U.S. patent number 3,586,841 [Application Number 04/799,379] was granted by the patent office on 1971-06-22 for boom load indicating system.
This patent grant is currently assigned to The Warner & Swasey Company. Invention is credited to Robert F. Griffin.
United States Patent |
3,586,841 |
Griffin |
June 22, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
BOOM LOAD INDICATING SYSTEM
Abstract
A boom load indicating system for a self-supporting boom which
determines the maximum bending stress on the boom and, in the case
of a telescoping boom, determines the bending stress on each
section of the boom. A strain transducer is mounted on the boom as
close as possible to the pivot where the boom lift force is
applied. Electrical analog circuitry indicates when bending stress
is approaching or at a maximum permissible value. When the boom is
operated at a substantial slewing angle, the predetermined maximum
permissible tipping moment provides the standard against which the
actual bending stress on the boom is compared.
Inventors: |
Griffin; Robert F. (South
Euclid, OH) |
Assignee: |
The Warner & Swasey Company
(Cleveland, OH)
|
Family
ID: |
25175755 |
Appl.
No.: |
04/799,379 |
Filed: |
February 14, 1969 |
Current U.S.
Class: |
702/43; 73/769;
73/849; 212/231; 708/800 |
Current CPC
Class: |
B66C
23/905 (20130101) |
Current International
Class: |
B66C
23/00 (20060101); B66C 23/90 (20060101); G06g
007/12 () |
Field of
Search: |
;235/151.33,184,185,151
;214/673 ;212/55 ;73/88.5,100 ;177/211 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Morrison; Malcolm A.
Assistant Examiner: Wise; Edward J.
Claims
: Having described my invention, I claim:
1. In combination with a boom which is subject to a substantial
bending moment, a strain transducer, and means operatively
connecting said transducer to the boom at a location effective to
produce on the transducer a strain which is substantially a
predetermined function of the bending moment on the boom, said
predetermined function is a substantial linear function of the
strain on the transducer with respect to the total bending moment
on the boom due to its own weight and the weight of any load
suspended from its outer end.
2. In combination with a self-supporting boom for supporting a load
from its outer end, boom lift means operatively engaging the
underside of the boom near its lower end for lifting and lowering
the boom, a strain transducer, and means rigidly attaching said
transducer to the underside of the boom in close proximity to the
location thereon where said boom lift means engages the boom so
that the strain measured by the transducer is substantially
linearly proportional to the total bending moment on the boom due
to the load and due to the weight of the boom itself.
3. The combination of claim 2, wherein said boom has a plurality of
extensible sections which interfit in succession along its
length.
4. The combination of claim 3, wherein said boom has telescoping
sections.
5. The combination of claim 3, and further comprising circuit means
operative in response to the output signal of said transducer and
in response to the individual lengths and weights of the boom
sections to determine the actual bending moment on each boom
section.
6. The combination of claim 5, wherein said circuit means comprises
electrical analog circuitry.
7. The combination of claim 5, wherein said circuit means comprises
means for producing reference signals proportional respectively to
predetermined maximum safe bending moments for each of the boom
sections, and means for comparing each of said reference signals
against a signal proportional to the actual bending moment on the
corresponding boom section.
8. The combination of claim 7, and further comprising indicator
means for each boom section operable by the comparison between each
reference signal and the corresponding actual bending moment signal
to provide an indication when the actual bending moment signal is
at least a predetermined percentage of the respective reference
signal.
9. An indicator system for use with a self-supporting boom having a
plurality of boom sections in succession outwardly from its base
and certain of which sections, at least, are extensible, said
system comprising means for sensing the bending moment on the boom
at a location thereon where the response of said sensing means is
substantially linearly proportional to the actual bending moment at
said location on the boom, an analog circuit including circuit
elements operatively connected to provide signals which are
substantially proportional to the lengths of the respective boom
elements, the circuit elements which correspond to the extensible
boom sections being variable in accordance with the individual
extended lengths of the latter, said analog circuit also including
circuit elements operatively connected to provide signals which are
substantially proportional to the weights of the respective boom
elements, and said analog circuit including means for combining the
output signal of said sensing means and said boom section length
signals and said boom section weight signals to provide for each
boom section a signal substantially proportional to the actual
bending moment thereon.
10. A system according to claim 11, and further comprising means
for producing reference signals proportional respectively to
predetermined maximum safe bending moments for the individual boom
elements, and means for comparing each of said reference signals
against the signal representing the actual bending moment on the
corresponding boom section.
11. In combination with a self-supporting boom having a base
section and a plurality of additional sections in succession
outwardly from the base section, at least certain of said sections
of the boom being extensible, strain transducer means operatively
coupled to the boom to determine the bending moment on the boom at
a particular location thereon, and circuit means operable in
accordance with the respective weights and extended lengths of the
several boom sections and the output of said strain transducer
means to produce a plurality of signals which individually are
representative of the bending moments on respective sections of the
boom.
12. The combination of claim 11, and further comprising boom lift
means operatively engaging the base section of the boom from below
for lifting and lowering the boom, and means rigidly coupling said
strain transducer means to the underside of said base in close
proximity to its operative engagement by said boom lift means so
that the strain measured by said strain transducer means is
substantially linearly proportional to the total bending moment on
the boom due to its own weight and the weight of any load suspended
from its outer end.
13. The combination of claim 11, wherein said circuit means is an
analog circuit including circuit elements operatively connected to
provide signals which are substantially proportional to the lengths
of the respective boom elements, the circuit elements which
correspond to the extensible boom sections being variable in
accordance with the individual variable lengths of the latter, said
analog circuit also including circuit elements operatively
connected to provide signals which are substantially proportional
to the weights of the respective boom elements, and said analog
circuit including means for combining the output signal of said
strain transducer means and said boom section length signals and
said boom section weight signals to provide for each boom section a
signal which is substantially proportional to the actual bending
moment thereon.
14. The combination of claim 13, and further comprising boom lift
means operatively engaging the base section of the boom at a
predetermined location for lifting and lowering the boom, and means
operatively connecting said strain transducer means to the base
section of the boom to measure the strain at the underside of the
boom in close proximity to the location where said boom lift means
is operatively connected to the base section of the boom.
15. The combination of claim 13, and further comprising means for
producing reference signals proportional respectively to
predetermined maximum safe bending moments for the respective boom
sections, and means for comparing each of said reference signals
against a signal proportional to the actual bending moment on the
corresponding boom section.
16. The combination of claim 15, and further comprising means for
producing a maximum safe tipping moment signal, and means for
comparing said tipping moment signal against the signal
proportional to the actual bending moment on the base section of
the boom when the slewing angle of the boom exceeds a predetermined
value while at the same time disabling the comparison between said
actual bending moment signal for the base section and the
corresponding reference signal which is proportional to the maximum
safe bending load for the base section.
17. A method comprising the steps of changing the length of a boom
for supporting an external load and having telescoping sections by
varying the telescopic relationship between sections of the boom,
sensing the length of the boom, changing the angular position of
the boom relative to a support surface by moving the boom
transversely to the support surface, sensing the angular position
of the boom relative to the support surface, sensing bending moment
induced in the boom under the combined effects of the external load
and weight of the boom, and generating a load signal which is a
function of the sensed bending moment in the boom, the sensed
length of the boom and the sensed angle of the boom relative to the
support surface.
18. A method comprising the steps of changing the length of a boom
having telescoping sections by varying the telescopic relationship
between sections of the boom with a resulting change in the bending
moment in the sections of the boom, changing the angular position
of the sections of the boom relative to a support surface by moving
the sections of the boom transversely to the support surface with a
resulting change in the bending moment in the sections of the boom,
generating a plurality of load signals each of which is associated
with a different one of the sections of the boom, and varying said
load signals in response to and as a function of changes in the
bending moment in the associated one of the boom sections resulting
from changes in the length of the boom and changes in the angular
position of the boom relative to the support surface.
19. A method as set forth in claim 18 further including the method
steps of generating a reference signal corresponding to a
predetermined maximum safe load, comparing said load and reference
signals, and generating an output signal when said load signal
exceeds a predetermined function of said reference signal.
20. A method as set forth in claim 18 further including the steps
of changing the magnitude of an external load supported by the boom
with a resulting change in the bending moment in each of the
sections of the boom, and varying each of said load signals in
response to changes in the bending moment in the associated one of
the boom sections resulting from changes in the magnitude of the
external load supported by the boom.
21. A method comprising the steps of changing the length of a boom
for supporting an external load and having telescoping sections by
varying the telescopic relationship between sections of the boom,
changing the angular position of the sections of the boom relative
to a support surface by moving the sections of the boom
transversely to the support surface, sensing bending moment induced
in a section of the boom under the combined influence of the
external load and weight of the boom, generating as a function of
the sensed bending moment a plurality of load signals each of which
is associated with a different one of the sections of the boom and
is representative of the bending moment induced in the associated
section of the boom by the external load, generating a plurality of
boom signals each of which is associated with a different one of
the sections of the boom and is representative of the bending
moment induced in the associated section of the boom by the weight
of the boom, and producing a plurality of combined signals each of
which is associated with a different one of the boom sections, each
of said combined signals being a function of the load and boom
signal associated with the same boom section as the combined signal
and being representative of the bending moment induced in the
associated boom section under the combined influence of the
external load and weight of the boom.
22. An assembly comprising a boom having a plurality of telescoping
sections, means for moving said telescoping sections of said boom
relative to each other to thereby vary the telescopic relationship
between said sections and the length of said boom, means for
varying the angular relationship of said boom to the support
surface by moving said boom toward and away from the support
surface, and circuit means for producing a signal representative of
the bending moment in said boom, said circuit means including means
for varying said signal in response to changes in length of said
boom and means for varying said signal in response to changes in
the angular relationship of said boom to the support surface.
23. An assembly as set forth in claim 22 further including means
operatively connected to said boom of engaging external loads of
different magnitudes, said circuit means including means for
varying said signal in response to changes in the magnitude of the
load engaged by said means for engaging external loads.
24. An assembly comprising a boom having a plurality of telescoping
sections, means for moving said telescoping sections of said boom
relative to each other to thereby vary the telescopic relationship
between said sections and the length of said boom, means for
varying the angular relationship of said sections of said boom
relative to a support surface by moving said boom toward and away
from the support surface, and circuit means for generating a
plurality of signals each of which is associated with a different
one of said sections of said boom and is representative of at least
a portion of the bending moment in the associated one of said boom
sections, said circuit means including means for varying at least
some of said signals in response to changes in the length of said
boom and means for varying at least some of said signals in
response to variations in the angular relationship of said sections
of said boom relative to the support surface.
25. An assembly as set forth in claim 24 further including means
operatively connected to said boom for engaging external loads of
different magnitudes, said circuit means including means for
varying at least some of said signals in response to changes in the
magnitude of the load engaged by said means for engaging external
loads.
26. An assembly as set forth in claim 25 wherein said circuit means
includes means for generating a plurality of reference signals each
of which is associated with one of said boom sections and is
representative of the magnitude of the maximum permissible bending
moment in the associated one of said boom sections, and means for
comparing each of said signals which is representative of the
bending moment in an associated one of said boom sections with the
reference signal which is associated with the same boom section and
for providing an output signal when a predetermined relationship
exists between the signal representative of the bending moment in
one of said boom sections and the reference signal associated with
the same boom section.
27. An assembly comprising a boom having a plurality of telescoping
sections for supporting an external load, means for moving said
telescoping sections of said boom relative to each other to thereby
vary the telescopic relationship between said sections and the
length of said boom, means for varying the angular relationship of
said boom to a support surface by moving said boom toward and away
from the support surface, a sensor means operatively connected with
said boom for sensing bending moment induced in a section of said
boom under the combined influence of the external load and weight
of said boom, first circuit means operatively connected to said
sensor means for generating a plurality of load signals each of
which is associated with a different one of said sections of said
boom and is representative of the bending moment induced in the
associated section of said boom by an external load, second circuit
means for generating a plurality of boom signals each of which is
associated with a different one of said sections of said boom and
is representative of the bending moment induced in the associated
section of said boom by the weight of said boom, and third circuit
means connected with said first and second circuit means for
generating a plurality of combined signals each of which is
associated with a different one of said boom sections and each of
which is representative of the bending moment induced in the
associated boom section under the influence of the external load
and the weight of the boom.
Description
In the past, various crane boom load indicating and warning systems
have been developed for indicating boom angle, boom radius, boom
length, boom load, etc. Representative of these systems are U.S.
Pat. No. 3,079,080, issued Feb. 26, 1963 to H. L. Mason and U.S.
Pat. No. 2,858,070, issued on Oct. 28, 1958 to Leon Scharff, both
of which relate to booms of the cable-supported type, as
distinguished from self-supporting booms, particularly telescopic
booms.
In the prior art systems for cable-supported booms, boom load
transducers have been located in a number of places. One place has
been in the boom support cable system as shown in U.S. Pat. No.
3,362,022, issued Jan. 2, 1968, to G. W. Mock et al. This
arrangement has the disadvantage that the measured force is not a
simple function of boom weight, load weight and boom angle, and
therefore does not readily lend itself to use as an input signal to
an analog computer.
Another place where load transducers have been located in systems
for cable-supported booms is at the outer end of the boom, coupled
to the lift line sheave. With this arrangement, the output signal
is proportional to the weight of the load, but the electrical cable
to the transducer is subject to mechanical abuse if mounted outside
the boom, and if applied to a telescoping boom it would present
playout and takeup problems.
Still another place where load transducers have been placed in
systems for cable-supported booms is in the lift cable, right at
the running block, or where the boom is single reeved, right at the
hook swivel. This placement has all the disadvantages of the one
last described, and in addition may subject the transducer to large
impact forces.
Another significant practical disadvantage of both of these
last-mentioned systems is that the readout indicates only the load
weight, and the operator must refer to load charts to determine
whether that load weight is safe for that particular boom length
and boom angle, which leads to the possibility of human error or
carelessness in the necessary correlation of this information.
The prior art systems have been primarily concerned with preventing
rigid, cable-supported boom cranes from tipping over. In
present-day cranes having self-supporting booms, particularly
telescoping booms, the structural strength of the boom, which
enables it to withstand the bending moment caused by the weight of
the load and the weight and length of the boom itself may be the
limiting factor, rather than tipping moment. The prior art systems
do not adequately satisfy the need for a system that will indicate
when a boom's structural limit is being approached, as well as when
a tipping condition is being approached.
In telescoping boom cranes, each boom section usually has a
different maximum allowable bending stress. Therefore, depending
upon the particular way in which a telescoping boom is extended,
the maximum allowable bending stress in one or the other of the
several boom sections may be the limiting factor on the permissible
load. Therefore, it is desirable to have a load indicating system
that is self-adjustable to indicate the maximum allowable load in
accordance with the way the boom sections are extended.
A principal object of this invention is to provide a novel and
improved boom load indicating system of improved accuracy for
self-supporting booms, particularly telescoping booms.
Another object of this invention is to provide a novel and improved
boom load indicating system for a self-supporting boom having a
strain transducer mounted on the boom at a location which insures
that the strain measured there is substantially linearly
proportional to the total bending moment on the boom, regardless of
the boom angle or, in the case of a telescoping boom, the boom
length or the particular manner in which the boom is extended.
Another object of this invention is to provide a novel and improved
boom load indicating system for a self-supporting telescoping boom
which responds to the bending moments in all of the individual
sections of the telescoping boom.
Another object of this invention is to provide a novel and improved
boom load indicating system for a self-supporting boom, especially
a telescoping boom, which includes transducers connected to control
the operation of analog computer circuitry whose output gives an
indication of the permissible boom load for the particular
conditions (e.g., boom angle and boom length) under which the boom
is then operating.
Another object of this invention is to provide a novel and improved
boom load indicating arrangement for a self-supporting boom in
which a boom strain transducer is mounted in such a manner as to
minimize the possibility of its being damaged.
Another object of this invention is to provide a novel and improved
indicating system for self-supporting booms which operates
automatically in accordance with either the permissible limit
imposed by the strength of the boom or the permissible tipping
moment due to slewing of the boom, depending upon which is the more
critical for the then-existing conditions of operation.
Further objects and advantages of the present invention will be
apparent from the following detailed description of certain
presently preferred embodiments thereof, which are illustrated
schematically in the accompanying drawings.
In the drawings:
FIG. 1 is a side elevational view of a vehicle-mounted,
hydraulically operated crane boom with which the present indicating
system is associated;
FIG. 2 is an elevational view of this vehicle-mounted crane boom,
viewed from the right end of FIG. 1 and with the boom support cab
and the boom turned 90.degree. from the FIG. 1 position and with
the boom lowered from the FIG. 1 position;
FIG. 3 illustrates schematically the boom in an extended position
and at a particular boom angle up from the horizontal;
FIG. 4 shows bending moment diagrams for the different boom
sections with and without a load suspended from the boom;
FIG. 5 is a schematic electrical circuit diagram of analog
circuitry for producing voltages proportional to the bending
moments on the individual boom sections due to their own weights
and extended lengths and for any boom angle, in the case of a boom
having all of its sections telescopable;
FIG. 6 is a similar diagram of analog circuitry for combining the
voltages produced by the FIG. 5 circuitry with the output signal of
the boom strain transducer to produce individual signals which are
proportional to the total bending moments on the respective
individual boom sections;
FIG. 7 is a schematic electrical circuit diagram showing analog
circuitry for the same general purpose as the FIG. 5 circuitry, but
for a boom in which not all of the boom sections are
telescopable;
FIG. 8 shows a portion of the circuitry used to provide the signals
proportional to the bending moments on the individual boom sections
due to the weight of the load, which signals are to be combined
with the output signals of FIG. 7 in the manner shown in FIG.
6;
FIG. 9 is a schematic electrical circuit diagram showing analog
circuitry for determining the bending moment on a boom having the
characteristic that the output signal of the boom strain transducer
is substantially the same for different boom lengths and boom
angles when the rated load for a certain length and boom angle is
applied;
FIG. 9A illustrates schematically a switching circuit for
selectively causing the FIG. 9 circuit to respond to the slewing
angle of the boom so as to indicate the maximum tipping moment,
instead of the maximum bending moment; and
FIG. 10 illustrates the preferred manner of mounting the strain
transducer on the underside of the base section of the boom in
accordance with the present invention.
Referring first to FIGS. 1 and 2, there are shown two views of a
vehicle-mounted hydraulic crane with telescoping boom 15 of known
construction. The boom has, in succession outwardly from the
supporting vehicle, a base section 16, an inner midsection 17, and
an outer midsection 18. If desired, the boom may be provided with a
fly section 19 and a jib 20, as shown in FIG. 3. The base section
16 of the boom is pivotally supported near its lower end from the
vehicle, as shown at P in FIG. 3.
The weight of the boom 15 plus the weight of any load hoisted by
the crane results in bending moments on the boom. These moments are
opposed by moments resulting from the weight of the vehicle 21 or
other carrier for the boom and the usual counterweight. If the sum
of the moments resulting from the weight of the boom and the load
exceed the opposing moments, the crane will tip over. A discussion
and analysis of tipping moment may be found in U.S. Pat. No.
3,079,080 to H. L. Mason.
Although tipping moment is a limitation that affects any mobile
crane, it is often less of a limitation than is the structural
strength of the boom. This is especially true with the
self-supported telescoping booms commonly used on hydraulic cranes.
These crane booms have no supporting cable structure and,
therefore, are limited in lifting ability by the magnitude of
bending moment they can withstand. This being the case, it is
desirable to accurately determine the bending moments in the boom
so that the crane operator may be provided with an indication when
the structural limits of the boom are being approached.
An important aspect of the present invention is directed to the
particular mounting of a strain transducer on the boom for sensing
the strain on the boom due to its own weight and the weight of the
load. I have discovered that ideally the boom strain transducer
should be mounted on the base section 16 of the boom near the pivot
22 where the boom-lift cylinder-and-piston units 23 operatively
engage this base section. There may be two such boom-lift
cylinder-and-piston units, one at each side of the base section of
the boom, or on small size cranes there may be one centrally
located boom-lift cylinder-and-piston. The boom strain transducer
is indicated generally by the reference character Tr in FIG. 1,
whereas this transducer is hidden in FIG. 2. The preferred
arrangement for attaching this transducer to the boom is described
in detail hereinafter with reference to FIG. 10.
I have discovered that there are substantially no stress
concentrations on the underside of the base section 16 of the boom
in the immediate vicinity of the pivot 22 and that the strain
measured in this region is substantially linearly proportional, at
all boom lengths and elevation angles, to the total bending moment
resulting from the weight of the boom and the weight of the
load.
In order to provide the crane operator with information in
meaningful terms, it is preferable to convert the output signal of
the boom strain transducer Tr to a reading in units with which the
operator is familiar, such as the percentage of rated load the boom
is lifting at each boom length and boom angle. The present system
includes electronic circuitry for performing certain analog
operations on signals that are functions of boom length, angle,
etc., in order to provide the crane operator with information which
is most readily informative to him.
FIGS. 5 and 6 show schematically a system for this purpose, in
accordance with the present invention, which performs these analog
operations in the most general case under consideration. The system
of FIGS. 5 and 6 is adapted for a telescoping boom in which each
boom section is telescopable, having a variable length, 1.sub.n ,
measured from the end of the next lower section, where n is a
number subscript denoting the particular boom section. With respect
to the base section 16 of the boom, 1.sub.1 is measured from the
pivot 22, as shown in FIG. 3. With respect to the inner midsection
17 of the boom, 1.sub.2 is measured from the outer end of the base
section 16, and so on. Each boom section 16, 17, 18, 19 and 20 has
weight, W.sub.n , which may be thought of as acting at the boom
section's center of gravity. The center of gravity of each boom
section 16, 17, 18, 19 and 20 is located a distance L.sub.n from
its outer end.
Each of the telescoping boom sections outwardly from the base
section 16 has a respective upwardly facing shoe U on the top at
its inner end. Each of the boom sections which telescopically
receives another boom section has a respective upwardly facing shoe
O on the bottom at its outer end. These shoes provide the support
surfaces between successive boom sections, and from FIG. 3 it can
be seen that the maximum bending moment in each telescoping boom
section will occur directly above the shoe O at the outer end of
the next boom section which receives it. In the base section 16,
the maximum bending moment will occur at the pivot 22. In the
general solution, it is also assumed that the boom is at and angle
.theta. above the horizontal and is supporting a load having weight
W.sub.z . Therefore, the vector components of the weights of the
load and boom sections normal to the boom are W.sub.z cos .theta.
and W.sub.n cos .theta., where the subscript n again refers to the
particular boom section under consideration.
Thus, the maximum moment in the jib 20 of boom 15 occurring at the
point 24 of its attachment to the fly section 19 is:
M.sub.5 = 1.sub.5 W.sub.z cos.theta. + (1.sub.5 - L.sub.5 ) W.sub.5
cos.theta. (1 )
It can be shown by superposition that the moment at any point in a
system is equal to the sum of all the individual moments acting
about that point. This is shown graphically in FIG. 4. Therefore,
the moments caused by the weights of the load and boom sections may
each be calculated separately. Letting the subscripts B and z refer
to the boom and load respectively, the equation (1 ) may be
rewritten as:
M.sub.5 = M.sub.5z + M.sub.5B (2 )
where:
M.sub.5z = 1.sub.5 W.sub.z cos.theta. (3 )
and
M.sub.5B = 1.sub.5 W.sub.5 cos.theta. - L.sub.5 W.sub.5 cos.theta.
(4 )
Considering now only the moments due to the weight of the boom
sections, the maximum moment in the fly section 19 due to its own
weight and the weight of the jib 20 will be:
M.sub.4B = (1.sub.4 + 1.sub.5 - L.sub.5 ) W.sub.5 cos.theta.
+ (1.sub.4 - L.sub.4 ) W.sub.4 cos.theta.
= 1.sub.4 W.sub.5 cos.theta. + (1.sub.5 - L.sub.5 ) W.sub.5
cos.theta.
+ l.sub.4 W.sub.4 cos.theta. - L.sub.4 W.sub.4 cos.theta.
= (1.sub.5 - L.sub.5 ) W.sub.5 cos.theta. + 1.sub.4 (W.sub.4 +
W.sub.5 ) cos.theta.
- L.sub.4 W.sub.4 cos.theta. (5 )
Since, from equation (4 ), the first term of the right side of
equation (5 ) is equal to M.sub.5B , it may be written:
M.sub.4B = M.sub.5B + 1.sub.4 (W.sub.4 + W.sub.5 ) cos.theta. -
L.sub.4 W.sub.4 cos.theta. (6 )
It may also be shown that:
M.sub.3B = M.sub.4B + 1.sub.3 (W.sub.3 + W.sub.4 + W.sub.5 )
cos.theta. - L.sub.3 W.sub.3 cos.theta. (7 )
M.sub.2B = M.sub.3B + 1.sub.2 (W.sub.2 + W.sub.3 + W.sub.4 +
W.sub.5 ) cos .theta.-L.sub.2 W.sub.2 cos.theta. (8 )
M.sub.1B = M.sub.2B + 1.sub.1 (W.sub.1 + W.sub.2 + W.sub.3 +
W.sub.4 + W.sub.5 ) cos.theta. - L.sub.1 W.sub.1 cos.theta. (9
)
The moments in each boom section due to the weight of the load may
be written as follows:
M.sub.5z = 1.sub.5 W.sub.z cos.theta. (10 )
M.sub.4z = (1.sub.4 + 1.sub.5 ) W.sub.z cos.theta. (11 )
M.sub.3z = (1.sub.3 + 1.sub.4 + 1.sub.5 ) W.sub.z cos.theta. (12
)
M.sub.2z = (1.sub.2 + 1.sub.3 + 1.sub.4 + 1.sub.5 )W.sub.z
cos.theta. (13 )
M.sub.1z = (1.sub.1 + 1.sub.2 + 1.sub.3 + 1.sub.4 + 1.sub.5 )
W.sub.z cos.theta. (14 )
The strain transducer Tr mounted on the bottom of the base section
16 of the boom as close as possible to the pivot 22 generates an
electrical signal that is a function of the strain to which it is
subjected. The strain in the mounting area is proportional to the
bending moment at this point on the boom. Letting the output of
transducer Tr be represented by T, we can write T = KM.sub.1 ,
where M.sub.1 is the moment at pivot 22 and K is a constant of
proportionality.
FIG. 5 is a schematic diagram of an electrical analog circuit that
will provide electrical signals representative of the moments in
the several boom sections due only to the weights of the boom
sections themselves.
A regulated power supply (not shown) provides a power supply
voltage of +E.sub.a for an amplifier 25 and also provides a power
supply voltage of -E.sub.a for one end of a cosine potentiometer
26. The opposite end of potentiometer 26 is at a fixed reference
potential E.sub.0 , such as by being grounded to the crane chassis.
The input shaft of the cosine potentiometer 26 is coupled to the
boom, as indicated schematically by the dashed line 27 in FIG. 5,
so that it is rotated in accordance with the boom elevation angle
.theta. and the schematically illustrated adjustable potentiometer
tap 28 is positioned in accordance with the cosine of .theta..
Therefore, the output E.sub.1 of potentiometer 26, 28 is equal to
-E.sub.a cos.theta. .
This signal is amplified by the amplifier 25, having a gain of
K.sub.1 , which may be unity or higher to provide subsequent
signals in a desired voltage range.
The output voltage Khd 1 E.sub.1 from amplifier 25 appears at point
29 in FIG. 5, and it is applied to a plurality of voltage dividers
equal in number to the number of boom sections whose individual
moments are to be determined. Each voltage divider is shown as
consisting of a variable resistor and a potentiometer resistance
connected in series between point 29, which is at the potential Khd
1 E.sub.1 , and point 30, which is at the reference potential
E.sub.0 . The voltage divider for the fifth boom section 20 (the
jib) includes variable resistor R.sub.50 and potentiometer
resistance R.sub.51 ; the voltage divider for the fourth boom
section 19 (the fly section) includes variable resistor R.sub.40
and potentiometer resistance R.sub.41 ; the voltage divider for the
third boom section 18 (the outer midsection) includes variable
resistor R.sub.30 and potentiometer resistance R.sub.31 ; the
voltage divider for the second boom section 17 (the inner
midsection) includes variable resistor R.sub.20 and potentiometer
resistance R.sub.21 ; and the voltage divider for the first boom
section 16 (the base section) includes variable resistor R.sub.10
and potentiometer resistance R.sub.11 . Each of the variable
resistors R.sub.50 , R.sub.40 , R.sub.30 , R.sub.20 and R.sub.10 is
manually adjusted so that the voltage at its juncture with the
respective potentiometer resistance R.sub.51 , R.sub.41 , R.sub.31
, R.sub.21 or R.sub.11 in the same voltage divider is proportional
to the sum of the normal forces due to the corresponding boom
section and all boom sections supported thereby (i.e., all
additional boom sections outward of this boom section). That
is:
E.sub.50 (W.sub.5 ) cos.theta. (15 )
E.sub.40 (W.sub.4 + W.sub.5 ) cos.theta. (16 )
E.sub.30 (W.sub.3 + W.sub.4 + W.sub.5 ) cos.theta. (17 )
E.sub.20 (W.sub.2 + W.sub.3 + W.sub.4 + W.sub.5 ) cos.theta. (18
)
E.sub.10 (W.sub.1 + W.sub.2 + W.sub.3 + W.sub.4 + W.sub.5 )
cos.theta. (19 )
It will be evident that since the input voltage to each voltage
divider (at point 29) already includes the cos.theta. factor,
resistor R.sub.50 is adjusted manually to have an ohmic value
inversely proportional to W.sub.5 , which is the known weight of
the fifth boom section 20. Similarly, resistor R.sub.40 is adjusted
manually to have a resistance value inversely proportional to
(W.sub.4 + W.sub.5 ), which is the sum of the known weight of the
fourth and fifth boom sections 19 and 20. The same technique is
used for determining the adjusted values of resistors R.sub.30 ,
R.sub.20 and R.sub.10 .
The adjustable contact of each potentiometer R.sub.51 , R.sub.41 ,
R.sub.31 , R.sub.21 and R.sub.11 is connected mechanically in any
suitable manner to its respective boom section so that the position
of the adjustable contact is proportional to the extended length,
1.sub.n , of its respective boom section. These mechanical
connections are indicated schematically by the dashed lines leading
from 1.sub.5 to the adjustable contact of potentiometer R.sub.51 ,
from 1.sub.4 to the adjustable contact of potentiometer R.sub.41 ,
and so on. Therefore, the voltages at the adjustable contacts of
the respective potentiometers may be expressed as follows:
E.sub.51 1.sub.5 (W.sub.5 ) cos.theta. (20 )
E.sub.41 1.sub.4 (W.sub.4 + W.sub.5 ) cos.theta. (21 )
E.sub.31 1.sub.3 (W.sub.3 + W.sub.4 + W.sub.5 ) cos.theta. (22
)
E.sub.21 1.sub.2 (W.sub.2 + W.sub.3 + W.sub.4 + W.sub.5 )
cos.theta. (23 )
E.sub.11 1.sub.1 (W.sub.1 + W.sub.2 + W.sub.3 + W.sub.4 + W.sub.5 )
cos.theta. (24 )
Five additional potentiometers R.sub.52 , R.sub.42 , R.sub.32 ,
R.sub.22 and R.sub.12 are also connected in parallel with each
other across the aforementioned voltage dividers R.sub.50 +
R.sub.51 , R.sub.40 + R.sub.41, etc. Each of these potentiometers
is set manually to provide a voltage at its adjustable contact
which is proportional to (L.sub.n W.sub.n cos.theta. ). Thus:
E.sub.52 L.sub.5 W.sub.5 cos.theta. (25 )
E.sub.42 L.sub.4 W.sub.4 cos.theta. (26 )
E.sub.32 L.sub.3 W.sub.3 cos.theta. (27 )
E.sub.22 L.sub.2 W.sub.2 cos.theta. (28 )
E.sub.12 L.sub.1 W.sub.1 cos.theta. (29 )
As shown in FIG. 5, the voltage E.sub.52 at the adjustable contact
on potentiometer R.sub.52 and the voltage E.sub.51 at the
adjustable contact on potentiometer R.sub.51 are applied to a
summing amplifier 32 which subtracts them to provide an output
voltage E.sub.53 , as follows:
E.sub.53 = e.sub.51 = e.sub.52
30 1.sub.5 w.sub.5 cos.theta. - L.sub.5 W.sub.5 cos.theta. (30
)
This, it will be recognized, is the expression developed previously
for M.sub.5B .
From FIG. 5, it will be apparent that the summing amplifier 33 is
connected to receive three input voltages, E.sub.53 , E.sub.42 and
E.sub.41 and to produce an output voltage E.sub.43 , as
follows:
E.sub.43 = e.sub.53 + e.sub.41 - e.sub.42 (31 ) which will be
recognized as the expression developed for M.sub.4B .
Similarly from an inspection of FIG. 5, it will be evident that
E.sub.53 = E.sub.43 + E.sub.31 - E.sub.32 = M.sub.3B (32 )
e.sub.23 = e.sub.33 + e.sub.21 - e.sub.22 = m.sub.2b (33 )
e.sub.13 = e.sub.23 + e.sub.11 - e.sub.12 = m.sub.1b (34 )
these signals, E.sub.53, E.sub.43 , E.sub.33 , E.sub.23 and
E.sub.13 , are applied as input signals to the correspondingly
designated input terminals in FIG. 6, now to be described. In
addition to these just-mentioned input signals, which are
respectively proportional to the moments of force on the individual
boom sections due to the weight of the boom itself, the FIG. 6
circuit also produces signals which are proportional respectively
to the bending moments on the individual boom sections resulting
from the weight of the load W.sub.z . The output signal T of the
boom strain transducer which is proportional to M.sub.1 = M.sub.1z
+ M.sub.1B , is amplified by a linear amplifier 34, the output of
which is labeled E.sub.14 .
The voltage E.sub.13 , proportional to M.sub.1B , is subtracted
from E.sub.14 in summing amplifier A'.sub.1 to provide as its
output voltage E.sub.15 . As already indicated, the voltage
E.sub.13 is received from the correspondingly labeled output
terminal of FIG. 5.
Referring to the moment diagrams of FIG. 4, the lowermost set of
straight-line curves shows the bending moments due only to the
weights of the individual boom sections for a given boom angle with
respect to the horizontal. The upper set of curves shows the
bending moments due to the weight of the load and the weights of
the individual boom sections for this same boom angle.
Referring to the lower set of curves, the bending moment due solely
to the weight of the outermost boom section, the jib 20, is
designated by the line 5B. The weight of this boom section may be
considered as concentrated at its center of gravity, designated by
the point C.G.5 along the abscissa. The support point for this boom
section is designated by the point S.sub.5 along the abscissa. The
respective centers of gravity and support points for the fourth,
third, second and first boom sections are similarly designated,
except that the support point for the base section 16 is designated
by reference numeral 22 to correspond to FIG. 3.
It will be apparent that the bending moment due to the weight of
the fifth section of the boom alone is a straight-line function,
increasing from zero at C.G.5 to a maximum at the support point 22
for the base section of the boom. The slope of line 5B is
proportional to the weight of the fifth boom section.
Line 4B in FIG. 4 shows the bending moment due solely to the weight
of the fourth boom section 19 plus the weight of the fifth boom
section. Line 4B is a straight-line function, intersecting the
bending moment line 5B for the fifth section at C.G.4 (the center
of gravity of the fourth section) and then sloping upwardly from
line 5B to a maximum at the support point 22 for the base section
of the boom. The difference between the slopes of lines 4B and 5B
is proportional to the weight of the fourth boom section 19
alone.
Similarly, line 3B in the lower set of curves in FIG. 4 shows the
bending moment due solely to the weights of the third, fourth and
fifth boom sections 18, 19 and 20. Line 3B follows a straight-line
function, having a minimum value at its intersection with line 4B
at C.G.3, the center of gravity of the third boom section 18, and
increasing linearly to a maximum value at the support point 22 for
the base section of the boom. The difference between the slopes of
lines 3B and 4B is proportional to the weight of the third boom
section 18 alone.
A similar analysis holds true for the bending moments 2B and 1B for
the second and first boom sections 17 and 16.
When a weight is suspended from the outer end of the boom, at a
point outward beyond the center of gravity C.G.5 for the outermost
boom section, the entire set of curves is displaced to a position
as shown by the upper set of curves in FIG. 5. The inclined base
line z of this upper set of curves shows the bending moment at
various points along the boom due solely to the weight of the
load.
Referring again to FIG. 6, in order to derive signals proportional
to the bending moments in each boom section due to the load, the
output E.sub.15 of summing device A'.sub.1 , which is proportional
to M.sub.1z , is fed to a voltage divider having five variable
resistors R'.sub.21 , R'.sub.22 , R'.sub.23 , R'.sub.24 and
R'.sub.25 . The generic expression for these resistors will be
called R'.sub.2n, where n corresponds to the final digit in the
subscript of the individual resistor. Each resistor R.sub.2n in
this voltage divider is mechanically coupled in any suitable manner
to the corresponding boom section so that it has a resistance
proportional to the instantaneous length 1.sub.n of that boom
section. This mechanical coupling is indicated schematically by
dashed lines associated with these resistors in FIG. 6. If the
constant of proportionality between the length 1.sub.n of each boom
section n and its associated variable resistor R.sub.2n is the same
for all of these resistors, then the voltage E.sub.n5 at the lower
end of each resistor R.sub.2n will be proportional to the bending
moment in that boom section due to the load.
Thus, E.sub.15 is proportional to M.sub.1z , and E.sub.25 ,
E.sub.35 , E.sub.45 and E.sub.55 are proportional to M.sub.22,
M.sub.3z , M.sub.4z , and M.sub.5z , respectively.
The FIG. 6 circuitry includes summing amplifiers A'.sub.2 ,
A'.sub.3 ', A'.sub.4 , and A'.sub.5 in which the voltages E.sub.25
, E.sub.35 , E.sub.45 and E.sub.55 are respectively summed with
voltages E.sub.23 , E.sub.33 , E.sub.43 and E.sub.53 , which are
proportional to M.sub.2B , M.sub.3B , M.sub.4B and M.sub.5B and are
produced in the circuit of FIG. 5, as already described in detail.
These summing operations produce voltages E.sub.24 , E.sub.34 ,
E.sub.44 and E.sub.54 at the respective outputs of amplifiers
A'.sub.2 , A'.sub.3 A'.sub.4 and A'.sub.5 which are proportional to
M.sub.2 , M.sub.3 , M.sub.4 and M.sub.5 , respectively. These
signals, along with the E.sub.14 output of amplifier 34, which is
proportional to M.sub.1 , represent the maximum bending moments in
the respective sections of the boom.
Each of these signals may be continuously or selectively monitored
or scanned, or compared to another signal representing the maximum
safe allowable bending moment in each boom section. For example, a
voltage comparator S.sub.2 may be arranged to compare the E.sub.24
signal with an E.sub.26 reference signal which represents the
maximum safe allowable moment in the second section of the boom.
Similar signal comparators S.sub.3, S.sub.4 and S.sub.5 are
provided for comparing the actual bending moment signal, E.sub.34,
E.sub.44 or E.sub.54, with a reference signal E.sub.36, E.sub.46 or
E.sub.56 representing the maximum allowable bending moment for the
same boom section. Each signal comparator may energize a respective
warning lamp Q.sub.2 , Q.sub.3 , Q.sub.4 or Q.sub.5 when the actual
bending moment signal for that boom section approaches the maximum
permissible value, such as by exceeding 85 percent of that value.
Also, each signal comparator may energize another indicator lamp
P.sub.2 , P.sub.3 , P.sub.4 or P.sub.5 when the actual bending
moment signal for that boom section reaches 100 percent of the
maximum permissible value, and when any of lamps P.sub.2, P.sub.3 ,
P.sub.4 or P.sub.5 is energized an audible alarm device X is also
energized.
The E.sub.14 output from amplifier 34 in FIG. 6, which is
proportional to the bending moment on the base section 16 of the
boom, is applied as one input signal to a voltage comparator
S.sub.1 where it is compared with a reference signal E.sub.16 .
When the boom extends substantially longitudinally of the vehicle
(i.e., not appreciably to one side or the other of the vehicle),
this E.sub.16 reference signal is obtained from the adjustable
contact of a potentiometer 35, whose manually adjusted setting
produces an E.sub.16 voltage that is proportional to the maximum
allowable bending moment on the base section of the boom.
However, when the boom is at a significant slewing angle (to one
side or the other of the vehicle) then the tipping moment may be
more critical than the bending moment of the boom. That is, the
crane may tend to tip over at a boom loading much lower than would
be critical to the structural strength of the boom itself under a
bending moment. In that case, the E.sub.16 signal will be obtained
for a second potentiometer 36 whose adjustable contact is
positioned in accordance with the sine of the slewing angle by
being mechanically coupled in any suitable manner to the boom
slewing equipment. Desirably, potentiometer 36 may be a sine
potentiometer.
The connections of the two alternatively used potentiometers 35 and
36 to the comparator S.sub.1 are such that the potentiometer whose
adjustable contact is at the smaller voltage will be connected to
provide the second input signal to comparator S.sub.1 , for
comparison therein against the E.sub.14 input signal. When the
slewing angle is zero or small, then the potentiometer 35 will
provide this second input signal to comparator S.sub.1 . When the
slewing angle is large, then potentiometer 36 will provide this
second input signal to comparator S.sub.1 .
Comparator S.sub.1 is arranged to turn on an indicator lamp Q.sub.1
when the bending moment signal E.sub.14 approaches the E.sub.16
signal, such as when it exceeds 85 percent of E.sub.16 . Also,
comparator S.sub.1 is arranged to turn on another indicator lamp
P.sub.1 and also to energize the audible alarm device X when the
E.sub.14 signal reaches 100 percent of the E.sub.16 signal.
The analog circuit of FIGS. 5 and 6 is provided with a test
arrangement which enables the operator to test it. The operator
will position the crane boom at a predetermined length, boom angle
and load and depress test switch C, which provides a voltage which
when added to the strain transducer output signal T in the FIG. 6
circuit, should produce a response of lamp P.sub.1 and the audible
alarm device X.
The analog circuitry of FIGS. 5 and 6 is a general solution
designed for use in conjunction with a boom having all of its boom
sections telescopically arranged. Simplification of this circuitry
is possible where the crane boom is as shown in FIG. 3, having a
base section 16 of fixed length and having an outermost section or
jib 20 which is not telescopable at all, but is arranged to be
manually attached to or removed from the fly section 19. Therefore,
for purposes of providing an electrical analog, the jib 20 is
either present in its full length or it is absent. Likewise, the
fly section 19, although it is telescopable, may not be powered,
and therefore it will be either in its fully retracted or its fully
extended position, but not inbetween. When retracted, its length
1.sub.4 is at a minimum but its weight W.sub.4 must still be
accounted for in calculating the bending moments due to the weights
of the boom sections.
Therefore, an electrical analog for such a boom may be provided as
shown in FIG. 7, which is simpler than that of FIGS. 5 and 6 in
that variable resistors and potentiometers are replaced by fixed
voltage sources. For example, as shown in FIG. 7, because the jib
section of the boom is not telescopable, R.sub.50 and R.sub.51 are
eliminated and R.sub.52 is set so that it provides a voltage
proportional to (1.sub.5 - L.sub.5 )W.sub.5 cos.theta. . A manually
operated switch SW.sub.1 may then be used to connect the E.sub.53
output of amplifier 32 to the positive input terminal of amplifier
33 when the jib 20 is attached to the fly section 19 and to
disconnect the E.sub.53 output of amplifier 32 from this input
terminal of amplifier 33 when the jib is detached.
In the voltage divider for the fourth boom section R.sub.41 is
replaced by fixed resistances or a pair of potentiometers R'.sub.41
and R".sub.41, as shown in FIG. 7, that may be manually set to
provide voltages proportional to the extended and contracted
positions, respectively, of the fly 19. A switch SW.sub.2 may be
provided to select between those two conditions.
Also, R.sub.10 and R.sub.11 are eliminated because the base section
16 of the boom is of fixed length, and R.sub.12 is set to provide a
voltage proportional to
1.sub.1 (W.sub.1 + W.sub.2 + W.sub.3 + W.sub.4 + W.sub.5 )
cos.theta. - L.sub.1 W.sub.1 cos.theta.
Except for these simplifications the FIG. 7 circuit is essentially
the same as that of FIG. 5 and therefore it is not necessary to
repeat the detailed description of its operation.
FIG. 8 shows modified circuitry for use in place of the input side
of the FIG. 6 circuit and in conjunction with the FIG. 7
circuit.
In FIG. 8, R'.sub.21 is fixed, instead of adjustable, because the
base section 16 of the boom has a fixed length.
Resistor R.sub.24 at the input side of FIG. 6 is replaced by a pair
of series-connected fixed resistors R'.sub.24 and R".sub.24 in FIG.
8, and a switch SW.sub.4 is connected across resistor R".sub.24 .
If fly section 19 of the boom is fully extended, then switch
SW.sub.4 is opened. If the fly section is fully retracted, then
switch SW.sub.4 is closed.
In FIG. 8, R'.sub.25 is fixed because the jib 20, if present on the
boom, is not adjustable in length, and a switch SW.sub.3 is
connected across R'.sub.25 . If the jib 20 is present on the outer
end of the boom, then switch SW.sub.3 is kept open. However, if the
jib is not present, then switch SW.sub.3 is closed to shunt
R'.sub.25 .
In FIG. 8, the variable resistors R'.sub.22 and R'.sub.23 , which
are proportional to the lengths of the respective boom sections,
may be provided by multiturn potentiometers driven by tag lines
affixed to the respective boom sections 17 and 18. However, if the
crane boom of FIGS. 1 and 2 is operated so that the inner and outer
midsections 17 and 18 are always extended and retracted equally,
then a fixed relationship is established between 1.sub.2 and
1.sub.3 , and therefore between R'.sub.22 and R'.sub.23 , so that
only one tag line is required. That is, if 1.sub.2 = 1.sub.3 , then
a single tag line affixed to the inner midsection 17 may adjust
both R'.sub.22 and R'.sub.23 equally.
It will be apparent, in view of the foregoing description that the
present analog circuitry may be modified to accommodate any number
of telescoping or removable boom sections. Such circuitry is
necessary to yield the desired information for the crane operator
where failure may occur in different boom sections subjected to
widely differing bending moments.
In one particular hydraulic crane, it was found that the output of
the strain transducer, Tr , was approximately the same value for
all boom lengths and boom angles when the rated load for the
particular length and angle was applied to the hook. So long as
this relationship holds true, a greatly simplified electrical
analog may be constructed, as illustrated in FIG. 9.
The strain transducer Tr affixed to the boom as described with
reference to FIG. 1 produces an output signal proportional to the
total bending moment in the boom. This signal is amplified by an
amplifier 40, which may be a solid state integrated circuit device.
A zero adjust signal may also be introduced via line 40a into
amplifier 40, which acts as a summing amplifier. In this manner,
the signal from transducer Tr caused by its preload may be
cancelled out, thereby providing a signal T' at the output of
amplifier 40 that is zero when the bending moment at the pivot 22
is zero, even though the transducer Tr is preloaded to some initial
strain.
The output T' of amplifier 40 will be a signal proportional to the
total moment in the boom. In order to translate this signal into
information meaningful to the operator, it is desirable to subtract
the effects of boom weight. The moment due to the load may be
represented by:
M.sub.z = M.sub.1 - M.sub.B
where:
M.sub.1 = total moment at the transducer
M.sub.B = moment due to weight of boom
M.sub.z = moment due to weight of load
The moment due to the weight of the boom may be represented by:
M.sub.B = W.sub.B L cos.theta.
where:
W.sub.B is the weight of the boom;
L is the distance from the pivot point 22 to the center of gravity
of the boom; and
.theta. is the vertical angle the boom makes with the
horizontal.
The circuit for generating the above relationship incorporates a
cosine function potentiometer 41, the input shaft of which is
rotated in accordance with the boom angle .theta., as indicated by
the dashed line 42 in FIG. 9. This may be done, for example, by a
damped pendulum, as described in U.S. Pat. No. 3,362,022, issued
Jan. 2, 1968, to G.W. Mock et al. The output of cosine
potentiometer 41 is equal to its input voltage multiplied by
cos.theta. . Because the weight of the boom is constant, the input
voltage to cosine potentiometer 41 may be made proportional to boom
weight W.sub.B so that the output of cosine potentiometer 41 is
proportional to W.sub.B cos.theta. . This output may be multiplied
by an amplifier 43.
The amplified signal from cosine potentiometer 41 provides the
input to a boom length potentiometer 44, the shaft of which is
rotated in accordance with boom length, as indicated by the dashed
line 45, such that it is representative of the distance from pivot
22 to the center of gravity of the boom. A spring loaded reel
having a tag line affixed to one of the telescoping boom sections
may be used to rotate the shaft of potentiometer 44 to provide this
relationship. Potentiometer 44 may be linear or it may be
constructed to more closely approximate the movement of the center
of gravity of the boom with boom extension.
The output of potentiometer 44 is proportional to M.sub.B = W.sub.B
L cos.theta. , but because a potentiometer has a gain equal to or
less than unity, preferably it is amplified by an amplifier 46 to
provide a signal Y of the proper magnitude. In practice, the gains
of amplifiers 43 and 46 may be adjusted so that the output Y of
amplifier 46 is of the proper magnitude.
Boom length will, of course, never be zero on a working crane.
Therefore, so that the resistance proportional to boom length does
not go to zero, a fixed resistor may be inserted in series with
potentiometer 44. Alternatively, the same offset effect may be
obtained by use of a separate signal L.sub.o added into amplifier
46, as shown in FIG. 9.
The output Y of amplifier 46, which is proportional to W.sub.B L
cos.theta. = M.sub.B , is used along with the output T' from
amplifier 40 as the inputs to summing amplifier 47 to produce a
signal T-Y which is proportional to M.sub.1 - M.sub.B = M.sub.z,
the moment due only to load.
This signal, T-Y, at the output of amplifier 47 may be operated
upon by a network 48 having a 1 /cos transfer function generator to
provide an indication on an indicating device 49 of load weight
W.sub.z .
However, networks with 1 /cos transfer functions are expensive, and
even if the crane operator knew the weight of the load, he would
still have to refer to load charts to determine whether he could
safely move the boom to a more extended length.
As noted above, in the particular crane boom that was studied, the
strain measured by transducer Tr was approximately the same for
each boom elevation and length at the respective rated loads. This
being the case, a maximum safe strain signal C may be used for all
boom elevations and lengths. In the analog circuit of FIG. 9, C may
be generated by a voltage divider 50 connected across a reference
voltage source. Because C is proportional to the maximum safe
strain, and therefore the maximum safe bending moment about pivot
22, a signal C-Y will be proportional to the maximum safe bending
moment less the actual bending moment due to the weight of the
boom, and this C-Y signal is proportional to the maximum bending
moment that may be placed on the boom due to the load. The C-Y
signal is provided by applying the C and Y signals to the opposite
input terminals of a summing amplifier 51, as shown in FIG. 9,
which subtracts them to provide the C-Y signal on its output line
52.
Since T-Y is proportional to the actual bending moment due to the
load, and C-Y is proportional to the maximum safe bending moment
which may be due to the load, the fraction T-Y/C-Y, expressed as a
percentage, is an indication of how much of the boom's capacity is
being utilized. A signal proportional to T-Y/C-Y may be generated
in a divider network of known design, represented schematically by
the box 53 in FIG. 9. The outputs of amplifiers 47 and 51 are
connected to the two input terminals of network 53 to produce an
output signal proportional to T-Y/C-Y, which output signal may be
applied to any desired indicating device 54, such as a galvanometer
type meter.
There may also be provided an indication as to when a certain
percentage, less than 100 percent, of the maximum safe bending
moment has been reached. This may be in the form of a triggering
circuit 55 that lights a lamp 56 when the signal proportional to
T-Y/C-Y reaches a predetermined percentage. A second triggering
circuit 57 may be adapted to light a second lamp 58 and sound an
audible alarm device 59 when 100 percent of the maximum safe
bending moment has been reached or exceeded.
In the FIG. 9 circuit, the value of the signal C obtained from
potentiometer 50 may be adjusted in accordance with the slewing
angle .psi. of the boom, as indicated schematically by the dashed
line 60. This may be done by changing the position of the
adjustable contact of potentiometer 50 in accordance with an
appropriate function of the slewing angle, .psi., such as the sine
of the slewing angle.
However, in accordance with the presently preferred embodiment of
the circuit depicted in FIG. 9, the value C is either one of two
values depending upon whether the slewing angle is small or large.
For example as shown in FIG. 9A, two alternatively used
potentiometers 50' and 50" may be provided and a selector switch 61
is connected between the adjustable contact on one or the other of
these potentiometers and the input side of the summing amplifier
51. This switch 61 is suitably coupled to the boom to sense the
slewing angle such that when a predetermined slew angle is exceeded
the switch 61 abruptly disconnects potentiometer 50' and connects
the other potentiometer 50" to the amplifier input. Thus, when the
boom extends over the end of the carrier vehicle the signal C is
established by potentiometer 50', but when the boom extends over
the side of the carrier vehicle the signal C is established by
potentiometer 50".
As already pointed out, the mounting of the strain transducer Tr on
the boom, particularly its attachment to the base section 16 near
the pivotal connection of the boom lift cylinder-and-piston units
to the boom, is an important feature of the present invention.
FIG. 10 shows the preferred structural arrangement for mounting the
strain transducer Tr, which is utilized in the electrical analog
circuits of the present invention as already described in detail.
Affixed to the bottom plate 62 of the base section 16 of the crane
boom 15, preferably by welding, are a pair of support blocks 63 and
64, which are spaced apart lengthwise of the boom. A cylindrical
hole 65 in block 63 is coaxial with a smaller cylindrical hole 66
in block 64, their common axis being parallel to the longitudinal
axis of crane boom 15.
A transducer housing bolt 67 is slidably received in the block
holes 65 and 66. The housing bolt 67 has a head 68, a proximal
shank portion 69 adjacent the head that is slightly smaller than
the block hole 65 so that it may easily fit therein, a mid shank
portion 70 that is slightly smaller in diameter than hole 66, and a
distal shank portion 71 approximately the same diameter as the mid
shank portion 70. The proximal shank portion 69 is screw-threaded
from the mid shank portion 70 approximately halfway to the head 68,
and is adapted to threadedly receive a first nut 72 at the inner
side of block 63. The distal portion 71 is screw-threaded to
receive nuts 73 and 74 at the inner and outer sides of block 64.
Suitable lock washers 72a, 73a and 74a are engaged between the
correspondingly numbered nuts and the adjacent sides of the
respective blocks.
A small bore 75 extends through the head 68 and proximal shank
portion 69 of the bolt into the mid shank portion 70. The boom
strain transducer Tr is cemented or otherwise fixedly secured in
this bore 75 at its inner end in such a manner that any strain on
the bolt is imparted to the transducer Tr . The cross section of
the mid shank portion 70 of the bolt in the vicinity of the
transducer Tr is made precisely uniform to insure against erroneous
signals that could be caused by nonuniform displacement under
stress. The bolt head 68 may be provided with an electrical
connector 76 adapted to seal the strain transducer within the bore
75 and to provide a convenient connection to an external cable for
connecting the lead-in wires of the transducer into the electrical
analog circuitry, as already described.
In use, the distal portion 71 of bolt 67 is inserted through the
block hole 65 and nut 72 slipped over this distal portion. Then nut
73 is screwed onto the distal portion 71 and all the way over to
the midportion 70 of the bolt. The distal portion is then inserted
through the block hole 66. Next, nut 72 is screwed onto the
proximal portion 69 of the bolt and is turned down tightly, locking
the bolt 67 to block 63. Then nut 73 is turned to the right in FIG.
10 along the distal portion 71 of the bolt until it abuts tightly
against block 64 to place the bolt shank between nuts 72 and 73 in
compression between blocks 63 and 64. When the desired
compressional preload has been achieved, nut 74 is screwed onto the
projecting end of distal portion 71 and is tightened against the
outer side of block 64.
The compressional preloading of bolt 67 is imparted to the strain
transducer Tr so that the latter is preloaded under compression
also. This compressional preload achieves two desirable results.
First, it prevents the output signal from the strain transducer Tr
from ever going negative under any normally encountered boom
conditions. This simplifies the electronic circuitry. Secondly, it
avoids some small errors that can occur in the zero strain area due
to internal stress in the bolt, blocks and boom.
While certain presently preferred embodiments of the present
invention have been described in detail with reference to the
accompanying drawings, it is to be understood that various
modifications, omissions, refinements and adaptions which depart
from the disclosed embodiments may be adopted without departing
from the scope of the present invention. For example, any of the
analog circuits disclosed herein may be modified by the use of
different circuit components to perform the analog functions
described.
* * * * *