Note: Descriptions are shown in the official language in which they were submitted.
CA 02317859 2000-09-07
Attorney Docket No.
LF-20867
TREADMILL MOTOR CONTROL
Field of the Invention
This invention generally relates to exercise equipment and in particular to
exercise
treadmills having an AC motor control system.
Background of the Invention
Exercise treadmills are widely used for performing walking or running aerobic-
type
exercise while the user remains in a relatively stationary position. In
addition exercise
treadmills are used for diagnostic and therapeutic purposes. Generally, for
all of these
purposes, the person on the treadmill performs an exercise routine at a
relatively steady and
continuous level of physical activity. One example of such a treadmill is
provided in U.S.
1 S Patent No. 5,752, 897.
Although exercise treadmills that use an AC motor to drive the belt have
reached a
relatively high state of development, they still have a number of problems
involving treadmill
frame resonance or vibration, low speed operation and power consumption.
Summary of the Invention
It is therefore an object of the invention to provide an exercise treadmill
having
improved AC motor control.
An additional object of the invention is to shift the phase relationship of a
three phase
drive signal applied by the motor controller to the motor in order to minimize
frame
resonance.
Still another object of the invention is to provide a method to minimize
frame resonance by staggering the phase relationship of the three phase drive
signal applied
by the motor controller to the motor.
A further object of the invention is to overcome operational problems of a
three phase
induction motor at low speed. By commanding the motor controller to present a
drive signal
of a frequency much higher than the desired motor speed to the motor at low
speeds, a high
slip, or difference between the driving signals circulating field and the
actual motor speed,
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CA 02317859 2000-09-07
will result thereby improving low speed performance of the treadmill.
Brief Description of the Drawings
Fig 1. is a perspective view of an assembled exercise treadmill according to
the
invention;
Fig. 2 is a block diagram of the control system for the treadmill of Fig. 1;
Fig. 3 is a schematic diagram of a motor control circuit for use with the
control
system of Fig. 2;
Fig. 4 is a phase diagram depicting a standard AC motor phase relationship;
Fig. 5 is a phase diagram illustrating an AC motor phase relationship
staggered from
the relationship of Fig. 5 used to minimize frame resonance of the treadmill
of Fig. I ;
Fig. 6 is a phase diagram illustrating an AC motor phase staggering
relationship used
to minimize frame resonance of the treadmill of Fig. 1;
Fig. 7 is a voltage vs. frequency diagram illustrating an AC motor drive
signal used to
improve low speed operation of the treadmill of Fig. l; and
Fig. 8 is a graph of torque vs. slip speed curves relating to the diagram of
Fig. 7.
Detailed Description of the Invention
Fig. 1 shows the general outer configuration of an exercise treadmill 10,
according to
the invention. The treadmill includes a control panel 12 having a set of
displays 14; a set of
workout program control buttons 16; a set of operational controls 18-22
including a pair of
time control buttons 18; a pair of incline control buttons 20 and a pair of
speed control
buttons 22; a numerical keypad 24; and a stop button 26. In addition, the
treadmill 10
includes such conventional treadmill elements as a belt 28, a deck 30 and an
inclination
mechanism 32 of the type described in U.S. Patent No. 6,095,951.
Fig. 2 is a representative block diagram of a control system 34 for the
treadmill 10.
The control system 34 is generally similar to the treadmill control systems of
the type shown
in Fig. 16 of U.S. Patent No.6,095,951 and controls an AC motor 38 having a
motor
controller 36 to propel the belt 28. The control system 34 uses a
microprocessor based
system controller 40 to control: the control panel displays 14 including the
message display
14; the user controls 16-22 and 26; the keypad 24, an optional remote display
42; and a
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CA 02317859 2000-09-07
remote keypad 44. In addition, the control system 34 serves to control a heart
rate monitoring
system of the type described in U.S. Patent No. 5,313,487 utilizing a set of
pulse sensors 46
and a deck or belt lubrication system 48 of the type shown in U.S. Patent No.
5,433,679 along
with the inclination mechanism 32. The control system also controls a user
detect or sense
system 50.
Fig. 3 provides an illustration of a preferred embodiment of the motor 38 and
the
motor controller 36 along with a power factor control circuit 52. Here, the
three phase AC
motor 38 having a set of three armature windings 54A-C is powered by the motor
controller
36 having a microprocessor 56 which controls a three phase inverter 58 that
includes a set of
six paired power or drive transistors 60A-B, 62A-B and 64A-B and six
associated
freewheeling diodes (not shown). Two of the drive transistors 60A-B, 62A-B and
64A-B are
connected to each of the armature windings 54A-C and are used for each phase
leg. Each
pair of the drive transistors 60A-B, 62A-B and 64A-B is in a complementary
manner by the
microprocessor 56. Normally the processor 56 controls the drive transistors
60A-B, 62A-B
and 64A-B to generate three identical but 120 degrees phase shifted waveforms
that are
applied to the armature windings 54A-C. The amplitude and frequency of this
waveform is
determine by the desired motor speed. In the preferred embodiment, a speed
sensor 68
integral with the motor 38 provides an input over a line 70 to the
microprocessor 56 to close
the motor speed control loop. The inverter 58 obtains power from a DC bus 72
which can be
derived from a 110 volt, two phase power source 74 which can be a ordinary
household
power line. In embodiments that do not use the power factor control circuit
52, the current
from the power source 74 is rectified and filtered to provide the DC bus
voltage on line 72.
For lower voltage AC power sources 74 (100-120VAC), the rectifiers and
capacitors (not
shown) are configured as a voltage doubter such that the DC bus voltage on the
line 72 is the
same for 120VAC with a doubter as it is for a 240VAC power source without the
doubter.
Fig. 4 is a phase diagram representing shifting in the phase relationship of
the three
phase drive signal applied by the motor controller 36 to the armature windings
54A-C in
order to minimize frame resonance. The motor controller processor 56 is
connected to the
system controller S0, as well as other components of the control system 34,
over a
communication bus 76. The processor 56 has direct control of the six drive
transistors 60A-
64B which apply to the motor armature windings 54A-C a three phase drive
signal of varying
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CA 02317859 2000-09-07
amplitude and frequency. This is accomplished by pulse width modulating the
three pairs of
drive transistors 60A-64B in a complimentary fashion which generates an
effective voltage at
any point in time. The three phases offset in time by 120° present a
space vector wave shape
voltage to the armature windings 60A-64B. The voltage/frequency relationship
is determined
by the motor's 38 volt/hertz curve and the feedback signal from the speed
sensor 68. In the
preferred embodiment other signals are also used including voltage, motor
current, and the
motor controller 36 temperature. It has been found that in a certain motor rpm
range, the
drive signal generated by the standard volt/hertz curve and space vector
modulation of the
motor controller 36 causes the housing of the motor 38 to vibrate at a
frequency
corresponding to the resonant frequency of the frame of treadmill 10. This
undesirable
resonance causes an objectionable audible noise to the user and nearby
observers besides
being transmitted to the user's feet through the belt 28. Since the vibration
is caused by the
motor 38 and amplified by the frame of the treadmill 10, one solution is to
smooth out the
motor 38 so as to not generate the vibrations. Another less desirable solution
is to make the
1 S frame of the treadmill 10 non-resonant which can be a very difficult and
costly proposition.
By modifying the drive signal from the standard space vector modulation scheme
it is
possible to reduce the vibrations generated by the motor 38. The preferred
technique uses
phase shifting, on a per revolution basis shifting the phase of the drive
signal to the motor
armature windings 60A-64B. In the preferred implementation of this approach
depicted in
the phase diagram of Fig. 4, at the zero crossing of the first phase 78, a
drive signal 80 is
shifted back in time a few percent, for example 2%, relative to that of an
unmodified drive
signal 82. At the next zero crossing 84, the first phase is shifted ahead in
time the same
percentage relative to that of unmodified drive signal 82. Preferably, all the
phases of all
three of the drive signals are shifted at the same point in time which means
the first phase at
0°, the second phase at 120° and the third phase at 240°.
This shifting behind, then shifting
ahead of the drive signal position relative to that of the unmodified drive
signal 82 can
substantially smooth out vibrations of the frame of the treadmill 10 when it
occurs at or
approximately near the resonant frequency of the frame of the treadmill 10. In
the preferred
embodiment, the phase shifting is controlled in such a manner by the processor
56 so as to be
inactive at a minimum motor speed. It then scales up in a linear fashion to a
target speed
corresponding to the resonant frequency of the treadmill 10 at which point it
is totally active.
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CA 02317859 2000-09-07
Then it scales down again in a linear fashion to a maximum motor speed where
it is again
inactive. This can insure smooth operation of the motor 38 and imperceptible
transitions from
inactive to active then inactive operation of the drive signal phase shifting
while substantially
reducing vibration induced in the frame of the treadmill 10.
Figs. 5 and 6 are phase diagrams illustrating an alternate method to minimize
resonance of the frame of the treadmill 10 by staggering the phase
relationship of the three
phase drive signal applied by the motor controller 36 to the motor 38. This
technique uses
phase staggering, that is, staggering the phases of the drive signal to the
armature winding
54A-C. This approach utilizes a change in the phase relationship from the
standard
relationship: a first phase 88 at 0°, a second phase 90 at 120°
and a third phase 86 at 240° as
shown in Fig. 5 to an anti-resonant phase relationship of where, for example,
the first phase
88 is at 0°, the second phase 90 is at 115 ° and the third phase
86 is at 245 ° as shown in Fig.
6. Preferably, the staggering of the drive signals by the processor 56 is
activated in a narrow
band around the resonant frequency of the frame of the treadmill 10. This
approach can
1 S result in a lower non-resonant vibration which is much less objectionable
to the user and
observers while greatly reducing the resonant vibration in the frame of the
treadmill 10.
Fig 7 is a voltage vs. frequency diagram illustrating an AC motor drive signal
from
the motor controller 36 that can be used to improve low speed operation of the
treadmill 10
where the standard relationship between voltage and frequency is shown by a
line 92. As
discussed above, a number of commercial treadmills use a three phase induction
motor driven
by a motor controller. In the preferred embodiment, the processor 56 has
direct control of the
six drive transistors 60A-64B in the motor controller 36 which apply to the
motor armature
windings 54A-C a three phase drive signal of varying amplitude and frequency.
The three
phases are offset in time by 120° as shown in Fig. 5 and present a sine
wave/space vector
voltage to the armature windings 54A-C. The voltage/frequency relationship is
determined
by the motors volt/hertz curve and the feedback signal from the speed sensor
68. The
classical voltJhertz curve uses the desired motor speed as the driving signal
frequency with
sufficient amplitude to provide adequate torque. Implementing a volt/hertz
curve where the
motor has sufficient torque at low speed however causes the motor 38 to cog
because of the
finite number of poles (not shown) in the motor 38 and because the speed is
not high enough
for a reasonable sized flywheel (not shown) attached to the motor 38 to dampen
out the
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CA 02317859 2000-09-07
vibrations. This cogging can cause large vibrations in the belt 28 which in
turn are very
uncomfortable to the user. The preferred solution is to smooth out the
operation of the motor
38 so it does not generate any low speed vibrations. Another less desirable
solution is to
increase the mass of the flywheel which can become very expensive. To
accomplish the
S preferred solution to this problem, the motor controller 36 applies drive
signals having a
frequency much higher than the desired motor speed to the motor armature
windings 54A-C
at low motor speeds. This generates high slip or difference between the
frequency of the drive
signals generating the circulating field in the motor 38 and the actual speed
of the motor 38.
This slip increases the frequency and decreases the amplitude of the cogging
to the point of
being smoothed out by the motor's flywheel. However, the higher frequency of
the drive
signals makes the motor 38 want to run at a much higher speed than desired.
Therefore, in
the preferred embodiment, the processor 56 using a feedback signal from the
speed sensor 68
dynamically controls the amplitude of the driving signals applied to the
armature windings
54A-C thereby keeping the motor 38 at the desired speed. A line 94 in Fig. 7
illustrates an
example of how the motor controller 36 can reduce the amplitude of the driving
signal to
control motor speed. Fig. 8 is a set of two torque vs. slip speed curves where
a curve 98
indicates the torque produced by the motor 38 without the reduction in drive
signal amplitude
and a curve 100 depicts the torque produced with the reduced amplitude as
shown by the line
94 in Fig. 7. An envelope 96 between lines 92 and 94 shows an example of the
area of
operation of the high slip, low speed operation of the motor 38 according to
this embodiment
of the invention. Also, it is preferred that as the speed of the motor 38
increases, the amount
of slip is reduced gradually and in a linear fashion shown by the volt vs.
frequency line 94.
Another significant feature of the invention relates to the use of the power
factor
control circuit shown in Fig. 3. In order to facilitate the use of the AC
motor such as motor
38 having sufficient horse power to drive the belt 28 at higher speeds while
using a lower
voltage power source such as the AC power sources 74, the power factor of the
input current
from the power source 74 is modified. In the preferred embodiment of the
invention, the
power factor control circuit 52 is inserted between the motor controller 36
and a rectifier 102
that in turn is connected to the two phase power source 74. In this case, the
power factor
control circuit 52 utilizes a boost converter having an inductor 104, a power
transistor switch
106 and an output rectifier diode 108. The transistor 106 is controlled by a
power factor
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CA 02317859 2004-06-02
controller IC 110, for example a MicrolinearTM ML4812, that programs the motor
control input
current on line 72 to follow the rectified input voltage from the rectifier
102 . This allows the
impedance of the load on the power source 74 to appear more purely resistive
thereby
improving the power factor of the current input into the motor 38. The power
factor control
circuit 52 provides two performance advantages when used with the motor
controller 36.
First, more power can be drawn from the power source 74 because the improved
power factor
reduces the current at a given load. Second, the increased voltage motor 38
makes it possible
to obtain higher torque out of the motor 38 at higher speeds.
It should be noted that the various features described above have been
described in
terms of their preferred embodiments in the context of the particular
treadmill 10, motor 38
and motor control 36 disclosed herein. The manner in which these features can
be
implemented will depend upon a number of factors including the nature of the
treadmill, the
AC motor and the motor control. For example, there are many different types of
electrical
circuits and components that would be suitable for implementing power factor
control and
motor control which would be functionally equivalent to the preferred
embodiments as well
as within the scope of this invention.
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