1. * GB780024 (A)
Description: GB780024 (A) ? 1957-07-31
Improvements in or relating to weighing apparatus
Description of GB780024 (A)
COMPLETE SPECIFICATION
Improvements in or relating to Weighing Apparatus
We, SCRIBBA:NS-REMP LIMITED, of Africa
House, Kingsway, London, W.C.2, a Com
pany registered under the laws of Great
Britain, and ANTHONY LEONARD HENDON,
of 96, Tuhbenden Lane, Orpington, Sent,
and PETER OLIVER ROGERS, of 14, Max
welton Close, Mill Hill, London, N.W.7,
both British subjects, do hereby declare
the invention, for which we pray that a
patent may be granted to us, and the
method by which it is to be performed, to
be particularly described, in and by the fofiowing statement
This invention relates to weighing apparatus.
The invention consists of a weighing apparatus comprising a
weigllt-earrying
platform mounted on a conducting coil located in a magnetic field by
anti-friction
bearings, the platform and coil being
adapted to be supported in a floating position by passing current
through the coil,
means for detecting movement of the coil when a weight is placed on
the platform, means responsive to the detecting means, for increasing
the current through the coil to restore the position of the coil and
means for indicating said variations in current.
The response time of the apparatus according to the invention can, be
made very much less than with normal type scales. For example, it is
relatively easy
to weigh l kilogram in, say, 100 milli

2. seconds. Weighing times of this order are important where weighings
are to be made
in quick succession, as in check-weighing
the output of certain production lines.
The apparatus is conveniently arranged
to indicate weight as a voltage; this is par
ticularly suited to the control of external
apparatus, for example automatie rejec
tion equipment, or for the continuous re
cording of weighings.
The invention will be further described with reference to the
aeeompanying drawinns.
Fig. 1 is an axial section of the mechanical part of an apparatus in
accordance with the invention;
Fig. 2 is a section on the line A-A of
Fig. 1;
Fig. 3 is a section on the line B-B of
Fig. 1;
Fig. 4 is a section on the line C-C of
Fig. 1;
Fig. 5 is a diagrammatic view showing the connection of the mechanical
part to the electrical circuit;
Fig. 6 is a circuit diagram of a suitable d.c. amplifier for use in
the circuit arrangement of Fig. 5;
Fig. 7 shows the connection of the arrangement of Fig. 5 to an
indicating voltmeter;
Fig. S shows the connection of the arrangement of Fig. 5 to a
difference- iiidieating voltmeter;
Fig. 9 shows the connection of the arrangement of Fig. 5 to
range-indicating, signalling devices;
Fig. 10 shows the connection of the arrangement of Fig. 9 to an
automatic feeding arrangement for articles to be weighed with
automatic stopping when an article of weight outside a range received;
Fig. 11 shows the connection of the arrangement of Fig. 9 to a feeding
arrangement with automatic rejection;
Fig. 12 shows the connection of the arrangement of Fig. 8 to control
the weight of material fed to containers on an average basis;
Fig. 13 shows the connection of the arrangement of Fig. 8 to control
the weight of material fed into each container;
Fig. 14 shows the connection of the arrangement of Fig. 5 for
generating a train of pulses corresponding in number to the weight to
be registered.
The function of the basic unit (see Fig. is is to provide a voltage
output which is accurately and linearly proportional to a weight (or

3. other force) applied to the unit.
Fundamentally, it relies on the fact that the force upon a coil placed
in a magnetic field is directly proportional to the current through
the coil; if this force is arranged to oppose the downwards force of
an applied weight, and the current is such that the system is in
equilibrium, then the current itself will he directly proportional to
the applied weight.
In the mechanical part (Figs. 1 to 4 a vertical shaft 1 carries a coil
2. wound upon a suitable former which is positioned freely in the
annular gap of a magnet 3.
Connected to shaft 1 in positioiis axially spaced and near the top
thereof are two shutter vanes 4 which control the amount of light
reaching two photocells 5 from a lamp 6. The shaft 1 has a screwed
thread 7 at the top by which various types of weighing platform niav
be attached there- to. The shaft 1 is supported by two bear- ing
systems in which a triangular section of the shaft is at each face in
contact with a roller bearing or bearings g. This allows very free
vertical movement of the system but prevents rotation of the shaft and
hence damage to the flexible coil leads, and prevents fouling of the
coil oii the magnet.
The magnet gap is 30 designed that the field is constant over the
range of move- ment of the coil, in practice. normally not more than
0.05 inches. In order to achieve this the winding length is made
greater than the actual gap depth, so that a coii- stant number of
turns are positioned in tlie main part of the field irrespective of
the shaft movement over its operating range.
An aluminium alloy coil former is used, but many other materials would
be suitable.
The application of a weight to weighing platform 9 (Fig. 5) upsets the
equilibrium of the system, and displaces the shaft 1, shutter vanes 4
and coil 2 downwards.
This movement of the shutter vanes 4 allows more light to fall on one
photocell 5 and less on the other. The two photo- cells 5 are
connected to a d.c. amplifier 10 (Fig. 5) in such a way that this
light change causes the current through the coil to increase, and
hence produce a force opposing that of the applied weight: thus
equilibrhim is re-established. The arrange- ment, in fact, forms a
closed loop servomechanism, and the current will always maintain such
a value as will hold the system in equilibrium. Since the current
change is proportional to the downward displacement of the shaft, the
gain of the amplifier must at least be such that the maximum required
current, and hence opposing force, can be obtained without the shaft
moving so far that the coil leaves the linear portion of the field.
The d.c. amplifier is shown in one form in Fig. 6. The photocells .)

4. are connected in opposition to @ id brim of cathode fol- lower valve
VI which has series connected cathode resistances R1 and R2 the centre
point of which is connected over resistance
R3 to the grid.
The cathode of valve V1 is connected over resistance R4 to the grid of
valve V2 which has an anode resistance R5.
The anode of valve V2 is connected to the grid of valve in the anode
circuit of which is connected the moving coil.
The output current of valve V3 is arranged to flow through a precision
resistor R6, producing a voltage V, which is directly proportional to
the applied weight.
By introducing rate-of-change feedback hito the amplifier by n means
of capacitor
C1 and resistance R7 connected in series between the cathode of valve
V3 and the grid of valve V2, the damping co-efficient can be adjusted
so that the system is critically damped. A metal coil former helps in
this respect in that it produces eddy current damping, and hence
generates a retarting force proportional to the velocity of the coil
former. In theory critical damping can only be obtained easily for one
particular value of applied weight, but in practice adequate damping
can be achieved r the whole range of we lit for which the @ so system
is designed.
Assuming critical damping and proper amplifier design, the current
will rise exponentially when a weight is applied. This 'rise time' can
be made as small as desired for any given weight by making the
amplifier gain sufficiently high.
A low-amplitude, low-frequency sine wave is permanently applied to the
moving coil, conveniently by application over condenser C2 to the
cathode of valve V2, so as to produce slight vibration of the moving
system. The purpose of this applied 'dither' is to break down friction
effects in the bearing system, and has the effect of increasing the
sensitivity.
The weighing unit described is capable of considerable modification
without affecting the basic principle of operation. Thus, an
electro-magnet could be used in place of the permanent magnet,
especially for handling heavy weights where a large magnet would be
needed. Detection of movement of the shaft could equally well be
carried out by causing the movement to vary an inductance or capacity.
The bear ing system may be of any type which allows free movement of
the shaft longitu- dinally, but prevents rotation of the coil.
In certain cases it might be desirable to allow, or cause, the
platform and hence the coil to rotate. In this case, the coil current
could be fed via slip rings.
The basic instrument B1 indicated generally in Fig. a, will normally

5. be comhined with ancillary apparatus of a type determined by the
particular application.
A direct reading of weight will be given by means of an accurate
voltmeter VMI. connected to the output (Fig. 7).
By using a centre-zero voltmeter =312
(Fig. 8), with one side connected to an adjustable reference voltage
from a potentiometer resistance PR fed by a stabilized voltage, the
system will show the difference between an applied weight and any
particular standard weight. In this case the reference voltage is
adjusted to give zero indication with the standard weight.
By means of suitab]e electronic circuitry for comparing the weigher
output voltage with one or more variable reference voltages,
representing reference weights, an applied weight may be shown as
falling in one of any number-of suitable weight ranges. For example
(Fig. 9), the weight may be presented as lying in one of three ranges,
the particular range being signi fied by means of a coloured lamp.
Thus, a green lamp G1 signifies that a weight lies within a certain
pre-set weight range determined 'Correct', whilst red and orange lamps
G2 and G3 respectively show that it is ]ower or higher in weight,
respectively, than the 'Correct' range. The lamps are operated by
contacts h and l of relays H and L which are in turn controlled by
contact of a polarised centre stable relay A which replaces the
voltmeter VM2 Fig. 8.
Articles to be weighed may be placed upon the weighing platform
manually, or may be fed automatically in succession. In an example
(Fig. 10) of the electronic weighing system with an automatic feed, in
which weights are indicated (as in Fig.
9) by means of three coloured lamps, it is arranged that the automatic
feed motor
FM for articles 11 on a main conveyor MC stops if an article is ]ow or
high in weight with respect to a 'Correct' range due to the operation
of relay H or relay l. The weight of the article may then be adjusted
manually until the weight is correct, i.e., lies within the pre-set
'Correct' range, whereupon the feed is automatically restarted.
As an alternative to stopping the automatic feed as in Fig. 10 if an
article is high or low in weight, a suitable mechanism. may be
automatically operated which will reject these articles from the
produc- tion line (Fig. 11). Thus with an arrangement otherwise
similar to that of Fig. 10 the operation of relay H or L can actuate
solenoids S which control a gate GA to deflect an article 11 onto
auxiliary conveyors ACI or AC2 away from the main extension conveyor
EC.
In a further example of the use of the basic weigher, the average
weight of material filled into a container is controlled by weighing.

6. In one snch example (Fig.
12) basins B are fed automatically to the weighing platform 9 of an
electronic weigher, and are filled with batter whilst on this platform
by means of a constant volume depositor D. The volume of the deposit
can be varied by altering the stroke of the depositor piston P.
Microswitch MS connected in series with resistrance R6 in the output
of the basic instrument BI to potentiometer PR is closed momentarily
once per cycle by means of a cam in the piston mechanism and timed to
operate at the end of a filling stroke any difference in voltage
between the weight and reference indicated by potentiometer
PR will then be transmitted over condenser C3 to an averaging circuit
AC. The average of a suitable number (say ten) of successive weighings
of filled basins is arranged to control the piston stroke by a piston
stroke adjusting mechanism PSALM operated by an electric motor EM
itself controlled hy a motor control circuit
MCC operated by an averaging circuit AC.
The piston adjusting motor normally remains stationary, but turns in
the appropriate direction to reduce the error should the average
weight differ from the pre-set reference. Thus the average weight of
the deposit is maintained constant, although individual weighings may
differ by an amount depending on the short term variations inherent in
the depositor itself.
Alternatively Fig. 13, the actual filling of the containers eould be
under direct control of the electronic weigher. Thus, filling is
started by the arrival of an empty basin operating a micro-switch MS
which operates a solenoid control circuit SCC which transmits a signal
to a control valve operating solenoids CVS which operates the control
valve CV of the depositor D.
When the comparison polarised relay A indicates the main basin as
filled to the correct weight it transmits a signal to the solenoid
control circuit SCC over contact a to bring about closing of the
control valve CV.
In either of these examples it may be necessary to allow for
variations in the weight of the containers, since the weigher output
represents the total weight of container and contents. In tliis case
additional electronic apparatus is used which in effect stores the
container weight and deducts it from the final total, thus indicating
true material weight. Such apparatus is described in our British
Patent No. 727,164.
The voltage output of the basic weigher is a continuous function ol
the applied weight. In certain cases, however, it may he advantageous
to convert the output into digital form. For example (Fig. 14, the
weigher output voltage may he compared by means of a suitable
polarised relax A as before with a reference voltage which is arranged

7. to increase from some initial value in equal and accurately controlled
steps. Pulses of voltage or current are produced by a voltage step
generator VSG each time a step occurs until the reference voltage has
climbed to a value equal (with- in the range of the step) to the
weigher output voltage. Thus the start level of the voltage staircase
is adjusted weigher output with no applied weight.
The application of a weight will then operate polarised relay A, which
detects unbalance between the weigher output and the voltage from the
step generator. Relay X thns starts the voltage staircase by oper-
ating the voltage step generator VSG at al and connects the output to
the pulse train output lead PTOL at a2. Relay A will release when the
voltage staircase has risen to a ralue equal to the weigher put, thus
disconnecting the voltage pulses from the pulse train output lead PTOL
at a2 and disconnecting the generator at a1. Each pulse represents a
small, known increment of weight, and the total weight is proportional
to the total number of pulses. This method of presenting the weight in
the form of a train of pulses is particularly suitable for the
operation of standard punched card calculating machines and automatic
typewriters, or other types of automatic computer.
What we elaim is:-
1. A weighing apparatus comprising a weight-carrying platform mounted
on a conducting coil located in a magnetic field by anti-friction
bearings, the platform and coil being adapted to be supported in a
floating position by passing current through the coil, means for
detecting movement of the coil when a weight is placed on the
platform, means, responsive tij the detection means, for increasing
the current through the coil to restore the position of theeoil and
means for indicat- in" said increase in current.
2. A weighing apparatus as claimed in
Claim 1 provided with means for imparting a vibration to the moving
system r lative to the hearing system with the object
@f reducing friction.
3. A weighing apparatus as claimed in
Claim 2 in which vibration is imparted to the suspended coil by
superimposing alternating current on the direct current norm
ally flowing through the coil.
4. A weighing apparatus as claimed in
Claim 1, 2 or 2 in which the coil former is of metal to give
eddy-current damping.
5. A weighing apparatus as claimed in
Claim 1, 2, 3 or 4 in which the means responsive to the detecting
means include an amplifier.
6. A weighing apparatus as claimed in Claim 5, in which @@@@-of-change
feed-back is provided in the amplifier.

8. 7. A weighing apparatus as claimed in any of the preceding claims in
which the means for detecting movement of the coil comprise a pair of
photocells fixed in relation to the magnetic field, a pair of
shutters, for screening the respective photocells, fixed in relation
to the coil in axially spaced positions and a light source for
illuminating the photocells.
8. A weighing apparatus as claimed in any of the preceding claims in
which a direct reading of weight is given by measurement of a voltage
corresponding to the increase in current.
9. A weighing apparatus as claimed in
Claim 8, in which the difference of the weight of an article to be
weighed from a standard weight, is indicated by comparison of the said
@@tace with a standard voltage.
10. A weighing apparatus as claimed in
Claim 9, in which the weight is indicated as lying within one of the
predetermined number of ranges of weight.
11. A weighing system having a conveyor for feeding articles to be
weighed comprising a weighing apparatus as claimed in Claim 10, in
which the feed is adapted to he arrested if the weight of an article
being weighed falls outside a particular range of weights.
12. A weighing system having a conveyor for feeding articles to be
weighed to a weighing apparatus as claimed in Claim 10, and a conveyor
for removing the articles which have been weighed in which a further
conveyor is provided for receiving articles from the weighing
apparatus and means operated from the weighing apparatus for
deflecting an article which has been weighed to the further conveyor
in the event that its weight falls outside a particular range of
weights.
13. A weighing system having means for filling containers and a
conveyor for feeding the container to a weighing apparatus as claimed
in f Claim 9. in which said difference in weigh indication is used to
control the weight of material being filled into the containers.
14. A weighing apparatus substantially as deserihed with reference to
and shown in Figs. 1 to 5 and 7, 8 or 9.