Note: Descriptions are shown in the official language in which they were submitted.
CA 02295451 2000-O1-11
ROTARY AND LINEAR MOTOR
FIELD
The present invention relates to a combination
rotary and linear motor which uses a modified brushless DC
motor.
BACKGROUND
There are a number of different configurations
which provide a combination of rotary and linear motion.
U.S. Patent No. 4,570,254 issued to Agostini discloses a
l0 drive motor for a compact disc player which uses permanent
magnets for both the rotor and the stator and employs
magnetic attraction to move the rotor from an intermediate
position to a playing position. The use of an axial
magnetic attractive force results in a movement, which
requires external stops to achieve a final position, as
there is no control over the force itself.
There are a series of patents represented by U.S.
Patent No. 5,627,418 issued to Satomi et al. which employ a
stepping motor having a stator and a mover having a series
of laminated iron plates shaped to provide inner stator
teeth along a direction of the motor axis and teeth in the
circumferential direction and outer mover or rotor teeth
opposed to said stator teeth. Permanent magnets are
incorporated into the stator or rotor. Energizing the
stator causes rotary motion and stepped linear motion.
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Aside from being complex to produce, such motors do not
provide continuous precise motion control.
U.S. Patent No. 3,394,295 issued to Cory discloses
a pair of diametrically opposite coils which operate in
conjunction with an axially spaced apart pair of
diametrically opposite permanent magnets oriented at 90
degrees to the coils. The rotor has discrete energized
positions which are 90 degrees with respect to each other
and so also do not provide precise angular control.
Accordingly, it is an object of the invention to
provide an improved motor providing concurrent rotary and
linear motion. It is a further object of the invention to
provide a motor with both concurrent rotary and linear
motion and both of which are continuously precisely
controllable.
SUNJ~IARY OF THE INVENTION
According to the invention there is provided
a rotary and linear motor having a rotor, a casing and a
plurality of stator coils mounted to the interior of the
casing spaced from the rotor and radially spaced apart
around the rotor. A rotary and linear motor control circuit
is operative to receive input data signals and to process
these data signals so as to produce a plurality of pulse
width modulated output control signals. A multi-phase
inverter circuit is coupled to an output of the rotary and
linear motor control circuit and is responsive to the
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plurality of output control signals to drive the stator
coils and produce a desired z-axis displacement, z, and
rotor angular displacement,6.
The input data signals may include sensed linear
displacement, z, sensed rotor angle, 8, coil current sensor
outputs Ia, and Ib, and command axial displacement, z~, and
command rotary angle,
In another aspect of the invention there is
provided a method of controlling the linear and rotary
movement of a motor having a plurality of stator coils
radially spaced around a rotor, wherein the rotor has at
least one magnetic pole. The method includes transforming
the coil currents through the stator coils into a current,
Id, through a notional coil having a d-axis parallel to a
pole of the rotor and a current, Iq, through a notional coil
having a q-axis perpendicular to the d-axis. Next the
method involves employing current Id to control the angular
position of the rotor and employing current Iq to control
the linear position of the rotor.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages will be apparent
from the following detailed description, given by way of
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example, of a preferred embodiment taken in conjunction with
the accompanying drawings, wherein:
Fig. 1 is cross section taken transverse to the
axis of the motor for a three phase one pole motor;
Fig. 2 is a cross section of the motor of Fig. 1
through the motor axis;
Figs. 3a, 3b, and 3c are a set of three cross
sections of the motor of Fig. 1 showing Z-axis movement;
Fig. 4 is a graph showing the force along the z-
axis for various values of the d-axis component of motor
coil current;
Fig. 5 is a schematic diagram of the motor control
circuit;
Fig. 6 is a cross section of the motor along a
plane through its shaft with the linear displacement and
rotor angle sensor;
Fig. 7 is a circuit diagram for a three phase
inverter used for driving the motor;
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Fig. 8 is schematic diagram of the control
algorithm for the motor; and
Fig. 9 is a waveform diagram showing the effect of
5 coil voltages on the pulse width modulated signals to the
coils.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
Referring to Figs. 1 and 2, the rotary linear
motor 10 has a rotor 12 mounted rigidly to a rotor shaft 14
journaled to a motor casing 16. Three radially equi-spaced
coils 18a, 18b, and 18c are mounted to the interior
cylindrical surface of the casing 16.
Fig. 2 shows the cylindrical rotor as being made
of permanent magnet with a half-cylinder being the north
pole and the other half cylinder being the south pole. A
coordinate system, which moves with the rotation of rotor
12, has axes d and q with the d-axis bisecting the north
pole and directed outwardly from that pole and the q axis
being perpendicular to the d-axis. Using this coordinate
system one can define the following parameters:
Va= modulating signal voltage of signal modulating
carrier signal for coil 18a,
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Vb= modulating signal voltage of signal modulating
carrier signal for coil 18b,
Vc= modulating signal voltage of signal modulating
carrier signal for coil 18c,
Ia= current proportional to actual current through
coil 18a,
Ib= current proportional to actual current through
coil 18b,
Ic= current proportional to actual current through
coil 18c,
Id= d-axis component of effective motor coil current,
Vd= d-axis component of effective motor coil voltage,
Iq= q-axis component of effective motor coil current,
Vq= q-axis component of effective motor coil voltage,
a = coil length along direction of motor axis,
z = rotor displacement along z-axis,
8 = radial angle of displacement of rotor,
w = angular velocity of rotor (~=dA/dt),
~r = magnetic flux from the rotor,
T = torque on rotor,
F = force on rotor along Z-axis,
R = resistance of stator coils,
Ld = inductance due to the time variation of current
along the d-axis,
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Lq = inductance due to time variation of current along
the q-axis,
c1 = coefficient of back emf,
c2 - coefficient of torque.
It should be noted that Id is the current through a notional
coil whose axis is aligned along the d-axis and Iq is the
current through a notional coil whose axis is aligned along
the q-axis.
The motor of Figs. 1 and 2 is a three phase
structure but the stator can be wound as a 4 or 5 phase
structure. Similarly although a one pole rotor is shown, a
two, or four pole rotor could be used.
Equations for voltage, torque and z-axis force can
be approximated as follows:
Vd = RId + Ld dId/dt + c~ LqIq + c1 ~r dz/dt,
Vq = RIq + Lq dIq/dt - (~ Ld Id + Qtr (~,
T = c2 ~r Iq, and F = f(z,Id),
Va = Vd Cos (8) + Vq Sin (8) ,
Vb = Vd Cos(6-2n/3) + Vq Sin(6-2n/3),
Vc = Vd Cos(6-4n/3) + Vq Sin(6-4n/3),
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Ic= -(Ia + Ib),
Id= (IaCos(6) + IbCos(8-2n/3) + IcCos(6-4n/3))*0.66,
Iq= (IaSin(6) + IbSin(8-2~/3) + IcSin(6-4n/3))*0.66.
Considering that F is a function only of the position along
the z-axis and the current Id, when there is d-axis current,
Id, and a displacement of the rotor along the z axis as seen
in Figures 3b, and 3c, then there will be a net force F
which tends to pull the rotor in or push it out of alignment
l0 with the coils along the z-axis of the motor.
Referring to Fig. 4, the relationship of force F
and f(z,Id) for various values of Id is shown. It will be
seen that the force F is zero at z=0 but rises steeply to a
flat portion which, at large positive and negative values of
z, falls off towards zero. Consequently, operation on the
flat portion of the curves is preferred.
A q-axis current will create a torque on the rotor
so as to cause the rotor to turn. The z-axis force due to
Iq is almost nil. Because of the minimum of interaction by
Iq on the z-axis force and Id on the rotor torque, Id can be
used to control z-axis movement and Iq can be used to make
the motor turn.
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Referring to Fig. 5, the required z-axis position
is input as a command on line 22 while the required rotary
angle A is input on line 20. The rotary and linear motor
control algorithms are output on any of 6 outputs, namely,
PWMA, PWMA_, PWMB, PWMB_, PWMC, and PWMC- Here the acronym
PWM stands for pulse width modulation. Each of the signals
PWMA, PWMA-, PWMB, PWMB_, PWMC, and PWMC_ drive a three
phase inverter 26 for driving the rotary and linear motor
10. The output from the three phase inverter 26 on the
three lines 28 drives coil A, coil B and coil C,
respectively. The coil current in coils A and B are each
sensed and fed back to the motor control algorithms 24. As
seen in Fig. 6, axial displacement and rotor angle are each
sensed in the motor 10 by a linear displacement sensor 32
and rotor angle sensor 34 and fed back to motor control
algorithms 24 (see Fig. 5).
Referring to Fig. 7 there is shown the three phase
inverter for driving the rotary and linear motor. In this
case series connected field effect transistors (FETs) 36 and
38, 40 and 42, and 44 and 46 are coupled together in series
across an input capacitor 52. Outputs are taken from the
intersection of the pairs of FETs. If, for example, the FET
38 driven by PWMA_ goes ON, then the line to motor coil A is
drawn down to ground potential. If, however, the FET 36
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driven by PWMA goes ON, the line to motor coil A is raised
to Vo.
The motor control algorithms, shown in detail in
5 Fig. 8, have comparators 60 and 62. Comparator 60 subtracts
the measured value of "z" from the command value z and
sends the difference to Cz(S) 64. Similarly, comparator 62
subtracts the measured rotor angle 8 from the command value
A~ and the difference is sent to Ce(S) 66. The output from
10 Cz(S) 64 is the z-axis force Idc while that from Ce(S) 66 is
current Iqc. Generally, Cz(S) 64 and Ce(S) 66 take the form
of a PID Controller, where S is the Laplace Operator.
Another set of comparators 68 and 70 subtract Id from the
command value Idc and Iq from Iqc and sends the differences
to Cd(S) 72 and Cq(S) 74. Generally, Cz(S) 64 and Ce(S) 66
take the form of a PI Controller, where S is the Laplace
Operator. The values of Id and Iq are determined by
transformation block 80 from Ia and Ib, the coil current in
coils Ia and Ib in accordance with the following formulas:
Id=(IaCos(8) + IbCos(6-2~/3)+IcCos(6-4n/3))*0.66
Iq=(IaSin(6) + IbSin((6-2~/3)+IcSin(6-4~/3))*0.66
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Here Ic= -(Ia + Ib). The value of 8 is obtained from the
rotor angle sensor 34 (see Fig. 6) and fed to transformation
blocks 76 and 80. The output from Cd(S) 72 is Vd while that
from Cq(S) is Vq. Vd and Vq are directed to a
transformation block 76 which calculates the phase voltages
Va, Vb, and Vc which are calculated according to the
following formulae:
Va = Vd Cos(6) + Vq Sin(6)
Vb = Vd Cos (A-2n/3 ) + Vq Sin (6-27L/3 )
Vc = Vd Cos(8-4n/3) + Vq Sin(6-4n/3)
Thus, Va, Vb, and Vc have a sinusoidal waveform.
The PMW logic circuit 78 (see Fig. 8) uses Va, Vb, and Vc to
establish PWMA, PWMA-, PWMB, PWMB-, PWMC, and PWMC-
Referring to Fig. 9, Va causes the logic circuit 78 to
extend the time duration of the first half cycle by an
amount proportional to the value of Va and decreases the
time duration of second half cycle by a like amount. The
signals PWMA-, PWMB, PWMB-, PWMC, and PWMC- are determined
in similar way.
In operation, as seen in Fig. 5 and 8, command
values of the z-axis position "z~" and the rotary angle "A~"
are generated by a software program (not shown) programmed
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to carry out a sequence of operations. At the same time
actual values of "z" and "8" sensed by the linear
displacement sensor 32 and the rotor angle sensor 34 are
sent from the rotary and linear motor and sensors 10 to the
rotary and linear motor control algorithm 24. As discussed
above, the values of "z" and "z~" and "A" and "6~" are
compared and the difference fed into Cz(S) 64 and Ce(S) 66
to yield Idc and Iqc, respectively. Coil currents Ia and Ib
are transformed by transformation block 80 to produce Id and
Iq. Idc and Iqc are compared with Id and Iq by comparators
68 and 70, respectively, and the differences sent to Cd(S)
72 and Cq(S) 74 to yield Vd and Vq, respectively.
Transformation block 76 transforms Vd and Vq to produce coil
voltages Va, Vb, and Vc, on three separate lines, which are
applied to the PWM Logic circuit 78. The outputs from the
PWM Logic circuit 78 are applied to respective gates of FETs
36, 38, 40, 42, 44, and 46 (see Fig. 7). The outputs 81,
82, and 84 from the junction of the pairs of FETs 36 and 38,
40 and 42, and 44 and 46, respectively, are directed to the
motor coils 18a, 18b, and 18c. Current sensors 48 and 50
for coils 18a and 18b are in output lines 81 and 82,
respectively.
For a rotor having two or more poles and six or
more stator coils the operation would follow the principle
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as for a single pole-three stator coil configuration except
torque is increased with an increased number of poles and
associated stator coils.
Accordingly, while this invention has been
described with reference to illustrative embodiments, this
description is not intended to be construed in a limiting
sense. Various modifications of the illustrative
embodiments, as well as other embodiments of the invention,
will be apparent to persons skilled in the art upon
reference to this description. It is therefore contemplated
that the appended claims will cover any such modifications
or embodiments as fall within the true scope of the
invention.
