U.S. patent application number 10/306141 was filed with the patent office on 2003-04-24 for galvanometer unit.
Invention is credited to Brown, David C., Stukalin, Felix.
Application Number | 20030075769 10/306141 |
Document ID | / |
Family ID | 23715347 |
Filed Date | 2003-04-24 |
United States Patent
Application |
20030075769 |
Kind Code |
A1 |
Brown, David C. ; et
al. |
April 24, 2003 |
Galvanometer unit
Abstract
A galvanometer unit comprises a limited-rotation motor with a
load element such as a mirror attached to a shaft extending from
the motor. In a servo loop that controls the angular position of
the mirror, a position-sensor attached to the shaft provides
position feedback information. The sensor includes a rotor which is
positioned at the null point of the fundamental torsional resonance
mode of the rotating system, thereby essentially eliminating
feedback components resulting from the resonance. If the motor
armature is supported for rotation on ball or roller bearings, the
controller for the motor causes complete rotation of the armature
from time to time, thereby distributing bearing wear around the
bearing races and prolonging the useful life of the bearings.
Inventors: |
Brown, David C.;
(Northborough, MA) ; Stukalin, Felix; (Framingham,
MA) |
Correspondence
Address: |
Hoffman, Warnick & D'Alessandro LLC
Three E-Comm Square
Albany
NY
12207
US
|
Family ID: |
23715347 |
Appl. No.: |
10/306141 |
Filed: |
November 27, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10306141 |
Nov 27, 2002 |
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10097841 |
Mar 14, 2002 |
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10097841 |
Mar 14, 2002 |
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09432244 |
Nov 2, 1999 |
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6380649 |
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Current U.S.
Class: |
257/422 |
Current CPC
Class: |
F16C 41/04 20130101;
G02B 26/105 20130101; F16C 19/52 20130101 |
Class at
Publication: |
257/422 |
International
Class: |
H01L 029/82 |
Claims
What is claimed is:
1. A galvanometer unit comprising: A. a motor having a rotor
comprising; 1. an armature, and 2. first and second shafts
extending from said armature; B. a load element affixed to the end
of said first shaft remote from said armature; C. a controller for
causing the motor to reciprocally rotate the rotor, said controller
including a servo system for controlling the angular position of
the load element, said servo system including a position sensor for
sensing the angular position of the shaft, the position sensor
including; 1. a sensor rotor attached to the shaft for rotation
therewith, and 2. a stator assembly for sensing the angular
position of the sensor rotor; the sensor rotor being positioned at
the null point of the fundamental torsional resonance of the
rotating system that includes the motor rotor, the shafts and the
load element.
2. The galvanometer unit defined in claim 1 including means for
adjusting the position of the null point to coincide with the
position of the sensor rotor on the shaft.
3. The galvanometer unit defined in claim 1: A. in which the stator
assembly comprises first and second stators disposed around said
shaft, said stators having opposing faces and opposing electrodes
on said faces; B. in which the rotor is; 1. of a dialetric
material; 2. is affixed to said shaft between said opposing faces;
and 3. has radially extending blades; and C. including means for
sensing the capacitances between said electrodes.
4. The galvanometer unit defined in claim 3: A. including means for
clamping said stators together. B. including a cavity between the
opposing electrodes; and C. in which said rotor is disposed in said
cavity.
5. The galvanometer defined in claim 1: A. including first and
second bearings supporting the rotor for rotation, each of the
bearings having an inner race and an outer race; and B. in which
the controller includes means for providing full rotation of the
armature at predetermined times, thereby to distribute wear on said
bearings.
6. A galvanometer unit comprising: A. a motor, said motor having a
rotor comprising an armature and shafts extending therefrom; B.
bearings supporting said armature for rotation, said bearings
having inner races and outer races; C. a load element supported
from an end of one of said shafts remote from said armature for
rotation therewith; D. a controller for reciprocally rotating said
rotor, said controller including a servo system for controlling the
angular position of the load element; and E. means in said
controller for providing full rotation of said armature at
commanded times, thereby to distribute wear on said bearings.
7. The galvanometer unit defined in claim 6 including a rotation
stop unit for mechanically limiting the rotation of said rotor,
said stop unit including A. a stop pin projecting radially from one
of said shafts; B. a plurality of stop positioned to interfere with
said stop pin and thereby limit rotation of said rotor; and C.
means for 1. retracting said stops, thereby permitting unlimited
rotation of said rotor, and 2. repositioning said stops for
engagement by said stop pin, thereby limiting the rotation of said
rotor.
8. A galvanometer unit comprising, a stator and a rotor, A. said
stator including field windings, B. said rotor comprising an
armature including a non-conducting permanent magnet providing a
flux that interacts with fields generated by currents in said
windings to provide rotation of the rotor, C. said rotor further
including 1. shafts abutting the ends of the armature, and 2. an
electrically conducting sleeve enclosing said armature and the
portions of said shafts adjacent to said armature, said sleeve
being in electrical contact with said shafts and retaining said
shafts, whereby said armature and said shafts rotate together as a
single member.
9. A galvanometer unit comprising A. a limited-rotation motor; B. a
load reciprocally rotated by said motor; C. cooling unit for
removing heat from said motor, said cooling unit comprising 1. a
heat-transfer surface of said motor, 2. a heat-dissipation plate
having a first surface in intimate thermal contact with said motor
surface, and a second surface having fins projecting therefrom, and
3. a fan projecting air toward said second surface, perpendicularly
to said second surface, whereby air from said fan impinges on said
second surface and flows outwardly therefrom along said fins.
10. The galvanometer unit of claim 1 including a cooling unit for
removing heat from said motor, said cooling unit comprising A. a
heat-transfer surface of said motor, B. a heat-dissipation plate
having a first surface in intimate thermal contact with said motor
surface, and a second surface having fins projecting therefrom, and
C. a fan projecting air toward said second surface, perpendicularly
to said second surface, whereby air from said fan impinges on said
second surface and flows outwardly therefrom along said fins.
Description
FIELD OF THE INVENTION
[0001] This invention relates to an improved galvanometer unit.
More particularly, it relates to a galvanometer unit incorporating
a limited-rotation motor having an improved bearing life and
improved position control for high-speed actuation.
BACKGROUND OF THE INVENTION
[0002] A galvanometer unit to which the invention relates includes
a limited-rotation electro-magnetic motor having a permanent-magnet
armature that interacts with the fields generated by currents
through field windings. Motors of this type are often used in
scanners, in which a light-directing component, usually a mirror,
is attached to the motor shaft and reciprocal rotation of the motor
causes a light beam directed at the mirror to sweep back and forth
over a target surface.
[0003] Since the motor undergoes limited rotation, the rotor,which
comprises the armature and associated shafts, may be mounted on a
flexural pivot that acts as a torsional spring for motor rotation.
However, the motor to which the present invention relates
incorporates bearings to support the armature and the limitation on
rotation is provided by the servo system that controls the angular
position of the mirror. Galvanometer motors of this type have in
the past suffered from bearing wear, which degrades the accuracy of
light beam direction, ultimately reaching an unacceptable level and
requiring replacement of the scanner.
[0004] Another problem encountered with prior galvanometer motors
is the torsional resonance of the rotating system, i.e. the rotor,
the load, e.g. mirror, and any other rotating components. A
position sensor is connected to the shaft to provide position
feedback in the servo loop and the output of the sensor includes
components resulting from resonant twisting of the shaft. There are
several resonance modes and the pass band of the servo system must
be well below the lowest resonance frequency to avoid unwanted
feedback causing instability of the servo system. Other problems to
which the invention is directed are the desirability of stability
and high sensitivity of the position servo. A further problem is
the need for uniformity of temperature in the rotating system and
efficient removal of heat from the motor.
SUMMARY OF THE INVENTION
[0005] A galvanometer unit incorporating the invention supports the
armature on ball or roller bearings. A servo controller that
rotates the scanner to commanded angular positions is programmed to
cause the rotor to undergo one or more complete revolutions from
time to time. This changes the angular relationships between the
bearing balls or rollers and the inner and outer bearing races.
Bearing wear is thus shifted to different portions of the races and
wear is distributed around the races instead of being concentrated
in a single angular span. This materially increases bearing
life.
[0006] Preferably, also, the position sensor in the servo system is
located at a null point of the fundamental resonance mode of the
rotating system. Thus there is negligible feedback in the servo
system from this resonance. This permits operation of the scanner
at significantly higher speeds.
[0007] More specifically, the rotating system exhibits a
fundamental torsional resonance mode in which the instantaneous
angular velocities of the motor armature and the mirror are in
opposite directions. The frequency of this mode, as well as the
frequencies of higher order modes, is a function primarily of the
rotational inertias and torsional stiffnesses in the rotating
system. The fundamental mode has a single null at an axial position
on the shaft determined by the physical parameters of the rotating
components. The output of a sensor located at the null position
contains a negligible frequency component corresponding to the
fundamental resonance mode. Therefore, the pass band of the servo
system, one of whose imputs is the angular position indicated by
the sensor, can be increased to a frequency closer to the
fundamental resonance than is practical in prior systems.
[0008] A further improvement is provided by the use of a capacitive
position sensor that is thinner than prior sensors. This reduces
the length of the shaft linking the scanning mirror to the motor,
which results with a corresponding increase in shaft stiffness.
This in turn increases the various resonances, including the
fundamental resonance frequency, again permitting an increase in
the pass band of the servo system.
[0009] A novel rotor structure and method of fabricating it
contribute both to torsional stiffness and high electrical and
thermal conductivity between the armature and the shafts in the
rotating system. This facilitates grounding of the rotor to prevent
the buildup of static charges and it also provides for temperature
uniformity so as to minimize differential thermal expansion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The invention description below refers to the accompanying
drawings, of which:
[0011] FIG. 1 is an isometric partly exploded, view of a
limited-rotation system incorporating the invention, along with a
schematic view of the controller;
[0012] FIG. 2 is an exploded view of the scanner;
[0013] FIG. 3A is a longitudinal cross section of the position
sensor used in the controller;
[0014] FIG. 3B illustrates the grounding brush used to ground the
rotating parts;
[0015] FIG. 4 illustrates a prior rotor, showing the connection
between the motor armature and the stub shafts;
[0016] FIG. 5A depicts the rotor assembly prior to making it to
final form;
[0017] FIG. 5B illustrates configuration of a crush grinder used in
guiding the rotor assembly;
[0018] FIG. 5C depicts the finished rotor; FIG. 6 is an enlarged
view of the cooling module used to cool the galvanometer motor;
[0019] FIG. 6 is an enlarged view of the cooling module used to
cool the galvanometer motor; and
[0020] FIG. 7A-7C depicts the stop mechanism used to limit rotation
of the motor;
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
[0021] As shown in FIG. 1, a scanner incorporating the invention
includes a motor 10 that reciprocally rotates a beam-directing
device, such as a mirror 12, by way of a shaft 14. The shaft 14
passes through a position sensor 16 that provides electrical
signals indicative of the angular position of the mirror 12. A
controller 18 connects to the motor and sensor unit through a
terminal block 20 on the motor. A cooling module 21 is attached to
the motor 10 to remove heat therefrom.
[0022] With reference to FIG. 2, the motor 10, which is enclosed in
a housing 23, includes a stub shaft 22 extending from the motor
armature (not shown in FIG. 2) at the front end of the motor. The
shaft 22 rotates in a ball or roller bearing 24, which has inner
and outer races (not shown). At the rear end of the motor, the
armature is similarly supported by a shaft and a bearing (not
shown). The shaft 14 has a threaded end 14a that is threaded into a
bore 22a of the shaft 22 and preferably soldered in place. When
connected in this manner, the shafts 14 and 22 function as a
unitary shaft supporting the mirror 12.
[0023] The position sensor 16 has a cylindrical housing 30 that is
fastened to the motor 10 by means of pins 32 that extend through
the housing 30 into corresponding holes in the motor housing. The
sensor unit includes a dielectric rotor 36 and a stator assembly
comprising a pair of fixed stators 33 and 34. The stators 33 and 34
are fastened together and to the housing 30 by bolts 38 extending
from a shoulder 39 in the housing.
[0024] As best seen in FIG. 3A, the stator 33 comprises a metallic
ring 40 to which a ceramic disk 41 is bonded. The inner surface of
the disk 41 is covered with a continuous metallic layer to provide
a common electrode 42 connected to an electronic module 43.
Similarly, the stator 34 comprises a metallic ring 44 to which a
ceramic disk 46 is bonded. The inner surface of the disk 40 carries
a plurality of electrodes 48 connected to an electronic module 49
mounted on the opposite surface of the disk.
[0025] A cavity 50 between the disks 41 and 46 accommodates the
rotor 36, which is affixed to the shaft 14 by compression between a
shoulder 14a (FIG. 1) and the stub shaft 22 when the shaft 14 is
assembled to the stub shaft.
[0026] The rotor 36 has a set of radially extending blades 36a-36d
(FIG. 2) that function in a conventional differential-capacitor
arrangement. For example, the module 43 may apply an AC signal to
the electrodes 42, with the module 49 comparing the capacitive
currents that pass through the rotor blades 36a-36d to the
electrodes 48. Position sensor output signals and power for the
sensor 16 pass between the controller 18 and the modules 43 and 49
by way of the terminal block 20 and ribbon cables 50 and 51.
[0027] The rotating system exhibits a torsional resonance which is
a function of several parameters, such as the magnitudes and
positions of the stiffnesses and moments of inertia in the rotating
system. The fundamental resonance mode, which has the largest
amplitude, is one in which the rotations of the motor armature and
the mirror 12 are 180.degree. out of phase, i.e., they rotate in
opposite directions. Between these two components, there is a node
at which there is zero rotation at the fundamental resonance
frequency. The sensor rotor 36 is positioned at this node. Its
output, therefore, contains a negligible component resulting from
fundamental resonance mode of the rotating system. Accordingly, the
position feedback from the sensor unit 16 to the servo components
in the controller 18 is essentially devoid of this component and
the bandwidth of the servo system can therefore extend through the
fundamental resonance frequency.
[0028] It is impractical to determine the location of the null
point of the fundamental resonance and then install the sensor
rotor 36 at that location. Therefore, we prefer to tailor the shaft
14 to the mechanical characteristics of the mirror 12 so as to
position the null point at the location of the sensor rotor 36. For
example, if the mirror has a relatively large moment of inertia,
the shaft 14 might be made stiffer than would be the case with a
mirror having a smaller moment of inertia. This is a preferable
arrangement for production of substantial quantities of identical
scanners, since identical mirrors and identical shafts can be
produced at relatively low cost.
[0029] On the other hand, for a single scanner one might assemble
all the components of the scanner, with the rotor 36 positioned at
a location known to be on the left (FIG. 2) of the null point. A
mass in the form of a collar 52 is then secured around the shaft 14
so as to move the null point to the left. The collar is moved along
the shaft until the null point is positioned at the sensor rotor
36. The correct collar position can be determined by energizing the
motor 10 in an open loop configuration at the fundamental resonance
frequency, and ascertaining the amplitude and phase of the output
of the sensor unit 16. The collar 52 is moved accordingly and the
process is repeated until there is a negligible output from the
sensor unit 16.
[0030] Preferably, the sensor rotor 36 is made of ceramic material.
It can thus be made thinner, yet stiffer, than the prior sensor
rotors. Also with the rotor 36 and the stator disks 42 and 46 made
of ceramic material, these parts are relatively thin and they also
exhibit negligible dimensional changes in response to changes in
temperature and humidity. This materially improves the stability
and precision of the sensor. The gaps between the rotor 36 and the
disks 42 and 46 can thus be made relatively thin, with a
corresponding increase in the signal-to-noise ratio of the position
sensor and shortening of the shaft 14. The reduced thickness of the
rotor, stator disks and gaps allows a reduction in the overall size
of the galvanometer assembly. Further, the shaft 14 can be
shortened, resulting in increased shaft stiffness and a concomitant
increase in the torsional resonant frequencies.
[0031] The controller 18 (FIG. 1) preferably includes a
microprocessor 53 that operates in accordance with instructions
stored in a non-volatile memory 54. The microprocessor positions
the scanning mirror 12 in response to input commands at a terminal
18a. In a servo arrangement the controller 18 receives position
feedback signals from the sensor unit 16 and uses these signals,
together with the command signals in controlling the motor drive
current. The controller 18 also includes other components, e.g.
analog/digital and digital/analog converters (not shown in the
circuit diagram).
[0032] In accordance with instructions recorded in the memory 54,
the processor 53 records the total number of cycles of the limited
rotation of the motor 10 in a register 55.
[0033] When the cycle count reaches a predetermined number, the
controller causes the motor 10 to undergo one or more complete
revolutions. This changes the relative ballrace positions in the
bearings 24 so that wear on the bearing races is shifted to an
angular range in unworn portions of the races. As set forth above,
this prolongs the useful life of the bearings. The register 55 may
be a hardware register as shown in FIG. 1, or, if the memory 52 is
non-volatile, it may be a location in that memory.
[0034] FIG. 4 illustrates a conventional mode of attachment of a
motor armature 60 to the stub shafts 22. Each of the shafts is
provided with a cup-like extension 22b that closely fits over an
end of the armature 60. The parts are secured together by an
intervening elastomeric adhesive. This arrangement results
relatively low thermal and electrical conductivity between the
rotor 60 and the shaft 22. Moreover, the relatively low rigidity of
the coupling between the rotor and shaft contributes to low
torsional resonance frequencies of the rotating system.
[0035] In FIGS. 5A-5C I have illustrated a novel rotor and a method
of fabricating it that overcome these problems. As shown in FIG.
5A, cylindrical shaft blanks 22 are positioned against the ends of
the armature 60. These parts are inserted into a sleeve 64 and
secured to the sleeve with a high-conductivity solder such as a
silver-tin eutectic. The solder bond covers the entire opposing
surfaces of the sleeve 64 and the parts enclosed therein. The
sleeve 64 is of a material, such as copper, characterized by high
thermal and electrical conductivity.
[0036] Next, the assembly is ground on a centerless grinder.
Finally, it is crush ground in a grinder whose cylinders are
depicted in FIG. 5B. Specifically, the crush grinder comprises a
grinding cylinder 66 in the form of a right cylinder and a cylinder
68 whose cross section is the negative of the axial cross section
of the finished rotor. This results in a rotor 60, as depicted in
FIG. 5C, in which the sleeve 64 provides a rigid connection between
the shafts 22 and the armature 62, and, further, provides high
thermal and electrical conductivity between the armature and the
shafts. This provides a uniform temperature throughout the rotor
and, further, permits grounding of rotor anywhere along its
length.
[0037] A further advantage the rotor construction is the conductive
paths provided by the sleeve 64. They operate as a shorted turn
that reduces the inductance of the armature windings and thus
decreases the voltage required to drive the motor 10.
[0038] If the scanner is used in a two-axis system with separate
scanners providing beam movement along the respective axes,
rotation of the mirror 12 beyond a limited range during may cause
contact between the mirror 12 and a mirror on the other scanner.
Accordingly, mechanical stops are usually provided to prevent
excessive rotation. In that case, the scanner will be removed from
the two-scanner assembly, and the steps removed before undertaking
full revolution of the motor 10. The stops must also be
subsequently reassembled to the rotor. The structure depicted in
FIGS. 7A-7C overcomes these problems.
[0039] More specifically, as shown in FIG. 7A, a stop pin 80
extends through the rear motor shaft 22. The pin 22 coacts with a
set of limit pins 82 disposed in a stop assembly 84 (FIG. 7B)
affixed to the motor housing 23. The limit pins 82, which extend
from a solenoid plunger 86, are arrayed as depicted in FIG. 7C,
which in the solid line, depicts the pin 22 in the neutral position
of the rotor 60 (FIG. 7A) and, in the dashed lines, the limits of
rotation of the rotor defined by the positions of pins 82.
[0040] The plunger 86 is urged to the left (FIG. 7B) by a spring 88
to bring the limit pins 82 to the position shown by the dashed
line, so that they limit rotation of the rotor 60 as depicted in
FIG. 7C. In a two-axis system, the mirror on the other scanner is
temporarily rotated to a position where it will not interfere with
full rotation of the mirror 12 (FIG. 2). The solenoid coil 90 is
then energized, either manually or by the controller 18, to retract
the plunger 86 and the limit pins 82 to their illustrated position
and thus permit full rotation, as described above, to change the
relative positions of the races and balls and in bearings 24.
[0041] As shown in FIG. 6, the cooling module 21 includes a grooved
plate 70 in close thermal contact with the motor 10 and a fan unit
72, positioned above the plate 70, that projects air toward the
plate. The grooves in the plate 70 are relatively shallow, and, as
is well known, this configuration provides efficient cooling with a
negligible velocity of the air exiting from the module. With this
arrangement, cooling of the motor 10 does not result in appreciable
air currents in the optical path, which would degrade the accuracy
with which the scanner positions light beams. Furthermore, it
imparts negligible vibration to the system, thereby minimizing
vibration as a source of error in positioning the beam reflected by
the mirror 12.
[0042] As shown in FIGS. 3A and 3B, connection of the rotor to
system ground is accomplished by a brush 78, affixed to the rear
surface of the ceramic disk 41, and connected to the electronic
module 43. The brush 78 is a generally U-shaped spring fashioned
from a material such as a gold alloy. A pair of inwardly extending
contact bends 78a and 78b are thus urged inwardly against a slip
ring 92, of like material, affixed to the shaft 14. This maintains
a reliable electrical connection to the shaft 14 and thus with the
entire rotor.
* * * * *