Note: Descriptions are shown in the official language in which they were submitted.
CA 02507856 1998-05-O1
CONVEYOR WITH INDEPENDENT CONTROL OF MULTIPLE PALLETS
This is a divisional of Canadian Patent Application No. 2,288,223, filed
May 1, 1998.
FIELD OF INVENTION
The invention generally relates to conveyor systems, and more specifically to
conveyor systems in the form of modular linear motors having multiple moving
elements
under independent control.
BACKGROUND OF INVENTION
There are a number of fundamental limitations with well-known conventional
conveyor systems which employ a belt for transporting pallets between
processing
stations. First, the speed of the belt is typically quite limited. This is
largely due to the
fact that the pallets are typically stopped, e.g., in order to be processed at
a
processing station, by mechanical stop mechanisms. Thus, if the belt conveyor
is
operated at a high speed, the strong impact between a pallet and mechanical
stop is
likely to jar whatever parts the pallet may be carrying for processing.
Second, it is
generally not possible to vary the acceleration and velocity profiles for
individual
pallets. For instance, if a first pallet is empty and a second pallet is
loaded with
delicate parts, it is generally not possible to aggressively accelerate the
first pallet to a
high speed while controlling the second pallet using more gentle acceleration
and
velocity profiles. This limitation affects the latency and possibly the
throughput of the
manufacturing line. Third, the belt conveyor is typically not bidirectional,
which may
result in a suboptimal design of the manufacturing line. Fourth, the belt
conveyor
typically provides limited flexibility or programmability, such as being able
to very
quickly change the positions of processing stations. Finally, the data
acquisition
capabilities provided by the belt conveyor are typically quite limited. For
example, it is
typically not possible to know where the pallets and their constituent loads
are located
along the conveyor at all times. Thus, for instance, it may be difficult to
know how
many pallets are queued at a particular processing station. For these and
other
reasons, a conveyor system having multiple moving elements or pallets under
substantially independent control may be desirable for various types of
applications.
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CA 02507856 1998-05-O1
Conveyor systems having multiple pallets under substantially independent
control are known in the art, but suffer from a variety of limitations. For
example, U.S.
Patent No. 4,841,869 issued June 27, 1989 to Takeuchi et al. discloses a
conveyor
system utilizing a linear induction motor, comprising a conveyor cart and a
guide rail
for movably supporting the conveyor cart. The guide rail includes primary
coils, and
the conveyor cart includes a flexible secondary conductor extending
longitudinally of
the cart so as to follow the guide rail. The primary coils comprise a station
primary coil
disposed at each loading and unloading station for stopping and starting the
conveyor
cart, two primary coils adjacent opposite ends of the station primary coil for
decelerating the conveyor cart that is to be stopped at the station by the
station
primary coil and for accelerating the conveyor cart having started from the
station to a
target running speed, and a plurality of intermediate accelerating primary
coifs
disposed between two adjacent stations for accelerating the conveyor cart to
maintain
the latter at the target running speed.
A major shortcoming with the Takeuchi et al. system is that the carts or
pallets
thereof cannot be positioned to stop at any point along the conveyor, but only
where
the linear motors thereof are disposed. This makes changing the location of a
station
a troublesome endeavour. In addition, the system is not capable of pinpointing
the
location of a moving pallet at any time. In view of these limitations, the
Takeuchi et al.
system does not feature truly independent and totalcontrol of multiple moving
elements.
U.S. Patent No. 5,023,495 issued June 11, 1991 to Ohsaka et al. discloses a
moving-magnet type linear d.c. brushless motor having plural moving elements
disposed for motion along a track. The track includes a coreless stator
armature
having a plurality of contiguously arranged coils thereon. Each moving element
includes a thrust-generating field magnet having P contiguous magnetic poles
of
alternating N and S polarity (i.e. polypolar magnet) having one side facing
the stator
armature. Each moving element may also include a polypolar position-detecting
magnet. The track includes a row of position/commutation sensors, each row of
position/commutation sensors being provided for detecting the magnetic poles
of only
the position-detecting magnet of a corresponding moving element. The
position/commutation sensors are used in control circuitry for generating an
electric
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CA 02507856 1998-05-O1
current in the stator armature to move the moving elements in predetermined
directions separately and independently.
The Ohsaka et al. system also has a number of shortcomings, particularly with
respect to the modularity or scaling properties of the system. First, due to
the fact that
a separate track of position/commutation sensors is required for each moving
element, the system can only accommodate a relatively small number of moving
elements. Second, the length of the linear motor is limited by a servocontrol
mechanism, described as a single microcomputer, which can only process and
accommodate a limited number of the position/commutation sensors and
associated
electric current generating control circuitry. Third, use of the magnetic
position-
detecting elements provides a relatively poor resolution for measuring the
position of
the moving element. Fourth, the winding arrangement of the stator armature is
essentially that of a linear stepper motor, which presents an uneven magnetic
reluctance along the stator armature resulting in relatively noticeable
cogging effects
and a jerky thrust production. Finally, the coreless design of the stator
armature also
results in a relatively low average thrust production which may not be
suitable for
typical conveyor system applications.
SUMMARY OF INVENTION
The invention seeks to avoid many of the limitations of the prior art in order
to
provide a linear motor for a conveyor system having multiple moving elements
under
independent control, such that the conveyor system can be constructed out of
discrete, self-contained, modular track sections, with little practical
restriction on the
length of the conveyor system or the number of pallets controlled thereby.
In a first aspect there is provided an apparatus for detecting the position of
a
moving element relative to a stationary element. The apparatus comprises a
plurality
of linear encoder readers spaced generally along the stationary element at
fixed
positions relative thereto. A device is readable by the linear encoder
readers. The
readable device is mounted on the moving element and has a length which is
greater
than the spacing between any given pair of adjacent linear encoder readers.
Guide
means are provided for aligning the readable device in order to interact with
the linear
encoder readers. Processing means are connected to each linear encoder reader,
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CA 02507856 1998-05-O1
for associating the readable device with only one linear encoder reader in a
state of
interaction with the readable device at any time and for resolving and
providing a
reading of the overall position of the moving element based on the fixed
position of
the associated linear encoder reader and a relative position of the readable
device in
relation to the associated linear encoder reader.
According to further embodiments of this aspect, the readable device can be
an optical strip and the linear encoder readers can comprise optical read
heads.
Alternately, the readable device can be a magnetic strip and the linear
encoder
readers can comprise magnetic detectors. The linear encoder readers can be
substantially equidistantly spaced along the stationary element and the
readable
device can have a length which is greater than the spacing between adjacent
linear
encoder readers and less than the spacing between three linear encoder
readers. In
a condition where the readable device is associated with a given linear
encoder
reader and simultaneously begins to interact with an adjacent linear encoder
reader,
the processing means is preferably operative to switch the association of the
readable
device with the given linear encoder reader to the adjacent linear encoder
reader
once the readable device has reached a pre-specified distance through the
given
linear encoder reader, or the adjacent linear encoder reader. The processing
means
can be operative to initialize the adjacent linear encoder reader prior to the
interaction
of the readable device with the adjacent linear encoder reader. Immediately
after the
association of the readable device is switched to the adjacent linear encoder
reader,
the processing means can be operative to require the readable device to
backtrack
for at least a minimum distance before the association of the readable device
is
switched back to the given linear encoder reader, to thereby provide a
hysteresis
effect.
In a second aspect, the present invention provides an apparatus for detecting
the positions of plural moving elements relative to a stationary element. The
apparatus comprises a single row of linear encoder readers spaced generally
along
the stationary element at fixed positions relative thereto. A device readable
by the
linear encoder readers is mounted on each moving element, and each readable
device has a length which is greater than the spacing between any given pair
of
adjacent linear encoder readers. Guide means are provided for aligning the
readable
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CA 02507856 1998-05-O1
devices in order to interact with the single row of linear encoder readers.
Plural
processing means are connected to each linear encoder reader to associate any
given readable device with only one linear encoder reader at any time and to
resolve
and provide a reading of the overall position of the corresponding moving
element
based on the fixed position of the associated linear encoder reader and a
relative
position of the given readable device in relation to the associated linear
encoder
reader.
According to further embodiments of this aspect, the readable device can be
an optical strip and the linear encoder readers can comprise optical read
heads. The
readable device can be a magnetic strip and the linear encoder readers can
comprise
magnetic detectors. The linear encoder readers can be substantially
equidistantly
spaced along the stationary element and each readable device can have a length
which is greater than the spacing between adjacent linear encoder readers and
less
than the spacing between three linear encoder readers. Each moving element can
be
sized longer than its corresponding readable device in order to preclude
readable
devices of adjacent moving elements from interacting with the same linear
encoder
reader. In a condition where a given readable device is associated with a
given linear
encoder reader and simultaneously begins to interact with an adjacent linear
encoder
reader, the plural processing means can be operative to switch the association
of the
given readable device with the given linear encoder reader to the adjacent
linear
encoder reader once the given readable device has reached a pre-specified
distance
through the given linear encoder reader, or the adjacent linear encoder
reader. The
plural processing means can be operative to initialize the adjacent linear
encoder
reader prior to the interaction of the given readable device with the adjacent
linear
encoder reader. Immediately after the association of the given readable device
is
switched to the adjacent linear encoder reader, the plural processing means
can
require the given readable device to backtrack for at least a minimum distance
before
the association of the given readable device is switched back to the given
linear
encoder reader, to thereby provide a hysteresis effect.
In a further aspect, there is provided an apparatus for detecting the position
of
a moving element relative to a track that comprises a series of stationary
units. The
apparatus comprises a plurality of linear encoder readers spaced generally
along
CA 02507856 1998-05-O1
each of the stationary units at fixed positions relative thereto. A device
readable by
the linear encoder readers is mounted on the moving element and has a length
which
is greater than the spacing between any given pair of adjacent linear encoder
readers. Guide means are provided for aligning the readable device in order to
interact with the linear encoder readers. Processing means are provided for
each
stationary unit. The processing means are connected to each linear encoder
reader
of the respective stationary unit to associate the readable device in a state
of
interaction with only one linear encoder reader at any time. The association
occurs
only when the readable device has passed through the one linear encoder reader
a
pre-specified non-zero distance. The processing means also resolve and provide
a
reading of the overall position of the moving element based on the fixed
position of
the associated linear encoder reader and a relative position of the readable
device in
relation to the associated linear encoder reader. The processing means of each
stationary unit are in communication with the respective processing means of
the
adjacent stationary units, and are adapted to transfer the resolving and
providing a
reading of the overall position of the moving element to the processing means
of an
adjacent stationary unit when the readable device has passed a pre-specified
non-
zero distance through a linear encoder reader of the adjacent stationary unit.
According to embodiments of this aspect, the readable device can be an
optical strip and the linear encoder readers comprise optical read heads. The
readable device can be a magnetic strip and the linear encoder readers
comprise
magnetic detectors. The linear encoder readers can be substantially
equidistantly
spaced along the track and the readable device can have a length which is
greater
than the spacing between adjacent linear encoder readers and less than the
spacing
between three linear encoder readers. In a condition where the readable device
is
associated with a given linear encoder reader and simultaneously begins to
interact
with an adjacent linear encoder reader, the processing means can operate to
switch
the association of the readable device with the given linear encoder reader to
the
adjacent linear encoder reader once the readable device has reached a pre-
specified
distance through the given linear encoder reader, or the adjacent linear
encoder
reader. The processing means can operate to initialize the adjacent linear
encoder
reader prior to the interaction of the readable device with the adjacent
linear encoder
reader. Immediately after the association of the readable device is switched
to the
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CA 02507856 1998-05-O1
adjacent linear encoder reader, the processing means can also operate to
require the
readable device to backtrack for at least a minimum distance before the
association
of the readable device is switched back to the given linear encoder reader, to
thereby
provide a hysteresis effect.
In yet another aspect, the present invention provides an apparatus for
detecting the positions of plural individually controlled moving elements
relative to a
stationary element. The apparatus comprises a single row of linear encoder
readers
spaced generally along the stationary element at fixed positions relative
thereto. A
device readable by the linear encoder readers is mounted on each moving
element,
and has a length which is greater than the spacing between any given pair of
adjacent linear encoder readers. Guide means are provided for aligning the
readable
devices in order to interact with the single row of linear encoder readers.
Plural
processing means are connected to each linear encoder reader to associate any
given readable device with only one linear encoder reader at any time and only
when
the given readable device has passed a pre-specified non-zero distance through
the
one linear encoder reader. The processing means also resolve and provide a
reading
of the overall position of the corresponding moving element based on the fixed
position of the associated linear encoder reader and a relative position of
the given
readable device in relation to the associated linear encoder reader.
According to embodiments of this aspect, the readable device can be an
optical strip and the linear encoder readers can comprise optical read heads.
Alternately, the readable device can be a magnetic strip and the linear
encoder
readers can comprise magnetic detectors. The linear encoder readers can be
substantially equidistantly spaced along the stationary element and each
readable
device can have a length which is greater than the spacing between adjacent
linear
encoder readers and less than the spacing between three linear encoder
readers.
Each moving element can be sized longer than its corresponding readable device
in
order to preclude readable devices of adjacent moving elements from
interacting with
the same linear encoder reader. In a condition where a given readable device
is
associated with a given linear encoder reader and simultaneously begins to
interact
with an adjacent linear encoder reader, the plural processing means can
operate to
switch the association of the given readable device with the given linear
encoder
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CA 02507856 1998-05-O1
reader to the adjacent linear encoder reader once the given readable device
has
reached a pre-specified distance through the given linear encoder reader, or
the
adjacent linear encoder reader. The plural processing means can operate to
initialize
the adjacent linear encoder reader prior to the interaction of the given
readable device
with the adjacent linear encoder reader. Immediately after the association of
the
given readable device is switched to the adjacent linear encoder reader, the
plural
processing mean can require the given readable device to backtrack for at
least a
minimum distance before the association of the given readable device is
switched
back to the given linear encoder reader, to thereby provide a hysteresis
effect.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other aspects of the invention are discussed in greater
detail below with reference to the drawings, provided for the purpose of
description
and not limitation, where like objects are referenced by like reference
numbers, and
wherein:
Fig. 1 is an isometric view of a portion of a modular conveyor system, in
accordance with the preferred embodiment, wherein multiple pallets move over a
track;
Fig. 1;
Fig. 2 is an exploded view of the system shown in Fig. 1;
Fig. 3 is a cross-sectional view of the conveyor system taken along line III-
III in
Fig. 4 is a plan view of an individual polyphase-like coil set employed in the
conveyor system in accordance with the preferred embodiment;
Fig. 5 depicts a conduction cycle of an individual coil shown in Fig. 4 in
relation
to the corresponding movement of a pallet thereover, in accordance with the
preferred embodiment;
Fig. 6 is a system block diagram of a preferred distributed control
architecture
for controlling the conveyor system of Fig. 1 and each section thereof;
Fig. 7 is a hardware block diagram of preferred electronic circuitry used to
control each conveyor system section shown in Fig. 6;
Fig. 8 is an electronic schematic diagram illustrating various portions of the
electronic circuitry shown in Fig. 7 in greater detail;
Fig. 9 is a system block diagram illustrating a servocontrol system according
to
the preferred embodiment for controlling pallets in each conveyor system
section;
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Fig. 10 is a flow chart of a digital p.i.d. position control loop employed in
the
servocontrol system of Fig. 9;
Fig. 11 is a flow chart relating to a commutation controller employed in the
servocontrol system of Fig. 9;
Fig. 12 is a diagram of a preferred scheme employed by the servocontrol
system of Fig. 9 for demultiplexing linear encoders spaced along each conveyor
system section in order to resolve the position of a given pallet therein;
Fig. 13 is a state transition diagram in relation to the demultiplexing scheme
of
Fig. 12;
Fig. 14 is a diagram of a preferred scheme for synchronizing the servocontrol
systems (each shown in Fig. 9) of adjacent conveyor system sections in order
to
smoothly control the movement of a pallet thereacross; and
Fig. 15 is a state transition diagram in relation the synchronization scheme
of
Fig. 14.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 illustrates a portion of a modular conveyor system 20, in accordance
with the preferred embodiment. The system 20 features multiple pallets or
moving
elements 22 (only one is illustrated) which are constrained to ride or travel
along a
continuous, stationary, track 24.
The description of the conveyor system 20 is organized as follows: (1 ) an
introduction to the operating principles thereof; (2) brief description of the
physical
structure of the system, which comprises a plurality of track sections or
units 26; (3)
description of the preferred electromagnetic structure of the system; (4)
introduction
to a preferred distributed control architecture for control of the system; (5)
detailed
description of a preferred servocontrol system for each track unit 26; (6)
detailed
description of a preferred servocontrol subsystem for detecting the position
of each
pallet 22 along each track unit 26; (7) detailed description of a method
according to
the preferred embodiment for synchronizing the servocontrol systems of
adjacent
track sections 26 when any given pallet 22 crosses therebetween.
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CA 02507856 1998-05-O1
Principle of Operation
Referring additionally to Figs. 2 and 3, each pallet 22, as explained in
greater
detail below, houses a plurality of permanent magnets 28 disposed to provide a
magnetic flux depicted by vectors (30 and 31 ) orientated normal to the track
24. The
track 24, as explained in greater detail below, houses a stator armature 32
comprising
a plurality of embedded coils 35 which are individually excited so that an
electrically
induced magnetic flux (depicted by vectors 36 and 37) produced by the stator
armature 32 is located only beneath a given pallet 22 to be controlled, in a
direction
normal thereto, without affecting adjacent pallets. The motive force for
translating
each pallet 22 arises from the magnetornotive (MMF) force produced by each
pallet
and the stator armature, i.e., by the tendency of the corresponding magnetic
fluxes
provided by the stator and pallet to align. Servocontrol means, as described
in greater
detail below, enable separate and independent moving MMFs to be produced along
the length of the track 24 for each pallet so that each pallet 22 can be
individually
controlled with a trajectory profile that is independent of any other pallet.
The
servocontrol means employs a contactless pallet position-detecting subsystem,
as
described in greater detail below. Structurally, the conveyor 20 may thus be
broadly
classified as a moving magnet type linear brushless motor having multiple
moving
elements.
Phvsica! Structure
Mechanically, the track 24 is composed of a plurality of track sections or
units
26 which are mechanically self-contained and quickly and easily separable from
one
another so as to be modular in nature. In the preferred embodiment, the track
units 26
are mounted on a substrate (not shown) so as to merely align and abut one
another
in order to form the continuous track 24. This preferred feature requires that
stator
armature coils 35 from one track unit not overlap or project onto the stator
armature of
an adjacent track unit, as explained in greater detail below. Also, each track
unit 26
houses all of the electronic circuitry 38 required to control the track unit.
As seen best in Figs. 2 and 3, each pallet 22 includes an extension 40 onto
which is mounted a relatively long, graduated, optically reflective strip 45.
The
extension 40 is disposed such that the reflective strip 45 interacts with
contactless,
optical linear encoder read heads 50 mounted to a corresponding extension 46
CA 02507856 1998-05-O1
depending from a side wall 48 of the track 24. With the aid of flap 52, this
inter-
engaging structure protects the optical components 45 and 50 from the traffic
on the
track and assists in precluding ambient light, i.e., light interference or
noise, from
falsely triggering the optical linear encoder read heads 50. The optical
components 45
and 50 are employed in the pallet position-detecting subsystem explained in
greater
detail below. At this point, it should be appreciated that by placing the read
heads 50
on track 24 and not on pallets 22, the pallets are not tethered in any way and
thus
their mobility is not restricted.
Each pallet 22 features load-bearing wheels 54 which ride along rails 56 of
track 24. Each pallet also features spring-loaded bearings 58 for constraining
the
pallet to stay on the rails 56 and maintain the alignment between optical
components
45 and 50.
Electromagnetic Structure
The magnetic structure of each pallet 22 comprises at least two thrust-
producing permanent magnets arranged in alternating North-South sequence. The
permanent magnet material, which may include Neodymium-Iron-Boron, Alnico and
ceramic (ferrite) base magnets, is selected on the basis of air gap flux
densities
required and the physical dimensions of the pallet magnetic structure. In the
preferred
embodiment, each pallet 22 carries two Neodymium-Iron-Boron permanent magnets
28 spaced apart by pole pitch P. This provides each pallet with a permanent
magnet
pole pair 60 which provides magnetic flux vectors 30 and 31 pointing in
opposite
directions. For reasons explained shortly below, and referring additionally to
Fig. 5,
the pole pitch P is preferably approximately equal to 2D/3, where D is the
overall
width of the permanent magnet poles pair, and the width, W, of each magnet 28
is
preferably approximately D/3. The permanent magnet pole pair 60 abuts a
magnetic
backplate (Fig. 2) and these components are preferably mounted in a cavity 64
of
pallet 22 such that end portions 66 of the pallet body function as dead poles
which
magnetically isolate the pallet permanent magnet pole pair 60 from the
permanent
magnet pole pair of any adjacent pallet.
The magnetic structure of the stator armature 32 comprises a yoke 68,
constructed out of electrical steel, which features a plurality of
substantially
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CA 02507856 1998-05-O1
equidistantly spaced slots 70 disposed in relative close proximity to one
another. A
representative slot spacing is 3 mm and representative slot dimensions are
1.5 x 7 x 75 mm. The turns of the stator armature coils 35 are mounted in the
yoke
slots.
The turns of each coil are formed (Fig. 4) into two legs 72 and 72' which are
spatially distributed over a specified number of yoke slots 70. The coil legs
72 and 72'
provide electrically induced, magnetic flux producing pole pairs that produce
magnetic
flux vectors 36 and 37 pointing in opposite directions. The spatial
distribution of coil
legs 72 or 72' reduces cogging effects caused by uneven reluctance and, in
comparison to a non-spatially distributed coil leg or electrically induced
pole, enables
a smoother thrust production along the stator armature 32.
The electrical pole pitch (Fig. 5) of each coil 35 is substantially equal to
the
mechanical pole pitch, P, of each pallet permanent magnet pole pair 60. In the
preferred embodiment, the width of each coil leg 72 or 72' is approximately
equal to
the width, W, of each pallet permanent magnet 28, whereby the overall width of
each
coil 35 approximately equals the overall width, D, of pallet permanent magnet
pole
pair 60.
The coils 35 are arranged as a sequence of individual polyphase-like windings
or coil sets, wherein coils in each set are overlapped such that the coil
centres are
spaced apart a distance P/p, where p is the number of quasi-phases. The
preferred
embodiment, as seen in Fig. 2 and in Fig. 4 (which is a plan view of a coil
set taken in
isolation), features a two phase-like arrangement, wherein each poiyphase-like
winding or coil set (hereinafter "coil pair 75") comprises two overlapping
coils 35
having their centres 76, 77 spaced apart by a distance P/2. Since the width,
W, of the
leg 72 or 72' of each coil 35 is D/3, and the width of the empty inner space
of the coil
is also D/3, it will be seen from Figs. 2 and 4 that one of the legs 72 or 72'
of each coil
35 in coil pair 75 substantially occupies the empty inner space 78 of the
counterpart
coil such that there are no unfilled yoke slots 70 spanned by the coil pair.
In addition,
the coil pairs are arranged to be immediately adjacent to one another such
that there
are no unfilled yoke slots 70 in an inter-coil pair region. This arrangement,
in
combination with the spatial distribution of the turns of each coil leg,
enables the
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CA 02507856 1998-05-O1
stator armature 32 to present a relatively uniform reluctance in order to
minimize
cogging effects.
Another advantage provided by the individual polyphase-like windings or coil
sets ties in the fact that the track 24 can be modularly constructed in
discrete sections
as described above such that no coil from one stator section overlaps,
projects or
otherwise encroaches upon an adjacent stator section. In contrast, a
conventional
convolute polyphase a.c. stator winding has an essentially endless coil
overlapping
arrangement such that turns cannot be mechanically separated.
In alternative embodiments, a coil set may comprise a short segment of a
conventional polyphase a.c. winding, preferably provided that length of each
segment
is approximately equal to the length of the magnetic structure of the pallet.
Thus, a
stator armature according to this embodiment comprises a series of
individually
controlled polyphase a.c. windings.
The magnetic circuit provided by the pallet and stator armature is as follows
(Fig. 2): the magnetic flux circulates through the pallet backplate 62,
through the
permanent magnets 28, across an air gap to and through the stator armature
poles
(i.e, coils 35), through the yoke 68, back through the stator poles, and back
through
the permanent magnets 28, returning to the pallet backplate 62.
Fig. 5(b) illustrates a conduction cycle 80 for a single coil 35 of any given
coil
pair 75. Fig. 5(a) indicates that the conduction cycle 80 begins just as a
leading edge
82 of pallet permanent magnet pole pair 60 (shown in solid lines) reaches a
leading
outer turn of the coil 35 and terminates just as a trailing edge 84 of the
pallet pole pair
60 (shown in stippled lines) passes over a receding outer winding of the coil.
Distances along the position axis of Fig. 5(b) correspond to the relative
distance
between a centre point 86 of coil 35 and a centre point 87 of the pallet pole
pair 60.
The conduction cycle 80 corresponds to a 540 degree electrical cycle. It
should also
be noted that the preferred conduction cycle illustrated in Fig. 5(b), in
association with
the design of the stator armature 32 as described above, yields a relatively
constant
MMF, having a ripple of only about 5-10%.
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CA 02507856 1998-05-O1
Distributed Control Architecture
Fig. 6 is a high level abstraction of a preferred control architecture
employed in
the conveyor system 20. Architecturally, the conveyor system 20 is partitioned
into a
plurality of control zones, each of which corresponds to one track section
unit 26,
which is placed under the control of a local section controller 90. The
section
controllers 90 are connected to one another in a peer-to-peer communications
network such that each section controller 90 is connected to a preceding and
following section controller through high speed communications links 92.
Each section controller 90 is also connected to a central controller 94, such
as
a computer workstation, through a supervisory network employing a multi-drop
bus
96. The central controller 94 and supervisory network provides an efficient
means for
initializing the section controllers. The central controller may also
communicate
destination data to the section controllers for the pallets (which are
preferably
uniquely addressed) and receive acknowledgement messages in return when
pallets
have reached their destinations. As such, the central controller may be used
for
process (i. e. manufacturing-fine) control. The central controller also
fulfils a
supervisory diagnostic role by monitoring the section controllers (e.g., by
engaging in
a continuous polling process) in order to determine whether any section
controller has
failed.
Each section controller 90 may also (but does not necessarily) include a cell
port 98 for interfacing section controller 90 to a station controller such as
an external
programmable logic controller (PLC) 100. The PLCs provide manufacturing-line
station-processing instructions to the track 24, such as directing the next
destination
for a pallet along the track, or providing station-specific motion
instructions in respect
of a given pallet stopped adjacent to or in a processing station (not shown).
For
instance, a typical two-axis station controller or PLC operates by providing
pulse
signals in order to synchronize the motion of a pallet along the track with
the motion
of a station end effector or dispenser moving along a transverse axis, whereby
each
pulse represents an incremental pallet move command. It will be appreciated
that the
provision of the station controller or PLC reduces the amount of bandwidth
that would
otherwise be required to communicate this information to the central
controller 94,
14
CA 02507856 1998-05-O1
thereby substantially eliminating a potential limitation on the length and
processing
capabilities of the conveyor system.
As illustrated, each section controller 90 is connected to all of the stator
armature coils 35 in the corresponding track unit 26 and, as described in
greater
detail below, is responsible for commutating the coils in the control zone in
accordance with an independent trajectory or "move" command for each pallet
located therein. However, unlike a conventional convolute two phase stator
armature
winding, the commutation is complicated by the fact that a given pallet, (such
as
illustrated pallet 22') may straddle two coil pairs 75 whereby both coil pairs
have to be
simultaneously excited in order to produce a suitable moving MMF along the
track 24.
Each section controller 90 is also connected to all of the optical read heads
50
situated in its control zone. The section controller is responsible for
resolving the
absolute position of each pallet 22 located in its control zone, as described
in greater
detail below.
Servocontrol System
Fig. 7 is a hardware block diagram illustrating the major components of a
given
section controller 90 which, in accordance with the preferred embodiment,
physically
comprises a control board 102 and two power boards 104. The control board 102
includes an ADSP2181 digital signal processor (DSP) 105, commercially
available
from Advanced Micro Devices of Norwood, MA, U.S.A., and associated program
memory 106. The DSP 105 includes two on-chip serial ports 108 for providing
the
communication link interfaces 92 to adjacent preceding and following section
controllers. A separate micro-controller 110 provides an interface to the
supervisory
network 96 which links the section controller 90 to the central controller 94.
A field
programmable gate array (FPGA) 112 is used to interface the cell port 98 with
the
local PLC 100. The FPGA 112 is also used to interface the optical read heads
50 with
the DSP 105.
The power boards 104 comprise a plurality of current amplifiers 114, one for
each coil 35 controlled by the section controller. (There are eighteen coils
in the
illustrated embodiment.) Each current amplifier 114 comprises an inverter such
as a
CA 02507856 1998-05-O1
two phase or H-bridge 116, drivers 118 for converting logic level signals to
analog
level signals in order to drive the power switches of the H-bridge, and
current sensing
circuitry 120 for sensing the coil current. Each power board also includes an
FPGA
122 which is used to interface the DSP 105 with the current amplifiers 114.
More
particularly, as shown in Fig. 8 which illustrates a given current amplifier
and its
associated FPGA circuitry, the FPGA 122 provides a latch 124 (for each coil 35
controlled by a given power board), addressable by the DSP 105, for storing a
pulse-
width modulated (PWM) duty cycle value used to drive the H-bridge 116. The
latch
124 is connected to a fixed frequency PWM generator 126 which operates by
comparing the value stored in the latch 124 with a continuously cycling
counter 128
and setting an output signal 130 accordingly. The output signal 130 and a
complementary signal 132 are connected to the drivers 118 so as to control the
base
inputs of power MOSFET devices 134 employed as switching elements in the H-
bridge 116.
The current sensing circuitry 120 comprises a current sensor 136 which is
used to measure the current flowing through a given coil 35 for all
commutation
phases of the H-bridge. A suitable current sensor is disclosed for instance in
U.S.
Patent No. 5,874,818 to Schuurman and assigned to an assignee of the instant
application. A variety of alternative current sensing devices may be used,
such as
current transformers or open and closed loop Hall effect devices. The output
of the
current sensor 136 is connected to an analog filter 138 which is connected to
an
analog multiplexer 140 (not shown in Fig. 7). The analog multiplexer 140
multiplexes
the current sensing signals from multiple current sensors associated with the
other 0
current amplifiers 114 located on the power board 104 and provides these
signals to
an analog to digital converter (A/D) 142 which is connected to a latch 144
addressable by the DSP 105. The FPGA 122 provides a channel selection means
146 for continuously sampling the current sensing signals from each current
amplifier
1 14. The FPGA 122 also provides circuitry 148 for generating the appropriate
control
signals to 5 the A/D 142. It will be noted (Fig. 7) that since each section
controller 90
comprises two power boards 104 each carrying AID 142, the DSP 105 can operate
in
a pipelined manner so that two coil current readings can occur substantially
simultaneously.
16
CA 02507856 1998-05-O1
Each power board 104 also includes a temperature sensor 147 and a voltage
sensor 149 which are connected to the A/D 142 and interfaced to the DSP 105 by
the
FPGA 122. The central controller 94 periodically polls each section controller
90 in
order to obtain diagnostics data provided by these sensors.
The DSP 105 of each section controller 90 is used to implement a closed-loop
digital servocontrol system which is shown in systemic form in Fig. 9. The
servocontrol system comprises a trajectory generator 150, as known in the art
per se,
for computing a pallet position set point vector S (S,, S2, ..., SK), where
component or
signal represents the position set point for a given pallet located in the
control zone
serviced by the given section controller and K is the number of pallets in the
control
zone at any given time. The trajectory generator 150 produces set points for
each
pallet in accordance with prespecified acceleration and velocity profiles for
the pallets
which are downloaded by the central controller 94 to the section controller 90
during
system initialization. For example, the trajectory generator 150 may employ a
trapezoidal acceleration profile to smoothly accelerate the pallet from an
initial rest
position to a terminal velocity and then smoothly de-accelerate the pallet to
a
destination position. In the preferred embodiment, the pallet position set
point vector
S is computed at a rate of approximately 1 KHz.
The pallet set points are compared against the measured positions, X (X,,
X2, ..., XK), of the pallets as determined by a pallet position feedback
subsystem 152
which also samples pallet positions at a rate of approximately 1 KHz. This
comparison results in the computation of a pallet position error vector D S
(OS1,
~SZ, ..., ASK). The pallet position error vector OS is fed into a position
compensator
154 which computes a force vector, F (F~, F2, ..., FK), specifying the force
required to
be applied to each pallet in order to minimize the pallet position error. The
force
vector F is also computed at a rate of about 1 KHz.
In the preferred embodiment, the position compensator 154 employs a well-
known proportional, integral, derivative (p.i.d.) control law, however
alternative control
methods such as the state space technique may employed. Fig. 10 shows
psuedocode for implementing a digital p.i.d. control loop in respect of one
pallet. It will
17
CA 02507856 1998-05-O1
be seen that in order to compute a derivative term, D term, of the p.i.d.
control law,
the p.i.d. control loop employs an error history buffer or array E[1..q] for
retaining a
set {0S;[r), DS;[r-1], DS;[r-2), ..., DS;[r-q]} of position errors where r
represents a latest
received pallet position error and q corresponds to the size of the buffer. In
addition,
the p.i.d. control loop employs an accumulator, I term, for storing the
integral term of
the p.i.d. control law. This data assumes special significance when a pallet
moves
across control zones, as discussed in greater detail below.
The force vector F and pallet position vector X are fed (Fig. 9) into a
commutation controller 155 which provides current set point data for the coils
35.
Fig.11 illustrates a preferred commutation control algorithm executed by the
commutation controller 155. Processing steps 158 and 162 set up nested loops.
The
inner loop is executed N times, where N is the number of coils 35 controlled
by
section controller 90. The outer loop executes the inner loop K times, where K
is the
number of pallets presently located in the current control zone. At processing
step
160 in the outer loop, the commutation controller 155 computes the centre
point,
CPP(i), of permanent magnet pole pair 60 for pallet(i), 1 < i <_K. (See
additionally Fig.
5). This computation is based on (a) input parameter or signal X;, the
measured
position of pallet(i), which, as described in greater detail below, is
measured at a
different reference point than the pallet pole pair centre point 87; and (b) a
constant
which is dependent upon the physical dimensions of pallet(i). At processing
step 164
in the inner loop (Fig. 11 ), the commutation controller 155 computes the
relative
distance, RD(j), between centre point CPP(i) of pallet(i) and the centre
point, CPC(j),
of a given coil, coil(j), 1 < j < N. At step 166, a check is made whether or
not
-D <_RD(j) <_D. This, as described above with reference to Fig. 5, indicates
whether or
not the pole pair 60 of pallet(i) is situated above coilQ). If the pole pair
60 of pallet(i) is
not situated above coil(j), flow control is passed to the next iteration of
the inner loop.
If the pole pair 60 of pallet(i) is situated above coil(j), then (Fig. 11 ) at
steps 168, 170
and 172 the commutation controller respectively reads a table 180
corresponding to
the conduction cycle 80 (Fig. 5) to extract a nominal current set point;
scales the
nominal current set point by input parameter F;, the required force for
pallet(i); and
updates a current set point table 182. This process is repeated for each
pallet in the
control zone in order to provide a current set point vector ~s (Is,, Is2, Iss,
..., IsN). The
current set point vector ~s is computed or updated at a 20 KHz rate.
18
CA 02507856 1998-05-O1
The current set point vector ~S is compared (Fig. 9) to an actual or measured
coil current vector ~A (IAA, I~, IA3, ..., IAN) generated by the current
sensing circuitry
120 in order to compute a current error vector ~~(~I,, X12, 013, ..., DIN) at
a 20 KHz
rate. The current error vector ~~ is fed into a current compensator 184 which
computes a PWM duty cycle value for each current amplifier 114 of each coil 35
using
a proportional, integral (p.i.) control law well known in this art. In the
foregoing
manner, the commutation controller 155 applies the conduction cycle 80 to the
necessary stator armature coils 35 in order to provide a moving MMF for a
given
pallet in the control zone, even when the pallet straddles two coil pairs 75.
Pallet Position Feedback Subsystem
The pallet position feedback subsystem 152 which supplies measured pallet
position data to the trajectory generator 150, position compensator 154 and
commutation controller 155 is now discussed in greater detail. Referring to
Figs. 6, 7
and 12, when the reflective strip 45 of a given pallet 22 moves over a given
optical
read head 50, two 90° out-of-phase signals are produced and quadrature
decoding
circuitry 186 causes a counter or register 188 associated therewith to count
up or
down in accordance with the direction of travel of the reflective strip 45.
For example,
if a 400 lines-per-inch graded reflective strip moves one inch through a given
optical
read head 50, such movement will cause the associated counter 188 to change by
+/- 400, depending on the direction of travel. The optical read head 50 and
decoding
circuitry 186 and 188 (hereinafter "encoder") as well as the associated
reflective strip
45 are commercially available, for instance, from the Hewlett Packard Company
of
Santa Clara, CA, U.S.A.
As depicted in Fig. 6, each control zone features a plurality, M, of the
optical
read heads 50 which are substantially equidistantly spaced at a distance, E,
along
every track unit 26. The length, R, of the reflective strip 45 is such that R
is greater
than E by a pre-determined amount, XR. Thus, the reflective strip associated
with any
given pallet can engage or trigger two encoders simultaneously at various
points
along the track. In addition, the length, L, of the pallet itself is at least
equal to or
greater than R in order to ensure that a reflective strip associated with an
adjacent
19
CA 02507856 1998-05-O1
pallet does not intertere with the given pallet. In other words, the length L
is chosen to
ensure that no two reflective strips can trigger the same encoder.
As shown in Fig. 7, the FPGA 112 of each section controller 90 interfaces the
linear encoders with the DSP 105 thereof. The DSP provides a parallel
processing
means for sampling the encoders and resolving the position of each pallet
located in
the associated track unit at a rate of approximately 1 KHz. Broadly speaking,
the
processing means associates the reflective strip 45 of any given pallet with
only one
encoder at any time so that the absolute position of the given pallet can be
calculated
based on a fixed position of the associated encoder (or more specifically its
read
head 50) and a relative position of the reflective strip in relation to the
associated
encoder.
In addition, when the reflective strip simultaneously engages two encoders, at
some point, as described in greater detail below, the processing means
transfers or
hands-off the association or "ownership" of the pallet from the current
encoder to the
adjacent engaged encoder. In this manner, the position of a given pallet can
be
continuously tracked across the control zone. When a pallet crosses control
zones, a
similar process occurs, with the addition that the adjacent section controller
creates a
data structure to keep track of the position of the given pallet, and at some
point as
described in greater detail below, once the hand-off is completed, the data
structure
for the pallet in the (now) previous control zone is deleted.
Figs. 12 and 13 depict a method according to the preferred embodiment for
accomplishing the hand-off or transfer of the ownership of a given pallet
between
adjacent encoders. More particularly, Fig. 12 depicts how a given encoder may
assume various control states, and Fig. 13 is a diagram of an associated state
transition table. In the illustrated embodiment, the reflective strip 45
features 3300
graduations, i.e. 3300 counts from start to finish, and a control zone
features seven
encoders (addressed from enc=0 to enc=6).
A "zone 2" state 200 represents a steady state condition wherein the
reflective
strip of a given pallet i engages a given encoder, encoder(n), and is not yet
near
encoder(n-1 ) or encoder(n+1 ). Considering the situation where the given
pallet moves
CA 02507856 1998-05-O1
to the right in Fig. 12, at some point (i.e. when count = 3060) the leading
right edge of
the associated reflective strip moves right into a "right-reset" state 202
where the
adjacent right encoder(n+1 ) is continuously reset to zero in preparation for
the hand-
off. The reflective strip then enters a "zone 3" state 204 (at count = 3120).
At some
point in this state, the leading edge of the reflective strip engages
encoder(n+1 ) which
begins its count reflecting the distance the leading edge of the reflective
strip has
passed therethrough. However, encoder(n) still owns the given pallet. The
ownership
continues until the leading edge of the reflective strip reaches a "right hand-
off' state
206 (at count = 3240). Somewhere in this state, depending on the rate the DSP
105
samples the encoders, the ownership of the given pallet is handed-off to
encoder(n+1 ). The transfer of ownership is shown in the changing state of
table 220
(Fig. 12) before and after the hand-off (where i represents the given pallet).
A similar process occurs when the given pallet moves leftward. "Reset-left",
"zone 1 ", and "left hand-off' states 208, 210 and 212 are the respective
counterparts
to the "reset-right", "zone 3", and "right hand-off' states 202, 204 and 206.
The preferred method provides a hysteresis effect when the given pallet
backtracks soon after the hand-off is accomplished. The extra distance XR by
which
the length R of each reflective strip exceeds the encoder spacing E enables
the
control state patterns 215 and 215' (Fig. 12) associated with each encoder to
overlap
and be partially temporally conterminous, as illustrated. The relative lengths
and
positions of the control states or zones are selected such that when the hand-
off is
effected, encoder(n+1 ) is in the "zone 1" control state 210. if during this
state the
given pallet backtracks, it must traverse at least a minimum hysteresis
distance H
backwards before the ownership of the given pallet is transferred back to
encoder(n).
The hysteresis effect provides for a more stable pallet position feedback
system by
preventing the oscillation or flip-flopping of hand-offs when a pallet
straddles two
encoders and is commanded to move relatively small distances to and fro. Such
a
condition could occur, for instance, when the pallet is located at a
processing station
and the motion of the pallet along the axis of track 24 is co-ordinated by the
PLC 100
with the motion of a station end effector or dispenser moving along a
transverse axis.
21
CA 02507856 1998-05-O1
The preferred method is carried out by each section controller 90 for each
pallet located in the corresponding control zone.
Those skilled in the art will appreciate that devices other than the optical
linear
encoder reader 50 and the reflective strip 45 may be used in alternative
embodiments. For example, the passive readable device can be a magnetic strip
and
the linear encoder readers can be corresponding magnetic detectors. Such an
alternative embodiment could provide very fine resolution, e.g. graduations of
about a
micron, however the cost of such linear encoders is typically very high and
may not
be required for most applications given the good resolution, typically a
thousandth of
inch, provided by the optically reflective strips.
Synchronizing Servocontrol Systems
The length of track 24 that a given section controller 90 can control is
limited
by various practical considerations, thereby complicating the production of
moving
MMFs for the pallets, which have to cross control zones. Accordingly, the
preferred
embodiment provides a means for synchronizing the servocontrol systems of
adjacent section controllers and for passing control of a pallet crossing
therebetween.
Figs. 14 and 15 depict a method and protocol according to the preferred
embodiment for synchronizing the servocontrol systems of adjacent section
controllers and for passing control of a given pallet i crossing control
zones. Fig. 14
depicts various control states assumed by section controller(n) of track
section or
control zone N and section controller(n+1 ) of track section or control zone
N+1 as a
given pallet crosses from zone N into zone N+1, and vice versa. Fig. 15
illustrates an
associated state transition table followed by each of controller(n) and
controller(n+1 ).
A "Solo Pallet" state 250 represents a steady state condition when the given
pallet is fully under the control of one section controller.
When the given pallet moves to the right in Fig. 14 from zone N to zone N+ 1,
the leading right edge of the associated reflective strip reaches a point t~
which is
considered to be near to zone N+1. Upon the occurrence of this event, a
message,
termed PM CREATE, is transmitted by controller(n) to controller(n+1 ) over the
peer-
22
CA 02507856 1998-05-O1
to-peer communication link 92 using a predetermined handshaking protocol (for
ensuring reliable communication), and controller(n) enters a "Pallet Struct"
state 252.
Correspondingly, controller(n+1 ) receives the PM CREATE message and enters a
"Pallet Ready" state 260. During the time period t~ - t2 represented by the
substantially conterminous states of the section controllers, the following
events
occur: (1 ) controller(n+1 ) creates or initializes a data structure for the
given pallet;
and (2) controller(n) passes various static data from its data structure
representing
the given pallet to controller(n+1 ) over the peer-to-peer communications link
92 in
accordance with a predetermined communications protocol. This data includes
information such as the pallet destination point, current velocity and
acceleration,
maximum permissible velocity and acceleration, length, number of magnets and
offset data, and envelope or buffer space data for collision avoidance
purposes.
At point t2, the leading edge of the pallet permanent magnet pole pair 60
reaches the leading turn of a border coil pair located in zone N+1. (See, for
example,
Fig. 5(a).) Upon the occurrence of this event, a message, termed PM COILSTART,
is
transmitted by controller(n) to controller(n+1 ), and controller(n) enters a
"Send Coil
Control" state 256. Correspondingly, controller(n+1 ) receives the PM
COILSTART
message and enters a "Receive Coil Control" state 258. During the time period
t2 - t3
represented by the conterminous states of the section controllers,
controller(n) is still
responsible for executing the position control loop for the given pallet,
which includes
computing a force set point component F; for the given pallet and measuring
the
position X; thereof. Controller(n) uses this data as described above to
regulate the
border coil pair 75 in zone N. The force set point F; and position X; are also
communicated to controller(n+1 ) at a rate of approximately 1 KHz over the
peer-to-
peer communication link 92. Controller(n+1 ) uses this data in its commutation
controller 155 and current compensator 184 in order to produce current step
point
components I~ and 12 in zone N+1 and regulate the border coil pair 75 in zone
N+1 so
as to properly servocontrol the given pallet. In this manner, controller(n)
and
controller(n+1 ) are synchronized to cooperatively execute the current control
loop for
the given pallet by regulating the border coil pairs in their respective
zones.
At point t3, the leading edge of the reflective strip of the given pallet
reaches a
point, as described above, where the ownership of the given pallet should be
handed
23
CA 02507856 1998-05-O1
off from a border encoder in zone N to a border encoder in zone N+1. Upon the
occurrence of this event, a message, termed EM CHANGE ACTIVE ENCODER, is
transmitted by controller(n) to controller(n+1 ), and controller(n) enters the
"Receive
Coil Control" state 258. Correspondingly, controller(n+1 ) receives the
EM CHANGE ACTIVE ENCODER message and enters the "Send Coil Control"
state 256. During the time period t3 - t4 represented by the substantially
conterminous
states of the section controllers, a number of steps occur:
(1 ) The dynamic or memory-based data used by controller(n) for the
position control loop of the given pallet is transferred over to
controller(n+1 ). In the preferred embodiment this comprises (a) the
accumulator, I term; and (b) a portion of the error history buffer E[2..q]
in respect of the set {DS;[r-1 ], DS;[r-2], ..., DS;[r-q]} of position errors
for
calculating the derivative term of the p.i.d. control law.
(2) The dynamic or memory-based data used by controller(n) to generate
the trajectory of the given pallet is transferred over to controller(n+1 ). In
the preferred embodiment this comprises up-to-date velocity,
acceleration, position and time base data.
(3) Controller(n) sends controller(n+1 ) a message effective to transfer the
ownership of the given pallet from the border encoder in zone N to the
border encoder in zone N+1. This change of state is also shown in
Fig. 13 where, for instance, when the border encoder (enc=6) is in the
"zone 3" state 204 and moves right into a hand-off zone, the border
encoder enters into a "Right Hand-off Message" state 216 where the
zone-crossing, controller ownership transfer message is transmitted.
(4) Once step (3) is effected, controller(n+1 ) becomes responsible for
executing the position control loop for the given pallet, which includes
computing the force set point component F; for the given pallet and
measuring the position X; thereof. Controller(n+1 ) uses this data as
described above to regulate the border coil pair in zone N+1. Now,
controller(n+1 ) communicates the force set point F; and measured
position X; to controller(n) at a rate of approximately 1 KHz over the
peer-to-peer communication link 92. Controller(n) now uses this data in
its commutation controller 155 and current compensator 184 in order to
24
CA 02507856 1998-05-O1
produce current step point components IN and IN.~ in zone N and
regulates the border coil pair in zone N so as to properly servocontrol
the given pallet. In this manner, controller(n) and controller(n+1 ) remain
synchronized to continue to cooperatively execute the current control
loop for the given pallet.
At point t4, the trailing edge of the pallet permanent magnet pole pair 60
passes the last turn of the border coil located in zone N. Upon the occurrence
of this
event, a message, termed PM COILSTOP, is transmitted by controller(n+1 ) to
controller(n), whereby controller(n+1 ) enters into a "Pallet Control" state
254 and
controller(n) enters into the "Pallet Ready" state 26. As soon as this point
is reached,
position control loop set point data is no longer transferred from
controller(n+1 ) to
controller(n). Since there is no longer any need to regulate the border coil
in zone N.
At point t5 controller(n+1 ) enters into the steady "Solo Pallet" state,
wherein a
message, termed PM DESTROY, is sent to controller(n) to terminate its data
structure for the given pallet.
fn the preferred method, the point at which any of the above described section
controller states is triggered or entered into differs depending upon the
direction the
given pallet is moving. This provides a hysteresis effect, similar to that
described
above, for enabling a more stable control system by preventing the inefficient
oscillation or flip-flopping between states when a pallet straddles two track
sections
and is commended to move relatively small distances to and fro.
The above process has been described at one border between track units. A
similar process can simultaneously occur at the opposite border between track
units
when a pallet travels thereacross.
It will be appreciated by those skilled in the art that while the preferred
embodiment passes a position error minimizing signal such as F; between
adjacent
section controllers when a pallet crosses control zones, an alternative
embodiment
may instead compute the current set points for the coil pair in an adjacent
control
zone which are spanned by a crossing pallet, and pass this data to the
adjacent
section controller. The current set point signals are linearly related to the
position
CA 02507856 1998-05-O1
error minimizing set point or signal, and both types of signals can be viewed
as
instances of coil regulating signals. The advantage of the preferred
embodiment is
that less information has to be passed over the relative slow (compared to the
processing speed of the DSP 105) serial communication link 92.
The preferred conveyor system 20 provides a number of advantages over the
herein-disclosed prior art. For instance, the electromagnetic structure of the
conveyor
system provides relatively smooth thrust production capabilities, and the
conveying
speed is much improved over typical belt conveyor systems. For example, in a
prototype system developed by the applicants, the pallets attained a 2g
acceleration
and steady velocity of 2 m/s. In addition, the pallet position-detecting
subsystem
enables the absolute position of each pallet to be determined at high
resolution at all
times anywhere along the track, thereby enabling the pallets to be precisely
positioned to any point along the track. Furthermore, the preferred
distributed control
system enables each pallet to be individually and separately controlled yet
interface
with manufacturing process controllers. Finally, these elements, in
combination with
the physical structure of the conveyor system, enable it to be constructed out
of
discrete, self-contained, modular track sections, with little practical
restriction on the
length of the conveyor system or the number of pallets controlled thereby.
The preferred embodiment has been disclosed with a certain degree of
particularity for the purpose of description but not of limitation. Those
skilled in the art
will appreciate that numerous modifications and variations can be made to the
preferred embodiment without departing from the spirit and scope of the
invention.
26
