Load cell

Details Load cell Load cell

2003-10-31

2004-12-21

Noori, Max (Department: 2855)

Measuring and testing

Dynamometers

Responsive to force

Reexamination Certificate

active

06832528

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a load cell with a force transducer for recording a weight, the force transducer having a part which does not deform under loading and a force introduction part with an elastically deformable part, the elastically deformable part and the non-deforming part having in a measuring portion a defined spacing in relation to each other which changes under loading, with a sensor arrangement with an inductively operating sensor element, which is disposed in the measuring portion opposite a signaling face, in order to detect changing of the spacing as an electrical signal, and with a circuit for converting the electrical signal into a weighing signal.
Such a load cell is known for example from DE 44 20 691 C1.
Other known force transducers are in many cases based on the deformation being detected as a surface expansion. Typical of these are strain-gage transducers, in which the surface expansion occurring when there is deformation is converted into a change in electrical resistance.
These strain-gage transducers achieve very high degrees of accuracy, but can be protected against ambient influences only with great effort, since the strain-sensitive sensor elements are located directly on the elastically deforming part and covering of the sensors has the concomitant effect of force bypasses, which have direct adverse effects on the properties of the transducer.
In the case of capacitively operating load cells, the elastically deformable part is connected to an electrode of a plate capacitor, deformation of the elastically deformable part caused by the force to be measured producing a change in capacitance.
The inductively operating load cell known from DE 44 20 691 C1 is designed specifically for applications in weighing technology and proves to be unsuitable under extreme ambient conditions, in particular such as occur for example in a motor vehicle when detecting the weight of occupants, since it is too susceptible to interference.
For such an application, a load cell must have the following special properties:
The load cell must have compact outside dimensions, in order to conform to the very confined installation space in the case of such applications. At the same time, it must be mechanically stable, to be able to withstand the very great forces possible in the operation of a motor vehicle.
The load cell must be designed with respect to the sensor element and measured value conversion in such a way that the very high ambient field strengths possible in a motor vehicle do not have any harmful influence on the measuring accuracy. For instance, electrical field strengths of up to 200 V/m can occur in a motor vehicle, an extremely high value in comparison with industrial ambiences of 10 V/m.
The load cell must be capable of operating in a large operating temperature range of from −40° C. to +85° C. with rapid temperature changes and also possible condensation.
The load cell must be able to withstand very high numbers of stress cycles, as occur in the operation of a vehicle due to the forces of acceleration, without the technical measuring properties changing.
In the course of introducing airbags as standard for the passenger seat in motor vehicles there arises the problem of detecting the occupancy of the seat, so that the passenger airbag is released for firing only in those cases in which the seat is occupied. Firing of the passenger airbag when the passenger seat is not occupied not only has the consequence that an additional increase in pressure in the vehicle is unnecessarily caused in the event of an accident by the firing of the passenger airbag, but also that, due to the integration of the passenger airbag into the dashboard, considerable repair measures are necessary in the passenger compartment of the vehicle following firing of the airbag.
Various problems which have been observed when airbags are fired with small children or youngsters traveling on the passenger seat have ultimately led to the requirement that, in the event of an accident, an airbag must be operated in a way which is adapted to the person sitting on the vehicle seat.
This gives rise to the problem of determining the necessary data for firing the airbag in a way appropriate for requirements.
One possible reference point is to detect the weight of the respective vehicle occupant, the size and physical robustness of the occupant then being concluded on the basis of a weight determination.
The invention therefore relates in particular to a load cell which can be used in ambiences with extreme conditions, for example in a motor vehicle.
In terms of measuring technology, the vehicle interior is extremely problematical, since not only do interference signals of the engine ignition (electromagnetic fields with high field strengths) occur to a considerable extent and have to be eliminated in signaling terms, but it is also necessary with respect to the functional capability of sensors that are to be used to take into account extreme vibrations, temperature conditions in a range from, for example, −40° C. to +85° C., rapid temperature changes and also atmospheric humidity ranging right up to the dew point.
Furthermore, account must be taken of the fact that not only the weights of the occupants act on the seat but also, when the vehicle accelerates and, in particular, decelerates suddenly, forces which are many times greater, not to mention the forces occurring in an accident situation, which likewise must not lead to malfunctioning of the sensors.
As well as this there are the various adjusting possibilities for a vehicle seat which are provided in modern motor vehicles and may only influence the measuring result within defined limits.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to develop a load cell of the type described at the beginning further in such a way that it can be used in particular in conditions which are very difficult in terms of measuring technology, and in particular under the other special ambient conditions within a vehicle when detecting the weight of occupants, and the weighing signal of which is substantially uninfluenced by this.
This object is achieved in the case of the load cell stated at the beginning according to the invention by the force transducer having a recess in the elastically deformable part or the non-deforming part in the region of the measuring portion, by the sensor element being disposed in the recess such that it is shielded against external electromagnetic fields and encapsulated, and by the load cell operating on the eddy current principle.
The load cells operating inductively according to the invention can be constructed in such a way that they are very insusceptible to interference and in particular also very robust, which is of special significance in the case of installation in a seat of a motor vehicle, since great forces can act on the seat even in normal driving operation, during acceleration and deceleration. In the event of an accident, the forces acting on the load cell are many times greater.
On account of the sensor element being disposed in a recess in the measuring portion of the force transducer, protection against mechanical effects on the sensor element is obtained. Since, furthermore, the force transducer is generally produced from metallic material, for example steel or aluminum, shielding against electromagnetic interference fields is also provided as a result.
In spite of the shielding against electromagnetic interference fields, the sensor arrangement is permeable to electromagnetic fields on its side facing the signaling face, dependent on its functional type.
On account of the real part of the measuring signal being evaluated (complex permeability) and the imaginary part ignored, shielding against electrical fields is of no importance. There is no need for shielding intended for this purpose.
The encapsulation allows influences of temperature and atmospheric humidity to be suppressed or even eliminated entirely.
The load cell according to the invention

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