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Патент USA US3100551

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Aug. 13, 1963
Filed July 17, 1961
27.74%, M w
Patented Aug. 13, 1963
3,109,546 '
Johannes F. Cramwinckel, Voorhurg, Netherlands, as
signor to Maatschappii van lderkel’s Patent N.V., Rot
terdam, Netherlands, a company of the Netherlands
Filed July 17, 1961, Ser. No. 124,436
3 Claims- ‘(CL 177-211)
makes it possible to measure with a high degree of ‘ac
curacy (better than .1 of one percent).
’ Accordingly, it is an object of the present invention to
provide a measuring circuit capable of producing excep
tionally accurate measurements in connection with use
Claims priority, application Germany July 15, 1960
this effect is overcome through the parallel connection of
the tertiary coils. The circuit of the presentinvention
on a crane scale.
I Another object of this invention is in the provision of
This invention relates in general to a measuring cir-‘ 10 a circuit for use in connection with measuring devices
such as load cells wherein a compensation bridge is pro
cuit, and more particularly to a'measuring circuit espe-_
vided in combination with the load cell but mounted at
cially useful in connection with crane scales ‘for measuring
a remote location, and wherein a transformer is provided
Still more particularly, the invention relates to a strain
for the load cell and one for the compensation bridge
gauge measuring circuit constructedin accordance with 15 and thereby providing a compensating transformer feed
which produces a highly accurate measuring unit.
the zero compensation method with alternatingécurrent
' Other objects, features, and advantages of the inven
supply, and wherein the measuring and compensation ele
tion will be apparent ‘from the ‘following detailed dis
ments are spaced apart a relatively large distance, es
closure, taken in conjunction with‘ the accompanying
pecially as maybe encountered with crane scales.
sheet of drawing, wherein like reference numerals refer to
The present invention includes a load cell having strain
gauges which is mounted on a hoisting cable of a crane
and having connected thereto a crane hook which is
adapted to be connected to elements to be weighed. Thus,
the crane scale may be used to weigh various relatively
like parts, in which:
The single FIGURE of the drawing is a schematic View
of the circuit in accordance with the invention.
Referring now to the drawing, a pair of identical trans
A compensating circuit is provided 25 formers 1 and 2. are provided in connection with the
measuring circuit, wherein‘ the transformer 1 is located
which consists generally of a Wheatstone bridge with a
with the compensation bridge and the transformer 2 is
servo-motor driven potentiometer that may be mounted in
heavy elements.
located with the load cell. The transformer 1 includes
a primary coil 11, a secondary coil 12, and a tertiary coil
the compensating circuit and load cell, cables having great 30 13, while‘the transformer 2 includes a primary coil 21,
a secondary coil '22 and a tertiary coil 23'. The primary
lengths such as up to 300' feet may be needed with small
coils 11 and 21 are connected in series with an alternating
diameter and high flexibility, while commutator rings or
current voltage supply 3, through conductors 4 and 5,
contacts may be also incorporated in between same. With
whereby the same ‘current supply is fed to both trans
cable lengths of over 300 feet, the cable resistance can
be such that changes in this resistance with temperature 35 formers so that the current in both are equal. If desired,
any other method of attaining equal current in the trans
would have a marked in?uence on the measuring accuracy
formers may be employed.
of the scale.
'The transformer '1 feeds a compensation bridge 10
The present invention employs a compensating trans
through the secondary coil 12, while the transformer 2
former arrangement wherein a transformer is included in
feeds a load cell 20 through the secondary coil 22. Bridge
the'load cell construction and in the compensation bridge.
outputs 14 and 15 of the load cell and 14a and 18 of the
Heretofore, where transformers have been used in this
compensation bridge are connected in series and to the
manner, the primary coils or windings of the transformers
input of an ampli?er 16. Further, the conductor 6 inter
have been connected in parallel and to a voltage source
connects the outputs 14 and 14a, while a conductor 7
with low internal resistance, whereby the secondary volt
ages are in phase and have the correct wave form. 'How 45 interconnects the output 15 with the input of the ampli- .
?er 16. The tertiary coils 13‘ and 23 of the transformers
every, this arrangement is disadvantageous since the pri
are connected in parallel through the conductors 8 and 9.
mary loss-impedance appears in series withv the trans
A servo-motor 17 is powered [from the output of am
former coupling impedance and the load, and both im
pli?er 16, and drives the wiper of a potentiometer 1S and
pedances are variable which might change the ratio be
the cabin of the crane or on the floor in a remote loca
tion from the crane hook. Thus, to electrically connect
tween both secondary voltages.
The above disadvantage is overcome in the present in
a pointer 19 of an indicator until the voltages on the
bridges 14-~15 and 14a—18 are of the same amplitude
vention wherein at least two transformers are provided,
one for the load cell and one for the compensation bridge,
and in opposite phase. Then the conductors v6 and '7 are
current~free as soon as the system has balanced, so the
and whereby the primary windings of both transformers 55 cable resistance has no in?uence on the circuitry.
The load cell .20 and the transformer 2 are mounted as‘
are being supplied from a constant current source or are
connected in series, whereby the same current ‘?ows
through both primary windings, the secondary windings
are connected to the load cell and compensation bridge,
and a tertiary winding is provided on each transformer.
The tertiary windings are connected in parallel and there
'fore the tertiary currents are practically zero. Separate
conductors may be provided to supply a constant current
to both primary coils of the transformers, or the primary
coils may be connected in series. ’ The conductors con
necting the tertiary coils carry currents only if both volt
ages are unequal or the wave forms are incorrect.
current in these conductors is small, the voltage that ape
pears over the conductor resistance is also small and
a unit, and have a crane hook 30 extending therefrom for
receiving loads to be weighed.
The pointer 19 on the indicator, being also driven by
' the motor 17 indicates the displacement of the potentiom
eter wiper in accordance with the applied weight on the ‘
crane hook 30.
Inasmuch as the transformers are supplied with alter
nating current by means of the alternator 3, current
variations have the same effect on both transformers and
65 therefore do not change the relation between the output
voltages, and therefore such variations do not cause errors.
The conductors 8 and ‘9 are practically current-free, but
with impedance changes a small current 1k. will appear
which merely serves to overcome the differences in the
there-fore both tertiary windings have nearly the same volt 70 output voltages of both transformers and is always of a ,
age. This ca-usesharmonics to be generated in the trans
formers (the permeability being current-dependent) but
small dimension compared with the current Is. In ter
tiary 00115 13 and 23, onlya small current 1;; is present,
causing only a small voltage drop over the resistance in
the tertiary coils and the conductors connecting same so
having a load cell, and an ‘indicator element having a
compensation bridge wherein the elements are separated
by a relatively large distance from’ each other, said device
including a pair of transformers, each transformer hav
ing primary, secondary and tertiary coils, one transformer
Thus, due to the foregoint , it is seen that a highly ac~
located with the weighing element and having its sec
curate measuring circuit is provided in connection with
ondary connected to said load cell and the other trans
strain gauges and especially useful on crane scales.
former located with the indicator element and having its
It will’ be understoodrthat modi?cations and variations
secondary connected to said compensation bridge, means
may be effected without departing from the scope of the
novel concepts of the present invention, but it is under 10 for supplying equal power to said transformer primary
stood that this application ‘is to be limited only by the
coils, and means for connecting said tertiary coils in
scope of the appended claims.
The invention is hereby claimed as follows:
3. A measuring device comprising a load cell and av '
1. A measuring device comprising a weighing element
compensation bridge connected in series, a pair of trans
and an indicator element wherein the elements are sepa 15 formers, one with the load cell and the ‘other with the
rated a relatively large distance from each other, said
compensaton bridge, each transformer having primary,
weighing elementincluding a load cell and a transformer
secondary and tertiary coils, means supplying a constant
that both driving clectromotive forces in coils 13 and 23
are almost equal.
supplying power thereto through its secondary coil, said
current to the primary coils of said transformers, means
indicator element including a compensation bridge, a
connectingsaid tertiary coils in parallel, means. connect- ’
transformer supplying power to said bridge through a 20 ing said secondary coils to said load cell and compensation
secondary coil, a potentiometer in said compensation
bridge respectively, means for balancing said compensa
bridge having a wiper arm, an ampli?er, an indicator hav~
tion bridge, means reading the weight applied to said
ing an indicator needle, a servo-motor driving said wiper
load cell, and means connecting said load cell and com
arm and indicator needle, said compensation bridge and
pensation bridge in series to said reading means.
load cell-being connected in series and to the input of 25
References Cited in the ?le of this patent
said ampli?er, and the primary coils of said transformers
being connected in series with a source of alternating
current voltage, and a tertiary coil in each transformer,
Alexander ____________ __ May 17, 1955
said tertiary coils being connected in parallel.
Golding _____________ __ Dec. 22, 1959 2. A measuring device comprising a weighing element 30 2,937,861
Maring __________ __'___._ May 24, 1960
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