GB2081026A

GB2081026A - Electromagnetic Vibrating System

Info

Publication number: GB2081026A
Application number: GB8109711A
Authority: GB
Original assignee: Popper Engineering Ltd
Current assignee: Popper Engineering Ltd
Priority date: 1980-03-31
Filing date: 1981-03-27
Publication date: 1982-02-10
Also published as: AR226886A1; ES8204214A1; IT8120820A0; BE888187A; IT1137106B; BR8101887A; ES500837A0; AU6890981A; IL59740A0; FR2479034A1; ZA811945B; JPS56155512A; CA1189559A; GB2081026B; DE3112569A1

Abstract

An electromagnetic vibrating system comprises an electromagnet- armature combination producing vibration and including first and second elements (14 and 16) defining a gap (18) therebetween, a base member (10) supporting the first element (16), a member (12) to be vibrated which is connected to the second element (14), and non-linear spring apparatus (20) for storing substantial energy of said combination as it approaches its minimum gap and converting it to kinetic energy of the member to be vibrated and for also preventing "knocking". The system of Fig. 2A is non-linear in that linear springs 20 do not engage at all separation distances. Springs may be of different lengths (Fig. 2B) and/or have different characteristics. Additionally, leaf springs 11 are included. Alternatives are detailed (Figs. 3A-16) including leaf spring and elastomeric pillow non-linear arrangements. Applications are specified. <IMAGE>

Description

SPECIFICATION Electromagnetic Vibrating System The present invention relates to vibrating systems generally and more particularly to electromagnetic vibrating systems.

Vibrating systems are known and used in a great variety of forms for various functions such as feeding, conveying, screening, sorting, shaking and discharging bulk materials. The source of vibration for such systems may be mechanical, such as an electric motor rotating eccentric masses, hydraulic, pneumatic or electromagnetic.

Electromagnetic systems are favored in most vibrating systems since they enable relatively simple and inexpensive constructions to be employed, requiring neither bearings in the case of electric motors, nor seals or valves as in hydraulic and pneumatic systems, and enable simplified amplitude monitoring and control.

Despite these advantages, electromagnetic vibrating systems have a serious inherent deficiency, which will now be described: Two types of electromagnetic vibrating structures are in common use. One employs a moving permanent magnet which passes through an ACenergized coil. This structure, which is normally used in loud-speakers is inefficient from an energy standpoint, since it wastes magnetic flux.

The second commonly employed structure employs an armature plate which is attracted head-on by a wound core. This structure avoids the energy inefficiency of the above-described moving magnet structure and is capable of producing relatively large forces. This structure, which is commonly used in materials handling applications, has the inherent limitation that its amplitude is limited by the maximum gap permitted between the armature and coil. The gap is limited by the amount of current that can be carried by the coils of the electromagnet. If the gap is increased, the current is increased correspondingly.Thus for a given gap, even though relatively high currents could be employed and magnet energies could be drawn from the magnet, the operating current and thus the magnet's energy output is deliberately kept low to prevent knocking of the armature against the core when the amplitude reaches the gap dimensions.

It may thus be appreciated that electromagnetic systems are significantly limited in their operation due to the physical limitations of the gap.

In order to overcome this problem it has been proposed to employ an electronic system for reducing the current supplied to the electromagnet as the gap between the electromagnet and the armature decreases. This is not desirable since it decreases the magnetic force and energy available to maintain the system in motion.

It has also been suggested to employ damping springs in association with electromagnetic systems to control the vibration thereof. The provision of damping springs negates the purposes of the present invention since it inhibits the reaching of high amplitudes and reduces the force and energy available for operating the vibrating system. Therefore, insofar as possible, in accordance with the present invention springs with relatively small damping are employed.

It is known from U.S. Patent 2,187,717 to employ a non-linear spring in association with an electromagnetic vibrating system. The purpose of this spring in the disclosed system is to control the natural frequency of the system and to maintain the system close to resonance notwithstanding variations in the load.

The frequency of such a system is given by the expression c Frequency= -=f (1) m where c=spring rate (2) and c is the ratio between change in force and change in displacement at any given point along the force/displacement curve of a spring.

It may thus be appreciated that the frequency can be maintained constant by suitable selection of the amplitude range of a non-linear spring at which desired force/displacement characteristics for a given mass are present. U.S. Patent 2,187,717 suggests the provision of selectable pretensioning means operative in response to a changing mass so as to match the force/displacement characteristics to the changing mass in order to keep the natural frequency constant in accordance with equation (1). Variations in the frequency of such vibrating systems are undesirable.

It is also appreciated that in order to match the force/displacement characteristics to a changing mass as suggested in U.S. Patent 2,187,717, the non linearity of the spring must be moderate enough to enable a constant spring rate c to be maintained within the range of amplitude of the system for any given mass. As will be described hereinafter, in accordance with the present invention, the spring rate c is not held constant within the amplitude range of the system. In accordance with an embodiment of the present invention, the spring rate c changes by at least twice its magnitude at rest (zero amplitude) and only in the direction of increasing rate with decreasing gap. If the electromagnet acts on only one side of the armature, the non-linearity is only along a portion of the pathway of the armature. In contrast, the aforesaid U.S.Patent 2,187,717 provides non-linearity along the entire range of motion of the armature.

It is also noted that the non-linear spring of the system in U.S Patent 2,187,717 is a highly damping spring due to its high internal friction as a result of rubbing between its leaves. In contrast, the present invention calls for springs having low damping characteristics.

U.S. Patent 4,235,1 53 shows a linear motion, electromagnetic force motor employing a non linear spring to oppose a rising magnetic force in order to stabilize the system. This patent does not disciose a vibratory system but instead deals with positioning apparatus which operates in a D.C.

mode. As such, it does not address the problem described hereinabove, the physical impacting between a vibrating member driven by an electromagnet and another member under high amplitude conditions.

It is noted in this connection that in a D.C.

system, reduction in the gap greatly increases the magnetic force. In an AC vibratory system it has been found that this force increase is not present, since as the gap is closed, the inductance, which opposes current flow, increases. A generally constant magnetic force is thus displayed by vibratory electromagnetic systems.

It is object of the present invention to provide apparatus which overcomes the above-described difficulties and which gives high amplitude operation at relatively low power.

In accordance with the present invention an electromagnetic vibrating system comprises an electromagnet-armature combination producing vibration and including at least first and second elements defining a gap therebetween, a base member supporting the first element, a member to be vibrated which is connected to the second element, and non-linear spring apparatus associated with the base and the member to be vibrated including apparatus for storing substantial energy of the electromagnet-armature combination as it approaches its minimum gap and converting it to kinetic energy of the member to be vibrated.

Further in accordance with an embodiment of the present invention there is also provided linear spring apparatus associated with the base and the member to be vibrated and the electromagnetarmature combination is operative to provide vibration of the member to be vibrated close to resonance of the linear spring apparatus.

Additionally in accordance with the invention, the energy of the electromagnet-armature combination that is stored by the non-linear spring apparatus includes kinetic energy of the member to be vibrated.

The present invention is illustrated by way of example in the drawings, in which: Figs. 1 A, 1 B and 1 C are displacement versus time curves illustrating the operation of apparatus constructed and operative in accordance with an embodiment of the present invention; Figs. 2A and 2B are pictorial illustrations of a vibrating system constructed and operative in accordance with an embodiment of the present invention; Figs. 3A, 3B, 3C, 3D and 3E are illustrations of variations of a vibrating tray type system constructed and operative in accordance with an embodiment of the present invention; Figs. 4, 5, 6, 7, 8, 9, 10 and 11 illustrate examples of types of non-linear spring constructions useful in vibrating systems constructed and operative in accordance with the present invention;; Fig. 12 illustrates a vibrating tray type system constructed and operative in accordance with an embodiment of the present invention and employing a non-linear Kappa spring; Fig. 1 3 illustrates another embodiment of vibrating system constructed and operative in accordance with an embodiment of the invention and employing a Kappa spring; Fig. 14 is an illustration of a Kappa spring useful in the invention; Figs. 1 5A and 15B are illustrations of a modified Kappa spring useful in the invention; and Fig. 1 6 is an illustration of a non-linear spring construction useful in vibrating systems constructed and operative in accordance with the present invention.

Befdre proceeding to a detailed description of the invention, a brief explanation of the problem of knocking in moving armature type electromagnetic systems will be presented, with reference to Figs. 1 A, 1 B and 1 C. Fig. 1 A shows a displacement curve of a moving armature electromagnetic system in which the displacement amplitude is equal to the maximum gap separation. Fig. 1 B shows a theoretical amplitude that could be realized with the electromagnetic system of Fig. 1 A were it not for the physical limitation Imposed by impacting of the armature against the electromagnet. Fig. 1 C shows the amplitude of an electromagnetic system constructed and operative in accordance with the present invention wherein the amplitude of the electromagnetic system during the far half cycle of the armature is increased beyond the gap limits.The techniques for producing this desired result will be described hereinafter.

The present invention will now be described with particular reference to the drawings. It is noted that while most of the drawings relate to a particular type of vibrating system, the vibrating tray or screen type, the invention is not limited to such types and is applicable to all suitable electromagnetic vibrating systems.

Fig. 2A illustrates a vibrating system to general construction comprising a base 10. A mass to be vibrated 12 is mounted onto base 10 as by leaf springs 11. An armature 14 is fixedly mounted onto mass 12. Also mounted onto base 10 is en electromagnet 16, disposed in facing relationship to armature 14 and defining a gap 18 therebetween. A pair of low-damping coil springs 20 are mounted onto base 10 facing mass 12 but separated therefrom by a distance L at rest. The distance L is typically one-third of the gap distance at rest.

It is a particular feature of the present invention that springs 20 do not contact mass 12 except when mass 12 approaches the electromagnet 16.

i.e. when the gap 18 is at a minimum. Thus it may be appreciated that springs 20 may be linear springs which in this constructlon constitute a non-linear spring system or sprIng means due to the fact that they are engaged only during part of the relative displacement of the members.

In the embodiment of Fig. 2A both springs 20 are of the same length and contact mass 12 at approximately the same point. An alternative embodiment is illustrated in Fig. 2B where instead of a pair of identical springs 20, there are provided springs 21 and 23 of differing length, such that spring 21 is longer than spring 23 and contacts mass 12 at an earlier point along its displacement path than does spring 23. It may thus be appreciated that by using a plurality of springs which engage the vibrating system along different portions of the displacement path, a desired force/displacement characteristic can be constructed. Alternatively or additionally combinations of springs having different characteristics may be employed.

The operation of the apparatus illustrated in Fig. 2A will be better understood by considering the displacement curve of Fig. 1 C which, as noted above, indicates that the amplitude is limited in one direction but not in the other. This limitation is achieved by the action of a non-linear spring system embodied in springs 20 of the embodiment of Fig. 2A. The non-linear spring system prevents armature knocking against the magnet and is operative to store the high speed kinetic energy of the moving elements as well as the electromagnetic energy of the magnet, until the armature comes to a standstill momentarily at its minimum gap.Immediately thereafter the armature is repelled from the electromagnet while the non-linear spring system releases the stored energy such that the armature is thrown forcefully towards the opposite direction with a high amplitude. The linear springs 11 bring the armature back to its zero position and the next cycle begins.

It is appreciated that the force produced by the linear springs 11 increases rather slowly with displacement, while the force produced by springs 20, which is applied only after a displacement L from the zero position upon contact between the springs 20 and mass 12, increases quickly with displacement. It is thus apparent that the resultant spring force of the spring system comprising springs 11 and 20 corresponds substantially to the force required to absorb the kinetic energy of the moving parts and the magnetic energy of the electromagnet-armature combination before the gap reaches zero.

In the illustrated embodiment of the present invention a non-linear spring system is produced by a combination of two linear springs.

Alternatively one or more non-linear springs may be employed, alone or in combination with linear springs.

It is a general requirement that throughout the displacement cycle of the system the natural frequency of the vibrating system including the springs and masses corresponds generally to the required frequency of the electromagnet.

The provision of the spring system comprising springs 20 is a primary feature of the invention, since these springs act to prevent "knocking" of the apparatus at high amplitudes and to store the electromagnetic and kinetic energy as the gap approaches zero and to accelerate the vibrating mass in the other direction at high speed.

For the purposes of clarity and distinction from the prior art, the terms "non-linear spring system" and "non-linear spring means" will be employed throughout the specification and claims to refer to a spring system or spring means wherein the spring rate c changes within the operative amplitude range of the springs at any given time.

This definition thus excludes the structure disclosed in U.S. Patent 2,187,717 wherein c remains substantially constant within any given operative amplitude range.

It is a primary feature of the invention that the spring system displays low internal damping or friction in order that the spring system is operative to convert substantially all of the magnetic energy absorbed thereby to kinetic energy of the mass to be vibrated. Thus the spring system must not act as a damper, resulting in energy wastage.

In practice it is seen that in order to correspond to the force required to decelerate the mass to zero speed without impacting against the magnet, the spring system must be non-linear.

It is noted for the sake of clarity that although the above discussion has been concerned with a system in which an electromagnet is stationary and an armature moves with respect thereto, the invention is equally applicable to the converse system wherein the armature is stationary and the electromagnet moves. Generally stated, the invention as described above is applicable to relative motion between the electromagnet and its armature including motion of either or both the armature and electromagnet.

Reference is now made to Figs. 3A-3E which illustrate various constructions of vibrating table type electromagnetic vibrating system. For the sake of clarity and conciseness, the elements common to all of the various constructions will be described only in connection with Fig. 3A and the same reference numerals will be used for identical parts in all of Figs. 3A-3E.

The illustrated vibrating table type electromagnetic vibrating system comprises a vibrating base 22, which is mounted by means of soft springs 24 onto a fixed support 26, such as a floor. A tray 28 which it is desired to vibrate along an axis 30 is mounted onto vibrating base 22 by means of leaf springs 32 and 34 which generally satisfy equation (1).

An electromagnet 36 is mounted onto vibrating base 22 and an armature 38 is mounted on tray 28 in facing relationship to electromagnet 36 across a gap 39. A non-linear spring system 40, shown schematically, is associated with vibrating base 22 and tray 28 for substantially storing the magnetic energy produced as the gap decreases to its minimum separation. Various embodiments of non-linear spring systems 40 will be described in detail hereinafter in connection with Figs. 4-11.

The embodiment illustrated in Fig. 3A is characterized in that the magnet 36 and armature 38 are arranged along an axis which is parallel to the axis of motion 30 of the vibrating tray 28.

In contrast, the embodiment of Fig. 3B is characterized in that the magnet and armature are aligned horizontally and thus at an angle P with respect to axis 30. This construction has the advantage of simplicity in manufacture as well as the additional important advantage that the gap is maintained relatively small. This may be appreciated by noting that for a maximal displacement of tray 28 of 2A, the maximal gap between the magnet and the armature in the embodiment of Fig. 3B will be 2A cos ,B as compared with a maximal gap of 2A for the embodiment of Fig. 3A.

The embodiment of Fig. 3C is similar to that of Fig. 3B except in that a pair of electromagnets 42 and 44 are employed and disposed on opposite sides of armature 38 across respective gaps 46 and 48. An electronic control 50 is provided for directing current to either one of the electromagnets, where a soft iron armature is employed. Alternatively, where the armature is a permanent magnet, the electronic control successively changes the polarity of the electromagnets.

The embodiment of Fig. 3D is similar to that of Fig. 3A and illustrates one example of a substantially non-damping spring system constructed and operative in accordance with an embodiment of the invention. Here a leaf spring 52 is mounted onto base 22. Spring 52 may be assumed to be linear and to have very low internal friction.

An impacting member 54 is associated with tray 28 and arranged to engage spring 52 only during that portion of the displacement cycle of the apparatus when gap 39 is approaching its minimum separation. During this portion of the cycle, spring 52 absorbs substantially all of the magnetic and kinetic energy produced and converts substantially all of it to kinetic energy of the vibrating elements in accordance with the invention. It is specifically noted that in this embodiment, a linear spring is employed to provide a non-linear, discontinuous spring system.

It is noted also that impact member 54 may be formed of an elastomer such as rubber in order to reduce impact noise.

The embodiment of Fig. 3E is in effect a combination of the embodiments of Fig. 3D and Fig. 3C in that a pair of magnets 42 and 44 are employed. In this embodiment the analogy to the spring system of Fig. 3D is a two sided substantially non damping spring system 56, typically comprising low internal friction rubber pads associated with first and second impacting members 58 and 60 mounted onto tray 28.

It is noted that the employment of a pair of magnets as in the embodiments of Fig. 3C and 3E effectively limits the amplitude of armature vibration to the overall distance between the two magnets. This arrangement does, however, have the advantage that one of the magnets, remote from the zero line may be energized only to assist the other magnet for high amplitude operation without requiring the provision of an additional armature. A highly symmetric oscillation may be provided in this manner, which is particularly desirable for certain operations, such as screening.

It should be appreciated in connection with all of the embodiments illustrated In Figs. 3A-2E that tray 28 may be alternatively a screen or any other desired element and that the springs employed may be alternatively any other type of spring such as coil springs, elastomer springs or leaf springs. All of these various constructions are exemplary also of other types of vibrating systems in which the invention may be employed.

Reference is now made to Figs. o1 11 which illustrate various exemplary embodiments of spring systems useful in the present invention.

Fig. 4 shows a conventional leaf spring 60 associated with a curved support 62. When the tray 28 is deflected in the direction of the solid arrow 64, the leaf spring 60 leans against the curved support 62, thus decreasing its free length by a distance "s" as illustrated in Fig. 6 with the result that the spring is effectively stiffened.

Deflection of tray 28 in the opposite direction produces no such stiffening. Thus a non-linear spring system operative as a linear spring in one direction is provided.

Fig. 5 illustrates a variation on the embodiment of Fig. 4 wherein first and second curved supports 66 and 68 are provided, both of which engage the leaf spring 60 during its deflection in the direction of the solid arrow 64 and neither of which engages the leaf spring 60 when it is deflected in the opposite direction.

Fig. 7 and 8 illustrate a non-linear coil spring 67 in respective compressed and at rest orientations. As seen in Fig. 8 the spring 69 is a variable pitch spring. Under compression the lower pitch coils initially are compressed solid reducing the active spring length, and thus increasing the spring stiffness along a portion of its deflection cycle.

Fig. 9 illustrates a conical spring which is also a variable pitch spring. Here the larger and thus softer coils are initially compressed and lean against a support 70, leaving the smaller coils which define a stiffer spring.

Fig. 10 illustrates a further embodiment of, non-linear spring system comprising an elastomeric pillow 72, formed of rubber, polyurethane, or any other suitable material. The pillow 72 is seated in a generally conical cup 74.

The elastomeric materials employed in pillow 72 have the property that they are resilient in one direction so long as they can expand in another perpendicular direction.

When the pillow is not compressed, it sits loosely within the cup 74. When a compressive force is applied to the pillow along an axis 76, the pillow is forced up against the inner walls of the cup, effectively stiffening the spring system.

The spring system illustrated in Fig. 10 has advantages of simplicity of construction and very high energy storage capacity for a relatively small spring volume. It suffers from the disadvantage that its rate depends on temperature and aging of the elastomeric material.

Fig. 11 illustrates a non-linear spring system which is the analog of the embodiment of Fig. 4 for use with a two magnet vibrating system as illustrated, for example in Fig. 3C. The spring system employs first and second curved supports 78 and 80 mounted on opposite sides of a leaf spring 82, for reducing the effective length thereof as a function of displacement in either direction.

Fig. 12 illustrates an electromagnetic vibrating system of the vibrating tray type in which the non-damping non-linear spring system comprises a Kappa spring. The construction of Kappa springs is described in applicant's U.S. Patent 4,129,290.

This particular Kappa spring is constructed as illustrated generally in Fig. 14, having a central leaf 85 and a pair of side leaves 87 and 89 connected to the central leaf 85 by connecting members 91 and 93. Lr denotes the free length of each of the side leaves 87 and 89 as illustrated and L2 denotes one-half of the free length of central leaf 85. B illustrates the free length of the connecting member 91 and 93, which are typically identical, measured as shown. When L,+L2=3B, a linear spring is produced.

In accordance with the present invention 3B is selected to be greater than La+L2. There is thus produced a Kappa spring whose spring rate increases with displacement under compression and decreases with displacement under tension.

Reference is now made to Fig. 1 3 which shows a vibrator comprising a vibrating base 84 mounted on a supporting surface by means of soft springs 86 and supporting an electromagnet 88.

A first vibrating mass 90 is supported by a plurality of non-linear Kappa springs 92, of the type described hereinabove, on base 84. A plurality of linear coil springs 94 are also mounted on base 84 and extend towards mass 90 and are spaced therefrom by a separation 96 which is equal, at rest, to approximately one-third of the electromagnet-armature gap 98 defined between electromagnet 88 and a facing armature supported on mass 90. A further vibrating mass 99 is mounted on mass 90 by means of springs 100. The illustrated vibrator may have many possible uses, for example in settling poured concrete and serves as an example of various uses for embodiments of the invention.

It is noted that the non-linear spring means illustrated in Figs. 4-6 and 11 constitute a preferred embodiment of non-linear spring means for use in accordance with the invention. These constructions may be characterized in that they comprise a curved support surface defining a circular section. The force/displacement characteristics of such systems provide a linear range which enables them to satisfy equation (1) and a non-linear range at greater displacements which enable them to correspond to the force/displacement characteristics required herein.

It is appreciated that the introduction of a nonlinear spring having the performance characteristics seen in Fig. 1 C provides a system whose natural frequency is amplitude dependent.

This is not the case with linear springs. In practice, however, the amplitude dependence does not affect performance since in most applications there is a constant variation in mass which contributes a non-linear element in any event which has the same amplitude dependency of the frequency as its result.

It may be desired in certain applications to provide an electronic trigger circuit for providing electrical impulses to the magnet such that the system automatically operates at resonance with maximum amplitude at a minimum current. The addition of suitable feedback circuitry can enable this apparatus to be used to control the amplitude.

Reference is now made to Figs. 1 5A and 15B which illustrate a modified version of the Kappa spring of Fig. 14 in respective at rest and compressed orientations. For convenience, common numbering is used for common elements appearing also in Fig. 14. The modification illustrated- in Figs. 1 5A and 15B is the provision of elastomer impacting pads 102 and 104 in facing orientation onto connecting members 91 and 93. These impacting pads provide an additional energy storage and conversion device which comes into play only at a predetermined compression level, when the pads initially contact.

If the springs of Figs. 15A and 1 5B were used to replace the springs 92 in the vibrating system of Fig.13, for example, the elastomer pads would provide kinetic and magnetic energy storage during only the portion of the travel cycle at which the electromagnet gap is at a minimum. In such a way they provide desired non-linearity for the system.

It is appreciated that instead of a pair of elastomer pads as shown, only one may be employed for impact against a rigid member. The elastomer pads may be formed of rubber or any other suitable material. As a further alternative they may be replaced by other types of springs which are characterized in that they achieve compressive contact only along a portion of the travel range of the vibrating system.

Reference is now made to Fig. 1 6 which illustrates an embodiment of non-linear spring system which comprises a linear coil spring 110 which is attached at one end to a rubber pad 11 2 and at a second end to a supporting surface 114.

A generally cylindrical rubber pad 11 6 is located interiorly of coil spring 110 and is selectably positioned onto an adjustable supporting bolt 11 8 which threadably engages a counter nut 120 for selectable mounting of the pad 11 6 with respect to the pad 112. The rubber pad 116 may be retained in place by means of a locating projection 122 extending outwardly from bolt 118.

It may be appreciated that the structure of Fig.

1 6 provides an adjustable non-linear spring system. By suitable adjustment of the position of bolt 118 the portion of the range of travel of pad 112 over which pad 116 acts as an energy absorber can be selected. Thus the non-linear spring system of Fig. 16 can be considered as a mechanically programmable non-linear spring system. It may also be appreciated that a plurality of elastomer pads, each mounted on an adjustably positionable support may be used to provide a multi-step adjustable non-linear spring system. Such a system is particularly useful in the apparatus of the present invention but is not limited to such applications.

It will be appreciated by persons skilled in the art that the invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow:

Claims

1. An electromagnetic vibrating system comprising .

an electromagnet-armature combination producing vibration and including at least first and second elements defining a gap therebetween; a base member supporting said first element; a member to be vibrated, said member connected to said second element; and non-linear spring means associated with said base and said member to be vibrated including means for storing substantial energy of said electromagnet armature combination as it approaches its minimum gap and converting it to kinetic energy of the member to be vibrated.

2. A system according to claim 1 and wherein said non-linear spring means displays increasing stiffness as a function of increasing deflection amplitude.

3. A system according to either of the preceding claims and wherein said non-linear spring means is operative over a non-linear range thereof.

4. A system according to any of the preceding claims and wherein said member to be vibrated comprises a vibrating tray.

5. A system according to any of claims 1 to 3 and wherein said member to be vibrated comprises a screen.

6. A system according to any of the preceding claims and wherein said non-linear spring means interconnects said base and said member to be vibrated.

7. A system according to any of claims 1 to 5 and where said non-linear spring means does not interconnect said base and said member to be vibrated, and makes contact therebetween only as said combination approaches its minimum gap.

8. A system according to any of claims 1 to 7 and wherein said non-linear spring means comprises a plurality of linear springs which are engaged successively with increasing deflection amplitude.

9. A system according to any of claims 1 to 7 and wherein said non-linear spring means comprises a non-linear spring.

10. A system according to any of claims 1 to 7 and wherein said non linear spring means comprises spring means having a linear characteristic around its rest position and a nonlinear characteristic at high amplitude deflection in at least one direction.

11. A system according to any of claims 1 to 7 and wherein said non linear spring means has a non-linear characteristic at high amplitude deflection in a single direction.

12. A system according to any of claims 1 to 7 and wherein said non linear spring means comprises a leaf spring associated with a curved supporting surface for engagement therewith at a predetermined deflection amplitude in at least one direction.

13. A system according to any of claims 1 to 7 and wherein said non linear spring means comprises a leaf spring associated with a plurality of curved supporting surfaces for engagement therewith at a predetermined deflection amplitude in at least one direction.

14. A system according to any of claims 1 to 7 and wherein said non linear spring means comprises a leaf spring associated with at least one curved supporting surface for engagement therewith at a predetermined deflection amplitude in a single direction.

1 5. A system according to any of the preceding claims and wherein said combination includes a single electromagnet and a single armature.

1 6. A system according to any of claims 1 to 1 4 and wherein said combination includes a pair of electromagnets and a single armature.

17. A system according to any of the preceding claims and wherein said combination Is arranged along an axis parallel to the axis of vibration of the system.

18. A system according to any of claims 1 to 1 6 and wherein said combination is arranged along an axis angled with respect to the axis of vibration of the system.

1 9. A system according to any of claims 1 to 7 or 1 5 to 1 8 and wherein said non-linear spring means comprises a non-linear Kappa spring.

20. A system according to any of claims 1 to 7 or 1 5 to 1 8 and wherein said non-linear spring means comprises a pneumatic spring.

21. A system according to any of claims 1 to 7 or 1 5 to 18 and wherein said non-linear spring means comprises an elastomer spring.

22. A system according to claim 19 and wherein said Kappa spring comprises an elastomer spring.

23. A system according to claim 1 and wherein said non-linear spring means is operative to store substantially all of the magnetic and kinetic energy of the vibrating system.

24. A system according to claim 1 and also comprising linear spring means associated with said base and said member to be vibrated, said electromagnet-armature combination being operative to provide vibration of said member to be vibrated close to resonance of said spring means.

25. An electromagnetic vibrating system substantially as described with reference to or as shown by Fig. 2A or2B or3A or 3B or 3C or 3D or 3Eor4and 6 orS or7 and 8 or9 or lOor 11 or 12 and 140r130r15Aand l5Bori6ofthe Drawings.