GB2107062A - Electrical weighing systems - Google Patents

Abstract

A weighing system comprises a weigh cell having strain gauge sensors in a bridge (16) which provides an output voltage which is a measure of the weight (14) on the cell. The bridge (16) is powered by a voltage source (17). The voltage signal provided by the bridge (16) is received via amplifier (18) by a voltage-frequency converter (20) and is transformed to a variable frequency signal dependent on the weight (14). Processing circuitry (12) which provides the weight indication receives the variable frequency signal from converter (20) and a reference frequency signal which can be provided by voltage-to- frequency converter (22) driven by a reference voltage source (24). Source (24) may be a predetermined fraction of reference source (17) in which case a calibration network is connected in parallel with bridge (16) and selectively switched by a switch to the input of converter (20) to provide predetermined frequency values equivalent to respective predetermined values of weight (14), see Fig. 3. <IMAGE>

Description

SPECIFICATION Weighing systems This invention relates to weighing systems.

Known weighing systems basically fall into two classes determined by the operating principles of the weigh cell. In one class the weigh cell incorporates strain gauge sensors which produce a variable amplitude output signal whereas the other class of weigh cell incorporates vibrating string sensors which produce a variable frequency output signal. In both classes the output signal is processed to provide the usable weight data to an output device which may for example be a digital display device and accordingly the known weighing systems have processing facilities between the weigh cell and the output device.

It is an object of the present invention to provide an interface circuit for a weigh cell incorporating strain gauge sensors to render such weigh cell compatible with the processing facilities and output device appropriate to a vibrating string sensor weigh cell.

Accordingly the present invention provides an interface circuit for a weigh cell incorporating strain gauge sensors, comprising a voltage-tofrequency converter arranged for receiving a variable amplitude output signal from a weigh cell incorporating strain gauge sensors, and means providing a reference frequency whereby processing circuitry may be arranged to derive weight data from the output signal of the voltageto-frequency converter and the reference frequency.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 illustrates a weighing system with a known form of vibrating string weigh cell; Fig. 2 illustrates a weighing system according to the present invention; Fig. 3 illustrates a modification of Fig. 2; and Fig. 4 illustrates a modified detail of Fig. 3.

The principle of a vibrating string type weighing system is illustrated in Fig. 1. The weight cell 10 generates two frequencies f1,f2, the ratio of which is a measure for the weight on the cell. The weight indicator 12 uses these two frequencies, f, and f2, to determine their ratio, and then determines the weight of article 14 out of this ratio. Indicator 12 comprises an AND gate 1 2A to which frequency f1 is fed directly, being the frequency which increases in value due to the presence of weight 14, whereas frequency f2 is divided by a fixed number N in divider 128 prior to being fed to gate 1 2A, being the frequency which decreases in value due to the presence of weight 14.The output of gate 1 2A is fed to a counter 1 2C which counts the frequency pulses applied thereto during a time interval equal to N/f2. This time interval could be fixed by fixing f2, so that f2 is independent of the weight 14, but by making the time interval related to f2 the effects of temperature and other perturbing effects are eliminated from the weight determination. Divider 128 and counter 1 2C are each controlled by a 'clock' signal on line 1 2D which acts intermittently to reset divider 1 2B and counter 1 2C. When divider 1 2B is reset its output goes high and remains high until it has counted N of the f2 pulses when the output goes low.When the output of divider 128 is high gate 1 2A is enabled and therefore counter 1 2C receives and counts the f, pulses until such time as the output of divider 1 28 goes low when the count terminates.

Thus the count determined by counter 1 2C equals fi N.- f2 and is therefore a measure of weight 14. This sequence of operation is of course repeated by the next clock signal on line 1 2D. the clock signal may be free running or simply initiated as a single pulse, or predetermined number of pulses, by an operator.

Fig. 2 illustrates a strain gauge based weighing system according to the present invention. This system incorporates the same weight indicator 1 2 as described in Fig. 1. As is known, the strain gauge bridge 1 6 provides an output signal V1, of which the amplitude is a measure for the weight on the cell but is usually so small that it needs amplification by amplifier 18.

Unlike the usual A/D converter of the prior art, the system of Fig. 2 uses a voltage to frequency converter 20 to transform the amplified output signal of the strain gauge bridge to a frequency signal f1, of which the frequency is dependent upon the measured weight.

To obtain compatibility with the aforedescribed vibrating string system it might be necessary that frequency f1 is not equal to zero when weight 14 is zero, but this could be easily obtained by introducing an offset voltage somewhere in the circuit before the V/F converter 20. A reference frequency f2 is obtained using a second V/F converter 22 fed by a stable reference voltage 24.

It would aiso be possible to feed this second V/F converter 22 by the bridge supply voltage 17, or rather a part thereof, which would have the advantage that the system becomes a ratio measuring system, eliminating the influence of any non-stability of the bridge supply voltage 1 7.

Another method of obtaining f2 would be to use a stable (crystal based) oscillator. In this case of course a very stable bridge supply voltage would be required, and if this method is used, it is possible to feed the bridge circuit 1 6 from the reference voltage required in each V/F converter 20, 22, (although not shown in Fig. 2) which would again reduce the system to a ratio measuring system. The signals f1 and f2 are then fed to weight indicator 1 2 having the same construction and function as that in Fig. 1.

The system of Fig. 2 is very suited for automatic calibration as is shown in Fig. 3. In this case a voltage dividing network 25 is connected in parallel with the bridge supply voltage 17. The input of the amplifier 1 8 is then, from time to time switched over from the strain gauge bridge 1 6 to a first output of the divider network 25, or to a second output of the divider network 25 by means of a switch 26. The first output of network 25 short-circuits the input terminals of amplifier 18 to provide a common mode voltage which determines a first value of t8 1 equivalent to a first predetermined value of weight 14.This value off1 will not be zero nor will it correspond to the value off1 when weight 14 is zero which because of unavoidable imbalance of the Wheatstone bridge circuit 1 6 and the presence of mechanical weightimposing components forming part of cell 10 will not be zero. The second output of the network 25 provides a defined portion of the bridge supply voltage 1 7 to amplifier 1 8 which is equivalent to a second value off1 at a second predetermined value of weight 14.

Switch 26 therefore inputs two frequencies, through the f1 channel, to the weight indicator 12 which then determines whether or not these frequencies are equal to the frequencies that were determined when the load cell incorporating bridge 1 6 was manufactured. These original manufacturing frequencies are stored within the system, for example in a PROM and if a difference is established, this this difference is used to compute a correction factor for f1 and/or the ratio f1/f2 during subsequent use of the system in determining an unknown weight. Of course the weight indicator 12 initiates the switching of switch 26 by means of a signal on line 28 to a switch actuator 29.

It will be evident that the dividing network 25 must meet high standard, since it determines the accuracy and stability of the entire weighing system, and network 25 enables the following errors to be compensated for: a) drift of the bridge supply voltage 1 7, b) drift of the amplifier 18, c) drift of the voltage-to-frequency converter 20, d) drift of the circuit 22 that generates f2.

Fig. 4 illustrates a modified form of the network 25 shown in Fig. 3 utilising fewer components but functionally similar.

The system according to the present invention can be used where full compatibility with vibrating string type cells are used as well as where only partial compatibility is requiired, for example, in a microcomputer based system where the hardware is compatible but the software not.

Transfer of weight data in the present invention occurs through a frequency modulated system which enhances reliability of data transfer in relation to the hitherto conventional analogue signal transfer.

Claims (filed 8-9-82) 1. A weighing system comprising a weigh cell incorporating strain-gauge sensors and processing circuitry arranged to derive weight data from the weigh cell output signals, wherein an interface circuit is electrically connected between the weigh cell and the processing circuitry, said interface circuit comprising a voltage-to-frequency converter arranged for receiving a variable amplitude output signal from the weigh cell, and means providing a reference frequency, and the processing circuitry is arranged to derive the weight data from the output signal of the voltage-to-frequency converter and the reference frequency.

2. A weighing system as claimed in claim 1, wherein said means providing a reference frequency comprises a reference voltage source and a voltage-to-frequency converter.

3. A weighing system as claimed in claim 2, wherein the strain-gauge sensors of the weigh cell are arranged on a bridge circuit which is powered by the said reference voltage source.

4. A weighing system as claimed in claim 3, wherein a calibration network is connected in parallel with said reference voltage source and a switching arrangement is provided selectively to connect the voltage-to-frequency converter of the interface circuit either to the output of the bridge circuit or to the output of the calibration network.

5. A weighing system as claimed in claim 1, and substantially as hereinbefore described with reference to any one of Figs. 2-4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   input of the amplifier 1 8 is then, from time to time switched over from the strain gauge bridge 1 6 to a first output of the divider network 25, or to a second output of the divider network 25 by means of a switch 26. The first output of network 25 short-circuits the input terminals of amplifier 18 to provide a common mode voltage which determines a first value of t8 1 equivalent to a first predetermined value of weight 14. This value off1 will not be zero nor will it correspond to the value off1 when weight 14 is zero which because of unavoidable imbalance of the Wheatstone bridge circuit 1 6 and the presence of mechanical weightimposing components forming part of cell 10 will not be zero.The second output of the network 25 provides a defined portion of the bridge supply voltage 1 7 to amplifier 1 8 which is equivalent to a second value off1 at a second predetermined value of weight 14.

   Switch 26 therefore inputs two frequencies, through the f1 channel, to the weight indicator 12 which then determines whether or not these frequencies are equal to the frequencies that were determined when the load cell incorporating bridge 1 6 was manufactured. These original manufacturing frequencies are stored within the system, for example in a PROM and if a difference is established, this this difference is used to compute a correction factor for f1 and/or the ratio f1/f2 during subsequent use of the system in determining an unknown weight. Of course the weight indicator 12 initiates the switching of switch 26 by means of a signal on line 28 to a switch actuator 29.

   It will be evident that the dividing network 25 must meet high standard, since it determines the accuracy and stability of the entire weighing system, and network 25 enables the following errors to be compensated for: a) drift of the bridge supply voltage 1 7, b) drift of the amplifier 18,

   c) drift of the voltage-to-frequency converter 20, d) drift of the circuit 22 that generates f2.

   Fig. 4 illustrates a modified form of the network 25 shown in Fig. 3 utilising fewer components but functionally similar.

   The system according to the present invention can be used where full compatibility with vibrating string type cells are used as well as where only partial compatibility is requiired, for example, in a microcomputer based system where the hardware is compatible but the software not.

   Transfer of weight data in the present invention occurs through a frequency modulated system which enhances reliability of data transfer in relation to the hitherto conventional analogue signal transfer.

   Claims (filed 8-9-82) 1. A weighing system comprising a weigh cell incorporating strain-gauge sensors and processing circuitry arranged to derive weight data from the weigh cell output signals, wherein an interface circuit is electrically connected between the weigh cell and the processing circuitry, said interface circuit comprising a voltage-to-frequency converter arranged for receiving a variable amplitude output signal from the weigh cell, and means providing a reference frequency, and the processing circuitry is arranged to derive the weight data from the output signal of the voltage-to-frequency converter and the reference frequency.
2. 2. A weighing system as claimed in claim 1, wherein said means providing a reference frequency comprises a reference voltage source and a voltage-to-frequency converter.
3. 3. A weighing system as claimed in claim 2, wherein the strain-gauge sensors of the weigh cell are arranged on a bridge circuit which is powered by the said reference voltage source.
4. 4. A weighing system as claimed in claim 3, wherein a calibration network is connected in parallel with said reference voltage source and a switching arrangement is provided selectively to connect the voltage-to-frequency converter of the interface circuit either to the output of the bridge circuit or to the output of the calibration network.
5. 5. A weighing system as claimed in claim 1, and substantially as hereinbefore described with reference to any one of Figs. 2-4.