changes in magnetic force, in order to direct the course of a solenoid armature toward soft landing, without demanding rapid changes in the total magnetic flux linkage, n&PHgr;. With a reduced tendency toward rapid “flux slewing,” i.e. ramping flux up or down at a rate limited by power supply voltage, the electronic controller can gain better control of mechanical motion. Some of this control advantage can be traded off for a reduced peak current draw, by increasing the winding count, n. The improved electromagnetic design does not “solve” the control problem, but it makes it far more tractable, within engineering and economic constraints.
In order to control solenoid force and therefore cumulatively influence mechanical momentum, one must cumulatively vary magnetic momentum in order to
Bergstrom Gary E.
Seale Joseph B.
Donovan Lincoln
Pierce Atwood
Rojas Bernard
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