U.S. patent number 3,870,160 [Application Number 05/265,329] was granted by the patent office on 1975-03-11 for crane safe load indicator.
This patent grant is currently assigned to Pye Limited. Invention is credited to Bernard David Francis Hutchings.
United States Patent |
3,870,160 |
Hutchings |
March 11, 1975 |
CRANE SAFE LOAD INDICATOR
Abstract
A crane safe load indicator for determining the safe load
supported by a pivoted boom which can be luffed. The indicator
functions by comparing a signal representative of the total turning
moment of the boom about its pivot in terms of the angle included
between the boom and a hydraulic ram which is supporting it, with a
reference signal which is representative of the maximum safe
turning moment of the boom about its pivot for the boom luff angle
and load radius currently obtaining. A modification of the
indicator provides a safe load indication when a fly jib is mounted
at the end of the boom. In this modification account is taken of
the change in centre of gravity of the boom fly jib and load due to
change in boom extension.
Inventors: |
Hutchings; Bernard David
Francis (Cambridge, EN) |
Assignee: |
Pye Limited (Cambridge,
EN)
|
Family
ID: |
10301038 |
Appl.
No.: |
05/265,329 |
Filed: |
June 22, 1972 |
Foreign Application Priority Data
|
|
|
|
|
Jun 25, 1971 [GB] |
|
|
30024/71 |
|
Current U.S.
Class: |
212/278; 177/147;
340/685; 701/124 |
Current CPC
Class: |
B66C
23/905 (20130101); G06G 7/28 (20130101); B66C
2700/084 (20130101) |
Current International
Class: |
G06G
7/00 (20060101); G06G 7/28 (20060101); B66C
23/00 (20060101); B66C 23/90 (20060101); B66c
013/16 () |
Field of
Search: |
;212/39 ;340/267C
;338/89 ;235/151.33 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,151,645 |
|
Jul 1963 |
|
DT |
|
1,000,613 |
|
Aug 1965 |
|
GB |
|
Primary Examiner: Blunk; Evon C.
Assistant Examiner: Nase; Jeffrey V.
Attorney, Agent or Firm: Trifari; Frank R. Franzblau;
Bernard
Claims
What we claim is:
1. A safe load indicator system for use with a crane or other
lifting apparatus having a pivotally mounted boom and extensible
boom supporting means comprising, means for producing a first
signal representative of the actual total turning moment of the
boom about its pivot by determining the angle included between the
extensible boom supporting means and the boom and the reaction
sustained by the extensible boom supporting means in supporting the
boom and a load suspended therefrom, means for deriving a second
signal determined by the boom luff angle and the length of the boom
and which is indicative of the current load radius, means
responsive to said second signal for producing a reference signal
representative of the maximum safe turning moment of the boom about
its pivot and corresponding to the current load radius, and means
responsive to the first and the reference signals for providing an
indication of the available lifting capacity.
2. A safe load indicator system for use with a crane or lifting
apparatus having a pivotally mounted extensible boom and extensible
boom supporting means comprising, transducer means for producing a
first signal which is a function of the reaction by the extensible
boom supporting means in supporting the boom and any load suspended
therefrom, angle sensing means for modifying said first signal in
accordance with the sine of the angle included between the
extensible boom supporting means and the boom to produce a first
resultant signal which is representative of the actual total
turning moment of the boom about its pivot boom angle sensing means
for producing a second signal which is proportional to the cosine
of the boom luff angle, boom length sensing means for modifying
said second signal in accordance with the boom length to produce a
second resultant signal which is representative of load radius,
load/radius law generator means adapted for operation in accordance
with the load/radius rating of the crane and responsive to said
second resultant signal to produce a reference signal which is
representative of the turning moment of the boom about its pivot
for lifting a maximum safe load at the load radius represented by
said second resultant signal, and means responsive to said first
resultant signal and said reference signal to produce an output in
accordance with any difference therebetween and thus between actual
total turning moment and maximum safe turning moment.
3. A safe load indicator system for use with a crane or other
lifting apparatus having a pivotally supported boom, a fly jib and
extensible boom supporting means comprising, means for producing a
signal representative of the actual reaction sustained by the
extensible boom supporting means in supporting the boom, fly jib
and any load suspended therefrom at the current boom luff angle,
means for producing a reference signal representative of the
computed boom supporting means reaction which would be required to
sustain the extensible boom supporting means at the boom luff angle
currently obtaining with the maximum safe load suspended from the
fly jib for said luff angle, and means responsive to said signals
for providing an indication of the available lifting capacity.
4. A safe load indicator system as claimed in claim 3, wherein the
boom is telescopic to vary its length, and further comprising means
for modifying said reference signal in accordance with the boom
extension.
5. A safe load indicator system as claimed in claim 4, further
comprising means for further modifying said reference signal as a
function of the shift of the combined centre of gravity of the boom
fly jib and load due to a change in the boom angle.
6. A safe load indicator for use with a crane having a pivotally
supported extensible boom, a fly jib and extensible boom supporting
means comprising, transducer means for producing a first signal
which is a function of the actual reaction force sustained by the
extensible boom supporting means in supporting the boom, fly jib
and any load suspended therefrom, boom angle sensing means for
producing a second signal which is proportional to the boom luff
angle, first load/angle law generator means adapted for operation
in accordance with the fly load/boom angle rating of the crane and
responsive to said second signal to produce a first reference
signal which is representative of the extensible boom supporting
means reaction which would be required to sustain the boom at
maximum extension with the maximum safe fly jib load at the boom
luff angle represented by said second signal, boom length sensing
means for modifying said first reference signal in accordance with
the boom length, second load/angle law generator means also adapted
for operation in accordance with the fly load/boom angle rating of
the crane and responsive to said second signal to produce a second
reference signal which is representative of a change in the centre
of gravity of the boom, fly jib and load for a change in boom
angle, means for producing a resultant reference signal in response
to the modified first reference signal and the second reference
signal, and means responsive to said first signal and said
resultant reference signal to produce an output in accordance with
any difference therebetween and thus between the actual reaction of
the extensible boom supporting means due to a fly jib load and the
maximum safe computed reaction due to said fly jib load.
7. A safe load control system for a crane having an extensible boom
pivotally mounted on a support structure and extensible boom
supporting means for adjusting the boom luff angle comprising,
means for producing a first signal which is proportional to the
reaction of the extensible boom supporting means, means for
modifying said first signal as a function of the sine of the angle
formed between the boom and the extensible boom supporting means to
produce a first resultant signal, means for producing a second
signal which is a function of the cosine of the boom luff angle,
means for modifying said second signal as a function of the boom
length to produce a second resultant signal indicative of the
actual load radius, means responsive to said second resultant
signal for computing a reference signal representative of the
maximum safe turning moment of the boom about its pivot and
corresponding to the load radius indicated by said second resultant
signal, and means responsive to said first resultant signal and
said reference signal for indicating the available lifting capacity
of the crane at the current load condition.
8. A control system as claimed in claim 7 wherein said first signal
modifying means comprises a potentiometer having a sine law
resistance variation and a slider adapted to be coupled to the
extensible boom supporting means to move proportional to the
extension of said extensible boom supporting means.
9. A control system as claimed in claim 8 wherein said second
signal producing means comprises a second potentiometer having a
cosine law resistance variation and a slider adapted to be coupled
for movement with the boom and proportional to the boom luff angle
and said second signal modifying means comprises a third
potentiometer having a linear resistance variation and a slider
adapted to be coupled to the boom for movement proportional to the
boom extension.
10. A control system as claimed in claim 7 further comprising a
first switch for selectively connecting said first signal or the
first resultant signal to an input of said indicating means, means
for producing a third signal which is a function of the boom luff
angle, means responsive to said third signal for producing a second
reference signal indicative of the computed maximum safe boom
supporting means reaction at the current boom luff angle and which
is in accord with the load/boom angle rating of the crane, and a
second switch operated in synchronism with the first switch for
selectively connecting said first and second reference signals to
an input of said indicating means.
Description
This invention relates to a load indicator arrangement for use with
cranes, derricks and other lifting apparatus of the type having a
pivoted boom which can be luffed by an hydraulic ram or other boom
supporting means.
A typical crane of the above type has a boom comprising a plurality
of telescoping sections, of which the lowermost is pivoted to a
base unit for luffing movement by means of a hydraulic ram which is
also pivoted at one end to the base unit and has its other end
pivoted to a point on the lowermost section so as to support the
boom at an angle (the luff angle) to the horizontal which is
determined by the extension of the ram. The base unit is normally
arranged to slew through the whole or part of a circle about a
vertical axis. As an alternative to the hydraulic ram, the boom can
be supported by a winch cable which is secured to its outer end and
which can be wound in and out to luff the boom. For this
alternative, the boom is not usually telescopic.
In a basic mode of operation of the crane, a load is supported by a
hoist rope passing over a sheave at the outer end of the boom. The
crane can lift loads located within a range of radii measured from
its slewing centre.
In general, where the boom is telescopic, any one of a plurality of
combinations of boom extension and luff angle may be employed to
bring the sheave vertically above a load located at a given radius
from the slewing centre. In the case of a non-telescopic boom,
different radii can be achieved only by varying the luff angle.
The maximum permitted (safe) load at a given radius is a function
of the crane design. The crane manufacturer provides a rating curve
or table which relates the maximum safe load for each value of
radius. In the case of a telescopic boom, the rating curve or table
also relates maximum safe load to boom extension for each value of
radius because the maximum safe load will vary with boom
extension.
For the handling of relatively light loads it is the practice to
secure an extension or fly jib to the head of the outer section of
the boom and to pass the hoist rope over a sheave at the outer end
of the fly jib. This permits the crane to reach loads at a greater
radius than when operated in the basic mode. Generally, the fly jib
is of relatively light construction compared with the boom so that
the latter can support any load which may be imposed on it by the
fly jib irrespective of its extension (in the case of a telescopic
boom) and luff angle. The maximum safe load for fly jib operation
is thus a function of the design of the fly jib and varies with the
angle which the fly jib makes with the horizontal. Since the fly
jib is secured at a fixed angle with respect to the axis of the
boom, the maximum safe load for fly jib operation is related to the
boom luff angle. The crane manufacturer also provides a rating
curve or table relating maximum permitted (safe) load for fly jib
operation to luff angle.
It is an object of the present invention to provide a crane or
other lifting apparatus of the above type with a safe load
indicator for indicating the actual load supported by the crane
relative to the maximum safe load and for providing a warning
signal if the actual load equals or exceeds the maximum safe load
both when the crane is operated in the basic mode and when it is
operated using an extension or fly jib.
According to a first aspect of the present invention a safe load
indicator arrangement for use with a crane or other lifting
apparatus of the type specified comprises means for producing a
signal representative of the actual total turning moment of the
boom about its luffing pivot in terms of the angle included between
the boom supporting means and the boom and of the reaction
sustained by the boom supporting means in supporting the boom and
any load suspended from it, means for producing a reference signal
representative of the maximum safe turning moment of the boom about
its pivot for the boom luff angle and load radius currently
obtaining, and means responsive to these signals to produce a
resultant output signal which can be utilised to provide an
indication of the available lifting capacity.
The word "reaction"is used herein to signify the force to which the
boom supporting means is subjected in supporting the boom (and
load). If the boom supporting means is an hydraulic ram then the
force would be a function of the fluid pressure in the ram, whereas
if the boom supporting means is a winch cable then the force would
be a function of the strain to which the winch cable is subjected.
The reaction sustained by the boom supporting means can thus
readily be determined as an electrical signal by a pressure
transducer or a resistance strain gauge transducer which is
appropriately mounted to suit the crane design.
In carrying out the invention according to this first aspect
thereof the load indicator arrangement can comprise transducer
means for producing a first signal which is a function of the
reaction sustained by the boom supporting means in supporting the
boom and any load suspended from it, angle sensing means for
modifying said first signal in accordance with the sine of the
angle included between the boom supporting means and the boom to
produce a first resultant signal which is representative of the
actual total turning moment of the boom about its pivot, boom angle
sensing means for producing a second signal which is proportional
to the cosine of the boom luff angle, boom length sensing means for
modifying said second signal in accordance with the boom length to
produce a second resultant signal which is representative of load
radius, load/radius law generator means adapted for operation in
accordance with the load/radius rating of the crane and responsive
to said second resultant signal to produce a reference signal which
is representative of the computed total turning moment at the boom
pivot for maximum safe load at the radius represented by said
second resultant signal, and means responsive to said first
resultant signal and said reference signal to produce an output in
accordance with any difference therebetween and thus between actual
total turning moment and maximum safe computed turning moment.
The invention according to this first aspect is applicable to to
aforesaid basic mode of crane operation.
According to a second aspect of the present invention a safe load
indicator arrangement for use with a crane or other lifting
apparatus of the type specified and adapted for operation using a
fly jib comprises means for producing a signal representative of
the actual reaction sustained by the boom supporting means in
supporting the boom and fly jib and any load suspended from the fly
jib, means for producing a reference signal representative of the
computed boom supporting means which would be required to sustain
the boom supporting means at the boom luff angle currently
obtaining with the maximum safe load for that luff angle suspended
from the fly jib, and means responsive to these signals to produce
a resultant output signal which can be utilised to provide an
indication of the load.
In carrying out the invention according to this second aspect in
the case where the boom is telescopic, means may be provided for
modifying said reference signal in accordance with boom extension
because the signal indicative of boom supporting means reaction is
also a function of boom extension in that it varies with a change
in boom extension. Also, is is preferable in order to achieve
greater accuracy of fly jib load indication to further modify said
reference signal as a function of the shift of the combined centre
of gravity of the boom fly jib and load due to a change in boom
angle.
More specifically, the load indicator arrangement may comprise
transducer means for producing a first signal which is a function
of the reaction sustained by the boom supporting means in
supporting the and fly jib and any load suspended from it, boom
angle sensing means for producing a second signal which is
proportional to the boom luff angle, first load/ angle law
generator means adapted for operation in accordance with the fly
load/boom angle rating of the crane and responsive to said second
signal to produce a first reference signal which is representative
of the boom supporting means reaction which would be required to
sustain the boom at maximum extension with the maximum safe fly jib
load at the boom luff angle represented by said second signal, boom
length sensing means for modifying said first reference signal in
accordance with the boom length, second load/angle law generator
means also adapted for operation in accordance with the fly
load/boom angle rating of the crane and responsive to said second
signal to produce a second reference signal representative of
change in the centre of gravity of the boom, fly jib and load for
change in boom angle, means for producing a resultant reference
signal in response to the modified first reference signal and the
second reference signal, and means responsive to said first signal
and said resultant reference signal to produce an output in
accordance with any difference therebetween and thus between the
actual fly jib load and the maximum safe fly jib load.
In order that the invention may be more clearly understood, an
embodiment thereof will now be described, by way of example, with
reference to the accompanying drawings, of which:
FIG. 1 is a schematic drawing of a crane having a telescopic boom
arranged for luffing;
FIG. 2 is a typical rating curve for the crane of FIG. 1;
FIG. 3 is a schematic drawing of the crane of FIG. 1 with an
extension or fly jib;
FIG. 4 is a typical rating curve for the crane of FIG. 3;
FIG. 5 is a block schematic diagram of a safe load indicator
arrangement according to the invention;
FIG. 6a is a block schematic diagram of a law generator of the safe
load indicator arrangement and
FIG. 6b is a graph illustrating the operation of the law generator
of FIG. 6a.
Referring first to FIG. 1, the crane there shown has a boom
indicated generally by the reference numeral 1 and comprising a
lower section 2, an intermediate section 3 slidable telescopically
within the upper end of the section 2 and an upper section 4
slidable telescopically within the upper end of the section 3. An
hydraulic extension ram, not shown in the drawing, is provided to
vary the position of the section 3 with respect to the section 2
and a similar ram positions the section 4 with respect to the
section 3, so that the overall length of the boom 1 may be adjusted
to any desired value between a maximum and minimum limit.
The lower end of the boom section 2 is pivoted to a horizontal base
unit 5 at a point 6 so as to permit luffing movement of the boom.
An hydraulic luffing ram 7 (extensible boom support) has one end of
its cylinder 8 pivoted to the base unit 5 at a point 9 and its
piston rod 10, which extends through the other end of the cylinder
8, pivoted to the boom section 2 at a point 11. The longitudinal
axis of the boom 1 makes an angle .theta. (the luff angle) with the
horizontal base unit 5, .theta. being variable by varying the
extension of the luffing ram 7. A load is suspended by a hoist rope
12 which passes over a sheave (not shown) provided at a point 13 at
the outer end of the upper boom section 4 to a hoist mechanism
(also not shown). The base unit 5 may be mounted by means not shown
for slewing abou a vertical axis 14.
The crane is rated by the manufacturer in terms of the maximum safe
loads which may be lifted at different radii 15 from the slewing
centre 14. A typical rating curve 16 for a crane of the type
described with reference to FIG. 1 is shown in FIG. 2. The load
radius is variable between a minimum value 17 obtained with the
luffing ram 7 fully extended and the boom sections 3 and 4 fully
retracted and maximum value 18 obtained with the luffing ram 7 at a
minimum extension and the boom sections 3 and 4 fully extended. An
intermediate radius such as 19 may be obtained by partially
retracting the boom section(s) 3 and/or 4 while maintaining the
luffing ram 7 fully extended, by partially retracting the luffing
ram 7 while maintaining the boom sections 3 and 4 fully extended or
by partially retracting both the luffing ram 7 and the boom
sections 3 and 4.
The crane as arranged for operation using an extension or fly jib
will now be described with reference to FIG. 3, in which the same
references as in FIG. 1 are employed for integers common to the two
FIGURES. A boom 1 comprising a lower section 2 and telescopically
extensible sections 3 and 4 is pivotally mounted on a base unit 5
at a point 6 and is supported by an hydraulic luffing ram 7 having
a cylinder 8 pivotally mounted on the base unit 5 at a point 9 and
a piston rod 10 pivotally connected to the lower-most boom section
2 at a point 11. As in FIG. 1, the base unit 5 is mounted for
slewing about a vertical axis 14.
A fly jib 21 is secured to the outer end of the boom section 4 at
point 13. The fly jib 21 is shown at an angle .beta. with respect
to the longitudinal axis of the boom 1, such that the longitudinal
axis of the fly jib 21 is inclined to the horizontal at an angle
somewhat less than the luff angle .theta., but the angle .beta. may
be zero, i.e. the fly jib 21 may be collinear with the boom 1. The
hoist rope 12 now passes over the sheave provided at the outer end
22 of the fly jib 21.
As was stated previously, the strength of the boom and the
stability of the crane as a whole are such that the maximum load
which the fly jib 21 can impose at the point 13 can be sustained
with any combination of boom extension and boom luff angle .theta..
The maximum safe load which may be supported by the hoist rope 12
is therefore determined by the strength of the fly jib 21 and the
angle (.theta. - .beta.) which it makes with the horizontal. Since
.beta. is constant the crane may therefore be rated in terms of
boom luff angle .theta.. A typical rating curve 23 for a crane of
the type described with reference to FIG. 3 is shown in FIG. 4.
A load indicator arrangement for use when the crane is adapted
either for operation in the basic mode (FIG. 1) or for fly jib
operation (FIG. 3) will now be described with reference to FIG.
5.
In this arrangement, a reference signal generator 24, for example,
a 700 Hz square wave oscillator, provides a stable signal voltage
V. The signal V is supplied to a transducer 25 which is connected
to the luffing ram 7 and is adapted to produce an output signal P
which is a function of the reaction sustained by the ram in
supporting the boom (1) and any load suspended from it. When the
ram 7 is of the single-acting type, the output of the transducer 25
is a function of (e.g., proportional to) hydraulic fluid pressure
below the ram piston 10. For a double acting ram, the transducer
output is a function of (e.g., proportional to) the difference
between the pressures below and above the ram piston 10, modified
by the ratio of the effective areas of the lower and upper sides of
the piston.
The signal P is applied via an amplifier 26 to an input terminal of
a ram angle sensor unit 27, comprising a potentiometer having a
resistance track 28. The ends of the track 28 are connected to
ground via respective resistances 29 and 30, and the signal P is
connected to a tap point 31 intermediate the ends of the track 28.
A slider 32 makes contact with the track 28. The potentiometer body
is mounted in fixed relation to the boom 1 and the slider 32 is
mechanically coupled to the luffing ram 7 so that it moves over the
track 28 when the angle .phi.included between the boom 1 and the
ram 7 changes with changing extension of the ram. The track 28 is
graded according to the sine law such that the signal M appearing
at the slider 32 is proportional to the sin .phi.. The resistors 29
and 30 serve to limit the range of values of sin .phi.to that
corresponding to the range of values of the angle .phi. permitted
by the construction of the crane.
Inspection of FIG. 1 will show that the actual total turning moment
of the boom 1 about the pivot point 6 is balanced by the reaction
of the ram 7 multiplied by sin .phi.. The signal M is therefore a
function of this actual turning moment.
In an alternative arrangement of the ram angle sensor unit 27, the
potentiometer is replaced by a plurality of fixed resistors
connected in series, the junctions between successive pairs of
resistors being connected to successive fixed contacts of a
multi-way rotary switch. The relative values of the fixed resistors
are chosen according to the sine law to produce a stepped
potentiometer chain corresponding to the potentiometer track 28.
The moving contact of the rotary switch is then equivalent to the
potentiometer slider 32.
The output signal M produced by the ram angle sensor unit 27 is
connected to a first fixed contact of a changover switch 33. The
second fixed contact of the switch 33 is connected to the output of
the amplifier 26 and the moving contact is connected to an input of
a summing amplifier 34. The output of the amplifier 34 is connected
to a meter 35 and to an input of an alarm unit 36.
The signal V produced by generator unit 24 is also fed to an input
of a boom angle sensor unit 37 comprising a potentiometer 38 having
a cosine law and a potentiometer 39 having a linear law, connected
in parallel between the signal line and ground. The potentiometers
38 and 39 are mounted for movement with the boom 1, and their
respective sliding contacts 40 and 41 are gravity actuated so that
there appears at the slider 40 a signal V1 proportional to the
cosine of the angle .theta. which the boom makes with the
horizontal (the luff angle), and at the slider 41 a signal V2
proportional to the angle .theta.. Conveniently, the boom angle
sensor unit 37 may comprise two inclinometers of the type described
in our British Pat. No. 965,017.
The slider 40 is connected to a first fixed contact of a changeover
switch 42, and the slider 41 is connected to an input of a
load/angle law generator unit 43, to an input of a load/angle law
generator unit 44 and to a meter 45. The output of the unit 43 is
connected to a second fixed contact of the changeover switch 42,
and the moving contact of the switch 42 is connected to an input of
a boom extension sensor unit 46. Unit 46 comprises a potentiometer
47 and a fixed resistor 48. The slider 49 of the potentiometer 47
is coupled to the boom 1 so as to be driven from the end of
potentiometer 47 adjacent resistor 48 to the opposite end as the
boom extension is varied from minimum to maximum. Resistance values
of the linear potentiometer 47 and the resistor 48 are chosen so
that the total series resistance is proportional to the boom length
when fully extended and the resistor 48 is proportional to the boom
length when fully retracted. It will be appreciated that for
applications in which the boom length is constant, the
potentiometer 47 may be replaced by a fixed resistor.
The slider 49 is connected to the moving contact of a changeover
switch 50. A first contact of the switch 50 is connected to a meter
51 and to an input of a load/radius law generator unit 52. A second
fixed contact of the switch 50 is connected to an input of a
summing amplifier 53. The output of unit 44 is also connected to
the input of the amplifier 53.
Preferably, all of the changeover switches 33, 42, 50 and 54 are
ganged together for operation so that the moving contacts of all
the switches connect with their respective first fixed contacts or
with their respective second fixed contacts.
The operation of the system is as follows:-
When the crane is operated in the basic mode (i.e., without the fly
jib 21), all of the switches 33, 42, 50 and 54 are set as shown in
FIG. 5.
As described hereinbefore, the signal P which is a function of the
reaction sustained by the luffing ram 7 in supporting the boom 1
and the load is fed to the ram angle sensor unit 27 where it is
multiplied by the sine of the angle .phi. included between the
luffing ram 7 and the boom 1 to produce the signal M.varies.P sin
.phi., which is a function of the actual total turning moment of
the boom 1 about the pivot 6. The signal M is fed to the input of
the summing amplifier 34.
The boom angle sensor unit 37 produces an output V1 proportional to
cos .theta., where .theta.is the boom luff angle, which is fed to
the extension sensor unit 46. The output unit 46 is a signal R
proportional to the boom extension multiplied by cos .theta., i.e.
proportional to the load radius, i.e., the horizontal distance 55
(FIG. 1) between the load and the boom pivot point 6. The signal R
is fed to the meter 51 which is suitably scaled to indicate the
radius 15 of the load from the slewing centre 14. The zero of the
meter 51 is offset, either electrically or mechanically, by an
amount corresponding to the distance 56 between the boom pivot
point 6 and the slewing centre 14. The offset may be required to be
of either sign, dependent on the design of the crane. In some
designs the boom pivot is on the opposite side of the slewing
centre of the load as shown in FIG. 1, in other designs it is on
the same side.
The signal R is also fed to the input of the load/radius law
generator unit 52. This unit has a characteristic related to the
crane manufacturer's Load/Radius rating curve (16, FIG. 2) such
that its output signal is proportional to the a computed turning
moment at the boom pivot when lifting the maximum safe load at the
radius corresponding to the input signal R.
It will be appreciated that a different load/radius law generator
unit is required for each different type and/or capacity of crane,
to which differing manufactures'ratings apply. For this reason,
unit 52 is preferably arranged as a self-contained unit provided
with means of connection to the rest of the equipment of a type,
e.g., plug and socket connectors or terminal blocks, which permit
of easy replacement. Similar considerations apply to the units 43
and 44. A more detailed description of a typical law generator unit
will follow hereinafter with reference to FIG. 6.
The output signal from the load/radius law generator 52 is fed via
the changeover switch 54 to the input of the summing amplifier
34.
The signals reaching the input of the summing amplifier 34 are
therefore a signal proportional to the actual total turning moment
at the boom pivot and a reference signal proportional to the
computed turning moment which would be produced by the crane
lifting its maximum safe load at the same radius as the actual
load. The gains of the various units of the equipment are selected
so as to render the constants of proportionality the same for the
two signals. These two signals also may be arranged to have
opposite polarities, e.g., by arranging unit 52 to produce a
complement signal.
When, therefore, the crane is lifting its maximum safe load, the
net input to the amplifier 34 is zero, producing zero output, which
is indicated at the calibration point of the scale of the load
meter 35 (the meter zero being offset mechanically to this point
during manufacture).
Increase of load above the rated maximum will produce a net input
of one polarity and a corresponding output from the amplifier 34
which will drive the meter 35 to indicate in the overload region of
its scale. Lesser loads will produce a net input and corresponding
output of opposite polarity from the amplifier 34, thereby driving
the meter 35 into the safe region and thus indicating available
lifting capacity.
The alarm unit 36 is arranged to give an audible and/or visible
alarm signal when the output of the amplifier 34 is zero or of the
one polarity aforesaid, i.e., when the actual load equals or
exceeds the maximum safe load, and may also comprise a trip circuit
to immobilise the hoist mechanism of the crane under these
conditions.
When the crane is required to perform a fly jib operation, the
changeover switches 33, 42, 50 and 54 are set to the position
opposite to that shown in FIG. 5.
The signal P, which is a function of the reaction sustained by the
luffing ram 7 in supporting the boom, fly jib and load, is
generated in the manner hereinbefore described and is fed from the
output of the amplifier 26 via the switch 33 to the input of the
summing amplifier 34.
The output signal V2 of the boom angle sensor unit 37 is displayed
by the angle meter 45 and is also fed to the input of the
load/angle law generator 43. This unit has a characteristic which
is related to the manufacturer's fly Load/Boom angle rating curve
for fly jib operation (23, FIG. 4), such that the output of unit 43
is a signal which is of opposite polarity to the signal P and is
proportional to a computed ram reaction which would be required to
sustain the boom at maximum extension with the maximum safe load
suspended from the fly jib at the boom luff angle represented by
the output signal V2.
The signal from unit 43 is fed via the switch 42 to the boom
extension sensor unit 46, where it is modified to supply at the
slider 49 a signal which is also a function of boom length, and
which is fed via the switch 50 to the input of the summing
amplifier 53.
A further signal is fed to the input of the amplifier 53 for the
following reason. The reaction, sustained by the luffing ram when a
load is supported by the fly jib varies not only with boom angle
and with boom extension, but also as a second-order function of
load due to the position of the combined centre of gravity of the
boom, fly jib and the load varying with variation of boom
angle.
The output signal V2 of the boom angle sensor unit 37 is fed to the
load/angle law generator unit 44 which has a characteristic related
to the rated load/boom angle curve 23 and produces an output signal
which is fed to the summing amplifier 53 and is proportional to the
change in the ram reaction, for maximum rated load, due to the
shift in the centre of gravity for change in the boom luff angle as
represented by the output signal V2.
The two signals into the summing amplifier 53 are opposite in
polarity to the signal P so that the output from the summing
amplifier 53 is therefore exactly the complement of the signal P
when the maximum safe jib fly load is lifted at any boom angle and
extension. This output signal is fed via the switch 54 to the input
of the summing amplifier 34.
The total signal input to the amplifier 34 when the system is
adapted to a crane performing fly jib operation therefore varies in
a manner similar to that described hereinbefore in relation to the
system adapted to a crane performing the basic mode of
operation.
Without the further signal from the unit 44, the maximum safe fly
jib load would not be so accurately computed because, at a given
boom length, the ram reaction required to sustain the boom, fly jib
and any load supported by it would be a miminum at maximum boom
angle, but as the boom angle is decreased the reaction would not
increase in direct proportion due to the change in the position of
the combined centre of gravity. In fact, the reaction would be
somewhat greater than a direct proportional increase. The actual
reaction signal P includes this factor but the signal computed from
the law generator unit 43 and modified by the extension sensor unit
46 does not and is somewhat less than it should be by a factor
related to the change in the combined centre of gravity for a
change in boom angle. This could give rise to a condition in which
the output from the summing amplifier 53 is less than the signal P
to produce a difference signal signifying safe fly-jib loading
when, in fact, the signal P is related to a fly-jib loading which
is greater than the permitted maximum.
The law generator units 43, 44 and 52 are each specific to a
particular type of crane. They are, however, similar in that each
is required to accept an input signal variable over a range of
values and to provide an output signal related to the input signal
according to a known law. In any given instance the law may not be
a simple mathematical relationship, but will be expressed
graphically in the form of a curve relating values of input and
output signals. The curve may be complex and include sudden changes
or even reversals of slope.
The circuit arrangement shown in FIG. 6a is a typical example of a
law generator and illustrates the generation of an output signal
which is a complex function of an input variable.
An operational amplifier 55 has a feedback resistor 56 and input
paths 57-61 inclusive, of which the paths 57-60 inclusive are
connected to an input signal Vin, assumed in this instance to vary
over the range 0V to +5V. The input 61 is connected to a reference
voltage source (-5V) via a present potentiometer.
Input path 57 comprises a resistor 62, so that the current I.sub.1
flowing through 62 to the input of the amplifier 55 (which input is
a virtual ground) varies linearly with the input signal Vin as
shown by the graph 63 in FIG. 66b.
Input path 58 comprises series resistors 64 and 65 with a switching
circuit comprising the transistors 66 and 67 connected to their
common point. The current i.sub.2 flowing through the path 58 to
the input of amplifier 55 increases linearly with an increase of
the input signal Vin until the potential at the emitter of
transistor 66 causes the latter to conduct. Further increase of Vin
thereafter increases the current through transistor 66. The point
at which the transistor 66 first conducts is determined by the
setting of the potentiometer 68 connected across a reference
voltage supply.
The current I.sub.2 therefore increases linearly with an increase
of the input signal Vin up to a point determined by the setting of
the potentiometer 68 and is thereafter constant, as shown by the
curve 69 in FIG. 6b.
The input path 59 comprises two series resistors and a transistor
switching circuit similar to those of the path 58 and also an
inverting amplifier 70. It therefore supplies a current I.sub.3
which increases linearly up to a value determined by the
potentiometer 71 and is thereafter constant, but is of the opposite
polarity to the currents I.sub.1 and I.sub.2 as shown by curve
72.
The path 60 comprises a series resistor and a series switching
circuit. No current flows in path 60 until the input voltage
reaches a value, determined by the setting of the potentiometer 73,
at which the switching circuit commences to conduct. Thereafter the
current I.sub.4 increases linearly with an increase of Vin, as
shown by curve 74.
The path 61 comprises a series resistor connected to a source of
constant voltage determined by the setting of the potentiometer 75.
The current I.sub.5 in path 61 is therefore constant, as shown by
curve 76.
The total input current I.sub.TOT at the input of the amplifier 55
is the algebraic sun of the currents I.sub.1 to I.sub.5 inclusive.
I.sub.TOT varies with the input signal as shown by the curve
77.
The output voltage V.sub.OUT of amplifier 55 is always such as to
provide a current through the feedback resistor 56 equal and
opposite to the input current I.sub.TOT. The current voltage
V.sub.OUT therefore changes with change of input voltage Vin
according to the curve 77, but with opposite sign.
It is apparent that any required law can be approximated with a
desired degree of accuracy by a suitable combination of input paths
to an operational amplifier.
* * * * *