METASTRUCTURE WITH VIBRATION INSULATION PROPERTIES

Technical field

This invention relates to a metastructure with vibration insulation properties.

Background art

Vibrational measurements are a reference point for structural dynamics, for fault diagnosis and product quality assessment.

Vibrational measurements are usually performed with accelerometers typically with piezoelectric technology connected to the structure in question.

The connection of the accelerometers to the structure is usually made with adhesive or beeswax, magnets or threaded pins.

However, the process for installing these sensors may take a very long time and, in some cases, may be irreversible or even not allowed.

Portable vibration probes have been widely used in the past, even though their performance (analysis range and overall accuracy) was lower than in fixed installations.

The advent of robots and the advances of electronics have sparked a new interest in portable vibration probes.

A portable vibration probe consists of a handgrip, an uncoupling element and an accelerometer.

The probe design is critical to ensure contact between the accelerometer and the structure to be tested.

The uncoupling element must avoid the introduction of spurious vibrations during the test, due to manipulation by a human operator or a robot, and ensure contact between the surface under examination and the accelerometer.

By way of example, a known uncoupling element is made of conventional material such as rubber. Aim of the invention

In this context, the need has been felt to create a metastructure with vibration insulation properties as described in independent claim 1 .

Advantageously, the metastructure according to the invention has a wide range of frequencies in which the propagation of the waves is not allowed (approx. 10 kHz), increasing the range of applicability compared with prior art solutions, and a low frequency of opening the bandgap (close to 1 kHz). Another advantage is a compact and lightweight design and at the same time an adequate structural strength.

Brief description of the drawings

The technical features of the invention are clearly described in the claims below and its advantages are more apparent from the non-limiting description which follows of a preferred, non-limiting embodiment of a metastruttura with vibration insulation properties as illustrated in the accompanying drawings, in which:

- Figure 1 is a schematic perspective view of a primary cell of a metastructure according to the invention;

- Figure 2 is a schematic perspective view of a basic unit of the primary cell of Figure 1 ;

- Figure 3 illustrates the basic unit of Figure 2 with some parts cut away to better illustrate others;

- Figure 4 is a schematic perspective view of a supporting grid of the cell of Figure 1 ;

- Figure 5 is a schematic perspective view of a metastructure having three primary cells arranged one after another according to a second embodiment;

- Figures 6 and 7 schematically show spatial and angular references in order to better clarify the spatial orientation of rod-shaped elements of the grid of Figure 4; - Figure 8 is another schematic perspective view of a primary cell of the metastructure wherein the basic units have been indicated on the basis of their spatial orientation.

Detailed description of preferred embodiments of the invention

With reference to the accompanying drawings, the numeral 1 denotes a metastructure with vibration insulation properties according to the invention.

The metastructure 1 is an artificial material the microstructure of which is designed to give certain physical properties, according to this invention for vibration insulation.

The metastructure 1 comprises at least one cell 2 comprising a plurality of basic units 3.

Preferably, the cell 2 comprises eight basic units 3 positioned relative to each other in such a way as to form a cell 2 with a cubic shape.

Each basic unit 3 comprises a hollow body 4 for housing at least part of a supporting grid 5 of the hollow body 4.

The hollow body 4 has a plurality of faces 6, contiguous to each other along respective edges 7, defining four pyramids 8 each having a respective base 9 located inside the hollow body 4.

More specifically, the four pyramids 8 of each basic unit 3 are positioned on opposite edges in pairs.

The supporting grid 5 of each basic unit 3 comprises a central zone 10 from which four rod-shaped elements 1 1 extend, each of which supports a base 9 of a respective pyramid 8 of the hollow body 4.

With reference to the cell 2, the grid 5 is formed by the supporting grids 5 of the eight basic units 3 which constitute the cell 2.

As will be specified below, the supporting grids 5 of the eight basic units 3 are connected together.

Four rod-shaped elements 1 1 define a first group of beams 13.

Taking each basic unit 3 as reference, it is possible to determine a Cartesian system x, y and z as shown in Figure 6.

The origin of the axes is defined by the intersection of the planes of symmetry PS1 , PS2 and PS3 schematically illustrated.

Considering this reference system and the angles 0 [deg] and <|) [deg], as indicated in Figure 6, it is possible to indicate the position of each rodshaped element of the four rod-shaped elements 11 of the first group of beams 13 as schematically illustrated below.

A further four rod-shaped elements 12 extend from the central zone 10, each of which has an end 12a which extends outside the hollow body 4 at a corner zone 18 of a respective pyramid 8.

The rod-shaped elements 12 define a second group of beams 14.

With reference to the four rod-shaped elements 12 of the second group of beams 14, each of which extends along a direction parallel to the direction of an edge 7 contiguous with two faces 6 of the hollow body 4.

Considering the Cartesian system x, y and z and the angles 0 [deg] and <|) [deg], it is possible to indicate the position of each rod-shaped element of the four rod-shaped elements 12 of the second group of beams 14 as schematically illustrated below.

The rod-shaped elements 11 of the first group of beams 13 have the same cross section.

The rod-shaped elements 12 of the second group of beams 14 have the same cross section.

The rod-shaped elements 11 of the first group of beams 13 have a cross section which is greater than the cross section of the rod-shaped elements 12 of the second group of beams 14.

The end 12a of a respective rod-shaped element 12 of the second group 14 of a basic unit 3 is connected with a respective end 12a of a respective rod-shaped element 12 of the second group 14 of another basic unit 3.

The supporting grid 5 of a respective basic unit 3 has a greater flexibility with respect to the relatively stiffer hollow body 4.

In other words, the hollow body 4 defines the mass of the basic unit 3.

Preferably, the material of the cell 2, and therefore of the rod-shaped elements 11 , 12 and of the hollow body 4, is of polymeric origin.

In particular, polyamide having an elasticity modulus of 1.70 GPa, a shear modulus of 1 .24 GPa and a density of 900 kg/m3.

With reference to the first group of beams 13, the cross section of each rod-shaped element 11 has a circular cross section along at least one stretch of the rod-shaped element 11 .

With reference to the second group of beams 14, the cross section of each rod-shaped element 12 has a circular cross section along at least one stretch of the rod-shaped element 12.

The circular cross section of each rod-shaped element 11 of the first group of beams 13 is greater than the circular cross section of each rod-shaped element 12 of the second group of beams 14.

According to a preferred variant embodiment, having defined the length of one side “I” of the cubic configuration of the cell 2, the thickness of each rod-shaped element 11 of the first group of beams 13 and the thickness of each rod-shaped element 12 of the second group of beams 14 is a function of the length “I”. In particular, the thickness of each rod-shaped element 11 of the first group of beams 13 is equal to 0.2 x “I” and the thickness of each rodshaped element 12 of the second group of beams 14 is equal to 0.04 x “I”.

The length "I" of the side of the cubic configuration of the cell 2 varies within a range of between 16 mm and 40 mm.

The thickness of each rod-shaped element 11 of the first group of beams 13 varies within a range of between 4 mm and 10 mm.

The thickness of each rod-shaped element 12 of the second group of beams 14 varies within a range of between 1 mm and 2 mm.

More specifically, it should be noted that the proposed measurement intervals for the length of the side of the cell 2, the thickness of each rodshaped element 11 of the first group of beams 13 and the thickness of each rod-shaped element 12 of the second group of beams 14 are an alternative definition to the above-mentioned formulations.

With reference to each basic unit 3, it has a symmetrical structure on diagonal planes.

In particular, each basic unit 3 has six planes of symmetry (PS1 , PS2, PS3, PS4, PS5 and PS6), as schematically illustrated in Figure 7.

Each basic unit 3 has three anti-symmetric planes each of which is an intermediate plane parallel to a basic face.

With reference to the cell 2, this also has six planes of symmetry, in a similar fashion to the planes of symmetry of the basic unit 3.

If the Cartesian system x, y and z of the basic unit and the angles 0 [deg] and (|) [deg] are taken as reference, as indicated in Figure 6, it is possible to indicate the position of each unit of the eight basic units 3 as schematically described below.

It should be noted that, in the cell 2, the basic unit 3 is rotated in such a way that the rod-shaped elements 11 of all the basic units 3 are joined to the centre of the cell 2, see Figure 4.

In use, the metastructure 1 preferably comprises three cells 2 arranged one after another in such a way as to form a parallelepiped shape.

According to a first alternative embodiment, the metastructure 1 is defined by the same polymeric material of origin.

By way of a non-limiting example, the metastructure 1 is obtained by means of an additive manufacturing process.

According to a second alternative embodiment, each hollow body 4 comprises at least one base portion 16 consisting of a material of polymeric origin and an end portion 17 consisting of a metallic material, in particular the end portion has a tetragonal shape.

In an experimental manner, a prototype of the metastructure 1 according to the invention has been made consisting of three cells arranged together in such a way as to be stacked and form a parallelepiped.

The metastruttura 1 was made of polyamide PA 2200 by means of a selective laser sintering process (SLS).

The lower surface of the fabricated metastructure was glued onto an electrodynamic exciter, or “shaker,” of the Data Physics V20 type having a winding resonance of approximately 12 kHz.

Two miniaturised monoaxial accelerometers, of the PCB 352C23 model, have been fixed on the plate of the shaker and on the upper surface of the metastructure 1 to detect the input and output signals.

Their sensitivities are 5.39 mV/g and 5.11 mV/g, respectively, with a frequency range of 2 to 10,000 Hz. A signal acquisition/generation system, of the mobile Siemens LMS SCADAS type, was used to read the signal of the two accelerometers and to provide the input signal (that is, the white noise) to the shaker using an amplifier (Gearing & Watson Power Amplifier PA 30).

For the acquisition, the chosen bandwidth is 0-10,000 Hz (to stay below the resonance of the shaker and within the accelerometer measurement field) with a frequency resolution of 1 .25 Hz.

The number of means used for each measurement is 64.

For the accelerometer signals, the Hanning window was chosen to avoid harmonic noise problems.

The experimental transmission diagram is obtained directly from the Siemens Simcenter TestLAB software by calculating the ratio between output signals and the input signals.

The experiments clearly demonstrate the design strategy: the attenuation of more than two orders of magnitude is evident in the frequency range from about 1478 Hz to 10,000 Hz (which is the upper operating limit frequency for the accelerometers, considering a linearity of 5%).

Moreover, the metastructure 1 is tested under operating conditions. In this configuration, an accelerometer (which measures the response in operating conditions) is attached to the lower surface of the metastructure 1 , whilst the upper surface is fixed to a worm screw in a manual press.

The metastruttura 1 is pushed against the plate of the shaker, maintaining the contact between the response accelerometer and the test surface, on which a control accelerometer is installed by means of beeswax.

The excitation is delivered to the shaker in a controlled way: a feedback circuit on the control accelerometer makes it possible to maintain the amplitude of the acceleration constant and equal to 1 g without the occurrence of problems linked to the resonances of the system.

By calculating the ratio between the signal recorded by the accelerometer which measures the response (positioned on the bottom of the metastructure 1 ) and that recorded by the control accelerometer, called the Interfering Effect (IE), it is possible to assess the intrusiveness of the metastructure 1 on the measurement.

A ratio of one, that is to say, IE equal to 1 , indicates that both accelerometers measure the same acceleration (ideal condition).

The graph below shows that the probe made with the metastructure 1 has a constant response of up to approximately 7 kHz, close to an IE of 1 .1 .

Comparing the response of the metastruttura 1 with the response of a commercial silicone probe, it can be inferred that the response of the metastruttura 1 is better more than 2 kHz compared with the commercial silicone counterpart.

The metastructure 1 has a constant response up to approximately 7 kHz; above 7 kHz it is no longer constant but deviates less than the commercial silicone solution.

For this reason, the metastructure 1 has less of an influence on the accelerometer response and the measurement is performed correctly.

The metastruttura 1 according to the invention is an innovative metastruttura as a promising solution for vibration test applications.

In particular, it was designed to be used as a manual probe, thus guaranteeing reduced dimensions, lightness, flat transmission diagram and a low-pass filter behaviour in the frequency range of interest, that is to say, 1 -10 kHz. io

Using the separation mechanism of the methods recently proposed in the literature, the metastructure 1 has reduced dimensions (that is, 4 cm of single cell 2) which shows a dynamic low-pass filter response, in the frequency range from 1478 Hz to 10,000 Hz, and an adequate structural strength for the application considered.

Experimental tests have shown that a prototype made up of 1x1x3 unitary cells, manufactured with SLS process, has an attenuation effect of unwanted vibrations of more than two orders of magnitude.

In addition to being a promising solution for vibration test applications, the metastruttura 1 is a good candidate for a future marketable manual probe.