Electrical generator or motor structure – Dynamoelectric – Reciprocating
Reexamination Certificate
2001-06-01
2002-09-10
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Reciprocating
C335S274000, C359S198100, C359S199200, C318S114000, C235S462320, C310S029000
Reexamination Certificate
active
06448673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to electromechanical reciprocating rotary motion devices; and in particular to reciprocating angular motion actuators for producing constant amplitude, variable frequency, substantially triangular waveform motion profiles suitable for optical scanning applications.
2. BackGround Art
The use of an oscillating mirror and associated motor assembly is well known in the art for effecting a beam sweeping action. A characteristic of such devices, whether the motor producing the oscillation is a stepper motor or a galvanometer type motor, as is commonly the case, force, generated by current flowing through the motor windings must be used to decelerate the scanning motion and then reverse it. This necessarily generates heat, which is a significant problem in a very small device such as a galvanometer scanner. This heating, for a sinusoidal scan waveform, is proportional to the fourth power of scan frequency, and the square of the scan angle.
position
:
θ
sin
ω
t
velocity
:
ω
θ
sin
ω
t
accelleration
:
-
ω
2
θ
sin
ω
t
=
τ
j
τ
=
iK
2
∴
i
=
ω
2
θ
j
K
2
P
=
i
2
R
T
=
T
case
+
R
th
(
ω
2
θ
j
K
t
)
2
R
coil
Many mechanical schemes have been employed to reduce the motor current and associated heat problem.
In Khowles U.S. Pat. No. 4,958,894, the excitation of an electromagnetic coil operating on a magnet at the end of a pivot arm extending off the mirror, is coordinated with the end-of-travel engagement of the magnet with one or the other of two resilient bumpers between which it travels, imparting a reversing bounce and resulting in the oscillation of the pivot arm and mirror. This bumper variation produces a faster reversal and lowers the required energy.
In Culp's U.S. Pat. No. 5,066,084, there is disclosed a constant velocity scanning apparatus in which the mirror oscillations are maintained with end-of-travel piezo motion actuators in combination with end-of-travel, resilient “energy absorbing and releasing contacts” analogous to the rubber bumpers of Khowles. Howe's U.S. Pat. No. 3,678,308, illustrates another variation on an oscillating scanner that employs mechanical springs to provide an end-of-travel bounce in the oscillating motion of the mirror.
These all involve scanning systems with mechanical springs defining the end of travel, and demonstrate well the general idea that opposing springs can be employed to conserve energy within a mechanically oscillating device. They use varying geometries and may also use modified motor drive current schemes for a coordinated effect on reducing average motor current while maintaining a satisfactory output waveform of the device.
It is instructive to look at U.S. Pat. No. 5,424,632, as illustrative of a common moving magnet scanner. The '632
FIG. 1
is described as a schematic view of a galvanometer used in a laser scanning system, illustrating the mirror, motor, and a position transducer. In the '632
FIG. 2
, torque motor
17
includes a magnetically permeable outer housing
28
that holds the stator
51
consisting of windings
31
on bobbin
50
. Permanent magnet rotor
100
is rotably mounted within the stator. Stator windings 31 in the '632
FIGS. 3 and 8
is the coil where the heat of concern is generated. This heat is dissipated radially through the device.
The achievable flux density of the stator magnet
27
as well as the resistivity of winding
31
are subject to fundamental material constraints. The achievable acceleration of this system is a function of the aspect ratio of the magnet (length to diameter) and proportional to 1/(magnet radius), to first order. This means that larger structures allow lower RMS (root mean square) acceleration. RMS acceleration is defined over the relevant thermal time constants. In other words, it is the maximum acceleration at which the device can be run without heat-induced damage and eventual failure. In theory, one can put an arbitrarily large stator current, ignoring demagnetizing of the magnet, for an arbitrarily short time, but when attempting to execute a repetitive waveform, the device would simply reach a certain steady state.
FIGS. 1 and 2
of the '632 disclosure are included herein as prior art
FIGS. 9 and 10
respectively.
Another area of art which readers may find instructive is that of resonant scanners. These scanners use a more or less linear spring, and constitute a mechanical oscillator in which energy is continually converted back and forth between kinetic energy (stored in the rotating mass) and potential energy (stored in a torsional spring). These can achieve very high efficiencies, as the motor only has to supply system losses, but they have two fundamental constraints. First, the frequency is constrained to the resonant frequency of the system. The frequency can be tuned, to some extent, such as by changing the temperature of the spring or making other mechanical adjustments to the design. There are patents to this effect. Second, the mechanical output motion must be sinusoidal, or very nearly so.
Dostal's U.S. Pat. No. 3,609,485, is a resonant torsional oscillator for optical scanning or other vibratory action at a high amplitude and constant rate. This patent is cited in many torsional resonant scanners, an example of which is Corker's U.S. Pat. No. 3,642,344, Optical Scanner Having High Frequency Torsional Oscillator. The problem with all of the resonant torsional oscillators is that they give sinusoidal motion, and are essentially constant frequency devices, being tunable over a narrow range by varying temperature or otherwise varying the spring rate of the spring.
In summary, there remains room for improvement in the design and operation of bi-directional reciprocating galvanometer scanners and similar reciprocating motion devices to reduce power requirements and minimize heat generation through the use of design features that provide for passive energy conservation in the change of direction phase of motion.
SUMMARY OF THE INVENTION
The invention may be most simply described as a reciprocating rotary action actuator consisting of a motor coupled to a rotor and stator where the stator has a ring magnet and a pair of soft iron pole pieces that concentrate the flux of the ring magnet into a concentric set of narrow, uniformly spaced, axially oriented, magnetic flux fields intersecting the rotor's field of travel. The rotor has small permanent magnets embedded in the periphery of a nonconductive, nonmagnetic rotor core, where the magnets are of the same number and spacing as the stator's magnetic flux fields, there being at least one and preferably two or more with equal spacing. The magnets are pole oriented axially opposite the flux fields of the stator pole pieces, so that upon rotation, the rotor magnets encounter the stator flux fields at each end of rotor travel, creating an opposing force that reverses the angular direction of the rotor with minimal requirement for motor current. The device can be incorporated into a galvanometer scanner or other devices with similar reciprocating rotary action requirements.
More particularly, the invention encompasses a high speed reciprocating angular motion device, adaptable to electrically powered optical scanning and other applications where frequency, amplitude, load moment of inertia, and scan efficiency are generally limited by thermal considerations of the actuator. The limitations of the prior art are overcome by combining a bi-directional, electrical drive actuator for driving a reciprocating scanner rotor with high efficiency, while a preferably passive, energy transformation mechanism, the equivalent of a set of hi-K (spring constant) bumpers or sprin
Brown David C.
Nussbaum Michael B.
Stukalin Felix
Antonelli Terry Stout & Kraus LLP
GSI Lumonics Corporation
Jones Judson H.
Ramirez Nestor
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