WO2009089836A1 - Tubular sensor capacitive load cell - Google Patents

Abstract

The invention relates to a precision load cell with an elastic body comprising a cylindrical body connected to a load receiving part by a membrane provided with one or more tubular members with non contacting sensors for sensing the deformation of the tubular members due to the load or force to be measured.

Description

Tubular sensor capacitive load cell

The invention relates to load cells for measuring mechanical loads and forces, comprising an elastic body fitted with sensors for measuring the strain in the elastic body or the deformation of the elastic body in response to the load or force to be measured.

The invention relates more specifically to a precision load cell with an elastic body comprising a cylindrical body connected to a load receiving part by a membrane provided with one or more tubular members, one or more cylindrical cavities in the cylindrical body and sensor means mounted in the cavities adapted for sensing the deformation of the tubular member due to the load or force to be measured.

The load cell disclosed in figure 1 is a widely used load cell of the aforementioned type, fitted with strain gage sensor means which, mounted at the end surfaces of the tubular members, measures the deformation of the tubular member due to the load or force applied to the load cell.

Because of the well known limitations of the strain gage sensor technology this load cell is sensitive to shocks and overloads and it is well known that the maximal allowed strain in the elastic body of around 0.001 limits the change of parameter to 0.002 which, when multiplied with the voltage applied to the strain gages, results in a rather low signal of typically 20 mV.

Furthermore, the circular strain gages in this well known load cell type have to be placed with a very high accuracy, concentrically on the surfaces of the tubular members, in order to provide load cells insensitive to eccentrically applied loads and in practice it is extremely difficult to obtain a high yield of the production.

US4811610 A (Gassmann) is a further example of prior art with the abovementioned limitations.

It is the object of the invention to provide load cells of the aforementioned precision type, fitted with sensor means which are not in contact with the elastic body, whereby a high degree of tolerance to overloads is ensured, and which provide a substantial change of the parameter of the measurement, which results in a high level of the signal. Furthermore it is the object of the invention to provide load cells where the sensor means are simple to mount accurately, resulting in load cells which at the same time are insensitive to eccentrically applied loads and easy to produce.

According to the invention this object is achieved by a load cell of the initially mentioned precision type with an elastic body comprising a cylindrical body connected to a load receiving part by a membrane provided with one or more tubular members with sensor means, one or more cylindrical cavities formed by said cylindrical body, said load receiving part and flexible membranes and non contacting sensor means mounted in the cavities adapted for sensing the deformation of the tubular members due to the load or force to be measured.

SUBSTITUTE SHEET This way and according to the invention the load or force to be measured may be measured with high precision in environments with overloads and shock loads and by applying capacitive sensor means the change of parameter may be as high as 20% which provides very high signals compared to strain gage sensors.

In preferred embodiments of the invention, the capacitive sensor means are mounted on supports designed to be easy to adjust for concentricity of the measuring surface of the capacitive sensor means and the tubular member.

In other preferred embodiments of the invention the capacitive sensor means are mounted on supports designed to be easy to adjust for concentricity while at the same time providing compensation to eccentric loads.

In another preferred embodiment of the invention, the material for the capacitive sensor means and the mounting of the capacitive sensor means are chosen to provide automatic compensation for the temperature dependence of the modulus of elasticity E of the elastic body.

In still another preferred embodiment of the invention, the membrane connecting the cylindrical body to the load receiving part is situated laterally nearly at the same level as the load button whereby the load cell is mechanically stable even when heavy side loads are encountered.

In a preferred embodiment of the invention, differential sensor means are applied to the same sensor carrier in order to provide a simple low cost load cell.

In another preferred embodiment of the invention, the tubular elements are provided with slits to facilitate the deformation for low forces or loads to be measured.

In still another preferred embodiment of the invention, the tubular elements are provided with grooves to compensate creep and hysteresis of the elastic body.

In all embodiments of the invention a signal processing unit may advantageously be mounted in the cylindrical cavity with the signal being conducted to the outside of the cylindrical body through a cable conduit or a suitable connector and in all embodiments of the invention the cylindrical cavities may advantageously be closed by flexible membranes in order to separate the sensor means from the environment.

The advantage obtained by the disclosed capacitive sensor systems is the very high sensitivity to the forces and loads applied to the load cells and the non contacting measuring principle, which gives a high tolerance to shocks and overloads as only the elastic body is loaded and not the sensor system. Because of the high sensitivity of capacitive sensor systems the elastic body is stressed far below the levels used by strain gage load cells.

Figure 1 shows, as prior art, a load cell with an elastic body comprising a cylindrical body connected to a load receiving part by a membrane provided with two tubular members with strain gages applied to the end surfaces of the tubular members sensing the deformation of the tubular members due to the load or force to be measured.

Figure 2 shows a horizontal cross section of the load cell elastic body of figure 1.

Figure 3 shows the deflection, exaggerated for clearness, of the membrane 3 and the tubular elements 4, 5.

Figure 4 shows a load cell according to the invention with capacitor electrode rings with electrodes facing the inner and outer surfaces of the tubular members.

Figure 5 is a horizontal cross section of a load cell according to the invention where the elastic body and the inner and outer capacitor electrode rings are shown.

Figure 6 shows a version of a capacitance load cell according to the invention with the capacitor electrode rings mounted with mounting means at the surfaces of the cylindrical body and the load receiving part.

Figure 7 shows a version of a capacitance load cell according to the invention with the capacitor electrode rings mounted with springs at the end surfaces of the tubular members.

Figure 8 shows a version of a capacitance load cell according to the invention with the capacitor electrode rings mounted on tubular mounting means fastened to the surfaces of the membrane.

Figure 9 shows the tubular mounting means with a flexible flange.

Figure 10 shows, exaggerated for clearness, the angular deformation of a cross section of the membrane and the tubular member.

Figure 11 shows a version of a capacitance load cell according to the invention with capacitor electrode rings mounted to face only the inner surfaces of the tubular member.

Figure 12 shows a version of a capacitance load cell according to the invention with a tubular member mounted at only one side of the membrane and with capacitor electrode rings mounted to face both the inner and outer surface of the tubular member. Figure 13 shows a version of a capacitance load cell according to the invention with a tubular member mounted at only one side of the membrane and with a capacitor electrode ring mounted to face only the inner surface of the tubular member.

Figure 14 shows a horizontal cross section of version of a capacitance load cell according to the invention with a tubular member with slits.

Figure 15 shows a version of a capacitance load cell according to the invention provided with grooves machined in the tubular members.

Figure 16 shows a version of a capacitance load cell according to the invention where the part of the membrane between the cylindrical body and the outer perimeter of the tubular members and the part of the membrane between the inner perimeter of the tubular members and the load receiving part have triangular cross sections which are joined by a thin flexible section.

The load cell in figure 1, demonstrates prior art.

The elastic body comprising the cylindrical body 1, is connected to the load receiving part 2, by the membrane 3, which is provided with the tubular members 4, 5. In load cells according to prior art, circular strain gages are bonded to the cylindrical end surfaces of the tubular elements 4, 5.

The load cells according to prior art does suffer from the well known limitations of the strain gage technology such as sensitivity to overloads and it is also readily seen that the mounting of the circular strain gages on the end surfaces of the tubular elements is a difficult task.

Furthermore the physical dimensions of the strain gages places a lower limit on the thickness of the tubular elements, whereby a limit is placed on the lowest load cell capacity possible with this technology.

Figure 2 is a horizontal cross section of the elastic body of the load cell of figure 1.

Figure 3 is showing the deformation, exaggerated for clearness, of the membrane 3 and the tubular elements 4, 5 of the load cell of figure 1.

Figure 4 shows a load cell according to the invention with the capacitor electrode rings 7, 8, 9, 10 with the electrodes 6 facing the tubular element.

The capacitor electrodes may be metallic rings supported by insulating mounting means in the shown positions or preferably ceramic rings with thick film electrodes 6.

Figure 5, is a horizontal cross section of the load cell of figure 4. In figure 6, a load cell according to the invention is shown with the capacitor electrode rings 7, 8, 9, 10 mounted by the mounting means 11 to the inner surface of the cylindrical part 1 and the surface of the load receiving part 2.

This mounting method provides a stable mounting of the capacitor electrode rings, but it is readily seen that the eccentric load, as shown applied to the load cell, will tilt the load receiving part 2 to the left and at the same time tilt the tubular element 4, 5 excessively to the right and because the characteristic of distance sensing capacitive sensors is non linear an error will result.

In figure 7, the capacitor electrode rings 7, 8, 9, 10 are mounted with springs 12 which allow the ends of the tubular elements to expand respectively to contract relative to the fixed diameters of the capacitor electrode rings 7, 8, 9, 10, when a load is applied to the load receiving part 2.

It is readily seen that an eccentric load will only have an insignificant effect, because the capacitor electrode rings will tilt together with the tubular elements 4, 5 and the average change of distance will be measured.

By adjusting the springs 12 it is easy to center the capacitor electrode rings in relation to the tubular element 4, 5.

In the load cell according to the invention, shown in figure 8, the capacitor electrode rings 7, 8, 9, 10 are mounted on tubular mounting means 13, which are fixed to the membrane 3, at suitable positions.

The tubular mounting means are shown in figure 9, with the flexible flange 14, which is fixed to the membrane 3.

In figure 10, the angular deflection x of the membrane 3 is shown with the angular deflection y of the tubular element 4, 5.

It is readily seen that the deflection x necessarily is higher than the deflection y which means that the tubular elements 13, with the capacitor electrode rings are deflected away from the tubular element when an eccentric load is applied to the load cell.

In the load cell disclosed in figure 11, only the capacitor electrode rings 7 and 9 are mounted and the tubular elements 4 and 5 are shown tapered to provide an optimal deformation when a load is applied to the load cell.

If the capacitor electrode rings 7 and 9 are ceramic rings with thick film electrodes 6 facing the tubular elements 4, 5 the lower coefficient of thermal expansion of the ceramic relative to the coefficient of the material of the tubular element increasing temperatures will increase the distance between the electrodes 6 and the tubular elements 4,5. An increasing distance will lower the sensitivity of the capacitive sensor and hereby enable a compensation of the decrease of the elastic coefficient of the material of the tubular elements with increasing temperature.

In the load cell according to figure 12, only one tubular element is implemented whereby the lateral distance between the load receiving part 2 and the membrane 3 may be kept low, which gives the load cell the ability to tolerate high side forces.

In figure 12 a signal processing module 15 and a membrane 16 are shown.

In figure 12 , the electrodes 6 are shown placed at different positions relative to the length of the tubular element 5, whereby the linearity of the capacitive measurement may be tailored.

These elements may be implemented in all embodiments of the invention.

In figure 13, a load cell is disclosed with only one capacitance electrode ring which provides a differential signal because the deflection of the tubular element 5 is higher at the ends.

In figure 14 the tubular elements are shown with slits to facilitate the deformation at low capacities for the load cell.

In figure 15 an embodiment of the invention is shown with grooves in the tubular elements.

At the groove, the elastic material will be stressed at a high level and the material at the groove will creep.

The creep of the material, when the groove is placed at a suitable position on the tubular element will result in a relaxation of the deflection of the ends of the tubular element and thus compensate the general creep of the load cell.

In the embodiment of the invention shown in figure 16, where the part of the membrane between the cylindrical body and the outer perimeter of the tubular members and the part of the membrane between the inner perimeter of the tubular members and the load receiving part have triangular cross sections which are joined by a thin flexible section, it will be possible to choose a combination of the dimensions of these sections which result in a combined stress in the thin flexible section which is very low even if the shear stress of the thin flexible section is rather high.

Instead of the capacitor electrodes, inductive sensors in the form of coils wound on coil carriers and facing the surfaces of the tubular elements may, according to the invention, be used as eddy current sensors.

The advantage obtained by inductive sensor systems, is the possibility of functioning in extreme environments and the non contacting measuring principle which gives a high tolerance to shocks and overloads as only the elastic body is overloaded and not the sensor system. Due to the fact that preferred embodiments of the invention has been illustrated and described herein it will be apparent to those skilled in the art that modifications and improvements may be made to forms herein specifically disclosed.

Accordingly, the present invention is not to be limited to the forms specifically disclosed.

For example all the disclosed embodiments may be combined to provide load cells optimized to special applications and in all embodiments the circular electrodes may be divided in segments, preferred will be segments of 60, 90 or 120 degrees. The division of the electrodes in segments will provide the possibility of an individual measurement on each segment, whereby a highly accurate compensation of the small residual errors resulting from eccentrically applied loads may be obtained.

Claims

Claims

LA precision load cell with an elastic body comprising a cylindrical body connected to a load receiving part by a membrane provided with one or more tubular members with sensor means, one or more cylindrical cavities formed by said cylindrical body, said load receiving part and flexible membranes characterised in that said sensor means are non contacting sensors, mounted in said cavities and adapted for sensing the deformation of the tubular members due to the load or force to be measured.

2. Load cell according to claim 1, cha r a c t e r i z ed in that the sensor means are capacitive.

3. Load cell according to claim 1, cha r a c t e r i z ed in that the sensor means are inductive.