[FIELD OF THE INVENTION]

The invention relates to load cells for measuring loads, comprising an elastic body fitted with sensor means to detect the strain or the deformation of the elastic body in response to the load to be measured.

The invention relates more specifically to a precision load cell comprising an elastic body where the elastic body comprises a first beam and a second beam that is positioned opposite to the first beam, a base end and a load receiving end that is positioned oppo- site the base end where the first and the second beams are connected to the base end and to the load receiving end via flexure points that provide sections for elastic deformation, further a sealed cavity comprising a flexible wall and sensor means for measuring the elastic deformation of the elastic body in response to the load to be measured.

[BACKGROUND] Numerous sensor types are designed with sensor means adapted in a sealed cavity with the sensor means activated by a lever brought through a flexible wall in the sealed cavity. The flexible wall protects the sensor means from humidity and corrosion. Examples are joy sticks and simple force sensors, but precision measurements are in prior art only achieved by precision load cells provided with a parallelogram. Patent EP1451538, describes a precision load cell with sensor means mounted in a sealed cavity placed in the base end of the load cell.

The load cell, according to the patent EP1451538 does in practice display some problems.

One problem arises from temperature induced distortions of the walls of the sealed cavity for example when the rays of the sun shines on one of the walls, increasing the dimensions of this wall and hereby distorting the sealed cavity and influencing the sensor means, which give a transient shift of the zero. Because of the comparably extended geometry of the cavity with an appreciable distance between the walls, temperature equalization will not be complete, take a certain time and the zero signal will therefore only partly, and then slowly return to the correct value.

A further problem is the temperature rise above ambient temperature of the sensor means, caused by the power generated by the electronic signal processing circuits, which normally are mounted in the base end of the load cell near the sensor means. This temperature rise is difficult to compensate for in the zero and slope of the signals of the load cell as it is added to the temperature of the environment.

Another problem is the changing distortion of the cavity and the sensor means through a changing eccentricity of the loads applied on the load cell, which changes the forces in the two beams of the parallelogram. Because the beams are directly hinged on the sealed cavity, the changing forces in the beams will result in varying deformations of the walls of the sealed cavity and hereby influence the sensor means. This changing deformation of the walls of the sealed cavity, with changing eccentricity of the applied loads reduces the obtainable accuracies of the load cell.

Still another problem is the distortion of the sealed cavity and the sensor means when the mounting screws in the base end are tightened for mounting the load cell.

US Patent Application Publication No. US 2003/0111277 describe a number of precision load cells with sensor means placed in a sealed cavity in the base end of the load cell and with a lever connecting a flexible wall of the sealed cavity in the base end to the load receiving part.

The load cells according to US 2003/0 11277 displays the same problems as the load cell according to EP1451538.

US patent 5,052,505 and US Patent Application Publication No. US 20 /0232393, both describe load cells where a sealed cavity for mounting sensor means is created by milling a recess in one of the beams of the parallelogram and closing the cavity with a cover.

Load cells according to these patents do both display a problem common to this type of load cell where strain gages are applied to the flexure points of the parallelogram. When a load cell of this design is loaded, the strain gage at one flexure point of a beam will see a positive strain from the bending of this flexure point while a strain gage at the other flexure point of the same beam will see a negative strain. Besides the bending strains there will however also appear strains in the flexure points from the longitudinal forces in the direction of the beam. These forces and strains will have the same sign in both flexure points of a beam and should ideally cancel each other when the signal of the load cell is measured in the normal Wheatstone bridge coupling of the strain gages. Inaccuracies in the strain gages themselves and in the bonding of the strain gages will however result in measurement errors with changing eccentricity of the load applied to the load cell, and these errors are tedious and time consuming to correct with filing or grinding of the flexure points.

It is the object of the invention to provide load cells with sealed cavities with sensor means which are thermally insulated from other parts of the load cell and which are not influenced by variations in the forces in the beams when varying the eccentricity of the load applied to the load cell and which are not influenced by tightening the mounting screws for the load cell.

[GENERAL DESCRIPTION]

According to the invention, this object is achieved by providing a precision load cell comprising an elastic body where the elastic body comprises a first beam and a second beam that is positioned opposite to the first beam, a base end and a load receiving end that is positioned opposite the base end where the first and the second beams are connected to the base end and to the load receiving end via flexure points that provide sections for elastic deformation, and a sealed cavity comprising a flexible wall and sensor means for measuring the elastic deformation of the elastic body in response to the load to be measured with the sealed cavity placed in the first beam and/or the second beam with the flexible wall and sensor means placed in the body of the beam between the flexure points of the beam. The precision load cell further comprising a lever having a first end and a second end, where the first end is connected to the flexible wall of the sealed cavity and the second end is connected to one or more of the opposite beam, the base end or the load receiving end, in order to transform any relative movement of the second ends of the lever into deformations of the flexible wall in response to the load to be measured. This way and according to the invention the load to be measured, may be measured by various types of sensor means placed in a sealed cavity, which because of a limited geometry and close thermal coupling of the walls provides a fast and nearly complete temperature equalization of the elastic body around the sealed cavity. Furthermore the thermal insulation between the beams and the base end and the load receiving end provided by the flexure points of the beams, limits influx of heat into the sealed cavity and the sensor means. The low strains in the solid walls each side of the sealed cavity placed in a beam provides very low changes of the deformation of the flexible wall of the sealed cavity under varying eccentricity of the load. With the sealed cavity and the sensor means placed in a beam between flexure points no influence on the measurement is seen when tightening the screws for mounting the solid base end and solid load receiving end in an application of the load cell.

In an embodiment of the load cell according to the invention the sealed cavity is placed in the first beam with the flexible wall and sensor means placed in the body of the beam between the flexure points of the beam. The load cell further comprising a lever having a first end and a second end, where the first end is connected to the flexible wall of the sealed cavity and the second end is connected to the opposite beam in order to transform any relative movement of the opposite beam into deformations of the flexible wall in response to the load to be measured. The advantage gained by this embodiment is a sealed cavity with sensor means insulated from humidity, thermal and stray mechanical stresses and a simple activation of the sensor means through the relative displacement of the beams transferred by the lever.

In an embodiment of the load cell according to the invention the sealed cavity is placed in the first beam with the flexible wall and sensor means placed in the body of the beam and between the flexure points of the beam. The load cell further comprising a first lever having a first end and a second end, where the first end is connected to the flexible wall of the sealed cavity and the second end is connected to the opposite beam through a flexure point and a second lever connected to the opposite beam in order to transform any relative movement of the opposite beam into deformations of the flexible wall in response to the load to be measured. The further advantage gained by this embodiment is the possibility to tailor the deformations of the flexible wall according to the specific applications of the load cell by varying the lengths of the levers.

In an embodiment of the load cell according to the invention the sealed cavity is placed in the first beam with the flexible wall and sensor means placed in the body of the beam between the flexure points of the beam. The load cell further comprising a lever having a first end and a second end, where the first end is connected to the flexible wall of the sealed cavity and the second end is connected to the load receiving end in order to transform any relative movement of the load receiving end into deformations of the flex- ible wall in response to the load to be measured.

In an embodiment of the load cell according to the invention the sealed cavity is placed in the first beam with the flexible wall and sensor means placed in the body of the beam between the flexure points of the beam. The load cell further comprising a lever having a first end and a second end, where the first end is connected to the flexible wall of the sealed cavity and the second end is connected to the base end in order to transform any relative movement of the base end into deformations of the flexible wall in response to the load to be measured.

In an embodiment of the load cell according to the invention sealed cavities are placed in the first beam with flexible walls and sensor means placed in the body of the beam between the flexure points of the beam. The load cell further comprising levers having first ends and second ends, where the first ends are connected to flexible walls of the sealed cavities and the second ends are connected to the opposite beam and/or to the load receiving end and/or to the base end in order to transform any relative movement of the second ends of the levers into deformations of the flexible walls in response to the load to be measured.

In an embodiment of the load cell according to the invention sealed cavities are placed in the first beam and in the second beam with flexible walls and sensor means placed in the bodies of the beams between the flexure points of the beams. The load cell further comprising levers having first ends and second ends, where the first ends are connected to flexible walls of the sealed cavities and the second ends are connected to the flexible walls in the opposite beam and/or to the opposite beam and/or to the load receiving end and/or to the base end in order to transform any relative movement of the second ends of the levers into deformations of the flexible walls in response to the load to be measured.

The advantage gained by this embodiment is the possibility to have dual independent measuring systems for critical applications of the load. In embodiments of the load cells according to the invention first levers connected to flexible walls protrudes through the flexible walls into the sealed cavities for the activation of sensor means.

In embodiments of the load cells according to the invention first levers connected to flexible walls of sealed cavities activates sensor means in the sealed cavities by deforming the flexible walls.

In embodiments of the load cells according to the invention, the sensor means are ca- pacitive and/or inductive and/or resistive and/or optic.

In embodiments of the invention flexure points may be inserted anywhere in the levers for tailoring the deformations of the flexible walls in response to the load to be measured. In embodiments of the invention a flexible wall may have different thicknesses at certain areas in order to facilitate the deformations of the flexible wall in response to the load to be measured.

In embodiments of the invention a flexible wall may have different thicknesses at certain areas in order to facilitate the measurement of the strain in the flexible wall in response to the load to be measured.

In an embodiment of the load cell according to the invention, electronic signal processing circuits are mounted in the sealed cavity and/or in the base end and/or the load receiving end of the load cell.

The advantage gained by this embodiment is that only a few, and then only low power components of the signal processing circuits are mounted in the sealed cavity while the heat generating higher power parts of the signal processing circuits are mounted in the base end. From the base end, the heat may quite easily be dissipated to the environment and will not reach the sealed cavity and the sensor means because of the thermal insulation provided by flexure points of the beams.

In an embodiment of the load cell according to the invention, electronic signal processing circuits which are mounted in the sealed cavity are connected to electronic signal processing circuits in the base end of the load cell through connections adapted in flexure points of the beams of the load cell.

By placing the connections and the preferably laser welded covers as close as possible to the neutral layer in the material of the flexure point, which is around half thickness of the flexure point, the connection and the cover are only bended and not stretched or compressed when the flexure point undergoes deformations, when the load cell is loaded and this reduces any influence on the measurement to an insignificant size.

By increasing the thickness of a flexure point where the connection between the electronic processing circuits is placed, the machining of the groove in the flexure point and the laser welding of the cover is easier to perform.

[BRIEF DESCRIPTION OF DRAWINGS]

The invention is explained in detail below with reference to the drawings, in which

Fig. 1 shows, as prior art a simple sensor with a lever protruding through a flexible wall into a sealed cavity in the sensor body for activating sensor means.

Fig. 2 shows, as prior art, a precision load cell with an elastic body, comprising a base end with a sealed cavity, a load receiving part and a parallelogram consisting of two beams connecting the base end and the load receiving part, and a lever connecting the load receiving part with a flexible wall of the sealed cavity for activating sensor means, adapted in the sealed cavity in the base end.

Fig. 3 shows, as prior art, a precision load cell with an elastic body, comprising a base end, a load receiving part and a parallelogram consisting of two beams connecting the base end and the load receiving part, with a sealed cavity in one of the beams with strain gages applied to the flexure points of the beam. Fig. 4 is a basic embodiment of the invention with an elastic body, comprising a base end, a load receiving part, a first beam with a sealed cavity with a flexible wall and sensor means activated by a first lever protruding through the flexible wall of the sealed cavity and connected to the second beam through a flexure point and a second lever.

Fig. 5 is an example of capacitive sensor means.

Fig. 6 is a basic embodiment of the invention with an elastic body, comprising a base end, a load receiving part, a first beam with a sealed cavity with a flexible wall and sensor means activated by a first lever connected to the flexible wall of the sealed cavity and connected to a second beam through a flexure point and a second lever.

Fig. 7 is a basic embodiment of the invention in a situation with the elastic body loaded.

Fig. 8 is a basic embodiment of the invention in another situation with the elastic body loaded and the flexure point closed.

Fig. 9 is an embodiment of the invention with electronic signal processing circuits divided between the sealed cavity and the base end of the load cell.

Fig. 10 A is an embodiment of the invention with the electronic signal processing circuits in the sealed cavity and the signal processing circuits in the base end of the load cell connected through connections adapted in flexure points of the beams.

Fig. 10 B is the embodiment of the invention of Fig. 10A showing the cover for the sealed cavity.

[DETAILED DESCRIPTION OF DRAWINGS]

The sensor in Fig. 1 shows, as prior art, a simple sensor where the lever 1, is protruding into the sealed cavity 2, placed in the sensor body 3, through the flexible elastic wall 4, for activating the capacitive sensor means, by moving the grounded vane 5, in relation to the capacitance sensors 6 an 7 when the lever 1 is activated. The capacitance sensors 6 and 7 are connected to the capacitance measuring circuit 8.

This type of sensor, with different sensor means, is widely used as a joy stick for con- trolling machinery and the flexible wall protects the sensor means from humidity and corrosion in machinery in tough environments.

It is easily seen that a sensor according to Fig. 1 is quite unsuitable as a load cell because the forces for activating the sensor means are dependent on the length of the lever 1. The load cell of Fig. 2 shows, as prior art, a precision load cell with an elastic body, comprising a base end 1 with a sealed cavity 2, a load receiving part 3, and a parallelogram consisting of the two beams 4 and 5, connecting the base end 1 and the load receiving part 3, a lever 6 connecting the load receiving part 3 with a flexible wall 7 of the sealed cavity 2, for activating sensor means 8, adapted in the sealed cavity 2, which also contains the signal processing circuit 9.

The heat generated by the signal processing circuit 9 is causing a temperature rise of the sealed cavity 2 which will be added to the temperature of the environment and hereby make the temperature compensation difficult.

A changing eccentrically applied load on the load receiving part 3, will give changing forces in the beams 4 and 5, which again will give dimensional changes of the sealed cavity 2 and hereby influence the measurement.

With the comparably thin walled sealed cavity it is also difficult in practise to avoid distortions of the sealed cavity and shifts of the zero signal when tightening the screws for mounting the base end of the load cell in the application. The load cell of Fig. 3 shows, as prior art, a precision load cell with an elastic body, comprising a base end 1 , a load receiving part 2 and a parallelogram consisting of the two beams 3 and 4, connecting the base end 1 and the load receiving part 2, a sealed cavity 5 in the beam 3 with strain gages 6 and 7, applied in the flexure points 8 and 9. A cover 10 seals the cavity 5 and the strain gages 6 and 7 from the environment. When a load is applied to the load receiving part 2, the strain gage 6 will be stretched by the bending of the flexure point 8, while the strain gage 7, will be compressed by the bending of the flexure point 9 and it is these strains which result in the signals from the strain gages which are normally connected in a Wheatstone bridge configuration. Lon- gitudinal forces will however also be present in the beams; but they will be the same at both ends of the beam and will give strains of the same sign in the gages which ideally will cancel in the Wheatstone bridge configuration. Even relatively small differences in the characteristics and the bonding of the strain gages will however give errors when a changing eccentricity of the load give changing longitudinal forces in the flexure points. The invention will now be described in further details with reference to Fig. 4, which is a basic embodiment of the invention with an elastic body, comprising a base end 1 , a load receiving part 2, the beam 3 with the sealed cavity 4 and the beam 5, each beam with flexure points 6 each end, a lever 7 protruding into the sealed cavity 4 through the flexible wall 8 for activating the capacitive sensor means 9, adapted in the sealed cavity 4, with the lever 7 connected to the beam 5 through the flexure point 10 and the lever 11.

The load cell according to Fig. 4 will only show small measurement errors from transient temperature shifts of the environment because of a rather limited size and comparably thick walls of the sealed cavity. Errors from heat dissipated from the electronic circuits mounted in the base end as shown in Fig. 9 will be insignificant because of the thermal insulation provided by the small cross section of the flexure points.

Errors from changing eccentricity of the load applied to the load cell are reduced significantly because the solid sidewalls of the sealed cavity will absorb the longitudinal forces in the beams without the flexible wall and sensor means being influenced.

Errors caused by tightening the mounting screws are strongly reduced because of the mechanical insulation provided by the flexure points between the solid base end, the solid load receiving end and the sensor means placed in the sealed cavity in the body of the beam. Fig. 5 is an example of capacitive sensor means with the flat U shaped spring 1 , the insulating body 2 with conducting areas 3 and 4 for connecting to suitable capacitance measuring circuits and with the anchor points 5 and 6 which are for example to be welded to the end of the lever 7 of Fig. 4 and to a suitable point in the sealed cavity 3. Fig. 6 is another basic embodiment of the invention with an elastic body, comprising a base end 1 , a load receiving part 2, a beam 3 with the sealed cavity 4 and the beam 5, each beam with flexure points 6 each end. A lever 7 connected to the flexible wall 8 of the sealed cavity 4 activates the strain sensor means 9 for measuring the deformations of the flexible wall when the lever 7 is connected to the beam 5 through the flexure point 10 and the lever 1 .

The load cell according to Fig. 6 will show the same robustness against unwanted stray influences as the load cell according to Fig. 4.

Fig. 7 is an embodiment of the invention of Fig. 4 where the elastic body is loaded while the flexure point 10 is opened. The deflections of the beams 3 and 5 are shown exaggerated to demonstrate how the levers 7 and 11 are shifted in relation to each other, but also that the flexible wall 8 does not undergo deformations with the flexure point 10 opened.

Fig. 8 is the basic embodiment of the invention of Fig. 4 where the elastic body is loaded, the flexure point 10 is closed and the deflections of the beams 3 and 5 are shown exaggerated to demonstrate how the flexible wall 8 undergoes deformations through the angular deflection of the lever 7 when the lever 7 is connected to the beam 3 through the flexure point 10.

It is seen from Fig. 7 and Fig. 8 that the angular deflection of the lever 7 is a simple function of the angular deflections of the beams 3 and 5 only, which is a prerequisite for a stable measurement.

Fig. 9 is a basic embodiment of the invention where the electronic signal processing circuits are divided in a circuit 13, which is mounted in the sealed cavity 4 and a circuit 14, which is mounted in the base end 1 of the load cell. By this embodiment, where the components of the circuit 13 in the sealed cavity 4 are preferably only a few low power components which generate essentially no rise of the temperature, only the temperature shifts of the environment have to be compensated.

The heat generated by the possibly higher power parts of the signal processing circuit 4, which is mounted in the base end 1 , may quite easily be dissipated to the environment and will not reach the sealed cavity because of the thermal insulation provided by flexure points 6 of the beams.

Fig. 10A is another basic embodiment of the invention where the connection 15 between the circuit 13 in the sealed cavity 4 and the signal processing circuit 14 in the base end 1 is adapted in a flexure point of a beam.

The connections 15 are preferably placed in grooves milled in a flexure point 6, with the groove preferably being sealed together with the sealed cavity 4 by the preferably laser welded cower 16.

Fig. 10B is showing the cover 16 of the load cell of Fig. 10A