Note: Descriptions are shown in the official language in which they were submitted.
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BRUSHLESS MOTOR SPEED CONTROL SYSTEM
DESCRIPTION
FIELD OF INVENTION
This invention relates to a novel device in the general field of motor control
systems, and more
specifically to one which permits a wider range of speeds for brushless motors
as used in
pan-tilt-zoom surveillance cameras.
BACKGROUND OF THE INVENTION
Surveillance cameras are commonly directed to move at high speed so they can
identify and track
a potential intruder as early as possible. However, it has been found that
very slow moving
threats are not always detected by some surveillance cameras because they are
not capable of
operating at very slow tracking speeds, and because slow speed operation
presents unique
technological challenges to produce recognizable images. Motorized panning and
tilting of
surveillance cameras at a wide range of speeds require motors that can operate
smoothly at both
high and low speeds. While high speed operation can effectively use brushless
motors in block
commutation modes, low speed operation using the same mode can degrade the
quality of the
image due to the lower frequency of the stepping function. This is especially
apparent when a
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camera is panned or zoomed to follow a target that has a large down range
movement as opposed
to a large movement across the horizon. Fine movements of a slow-moving camera
that are not
smoothed out can cause noticeable stuttering and vibration that create
blurring or
misinterpretation of surveillance images. There is a need for a surveillance
camera motion
control system that operates smoothly at both high and low speeds, and does
not require
expensive digital encoding methods to enable smooth and precise motor control
timing at low
speeds. Some attempts to solve this problem have been found in the prior art
and will now be
described.
One prior art attempt to solve this problem is a controller used for a
variable speed fridge
compressor (U.S. Pat. No. 7,102,306). While both high speed block commutation
and low speed
sine commutation are used to provide a wider speed range, the smoothness of
low speed
operation is not sufficient to operate long range surveillance cameras.
Another prior art attempt
to solve this problem is a controller used for a variable speed dentist drill
(U.S. Pat. No.
6,091,216). Again, both modes are used to provide both speed ranges, but the
speed variation at
slow speed ranges is not sufficient to prevent blurred images for long range
surveillance camera
control applications. In summary, there is still a need for a Brushless Motor
Speed Control
System which provides a sufficiently smooth operation for consistently clear
long range
surveillance imaging when used at low speeds, but is also capable of high
speed camera
movement when needed, and which combines these elements efficiently, at low
cost, and with
fewer extraneous components.
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BRIEF SUMMARY OF THE INVENTION
The Brushless Motor Speed Control System is designed to provide smooth and
precisely
controlled low and high speed motor driving, of a very wide speed range,
employing the same
controller. The issue with brushless motors employing hall sensors for high
speed block
commutation timing is that the hall sensors do not provide enough resolution
to drive the motor
consistently at low speeds. One solution is to run the motor synchronously at
speeds below
approximately 200 rpm. If this is done using the same block commutation
timing, the resulting
rotation is very notchy as the motor jumps from one magnet position to the
next. By
implementing a sinusoidal synchronous commutation at low speeds, this
notchiness is removed.
The disclosed speed control system was designed because the camera
surveillance industry is
switching from older style brush motors to brushless motors. Most new
surveillance cameras use
BRUSHLESS motors to pan, tilt, and zoom (PTZ) and these motors operate by the
block
commutation method using pulse-width modulation (PWM) motor control circuitry.
At high
speeds, this method operates smoothly, but at low speeds, they cannot create
an image without
noticeable blurring caused by the lowered positional discrimination in this
mode. Smooth low
speed operation of brushless motors is possible by operating as a stepper
motor with sinusoidal
synchronous commutation, but the seamless integration of these two
complimentary
commutation modes is not available using presently available methods and
technologies.
A key aspect of this invention is the control of brushless motor speeds using
the functional
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integration of two commonly employed commutation methods. Smooth low speed
control is
achieved without the need for expensive digital encoders, by synchronously
driving three PWMs,
sine wave modulated. Smooth high speed control employs trapezoidal Hall Effect
feedback
looping with programmable controller PID (Proportional, Integrated,
Differential) error
correction. Long range surveillance cameras require PTZ motors able to operate
at any speed
without image blurring due to unneeded motions caused by limitations of
commutation modes or
transition between modes. Therefore a means to ensure smooth transition
between low and high
speed commutation mode is also necessary and is incorporated in this control
system.
With the introduction of advanced programmable controllers employing greater
peripheral
functionality and including integrated PWM drivers, much smoother operation of
dual mode
brushless motor controllers is now possible. New software capabilities with
greater sampling
rates not only peimit smoother operation in both commutation modes, but also
smoother
transition between modes. In this design, no expensive digital encoders are
required at low
speeds, yet fine motor resolution is possible, enabling clear images even at
rotation rates of one
per month or less. At the same time, a high speed mode, with nominal speeds of
up to 100
degrees per second, is available without the speed limitations and overheating
of the stepper
motor mode. The resulting control system requires fewer complex and expensive
parts, a simpler
implementation, and provides an exceptionally wide speed range suitable for
long range
surveillance imaging of consistent clarity.
The invention thus is essentially a brushless motor control system for
providing a wide range of
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smooth and precisely controlled low and high speeds for pan-tilt-zoom
surveillance cameras, in
which a brushless motor is controlled by a programmable controller: a) in a
low speed mode by
sinusoidal synchronous commutation; b) in a high speed mode by block
commutation; c) in a
transition phase from the low speed mode to the high speed mode by modulating
integrated
pulse-width modulation (PWM) square waves with sine waves.
The system achieves smooth low speed control without the need for digital
encoders, by the
programmable controller synchronously driving multiple PWMs, sine wave
modulated, in the
low speed mode.
Examples of the "programmable controller" would be i) a microcontroller,
running software fed
to it, ii) a field programmable gate array (FPGA), configured to control the
motor's circuits, or iii)
an application specific integrated circuit (ASIC), dedicated to controlling
the motor's circuits.
=
In one preferred embodiment: a) appropriate high speed control is achieved by
a motor driver
card that receives Hall sensor outputs from the brushless motor and provides
appropriate PWM
=
output to drive the brushless motor at a selected speed; b) the high speed
mode employs
trapezoidal Hall Effect feedback looping with programmable controller
Proportional, Integrated,
Differential (PID) error correction; c) a camera control card issues Video
System Control
Architecture formatted commands on a TTL serial bus to a Pan motor driver card
or to a Tilt
motor driver cards or to a camera; d) motor driver cards can command a motor
to go to a specific
position, report the current position by means of a digital resolver, drive at
a specific speed and
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direction, and stop; e) motor driver cards can command a motor to send back
data, such as
current motor speed and PWM levels and can set motor operational parameters; 0
the operation
parameters include one or more of closed loop gains, acceleration rates, and
brake force; g) an
auxiliary control card can allow for three independent integrated circuit bus
protocol ports so that
pan, tilt and zoom positions can be fed to it from each position, thereby
enabling external
dedicated secure control pathways for panning, tilting and zooming; h) a motor
driver card used
to regulate the speed of.panftilt/zoom (PTZ) motors as directed from an
external source such as a
camera control card or an auxiliary control card; i) the brushless motor
comprises a series of
stationary wire-wound stators affixed around a central rotating multipolar
magnet rotor that
generates rotary motion to move a surveillance camera around each axis when
provided with an
appropriately timed PWM output through the stators; j) Hall Sensors are
positioned around the
brushless motor to detect polarity changes as it rotates, and to provide speed
regulation feedback
signals to a programmable controller by means of a sensor output pathway; k)
the programmable
controller generates control signals for a PWM driver chip which supplies PWM
output for the
brushless motor; 1) a resolver driver chip detects and processes motor
positions for each axis and
forwards angular positional data to a programmable controller chip; in) a
sinusoidal waveform
oscillator generates sine waves for smooth motor speed in low speed in the low
speed mode; n)
in the low speed mode a sine wave is sent to each stator in the brushless
motor, and each
succeeding stator receives a sine input which is a number of degrees advanced
from that sent to a
previous stator; o) in the high speed mode, the brushless motor is driven by
each channel of a
PWM output, which is regulated by a feedback loop from a Hall sensor output;
p) in the
transition phase from the low speed mode to the high speed mode, the
modulation of the PWM
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square waves with sine waves is done by: i) having a center locked phase
alignment between
each wavelength, and by using a fixed frequency between wavelengths; ii) each
sine wave adds
to or subtracts from a standard PWM output and creates a modulated PWM
waveform; q) sine
commutation drives the brushless motor by means of sine wave input to stators,
the sine wave
being incrementally out of phase to each successive stator in increments that
are 360 degrees
divided by the number stators, whereby a rotor is attracted to each stator 14
in turn as the sine
wave input at a first stator increases to its maximum voltage, and then to a
second stator as its
voltage increases while the first stator's voltage decreases; r) the low speed
mode operates when
the brushless motor has an rpm number less than a presselected number in the
range of 200-300
rpm, the high speed mode operates when the brushless mote has an rpm number
greater than a
preselected number in the range of 200-300 rpm; and the transition phase
occurs at a transition
speed in the range of 200-300 rpm, in order to effect smooth operation of the
brushless motor
speeds above and below the transition speed; s) the high speed mode uses
closed loop feedback
which sends Hall sensor outputs to the programmable controller which
determines if a speed of
the brushless motor is high enough and then regulates that speed by means of
PID looping; t) a
result of PID looping determines a duty cycle of a square wave sent to a power
stage controller
which supplies a PWM driver, the driver sends a phased output to drive each
stator of the
brushless motor, the PID looping varies the duty cycle according to
proportional, integrated, and
differential speed error,. commutation is controlled by switching through a
commutation pattern
every time Hall sensors change polarity, and three PWMs run a common duty
cycle, but only one
PWM output is enabled at a time; u) the low speed mode varies a PWM duty cycle
according to
preset values held in a sine wave lookup table, position in the lookup table
is sequential and is
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switched by a timer interrupt to control speed of the brushless motor, each
stator thereof having
its own PWM output source, each source being a set number of degrees apart in
the sine table,
the PWM duty cycle being continuously varied in a sine wave pattern, and
torque being a fixed
factor applied to the sine wave lookup table values before loading into a PWM
duty cycle control
register; v) the low speed mode uses synchronous drive consisting of three
PWMs, sine wave
modulated; w) the transition phase uses H-Bridge PWM drivers, a fixed PWM
output frequency,
and a phase alignment that is center locked; x) if a speed regulation of the
brushless motor is due,
a new speed is calculated by taking a proportional and integral error between
a required speed
and a measured speed, from which a new value for a PWM duty cycle is then
calculated, and a
regulation loop is interrupt driven to ensure that it occurs regularly;
The brushless motor control system thus provides a wide range of smooth and
precisely
controlled speeds for pan-tilt-zoom surveillance cameras, using a seamless
combination of high
and low speed commutation modes for a brushless motor. The system preloads PID
and lookup
table registers used by a programmable controller for a smooth transition from
high speed mode
to low speed mode, and from low speed mode to high speed mode, phase locking a
sine position
during transitions, in order to give a surveillance camera an ability to
quickly move from one
target to another at up to 100 degrees per second yet track objects that are
moving very slowly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a general diagram of all relevant electronic and mechanical
elements of the
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Brushless Motor Speed Control System.
FIG. 2 shows a Motor Driver Card and how it connects to a motor.
FIG. 3 shows a Flow Chart illustrating the transition between modes, some of
the software
functions of its programmable controller chip, and how it controls the motor.
FIG. 4 shows the waveforms of the transition between modes.
FIG. 5 shows a schematic of the Motor Driver Card.
FIG. 6 shows a schematic of the Camera Control Card.
FIG. 7 shows a schematic of the TTL Serial Bus.
FIG. 8 shows a flowchart of basic motor operations.
FIG. 9 shows PWM output waveforms in high speed mode.
DETAILED DESCRIPTION
All elements will first be introduced by reference to figures, then the
functionality and
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interactions of each element with each other element will be described, and
finally the preferred
embodiment of the novel device will be described in detail.
FIG. 1 diagrams the entire Brushless Motor Speed Control System 10 wherein
each motor driver
card 22 receives Hall sensor outputs 20 from the motor 12, and depending on
the operating mode
and the required speed, provides the appropriate PWM output 28 to drive the
motor 12. A camera
control card 40 can issue VISCA (Video System Control Architecture) formatted
commands on a
TTL serial bus 30 to either Pan or Tilt motor driver cards 22, or to the
camera 38. Motor driver
cards 22 can command each motor 12 to go to a specific position, report the
current position by
means of a digital resolver 32, drive at a specific speed and direction and
stop (see FIG. 8). There
will also be commands to read back data such as current speed and PWM levels
and commands
to set motors operational parameters such as closed loop gains, acceleration
rates, brake force etc.
An auxiliary control card 42 can allow for three independent IC bus protocol
ports so that pan,
tilt and zoom positions can be fed to it from each source, thereby enabling
external dedicated
secure control pathways.
FIG. 2 details the motor driver card 22 used to regulate the speed of PTZ
motors 12 as directed
from external sources such as the camera control card 40 or the auxiliary
control card 42, by
means of the TTL Serial Bus 30. Each motor 12 includes a series of stationary
wire-wound
stators 14 affixed around its inner circumference. A central rotating
multipolar magnet called a
rotor 16 generates the rotary motion required to move the camera 38 around
each axis when
provided with an appropriately timed PWM output 28 through the stators 14.
Hall Sensors 18 are
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positioned around the motor 12 to detect polarity changes as it rotates, and
provide a speed
regulation feedback signals to the programmable controller 24 by means of the
sensor output 20
pathway. The programmable controller 24 generates the control signals for the
PWM driver chip
26 which supplies the PWM output 28 for the motor 12. The driver card 22 has
its own resolver
driver 34 chip which detects and processes motor 12 positions for each axis by
means of the
digital resolver 32, and forwards the angular positional data to the
programmable controller chip
24. Finally, a sinusoidal waveform oscillator 36 generates the sine waves 44
necessary for
smoother Low Speed Mode 48, which will be illustrated in FIGS. 3 & 4, and
explained in more
detail below.
FIG. 3 flowcharts the basic functional elements necessary to generate either
low speed mode 48
or high speed mode 50, and demonstrates a smooth transition between the two
modes. With the
exception of external inputs such as those from the TTL bus 30, sine
oscillator 36, or Hall sensor
outputs 20, as well as the power stage controller (PSC) 56, PWM driver 26, and
the motor 12, the
remaining elements on this flowchart illustrate programmable operations within
the onboard
programmable controller 24. In order to properly understand this flowchart,
these operational
terms need more complete definitions; therefore a detailed explanation of its
elements will be
postponed until the fuller description in the preferred embodiment below.
FIG. 4 illustrates by means of wave diagrams, the transition from low speed
mode 48 to high
speed mode 50. In low speed mode 48 a sine wave 44 is sent to each stator 14,
and each stator 14
receives a sine input which is 120 degrees advanced from that sent to the
previous stator 14. In
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high speed mode 50, the motor 12 is driven by each channel of the pwm output
28, which is
regulated by a feedback loop from the hall sensor outputs 20. (see FIG. 3) The
smooth transition
from low speed mode 48 to high speed mode 50 is created by modulating the PWM
square waves
46 with sine waves 44 as shown in FIG. 4. This is made possible by having a
center locked phase
alignment 62 between each wavelength, and by using a fixed frequency 60 (16
kHz, for instance)
between wavelengths. FIG. 4 shows by means of broken arrows where each sine
wave 44 adds to
(Al, Bl, Cl) or subtracts from (A2, B2, C2) the standard PWM output 28
(compare with FIG. 9),
and which creates a modulated PWM waveform which enables the transition from
low speed
mode 48 to high speed mode 50.
FIG. 5 shows a schematic of the motor driver card 22 using an Atmel 90 PWM
programmable
controller 24. FIG. 6 shows a schematic of the camera control card 40, and
FIG. 7 a schematic of
the TTL serial bus 30 circuitry. FIG. 8 flowcharts the non speed related motor
control operations,
and FIG. 9 shows the PWM output 28 waveforms in high speed mode 50.
The preferred embodiment of the brushless motor speed control system 10 will
now be described
in detail. In order to properly understand the core concepts of this
invention, especially as
illustrated by FIGS. 3 & 4, there should be consistent use of terminology
relevant to this field.
Therefore a brief review of the terms relevant to brushless motors, drive and
control methods and
speed modes will now be undertaken.
The two methods used to control the running speed of a motor are open loop
(synchronous) or
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closed loop (asynchronous or feedback). Open loop (synchronous) control is the
application of
direct power to the motor without feedback error correction. Varying the power
applied to the
motor directly varies the motor speed. Closed loop (asynchronous) control is a
method in which
the power input of a motor is adjusted by a control circuit which compares a
reference signal
with a feedback signal proportional to an output parameter (e.g., speed) of
the motor to modify
the power input of the motor so as to achieve or maintain some desired
operating condition of the
motor (e.g., constant running speed). The specific method of closed loop motor
control employed
in this invention will be discussed below.
The two methods used to drive (turn) an electric motor 12 are sine
(sinusoidal) or block
commutation. A commutator in this application is a means of electronic
switching that
periodically reverses the current of the stators 14 in an electric motor 12 in
order to efficiently
turn the rotor 16.
The sine commutation method drives the motor by means of sine wave 44 input to
each stator 14.
In order to turn the rotor 16, the sine wave 44 input at each stator 14 is out
of phase by that
fraction a circle divided by the number stators 14. If three stators 14 are
used, then each stator 14
is 120 degrees out of phase from the last stator 14. By this means the rotor
16 is attracted to each
stator 14 in turn as the sine wave 44 at one stator 14 increases to its
maximum voltage, and then
to the next stator 14 as its voltage increases while the previous stator's 14
voltage decreases. Thus
the motor 12 turns by means of the sine wave 44 commutation method.
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The block commutation method drives a motor 12 by means of square wave 46
input to each
stator 14. When driving a motor 12 by this method, a higher number of stators
14 are usually
required in order to prevent torque losses during rotation from one stator 14
to the next, which is
due to the on-off nature of square wave 46 inputs. The disadvantages of block
commutation are
the cost for the rotor position sensor, tachogenerator, and rotary encoder,
plus the torque jump
during switching between the individual phases, which appears as torque
ripple. Recent efforts to
remedy these problems include powering with sine-valued pulse-width modulation
(PWM).
While PWM block commutation commonly drives brushless DC motors 12 efficiently
at high
speeds, the square wave 46 input is incapable of driving at low speeds without
unwanted uttering
because the phase transitions are not smooth enough.
For purposes of this invention, two speed modes and two transition modes are
defined, with the
low speed mode 48 approximately 200-300 rpm or less, and high speed mode 50
approximately
200-300 rpm or greater. Transition modes are defined as the direction from
which a speed change
passes through the transition speed 70 range (approximately 200-300 rpm).
Therefore, LH mode
66 is defined as a low to high speed transition, and HL mode 68 is a high to
low speed transition.
As will be shown below, because two commutation methods are employed,
different methods are
required to effect a smooth transition depending on whether one is going from
sine to block or
block to sine commutation.
A brushless motor speed control system 10 with a wide speed range, including
ultra low speeds,
14
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requires a means to drive (turn) the motor efficiently, and methods to
smoothly control its speed
at all required ranges. As described above, the disclosed speed control system
10 uses block
commutation at high speed mode 48 and sine commutation at low speed mode 50.
Each of these
modes will now be discussed in detail, as well as the transition speed 70
modes required to effect
smooth operation at all speeds.
High Speed Mode:
The motor 12 is driven in high speed mode 50 by means of square wave 46 PWM
output 28. As
shown in FIG. 3, the speed is controlled with the closed loop feedback method
which sends hall
sensor outputs 20 to the programmable controller 24 which determines if the
speed is high
enough (above 200-300 rpm), and then regulates that speed by means of PID
looping. The result
determines the duty cycle 58 of the square wave 46 sent to the power stage
controller 56, which
supplies the PWM driver 26, which sends the phased output 28 to drive each
stator 14 of the
motor 12. PID looping varies the duty cycle 58 according to proportional,
integrated, and
differential speed error. Commutation is controlled by switching through
commutation pattern
every time hall sensors 18 change state (polarity). All three PWMs run the
same duty cycle, but
only one PWM output is enabled at a time.
Low Speed Mode:
The relationship between the position of the rotor 16 and stator 14, and the
time that the
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electromagnets change their polarity, is known as "timing". In high speed mode
50 the speed is
calculated from the timing between each hall sensor 18 interrupt sent to the
programmable
controller 24. But the hall sensors 18 are not perfectly spaced around the
motor 12 so there will
be some speed jitter. Averaging the sensor output 20 can diminish this effect,
while an optical
encoder could solve this problem. Most application notes state that sine wave
44 commutation
requires an optical encoder but this device adds a significant amount to the
price of the motor
controller. The disclosed method achieves the same speed range without the
need for an optical
encoder, and the hall sensors may be eliminated as well.
Synchronous mode varies the PWM duty cycle 58 according to preset values held
in sine wave
44 lookup table 52. Position in the LUT 52 is sequential and is switched by
timer interrupt 54
which controls speed in the low speed mode 48. Each stator 14 has its own PWM
output 28
source, each source is 120 degrees apart in the sine table, and the PWM duty
cycle 58 is
continuously varied in a sine wave 44 pattern. Torque is a fixed factor
applied to the sine wave
44 lookup table 52 values before loading into PWM duty cycle 58 control
register.
To drive a motor 12 in low speed mode 48 while employing the same feedback
loop of the high
speed mode would require a larger number of Hall Effect sensors for it to be
able to rotate
smoothly at ultra low speeds. Instead, as shown in FIG. 3, the motor 12 is
driven synchronously
in low speed mode 48, Using a timer interrupt 54 to advance the programmable
controller's 24
lookup table (LUT) 52. A timer interrupt 54 is used when a certain event must
happen at a given
frequency, such as causing the motor 12 to rotate synchronously. Therefore,
synchronous timing
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in low speed mode 48 is enabled by the timer interrupt 54, which provides the
appropriate duty
cycle 58 to drive each phase of the PWM output 28 to the motor 12. In order to
run at speeds
below 200 rpm, the motor 12 can be operated open loop in a stepper mode. In
this mode the
commutation is switched from one step to the next under timer interrupt 54 and
the hall sensors
18 are ignored. In summary, low speed mode 48 uses synchronous drive which
consists of three
PWMs sine wave 44 modulated. The ability to switch into stepper motor mode
once the speed
drops below 200-300 rpm means that there is effectively no lower limit to the
speed of the motor
12.
Transition Modes:
A key element to the smooth operation of a dual commutation mode speed control
system 10
requires that the transition between speed modes be smooth. This functionality
is made possible
by faster and more versatile programmable controllers, software programs, and
novel capabilities
of H-Bridge PWM drivers. As shown in FIG. 4, other elements that make a smooth
transition
possible are the use of a fixed PWM output 28 frequency 60, and a phase
alignment 62 that is
center locked. As outlined above, the two transitions possible are LH mode 66
and HL mode 68,
so we will now discuss the unique means to maintain consistent transition
speeds throughout
these ranges. The specifics of this discussion refer to FIGS. 3 and 4
throughout.
LH Mode:
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The transition from low speed mode 48 to high speed mode 50 is shown in FIG.
3, at the point
where the operating speed falls within the transition speed 70 range, the
programmable controller
24 pre-loads default error values into the PID regulation loop in order to
achieve a smooth
transition.
As shown in FIG. 4, the PWM output 28 goes from sine wave 44 to modulated
square wave 46
during the transition speed 70 range to unrnodulated square wave 46 in high
speed mode 50.
HL Mode:
The transition from high speed mode 50 to low speed mode 48 is made possible
by supplying a
similar set of preset values (see FIG. 3) to the sine wave lookup table 52
instead of using the
timer interrupt 54 to synchronize motor 12 driving. In this case, the current
hall sensor 18
position from high speed mode 50 is used to start the sine wave 44 pattern at
the nearest point in
its cycle so that it is synchronized to the rotation of the motor 12. While in
synchronous mode,
drive slip can be detected by monitoring the hall position sensors 20 relative
to the sine wave 44.
Note that there is also a baseline torque input (see fixed torque factor in
FIG. 3) added to the duty
cycle 58 in low speed mode 48.
Software Implementation:
Interrupts from the hall sensors 18 trigger routines that take speed
measurements by reading
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=
timer0. These are stored in an array so that a running average can be taken
from the last 24
samples. The position of the rotor 16 is also read from the hall sensors 18
and the next
commutation step is found and sent to the motor drive chip. At the same time
the current duty
cycle 58 value is read and loaded in the power stage controller (PSC) 56
hardware which
generates the PWM drive output 28 to the motor 12 windings.
Asynchronously, the main software loop runs and reads commands in from the TTL
serial bus 30
port, which are decoded and processed by the programmable controller 24. After
that, if a speed
regulation is due, the new speed is calculated by taking the proportional and
integral error
between the required speed and the measured speed. A new value for the PWM
duty cycle is then
calculated. Currently the PWM has 12 bit resolution and the error values are
16 bit signed. The
regulation loop could be changed to be interrupt driven to ensure that it
occurs regularly. Also the
PWM duty cycle 58 is only reloaded 24 times per revolution i.e. as each hall
interrupt occurs. As
speed variation cannot be measured between hall sensors, there is little point
in make PWM
adjustments more frequent.
As shown in FIGS. 1-3, the resolver 32 is an angular positioning sensor which
provides axis
positional data to the camera control card 40 by means of the resolver driver
chip 34, and this
allows the motor driver card 22 to turn a camera 38 to a specific position at
a specific speed. PTZ
position data is also sent to the auxiliary control card 42 on a high speed I
C bus from the camera
control card 40. The camera control card 42 manages the preset position
control loop and is
solely responsible for sending speed commands to the motor driver cards 22.
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The dual mode of operation provides a very large range of speed control giving
the camera the
ability to quickly move from one target to another at up to 100 degrees per
second and track
objects moving very slowly. For practical purposes that can be described as
one 360 degree
rotation per month, although the system could go slower if there was a need.
No product on the
market currently has anywhere near this range of speed of operation or ultra
low speed capability.
Minimum slow speed possible approaches zero rpm and is only limited by the
size of the counter
timer and the clock rates provided to it. Currently there are 64 steps in the
sine wave table and
the maximum rate we clock through is around 300 rpm with a motor using 4 pole
pairs. In the
current programmable controller 24 design, the bottom speed is set to 30 rpm
at the motor, but
the current design can operate down to 1 rpm. Motors with fewer pole pairs
will rotate faster.
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Top speed is limited by supply voltage and CPU processing speed. Currently we
can go beyond
the rated specification of the input stage of the gearbox, 10,000 rpm and have
measured speeds of
over 18,000 rpm. Again, motors with fewer pole pairs will rotate faster. In
the programmable
controller 24 design, the standard power supply voltage limits the top speed
of the motor 12, but
we aim to get up to 12,750 rpm. The current design will go up to nearly 18 k
but it needs 30V to
do it. Using a Maxon motor there are 24 interrupts per revolution so that the
CPU is clocking at
the maximum speed available.
There are three Hall sensors 18 in all of the motors 12 we have tested and
they commutate on the
rising and falling edges. The 4 pole pair Maxon motor gets 24 edges per
revolution. The
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CA 02631299 2016-07-25
limitations are the lookup table size which gives some granularity to the
slowest speeds, but one
can achieve 64 steps when on full zoom. So by storing a quarter of the sine
wave, a potential to
quadruple the resolution without using up any more space is possible. The sine
mode is limited in
its maximum speed because it is synchronous and it does not develop maximum
torque in the
motor, so there is quite a lot of heating in the motor, which means that its
basically operating at
maximum stall current all the time in this mode.
The core uniqueness of the brushless motor speed control system 10 is the
provision of a very
wide speed control range with minimal processing power and cost. This is
achieved by means of
seamless integration of commutation modes, preloading the PID & LUT registers
for a smooth
transition, and phase locking the sine position during transitions.
The foregoing description of the preferred apparatus and method of
implementation should be
considered as illustrative only, and not limiting. Other embodiments are not
ruled out or similar
methods leading to the same result. Other techniques and other materials may
be employed
towards similar ends. Various changes and modifications will occur to those
skilled in the art,
without departing from the true scope of the invention as defined in the above
disclosure, and the
following general claims.
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