GB2219096A - Weighing machine - Google Patents

Description

1 2219096 WEIGHING MACHINE The present invention relates to a weighing

machine and has particular reference to weight position sensing in a weighing machine.

In a known weighing machine, for example as disclosed in EP 00 55 633, strain gauges are provided for the determination of a load-induced turning moment. However, because.of the smallness of their output signal, considerable cost and complication is involved in the necessary electronic evaluating system. Moreover, capacitive spacing-sensors are provided to measure the horizontal movement of a pan carrier relative to fixed points of the machine housing. If the overall height of the weighing machine is to remain low, thenthis horizontal movement is very small and correspondingly only a small output signal results. In addition, the frequently nonlinear yielding of the guide rod parallel guide enters into the measurement result, so that the calculation of the turning moment is made more difficult. In both constructions, the overall height-of the weighing machine is appreciably increased by the sensors.

There is therefore a need for a mode of construction of weighing machine with corner load sensing which may entail a smaller expenditure for the sensors and an associated electronic evaluating system and not lead to increase in the overall height of the weighing machine.

According to the present invention there is provided a weighing machine comprising load pick-up means, a load pan bearing on the pickup means by way of a resilient support member, measuring means to measure a turning moment exerted on the pick-up means by a weight on the pan, a substantially planar plate mounted on the pick-up means, and at least three sensors arranged to measure vertical deformation, dependent on magnitude and position of the weight, of the support member relative to the plate.

The spacing of the sensors from a point of mounting of the supp6rt member on the load pick-up means can be made greater through utilisation of the vertical movement of the support member than in the case of utilisation of the horizontal movement. Thereby, the output signal of the sensors is substantially greater and the overall height of the weighing machine is hardly influenced. Due to mounting of the spacing sensors on a plate which is fastened to the load pick-up means, the yielding of an associated guide rod parallel guide does not result in a signal from the sensors, so that the properties of the guide do not enter into the measurement.

Embodiments of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a perspective overall view of a weighing machine embodying the invention; Fig. 2 is a schematic cross-sectional view of the weighing machine and a block schematic diagram of the electronic system, with a first form of corner load sensing; is a plan view of a support member of the machine; is a elevation of an optical sensor in a second form of corner load sensing; and is a cross-section, along the line V4 of Fig. 4, of the optical sensor.

Referring now to the drawings, there is shown in Fig. 1 a weighing machine comprising a housing 3, a weighin"g pan 34, an indicator 19, a taring key 12 and further operating keys 35.

Fig. 2 shows the interior of the weighing machine schematically in Fi g. 3 Fig. 4 Fig. 5 r T 4 3 - section, with the housing 3 largely omitted. The weighing system consists of a carrier 1 to which a load pick-up 2 is fastened to be movable in vertical direction by way of two guide rods 4 and 5 with hinges 6. The load pickup 2 transmits a force corresponding to the mass of the weighed load by way of a coupling element 9 to a load arm of a transmission lever 7. The lever 7 is mounted on the carrier 1 by a cross spring joint 8. A coil body with a coil 11 is mounted on a compensating arm of the lever 7. The coil 11 is disposed in an air gap of a permanent magnet system 10 and generates a compensating force. The magnitude of the compensating current through the coil 11 is regulated in such a manner by a position sensor 16 and a regulating amplifier 14 that equilibrium prevails between the weight of the weighed stock and the electromagnetically generated compensating force. The compensating current generates a measurement voltage, which is conducted to an anal og- to-di gi tal con- verter 17, across a measuring resistor 15. The digitalised result is taken over by a digital signal processing unit 18 and indicated digitally in the indicator 19. These parts of the weighing system of the electronic weighing machine can be generally known and have therefore been described only briefly.

The connection of the weighing pan 34 with the load pick-up 2 takes place by way of a lower pan or support member 20, which is constructed to be resiliently yielding and on the one hand is connected with the load pick-up 2 by way of a spacer member 27 and screws 28 and on the other hand carries the weighing pan 34 by way of support elements 30.

The deformation, which is dependent on the magnitude and position of the load on the pan 34, of the member 20 is determined by at. least three spacing sensors. Only one of these sensors, which is in the form of a flat coil 25, is recognisable in Fig. 2. The coil 25 is mounted on a plate 24 of insulating material, this plate 24 also being fastened to the load pick-up 2. The coil 25 functions as an eddy current sensor and measures the spacing from the member 20, which consists of an electrically conductive material such as sheet metal. The coil 25 is connected by way of a movable wiring connection 26 with an evaluating circuit 13, which derives a spacing signal from the change in impedance of the coil 25 and passes it on to the digital signal processing unit 18. The unit 18 can then correct corner load errors of the parallel guide, as described in DE-PS 30 03 862.

The coil 25 can, of ocurse, also consist of a conductor track applied in spiral shape on the upper side of the plate 24.

Due to the connection of the coils 25 to the load pick-up 2, only the deformation of the member 20 is measured, whilst load-dependent changes in position of both the member 20 and the plate 24 - for example, due to deformation of the guide rod parallel guide for the pick-up 2 - do not lead to change in the measurement signal of the coils 25. The plate 24 for mounting the coils 25 can be thin, since it need carry only the coils 25 and is not loaded by the load on the weighing pan.

Fig. 3 shows a preferred form of the support member 20 in plan view. The member 20 consists of a central part 22, which is connected by way of four screws 28 with the pick-up 2, and of four resilient arms 21, which carry the support elements 30 for the weighing pan 34. The bowing of each arm 21 is measured by a respective spacing sensor under- neath each arm 21.

An optical spacing sensor is shown in Figs. 4 and 5 as an alternative to the eddy current spacing sensor. Fig. 4 is a side elevation and Fig. 5 a section along the chain-dotted line V4 in Fig. 4. The optical sensor consists of an optical transmitter 32, which is mounted in a block 23 on the plate 24, and an optical receiver 33, which is mounted in a block 29 also on the plate 24. A light stop 31, which is connected with the resilient member 20, enters into the light path between the transmitter 32 and receiver 33. The light stop 31 varies the amount of transmitted light on perpendicular movement of the stop relative to the light path, so that a spacing-dependent output signal results at the receiver 33.

The characteristic of the sensor can be influenced and, for example, linearised by appropriate shaping of the light stop 31.

The two forms of spacing sensors described in the preceding are, of course, merely examples. Sensors of other kinds can also be used, for example capacitive sensors, differential transformers, reflection-optical or fibre-optical sensors and so forth. Equally, the form of construction of the support member 20 shown in Fig. 3 represents only one example.

The resilient support member and spacing sensors can also be used when the load pick-up is parallelly guided not by two guide rods, but by, for example, a lever arrangement in the form of a scale beam weigh- ing machine.

Claims (7)

1. A weighing machine comprising load pick-up means, a load pan bearing on the pick-up means by way of a resilient support member, measuring means to measure a turning moment exerted on the pick-up means by a - weight on the pan, a substantially planar plate mounted on the pick-up means, and at least three sensors arranged to measure vertical deformation, dependent on magnitude and position of the weight, of the support member relative to the plate.

2. A weighing machine as claimed in claim 1, wherein the support member comprises at least three support arms.

3. A weighing machine as claimed in either claim 1 or claim 2, wherein the sensors are arranged on the plate.

4. A weighing machine as claimed in claim 3, wherein the support member comprises electrically conductive material and each of the sensors comprises a flat coil and means to measure change in impedance thereof on change in spacing of the coil and the support member.

5. A weighing machine as claimed in claim 3, wherein each of the sensors comprises light emitting and receiving means to provide a light path and the support member is provided with a respective light stop associat- ed with each sensor and arranged to interrupt the light path of that sensor to a varying extent dependent on the deformation of the support member.

4 11 f L

6. A weighing machine substantially as hereinbefore described with reference to Figs. 1 to 3 of the accompanying drawings.

7. A weighing machine substantially as hereinbefore described with reference to Figs. 1, 2, 4 and 5 of the accompanying drawings.

e Published 1989 atThe Patent Office, State House, 66/71 High Ho1boMLDndonWC1R4TP. Further copies maybe obtaJnedfromThe Patent Office.

Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Gon. 1/87