Load-monitoring system for boom-type crane

A memory aboard a boom-type crane stores the coefficients of several polynomials of at least the fifth order closely approximating respective curves giving the maximum permissible load for different boom lengths as a function of the elevational angle of the boom (or the horizontal projection of its length). The set of coefficients read out under the control of a boom-length sensor is fed to a processor which calculates, for a given angle (or projection) as measured by another sensor, the numerical value of the maximum load corresponding to the selected polynomial. The actual value of the load, e.g. as detected by the hydraulic pressure of a jack engaging the boom at an intermediate point, is compared with the permissible maximum calculated by the processor; if that maximum is exceeded, an alarm is given.

FIELD OF THE INVENTION 
My present invention relates to a load-monitoring system for a crane of the 
boom or jib type, designed to alert an operator to the fact that the load 
moment of the crane is about to surpass a critical magnitude and/or to 
prevent directly the establishment of an overload situation. 
BACKGROUND OF THE INVENTION 
As is well known in the art, the effective load movement of such a crane 
depends not only on the weight of a load suspended from its hoisting cable 
but also on the length of its boom and its angle of inclination or 
elevation. The point at which this moment begins to impair the stability 
of the crane, when the same is designed as a mobile vehicle, is also 
determined in each instance by the dimensions of its base and, at least in 
the absence of lateral outrigger-type supports, by the azimuthal angle 
included between the boom and the vehicular axis. Reference in this 
connection may be made, for example, to U.S. Pat. Nos. 3,638,211 and 
3,740,534 as well as others listed therein as cited references. 
From the patents referred to above it is known to provide such a crane with 
a computer including a memory digitally storing information peculiar 
thereto, the memory being addressable by a boom-angle sensor and a 
boom-length sensor to read out a numerical value representing the maximum 
load that can be safely supported under the existing operating conditions. 
This value is compared with the magnitude of the load actually measured 
and an alarm signal is given if the measured magnitude approaches the 
stored value. 
Such a memory must have a large storage capacity in view of the many 
combinations of boom length, boom angle and possibly other parameters that 
have to be taken into account. 
It is also known, e.g. from British Pat. No. 1,107,116, to use an analog 
computer for the purpose of determining the maximum permissible load 
moment on the basis of signals measuring such parameters as the boom angle 
and the tension of a hoisting cable supporting the load. As more 
particularly described in that British patent, the computer may operate 
along a straight line or a curve--e.g. an arc of a circle or a 
parabola--which approximates a curve representing the permissible maximum 
load as a function of angle of elevation. A path corresponding to that 
straight line or curve can be traced by a lever or a slide coacting with 
suitable markings. 
A straight line or even a second-order curve such as a parabola, however, 
is only a rather rought approximation of the actual load characteristic of 
a crane of the kind here envisaged. In order to satisfy national and 
international regulations concerning crane safety, a control system of 
this simplified type would have to be so designed that the deviations lie 
on the "safe" side of that characteristic; this entails an 
underutilization of the load-carrying capacity in many instances. 
OBJECT OF THE INVENTION 
The object of my present invention, therefore, is to provide an improved 
load-monitoring system which does not require a memory of large storage 
capacity while allowing a good approximation of the actual load 
characteristic of a crane in order to prevent the establishment of 
overload conditions. 
SUMMARY OF THE INVENTION 
A system according to my present invention comprises first sensing means 
for generating a first signal representative of effective boom length, 
second sensing means for generating a second signal representative of the 
elevational angle of the boom, and a memory storing a plurality of sets of 
coefficients of polynomials of at least the fifth order closely 
approximating respective curves giving the maximum permissible load for 
different boom lengths as a function of variable parameter relating to 
boom inclination. That parameter could be the elevational boom angle 
itself but may also be the horizontal (or vertical) projection of the boom 
given by its length times the cosine (or sine) of that angle. In some 
instances it may even suffice to measure the extent to which the boom 
projects horizontally beyond its support, specifically its vehicular base. 
The memory is addressable by the first sensing means for reading out a set 
of coefficients, selected in response to the first signal, to a processor 
which is also connected to the second sensing means for calculating a 
numerical value of the polynomial defined by the selected set of 
coefficients which corresponds to the variable parameter as determined by 
the second signal. A comparator with inputs connected to the processor and 
to measuring means engageable with the boom, for determining the magnitude 
of a load supported thereby, generates an alarm signal whenever that 
magnitude exceeds a permissible limit determined by the calculated 
numerical value. The term "alarm signal", as here used, encompasses not 
only an indication given to an operator but also a possible command 
preventing the boom-positioning mechanism from inadmissibly changing the 
elevational angle. 
The memory, which is preferably of the read-only type, may be 
reprogrammable to modify the stored set of coefficients in accordance with 
structural changes of the crane to which the system is applied. 
The measurement of the actual load may be carried out, as in the system of 
the above-identified British patent, by a sensor responsive to the tension 
of the boom-hoisting cable. An alternative measuring device, also known 
per se, is a pressure sensor connectable with a cylinder of a hydraulic 
jack which serves to adjust the angle of elevation and which is anchored 
to a fixed base and to an intermediate point of the boom; see also my 
prior U.S. Pat. No. 4,185,280. The output of such a pressure sensor is a 
signal dependent not only on the live load suspended from the cable but 
also on the dead weight of the boom itself and any ancillary jib serving 
as an extension thereof. The relationship between that output signal and 
the elevational angle is a function which, as I have found, can be quite 
closely approximated by a polynominal of the fifth or the sixth order 
having five or six coefficients, respectively. Higher-order polynomials 
could, of course, be used, yet this would call for additional storage 
capacity which generally will not be necessary.

SPECIFIC DESCRIPTION 
Reference will first be made to FIG. 19 in which I have shown a mobile 
crane 10 of a type known per se, comprising an extendable main boom 11 
with telescoped sections and an ancillary boom or jib 12 articulated 
thereto at 13. A hoisting cable 14 passes around both booms and supports a 
hook 15 designed to carry a load not shown. Boom 11 is inclined to the 
horizontal at an elevational angle .alpha. which can be varied by a 
hydraulic jack 18 under the control of the crane operator. Jib 12 includes 
with boom 11 an angle .beta. which is adjustable with the aid of a cable 
16 deflected by a brace 17. The jack 18 is anchored to a platform 19 
which, together with the operator's cab, is rotatable in the usual manner 
about a vertical axis on a wheel-supported base 20. This base may be 
provided with outriggers, e.g. as shown in the two above-identified U.S. 
patents, which have not been illustrated. 
The variable length L of the main boom 11 is measured by a sensor 21 while 
its elevational angle .alpha. is determined by a sensor 22, both of which 
may be of the type described in the first two U.S. patents referred to. 
Another type of boom-length sensor, which can also be used, is described 
in U.S. Pat. No. 3,489,294. The jib 12 has a constant length L'. The 
elevation of boom 11 is controlled by the pressure of hydraulic fluid fed 
to jack 18, that pressure being measured by a sensor 23 on support 19. 
FIG. 20 shows a load-monitoring system according to my invention installed 
aboard the crane 10 of FIG. 19. The system comprises a preferably 
programmable read-only memory 24 with an address input extending from 
length sensor 21 and another address input originating at a manual 
selector 25 which is settable in various positions depending on the 
utilization of nonutilization of outriggers and of the ancillary boom or 
jib 12 as well as on the azimuthal angle by which the operator`s cab and 
the boom have been rotated from the forward-pointing position illustrated 
in FIG. 19. If the jib 12 is used, its angle of relative inclination 
.beta. will also be fed to the memory 24 by the selector 25. 
Memory 24 is divided into a number of sections, jointly addressable by 
sensor 21 and selector 25, each storing a set of coefficients assigned to 
a respective polynomial which approximates the actual values of the 
maximum permissible hydraulic pressure of jack 18 for different 
elevational angles .alpha. within a predetermined operating range. These 
actual pressures, as empirically determined for a multiplicity of values 
of .alpha., lie at more or less closely spaced points of a coordinate 
system which may be interconnected by straight lines or in stepped fashion 
to form a polygonal trace. In practice, that trace may be approximately 
linear in an intermediate part of the range but will significantly depart 
from linearity for the lowest and the highest values of .alpha.. 
A processor 26, receiving the coefficients read out from memory 24 under 
the control of sensor 21 and selector 25, plots from these coefficients a 
polynomial curve approximating the aforementioned polygonal trace within 
the range of variation of elevational angle .alpha.. The instantaneous 
magnitude of angle .alpha. is supplied by sensor 22 to processor 26 which 
on the basis thereof delivers a signal proportional to pressure P to a 
comparator 27. Sensor 23 supplies that comparator with a signal proportion 
to the actual hydraulic pressure P' which at no time should exceed the 
permissible value P. Comparator 27, therefore, emits an alarm signal A 
whenever the magnitude of P' closely approaches the value P; signal A may 
visually or audibly alert the crane operator and/or may inhibit the 
positioning mechanism from changing its elevational angle. 
The graphs of FIGS. 1-18 show in full lines respective curves representing 
the polynomial P(.alpha.) for different sets of parameters reflected by 
the output signals of sensor 21 and selector 25, with angle .alpha. read 
on the abscissa and pressure P read along the ordinate. Also shown in some 
of these graphs, in phantom lines, are segments of the empirically 
determined polygonal traces referred to above, to the extent that these 
traces deviate significantly from the associated polynomial curves. With 
the exception of FIGS. 12-15, these graphs apply to the crane 10 of FIG. 
19 stabilized by the nonillustrated outriggers; FIGS. 1-10 correspond to 
different lengths L of main boom 11, with FIGS. 12-15 relating to the same 
boom lengths as FIGS. 1-4 but without outriggers. FIGS. 11 and 16-18 
pertain to the presence of the ancillary jib 12 of length L' inclined at 
different angles .beta. to the main boom 11; in all the other instances 
the load is suspended directly from the end of that main boom. These 
examples are all for the forward-pointing position of FIG. 19. 
In the following Table I have listed the polynomial coefficients K.sub.0 
-K.sub.6 for the function P=K.sub.6 .alpha..sup.6 +K.sub.5 .alpha..sup.5 
+K.sub.4 .alpha..sup.4 +K.sub.3 .alpha..sup.3 +K.sub.2 .alpha..sup.2 
+K.sub.1 .alpha.+K.sub.0. The curves of FIGS. 1 and 3 are only polynomials 
of the fifth order, with K.sub.6 =0, while the others are all of the sixth 
order. The boom length L is variable between 11 and 34.6 meters while the 
pressure P goes up to about 160 bars. The operative range of elevational 
angle .alpha. may extend between about 20.degree. and 80.degree.. 
Processor 26 could be of the digital or the analog type. 
TABLE 
__________________________________________________________________________ 
FIG. 
K.sub.6 K.sub.5 K.sub.4 K.sub.3 K.sub.2 
K.sub.1 
K.sub.0 
__________________________________________________________________________ 
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__________________________________________________________________________ 
It is to be understood that my invention is also applicable to a crane in 
which the elevation of the boom is controlled by a hoisting cable, as in 
the above-identified British patent, whose tension is measured by a sensor 
as representative of the actual load. Curves generally similar to those of 
FIGS. 1-18, with tension instead of pressure plotted along the ordinate, 
would be used in such a system.