Weighing scales – Computer – Electrical
Reexamination Certificate
1998-05-27
2001-01-23
Gibson, Randy W. (Department: 2859)
Weighing scales
Computer
Electrical
C177S185000, C073S001130, C702S098000, C702S099000, C702S100000, C702S101000
Reexamination Certificate
active
06177637
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to systems for sensing the value of a physical property (such as force, temperature or movement) and having a plurality of sensors used in combination. One example of such a system is a weighing system where a plurality of load cells are employed to sense the magnitude of a load placed on a weighing platform.
In a weighing system, beneath the weighing platform is the bottomwork which includes the load cell or cells. Where there are multiple load cells, it is in practice impossible to guarantee an equal distribution of the load between them: some cells receiving a slightly larger weight than others. Due to imperfections in any load cell, the output signal will not be a linear function of the weight imposed on the cell. The unequal distribution of weight amongst the load cells will vary depending on the position of the load on the weighing platform. As a result, the non-linearity will introduce an error whose value will vary according to the positioning of the load on the weighting platform. This, in turn, will cause the weight readings to vary dependent on the position of the load. This is called cornering error and represents a small fraction of the load being measured.
Traditionally, two basic methods have been used to correct the errors. The first method is to wire all of the cells in parallel, each with its own empirically derived resistor added in order to compensate for differences between cells. This provides a single signal which can be measured by a low speed, high resolution analogue/digital (A/D) converter. However setting up of the compensation resistors is an iterative process that can be time consuming and expensive. This method can not correct for cornering errors caused by cell non-linearity.
The second method is to measure the output of each cell individually to the full accuracy requirement of the system. Measurements are taken by a single A/D converter that is switched to the output of each cell in turn. Such an arrangement is prone to vibration related interference. In order to avoid problems with typically low frequency vibration, a high speed A/D is required. The system can now resolve the weight on each individual cell allowing faster single pass calibration. In addition cell non-linearity can also be corrected. However high speed, high resolution A/D's are very expensive.
SUMMARY OF THE INVENTION
According to the present invention there is provided a sensing system comprising a plurality of sensors, switching means, a low pass filter, a first analogue to digital (A/ID) converter, and a second A/D converter capable of faster operation than said first A/D converter, said plurality of sensors for sensing the magnitude of a physical property, each sensor arranged to provide a separate output signal having an amplitude related to the magnitude of the physical property; said switching means arranged to sequentially connect said sensor output signals to the low pass filter, said low pass filter having an output to said first analogue to digital (A/D) converter, said switching means also arranged to sequentially connect the sensor output signals to said second A/D converter; wherein said first A/D converter is arranged to produce a first signal comprising information relating to the average of the physical property sensed by said plurality of sensors, and wherein said second A/D converter is arranged to produce a second signal comprising information relating to the distribution of sensor output signal amplitudes amongst the sensors.
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GEC Avery Limited
Gibson Randy W.
Kirschstein et al.
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