Technical field

The present invention relates in general to the sector of weighing devices for packaging lines and in particular to a load cell capable of eliminating the apparent weight due to the vibrations produced by the automatic machine on which it is installed.

Background

A load cell is a transducer used for measuring a force applied to a body. The most common application of load cells is in electronic weighing systems and in the measurement of mechanical compressive and tensile stresses.

A load cell generally comprises a measuring body made of metal, typically steel or aluminium, applied on which is a force that causes a deformation thereof.

In dynamic measurements of weight, so-called bending-beam load cells are used, which comprise a parallelepipedal body made in which is a butterfly-shaped through opening, which enables the body, appropriately mounted in cantilever fashion on a support, to bend with the movement of a parallelogram. As compared to load cells that operate in tensile or compressive mode, bending-beam load cells afford the advantage of not requiring any alignment between the loads applied and the axis of deformation of the body measuring body.

In a load cell with resistive strain gauges, the latter can be glued in appropriate points of the deflecting body in such a way that, by undergoing deformation, they modify their electrical resistance. The variation of voltage is the signal that is amplified and processed so as to be transduced into the value of the weight.

Dynamic measurement of weight is carried out on packaging lines provided with a plurality of apparatuses and devices that typically generate vibrations in a very wide frequency spectrum, which ranges from a few hertz up to several kilohertz, the accelerations of which give rise to an apparent weight that adds to the gravitational weight of the object to be measured.

The oscillatory forces transmitted to the load cells significantly worsen the standard deviation of the population of the averaged signals all the more the lower the frequency of the vibrations as compared to the sampling frequency .

In order to reduce the errors of measurement due to the vibrations, according to the known art, either hardware filtering or software filtering or else accelerometers are used for measuring the accelerations, and these are then subtracted from the measurement of the weight.

By way of example, EP 0 635 703 A1 discloses the combination of a weighing cell and a displacement sensor. The weighing cell includes a strain-inducing element having a strain-generating region, where strain is generated in response to application of a load thereto, and a strain gauge for detecting the strain generated in the strain-generating region. The displacement sensor includes a fixed rigid component, a movable rigid component forming a weight element, a generally elongated beam member rigidly secured at one end to the fixed rigid component and at the opposite end to the movable rigid component so as to extend between the fixed rigid component and the movable rigid component, and a displacement-detecting element mounted on the elongated beam member for outputting an electrical signal having an amplitude proportional to the amount of displacement of the movable rigid component in a direction generally perpendicular to the elongated beam member. The strain-inducing element of the weighing cell includes a fixed rigid body, configured for being secured to a base, and a movable rigid body, configured for receiving the load to be measured, and first and second beams, which are rigidly secured at opposite ends to the fixed rigid body and the movable rigid body and extend parallel to one another between the fixed rigid body and the movable rigid body. The displacement sensor is fixedly carried by the fixed rigid body of the weighing cell.

There exists the need to improve the technology in so far as the techniques so far known do not solve effectively the disturbance on the measurements of the weight due to the vibrations.

Summary of the invention

The technical problem posed that is solved by the present invention is consequently that of providing a system of load cells that will enable the drawbacks mentioned above with reference to the known art to be overcome .

The above problem is solved by a device according to Claim 1 and/or by a corresponding method according to Claim 10.

Preferred characteristics of the present invention form the subject of the dependent claims.

The load cell according to the invention comprises a bending-beam load cell which is mechanically coupled to a cantilever (springboard-like) element sensorized with strain gauges, in which the cell measures the weight and the cantilever measures the apparent weight to be subtracted from the measurement made by the cell. The instantaneous samples of the apparent weight, via a specific procedure, are synchronously subtracted from the samples coming from the load cell, thus eliminating the contribution thereof.

The bending-beam load cell and the cantilever, which are subjected to the same vibrations, are made in such a way that they oscillate with the same characteristic frequency .

The cantilever is set and calibrated with the known procedure used for strain-gauge load cells.

According to one embodiment of the invention, the cantilever may have various geometrical shapes, where its function is to oscillate in response to vibration.

Other advantages and characteristics, as well as the modes of use of the present invention will emerge clearly from the ensuing detailed description of some embodiments, which are provided purely by way of non-limiting example.

Brief description of the drawings

Reference will be made to the attached plates of drawings, wherein:

Figure 1A is a cross-sectional view that shows a load cell according to the invention;

Figure IB is a cross-sectional view that shows the sensor for measuring the apparent weight;

Figures 2A and 2B are cross-sectional views that show a cell with a cantilever element installed in an embodiment alternative to that of Figure 1A;

Figure 3 is a cross-sectional view that shows a cell with the cantilever element in the form of load cell; and

Figure 4 provides a simplified diagram of the measurement electronics.

Detailed description of preferred embodiments

With initial reference to Figure 1A, a load cell according to the invention is designated as a whole by the reference number 100 and comprises a deformable body 101, which has, for example, a generically parallelepipedal shape and made in which, in the transverse direction Y, is a butterfly-shaped through opening 102. The through opening 102 extends in the longitudinal direction X between a first shoulder 104 and a second shoulder 105 of the deformable body 101.

In the embodiment illustrated, the deformable body 101 is fixed by means of its shoulder 104, whereas the loading area is opposite thereto, where an area that is to receive a load in the vertical direction Z is provided, for example in the form of a plate. In the embodiment illustrated, the loading plate is set on the top of the deformable body 101 in the proximity of the second shoulder 105 and is fixed thereto, in an example of application, via a threaded through hole 108.

The configuration of the load cell illustrated is consequently that of a load cell of the bending-beam type.

In the embodiment illustrated, the bending-beam load cell 100 is fixed in cantilever fashion on a support (not shown) at the end of the shoulder 104 via a through hole 107.

The load cell 100 according to the invention is of the resistive-strain-gauge type and comprises in a known way an electronic control unit 300 housed in a remote position with respect to the cell.

According to the invention, the deformable body 101 receives within the through opening 102 a cantilever element 103 (referred to hereinafter for brevity simply as "cantilever") having, in the embodiment illustrated in Figure IB, generically the shape of a parallelepiped elongated in the longitudinal direction X which is bent to form an L-shape in the direction of the axis Z in a terminal part thereof where a through hole 109 is present for fixing to the shoulder 104 via a threaded screw as shown in Figure 1A.

In the embodiment illustrated, glued on the cantilever 103 on the opposite sides of its base are two strain gauges 110 and 111 positioned along the longitudinal axis X in the direction of greater deformation of the cantilever.

According to the invention, constrained to the end of the cantilever 103 that is free to vibrate is a metal mass 106, which can be slid along the length of the cantilever 103, not occupied by the strain gauges 110 and 111, and can be fixed thereto in the preselected position via a threaded through hole 112, the screw of which bears upon the cantilever 103.

As is known, a force applied on the loading plate constrained to the shoulder 105 via the threaded through hole 108 and acting in the vertical direction Z causes elastic bending of the deformable body 101, which undergoes deformation like a parallelogram whereas the cantilever 103, which is not loaded by any force, remains in its state of rest. Deformation of the bending body 101 is measured, in a known way, via strain gauges glued in positions corresponding to the thinner regions of the body 101, the signals of which are transduced by an appropriate electronics 300 shown in Figure 4 into the value of weight.

In the case where the load cell 100 is set in vibration by external oscillatory forces, for example because it is inserted in an automatic machine and fixed thereto via the threaded through hole 107, both the deformable body 101 and the cantilever 103 will forcedly enter into vibration at their respective characteristic frequencies.

Thanks to this configuration, the deformable body 101 will receive, in the plate connected to the shoulder 105, the object to be weighed, while at the same time the signals produced by the strain gauges of the cantilever 103 in forced vibration will be transduced into apparent weight.

The electrical signals coming from the strain gauges of the cantilever 103 and those coming from the strain gauges of the load cell 101 constitute two different sequences or threads, which, acquired synchronously, are processed in parallel by the electronics 300 via a measurement chain that comprises signal amplifiers 302, an analog-to-digital conversion 304, and a microcontroller 306, in which the firmware compensates the signals with respect to one another and averages them.

The compensation procedure will subtract from the thread coming from the load cell 101 the thread coming from the cantilever 103 and supplies a signal compensated by the effect of the vibrations.

In the embodiment illustrated, it is necessary for the bending-beam load cell 101 and the cantilever 103 to have the same characteristic frequency. According to the invention, it may be assumed that these two frequencies

are equal to one another, which means that

As is known, the first of the two equations yields the characteristic frequency f _T of the cantilever 103, whereas the second yields the characteristic frequency f _F of the bending-beam load cell, where: E is the Young's modulus of the material; b is the base, h the height, and L the length of the cantilever 103; and m is the weight of the mass 106. As regards the cell 101, K is its stiffness, whereas the mass M is the sum of the tare weight of the weighing machine plus the weight of the object being weighed.

From the equality of the two equations the cantilever 103 is sized; for example, the mass 106 is set.

In the embodiment illustrated, exact tuning of the frequencies is obtained by displacing along the transverse axis Y of the cantilever 103 the mass 106 until the same frequency is instrumentally obtained. The distance between the free end of the cantilever 103 and the mass 106 determines the range of measurements that the load cell 100 can carry out, keeping the vibrations compensated.

With reference to Figures 2A and 2B and to the embodiment illustrated, the load cell with compensation of vibrations 100 may be configured with the cantilever 103 fixed to the cell 101 via a spacer 109 on the top side (Figure 2A) or the bottom side (Figure 2B) of the shoulder 104 via the through hole 107.

According to the embodiment represented in Figure 3, a load cell 100 similar to the load cell 100 of Figure 2B envisages a deflecting body 202 identical to the deflecting body 201 as advantageous embodiment of the cantilever 103 where the deflecting body 201 is the same as the deflecting body 101 already illustrated.

In the embodiment shown in Figure 3, the two cells 201 and 202 are constrained together via screws that pass through the through holes 206 and 207 with interposition of an insert 203 that keeps them separate and free to vibrate .

According to this embodiment, the two cells 201, 202 will vibrate at the same characteristic frequency when the mass 210, fixed to the cell 202 via a screw that is tightened in the threaded through hole 208, is equal to the sum of the tare weight and of the weight of the object to be weighed .

In the embodiment illustrated in Figure 3, the cell 100 is mechanically connected to the machine via the same screws as those used for joining the two cells and via the through holes 206 and 207.

The load cells 100 according to the invention can be used in a weighing station of a line for packaging products in bulk form, for example ground coffee packaged in pods, medicines packaged in purposely provided containers, and in similar cases where the objects to be weighed always have the same mean weight.

The present invention has so far been described with reference to preferred embodiments. It is to be understood that other embodiments may be conceived, all of which fall within the same inventive idea, as defined by the extent of protection of the claims provided hereinafter.