U.S. patent number 3,913,690 [Application Number 05/468,249] was granted by the patent office on 1975-10-21 for crane load indicating arrangement.
This patent grant is currently assigned to Pye Limited. Invention is credited to Robert William Hubbard, Bernard David Francis Hutchings.
United States Patent |
3,913,690 |
Hutchings , et al. |
October 21, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Crane load indicating arrangement
Abstract
A crane load indicating arrangement for a crane having a luffing
boom and which provides a load indication in terms of total
effective load, the load indication being with respect to both
radius related duties and angle related duties. The arrangement
provides a load indication with and without a fly jib attached to
the end of the boom and also includes means for compensating for
boom deflection.
Inventors: |
Hutchings; Bernard David
Francis (Chelmsford, EN), Hubbard; Robert William
(Southend-on-Sea, EN) |
Assignee: |
Pye Limited (Cambridge,
EN)
|
Family
ID: |
10192328 |
Appl.
No.: |
05/468,249 |
Filed: |
May 9, 1974 |
Foreign Application Priority Data
|
|
|
|
|
May 16, 1973 [GB] |
|
|
23233/73 |
|
Current U.S.
Class: |
177/25.12;
177/45; 177/147 |
Current CPC
Class: |
B66C
13/50 (20130101); B66C 23/905 (20130101) |
Current International
Class: |
B66C
13/18 (20060101); B66C 13/50 (20060101); B66C
23/90 (20060101); B66C 23/00 (20060101); G01G
019/04 (); G01G 023/18 (); G01G 019/14 () |
Field of
Search: |
;177/45,145,147,151,1,25,136 ;214/2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller, Jr.; George H.
Attorney, Agent or Firm: Trifari; Frank R. Franzblau;
Bernard
Claims
What we claim is:
1. A load indicating arrangement for a crane having a pivoted boom
comprising, means for producing a first output signal
representative of the total turning moment of the boom about its
pivot in supporting a load, means for producing a second output
signal representative of the horizontal distance between the boom
pivot point and the load, means for producing a third output signal
which is the quotient obtained by dividing said first output signal
by said second output signal, said third output signal being thus
representative of total effective load, a law generator unit for
each mode of operation of the crane, each unit being adapted to
produce a fourth output signal representative of the maximum safe
load for the crane in the instant mode of operation, and means for
comparing said fourth output signal with said third output signal
to provide an indication of the actual crane load relative to the
maximum safe load.
2. An arrangement as claimed in claim 1, wherein said indication of
the actual crane load relative to the maximum safe load is given by
a meter calibrated in percentage of safe working load, said meter
being coupled to the output of an operational amplifier having said
third output signal and said fourth output signal applied jointly
to an input terminal thereof and which has its gain divided by said
fourth output.
3. An arrangement as claimed in claim 1, wherein said indication of
the actual crane load relative to the maximum safe load is given by
a meter having a zero which is offset mechanically to a calibration
point from which it is driven on the one hand into an overload
region of its scale when said third output signal is greater than
said fourth output signal and on the other hand into a safe region
indicating available lifting capacity when said third output signal
is less than said fourth output signal.
4. An arrangement as claimed in claim 1 further comprising, means
for producing a fifth output signal which is determined by the boom
deflection, and means for modifying the value of said second output
signal by summing therewith said fifth output signal.
5. An arrangement as claimed in claim 1 wherein said first output
signal producing means comprises, transducer means for producing an
output signal which is a function of the reaction sustained by the
boom supporting means in supporting the boom and any load suspended
therefrom, and angle sensing means for modifying said output signal
in accordance with the sine of the angle included between the boom
supporting means and the boom thereby to produce said first output
signal.
6. An arrangement as claimed in claim 1 wherein said means for
producing a second output signal includes means for producing a
sixth output signal which is representative of the length of the
boom, means for producing a seventh output signal which is
representative of the cosine of the boom luff angle, and means for
multiplying said sixth and seventh output signals to derive the
second output signal as the product thereof.
7. An arrangement as claimed in claim 6, including means for
producing an eighth output signal for modifying the value of said
sixth output signal by an amount which is representative of the
change in load radius error that occurs between maximum and minimum
boom extensions when an offset fly jib is fitted, together with
means for changing said amount as a function of boom luff angle,
and means for selecting the value of said sixth output signal
having regard to the effective boom length due to the offset fly
jib.
8. An arrangement as claimed in claim 1 wherein the crane is
adapted to provide at least two modes of operation and for modes of
operation involving radius-related duties each law generator unit
concerned is responsive to said second output signal, whereas for
modes of operation involving anglerelated duties each law generator
unit concerned is responsive to an output signal which is
representative of the boom luff angle.
9. An arrangement as claimed in claim 8, including switch means for
selectively connecting said second output signal or said boom luff
angle output signal to a law generator unit.
10. An arrangement as claimed in claim 1 further comprising, means
for producing a fifth output signal which is representative of the
position of the centre of gravity of the boom structure and varies
with a change in boom length, and means for combining said fifth
output signal with said third output signal to produce a corrected
third output signal.
11. An arrangement as claimed in claim 10 further comprising, means
for producing a sixth output signal which is representative of the
weight of the boom structure, means jointly responsive to said
fourth and sixth output signals to produce a seventh output signal,
and means for comparing a given percentage of said seventh output
signal with the output obtained from the comparison of said third
and fourth output signals to provide an indication on the actual
hook load relative to a percentage of the maximum safe hook
load.
12. An arrangement as claimed in claim 11, including means for
combining said third, fifth and sixth output signals to provide a
eighth output signal which is representative of actual hook
load.
13. A load indicator apparatus for a crane having a pivotally
mounted extensible boom comprising, means for producing a first
signal representing the total turning moment of the boom and its
load about its pivot, means for producing a second signal
representing the horizontal distance of the load from the boom
pivot point, means for combining said first and second signals for
producing a third signal representing the total effective boom
load, means for deriving a fourth signal representing the maximum
safe load for the crane for the prevailing crane operating
conditions, and means for comparing said third and fourth signals
to derive an output signal indicative of the available lifting
capacity of the crane.
14. A load indicator apparatus as claimed in claim 13 wherein said
crane is adapted to provide at least two modes of operation, one of
which is determined primarily by the load radius and the other by
the boom luff angle, means for producing a signal representing the
boom luff angle, said fourth signal deriving means including law
generator means having input means selectively connected to receive
said second signal or said luff angle signal in said one or the
other modes of operation, respectively.
15. A load indicator apparatus as claimed in claim 13 further
comprising means for producing a fifth signal representing the
position of the boom center of gravity, and means for combining
said third and fifth signals to correct the third signal as a
function of the change in the boom center of gravity with a
variation in the length of the extensible boom.
16. A load indicator apparatus as claimed in claim 13 further
comprising means for modifying the second signal for fly jib
operation of the crane comprising, means for producing a fifth
signal representing the boom length, means for producing a sixth
signal representing the boom luff angle, means responsive to said
sixth signal to provide a correction thereto, means for producing a
seventh signal representing the cosine of the boom luff angle, and
means for combining said fifth signal, the corrected sixth signal
and the seventh signal to derive the modified second signal.
17. A load indicator apparatus as claimed in claim 13 further
comprising, means for producing a fifth signal representing the
deflection of the boom under load, and means for modifying the
second signal by combining therewith said fifth signal, said third
signal being derived by combining the first signal with the
modified second signal.
18. A load indicator apparatus as claimed in claim 13 further
comprising boom support means, and wherein said first signal
producing means comprises, means for producing a fifth signal which
is a function of the reaction force sustained by the boom support
means, means for producing a sixth signal representing the sine of
the angle formed by the boom and the boom support means and means
for combining said fifth and sixth signals to derive said first
signal, and wherein the means for producing the second output
signal comprises, means for producing a seventh signal representing
boom length, means for producing an eighth signal representing the
cosine of the boom luff angle, and means for combining said seventh
and eighth signals to derive said second signal.
Description
This invention relates to a load indicating arrangement for use
with cranes, derricks and other lifting apparatus of the type
having a pivoted boom which can be luffed by a hydraulic ram or
other boom supporting means. It has a particular but non-exclusive
application to mobile cranes of the above type having an extensible
boom which can be slewed through the whole or part of a circle.
A typical mobile 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. One end of the ram is also pivoted to the base unit, and the
other end is 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
mounted on a road or rail chassis and is 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.
The chassis may be provided with outriggers or blocking girders,
which are carried in a stowed position when the crane is in road
trim, but which can be extended outwards from the chassis and have
their outer ends blocked up from the ground in order to increase
the crane's stability and to relieve the load on the road
wheels.
For basic duties of the crane, a load is supported by a hoist rope
or cable 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. For lifting light loads, a fly jib may be
secured to the outer end of the boom. This increases the radius of
action of the crane.
Such a crane has a number of possible modes of operation, for
example, blocked, free-on-wheels, and with or without fly jib. In
whatever mode the crane is operated, the load must be limited so
that the overturning moment which it produces does not imperil the
stability and also that no component part of the crane is
over-stressed.
When operating without a fly jib, the prime consideration is
stability. Stability is greatest when the outriggers are extended
and blocked up. In the free-on-wheels condition, stability is
frequently greater when the boom is extended over an end of the
chassis than when it is slewed to one side or the other, because
the wheel base length of the chassis is usually substantially
greater than its track width.
A fly jib is usually of much lighter construction than the main
boom to which it is secured, and is adapted to support only
relatively light loads. Over much of the radius of operation of the
crane, the strength of the fly jib is the limiting factor in
determining the maximum safe load, and the question of stability
does not arise. At large radii, however, when the main boom is
fully extended and at a small luff angle, the moment produced by a
load which is within the strength capability of the fly jib may
reach the stability limit.
The crane manufacturer prepares rating tables which give the
maximum permissible hook loads which the crane may lift. A separate
table is prepared for each possible mode of operation. In general,
for modes of operation involving basic duties, the safe hook load
is related to radius from the slewing centre (i.e. radius-related
duties). For duties involving the fly jib, the safe hook load may
be related to luff angle below a given value of radius (i.e.
angle-related duties) and to radii above that value, or be related
to luff angle for all radii.
In our co-pending U.S. patent application Ser. No. 468,764, there
is described such a load indicating arrangement in which for
radius-related duties a loading indication is produced in terms of
turning moment of the hook load about the boom pivot point, whereas
for angle-related duties a loading indication is produced in terms
of actual hook load.
In contrast, the present invention provides a load indicating
arrangement including means for producing a loading indication in
terms of total effective load. Such an indication can be in respect
of both radius-related duties and angle-related duties.
More specifically, there is provided according to the present
invention a load indicating arrangement for use with a crane or
other lifting apparatus of the type specified, which arrangement
can comprise, means for producing a first output representative of
the total turning moment of the boom about its pivot in supporting
a load, means for producing a second output representative of the
horizontal distance between the boom pivot point and the load,
means for producing a third output as the quotient obtained by
dividing said first output by said second output, said third output
being thus representative of total effective load, a law generator
unit in respect of each mode of operation of the crane, each unit
being adapted to produce a fourth output representative of the
maximum safe loading for the crane in the appertaining mode of
operation for the load radius or luff angle, as the case may be,
currently obtaining, and means for comparing said fourth output
with said third output to provide an indication of the actual crane
loading relative to the maximum safe loading.
In the above context, the term "weight of the boom alone" is meant
to embrace the weight of the boom with or without a fly jib,
together with the weight of the sheave, hoist rope, hook, etc.,
that is, the total weight of the structure that supports the load,
but excluding the weight of the load. The term "total effective
load" means the weight, at the hook, of the actual load, plus the
total weight of the structure acting through its centre of gravity;
that is, in the latter respect, an equivalent weight at the hook
which produces the same turning moment as the weight of the
structure acting through its centre of gravity.
In carrying out the invention said first output is preferably
determined 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. More specifically, transducer means can be provided for
producing an output which is a function of said reaction, together
with angle sensing means for modifying said output in accordance
with the sine of the angle included between the boom supporting
means and the boom to produce said first output.
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 a 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 stress to which the cable is subjected. The
reaction sustained by the boom supporting means can thus readily be
determined as an electrical signal by means of a pressure
transducer or a resistance strain gauge transducer which is
appropriately mounted to suit the crane design.
In order that said third output represents accurately the total
effective load, the arrangement preferably includes means for
producing a fifth output which is representative of the change in
the position of the centre of gravity of the boom structure for
change in boom length, together with means for combining said fifth
output with said third output to produce a corrected third output.
The arrangement also preferably includes means for correcting for
boom deflection (or bending) in the production of said second
output, to take into account the effective increase in the
horizontal distance of the load from the boom pivot point due to
boom deflection; that is, the effective increase in load
radius.
For modes of operation involving basic duties, each law generator
unit concerned is responsive to the second (radius) output
(corrected for boom deflection) which is representative of the
horizontal distance between the boom pivot point and the load (i.e.
radius-related operation), whereas for modes of operation involving
fly jib duties, each law generator unit concerned is responsive to
an output from a boom angle sensing means (i.e. luff angle-related
operation).
Each law generator unit may be brought into use selectively by
means of mode sensors which are adapted to be activated selectively
as the crane is set up for different modes of operation.
Alternatively, plug-in law generator units can be provided for each
mode of operation.
Means may also be provided to produce an output which is
representative of actual hookload and hook load be utilised to
operate a meter which is calibrated to show actual weight of load.
Other meters can be provided which are responsive to said radius
output and the output from the boom angle sensing means
respectively, to show load radius and luff angle, respectively.
In order that the invention and the manner in which it is to be
performed may be more fully understood, an embodiment thereof will
now be described, by way of example, with reference to the
accompanying drawings, of which:
FIG. 1 is a diagrammatic representation of a mobile crane,
FIG. 2 (which comprises FIGS. 2a, 2b and 2c laid side-by-side in
that order) is a block schematic diagram of load indicating
arrangement according to the invention; and
FIG. 3 is a schematic diagram of a law generator unit for use in
the arrangement of FIG. 2.
Referring first to FIG. 1, the mobile crane there shown has a boom
indicated generally by the reference numeral 1. The boom comprises
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. Extension means such as a hydraulic ram (not shown in
FIG. 1) is provided to position the section 3 with respect to the
section 2 and to position 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 a 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 1.
A hydraulic luffing ram 7 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 axis of the boom 1 makes an angle
.theta. (the luff angle) with the horizontal, .theta. being
variable by varying the extension of the luffing ram 7.
The base unit 5 is mounted upon a road vehicle chassis 12 and is
arranged for rotation with respect to the chassis about a vertical
axis on a slewing centre 13.
For basic duties of the crane,, a load is suspended by a hoist rope
14 which passes over a sheave (not shown) at the outer end of the
boom section 4 to a winding drum (also not shown). It will be seen
that by varying the extension of the boom and/or the luff angle the
horizontal distance R1 between the slewing centre 13 and the hoist
rope 14 can be varied so as to permit lifting of loads located
within a range of radii from the slewing centre.
For fly duties of the crane, a fly jib 15, shown in broken outline
in FIG. 1, is secured to the outer end of the boom section 4, and
the hoist rope 14' passes over a sheave (not shown) at its outer
end. For any combination of boom extension and luff angle, the
horizontal distance R2 between the slewing centre 13 and the hoist
rope 14' is greater than the corresponding value of R1.
A load suspended by the hoist rope 14 (14') exerts a turning moment
about the boom pivot point 6. To this is added the turning moment
exerted by the weight of the boom acting through its centre of
gravity 16. The total turning moment is opposed by the component
normal to the boom axis of the reaction of the luffing ram 7.
A load indicating arrangement for a mobile crane of the above type
will now be described with reference to FIGS. 2 and 3. The
arrangement will be described firstly in relation to basic duties
of the crane and additional features required in respect of fly
duties will follow.
Referring to FIG. 2, a reference signal generator 17, for example a
700 Hz square wave oscillator, provides a stable signal V of
constant voltage. This signal V is supplied to a transducer 18
which is connected to the luffing ram 7 (FIG. 1) and is adpated to
produce an output P which is a function of the reaction sustained
by this 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 18 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. For a double-acting ram, two transducers are
usually fitted to measure pressures above and below the ram piston,
and their outputs are combined electrically to produce a resultant
transducer output.
The signal P is applied via an amplifier 19 to an input terminal of
a ram angle sensor 20, comprising a potentiometer having a
resistive track 21. The ends of the track 21 are connected to
ground and the signal P is applied at a tapping point 22
intermediate the ends of the track 21. The potentiometer body is
mounted in fixed relation to the boom 1 and a slider 23, which
contacts the track 21, is mechanically coupled to the luffing ram 7
so that it moves over the track 21 when the angle .phi. included
between the boom 1 and the ram 7 changes with changing extension of
the ram. The track 21 is graded so that the signal appearing at the
slider 23 is proportional to sin .phi.. The slider 23 is connected
to an input terminal of an amplifier 24 which provides an amplified
output M proportional to P sin .phi., i.e. to the component of the
ram reaction normal to the boom 1. Output M is therefore also
proportional to the total turning moment of the boom about the boom
pivot point 6.
A boom extension sensor 25 comprises a potentiometer having a
resistive track 26 and a slider 27 which is mechanically coupled to
the boom so as to be driven over the track 26 as the boom extension
is varied from minimum to maximum. The end of the track 26
corresponding to maximum extension is connected to the negative
terminal of a stabilised reference supply (e.g. -5v), the other end
being connected to the Ov side of the supply. It is assumed for the
purposes of the present description that the load indicating
arrangement is energised by a -5V stabilised reference supply, but
it is to be understood that this voltage is given only as an
example and that the actual voltage supply required depends upon
the type of circuit elements used in the load indicating
arrangement. The slider 27 is connected to an input terminal of an
amplifier 28. There is also connected to this input terminal of
amplifier 28 a present potentiometer 29 connected across the -5V
reference supply. This potentiometer 29 is provided to facilitate
initial setting-up of the arrangement. The amplifier 28 gives an
output L proportional to the boom extension.
A boom angle sensor 30 comprises a potentiometer mounted for
movement with the boom 1 and having a resistive track 31 connected
across the -5v reference supply. A slider 32 is gravity actuated,
e.g. by a pendulum, so that it moves over the track 31 as the luff
angle changes when the extension of the luffing ram 7 is varied.
The slider 32 is connected to an input terminal of an amplifier 33
which gives an output .theta. proportional to the luff angle
.theta.. This output may be used to drive a meter 34, which is
scaled in terms of luff angle, and this output is also applied to a
cosine law generator unit 35. This unit 35 is preferably of a type
in which the slope of its input/output characteristic is modified
stepwise in accordance with changes in its input amplitude so as to
produce an overall characteristic comprising a plurality of linear
sections of differing slopes and approximately closely to a cosine
law. The resultant output from unit 35 is thus proportional to the
cosine of the luff angle .theta..
The boom extension output L produced by the amplifier 28 is fed via
a gain control element comprising a fixed resistor 36 and a preset
variable resistor 37 to an input terminal of a summing amplifier
38. Also fed to this input terminal is an output E which is a
function of the length of the boom when fully retracted and which
is obtained from a potential divider comprising a fixed resistor 39
and a preset variable resistor 40 connected in series across the
-5v reference supply.
The fully-retracted length of the boom is constant for any one mode
of operation of the crane, but may vary from mode to mode. As will
be described more fully later, a plurality of resistors such as
resistor 40 is provided, each one preset to the value appropriate
to a particular mode, and means represented by the dotted rectangle
including resistor 40 are provided for selecting the particular
resistor corresponding to each mode of operation.
For radius-related duties, the output E is made proportional to the
fully-retracted length of the boom in each mode.
The resultant output of the amplifier 38 is thus substantially
proportional to the total length of the boom structure from which
the load is suspended and is applied as a first input to an
analogue multiplying unit 42. The output of the cosine law
generator unit 35 is applied as a second input to the unit 42.
Thus, the unit 42 produces a resultant output R proportional to (L
+ E) cos .theta.. It can be seen from FIG. 1 that (L + E) cos
.theta. is the basic horizontal distance R between the boom pivot
point 6 and the load, and that it equals the sum of the radius R1
of the load from the slewing centre 13 and the distance D between
the slewing centre and the boom pivot point.
The output R is applied to an input terminal of a summing amplifier
43. There is also applied to this input terminal an output BDC from
a circuit element 44 which is provided to correct for boom
deflection (or bending). Any deflection of the boom will result in
an increase of the load radius, so that the output R is not truly
representative of the true load radius. The circuit element 44
comprises two analogue multiplying units 45 and 46. The outputs L
and E are summed to form one input to the unit 45 and the output
.theta. is applied to the unit 45 as a second input. The resultant
output (L + E) .theta. from unit 45 is thus proportional to the
product of the total length of the boom and the luff angle .theta..
This resultant output is applied as one input to the unit 46 and
the output M, proportional to the total turning moment of the boom,
is applied as a second input to this unit 46. A potentiometer 47 is
connected between the OV line and the output terminal of the unit
46. This potentiometer is preset on initial setting-up of the
arrangement to provide a resistance value appropriate to the
particular boom structure concerned. The output from the unit 46 is
the product of the output M and the output (L + E) .theta. and is
adjusted in magnitude by the setting of the potentiometer 47 to
form the output BDC which is a function of the boom deflection that
occurs for the boom length, luff angle and total turning moment
currently obtaining. As aforesaid, boom deflection results in an
increase in the radius that the load is at so that the output BDC
is summed with the output R at amplifier 43, which latter produces
a true radius output TR. A meter 48 is provided to indicate true
radius of the load in response to the true radius output TR.
The output TR is applied as one input to an analogue dividing unit
49, and the output M is applied to this unit 49 as a second input.
The unit 49 is responsive to these two inputs to produce an output
TEL which is representative of total effective load; that is, total
turning moment divided by true radius equals total effective load
at hook. This output TEL is applied to an input terminal of a
summing amplifier 50. A further output SL is produced by a mode
unit 51, which will be described presently, and this output SL is
also applied to the input terminal of the amplifier 50. For the
radius-related duties being described, the unit 51 is responsive to
the output TR to produce the output SL. This output SL is
proportional to the maximum safe total effective load which the
crane is permitted to withstand for the boom length and luff angle
that currently obtain (i.e. the load radius) in any particular mode
of operation. The output SL is arranged to have a polarity opposite
to that of the output TEL.
However, for a telescopic boom structure, it is not sufficient
merely to combine algebraically the outputs SL and TEL to provide a
net input to the amplifier 50 that can be utilised to give an
indication of the crane loading, because for radius-related duties,
a particular load radius can be attained with a variety of boom
extensions and luff angles. An equivalent weight at the hook which
produces the same turning moment as the weight of the boom
structure acting through its centre of gravity is determined by the
actual weight of the boom structure and by the position of the
centre of gravity. The latter will change as the boom extension is
varied and the change will be affected by the telescopic structure
of the boom. It can be shown that an expression for such an
equivalent weight has the form (F .+-. KL), where F is a constant
related to the weight of the boom structure, and KL is related to
the position of the centre of gravity of the boom structure, for a
given mode of operation, K being a constant for a particular boom
and L being the boom extension.
In order to provide the output K, the arrangement further comprises
a potential divider comprising a fixed resistor 53 and a preset
variable resistor 54 which are connected in series between the
output terminal of the amplifier 28 and the OV line. The value of
the resistor 54 is set to produce an output proportional to K at
the tapping point of the potential divider. Since the value of K
may vary from mode to mode of operation of the crane, a plurality
of resistors such as resistor 54, each one preset to the value
appropriate to a particular mode, is provided together with means
(to be described hereinafter) represented by the dotted rectangle
including resistor 54 for selecting the particular resistor
corresponding to each mode of operation. The outputs L and K are
applied to respective input terminals of an amplifier 55 which
produces the output .+-. KL.
In order to provide the output F, there is provided a further
potential divider comprising a preset variable resistor 56 and a
fixed resistor 57 connected in series across the -5v reference
supply, the value of the resistor 56 being set so as to produce an
output proportional to the constant F at the tapping point of the
potential divider. In this instance also a plurality of preset
resistors such as resistor 56 is provided for each mode of
operation, together with means represented by the dotted rectangle
including the resistor 56 for selecting the particular resistor
corresponding to each mode.
The output KL is applied as a further input to the amplifier 50.
When therefore, the crane has reached its maximum safe total
effective load in a particular mode of operation, SL = TEL .+-. KL
and the net input to the amplifier 50 is zero. The output of
amplifier 50 is consequently also zero and is indicated at the
calibration point of a safe working load meter 58 connected to the
output terminal of the amplifier 50, the meter zero having been
offset mechanically to this calibration point. Increase of total
effective load above the rated maximum (TEL .+-. KL>SL) will
provide a net input of one polarity and a corresponding output from
the amplifier 50 which will drive the meter 58 into an overload
region of its scale. Total effective loads less than the rated
maximum (SL>TEL .+-. KL) will produce a net input and
corresponding output from the amplifier 50 of the opposite
polarity, driving the meter 58 into a safe region of its scale and
so indicating available lifting capacity. The output of the
amplifier 50 may also be applied to an alarm unit 59 which is
adapted to produce an audible and/or visual alarm signal when the
maximum safe total effective load is reached or exceeded.
The arrangement may also include means to provide a preliminary
warning signal when the total effective load exceeds a
predetermined percentage of the maximum safe total effective load,
and/or trip circuits to cut off power to the hoist motor in the
event of an overload. However, it may be preferred that a
preliminary warning signal is provided when a predetermined
percentage of the maximum safe weight of actual hook load, as
distinct from total effective load, is exceeded. To provide this
latter facility, the arrangement includes a further amplifier 60 to
the input terminal of which the output of amplifier 50 is applied.
The output SL is applied to one input terminal of an amplifier 66
and the output F is applied to a second input terminal of the
amplifier 66. It is arranged that these outputs SL and F are of
opposite polarity so that the output of amplifier 66 is (SL + F).
This latter output is applied across a potentiometer 62 the slider
61 of which is connected to the input terminal of amplifier 60. For
the condition SL > TEL .+-. KL, the output of amplifier 50 is of
opposite polarity to the output from amplifier 66. Thus, the net
input to amplifier 60 becomes zero to cause an alarm unit 67 at the
output of amplifier 60 to operate when the output of amplifier 50,
which corresponds to available lifting capacity, reduces to a value
corresponding to a percentage of the output (SL + F), as determined
by the setting of the potentiometer 61. It is to be noted that the
percentage of the output (SL + F) never reduces to zero, even
though the output SL may do so, because of the contribution of the
output F. Thus, if a crane controlled by the arrangement is
operating at extreme values of radius or luff angle, the alarm unit
67 may operate to indicate very little lifting capacity is
available even before any load is put on the hook.
The outputs TEL, .+-. KL and F may also be combined at the input
terminal of a further amplifier 71 to provide a net input thereto
that is proportional to the weight of the hook load. The output of
amplifier 71 drives a meter 72 which is calibrated to indicate
actual hook load.
For angle-rated duties (i.e. usually using a fly jib) the unit 51
is responsive to the output .theta. to produce the output SL, relay
changeover contacts 52 being provided to select which of the
outputs TR and .theta. is applied to the unit 51. Thus, for
angle-related duties, the output SL is proportional to the maximum
safe total effective load which the crane is permitted to withstand
for the luff angle that currently obtains. The load radius is not
taken into consideration because the maximum safe loading is
limited by the strength of the fly jib. The outputs SL, .+-. KL and
F are utilised in the same manner as for radius-related duties to
drive the meters 58 and 72 and to operate the alarm units 59 and
67.
Unlike radius-related duties, for which the output E is made
proportional to the fully-retracted length of the boom in each
mode, for angle-related duties using an offset fly jib, the output
E is made different from such directly proportional value to take
into account the fact that, for any given luff angle and load, the
ratio of the turning moments due to the load alone (i.e. ignoring
the turning moment of the boom structure) for a fully-extended boom
and a fully-retracted boom is not equal to the ratio of the
corresponding load radii. This inequality may be appreciated by
considering the effect of telescoping the boom between
fully-extended and fully-retracted positions at different
luff-angles, from which it can be seen that the proportion of load
radius due to the offset fly jib is greater at large luff angles
than at small luff angles. A similar consideration applies to the
effect of the weight of the fly jib acting through its centre of
gravity.
Looked at another way, because of the offset of the fly jib, the
turning moment due to the hook load is not normal to the boom axis,
but is normal to a line joining the pivot point 6 (FIG. 1) to the
end of the fly jib. Similarly, the turning moment due to the weight
of the fly jib acting through its centre of gravity is not normal
to the boom axis. Therefore, the output M, which is derived from
the actual total turning moment that is considered as being normal
to the boom axis, does not accurately represent the total turning
moment when an offset fly jib is mounted on the boom. The error in
the output M varies with boom extension and with luff angle.
Therefore, in order to obtain an accurate value for the output TEL
when an offset fly jib is fitted, the output M is divided, not by a
true radius output which is directly proportional to the load
radius (i.e. the output TR as in radius-related duties), but by a
radius output which varies from the true radius output on the same
manner as the output M varies from the true total turning moment.
This is achieved to a sufficient degree of accuracy by adding an
increment related to the length of the fly jib to the output E,
this increment being provided by adjustment of the potentiometer
40. However, this would give the appropriate radius correction only
for one luff angle. In order to provide the appropriate radius
correction at any luff angle, the arrangement further comprises a
potentiometer 41 which is connected at one end to receive the
output .theta. and to the OV line at the other end. This
potentiometer 41 is preset to produce a corrective output J =
.theta./K, where k is a constant for any one mode of angle-related
operation. This output J is applied to the input terminal of the
amplifier 38, together with the output E and the output L. Thus,
the output R from the unit 42 is now (L + E + J) cos .theta.. As a
consequence, the output TR is no longer a true radius output, but
is corrected to give a sufficiently accurate value for the total
effective load output TEL. Because the output TR is no longer a
true radius output, the load radius indicating meter 48 is not used
for offset fly jib operation. However, this is not a serious loss
because the load radius is not significant for angle-related
duties.
The mode unit 51, which will now be described with reference to
FIG. 3, comprises a plurality of similar law generator units, each
adapted to provide an output which varies according to a
predetermined law. One law generator unit is provided for each
separate mode of operation which the crane can perform, and is
preset to a law corresponding to the manufacturer's rating curve
for that mode of operation. Means are provided to select the one
law generator unit corresponding to the mode of operation being
performed.
Referring to FIG. 3, a law generator unit 73 is carried on a
printed circuit board indicated by the broken line rectangle. The
circuit of this unit comprises a plurality of similar threshold
amplifiers indicated generally by the references 74, 75, 76 and 77.
A positive input V.sub.IN, which can be either the luff angle
output .theta. from amplifier 33 or the true radius output TR from
amplifier 43 (see FIG. 2), is applied to each threshold amplifier.
Considering first the threshold amplifier 74, the input V.sub.IN,
which passes through a contact RLIA of a relay RL1 which is
energised when the particular unit 73 is in use, is fed via an
input resistor 78 to an input terminal of an amplifier 79. A
negative bias signal is fed to the same input terminal via a
resistor 80 from the slider of a preset potentiometer 81 (Break 1)
connected between a -5v reference supply (via relay contact RLID)
and ground. The output terminal of amplifier 79 is connected to the
same input terminal thereof via a feedback circuit comprising a
resistor 82 and two diodes 83 and 84. The arrangement is such that
if the magnitude of the positive input V.sub.IN is less than the
magnitude of the negative bias signal, giving a net negative input
to the amplifier 79, the amplifier output tends to go positive.
This causes the diode 83 to conduct. Since the input to the
amplifier 79 is a virtual ground, the output is therefore clamped
substantially at ground potential (plus the voltage developed
across the low forward resistance of the diode 83) for all values
of the input V.sub.IN less than the value of the bias voltage set
by the potentiometer 81.
If the value of the input V.sub.IN is greater than the bias voltage
value, thus giving a net positive input, the output of amplifier 79
goes negative. Diode 83 is cut off, but diode 84 conducts,
connecting resistor 82 as a feedback resistor between the output
and input terminals of the amplifier 79.
Therefore, as the input V.sub.IN varies from zero to its maximum,
say -5v, the output of the threshold amplifier 74 remains
substantially zero until the input V.sub.IN reaches a value (the
threshold or break value) determined by the setting of the (Break
1) potentiometer 81. Thereafter, the output increases linearly with
a further increase of the input V.sub.IN, with negative polarity
and at a rate determined by the relative values of the feedback
resistor 82 and the input resistor 78.
The output of the threshold amplifier 74 is applied to a first
summing junction 85 via a resistor 86, and also to one end of a
Slope 1 potentiometer 87. The slider of the potentiometer 87 is
connected to a second summing junction 88 via a resistor 89.
The threshold amplifiers 75, 76 and 77 are similar to the amplifier
74 just described and are provided with respective
threshold-setting potentiometers 90 (Break 2), 91 (Break 3) and 92
(Break 4). Their outputs are applied to the first summing junction
85 via respective resistors 93, 94 and 95; and also to respective
potentiometers 96 (Slope 2), 97 (Slope 3) and 98 (Slope 4). The
sliders of the potentiometers 96, 97 and 98 are connected via
respective resistors 99, 100 and 101 to the second summing junction
88.
The input V.sub.IN is applied to the first summing junction 85 via
a resistor 102 and also to an "Initial Slope" potentiometer 103,
whose slider is connected to the second summing junction 88 via a
resistor 104.
A "Shift" potentiometer 105 is connected between ground and the -5v
reference supply, and its slider is connected to the second summing
junction 88 via a resistor 106.
The first summing junction 85 is connected via relay contact RLIB
to an input terminal of an amplifier 107 contained in the mode unit
51. The second summing junction 88 is connected via relay contact
RLIC to an input terminal of an inverting amplifier 108, whose
output terminal is connected, via a resistor 109 to the said input
terminal of amplifier 107.
The operation is as follows: ignoring for the present the second
summing junction 88 and the amplifier 108, the output of the
amplifier 106 depends on the contributions to the first summing
junction from the input V.sub.IN via resistor 102 and from the
threshold amplifiers 74, 75, 76 and 77.
As the input V.sub.IN increases from zero, current flows through
resistor 102, but until the input V.sub.IN reaches the respective
break points of the threshold amplifiers, their outputs all remain
zero. Consequently, the output of the amplifier 107 initially
increases linearly with the input V.sub.IN at a rate determined by
the relative values of a feedback resistor 110 and the resistor
102, and with negative polarity.
When the input V.sub.IN reaches the first break point, determined
by the setting of the potentiometer 81, the first threshold
amplifier 74 commences to give an output which increases linearly
with a further increase of the input V.sub.IN, and which is
negative going. The current flowing via resistor 86 into the input
terminal of the amplifier 107 is therefore of opposite polarity to
the current flowing via resistor 102. The net effect is that the
rate of rise of input current with increase of the input V.sub.IN
is reduced for values of the input V.sub.IN above the first break
point. Therefore, the rate of increase of the output of the
amplifier 107 is similarly reduced.
As the input V.sub.IN continues to increase it reaches successively
the second, third and fourth break points determined respectively
by the settings of the potentiometers 90, 91 and 92. At these
points, the threshold amplifiers 75, 76 and 77 commence in turn to
contribute to the input current to the amplifier 107.
The result is that a curve relating the output of the amplifier 107
to the input V.sub.IN, neglecting the amplifier 108, comprises five
linear sections whose slopes are progressively less. The break
points at which the slope changes are selected by adjustment of the
potentiometers 81, 90, 91 and 92.
Turning now to summing junction 88 and amplifier 108, it will be
seen that the inputs to this junction comprise a fraction of the
input V.sub.IN chosen by adjustment of the potentiometer 103 and
fractions of the outputs of the threshold amplifiers 74, 75, 76 and
77 selected respectively by adjustment of the potentiometers 87,
96, 97 and 98. Consequently, the curve relating the output of
amplifier 108 to the input V.sub.IN comprises five linear sections
whose slopes are progressively less, and which individually are
less than or equal to the slopes of the sections of the
corresponding curve for the amplifier 107. The break points of the
two curves are identical.
Since the output of the amplifier 109 is applied to the input
terminal of the amplifier 107, the overall output of the latter
amplifier is the difference between the two curves aforesaid.
Consequently, the overall characteristic is a curve comprising five
linear sections, both the slopes of the individual sections and the
break points at which the slopes change being adjustable. In
addition, the DC level of the characteristic may be varied by
adjustment of the Shift potentiometer 105, which modifies the
current into the summing junction 88.
The Break-potentiometers, the Slope-potentiometers and the
Shift-potentiometers are adjusted to produce an overall
characteristic which matches within close limits a crane rating
curve.
A law generator unit 73 is provided for each separate rating curve.
Each first summing junction 85 is connected via its respective
relay contact RLIB to the input terminal of the amplifier 107, and
each second summing junction 88 is connected via its respective
relay contact RLIC to the input terminal of the amplifier 108.
Selection circuits within the mode unit 51 ensure that only one of
the relays, such as relay RLI, is energised at any one time, so
that only one of the law generator units 73 is operational.
The selection circuits are arranged to energise the particular law
generator unit appropriate to the desired mode of operation of the
crane and may be automatic in operation. For example, sensors may
be provided to detect when the outrigger booms are extended and
blocked up. Only when the outrigger sensors are operated will a law
generator for blocked modes of operation be brought into circuit.
If the sensors are not operated, a law generator appropriate to
free-on-wheel modes of operation will be selected.
Similarly, for cranes whose fly jib duty ratings are overridden by
the main radius duty ratings for certain combinations of luff angle
and boom extension, the boom extension and luff angle will be
supplied to the selector circuits, and the law generator unit
selected will depend on the values of these signals.
The true radius output TR provided by the amplifier 43 (FIG. 2) is
supplied to the mode unit 51 and is connected to the inputs of
those law generator units 73 which are selected when the crane is
performing radius-related modes of operation. Similarly, the luff
angle output .theta. provided by the amplifier 33 is connected to
the inputs of those units which are selected for angle-related
modes of operation. In each case, the connection is via the relay
contact RLIA.
As previously mentioned, with reference to FIG. 2, the effective
length of the boom when fully retracted (output E), and also the
values of the constant outputs F and K will vary for different
modes of operation. In each of the law generator units 73 there is
provided a preset variable resistor 40 having one end connected to
the OV line and the other to one side of a relay contact RLIE. The
other side of the contacts RLIE of all the units 73 are connected
together and to the negative input terminal of amplifier 38 (FIG.
2) so as to connect the resistor 40 of the selected unit 73 in the
position shown in the dotted rectangle containing resistor 40 of
FIG. 2.
Similarly, each law generator unit 73 contains preset variable
resistors 54 and 56 connectible via respective relay contacts RLIF
and RLIG to the positions shown for the dotted rectangles
containing these resistors, respectively, in FIG. 2.
It is expected that a load indicating arrangement, as hereinbefore
described, which provides an available lifting capacity indication
in terms of total effective load at the hook, will have advantage
over other forms of load indicating arrangement, such as described
in our co-pending U.S. patent application Ser. No. 468,764, which
provides an available lifting capacity indication in terms of the
turning moment of the hook load about the boom pivot point, or in
terms of actual hook load, because it can afford a greater dynamic
range of operation for a wider range of crane sizes and thus has a
more general application to cranes, or the like, of the different
sizes. This may be explained as follows.
Consider first a crane which is rated to lift, say, 10 tons at a
radius of 15 feet, but only 0.5 tons at a radius of 50 feet. The
ratio of maximum hook load to minimum hook load is 10 tons to 0.5
tons (i.e. 20 : 1), while the ratio of maximum to minimum hook load
turning moment is (10 .times. 15 = 150) to (0.5 .times. 50 = 25)
tons feet (i.e. 6 : 1). Assuming the use in a load indicating
arrangement of operational amplifiers having a dynamic range of
0-5v, then in the first case 5v = 10 tons and 0.25v = 0.5 tons,
whereas in the second case 5v = 150 tons feet and approximately
0.8v = 25 tons feet. Assuming further that the operational
amplifiers have a standing error voltage of 0.5mv, then 0.5mv in 5v
gives a 0.01 percent error in the amplifier output and 0.5mv in
0.8v gives a 0.066 percent error in the amplifier output. If the
overall arrangement is required to have less than 1 percent total
error then these percentage errors might be acceptable, provided
that the percentage errors of amplifiers throughout the arrangement
did not accumulate to exceed the total percentage error.
However, in the case of a crane which is rated to lift, say 40 tons
at a radius of 15 feet and 0.4 tons at a radius of 100 feet, the
maximum to minimum hook load ratio is 100:1 and the maximum to
minimum hook load turning moment ratio is (40 .times. 15 = 600) to
(0.4 .times. 100 = 40) tons feet (i.e. 15 : 1). This second ratio
of 15 : 1 approximates to the first ratio of 20 : 1 for the smaller
crane, so that the percentage error for turning moment computation
for the larger crane is now slightly greater than the percentage
error for hook load computation for the smaller crane. Furthermore,
the hook load computation for the larger crane now gives 0.5mv in
50mv (i.e. the voltage value for 0.4 tons), so that the percentage
error in this instance is 1 percent which is the total percentage
error allowed for the arrangement.
Consider now an arrangement according to the invention which
provides safe load indications computed from total effective load
at the hook. If the boom structure of the smaller crane weighs 4
tons, so that the average effective weight of the boom at the hook
can be said to be 2 tons, then the total effective hook load ratio
between maximum and minimum for the smaller crane is (10 + 2) :
(0.5 + 2) .apprxeq. 5 : 1. If the boom structure of the larger
crane weighs 10 tons, so that the average effective weight of the
boom at the hook can be said to be 5 tons, then the total effective
hook load ratio between maximum and minimum for the larger crane is
(40 + 5):(0.4 + 5) .apprxeq. 9 : 1. From these figures, which are
given by way of example only, it can be seen that an arrangement
according to the invention would afford, on the assumptions made, a
dynamic range of operation for either crane with an acceptable
percentage error.
Also, with regard to improving dynamic range of operation, the
following modification may be made in respect of the meter 58 (FIG.
2c). This meter 58 indicates, as previously described, the
difference between the outputs (TEL + KL) and SL.
For "heavy" duties of a crane, both the outputs TEL and SL will be
large so that the difference between them can be large and the
meter sensitivity will be adequate. However, for "light" duties of
a crane, both the outputs TEL and SL will be small and the meter
sensitivity may be insufficient to provide an indication between a
"load" and a "no-load" condition. Therefore, to obtain
substantially the same meter sensitivity the gain of the amplifier
50 can be divided by the output SL, with the meter 58 being
calibrated in terms of percentage "safe working load" (within
limits).
* * * * *