U.S. patent application number 11/394778 was filed with the patent office on 2007-10-04 for methods and apparatus for commutating a brushless dc motor in a laser printer.
This patent application is currently assigned to Lexmark International, Inc.. Invention is credited to Aaron M. Lambert, Steven M. Turney.
Application Number | 20070229012 11/394778 |
Document ID | / |
Family ID | 38519993 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070229012 |
Kind Code |
A1 |
Lambert; Aaron M. ; et
al. |
October 4, 2007 |
METHODS AND APPARATUS FOR COMMUTATING A BRUSHLESS DC MOTOR IN A
LASER PRINTER
Abstract
In a laser printer, methods and apparatus include commutating a
brushless dc motor having three windings. A controller receives
discrete motor position signals, such as from hall-effect or FG
sensors, and extrapolates motor position between the signals. It
commutates the motor based on the extrapolated motor position and
updates motor position whenever an actual discrete signal is
received. Drive signals from the controller to the motor are such
that a current flowing in any of the three windings follows a
generally sinusoidal waveform. High and low switches are provided
per each winding of the three windings and are cumulatively
switched according to an extrapolated motor position based
multiplier applied to a pulse width modulation duty cycle. In this
regard, lookup tables, counters, registers and the like are
provided. An engine card of the printer includes an ASIC with a
power driver for use with generally off-the-shelf brushless dc
motors.
Inventors: |
Lambert; Aaron M.;
(Lexington, KY) ; Turney; Steven M.; (Lexington,
KY) |
Correspondence
Address: |
LEXMARK INTERNATIONAL, INC.;INTELLECTUAL PROPERTY LAW DEPARTMENT
740 WEST NEW CIRCLE ROAD
BLDG. 082-1
LEXINGTON
KY
40550-0999
US
|
Assignee: |
Lexmark International, Inc.
|
Family ID: |
38519993 |
Appl. No.: |
11/394778 |
Filed: |
March 31, 2006 |
Current U.S.
Class: |
318/466 |
Current CPC
Class: |
H02P 23/0004 20130101;
H02P 6/10 20130101 |
Class at
Publication: |
318/466 |
International
Class: |
H02P 3/00 20060101
H02P003/00 |
Claims
1. A laser printer, comprising: a brushless dc motor with three
windings and a plurality of sensors providing discrete signals
indicative of a position of the motor; and a controller connected
to receive the discrete signals, wherein the controller
extrapolates positions of the motor between the discrete signals
and updates a motor position upon receiving the discrete signals,
wherein the controller includes a PID and a commutator logic, the
commutator logic including a lookup table of extrapolated motor
positions.
2. The laser printer of claim 1, further including a high and low
switch per each of the three windings.
3. The laser printer of claim 1, further including a driver
connected to the controller to supply drive signals to each of the
three windings.
4. The laser printer of claim 3, wherein the drive signals cause a
current flowing in any of the three windings to follow a generally
sinusoidal waveform.
5. The laser printer of claim 1, wherein the plurality of sensors
providing the discrete signals includes at least one hall-effect
sensor.
6. The laser printer of claim 1, wherein the plurality of sensors
providing the discrete signals includes at least one encoder
signal.
7. The laser printer of claim 1, wherein the controller is
configured to calculate a pulse width modulation with variable duty
cycle per one of the three windings.
8. The laser printer of claim 7, wherein the controller is
configured to calculate a multiplier of the pulse width
modulation.
9. The laser printer of claim 7, wherein the controller is
configured to obtain a second and third pulse width modulation per
the other two windings of the three windings by time adjusting the
pulse width modulation per the one of the three windings.
10. (canceled)
11. A laser printer, comprising: a brushless dc motor with three
windings and a plurality of sensors providing discrete signals
indicative of a position of the motor; a high and low switch per
each winding of the three windings; a driver connected to supply
drive signals to each of the high and low switches per the each
winding; and a controller connected to the driver and to the motor
to receive the discrete signals, wherein the controller
extrapolates positions of the motor between the discrete signals
and applies an extrapolated position pulse width modulation duty
cycle and multiplier to the each winding via the driver so that a
current flowing in any of the three windings follows a Generally
sinusoidal waveform, wherein the controller further includes a
counter and a register value, the controller supplying a logic high
or low to the driver depending whether the counter is at a count
below or above the register value.
12. The laser printer of claim 11, wherein the controller is
further configured to update a motor position upon receiving the
discrete signals.
13. The laser printer of claim 11, wherein the extrapolated
position pulse width modulation duty cycle and multiplier includes
an off time for about one-third of a period.
14. (canceled)
15. A method of commutating a brushless dc motor in a laser
printer, the motor having three windings, comprising: by a
controller, receiving discrete position signals from the motor; by
the controller, extrapolating a motor position during times of not
said receiving the discrete position signals from the motor,
wherein the extrapolating includes communicating with a lookup)
table of commutator logic, the controller including a PID and the
commutator logic; applying drive signals to each winding of the
motor during the times of the not said receiving; and thereafter,
updating a motor position upon receiving another discrete position
signal.
16. The method of claim 15, wherein the applying further includes
causing a current flowing in any of the three windings to follow a
generally sinusoidal waveform.
17. The method of claim 15, further including calculating an
extrapolated position pulse width modulation and multiplier per
each of the three windings.
18. The method of claim 17, wherein the extrapolating further
includes calculating about 256 motor positions.
19. (canceled)
20. The method of claim 15, wherein the receiving the discrete
position signals from the motor further includes receiving at least
one encoder signal and at least three hall-effect sensor signals.
Description
FIELD OF THE INVENTION
[0001] Generally, the present invention relates to laser printers.
Particularly, it relates to brushless dc motors useful in
controlling motion of various components in the printer. In one
aspect, a quieter operating motor is contemplated. In another,
commutation of the motor includes extrapolating between position
feedback signals and making updates of same.
BACKGROUND OF THE INVENTION
[0002] As is known, laser printers use motors to impart motion to
various movable components, such as mirrors, belts, drums, paper
transport structures, etc. However, many modem motors rotate at
speeds with fundamental commutation frequencies and harmonics in
the audible range. To end users, this sometimes causes hearing
annoyance, especially considering motors often vibrate connected
structures such as metal frames. To avoid this, some manufacturers
have focused on increasing the number of poles of a motor or by
commutating with other than square wave signals, to name a few.
Unfortunately, increasing motor poles does not eliminate the
hearing annoyance because this just shifts the fundamental
frequency and harmonics to a higher value in the audible range.
With non-square wave commutation, motor feedback often requires
more than the three or so typically provided feedback sensor
signals. Both are also quite complex and relatively expensive.
[0003] Accordingly, there exists a need in the art for eliminating
motor noise in laser printers. It would be particularly useful if
such could be accomplished by smoothing motor commutation and doing
so with typical motor components. Naturally, any improvements
should further contemplate good engineering practices, such as
relative inexpensiveness, stability, low complexity, etc.
SUMMARY OF THE INVENTION
[0004] The above-mentioned and other problems become solved by
applying the principles and teachings associated with the
hereinafter described commutation of a brushless dc motor in a
laser printer. Specifically, methods and apparatus contemplate
utilizing standard or off-the-shelf motors with a commutation
scheme that extrapolates motor position at times between receipts
of discrete signals from position sensors of the motor and updating
the motor position whenever an actual discrete signal is received.
In this regard, a laser printer includes an engine card interfacing
with the motor and its typical hall-effect sensor signals, times
three, and an encoder, such as an FG (frequency generator)
signal.
[0005] In a representative embodiment, the engine card contemplates
an ASIC controller with a PID (proportional, integral, derivative)
controller and commutator logic. The PID logic produces a pulse
width modulation (PWM), in duty cycle, for application to any
winding of the motor. The commutator logic includes a lookup table
with a multiplier for the PWM. High and low switches are provided
per each winding of the three windings and are cumulatively
switched according to the extrapolated motor position PWM and the
multiplier. In this regard, lookup tables, counters, registers and
the like are provided.
[0006] These and other embodiments, aspects, advantages, and
features of the present invention will be set forth in the
description which follows, and in part will become apparent to
those of ordinary skill in the art by reference to the following
description of the invention and referenced drawings or by practice
of the invention. The aspects, advantages, and features of the
invention are realized and attained by means of the
instrumentalities, procedures, and combinations particularly
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The accompanying drawings incorporated in and forming a part
of the specification, illustrate several aspects of the present
invention, and together with the description serve to explain the
principles of the invention. In the drawings:
[0008] FIG. 1 is a diagrammatic view in accordance with the present
invention of the windings and switch control of a brushless dc
motor in a laser printer;
[0009] FIG. 2 is a graph in accordance with the present invention
of a preferred signal per a given winding of the brushless dc motor
in a laser printer;
[0010] FIG. 3 is a graph in accordance with the present invention
of a preferred current per each of the windings of the brushless dc
motor in a laser printer;
[0011] FIG. 4 is a diagrammatic view in accordance with the present
invention of a control system in a laser printer for commutating a
brushless dc motor;
[0012] FIG. 5 is a graph in accordance with the present invention
of a preferred lookup table in the control system of the laser
printer;
[0013] FIG. 6 is a table in accordance with the present invention
of the graph of FIG. 5;
[0014] FIG. 7 is a graph in accordance with the present invention
illustrating the timing of extrapolation of the position of a
brushless dc motor in a laser printer;
[0015] FIG. 8 is a graph and inset in accordance with the present
invention of timing for calculating an updated duty cycle for a
winding in a brushless dc motor in a laser printer;
[0016] FIG. 9 is a graph in accordance with the present invention
of a representative noise reduction from an actually commutated
brushless dc motor for a laser printer; and
[0017] FIG. 10 is a graph in accordance with the present invention
of the current in a winding of the brushless dc motor for rendering
the noise reduction feature of FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] In the following detailed description of the preferred
embodiments, reference is made to the accompanying drawings that
form a part hereof, and in which is shown by way of illustration,
specific embodiments in which the invention may be practiced. These
embodiments are described in sufficient detail to enable those
skilled in the art to practice the invention and like numerals
represent like details in the various figures. Also, it is to be
understood that other embodiments may be utilized and that process,
mechanical, electrical and/or other changes may be made without
departing from the scope of the present invention. In accordance
with the present invention, commutation of a brushless dc motor in
a laser printer is hereafter described.
[0019] With reference to FIG. 1, the three windings of a brushless
dc motor 10 for use in a laser printer are given as windings U, V,
and W. Per each winding, nodes 1, 2, and 3 provide a cumulative
input to the winding and such is defined by a high HI and low LO
switch. Further, the HI side includes a connection to a positive
+V.sub.OLTAGE value and such is on the order of about +24 V. On the
LO side, a ground connects and such corresponds to the ground of
the laser printer. During use, the HI and LO switch per each
winding is switched on and off to commutate the motor. Of course,
care must be taken that the HI and LO switch of any given node are
not both turned on simultaneously which would create a direct path
from the input power to ground. In turn, each winding is commutated
such that a current flowing therein follows a generally sinusoidal
waveform with each winding being adjusted in time from the other
windings. Because perfect sine waves have no higher harmonics in
the frequency domain, noise reduction of the motor, and attendant
laser printer, is achieved.
[0020] With reference to FIG. 2, representative waveforns applied
to the HI and LO switches per each winding are given. That is, the
HI switch is pulse width modulated (e.g., the HI switch is opened
and closed for variable periods of time), and is given as a
function of duty cycle, according to the waveform 10. The inverse
12 of the waveform is that which is applied to the LO switch and
cumulatively the waveforms correspond to a 100% duty cycle. Also,
because of the shape of the waveforms, their cumulative effect is
that of a sinusoidal current waveform through a given winding and a
sinusoidal waveform per a differential voltage between any two of
the three nodes. For instance, at time A, the HI switch has a duty
cycle of 40% while the LO switch has a duty cycle of 60%.
Similarly, at time B, the HI corresponds to 60% while the LO is
40%, and so on. As a result, the actual current through a given
winding is a substantially sinusoidal waveform 14 as seen in FIG. 3
(e.g., the difference between waveform 10 and another waveform 10
(not shown) at another of the nodes that has been shifted in time
by one-third of a period T). Sinusoidal waveforms 16 and 18, on the
other hand, are representative of those currents through the other
windings. As denoted, the sinusoidal current waveforms are
substantially identical with the exception of being shifted in
phase. It is also preferred that all three windings have current
flowing at any given time whereas the past typically had one of
three windings without current flowing at any given time. It should
be appreciated, however, that these graphs and waveforms presume
the HI and LO switches are ideal. Namely, an ideal switch is one
that has instantaneous switching time from open to closed and when
the HI switch is closed, the LO switch is opened, and vice versa.
Of course, the actual implementation of switches is such that there
is some measurable time delay that must be accounted for when
switching the HI and LO switches on and off.
[0021] With reference to FIG. 4, a laser printer of the invention
is given generically as 20. It includes a brushless dc motor 22 and
attendant electronics 24 per a typically off-the-shelf assembly 26
and the motor imparts motion to various movable components 28 in
the printer, such as mirrors, belts, drums, paper transport
structures, etc., for a variety of reasons. An engine card 30
includes a controller, in the form of an ASIC 32, a power driver 34
and a voltage and ground source 36 (on or off the engine card) for
variously commonly powering and grounding the ASIC 32, the power
driver 34, the assembly 26 and other components (not shown). Output
from the motor and connected to the controller are the three
typical hall-effect sensor signals H.sub.U, H.sub.V, and H.sub.W
and an encoder FG signal. As is typical with brushless DC motors,
the hall-effect sensors provide discrete signals indicative of six
states of the motor, to indicate position. The order of occurrence
of these states is dependent on motor construction and direction.
In a common embodiment, the discrete signals are six sensor states
0-5 and correspond to logic high or low given per the sensor signal
H.sub.U, H.sub.V, H.sub.W as 0,0,1; 0,1,0; 0,1,1; 1,0,0; 1,0,1; and
1,1,0, respectively. As skilled artisans know, however, these are
not the actual order of occurrence but are representatively
provided in this order according to binary counting. Unfortunately,
the motor position per a standard assembly 26 is not affirmatively
known at other than these six states and commutating the motor at
only one of these six states has lead to the problems of the prior
art.
[0022] Thus, the engine card and controller contemplate
extrapolating the motor position between these six states and
commutating the motor accordingly. Preferably, this includes the
power driver supplying drive signals D1, D2, and D3 to the
respective nodes 1, 2 and 3 of the motor (FIG. 1) to achieve the
current waveforms in the windings as seen in FIG. 3. In a
representative embodiment, the drive signals are voltage pulses
between ground and +V.sub.OLTAGE that vary regularly in voltage and
pulse width (PWM, in terms of duty cycle) and are variously on or
off. In a preferred embodiment: the on or off switching rate occurs
on the order of about 20 to about 35 kHz; the duty cycles vary
between 0 to 100% as in FIG. 2; and the voltage varies between
ground and +24 volts.
[0023] In a further representative embodiment, 256 discrete values
of the motor position (positions 0-255, inclusive) are found in a
lookup table T, 38, for a single commutation cycle of the motor and
such relate to providing input to the power driver to achieve the
desired drive signals. However, whenever an actual discrete signal
is obtained from the motor, the discrete signal is used to reset
the extrapolated position. In this manner, the proper phasing of
motor commutation is maintained.
[0024] For example, a PID logic 40 receives the FG signal from the
motor. In turn, it calculates a PWM in duty cycle for commutating
the motor. This calculation is well known. However, in combination
with the commutator logic 42, the calculated PWM is altered via a
multiplier per a given position. Then, the controller creates six
output signals 52, 54 and 56 (two per the U winding, both HI and
LO; two per the V winding, both HI and LO, and two per the W
winding, both HI and LO) serving as inputs to the power driver to
create the drive signals D1, D2, and D3. In practice, these outputs
(52, 54, and 56) are either a logic low or high. The HI outputs
depend upon whether a counter C, 43, exceeds a certain register
value 41 or not. If it exceeds the value, a logic low is output. If
it does not, a logic high is output. Typically, the value of the
register corresponds to a product of the commanded PWM from the
controller and a value of the table T chosen by the position. Also,
the counter counts down at a rate in increments proportionate to
the PWM rate. In this regard, because it is preferred that the PWM
exists between 20-35 kHz, the counting increment rates then occur
in increments of their inverse, or about 7 to 12.5 nanoseconds
(assuming a 12 bit counter). Naturally, a memory 44 is available to
the controller during use, including each of the logics 40 and 42,
and includes a variety of addresses, registers, or the like.
[0025] With more specificity, FIG. 5 shows a waveform 60 for a HI
switch of a winding U, V or W in terms of extrapolated motor
position (x-axis) and a multiplier (y-axis) for the PWM from the
PID controller. For instance, the position given as 0 corresponds
to a value of 126. Thus, a multiplier for the PWM would be the
value 126 divided by the highest possible state of the 256 states,
or 126/255. To simplify implementation, the actual value used was
126/256. Similarly, at positions 42 and 43, and again at 85 , the
value of waveform 60 is 255. In this instance, the multiplier would
be nearly 1 or 255/256. In another example, the multiplier would be
0 or 0/256 as between positions 150 and 235 (about one-third of the
entire waveform 60). Of course, other positions are possible. In
this regard, FIG. 6 shows a complete lookup table 38 of values for
the waveform 60.
[0026] As expected during commutation of a motor, actual discrete
signals from the hall-effect sensors will arrive at the controller
of the invention. Because 256 possible values of states are used in
extrapolating the motor, and because 6 states from the hall-effect
sensors are expected during this time, the value of 256/6 yields
42.67. Thus, it is expected that for every 42.67 extrapolated
positions of the motor, an actual discrete signal will arrive for
the motor. In practice, this then occurs at extrapolated positions
0, 42.67, 85.33, 128, 170.67, and 213.33 as given by the boxes in
the table.
[0027] Because each motor in a laser printer is imperfect relative
to the other motors in similar printers, the actual arrival of a
discrete signal from the hall-effect sensors will likely vary at
other than exactly 42.67 extrapolated positions. That is why the
controller of the engine card updates the extrapolated motor
position whenever the discrete signals arrive, regardless of when
they actually arrive. In this regard, FIG. 7 illustrates the
operation of the commutation position calculation in the presence
of infrequent and/or possibly inaccurate sensor position feedback.
With reference to FIG. 7, the Position (or y-axis) relates to the
extrapolated position or the x-axis of FIG. 6. The waveform,
however, is given as 70. Along the x-axis of FIG. 7, two items of
information are presented. That is, the commutator state
C.sub.state (e.g., one of the six states of the hall-effect
sensors) and clock ticks per a given engine card. In this regard,
hall-effect states 4, 5 and 6 (or beginning again at state 0), for
example, correspond to clock ticks of 500, 1200 and 1900,
respectively. Using 3 hall sensors, and assuming that their
placement about a motor is correct, the absolute position with
regard to commutation using the 256 point table is known at each
receipt of a hall state change (hall event). After each event and
up until the next event, the position is incremented from the known
position at a rate consistent with the time between the previous
two events. Assuming that the motor speed does not change and that
the hall sensors have been perfectly placed, the incremented
position should align perfectly with the absolute position set at
the next hall event. In the case that these assumptions do not
hold, the incremented position value will be either greater or
lesser than the absolute position that is set at the next hall
event and a discontinuity in the position will be evident as is
indicated in the jump from D to E. In other words, when the
hall-effect sensor V Hall arrived at clock tick 1200, the
extrapolated position was being given at position D, but the actual
position should have been position E. Thus, an update to the actual
position from the actual discrete signals of the motor needs to
occur. Conversely, at clock tick 500, the extrapolated position and
actual position needed no adjustment. Naturally, depending upon
motor speed, the slope of the curve 70 will vary. Also, at clock
tick 1900, the position starts over again at 0. Naturally, the
clock tick is selected according to useful operating parameters of
the laser printer and can vary as users deem appropriate.
[0028] The method indicated in FIG. 7 of position calculation and
absolute position reset at each hall event is susceptible to
inaccurate hall sensor placement which may cause discontinuities in
the position value even at a perfectly constant speed. The position
may alternately be set based on the FG signal encoder feedback and
the periods of event occurrence of this more consistent signal used
to determine the rate of position increment in between encoder
events. This method may yield a more consistent position signal.
The encoder is an incremental sensor and thus must still be used in
conjunction with hall feedback which is absolute. To implement
this, the absolute position is updated based on a particular hall
state at a frequency of once per motor revolution.
[0029] In FIG. 8, a representative PWM signal 80 at a frequency of
about 33 kHz with a duty cycle varying along the time axis from
about 25% to about 75% according to a sinusoidal waveform 82 is
illustrated. In the inset 84, the next PWM 86 (in duty cycle) is
calculated by multiplying the value from the table T at the present
position and the commanded PWM from the PID controller or other PWM
generator. This calculation for the next PWM duty cycle takes place
during the time of the present PWM period 85. This calculation
takes some amount of time in clock ticks to perform and thus must
begin at some finite time before the end of the present PWM cycle.
Naturally, the longer one waits to make the next calculation, the
closer the position signal used will be to the actual motor
position during said PWM cycle. This then allows a new position
based duty cycle to be used for each PWM period. Also, the counter
C (FIG. 4) is used in this regard.
[0030] In FIG. 9, two generally overlying waveforms 85, 87 are
given. In the first, a brushless dc motor was commutated with a
generally square waveform according to the prior art and noisy
harmonics 90, 92 and 94 in the audible frequency range appeared per
a given laser printer. In the second, the harmonics are fairly
gone. Thus, a noise decibel reduction of about 8.4, 8.1 and 4.6
decibel (dB) was noticed and a quieter operating printer achieved.
As a corollary, FIG. 10 shows overlaid actual current waveforms
used per a given winding (U in this instance) of the commutated
brushless dc motor to produce the noise graph of FIG. 9. As is
seen, the prior art waveform is 100 and is generally a square wave.
The instant invention, however, is given as 102 and generally
follows a sinusoidal shape. Again, sinusoids have no harmonics.
[0031] In any orientation, certain advantages of the invention over
the prior art are readily apparent. For example, standard
off-the-shelf brushless dc motors with typical hall-effect sensors
and FG signals can be used. This adds robustness and tends to lower
manufacturing costs. Less intuitively, motor extrapolated positions
can be used to provide stable commutation including updating the
motor position whenever an actual position signal is received.
Integrated assemblies in the form of an ASIC can replace the
modularity of certain prior art designs. This improves
manufacturability.
[0032] Finally, one of ordinary skill in the art will recognize
that additional embodiments are also possible without departing
from the teachings of the 14 present invention. This detailed
description, and particularly the specific details of the exemplary
embodiments disclosed herein, is given primarily for clarity of
understanding, and no unnecessary limitations are to be imported,
for modifications will become obvious to those skilled in the art
upon reading this disclosure and may be made without departing from
the spirit or scope of the invention. Relatively apparent
modifications, of course, include combining the various features of
one or more figures with the features of one or more of other
figures.
* * * * *