Eccentric load compensated load cell

The invention relates to a load cell for measuring mechanical loads and forces comprising an elastic body fitted with sensors for measuring the deflection of a membrane loaded by the load or force to be measured.

The load cell shown in figure 1 is a well known design for measuring compression forces or loads.

The normally cylindrical elastic load cell body 1 is placed with the rim 2 on a supporting structure and the load is applied to the membrane 3 through a load button 4 at the top of the load cell.

An insulating electrode carrier 5 is mounted in the cavity 6 of the load cell body 1, by means of the elastic supports 7.

A conductive layer 8 applied at the electrode carrier 5 forms, with the lower side of the membrane 3, a sensor capacitance which changes value when the membrane is deflected by the load or force to be measured.

An electronic module 9 converts the sensor capacitance to a signal which is brought to the outside of the load cell through a cable conduit.

The cavity 6 of the load cell is closed by the membrane 10.

The signal from the sensor measuring the deflection or the strain will, in known designs, be different from the correct value when the load is applied eccentrically or when the load has a force component not parallel to the axis of the load cell.

It is the object of the invention to provide capacitive load cells with electrodes arranged to compensate for eccentric loads or loads applied at an angle to the axis of the load cell.

According to the invention this object is achieved by arranging mechanically coupled conductive surfaces, deflected by the load or force to be measured, each side of an electrode carrier produced from insulating material with conductive electrodes mounted on each side of the electrode carrier to face the mechanically coupled conductive surfaces to form two or more sensor capacitances.

The mechanically coupled conductive surfaces are in preferred embodiments of the invention provided with means for the adjustment of the area or the gap of the sensor capacitances.

This way and according to the invention loads and forces applied eccentrically or with an angle to the axis of the load cell may be measured with a high accuracy.

The invention will now be described in further detail.

Figure 2 shows the elements of the known design of Figure 1 with the addition of the conductive surface 11 mechanically coupled to the conductive surface constituted by the lower side of the membrane 3.

The coupling is performed by mounting the inner circumference of the conductive surface 11 on the cylindrical part 12 of the elastic body 1.

The outer circumference of the mechanically coupled conductive surface 11 is in figure 2 mounted at the inner wall of the cylindrical part of the elastic body 1. hi this way the deformation of the coupled conductive surface 11 will closely follow the deformation of the membrane 3.

The conductive surface 11 forms a sensor capacitance with the conductive layer 13, applied to the lower side of the insulating electrode carrier 5.

Figure 3 shows the insulated electrode carrier 5 with the upper electrode 8. Figure 4 shows the insulated electrode carrier 5 with the lower electrode 13.

The electrode carrier 5 may preferably be produced of high stability ceramic material and the electrodes 8 and 13 may preferably be applied as silver electrodes by thick film technology.

The inner and outer diameters of the electrodes 8 and 13 may be equal or different, but the areas will preferably be concentric with the membrane 3.

These diameters and the distance between the electrode 8 and the membrane 3 and the distance between the electrode 13 and the coupled conductive surface 11 will be chosen as a combination for best linearity of the measurement.

Figure 5 shows an embodiment of the mechanically coupled conductive surface 11 with the means 14 for mounting the surface 11 to the inner wall of the cylindrical part of the elastic body 1 and the means 15 for coupling the conductive surface 11 to the cylindrical part 12 of the elastic body 1.

The cuts 16 in the coupled conductive surface 11 may tailor the deformation of 11 to enable a suitable relation between the deformation of the membrane 3 and the surface 11.

Figure 6 shows an embodiment of the mechanically coupled conductive surface 11 with the means 14 for mounting the surface 11 to the inner wall of the cylindrical part of the elastic body 1 and the means 15 for coupling the conductive surface 11 to the cylindrical part 12 of the elastic body 1.

The cuts 16 in the coupled conductive surface 11 may tailor the deformation of 11 to enable a suitable relation between the deformation of the membrane 3 and the surface 11.

The cuts 17 permits a difference in the coefficient of thermal expansion between the elastic body 1 and the mechanically coupled conductive surface 11 to be equalized when the ambient temperature changes.

The sensitivity of a capacitive load cell to eccentric loads and forces is due to the non linear relation of the distance between the electrodes of the sensor capacitance and the capacitance. With an eccentric load applied to the load cell of figure 1, the membrane will deflect mostly at the side where the load is applied and to a lesser degree at the opposite side of the membrane 3.

Because of the nonlinear characteristic of the sensor capacitance, with a higher sensitivity with a smaller distance, the load cell of figure 1 will tend to give a higher signal with an eccentric load.

The disclosed embodiment of the invention, shown in figure 2, relies on the coupled conductive surface 11 to be deflected in a manner closely matching the deflection of the membrane 3 when eccentrically loaded.

Because the electrodes 8 and 13 are coupled differentially to the capacitance measuring electronic module 9 the effect of the eccentric load is compensated to a high degree.

The inner and outer diameters of the electrodes 8 and 13 may be tailored to adjust the compensation.

Figure 7 shows an embodiment of the invention with the mechanically coupled conductive surface 11 mounted only at the inner circumference to the cylindrical part 12.

Hereby the mechanically coupled conductive surface 11 will follow the movement of the cylindrical part 12, but will not be deformed the same manner as the membrane 3.

It will for centric loads constitute one part of a differential sensor capacitance together with the electrode 13 and the other part of the differential sensor capacitance will be constituted by the membrane 3 and the electrode 8.

The embodiment of the invention, shown in figure 7, relies on the coupled conductive surface

11 to be tilted in a manner closely matching the tilting of the membrane 3 when eccentrically loaded.

By tailoring the inner and outer diameters of the electrodes 8 and 13 the compensation may be performed to a high degree.

The sensor electrode 18 will be more sensitive to the tilting of the mechanically coupled conductive surface 11 because of the greater distance to the center of the load cell, but will be equally sensitive to the sensor capacitances 8 and 13 for centric loading.

By tailoring the diameters and hereby the area of the sensor capacitance 18 it will be possible to use this signal in the electronic module to adjust the compensation to eccentric loads.

Figure 8 shows an embodiment of the mechanically coupled conductive surface 11 as preferably implemented in the disclosure according to figure 7.

The mechanically coupled conductive surface 11 is mounted on the cylindrical part 12 by means of 15 and the cuts 17 will enable differences in the thermal expansion between 11 and the cylindrical part 12 to be equalized.

The cuts 16 will enable the four segments shown of mechanically coupled conductive surface

11 to have the distance to the sensor capacitances 13 and 18 adjusted individually simply by bending one or more of them in a suitable manner.

Figure 9 shows a preferred embodiment of the load cell according to the invention where one additional mechanically coupled conductive surfaces 18 is placed on the cylindrical part 12 to provide a sensor capacitance with the electrode 8.

The advantage in this embodiment is due to the identical characteristics and deflection of the mechanically coupled conductive surface 11 and the mechanically coupled conductive surface 18.

Figure 10 shows the electrode carrier of a preferred embodiment of the load cell according to the invention where one or both of the electrode surfaces 8 and 13 are divided into two or more sections of electrode areas.

In figure 10, three sections are shown, and preferably, but not necessarily each section is measured separately by the electronic module 9.

The advantage in this embodiment is due to the possibility of tailoring the characteristics of the electrode areas separately.

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 the number and the position of the mounting means 14 and 15 and the cuts 16 and 17 in the mechanically coupled conductive surface 11 may be varied according to the application.

As another example the electrode carrier itself is not necessarily produced of insulating material, but could be produced of any suitable dimensionally stable material applied with insulated layers or insulated parts to support the capacitive electrodes.

Also the lateral groove between the membrane 3 and the load cell body 1 may be tailored to provide a sufficient deformation of the membrane 3 without transferring excessive stresses to the load cell body 1.