GB2176897A - Weighing machine - Google Patents

Abstract

A weighing machine comprises weight sensing apparatus including two identical weigh cells housed in a single frame and operating in conjunction with each other. Means are provided for combining the outputs 3 and 10 of the weigh cells in mutual opposition, one of the weigh cells being arranged to carry a package to be weighed while the second weigh cell carries a constant load equal to the nominal weight of the package. Such means obviate or reduce the effects of vibration upon the weigh cells. Resistors 11 and 12 may supply an offset for cancelling the constant portion of output 10. <IMAGE>

Description

SPECIFICATION Weighing machines The invention described below applies to weighing machines. It applies more particularly to the method of use of transducers on weighing machines such transducers being of a type which convert the weight of an article or package to an electrical output in the form of a voltage or current, for example transducers known as linear differential transformers, or strain gauges or electro-magnetic force balance weighers. Below we have used the term weigh cell to describe the embodiment of such transducers into a frame together with a substantially linear means of restraint which may include a suitable spring or electro-magnet.The invention applies to weighing apparatus in which articles are placed by hand on to the weigh cell when the rate of weighing will be relatively slow and it may apply more particularly to so called high speed machines wherein machinery is used to provide a rapid succession of articles, substantially one at a time, over a weigh cell.

The invention applies specifically to means to obviate or reduce the effects of vibration upon the weigh cell. Vibration may be present at an ambient level, i.e. within the foundation of the building in which the machine is used when vibration may communicate itself to the frame of the machine and thence to the frame of the weigh cell itself.

Again when machinery is used to convey the packages or series of packages across the weigh cell the vibration caused by the working parts of the conveying means may communicate itself again to the frame of the machine and thence to the frame of the weigh cell.

Such vibration as described above may often impart a component of movement which may be vertical or in such direction as to cause an undesirable output from the weigh cell, on the otherwise stationary frame of the weigh cell. The portion of the weigh cell which normally responds mechanically to a greater or lesser degree to the weight of a package being weighed, when it has reached a stable condition, tends to remain stationary due to its own inertia and the inertia of a package if one is present on the weigh cell. Relative movement is thus imparted to the transducer between the part attached to the frame and the normally responsive part of the transducer and an undesirable signal is generated by the transducer.

The vibration may again have a wide frequency spectrum and while isolating means may be used between the frame of the machine and that of the weigh cell it is not possible to isolate the weigh cell from all components of vibration and further as the isolating means is constructed to become increasingly effective it also becomes increasingly and objectionably costly.

It will be well known that a form of mechanical, or sometimes electrical, damping is either inherent in the design or the weigh cell itself, or may be deliberately added to the construction of the weigh cell. While in the present context this has a anti-vibration effect, its usefulness in this respect is usually limited, since a controlled weigh cell response is required, this response being adjusted according to the required rate of succession of articles over the weigh cell.

The transducers within the weigh cells above have a continuously variable electrical output at least over a useful working range, when the weight of a package is represented by a D.C. voltage or current at a convenient level. Again the transducer may provide a useful continuously variable A.C.

output when suitably connected.

According to the invention there is provided a weighing machine comprising weight sensing apparatus including two identical weigh cells operating in conjunction with each other.

Preferably means are provided for combining the outputs of the weigh cells in mutual opposition.

One of the weigh cells is preferably arranged to carry a package to be weighed while the second weigh cell carries a constant load equal to the nominal weight of a package.

Preferably the output signal of the second weigh cell carrying the nominal load is used in opposition to the output signal from said one weigh cell. A suitable signal may be used to offset the constant output portion of the signal from the second weigh cell carrying the nominal load.

In one embodiment the two weigh cells are housed together in a single frame.

By way of example, a specific embodiment in accordance with the invention is described with reference to the accompanying diagrammatic drawings in which: Figure 1 is a perspective view of a weighing machine having two identical weigh cells; and Figure 2 is a circuit diagram of means connecting the outputs of the two weigh cells.

Referring to the drawings, weighing means comprise two identical weigh cells 1 and 2 whose outputs are suitably connected. Weigh cell 1 is used in a conventional manner i.e. a package 4 to be weighed will be placed by hand or otherwise on to a platform 5. Its net output 3 will now, as described above, comprise a usually major signal due to the weight of the package 4 together with an output in response to any vertical component of vibration. The second weigh cell 2 is mounted preferably close to weigh cell 1 and in the same aspect to the frame 7 of the machine as weigh cell 1 so that it is subject as nearly as possible to the same vibration as weigh cell 1. Weigh cell 2 is now loaded with a load 8 whose weight is made equal to a nominal package weight. This load may be simply a mass of metal fixed to a suitable platform 9 of weigh cell 2.By this means, and by the adjustment of any weigh cell damping means which may be inherent in or provided on the respective weigh cell, the response of each weigh cell particularly with regard to vibration, is made substantially identical.

It will be appreciated that weigh cell 2 will provide an output 10 identical to that of weigh cell 1, when this carries a package with a precisely nominal weight.

Means are provided to combine the outputs of the two weigh cells. In this embodiment, the outputs 3,10 from the weigh cells 1,2 respectively are taken to the inverting and non-inverting input terminals of a summing amplifier shown in the diagram of Figure 2. The positive pole of the output 10 is connected via a resistor 11 to a negative bias supply so that by adjustment of a potentiometer 12 the constant positive output from 10 due to the load 8 may be offset when this is required. With the bias so adjusted the output from the amplifier will be a negative voltage proportional to the weight of package 4 only, since the vibrational responses from the two weigh cells will cancel each other. If the negative polarity of output 13 is objectionable it may be inverted by known means.

Again those skilled in the art will appreciate that the diagram shown is by way of example only and that the same net output may be achieved by other circuit arrangements. Furthermore, due to manufacturing tolerances in the weigh cells and associated components, zero conditions and scale values may require to be made adjustable by the addition of suitable controls.

It may be argued that such means of cancellation of vibrational effects will only be precisely correct when a package weight is precisely equal to the weight upon weigh cell 2. This is true. However, for a package to be within an acceptable band of weight, its weight will still be extremely close to the nominal weight and the deviation of the actual weight from the nominal weight will have a negligible effect upon the vibrational response of the weigh cell.

Some transducers such as linear differential transformers and strain gauges may be operated from an A.C. supply and it may then be convenient to the designer to use the A.C. outputs from the prime transducers of the weigh cell directly. In this event the pair of weigh cells are operated from the same supply so that there are no problems with differences of frequency, phase or supply voltage.

The connection of the outputs is then again as shown in Figure 2, but when an offset is required this has of course to be obtained from the A.C.

supply and must be of the appropriate phase.

It will also be appreciated that although Figure 1 shows the use of two similar weigh cells separately mounted on a rigid frame, the two weigh cell transducing components together with their springs or other restraints, their associated inertial masses and their damping means, may be housed within a single frame. Further in such a configuration the mass 8 may be contained within such a frame since it has no mechanical function external to the frame.

Claims (7)

1. A weighing machine comprising weight sensing apparatus including two identical weigh cells operating in conjunction with each other.

2. A weighing machine as claimed in Claim 1, wherein means are provided for combining the outputs of the weigh cells in mutual opposition.

3. A weighing machine as claimed in Claim 1 or Claim 2, in which one of the weigh cells is arranged to carry a package to be weighed while the second weigh cell carries a constant load equal to the nominal weight of a package.

4. A weighing machine as claimed in Claim 3, in which the output signal of the second weigh cell carrying the nominal load is used in opposition to the output signal from said one weigh cell.

5. A weighing machine as claimed in Claim 3 or Claim 4, in which a suitable signal is used to offset the constant output portion of the signal from the second weigh cell carrying the nominal load.

6. A weighing machine as claimed in any one of the preceding claims, in which the two weigh cells are housed together in a single frame.

7. A weighing machine substantially as hereinbefore described with reference to and as shown in the accompanying drawings.