Weighing apparatus with additional assembly for torsion compensation

A weight scale in particular a bath room scale contains strain gauges. To make the scale of simple design while being reliable and precise a single bar has at its ends a load receiving plate and support base. At least two strain gauges are fastened to the bar and are sensitive to bending of the bar. At least one another strain gauge is fastened to the bar, this being sensitive to twisting of the bar.

BACKGROUND OF THE INVENTION 
The invention concerns weighing apparatus of the type having strain guages, 
and it is particularly applicable to bathroom scales. 
It is intended that such apparatus which, while being of a simple design, 
structure and low-cost is nevertheless reliable and precise. 
For this purpose, the apparatus of the invention comprises a single bar 
integrated at its ends with a load receiving plate and with a support base 
respectively; at least two assemblies with strain gauges fastened to the 
bar sensitive to bending of the bar under the effect of the load applied, 
those assemblies, in combination with the zone of the bar with which they 
work and with a circuit to process the signals emitted by the gauges 
having the same measurement sensitivity for the same load applied. The 
apparatus further comprises at least a third assembly with a strain gauge 
fastened to the bar sensitive to twisting of the bar under the effect of 
the load applied and a circuit to process the signals emitted by the three 
strain gauge assemblies, further including means for combining the signals 
so as to eliminate in the outgoing signal the component resulting from 
strain of the bar in torsion. 
The first two bending-sensitive assemblies contain identical gauges working 
together with the bar in zones of the same section. 
Outside of the zones of measurement of the bending movement, the bar can be 
of variable section and/or composition, notably, at its fastening end or 
in the zone receiving the torsion-sensitive assembly. 
Preferably, however, the bar is of uniform section, e.g., polygonal 
(square, rectangular, etc.) or curved (round, elliptical, etc.) or 
profiled (T, U, I, H, [, L, etc.). 
The bar can be solid or tubular. 
According to one preferred embodiment, each assembly contains a single 
gauge, the gauge of each assembly sensitive to bending being parallel to 
the general direction of the bar and the gauge of the assembly sensitive 
to twisting being inclined in that direction, e.g., by 45.degree.. 
In practice, each of the three strain gauge assemblies is sensitive to both 
bending and twisting, but for different parts, so that, by elimination of 
the twisting component in the outgoing signal, the bending component 
forming that outgoing signal is essentially defined by the two 
bending-sensitive components and, for a small part, the by 
twisting-sensitive component.

DETAILED DESCRIPTION 
The weighing apparatus, notably, a bathroom scale, shown in FIGS. 1 and 2, 
contains a sensitive element 1 formed by a single cylindrical bar of 
uniform cross-section. Preferably, as illustrated, the bar 1 is of 
circular cross-section, but that section could be of another curved shape, 
e.g., elliptical or even polygonal, such as square or rectangular, or even 
profiled (T, H, I, U, [, L, etc.). 
At one end 2, the bar 1 is integrally connected to a load support plate 5 
by fastening a screw 3 through a clamp 4 to hold load support plate 5. 
This support plate may comprise an upper plate 6 and a fork- or 
frame-shaped supporting structure 7, to which the plate 6 is attached and 
which is directly connected on the end 2. 
At its other end 3, the bar 1 is integrally connected with a base 10 by 
fastening a screw 8 through a clamp 9 to base 10, whereby the weighing 
apparatus may rest on a support, such as the floor. The base 10 may have a 
lateral fitting 11, the upper edge of which is overhung by a lateral 
peripheral skirt of the plate 6. The base 10 may also be fork- or 
frame-shaped in order to provide a stable bearing. 
The plate 6 and the fitting 11 have been deleted from FIG. 2 for sake of 
clarity of the drawing. 
The bar 1 is generally horizontal, as are the support structure 7 and the 
base 10. The bar 1 can extend parallel to one side of the machine or, if 
necessary, along a diagonal or even along a diameter, if the apparatus is 
circular. 
When a load to be weighed is placed on the plate 6, it subjects the bar 1 
to bending and possibly twisting or torsion stresses. To measure those 
stresses, there are attached to the bar 1, generally by gluing, at least 
two longitudinal strain gauges 12 and 13 and at least one strain gauge 14, 
shown in dotted and dashed lines on FIG. 1, the direction of which is 
inclined on the longitudinal axis 15 of the bar. Preferably, the two 
longitudinal gauges 12 and 13 are on the same generatrix, e.g., on the 
upper or lower generatrix, and gauge 14 is tilted by 45.degree. on the 
axis 15 of the bar, so as to have maximum twisting sensitivity. 
If the load to be weighed is perpendicular to the axis 15, the latter works 
only in flexion, while if the load is off-center, the bar works in flexion 
and in torsion. In that case, the strain of the bar in torsion also acts 
on the longitudinal gauges 12 and 13, which then run the risk of 
distorting the measurement. The inclined gauge 14 is mainly sensitive to 
twisting and, to a lesser extent, to bending. Its function is to cancel 
the part of the signal of the longitudinal gauges 12 and 13 occasioned by 
twisting. 
The signals are treated by means of the extensometric bridge of FIG. 3. The 
gauges 12 and 13 are mounted in series. The terminals of the series 
circuit are connected to the terminals S and S', respectively, of the 
power supply in parallel on that series circuit. A second series circuit 
consisting of gauge 14 and an adjustable balancing resistor R.sub.e is 
connected in parallel to the first series circuit. The deflection signal 
is picked up between guages 12 and 13, on one side, and between gauge 14 
and resistor R.sub.e, on the other. To cancel the twisting component in 
the outgoing signal, a small resistor R.sub.c is provided in series with 
gauge 14. In the alternative and as shown in dotted and dashed lines, a 
large resistor R.sub.c is connected in parallel across gauge 14. 
In the embodiment of FIG. 4, a fourth strain gauge 17 is added to the 
gauges 12, 13 and 14 of FIGS. 1 to 3, inclined on the axis 15 of the bar 1 
by an angle opposite to that of gauge 14. 
The extensometric bridge of FIG. 5 differs from that of FIG. 3 by replacing 
resistor R.sub.e with gauge 17, the branches of the bridge containing 
gauges 14 and 17 including the small series resistor r.sub.c and r'.sub.c 
or large parallel resistors R.sub.c and R'.sub.c. 
The embodiment of FIG. 6 differs from that of FIG. 4 by the addition of a 
second pair of longitudinal gauges 18 and 19, situated, for example, on 
the same generatrix as but diametrically opposite to gauges 12 and 13, and 
by the displacement of position of gauge 17 which remains inclined on the 
axis 15 by an angle opposite to that of gauge 14. 
The extensometric bridge of FIG. 7 used with the arrangement of FIG. 6 
contains three series circuits mounted between the power terminals S, S'. 
These series circuits are a circuit containing longitudinal gauges 12 and 
13, a circuit containing longitudinal gauges 18 and 19, and a circuit 
containing inclined gauges 14 and 17, each associated with a small 
correction resistor r.sub.c and r'.sub.c in series (or with a large 
parallel resistor), the outgoing signal being picked up between the 
terminal common to gauges 12 and 13 and the terminal common to gauges 14 
and 17, an adjustable correction resistor R".sub.c being in parallel with 
the terminals picking up the outgoing signal, and another adjustable 
correction resistor R"'.sub.c being provided between the terminal common 
to gauges 18 and 19 and the terminal common to gauges 14 and 17. In 
practice, only one of the resistors R".sub.c or R"'.sub.c is used and its 
circuit can then be controlled, for example, by a switch. 
It will be evident to one skilled in the art that other bridge arrangements 
are possible. 
The relative positions of the longitudinal and inclined gauges on the bar 1 
are not generally decisive. For example, an inclined gauge could be glued 
on the bar at the same time as a longitudinal gauge to form a 
double-crossed gauge. 
In each of the embodiments described above, only one strain bar is used and 
the measurement errors induced by the longitudinal gauges on the twisting 
of the bar are eliminated by the inclined gauge or gauges. 
Although single-gauge assemblies and a bar of constant section have been 
described, other embodiments are possible. 
Thus, the bar may have a variable cross-section. That cross-section could 
even vary from one zone of measurement of the bending moment to the other, 
the correction then being made in the extensometric bridge, adapted for 
that purpose, so that the signals originating from the two assemblies 
sensitive to bending will be the same. It is also possible to provide 
several gauges per measurement unit.