Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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B~CKGROU~D OF THE INVENTION
This invention relates to a method and apparatus
for controlling the speeds of at least two motions so as to
maintain a prescribed rèlationship between them. More
particularly, the invention relates to a method and apparatus
for controlling the speeds of operation of two or more
sources of motive power, so as to maintain a prescribed
relationship between those speeds and thereby maintain a
desired speed relationship between motions driven by those
sources.
In a wide variety of industries and applications,
drive systems are utilized which require the use of two or
more sources of motive power, e.g. electric motors, wherein
it is mandatory that a given difference between speeds of
the two motors be maintained in order to achieve a given
result. The latter difference in speeds is generally
described in terms of the ratio of the speed of one motor
with respect to the speed of the other motor and is commonly
termed "draw ratio".
Generally spea~ing, the successive motors in a
drive system are in effect in a "master-slave" relationship
wherein a change in speed in a preceeding motor, the master
motor, will bring about a change in speed of a succeeding
motor, the slave motor, in order to maintain a prescribed
draw ratio. Where a plurality of motors are involved, this
relationship continues throughout the succession of motors
in the drive system in order to maintain the predetermined
draw ratios between each pair of motors. Such systems are
commonly used in continuous process manufacturing, such as
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in the manufacturing of synthetic fibers where a strand or
web of material is processed from a supply point through
various stages to the finished product. The speeds of the
various motors throughout the system will affect the
characteristics of the manufactured fiber. In such applications
the draw ratios vary over a very narrow range, and it can be
anticipated that no motor in the system will have difficulty
in meeting the demands placed on it for speed variation.
Examples of such systems are to be found in U.S. Patent Nos.
3,118,097 and 3,447,050.
Multimotor drive systems can find application,
as well, in apparatus where one or more motors in the system
will have difficulty in meeting the load or speed demand
placed on it. An example of such an application, which will
be described in greater detail hereinbelow, is an apparatus
wherein it is necessary to use more than one motor to drive
two or more mechanical members which are used to move an
implement through a prescribed path. For example, in a
power shovel it may be necessary to provide for the
simultaneous rotation and extension of the handle on which
- the dipper is mounted, and it will be necessary that there
be a given relationship between thé hoist speed, i.e., the
speed of rotation of the dipper handle and the crowd speed,
i.e., the speed of the longitudinal movement of the ~andle
in order to effect motion of the dipper through a predetermined
digging path. During this operation for a variety of reasons
it may be the case that a commanded one of the motors might
not be able to run at the o~nded speed; for example, in order
to maintain the prescribed speed ratio a motor may be
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commanded to operate at a speed which is beyond its maximum
safe design speed. In another situation the motor might not
have the capability of delivering the required horsepower at
the commanded speed. ~It also may be the case than an operator
in anticipating a given situation might cause a motor being
commanded to operate at one speed to operate at a lower
speed which would have the effect of taking the speed of the
motor outside of the prescribed ratio. In all of these
situations i the implement is to continue to move through
the desired path, the ratio of the hoist speea to the crowd
speed must be maintained and therefore, there must be a
change in speed of the commanding motor, as well as the
commanded motor.
In other applications involving excavators and the
like it may be necessary that the aforementioned speed ratio
might have to be varied over a relatively wide range during
normal operating cycles. Again, in order to accomplish this
it is necessary that the commanding motor be adjusted to
take into consideration other influences on the commanded
motor.
In the prior art control systems such as those
discussed hereinabove in continuous process manufacturing no
provision has been made for adjusting the speed of operation
of the commanding motor, when for some reason the commanded
motor is incapable of or, for example, has been manually
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adjusted to not respond or is caused to operate at a speed which
woulcl change the speed ratio to an undesirable value in the
absence of some change in the speed of operation of the
commanding motor. Thus, the prior art ratio control systems
make no provision for maintaining a given speed ratio between
two or more motors when one or more of the commanded motors is
unable to respond to cause the system to operate at the
prescribed draw ratio.
It is, therefore, an object of this invention to
provide means and method for coordinating the speeds of two
or more motions so as to establish and maintain a prescribed
relationship between those speeds.
Another object of this invention is to provide means
and method for coordinating the speeds of two separately driven
mechanical motions so that an implement moved by the joint
operation of the two motions will follow a prescribed path.
According to one aspect of the present invention there
is provided a method of maintaining a predetermined speed ratio
between at least two motions driven, respectively, by two
separately and independently controlled motive power sources,
the method including the steps of producing two control signals
respectively from the sensed speeds of the two motions and from
the value of the predetermined speed ratio and governing the
speed of operation of one of the two drive power sources driving
one of the motions with one of the control signals and separately
governing the speed of operation of the other power source
driving the other motion with the other of the two control speeds.
This method may be utilized, for example, in a power
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shovel or the like having an implement moveable over a path
determined by the relative speeds of the motions of at least
two mechanical members which drive the implement, there being
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provided an apparatus for producing a predetermined speeZ ratio
characteristic for operating the two members.
According to another aspect of the present invention
there are provided in such an apparatus.sources of motive
power for driving the two members, means for sensing the
relative positions of the two members for producing a.corresponding
ratio signal proportional to the relative speeds of the motions
of the two members, computing means responsive to the speeds of
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operation of the sources for producing control 5ignals having
values proportional to the ratio signal, and control means for
regulating the speed of operation of the sources in accordance
with the values of the control signals.
More specifically, the present invention resides in
method.and apparatus for controlling the speeds of at least
two sources of motive power, and thereby the motions they drive
by governing the speed of each source with a signal having a
value proportional to a predetermined ratio of one speed to the
other and either proportional or inversely proportional to the
speed of the other motor. The positions of the first and second
motions driven by the motors are sensed and a signal having a
value corresponding to the desired ratio of those speeds is
produced. Each motor is coupled to a speed transducer which
produces an electrical signal having a value corresponding to
the speed of that electrical motor~ Depending on which ratio
is used the signal corresponding to the value of the speed of
each motor is either multiplied by or divided by the speed ratio.
When such signals are derived, each is coupled to a motor other
than the one from which the speed was taken to perform the
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aforementioned computation. The results of these mathematical
operations are communicated to speed regulating devices which
govern the speeds of operation of the respective motors.
As indicated hereinabove, in many situations it is
desirable that, for example, an operator be able to control
one or both of the speeds of the motions in question. Accordingly,
a reference signal is generated for each speed to be so controlled,
and this reference signal is compared with the one of the
signals being supplied to the motor resulting from the signal
computation operation discussed hereinabove. In this case the
minimum of the values of these two signals is chosen for
governing the speed of the motor in question.
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The speed control mechanism described above yields
a means by which the path of an implement can be precisely
controlled when that implement is being moved by means of
more than one mechanical motion. Thus, for example, in
the case of a power shovel the hoist and crowd speeds can be
precisely controlled to produce a ratio of those two speeds
which will cause the dipper to operate over a predetermined
path.
BRIEF DESCRIPTION OF THE DRAWINGS
The principles of the invention will be more readily
understood by reference to the following detailed description
of a preferred em~odiment in conjunction with the drawings
which are briefly described as follows:
Figure 1 is a block-schematic diagram of a preferred
embodiment of a system for regulating the speeds of two
motors in accordance with the principles of the invention;
Figure 2 is a side view of a power shovel utilizing
the Figure 1 preferred embodiment of the invention and includes
a range drawing illustrating the dipper path resulting from
use of the invention;
Figure 3 is a partial side plan view showing portions
of the power shovel boom, dipper handle and yoke of the
Figure 2 power shovel and illustrating a preferred embodiment
of a mechanism for generating the speed ratio function for
the Figure 1 embodiment; and
Figure 4 is a graph illustrating the desired crowd
speed to hoist speed ratio for given positions along the
digging path illustrated in Figure 2 which produced the
flat, linear digging path as illustrated in that figure.
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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Figure 1 is a block-schematic diagram of a control
system for regulating the speeds of two motors in accordance
with the principles of this invention. System 10 includes
two sources of motive power, which in this embodiment are
electric motors, motor A,12, and motor B,14. In the con-
ventional manner the speeds of these motors are respectively
controlled by speed regùlator A,ll and speed regulator B,13.
As indicated, these speed regulation devices may be of
conventional construction and of a type particularly suited
to the specific kind of motor involved. Motor 12 and motor
14, respectively, are connected to provide the motive power
for mechanical drive A,16 and mechanical drive B,18; these
may be any form of mechanical load and one particular
structure for providing such loads is described hereinbelow.
A function generator 20, which might assume a variety of
forms, an- example of which is described in detail below,
continuously senses the mechanical motion and positions of
the two drives 16 and 18 and at line 2i produces a voltage
signal having a value corresponding, in this case, to the
desired ratiG of the speed of drive 16 to the speed of drive
18.
Simultaneously, the speeds of motors 12 and 14 are
sensed, respectively, by speed transducers 22 and 24 which
in this preferred embodiment might take the form of tachometer-
generators. An electrical signal having a value corresponding
to the speed of motor 12 is supplied from transducer 22 to
input 26a of an electr~bnic divider circuit 26. The output
from function generator 20 is supplied to input 26b. The
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electronic divider circuit in the conventional manner
produces a signal at 26c which is of a value corresponding
to the quotient of the signal appearing at input 26a divided
by the signal appearing at input 26b; this yields the
quotient of the speed of motor 12 divided by the speed ratio
produced from function generator 20. Correspondingly, the
output signal from transducer 24 having a value corresponding
to the speed of motor B is supplied to an input 28a of an
electronic multiplier 28. Input 28b also receives the
ratio signal from function generator 20. This circuit in
the conventional manner produces a signal having a value
corresponding to the product of input 28a multiplied by
input 28b or the product of the speed of motor 24 and the
ratio produced by function generator 20.
By means of the foregoing electronic computations,
control values, and corresponding signals, are arrived at
for, respectively, regulating the speeds of motor 12 and
motor 14 in accordance with the then existing desired ratio
of the speed of motor 12 to the speed of motor 14. These
values are then, respectively, compared with reference
values, and the results of these comparisons are used to
govern the speeds of the motors.
More particularly, the output signal appearing at
26c from divider 26 is coupled for regulation of the speed of
motor 14. The output signal appearing at 28c from multiplier
28 is coupled to the regulation circuitry of motor 12 for
regulating the speed of that motor. Of course, in the
conventional manner these signals actually are utilized by
speed regulators 11 and 13 to control the speeds of motors
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12 and 14 accordingly.
Elements 30 and 32 refer to sources of reference
control signals for operating the speed regulators 11 and 13.
In the preferred embodiment shown herein these sources of
control voltages are arranged to be manually adjustable so
that an operator can control, if desired, the speeds of one
or both of the motors. Generally, it is not possible where
a precise amount of manual control is desired for an operator
to effectively control both motions being driven, and the
iO invention described herein then assumes greater importance.
For controlling the speed of motor 12 the outputs
from 28c and from reference source 30 are individually
coupled to a minimum-seeking diode logic network 34, constructed
as shown in the drawing, which functions as a comparator to
select and pass on to speed regulator 11 the one of the
inputs it receives which has the minimum value. Accordingly,
motor 12 will be governed by the signal value which is the
lesser of the output from reference source 30 or the product
of the ratio output from fu~tion-:-generator 20 and-the speed of
~ motor 14. The speed of operation of motor 14 is governed
in a similar manner. That is, output 26c and an output from
reference source 32 are coupled to diode logic network 36
which selects and passes to speed regulator 13 the lesser of
the output from reference source 32 or the quotient of the
speed of motor 12 divided by the ratio output from function
generator 20. The latter signal value then in the conventional
manner causes speed regulator 13 to adjust the speed of
motor 14 accordingly if that should be necessary.
Figure 2 is illustrative of a specific application of
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the invention disclosed and claimed herein, as well as
illustrating an inventive technique for controlling the
digging operations of a power shovel through a prescribed
path. The specific form of power shovel used forms no part
of the invention, and it will be described herein in sufficient
detail only for an understanding of the principles of the
invention.
Referring to Figure 2 there is illustrated a power
shovel 40 generally including a crawler unit 42 having
rotatably mounted thereon a main frame 44, the rotatable
mounting being accomplished by a conventional roller circle
structure 43. A boom 46 is mounted on main frame 44 by
means of boom foot pins journaled therein at 45O
A dipper 48 is mounted on a dipper handle 50, as
shown, and rack 49 is carried on dipper handle 50, as shown,
for engagement with pinions (not shown) within a yoke 52.
This rack and pinion arrangement provides for longitudinal
movement of dipper handle 50 relative to boom 46. Yoke 52
is pivotally connected to boom 46 by means of a shipper shaft
54 which is journaled therethrough. This connectéd arrangement
of the yoke to the boom~and the placement of the dipper handle
wlthin the yoke provides for rotation of the dipper handle
50 about the axis formed by shaft.54 as shown in Figure 2.
A hoist motor and drum arrangement 58 provides the
drive for movement of hoist rope 59 about a boom point
sheave 60 rotatably mounted at the point of boom 46. The end
of the hoist rope is then connected to the dipper as shown
in Figure 2. This, therefore, provides the drive for the
rotational motion of the dipper and dipper handle about
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shipper shaft 5~, referred to herein as the hoist motion.
A crowd motor 56 along with a suitable gearing
arrangement is provided and in this embodiment is shown as
mounted on the boom 46. This crowd motor and gearing arrangement
provides the drive for the pinions (not shown) mounted on
shipper shaft 54 which meah with rack 49 to thereby provide
the drive for longitudinal motion of the dipper handle 50.
This motion is referred to as a crowd motion and the speed
of movement of dipper handle 50 is referred to as a crowd
speed.
Generally simultaneously, hoist mechanism 58 is
operated to cause rotation of dipper 48 and dipper handle 50
and yoke 52 about shipper shaft 54. The speed with which
this motion is carried out is referred to as hoist speed.-
It is the combination of these two motions.and the judicious
selection of a ratio of crowd speed to hoist speed which
causes the dipper and dipper handle to follow a prescribed
digging path as shown in Figure 2.
In conjunction with Figure 2 reference should now be
had to Figure 4 where the numerals l through 8 correspond to
like numerals in Figure 2. Curve 70 in Figure 4 is a plot
: of the crowd speed to hoist speed ratios which are necessary
: for the dipper to sequentially assume each of the positions
1 through 8 as those positions are illustrated in Figure 2.
As an example of the operation of system 10 to achieve
the operational characteristic illustrated in Figure 4, if
the hoist speed were to be maintained at a constant value,
perhaps by use of a manually selectable reference speed source,
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the crowd speed would have to be varied in a manner
substantially following curve 70. System 10 would operate
to achieve this variation of crowd speed. In such a
circumstance, however, it is likely that the crowd motor
could not achieve the high speeds required at midposition,
i.e., positions 3 through 5, so the hoist speed would have to
be reduced in order to follow the prescribed path. Accordingly,
system lQ would operate to reduce the speed of operation of
the hoist motor below the speed which the operator is
attempting to maintain at a constant value. By this means,
the desired speed ratio at those points in the prescribed path
are maintained ! and there will then be no deviation from
the prescribed path.
In the case of the operation of a power shovel of this
particular type it is probably only desirable that manipulation
of the dipper be automated only during that portion of the
entire cycle of operation which is illustrated in Figure 2
and 4. For purposes of understanding the general principles
of operation of the shovel, the operator would manually
position the dipper at any desired point on the prescribed
digging path, and perhaps, an actuating switch could then be
used to initiate the automatic operation. By means of
system 10, the dipper would move along the prescribed path.
When the dipper is full, the operator will deactivate system
10 and then manually control the dipper through the dumping
and return portians of the cycle.
Figùre 3 illustrates in detail portions of dipper
handle 50 and boom 46 adjacent yoke 52 and shipper shaft 54.
In addition there is illustrated a cam and roller arrangement
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for operating a potentiometer 64 to achieve the signal
output corresponding to output 21 from function generator 20
in system 10.
More particularly, the lower end of yoke 52 forms a
cam surface 60 extending around the periphery thereof. During
operation of the shovel yoke 52 will move about shipper
shaft 54 in a manner generally following the path of operation
of the dipper handle. Cam surface 60 which in this case is
shaped to provide the characteristic of curve 70 is engaged
by a roller 62 attached to an arm 63 which at its other end
is attached to a potentiometer 64. This potentiometer along
with a suitable voltage source then provides a voltage which
varies according to the prescribed path of operation for the
shovel by providing a voltage characteristic which generally
follows curve 70 in Figure 4. This arrangement then provides
a function generator which responds to the varying angle
between the dipper handle and the boom from which the desired
voltage ratio corresponding to the speed ratio is derived.
There are, obviously, a myriad of other ways in the
form of mechanical linkages, levers or the like which could
be used to drive potentiometers to perform this function.
In addition, the cam might be separately driven from yoke 52
by means of levers or the like. Other forms of electrical
components can be used to perform the functions of potentiometer
64. For example, digital techniques could be used, and in
this instance the potentiometer can be replaced with a
conventional digital resolver. Likewise, other types of
analog function generating devices suited to the particular
application can be utilized.
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The principles of this invention are described herein
above in connection with a preferred embodiment wherein the
system is utilized for the control of crowd and hoist drives
in a power shovel. It is contemplated, however, that the
invention has a wide variety of applications where it is
desirable to maintain a speed ratio between at least two
motions, whether that ratio is derived on the basis of
posi~ion, time or any other suitable parameter. While an
analog system is described, an analogous digltal system can
be used for realizing the principles of this invention.
Furthermore, while the preferred embodiment of the invention
described herein is described in connection with a conventional
type of power shovel it is contemplated that the invention
has application for a variety of other types of excavating
equipment, as well a$ in any environment where the maintenance
of a given speed ratio between two motions is desired.
Accordingly, the specific description given hereinabove is
not to be considered as limiting, and the invention should
be considered only as being Idefined by the appended claims.
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