Electricity: motive power systems – Synchronous motor systems – Hysteresis or reluctance motor systems
Reexamination Certificate
1999-12-06
2002-05-14
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Hysteresis or reluctance motor systems
C318S638000, C318S639000, C318S700000, C318S799000
Reexamination Certificate
active
06388417
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to variable reluctance dynamic force motors (or actuators) used to convert electrical energy into oscillating or reciprocating mechanical force.
2. Description of the Related Art
To produce a high reciprocating force with reasonable transduction efficiency, one often employs a variable reluctance electromagnetic motor (VR motor). In particular, VR motors are generally lighter and more efficient than electrodynamic (voice coil) motors capable of producing similar peak forces. However, VR motors, as commonly controlled, can exhibit substantial instabilities that must be overcome before useful operation is attained. These instabilities can be particularly severe when the motor stroke is a high percentage of the nominal gap length, a situation often encountered when driving dynamically reactive loads. Also, transient disturbances can also lead to instabilities in VR motors controlled according to common prior art methods.
Representative examples of the manner in which VR motors have traditionally been analyzed and operated can be found in U.S. Pat. Nos. 5,206,839 and 5,266,854 to Murray, and U.S. Pat. No. 5,375,101 to Wolfe et al. The method of control described in this prior art can be generally described as “current control.” The following analyses (and assumptions) are common to these patents:
1. The flux established in a gapped electromagnetic device is proportional to the current in the electromagnet coil. Since the actual current-to-flux relation is described by &phgr;~i/g (where &phgr; is the flux in the gap, i is the current in the coil, and g is the length of the air gap), this assumption is only valid when the gap length g is constant.
2. The force developed across the air gap of a gapped electromagnetic device is proportional to the square of the established flux (F~&phgr;
2
.
3. Therefore, F~i
2
.
If VR motors could be operated effectively by simply appealing to point
3
(namely, that F~i
2
), then one could generate an arbitrary force response by simply controlling the motor coil current to be the square root of the desired force. Unfortunately, for a dynamic force motor to deliver any real power to a load, the “movable” part must, in fact, experience displacement with respect to the “stationary” part (the “movable” and “stationary” parts of the motor are henceforth referred to as the armature and core, respectively). In other words, the gap length g in point
1
is not constant. Operation in this manner (allowing the motor parts to move) violates the “no gap motion” assumption expressed earlier, and, to a greater or lesser extent, invalidates the F~i
2
relationship. Or, put in another way, the actual relationship to consider is F~(i/g)
2
.
One way of overcoming this complication of the F~(i/g)
2
relation is to require that the motor air gap(s) remain nearly constant. For example, if a motor has a nominal 1 mm unenergized air gap, one might choose to limit relative dynamic motion to 0.05 mm, or a 5% excursion of the nominal gap length. It is generally accepted by those skilled in the art that if the gap motion is tightly constrained in this manner, then F~i
2
reasonably represents the behavior of the motor. Furthermore, schemes that establish a substantial bias current in the motor coils and then employ small-signal perturbation techniques with respect to this bias point are common in the prior art. Prior art vibrators, such as the one described in U.S. Pat. No. 3,775,626 to Brosch, have noted that the air gap length must, for some material handling applications, be set large enough to be robust to changes in the operating conditions of the vibratory material handling system.
However, those skilled in the art will also appreciate that, for a given desired dynamic force, one can minimize the corresponding motor coil currents by minimizing the length of the air gap between the armature and the core. To do this, and still have a motor that can experience relatively large strokes, one must allow large gap excursions (up to 100% of the nominal unenergized gap length). In the presence of large gap excursions, the F~i
2
relation is no longer adequate to describe the behavior of the motor, and one must deal with the added nonlinearity exhibited by F~(i/g)
2
.
An important attribute of the present invention (to be discussed in detail in another section) is the inherent stability that it provides in the context of VR motor control. In classical control theory, stability means that the output of the VR motor (either the relative positions of the armature and core, or the force generated between them) will not grow without bound due to a bounded input, initial condition, or unwanted disturbance. In other words, there will always be a reasonable, bounded relationship between the motor action requested via an input signal and the resulting actual motor action, even in the presence of external noise, transients, or other disturbances.
When evaluating the control stability consequences of the F~i/g)
2
relationship that has been discussed previously, it is instructive to consider a simple example. In a one-sided motor structure such as that shown in
FIG. 1
, start with an initial fixed coil current lo that flows through coil
4
, and an initial gap length
12
G
0
(denoted by label
12
in FIG.
1
). Thus, the initial force between the two moving motor parts (core
2
and armature
8
) will be, by F~(ig)
2
, proportional to (I
0
/G
0
)
2
. As this initial force acts to move the two motor parts closer together, the instantaneous gap length will decrease to values smaller than G
0
, and as the gap reduces, the instantaneous force will increase as 1/ g
2
, even while the coil current is held constant. With most common VR motors, the suspension stiffness is linear (F=−kx), but this increase in electromagnetic force is quadratic (1g
2
), so the net effect can be a rapidly increasing force that moves the motor parts together until they collide and clamp. Or, for motor designs such as those described in U.S. Pat. No. 5,266,854 to Murray, the suspension can be stiff enough so that for small displacements from the nominal gap, the motor is stable, but for larger displacements, one experiences such “runaway” instability. In particular, see column 3 and
FIG. 3
of U.S. Pat. No. 5,266,854 to Murray for further details. To more fully stabilize such “current-controlled” motors, one must vary the coil current in relation to the instantaneous gap length or measured magnetic flux via a feedback mechanism, and even these feedback circuits can suffer from bandwidth and gap displacement limitations.
Examples of prior art that addresses these concerns may be found in U.S. Pat. No. 5,621,293 to Gennesseaux, and “Parametric Modeling and Control of a Long-Range Actuator Using Magnetic Servo Levitation”, IEEE Transactions on Magnetics, authored by H. Gutierrez and P. Ro (September, 1998). In Gennesseaux, gap-motion-induced instabilities are compensated via two distinct means: 1) measuring the actual armature motion with a displacement sensor and compensating the coil current accordingly and 2) measuring the air gap flux with a Hall-effect flux sensor and compensating the coil current accordingly. In Gutierrez & Ro, the authors attempt to compensate not only the gap-motion nonlinearities, but also distortions to the first-order relations introduced by material non-idealities, flux density non-uniformity, and the like. In both of these cases, extensive and complex controllers are used to achieve gap excursions larger than is common in the traditional art, but in neither case is evidence presented that large gap excursions, such as gap excursions approaching 100% are possible. U.S. Pat. No. 3,219,919 to Snavely also uses a position transducer and feedback circuit to compensate the F~(i/g)
2
relation.
It is worth noting that, historically, there have been a few predominant applications for VR motors. The first is for acoustic transduction, particularly in the generation of high-amplitude sound waves for und
Foley & Lardner
Macrosonix Corporation
Nappi Robert E.
Smith Tyrone
LandOfFree
High stability dynamic force motor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High stability dynamic force motor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High stability dynamic force motor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2825528