Electricity: electrical systems and devices – Control circuits for electromagnetic devices – For relays or solenoids
Reexamination Certificate
1997-06-26
2001-03-27
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
For relays or solenoids
C361S154000
Reexamination Certificate
active
06208497
ABSTRACT:
BACKGROUND
1. Field of the invention
The present invention relates to systems and methods for controlling the movement of mechanical devices. More particularly, the present invention relates to the servo control of electromagnetic devices. Still more particularly, the present invention relates to the servo control of solenoids using the measurement of position and the approximation of position of the solenoid's armature to regulate movement of that armature. The present invention may be used in a variety of areas where lifting and/or propulsion is desired with minimum energy consumption.
2. Description of the Prior Art
A solenoid is a linear motor, inherently capable of efficient conversion of electrical to mechanical energy. In rotary motors, experience teaches that large size favors efficiency, and for a given size motor, the highest efficiency is obtained when there are very close clearances between stator and rotor parts and when operation is at high RPMs. Electrically speaking, a high frequency of magnetic reversals translates into a high rate of transfer of electromagnetic power. At low frequencies, resistive power losses wipe out efficiencies, while at constant magnitudes of peak magnetic flux, higher frequency translates into higher power transfer without significant increase in I
2
R resistive power loss. To avoid the eddy current losses associated with high frequency magnetic fields, rotary motors employ laminations in magnetic steels, or high-resistivity ferrite parts. Steels have a large advantage over ferrites at moderately low frequencies (particularly below 1 KHz) in their ability to handle flux densities up to about 2 Teslas, compared to ferrites at up to about 0.5 Teslas. The 4-to-1 advantage in flux density translates into a 16-to-1 advantage in energy density and magnetic force. Translating the rotary motor rules into the realm of solenoids, one can expect that efficient operation is fast operation. A fast solenoid must have a low shuttle mass, or alternatively, shuttle inertia may be cancelled by resonating its mass with a spring at the design operating frequency (as is done, e.g., in tuned magnetic vibrators for aquarium diaphragm pumps and barber clippers). As the counterpart of close clearances in rotary motors, solenoids operate efficiently at very short operating strokes, relying on high force and high frequency of operation to raise the power/weight ratio. Short strokes are effective only if, at the end of a power stroke, the entire magnetic circuit closes with minimal air gaps—a matter of efficient design. For a solenoid shuttle in non-resonant operation, a short stroke translates into a short stroke time, amounting to operation at high frequency and a high rate of change of magnetic flux, “&PHgr;,” as the magnetic gap closes. A high rate of change of flux, i.e., a large “d&PHgr;/dt,” translates into a high induced magnetic voltage in relation to resistive voltage. Induced voltage represents conversion between electrical and mechanical energy, while resistive voltage represents energy loss, so a large “d&PHgr;/dt” translates into high efficiency.
There are and will always be solenoids designed for utilitarian binary control operations, e.g., unlocking the downstairs front door: contexts where power efficiency is of minor importance and stroke length is a matter of feasibility and convenience, rather than a matter of efficient motor design. Magnetic steel solenoid parts are typically solid rather than laminated, because eddy current losses in dynamic operation are not a design consideration. Moving from the context of infrequent operation of a door latch to the very frequent operation of a print wire driver in a dot matrix print head, repetitive impact and consequent work hardening of the magnetic steel in a solenoid becomes a serious consideration. Magnetic materials for solenoids should ideally exhibit a low coercive force, i.e. a low inherent resistance to change in magnetic flux. In magnetic steels, low coercive force correlates with a large crystalline structure, attained through high temperature annealing to allow growth of large crystals. Annealed steels are mechanically soft and ductile, and their low-coercive-force property is described as magnetically soft. Repetitive stress and shock break up large crystals in steel, yielding a finer grain structure that is mechanically work-hardened and magnetically harder. Permanent magnets are optimized for high coercive force, or high magnetic hardness: the ability to retain magnetization against external influences. In solenoids, the mechanical work hardening of the steel takes place in a strong magnetizing field, leaving permanent magnetism in the solenoid circuit. The result is to cause the solenoid to stick in its closed position after external current is removed. This is a failure mode for print wire solenoids. A standard approach to keep solenoid parts from sticking is to cushion the landing at full closure, leaving an unclosed magnetic gap, typically through the thickness of the cushion material. This residual gap generates resistance to residual flux after power removal, reducing the tendency of the shuttle to stick closed. Residual magnetic gaps compromise efficiency in two ways: because the most efficient part of the magnetic stroke is approaching full gap closure, where the ratio of force to electric power dissipation is high; and because currents for maintaining extended closure must be made substantially higher to overcome the magnetic resistance of gaps.
Prior art techniques for servo control of solenoid motion and, more generally, magnetic actuation, are summarized well in the introductory section of U.S. Pat. No. 5,467,244, issued to Jayawvant et al: “The relative position of the object is the separation or gap between the control electromagnet and the object being controlled and in prior art systems is monitored by a transducer forming part of the control signal generator for the feedback loop. Such transducers have included devices which are photocells (detecting the interruption of a light beam by movement of the object); magnetic (comprising a gap flux density measurement, e.g. Hall plate); inductive (e.g. employing two coils in a Maxwell bridge which is in balance when the inductance of the coils is equal); I/B detectors (in which the ratio of the electromagnet coil current and magnetic flux is determined to provide a measure of the gap between electromagnet and object; for small disturbances the division may be replaced by a subtraction); and capacitative (employing an oscillator circuit whose output frequency varies with suspension gap).” Dick (U.S. Pat. No. 3,671,814) teaches magnetic sensing with a Hall sensor. In the succeeding description of “Apparatus for the Electromagnetic Control of the Suspension of an Object” Jayawant et al derive, from a generalized nonlinear electromagnetic model, a linearized small perturbation model for use in magnetic suspension of an object in the vicinity of a fixed target position. Specifically, they make use of what they call “I/B detectors” (see above quote) wherein the ratio of current “I” divided by magnetic field strength “B” provides an approximately linear measure of the magnetic gap. In text to follow, the ratio “I/&PHgr;” will be used in preference to “I/B” since inductive voltage measurements lead to a determination of the total flux “&PHgr;” rather than a local flux density “B.” Specifically, as noted by Jayawant et al, the time derivative “n·d&PHgr;/dt” equals the voltage electromagnetically induced in a winding of n turns linked by the magnetic flux “&PHgr;.” Thus, time integration of the voltage induced in a coil yields a measure of the variation in “&PHgr;,” and additional direct measurement or indirect inference of “I” leads to a determination of the ratio “I/&PHgr;” used to close the servo loop. Where electrical frequency is substantially higher than the frequency associated with solenoid mechanical motion, the ratio “I/&PHgr;” is also the ratio of the time derivatives “(dI/dt)/(d&PHgr;/dt),” so that a measurement of the hi
Bergstrom Gary E.
Seale Joseph B.
Fleming Fritz
McDermott & Will & Emery
Venture Scientifics, LLC
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