GB2256254A

GB2256254A - Damping device

Info

Publication number: GB2256254A
Application number: GB9111443A
Authority: GB
Inventors: Peter Huggins
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-05-28
Filing date: 1991-05-28
Publication date: 1992-12-02
Also published as: GB9111443D0

Abstract

A load cell 10 or other force measuring device has a damping device coupled to its input element. The damping device comprises parallel plates 21 and 22 and a body 30 of resiliently compressible material disposed between them and impregnated with liquid e.g. water. Plate 21 receives the load and plate 22 is coupled to the input element of the load cell 10: any changes in applied load (e.g. if the load impacts onto plate 21 under momentum) are damped by liquid being forced out of the body 30 under compression and reabsorbed as the body recovers. The body 30 may be a sponge which is enclosed with water in a plastics bag 23. <IMAGE>

Description

DAMPING DEVICE This invention relates to a damping device particularly for use with a load cell or other force-measuring device.

It is often necessary in industry to weigh products coming off a production line for quality control purposes.

However, owing to the settling time of the weighing device, it is not possible to weigh every product without significantly reducing the product flow rate.

Modern electronic weighing devices incorporate a force measuring device known as a load cell. A typical load cell comprises a plurality of strain gauges mounted on a beam disposed inside a chamber. When an object is weighed, it stresses the beam: the strain gauges mounted on the beam provide an electrical output signal proportional to the weight of the load. If the load is dropped or suddenly applied onto the weighing device, the beam may oscillate for over a second before the output signal settles to a correct value. It is this initial oscillation of the beam which causes an increase in the overall weighing time.

It is known to fill the internal chamber of the load cell with a viscous liquid to damp the oscillation and reduce the settling time. As the beam oscillates, the viscous liquid acts to absorb energy and damp the oscillations. By careful design of the shape of the load cell chamber and the beam, critical damping can be achieved to reduce the weighing times.

Load cells damped in the above-mentioned way reduce the settling time to 40 milliseconds. However, it is not possible to retrofit existing weighing devices with the same damping arrangement, nor is this arrangement applicable to load cells of a different construction.

We have now devised a damping device which can be fitted to many types of load cell or other force measuring device and may be retrofitted to existing devices.

In accordance with this invention there is provided a damping device comprising a first element for receiving a load, a second element for exerting an output force dependent upon the applied load, and a body of resiliently compressible material disposed between the first and second elements and impregnated with a liquid or other flowable substance and arranged to expel said liquid or substance when compressed and to absorb liquid or other flowable substance when recovering.

Preferably the impregnated body reabsorbs the same liquid, upon recovery, as it expelled when compressed, but it may instead expel liquid into one reservoir and absorb fresh liquid from another reservoir.

Preferably the impregnated body is enclosed within a flexible container (e.g. a plastic bag) which contains the liquid or flowable substance when expelled from the body under compression of the latter. A preferred liquid is water.

In use, the damping device damps the load force applied to the first or input element. The device may be provided at the input of a load cell used for weighing: the load to be weighed is placed on the first or input element and the second or output element is coupled to the load cell. The impregnated, resiliently compressible body of the damping device effectively damps the applied load, for example when the load impacts the input element with significant momentum.

Stops may be provided to limit the movement of the input element towards the output element of the damping device, to prevent damage to the device.

Also in accordance with this invention there is provided a force measuring device, having an input element for receiving a force and arranged to give an output signal dependent upon the force received by said output element, and a damping device coupled to said input element so that the applied force is received by the input element via the damping device and any changes in the applied force are damped before application to the input element.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a perspective view showing a load cell and a damping device for the load cell; and FIGURE 2 is a sectional view of the damping device when fitted to the input of a load cell.

Referring to the drawings, there is shown a weighing device having a load cell comprising a rectangular block 10 of metal formed adjacent one side with a large rectangular aperture 11. The aperture 11 is divided by a horizontal beam 12 on which four strain gauges 13, 13, 14, 14 are mounted.

Threaded holes 15 are formed through the top edge of the load cell above the aperture 11. The metal block 10 is mounted to a support 8 via a spacer 9 at its bottom corner opposite the aperture 11.

The load cell is provided with a damping device, comprising a first plate assembly which comprises a rectangular plate 21 with downwardly projecting side walls formed on two opposite edges, and a second plate assembly which comprises a rectangular plate 22 with upwardly projecting side walls formed on two opposite edges, the two plate assemblies being interlocked to form a chamber of cuboid-shape. The side walls of the upper plate assembly are formed with vertical slots 25 in their edges, and the side walls of the lower plate assembly are formed with through holes 31. Dowels 24 fit tightly into the holes 31 and project into the slots 25, so that the top and bottom plates 21,22 are free to move up and down in relation to each other through a limited distance.

A sponge 30 enclosed within a plastics material bag 23 is disposed inside the chamber which is defined between the two plate assemblies. The open pores of the sponge are impregnated with water before the bag 23 is sealed. The bag 23 is preferably a loose fit around the sponge 30, and spaces 32,33 are left between the sides of the bag 23 and the vertical sides of the chamber.

A load platform 29 is fixed to the plate 21, which is supported by the sponge 30. The second plate assembly has a bearer 28 spaced from its underside by packing pieces 27. The damping device is secured to the load cell by two bolts 26 which pass through the bearer 28 and are tightened into the holes 15 on the top edge of the load cell.

In use, when a load is placed on the load platform 29, the impact force due to its momentum causes the sponge 30 to be compressed between the two movable plates 21,22. As the sponge 30 is compressed, water is expelled from it, but remains contained within the bag 23, which can expand sideways into the spaces 32,33. The initial downwards movement of the applied load will stop, and the sponge will partially recover thus re absorbing some of the expelled water: the load may oscillate up and down several times before coming to rest. However, owing to the weight of the applied load, the sponge will not completely recover, but will remain partially compressed. The weight of the applied load is transmitted to the body 10 of the load cell, subjecting the body 10 to a turning moment about its mounting point 9 and thus stressing the beam 12.The strain gauges 13,13, 14,14 mounted on the beam 12 are connected into an electrical bridge which gives an electrical output signal proportional to the weight of the load.

If a load were to impact onto the platform 29 with a large momentum, the sponge 30 might be so greatly compressed that the bag 23 might burst. However, the dowels 24 within the slots 25 restrict the distance through which the two plate assemblies can move towards each other.

It will be appreciated that the load cell 10 may be used to measure forces other than weight, such as pressure.

A damping device, in accordance with this invention, is suited for use in all force measuring applications.

Whilst a load cell of the stressed-beam type has been described, any form of mechanical or electrical devices may be fitted with the damping device, e.g. a spring balance.

In cases where the load measuring device is mounted to a vibrating surface, a second damping device may be fitted between the device and that surface to prevent the measuring device from sensing the vibrations.

A damping device in accordance with this invention is able to reduce the settling time of a force measuring device by over 25 times.

Claims

CLAINS

(1) A damping device comprising a first element for receiving a load, a second element for exerting an output force dependent upon the applied load, and a body of resiliently compressible material disposed between the first and second elements and impregnated with a liquid or other flowable substance and arranged to expel said liquid or substance when compressed and to absorb liquid or other flowable substance when recovering.

(2) A damping device as claimed in claim 1, in which the impregnated body is enclosed within a flexible container which contains the liquid or flowable substance when expelled from said body under compression of the latter.

(3) A damping device as claimed in claim 1 or 2, in which the liquid with which the body is impregnated is water.

(4) A damping device as claimed in any preceding claim, in which said first and second elements comprise parallel plates.

(5) A damping device as claimed in claim 4, in which the two plates are slidable towards and away from each other between opposed side walls which enclose said impregnated resilient body.

(6) A damping device as claimed in any preceding claim, further comprising stop means to limit the movement of said first and second elements towards each other.

(7) A force measuring device comprising an input element for receiving a force and arranged to give an output signal dependent upon the force received by said input element, and a damping device as claimed in any preceding claim, the damping device having its second element coupled to said input element so that an applied force is received by said input element via the damping device.