GB1571785A - Electronic balances - Google Patents

Description

(54) IMPROVEMENTS RELATING TO ELECTRONIC BALANCES (71) We, PA MANAGEMENT CON SULTANTS LIMITED, a British Company, of Hyde Park House, 6()A Knightsbridge, London SW1X 7LE (formerly 2 Albert Gate, Knightsbridge, London SW1X 7JU), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to balances or weighing machines that employ electrical phenomena in their operation. Such apparatus are commonly referred to as electronic balances.

In one form of electronic balance a current is passed through a ooil to produce an electromagnetic field which tends to restore an unbalanced machine to its balanced condition.

The current requited to cause the machine to be maintained in its balanced condition is related to the weight which causes the unbalanced condition to occur. The value of the weight can thus be determined from the value of the current.

In one embodiment of such a balance a pan carrying a weight is attached to a magnet within whose magnetic field a coil carrying a current is arranged. Variations in the current passing through the coil can be used to exert a restoring force upon the pan via the magnet, according to the well-known formula:- F = Bil where F is the force produced, B is the flux density of the magnetic field produced by the magnet, i is the current flowing through the coil, and 1 is the length of conductor in the field, In practice it is found that the current flowing through the coil changes the value of B and thus the force becomes a complex function of i, and gives rise to a nonlinear balance.It has previously been proposed to incorporate com- pensating coils into the magnetic circuit with limited degrees of success.

We have found that the non-linear effect can be minimised by employing the combined effect of a pair of balanced force transducers, one of which is so arranged with respect to the other that the non-linear effects tend to cancel. Such an arrangement may be likened to a push-pull arrangement in the operation of electronic valves.

According to the invention there is provided a weighing machine or balance comprising a pair of permanent magnet assemblies and a pair of coWaperating coils, the coils each constituting a part of a unit which is movable in accordance with the weight of an object being weighed and the coils each being so wound, positioned for movement in the magnetic field of its respective magnet assembly, and arranged for connection to a source of current that current from the source when passed through the coils produces a field in each coil whose effect is to tend to oppose movement of the unit due to the weight of an object being weighed, the current passing through each coil producing a magnetic field which acts with respect to the magnetic field of its magnet assembly in the opposite sense to that in which the magnetic field produced by the other coil acts with respect to the magnetic field of its respective magnet assembly.

An embodiment od the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Fig. 1 is a partly schematic side elevation and a partly block schematic diagram of an arrangement illustrating the principle of opera- tion of an electronic balance, Fig. 2 is a graph illustrating the non hinegarity of a known arrangement, Fig. 3 is a section through a part of a balance assembly according to the invention, and F:ig5. 4 and 5 are electric aircuit diagrams for use with an assembly operating upon the principle described with reference to Fig. 3.

Referring to Fig. 1, there is shown diagrammatically a pan 1 supported upon a shaft 2, which extends through a transducer assembly 3 to a plate 4, to which it is attached. The plate 4 is suspended from the transducer assembly 3 by a suspension 5, referred to as a lower suspension. The transducer assembly 3 is suspended from a frame 6 of the machine by an upper suspension 7. The transducer assembly 3 includes a magnet arrangement (not shown) which is movable with the shaft 2 and a coil (not shown) which surrounds the shaft 2 and produces a magnetic field which acts in opposition to that of the magnet within the assembly 3.Current is supplied to the coil (not shown) in the assembly 3 from a servo amplifier 8 under the control of a position sensor 9, which detects movements of the pan d when a weight is placed on it, and causes the position of the pan to be maintained as a.

result of a compensating reaction between rhe magnetic fields of the coil and the magnet in the assembly 3.

It is apparent that any increase in the weight on the pan 1 requires a corresponding inaase in the current supplied by the servo amplifier 8 to the coil in the assembly 3 in order to maintain the position of the pan. The voltage develop4 by the current supplied by the servo amplifier 8 to the transducer assembly 3 is measured and displayed on a digital voltmeter 10 which is so calibrated that the value of the weight on the pan can be read directly. It will be appreciated that the position sensor can be optical, electrical or mechanical.

However, in view of the fact, previously mentioned, that the force exerted upon the pan due to the interaction of the magnetic !fie!lds is a complex function of the current in the coil, the actual weight on the pan does not correspond exactly to the reading obtained on the voltmeter 10 in the simple embodiment shown in Fig. 1.

A graph illustrating at 12 the ùnction of nonQineanty against the actual weight is shown in Fig. 2, where the actual weight is represented on the abscissa and the none linear part is represented on the ordinate.

An embodiment in accordance with the present invention will now be described with reference to Fig. 3, in which there is shown a coil fonner consisting of a disc 15 which supports a tlinder 16. On one side of the disc 15 there is wound on the cylinder 16 a coil 17 through which current is fed in the direction indicated by the normal convention. On the other side of the disc 15, there is wound on the cylinder 1L6 a coil 18 through which current is fed in the direction indicated by the normal convention.The coil fonner is supported by a stem or shaft with which it is displaceable and which passes through the centre of a magnet assembly comprising ring magnets 19 and 20, whose fields act in opposite directions, as indicated by arrows 2I and 22, circular disc pole pieces 23, 24 and 23a, 24a and cylindrical coupling members 25 and 26.

It will be noted that the assembly is mech anically symmetrical about the plane in which the disc 15 lies, but that since the "lift" current flows through the coils 17 and 18 to produce lift in the two similar halves of the assembly, there will be a degree of cancellation of the nunZinear effect referred to above, even though, because the fields of the magnets 19 and 20 act in opposite directions, the lift pro ducked by each half of the assembly is additive.

Typical parallel and series electrical circuit connections for the coils 17 and 18 are shown in Figs. 4 and 5 respectively. In each circuit there is shown a calibration resistor 28 and a linearity compensation resistor 29 which enables a further degree of compensation of nonlinearity effects to be obtained. The direction of currentflowineachof the coils 17 and 18 is shown by means of the usual convention.

The design shown uses a centre pole magnet assembly, but other magnetic circuit arrange scents, for example outside ring assemblies can be used. The use of a centre pole arrange ment is favoured for precision transducers, such as would be used in a magnetic balance, because low external magnetic fields are re quired to avoid performance changes due to the interaction of ferromagnetic objects with any external fields.

It will also be appreciated that, although the invention has been described with reference to a balance having a simple pan, it can be ap plied to balances employing two pans and a centre pivot. The system proposed employing a pair of permanent magnet assemblies and a pair of coils produces a more linear relation ship between the mechanical force and the electric current tin the coils than each iindividua coil possesses if operated alone.

WHAT WE CLAIM IS 1. A weighing machine or balance compris- ling a pair of permanent magnet assemblies and a pair of co-operating coils, the coils each con- stituting a part of a unit which is movable in accordance with the weight of an object being weighed and the coils each being so wound, positioned for movement in the magnetic field of its respective magnet assembly, and arranged for connection to a source of current that current from the source when passed through the coils produces a field in each coil whose effect is to tend to oppose movement of the unit due to the weight of an object being weighed, the current passing through each coil producing a magnetic field which acts with respect to the magnetic field of its magnet assembly in the opposite sense to that in which the magnetic field produced by the other coil acts with respect to the magnetic field of its respective magnet assembly.

2. A system as claimed in claims 1 including a resistive element in series and/or in parallel with a coil.

3. An electronic balance as claimed in claim 1 substantially as described herein with refer ence to Figs. I and 3 of the accompanyingr

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. assembly 3 includes a magnet arrangement (not shown) which is movable with the shaft 2 and a coil (not shown) which surrounds the shaft 2 and produces a magnetic field which acts in opposition to that of the magnet within the assembly 3. Current is supplied to the coil (not shown) in the assembly 3 from a servo amplifier 8 under the control of a position sensor 9, which detects movements of the pan d when a weight is placed on it, and causes the position of the pan to be maintained as a. result of a compensating reaction between rhe magnetic fields of the coil and the magnet in the assembly 3. It is apparent that any increase in the weight on the pan 1 requires a corresponding inaase in the current supplied by the servo amplifier 8 to the coil in the assembly 3 in order to maintain the position of the pan. The voltage develop4 by the current supplied by the servo amplifier 8 to the transducer assembly 3 is measured and displayed on a digital voltmeter 10 which is so calibrated that the value of the weight on the pan can be read directly. It will be appreciated that the position sensor can be optical, electrical or mechanical. However, in view of the fact, previously mentioned, that the force exerted upon the pan due to the interaction of the magnetic !fie!lds is a complex function of the current in the coil, the actual weight on the pan does not correspond exactly to the reading obtained on the voltmeter 10 in the simple embodiment shown in Fig. 1. A graph illustrating at 12 the ùnction of nonQineanty against the actual weight is shown in Fig. 2, where the actual weight is represented on the abscissa and the none linear part is represented on the ordinate. An embodiment in accordance with the present invention will now be described with reference to Fig. 3, in which there is shown a coil fonner consisting of a disc 15 which supports a tlinder 16. On one side of the disc 15 there is wound on the cylinder 16 a coil 17 through which current is fed in the direction indicated by the normal convention. On the other side of the disc 15, there is wound on the cylinder 1L6 a coil 18 through which current is fed in the direction indicated by the normal convention.The coil fonner is supported by a stem or shaft with which it is displaceable and which passes through the centre of a magnet assembly comprising ring magnets 19 and 20, whose fields act in opposite directions, as indicated by arrows 2I and 22, circular disc pole pieces 23, 24 and 23a, 24a and cylindrical coupling members 25 and 26. It will be noted that the assembly is mech anically symmetrical about the plane in which the disc 15 lies, but that since the "lift" current flows through the coils 17 and 18 to produce lift in the two similar halves of the assembly, there will be a degree of cancellation of the nunZinear effect referred to above, even though, because the fields of the magnets 19 and 20 act in opposite directions, the lift pro ducked by each half of the assembly is additive. Typical parallel and series electrical circuit connections for the coils 17 and 18 are shown in Figs. 4 and 5 respectively. In each circuit there is shown a calibration resistor 28 and a linearity compensation resistor 29 which enables a further degree of compensation of nonlinearity effects to be obtained. The direction of currentflowineachof the coils 17 and 18 is shown by means of the usual convention. The design shown uses a centre pole magnet assembly, but other magnetic circuit arrange scents, for example outside ring assemblies can be used. The use of a centre pole arrange ment is favoured for precision transducers, such as would be used in a magnetic balance, because low external magnetic fields are re quired to avoid performance changes due to the interaction of ferromagnetic objects with any external fields. It will also be appreciated that, although the invention has been described with reference to a balance having a simple pan, it can be ap plied to balances employing two pans and a centre pivot. The system proposed employing a pair of permanent magnet assemblies and a pair of coils produces a more linear relation ship between the mechanical force and the electric current tin the coils than each iindividua coil possesses if operated alone. WHAT WE CLAIM IS

1. A weighing machine or balance compris- ling a pair of permanent magnet assemblies and a pair of co-operating coils, the coils each con- stituting a part of a unit which is movable in accordance with the weight of an object being weighed and the coils each being so wound, positioned for movement in the magnetic field of its respective magnet assembly, and arranged for connection to a source of current that current from the source when passed through the coils produces a field in each coil whose effect is to tend to oppose movement of the unit due to the weight of an object being weighed, the current passing through each coil producing a magnetic field which acts with respect to the magnetic field of its magnet assembly in the opposite sense to that in which the magnetic field produced by the other coil acts with respect to the magnetic field of its respective magnet assembly.

2. A system as claimed in claims 1 including a resistive element in series and/or in parallel with a coil.

3. An electronic balance as claimed in claim 1 substantially as described herein with refer ence to Figs. I and 3 of the accompanyingr

drawings.

4. An electronic balance as claimed in claim 1, induding a circuit arrangement substantially as described herein with reference to Fig. 4 or Fig. 5 of the accompanying drawings.