GB2299715A

GB2299715A - Reciprocating motor and compressor incorporating the same

Info

Publication number: GB2299715A
Application number: GB9600687A
Authority: GB
Inventors: Wei-Min Zhang
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-04-03
Filing date: 1996-01-12
Publication date: 1996-10-09
Anticipated expiration: 2016-01-12
Also published as: GB2299715B; GB9600687D0

Abstract

A compressor (10; 100; 200; 300; 500) with a build-in reciprocating motor, comprises a cylindrical housing (20; 120; 210; 510) with two ends thereof fitted with two opposing electromagnets (30; 130; 230; 530), each has a circular inner pole (36; 136) and a coaxial annular outer pole (34; 134). A free piston (50; 150; 250; 560; 600) is disposed in the housing between the two electromagnets, dividing the interior of the housing into two chambers (I, II). The piston carries permanent magnet (40; 140, 145; 561; 610), providing inner and outer poles (44, 46; 141, 146) which have conical surface portions (43, 49; 141, 146) complementary with the corresponding poles (34, 36; 134, 136) of the electromagnets. Sliding pole pieces (630 and 660) can be used to increase the stroke length and reduce the piston's total weight. Valves (61, 63, 65; 161, 165) are fitted to form one-way flow passage connecting the inlet and the outlet of the compressor. In operation, the complementary surfaces of the electromagnets and the permanent magnets form concentric forces which drive the free piston axially while keeping it magnetically suspended. Buffer mechanisms, including air, spring and/or magnetic cushioning, are formed between the piston and each of the electromagnets to prevent direct physical impact between them. A movable support (280; 580) provides automatic adjustment of piston's stroke length in response to changes of output pressure. A circuit (285, 283, 212, 211, 221) is formed to circulate a lubricant for keeping the piston lubricated. Magnetic coupling arrangement (570, 545, 555) regulates magnetic flux interaction.

Description

Reciprocating Motor and Compressor Incorporating the Same Technical Field of Invention The present invention relates to a reciprocating motor and/or a compressor incorporating such a motor, and more particularly, to a reciprocating mechanism having a free piston with magnetic means so as to reciprocate in response to an alternating electromagnetic field.

Background of Invention It is known in the art that a conventional reciprocating compressor is driven by a rotary motor, in which a crank or cam mechanism is used to convert rotational movements of the motor rotor into reciprocating movements of one or more pistons. Such an compressor has the following drawbacks. Firstly, the arrangement is not efficient because its electric input has to drive a whole chain of mechanical parts, including unavoidably a motor rotor, a crankshaft, a piston-rod and a piston-head. It is not difficult to see that keeping such a chain of parts in operation per se consumes a lot of power before any useful work can be done to the medium to be compressed.Secondly, due to the conversion of rotary movements to reciprocating movements it is unavoidable that the piston-head is subject to strong side forces, which cause fiction and wear, and produce unwanted heat. In order to cope with this problem, it is necessary to incorporate further arrangements for supplying lubricant oil to the moving parts and keeping the mechanism cooled, which add more complications to the structure and further burden to the motor. Furthermore, due to the above reasons the parts used in such a compressor have to be made of high strength materials by precise machining processes, therefore high costs. Finally, the compressor, once built, has to work within a narrow range of rated working conditions with little flexibility to cope with changes of operation conditions or load.

Summary of Invention It is, therefore, a main object of the present invention to provide a reciprocating motor and/or compressor which overcomes the above problems and disadvantages.

According to one aspect of the invention, there is provided a reciprocating motor comprising: two driving members with magnetic pole means arranged coaxially opposing each other along a common axis; a reciprocating member with magnetic pole means disposed movably between and arranged coaxially with the pole means of said two driving members; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and the pole means of said reciprocating member have complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means of said driving members and said reciprocating member are arranged to form, in response to an electric input of said energising means, a push-and-pull driving force pair along said axis to cause said reciprocating member to reciprocate between said two driving members.

According to another aspect of the invention, there is provided a reciprocating compressor comprising: a cylindrical housing defining an inner space with two driving members with magnetic pole means arranged coaxially opposing each other along a common axis; a reciprocating member with magnetic pole means forming a piston, disposed in said inner space movable between and coaxial with said two driving members; valve means for forming at least one fluid passage into and out of said inner space; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and said reciprocating member has complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means of said magnetic driving members and said reciprocating member are arranged to form, in response to an electric input of said energising means, a push-and-pull driving force pair along said axis to cause said reciprocating member to reciprocate between said two driving members.

It is preferable that each of the two magnetic driving members is an electromagnet having annular inner and outer poles with conical pole faces, and the reciprocating member has complementary pole faces formed by permanent magnet. This arrangement reduces the magnetic resistance in the magnetic circuit during the reciprocating movements, and provides concentric driving forces to suspend or "float" the reciprocating member magnetically.

It is advantageous that the reciprocating member has a radially arranged permanent magnet which can have a shunt mechanism to regulate its magnetic flux to the driving members. Furthermore, sliding pole pieces can be arranged to improve the piston's flexibility and agility, so as to achieve a better energy efficiency. It is also preferable to provide a lubricant circulating circuit in the compressor to keep the piston and its sliding parts lubricated. Also, fluid dynamic and/or magnetic means can be used to cause the piston to rotate during its reciprocating movements, so as to improve its lubrication and reduce its wear.

It is advantageous that the buffer means are arranged to provide cushion effects to protect the reciprocating member at the end positions of its movements. The buffer mechanisms may include fluid cushion, spring cushion and/or magnetic cushion. Furthermore, the buffer mechanisms can also be used to balance the reciprocating member's movements in opposite directions to improve its energy efficiency.

It is advantageous that the driving members can have magnetic coupling means to selectively adjust the magnetic flux of each driving member according to the position of the reciprocating member.

It is advantageous that the mechanism can be powered by either AC or DC current.

The apparatus further comprises sensor means which adjust the AC current or reverse the DC current supply in response to the movement of said reciprocating member to each of two end positions along said axis. In this case the device can be made portable and powered by batteries.

It is preferable to have at least one of the driving members arranged in a manner that its position is adjustable along the axis in response to the output pressure, so as to provide automatic compensation to load changes.

The simple structure ofthe present invention make it suitable for mass production with low manufacturing costs. On the other hand, the fully "floated" piston reduces fiction hence ensure a long service life. In case of high pressure application, a plurality of the reciprocating compressors can be connected in series to build up output pressure, that is to say a wide range of application requirements can be met by combining standard models. The use of the magnetic coupling and magnetic shunt ensure high energy efficiency and at the same time avoids leakage of magnetic flux to the environment so that during operation the electromagnetic interference to environment is very small.

Brief Description of Drawings Further features, advantages and details of the present invention are to be described hereinbelow with reference to the preferred embodiments illustrated in the accompanying drawings, in which: Fig. 1 is a cross-sectional view taken along the central axis of a compressor according to a first embodiment of the present invention; Fig. 2 is a cross-sectional view taken along the plane B-B shown in Fig. 1; Fig. 3 is a cross-sectional view taken along the central axis of a compressor according to a second embodiment ofthe present invention; Fig. 4 is a cross-sectional view taken along the central axis of a compressor assembly according to a third embodiment ofthe present invention; Fig. 5 is a cross-sectional view taken along the plane C-C shown in Fig. 4;; Figs. 6A and 6B are cross-sectional views taken along the central axis of a compressor according to a fourth embodiment ofthe present invention; Figs. 7A and 7B are sectional views ofthe piston 560 shown in Figs. 6A and 6B; Figs. 8A and 8B are sectional views of the electromagnet 530 and its cushion arrangements 540 and 550, as shown in Figs. 6A and 6B; Figs. 9A and 9B are cross-sectional views taken along the central axis of a piston according to a fifth embodiment ofthe present invention, and Figs. 10A and 10B are sectional views of a magnet disc 610 and a magnet ring 640' taken along the planes C-C and D-D, respectively, as shown in Fig. 9A.

Detailed Description of Preferred Embodiments In this application, the illustrated embodiments are described as compressors for the sake of easy understanding. It should be understood that the same concept can be used for gas as well as for liquid, and also it can be used as a vacuum pump. The term compressor should be interpreted as covering all these applications, unless stated otherwise specifically.

Also, the same concept can be used to build a linear motor by simply removing valves and seal members. For this reason, there is no need for a separate description on how to change each compressor embodiment into a linear motor.

Figs. 1 and 2 illustrate a first preferred embodiment of the compressor according to the present invention. As shown, a compressor 10 has a cylindrical housing 20, two annular electromagnets 30 fitted to two axial ends of the housing 20, and a free piston 50 located in the inner space defined by the housing 20 and the two electromagnets 30. The piston 50 divides the inner space ofthe compressor 10 into two chambers I and II.

The housing 20 is a tubular cylinder which can have external ribs and/or fins to improve its mechanical strength and heat dissipation, especially when it is used as a gas compressor. The housing 20 can be made of any non-magnetic material, such as plastics, fibre-reinforced resin, ceramics, aluminium, copper, brass, stainless steel etc. In case of the application as a gas compressor in a cooling system, non-magnetic metals are preferable for good heat dissipation. A low friction and wear resistance coating (not shown) can be formed on the inner surface of the housing 20 by conventional methods.

The two electromagnets 30 have basically the same structure, i.e. each has an annular core with an outer pole 34 and a coaxial inner pole 36, which is connected to the outer pole by a connection portion 31. A toroidal coil 32 is fitted in the space between the two poles 34 and 36. The pole 34 has an outer surface which is formed by a conical portion 33 with functions to be described later, and a cylindrical portion engaging the inner surface of the housing 20.

Two seals 35 are fitted to this cylindrical portion to ensure an airtight engagement between the outer surface of the electromagnet 30 and the inner surface of the housing 20. The two electromagnets 30 are secured to the ends of the housing 20 by fixing members 21, shown as screws in Fig 1. The inner pole 36 has a central hole which is formed by a small through-hole portion 37, a cylindrical middle portion 38 and a conical portion 39 extending towards the free end of the pole 36. An one-way valve 61 or 65 is fitted to one end of the through hole 37 of each electromagnet 30 to cover the hole 37 forming an one-way fluid communication between the exterior ofthe housing 20 and each ofthe chambers I and II. Each ofthe poles 34 and 36 has a conical surface portion 33 or 39 which is directed towards the free piston 50.The leading edges of the poles 34 and 36 are separated by a non-magnetic member 321, which can be a part of the bobbin for the coil 32.

The core of the electromagnets 30 can be made of any "soft" magnetic material, such as iron, steel or ferrite, and it is preferable to make the core by moulding, casing or diepressing, using particles of magnetic material held together by a bonding agent, e.g. a resin.

The free piston 50 is axisymmetrical and also symmetrical about the central plane perpendicular to its axis, i.e. its left-hand half is a mirror image of its right-hand half The piston 50 has a supporting frame formed by a non-magnetic material, including an outer cylinder 51, an inner tube 53 and a central disc 55 connecting the outer cylinder 51 to the inner tube 53. As shown in Figs. 1 and 2, the material of the disc 55 is thicker than that of the cylinder 51 or tube 53, and a number of ribs 57 and 59 are formed to further strengthen the connection between the cylinder 51 and the disc 55 and between the tube 53 and the disc 55.

The central disc 55 is perpendicular to the central axis of the cylinder 51 and the tube 59, which are coaxial, so that the disc 55 divides the inner space of the compressor 10 into the two chambers I and II. At each end of the outer surface of the cylinder 51 there is a seal 52 made of low friction and wear resistance material for forming an airtight engagement with the inner surface of the housing 20. To further reduce the frictional resistance between them, the seal 52 can be formed by a number of spiral ridges. At the central point of the disc 55 there is a through hole 47 which forms the only fluid communication between the two chambers I and II separated by the piston 50. An one-way valve 63 is fitted to the hole 47 to control the fluid flow through the hole.

The piston 50 carries, on each side thereof, a permanent magnet 40 formed in a structure which is generally complementary to the configuration of the two poles of a corresponding electromagnet 30. That is to say that each permanent magnet 40 has a central pole 46 with a conical outer surface portion 49 corresponding to the conical surface portion 39 of the inner pole 36 of the electromagnet 30, and an annular outer pole 44 with a conical inner surface portion 43 corresponding to the conical outer surface portion 33 of the outer pole 34 of the electromagnet 30. The central and outer poles 46 and 44 are magnetically connected to each other by a magnetic layer 41 formed on the non-magnetic disc 55, so that the two magnets 40 are magnetically insulated from each other.As shown in the left-hand side of Fig 1, the permanent magnet 40 carried by the piston 50 is attracted towards the electromagnet 30 at left end ofthe compressor 10 so that the corresponding poles ofthe two magnets are brought close to each other. The leading end of the tube 53 has a seal 54 which forms an airtight contact with the side wall of the middle portion 38 of the inner pole 36 of the left-hand electromagnet 30.

The supporting frame of the piston 50 can be formed by moulding, casting or diepressing. It is preferable to use the same material for making the frame as that for making the housing 20 so that two of them have the same thermal expansion during the operation of the compressor 10. The permanent magnets 40 carried by the piston 50 can be formed by a separate casting or pressing step after the supporting frame has been made. It is preferable to use rare-earth material for forming the permanent magnets 40, so as to have high magnetic strength and low weight. The permanent magnets 40 can be formed as a thin layer on the frame or even as a film of magnetic coating, as long as they provide a strong enough magnetic field and the required general configuration. An extra coating of a magnetic conductive material can be formed on the surface of the central tube 53 and the cylinder 51 before the permanent magnet is formed on top of it, to make their axial ends magnetically conductive.

It should be noted that since both the housing 20 and the supporting frame of the piston 50 are made of non-magnetic material, the two pairs of the permanent magnets 40 and the electromagnets 30, which defines the two chambers ofthe compressor 10, are magnetically separated and well insulated from each other and from the environment of the compressor 10.

The match of the configurations in each magnet pair and the magnetic insulation from the environment ensure that the distribution of the magnetic field is concentrated between the two magnets of each pair so as to provide maximum attracting or repelling force, according to the polarity ofthe opposing poles ofthe magnets.

The magnetic polarity of the inner and outer poles of the electromagnets 30 and the permanent magnets 40 are arranged in a manner that when the two electromagnets 30 are energised at the same time, they provide a combined push-and-pull force onto the piston 50, so that the piston is forced to move towards one end of the compressor 10, (to the left-hand end as shown in Fig 1), while when the electric current to the two electromagnets is changed to an opposite direction, the push-and-pull force will also change to an opposite direction to force the piston to move to the other end. One way of arranging the magnetic polarity is to keep the inner pole 36 of the two electromagnets 30 at the same polarity while the two inner poles 46 of the magnets 40 are in different polarity.For example, the left-hand one is S and the right-hand one is N as shown in Fig 1, so as to form a push-and-pull combination.

The conical surface portions 33 and 39 of the electromagnets 30 and the conical surface portions 43 and 43 of the permanent magnets 40 have the effects of reducing the air gap in the magnetic circuit formed by the pair of an electromagnet 30 and a permanent magnet 40, while at the same time allowing a relatively long stroke length for the piston movement.

As shown in Fig 1, the lengths of the maximum air gaps are 1z and 12. respectively, while the stroke length is 13, which is significantly longer. This arrangement ensures that closed magnetic circuits are formed between each of the electromagnets and the corresponding one of the permanent magnets within the housing so that it reduces the magnetic resistance in the magnetic circuits, therefore improve the system efficiency. On the other hand, when the piston is at one of its end positions, one pair of magnets would be at the status of having a minimum gap, as the left-hand pair shown in Fig. 1. At this status the pair would have the maximum repelling force when the current direction of the coil 32 is changed over.This maximum repelling force would compensate the attracting force of the other pair which have the maximum gap at that status. As the gap increases at one side ofthe piston the one at the other side decreases so the combined push-and-pull force by the two sides would be stable.

The coaxial and nested configuration of the electromagnets 30 and the permanent magnets 40 has the effects of forming an annular magnetic flux between themselves without magnetic leakage to the exterior of the housing and which is evenly distributed around the axis of the piston movement. This will in turn form an evenly distributed concentric driving force between each magnet pair. Under the effect of such a driving force, the piston is magnetically suspended to operate in a floated and at the same time axially aligned status, therefore the friction between the outer surface of the piston 50 and the inner surface of the housing 20 is minimized, producing increased efficiency and reducing the wear of the seals. It should be noted that except the valve members the piston 50 is the only movable component, therefore the only component subject to wear during the compressor's operation.The above 'oating" effects of the annular magnetic field would prolong the service life of the compressor significantly.

The fluid circuit through the compressor 10 is formed by three one-way valves 61, 63 and 65 which connect the inlet end ofthe compressor 10 (right-hand end as shown in Fig. 1) to chamber II, then to chamber I and finally to the outlet end of the compressor. The valves 61, 63 and 65 can be any conventional type, such as flap valve, disc valve or ball valve.

As shown in Fig. 1, the seal 54 at left-hand free end ofthe central tube 53 ofthe piston 50 also serves as a valve to stop fluid flow from the chamber I to the outlet valve 65, therefore the remaining space of the chamber I becomes a "dead" space because the fluid in the space is trapped and cannot flow out. This is designed to work as a cushion to prevent the piston 50 from hitting the electromagnet 30 at the left-hand end of the compressor. The seal 54 at the other end of the piston works in the same way in an opposite stroke of the compressor. The size of this buffer space depends on whether the fluid is gas or liquid, and it can be easily adjusted by changing the axial position of the seals 54 along the tube 53.In case of a gas compressor, the space needs to be relative larger to allow the gas trapped in the space to be compressed so the seals 54 are positioned at leading edge of each end of the tube 53, and the space would work as a gas spring, while in case of a liquid pump, the space can be smaller and the seal 54 can be positioned backwards or even omitted because the narrow gap between each end of the tube 53 and the inner wall of the corresponding hole 38 would be small enough to restrict flow rate of the liquid so as to effectively buffer the piston's movement.

When the two strokes of the piston 40 are subject to different resistance, e.g. due to higher output pressure which produces higher resistance to the output stroke, the seals 54 at the two opposite ends of the tube 53 can be located at different positions relative to the respective leading edge so as to have slightly different buffering effect which balances the two strokes of the piston movement.

It is clear now that although the piston 50 itself is made of light material and carries highly fragile permanent magnets, it is nevertheless fully protected and highly durable due to the evenly distributed driving forces and the fluid buffer arrangement.

For the purposes of operation control, a sensor 70 is fitted in the through-hole 38 of each electromagnet 30, for detecting the position of the piston 50. The sensor 70 can be simply a series of electric contacts which are connected when in contact with the seal 54 which is made conductive. The sensor can also be capacitive or magnetic type, which produces sensing signals without physical contact. The output signals from the sensor 70 are used to control the current to the coil 32 ofthe electromagnet 30 in response to a change of load. For example, when the electromagnet 30 is powered by an AC input, the signals from the sensor 70 are compared with the phase of the input current to see whether the current is too large or too small.In case the input current is too large, the magnetic driving force to the piston would also be large so the piston 50 would move faster and arrive at the end position earlier, while when input is too small, the magnetic force would be small and move the piston slowly, so it arrives at the end position later, or even not arrive at the position of the sensor at all.

This would be checked by the sensor, and the input current can be adjusted accordingly.

When DC power is used, the signals from the sensors 70 are used to switch the direction of the DC input, causing the piston to reciprocate.

Fig. 3 shows a second embodiment of the present invention. In Fig. 3, a compressor 100 has a cylindrical housing 120 and two electromagnets 130 fixed via supporting plates 121 to each axial end of the housing. Inlet valves 161 are also fitted on the plates 121. Each electromagnet 130 has an annular core providing two poles 134 and 136. A coil 132 is fitted between the two poles 134 and 136. A central conical cavity 139 is formed in the central pole 136. A piston member 150 is located between the two electromagnets. The piston member 150 has a central permanent magnet 140 having two conical tips, a cylindrical permanent magnet 145 and an annular member 153 of non-magnetic material for securing the tow magnets 140 and 145 together. The magnet 145 is surrounded by a cylinder 151 which is made of non-magnetic material, preferably the same material as the housing 120. The cylinder 151 has seals 156, similar to that of the first embodiment. The piston member divides the internal space of the housing 120 into two separate chambers I and II which are sealed from each other by the sealing members 156. The sealing members 156 also provide low fiction contact between the external surface of the cylinder 151 and the inner surface of the housing.

The tips of the permanent magnet 140 are shaped to fit into the central cavity 139 of the electromagnets 130 when the piston is attracted to one of the electromagnets. The pole direction ofthe two electromagnets 130 are arranged in the same manner, that is to say, when an electrical current is supplied to each ofthe coils 132, the central poles 136 are ofthe same magnetic polarity, say both of them are North. In this case the piston 150 is attracted at lefthand side as shown in Fig. 3 while at the same time repelled at the right-hand side. This combined push-and-pull force moves the piston to the left-hand side. By changing the direction ofthe current in the coils 132, the movement ofthe piston will also be changed.The inlet valves 161 are arranged at two ends so that when the piston 140 is moved from right to left, the valves 160 at right end are opened to let the fluid into the chamber II while at the same time the outlet valve 165 at the left end is opened to let the fluid to be forced out of the chamber I. When the piston arrives at the end position as shown in Fig. 3, the leading edge of the outer cylinder 151 blocks the outlet valve 165 to form a buffer space between the piston and the electromagnet, which works in a similar way as the first embodiment.

In this embodiment the second permanent magnet 145 is not necessary for its operation. When the magnetic field provided by the magnet 140 is strong enough, the magnet 145 can be replaced by a magnetically conductive member. It should be noted that this embodiment provides a well balanced operation in the sense that the piston's left and right strokes have exactly the same fluid compression function, and are subject to the same moving resistance. When two identical compressors 100 are arranged end to end along the same axis, and their pistons are made to move always in opposite directions, e.g. by connecting the electric supply in opposite direction, the pair would provide a highly balanced arrangement in which the impacts by the two pistons always cancel each other so the pair as a whole cause virtually no vibration.Obviously, the arrangement would be highly beneficial to the applications where low noise and vibration are required.

Fig. 4 shows a compressor assembly according to a third preferred embodiment of the present invention. Fig. 5 is the cross-sectional view taken along the plane C-C in Fig. 4, showing details ofthe piston 250.

In Fig. 4, the assembly is a multistage compressing arrangement formed by two serially connected compressors, 200 and 300, which are of the similar structure. The difference between them is that the first stage compressor 200 is one size larger than the next stage compressor 300, so a gaseous working medium can be progressively compressed. If however, the working medium is liquid, the chain should be made of pumps of the same size for the liquid is not compressible, but the operating principles are the same. Obviously, further stages of compressors can be connected into the chain when a higher output pressure is required, and adapter members can be used in the chain between compressors. In the following description, only the details of the compressor 200 are explained.

The compressor 200 has a tubular housing 210 with a lining member 220, two electromagnets 230, a free-piston 250 and a movable support 280. The electromagnets and the piston work in the similar manner as in the previous embodiments.

In Fig. 5, the housing 210 has on its inner surface grooves 211 which are connected at their top ends to the small through-holes 221 formed on the lining member 220 in Fig. 4. The lining member 220 is secured inside the housing 210 by thermal fitting, i.e. by fitting a cool lining 220 into a heated and expanded casing 210 so that when it cools down the housing grips the lining firmly. At their lower ends a chamber 212 is formed between the casing and the lining, and the small grooves 211 form internal channels connecting the chamber 212 to the through-holes 221 for lubricant circulation.

The one-way fluid communication through the compressor 200 is formed by the inlet valves 261, piston valves 263 and an outlet valve 265. As shown in Fig. 5, the piston valves are formed by a number of angled through-holes 262 formed on the piston disc, which holes direct fluid flow in outward tangential directions, and the arrangement of a flap member indicated by the dot line in Fig. 5 encourages such outward flow. The effects of these tangential flows are to cause the piston to rotate, as shown by the arrow sign R Between the piston 250 and each of the two electromagnets 230 and 230', a buffer mechanism is formed by seals 271 and 273.When the piston 250 moves upwards to the end position as shown in Fig. 4, the seal 271, on the inner surface ofthe piston cylinder, acts with the outer cylindrical surface of the top electromagnet 230 to form an annular gas cushion between the leading edge of the piston cylinder and the inlet valves 261 on the supporting plate of the top electromagnet 230. At the same time, the inner seal 273 engages the central cavity of the electromagnet 230 to form a central gas cushion in the top cavity 237. These two cushions protect the piston from hitting the upper electromagnet 230. When the piston moves downwards to the lower end, the other inner seal 273 would block the outlet valve 265, forming a cushion between the piston disc and the top end of the lower electromagnet 230'.

Lubricant liquid 285 collected around the lower electromagnet 230' would also buffer the piston, protecting it from hitting the bottom electromagnet.

A movable support 280 is used to carry the lower electromagnet 230' and the outlet valve 265. The support 280 has a base member 281, a sleeve member 282 and a biasing spring 284, which urges the base member away from the piston. The leading edge of the sleeve member 282 forms a sliding contact with the lower edge of the lining member 220, and between them a number of grooves are formed to provide fluid communication with the chamber 212, in which is located the spring 284. The sleeve member 282 also serves as a stopper, when it contacts with the top edge ofthe chamber 212, to define the upmost position ofthe support 280. The range of movement for the support 280 is shown by the double-arrow sign 286.In operation, when the output pressure is low, e.g. during the start-up of the compressor, the spring 284 urges the support 280 downwards against the leading end of the next stage compressor 300, and at this position the piston has the longest stroke length, so the compressor has the largest throughput. As the output pressure is gradually built up, due to the compressor's operation, the total force on the bottom surface of the base 281 would eventually become larger than the biasing force ofthe spring 284, so the support 280 would be forced by the gas pressure on its lower surface to move upwards. This movement reduces the stroke length therefore reduces the output by each piston stroke.Furthermore, due to the reduced stroke length, the average magnetic gap in the magnetic circuit is also reduced, with the effect of an increased driving force, producing a higher output pressure. When there is a big enough drop of the output pressure, e.g. due to an increased release of the compressed medium at the system outlet, the support 280 would immediately resume its original position by sliding down in the housing 210 under the biasing force of the spring 284, then the compressor is ready to work in its maximum output rate. That is to say, by this adjustment in response to the changes of output pressure, the compressor changes automatically from a lowpressure high-output operation to a high-pressure low-output operation, or vice versa. This automatic adjustment becomes more beneficial when a number of compressors are connected serially in a multistage arrangement, in which each of them can adjust its own rate to match with the others in the chain so that the load is evenly distributed over the whole chain.

A lubricant liquid 285 is used in the compressor 200 for keeping the outer surface of the piston 250 and the inner surface of the lining 220 lubricated, and the liquid would end up in the collecting area around the lower electromagnet 230'. A lubricant circulating circuit is formed by the grooves 283 formed on the inner surface of the sleeve member 282, the spring chamber 212, the grooves 211 formed between the housing 210 and the lining 220, and the through-holes 221 which return the lubricant back to the piston. The grooves 211 and 283 are made small enough so that the lubricant is sucked into the grooves 283 and 211 mainly by their capillary effects.When the piston 250 moves downwards, on the one hand the increased pressure on the liquid surface would force the liquid into the chamber 212 then to enter the capillary grooves 211, while on the other hand the holes 221 would be exposed to the low pressure side of the piston, allowing the liquid to come out of the grooves. When the piston moves upwards in a return stroke, the valves 263 would open, causing the piston to rotate as mentioned above, therefore to spread the lubricant evenly around the whole inner surface of the lining 220, forming a film of lubricant between the outer surface of the piston and the inner surface of the lining 220. This film also improves the sealing around the piston.It is worth mentioning that the position of the holes 221 are selected so that they are covered when the piston is at its upper position, therefore not causing any gas leakage between the two sides of the piston.

Figs. 6A to 8B show a compressor 500 according to a fourth preferred embodiment of the present invention. Figs. 6A and 6B are cross-sectional views taken along the central axis of the compressor showing the positional changes of the different parts of the compressor during its operation. Fig. 7A shows more details of the piston 560, and Fig. 7B is the crosssectional view taken along the plane B-B in Fig. 7A. Similarly, Figs. 8A and 8B show the details ofthe electromagnet 530 and the buffer arrangements 540 and 550.

As shown in Fig. 6A, the compressor 500 has a tubular housing 510 with a lining member 511 similar to that shown in Figs. 4 and 5, a free piston 560 with a movable magnetic shunt mechanism 570, two electromagnets 530 and 530', one supported by an end plate 520 and the other by a movable support 580. Each ofthe electromagnets 530 and 530' has outer and inner buffer mechanisms 540 and 550, or 540' and 550' for preventing the free piston 560 from hitting the electromagnets directly. The end plate 520 and the movable support 580 also provide the fluid inlets 521 and 523 and outlets 582, respectively. The lining member 511, which is made of non-magnetic steel, and the movable support 580 are similar to and operate in the same way as the members 220 and 280 shown in Fig. 4 to provide lubricant circulation and automatic stroke length adjustment.It is worth mentioning that when the compressor 500 is used to work with a liquid, e.g. it can be used as a hydraulic pump for high pressure and leak-free applications, the piston can be lubricated by the working medium therefore the lubricant circulation arrangement would not be needed. In this case further outlet holes can be formed close to the outer periphery ofthe support 580 to match the holes 521 on the top end plate 520.

Generally speaking, in operation the piston 560 moves between two end positions under the driving forces by the two electromagnets 530 and 530', similar to the previous embodiments. When analysed from the viewpoint ofthe fluid compression, the piston's down strokes suck in fluid through the inlet holes 521 and 523 on the top plate 520, and at the same time drive the fluid already in the compressor out ofthe outlet holes 582 in the bottom support 580. In contrast, the upward strokes merely force the fluid from upper side of the piston to its lower side without producing any output. That is to say, the two strokes are unbalanced with most of the actual work done by the downward strokes and with its upward strokes simply as return movements. Such unbalanced piston movements reduce the compressor's energy efficiency.In order to tackle this problem, arrangements are made in the compressor 500 to balance the two types of strokes by converting the kinetic energy of the upward piston movements into energy reserve in different buffer mechanisms, which is then released during the piston's downward strokes to make the compressor operation more efficient. The buffer arrangements include the inner and outer buffers 540, 550, 540' and 550' associated with the two electromagnets 530 and 530' and the springs in the magnetic shunt mechanism 570 of the piston 560.

As shown in Fig. 7A, the free piston 560 has a main permanent magnet 561 in the shape of an annular disc. The polarity of the magnet 561 is in radial direction, i.e. with its outer periphery as the south S, and the inner periphery as the north N. The thickness of the disc 561 increases from its outer periphery inwards to ensure that the inner and outer pole faces are of the similar sizes and the cross sectional area perpendicular to the magnet flux maintains unchanged along the flux direction to avoid local flux saturation. A cylindrical magnetic member 562 is secured to the outer periphery of the disc 561, serving as the pole piece for the outer pole and a generally tubular pole piece 563 is secured to the inner pole, so that the magnet 561 has its two pole pieces 562 and 563 arranged coaxially to form generally radially directed magnetic flux, as shown by the dash lines in Fig. 7A.

Each side surface of the annular disc 561 is covered by a protection member 564 or 564', made of plastics or rubber, to keep the disc 561 sandwiched therebetween to protect this relatively brittle member from being damaged by mechanical shocks and also to hold the disc. and its two pole pieces together. The two members 564 and 564' can be formed by injection moulding, preferably by using gasified plastics or foamed rubber, after the disc and the pole pieces having been assembled so as to form an integrated structure of good mechanical strength and light weight. The members 564 and 564' also define smooth surfaces which closely match with the pole fices of the two electromagnets, to reduce any "dead space" between the piston faces and the driving electromagnets during their operation. The member 564' also has a grove in which is fitted a valve member 566. As also shown in Fig.

7B, a number of angled through-holes 565 are formed in the disc 561 and the protection members 564 and 564', which are covered by the valve member 566 to form a series of oneway flap valves, which cause the piston 560 to rotate in direction R in Fig. 7B during its operation, in a way similar to the previous embodiment.

The inner pole piece 563 has a central supporting part 567 which carries a magnetic shunt mechanism 570. The shunt 570 is formed by two permanent magnets 571 and 571' connected together by a non-magnetic bar 572. The bar 572 is movably supported by the part 567 which has seals to prevent any leakage through the sliding engagement between them.

Two springs 573 and 574 are each used between the part 567 and one ofthe magnets 571 and 571' to provide biasing forces. Low friction bushing is used for the springs to allow the shunt to rotate relative to the piston. The springs 573 and 574 are made of magnetic steel so they also provide magnetic connection, via the inner pole piece 563, between the magnet 571 or 571' and the disc magnet 561. The polarity ofthe shunt magnets are arranged in a way that both of them are magnetically attracted by the inner pole of disc magnet 561. The leading face ofthe shunt magnet 571 or 571' is covered by a soft layer, such as rubber, to protect it.

The magnetic circuit formed by the piston 560 has two branches, each includes an air gap between a leading edge ofthe pole piece 562 and the pole face ofthe magnet 571 or 571' for acting with a corresponding electromagnet, and the main magnet 561 serves as the common route shared by the two circuit branches. In Fig. 7A, the magnetic shunt 570 is at a neutral position relative to the disc magnet 561, where the two circuit branches have equal flux distribution indicated by the dash lines. However, this balanced flux distribution is unstable because a very slight axial movement ofthe shunt 570 relative to the disc 561 would increase the flux in one branch at the expense of the other. In other words, from the viewpoint of the flux distribution the shunt 570 serves as a magnetic switch which decides by its axial position that which circuit branch would have a bigger share of the total flux. This arrangement is also used as a buffer mechanism by having a natural status predetermined for the magnetic switch by the two springs 573 and 574 which are arranged to form a push-andpull pair, i.e. the upper spring 573 is a compression one which tends to expand and push the magnet 571 upwards and away from the disc magnet 561, while the lower spring 574 is an extension one which tends to contract and pull the magnet 571' towards the disc magnet 561.

That is to say, when not subjected to external forces the shunt 570 would naturally end up at a position where the magnet 571' is attracted to and engages with the lower end of the inner pole piece 563, pushing the other magnet 571, via the bar 572, to a position far away from the top end of the inner pole piece 563. At this position the flux of the lower branch would be much larger than that ofthe upper branch. In operation the piston has to overcome the biasing forces of the springs 573 and 574 in its return strokes and then release the force during its output strokes. Furthermore, the axial movement ofthe shunt 570 helps to switch the main magnetic flux from one end of the piston to the other so as to act with a corresponding electromagnet 530 or 530' in a more efficient way, as to be described later.

Fig. 8A is a partial sectional view with the right half of the electromagnet 530 and the outer buffer 540 shown in section and their left half in front view. It also shows details of the top end plate 520 and the inner buffer 550. Details ofthe corresponding members 530', 540' and 550' at the bottom end of the compressor are basically the same, unless to be described otherwise.

More specifically, the end plate 520 is secured onto the housing 510 and it carries the electromagnet 530 and its buffer mechanisms. A series of outer inlet holes 521 and a series of inner inlet holes 523 are formed on the end plate 520, corresponding to the outer periphery and the central hole of the annular electromagnet 530, and being covered by flap member 522 or 524 to form one-way valves which supply fluid into the compressor. Details of the flap members 522 and 524 are not shown in Fig. 8B but they are similar to the flap member 566 shown in Fig. 7B. The new features of the electromagnet 530 are the angled slots 536 and 537 formed on its outer and inner conical pole faces, and the slots 534 and 535 formed on the opposite end, all in axial direction.The slots are formed to facilitate the fluid flow from the holes 521 and 523 into the compressor chamber and at the same time helping to cool the electromagnet 530 and to reduce eddy currents in the core material. It should be noted, as also shown in Fig. 6B, that the slots 534 and 536 on the surface of the outer pole 531 are not connected to one other, neither the slots 535 and 537 on the surface ofthe inner pole 533. On each of the surface a narrow band free of any slots remains between the two sets of slots, which co-operates with a seal member 547 or 557 of the two buffer 540 and 550 to provide air cushion effects.

Associated with the electromagnet 530 are the outer buffer 540 surrounding it and an inner buffer 550 in its central hole. The buffer 540 includes a cushion ring 541 biased downwards by an expansion spring 549. The ring 541 has a number of retaining fingers 542 extending in axial direction, each having a hooked tip 543 fitted into one of the slots 534 to define the range ofthe buffer's movement by the length ofthe slots 534, as shown in Figs. 6A and 6B. The ring 541 with its finger members 542 is made of a non-magnetic and rigid material, such as aluminium or plastics, to provide good mechanical strength. The ring 541 carries a flux coupler 544 made of a magnetic material, which is in the shape of a ring carrying a number of fingers 545 each with a hooked tip 546.The fingers 545 are made of flexible and resilient material so that the fingers can bend easily when attracted by magnetic force. The tips 546 are rigid and they also have the function of limiting the buffer's upward movement by engaging the leading edge ofthe outer pole 531. The magnetic fingers 545 are configured to fit closely to the conical pole face of the outer pole 531 when the cushion ring is in the contracted position, as shown in Fig. 8A. Shallow groves can be formed on the conical surface so that the fingers 545 can fit into them to provide a smooth pole face to match with the complementing face of the piston 560, as mentioned above. The sealing ring 547 is clamped between the ring 541 and the ring 544, to form an air cushion when it engages the annular band on the surface ofthe outer pole 531.

Similar to the outer buffer 540, the inner buffer 550 has a cushion ring 551 of a nonmagnetic and rigid material, biased by a spring 559. The ring 551 has a number of retaining fingers 552 each having a hooked tip 553 fitted into one of the slots 535 to define its movement. A flux coupler 554 has a number of resilient fingers 555 each with a hooked tip 556 to match with a corresponding tip 546 ofthe outer buffer 540. A sealing ring 557 is fitted between the rings 551 and 554, for forming an air cushion in the central hole of the electromagnet 530. The inner buffer 550 also includes a permanent magnet 558 with a polarity repulsive to that ofthe shunt magnet 571 for providing magnetic cushion to the piston 560. The operations ofthe buffers 540 and 550 are to be explained below.

Now the operations of the compressor 500 are to be described with reference to Figs.

6A and 6B, in which different positions of the piston 560 are shown together with the changes ofthe cushion and driving mechanisms.

Firstly, the operation ofthe buffer mechanisms is explained in details.

Fig. 6A shows that the piston 560 is moved to its up end position where it is well cushioned by the two buffers 540 and 550, which provide air cushion by the effects of the seals 547 and 557, spring cushion by the compressed springs 549 and 559 and magnetic cushion by the central magnet 558 which expels the shunt magnet 571. All these cushion effects are at the same time working as energy reserves which is to be released when the piston starts the downward stroke. In Fig. 6B, the piston 560 is in the middle of its downward stroke, as indicated by the arrow A. At the this particular position, in addition to the magnetic driving force by the electromagnets, the piston's outer cylinder pole piece is pushed downwards by the outer cushion ring 541 and the shunt 570 is forced down by the inner buffer 550.The piston is moving to the bottom end, where the main cushion effect will be provided by the resistance of the compressed fluid, which is being forced out through the outlet holes 582, together with the effects of the buffers 540' and 550' at the end of the movement. In addition to these, the lubricating liquid surrounding the lower outer periphery of the electromagnet 530' also helps to buffer the piston's final movement, therefore air cushion effects are not provided by the buffers 540' and 550', i.e. their rings 547' and 557' do not have sealing effects.

Since the working load of the piston's upward strokes is smaller than that of the downward strokes, as mentioned before, the upper cushion springs 549 and 559 are selected to be stronger than the springs 549' and 559'. That is to say, the piston's upward strokes are powered basically by the push-and-pull force of the electromagnets alone and the piston's upward kinetic energy is converted into the elastic energy of the springs 549 and 559 for helping the piston's down stroke. Further compensation is made by the permanent magnets 558 and 558', which are arranged with the permanent 558 opposing the piston's upward movement while the magnet 558' attracting its downward movement, forming another pushand-pull pair. Finally the springs 573 and 574 of the shunt mechanism 570 also urges the piston downwards, as mentioned above.Due to these compensating arrangements, a significant portion of the push-and-pull force by the two electromagnets for piston's upward strokes is converted to make the piston's downward strokes more powerfuL therefore effectively balanced the piston's movements.

Now the magnetic couplings and their automatic flux switching mechanisms are described in details.

As shown in Fig. 6A, the piston 560 is driven by the combined force of the two electromagnets to the top end position where the upper branch of the piston's magnetic circuit, including the disc magnet 561 and the top shunt magnet 571, is included into the magnetic circuit of the electromagnet 530, which attracts the piston. At the same time, the magnetic circuit of the bottom electromagnet 530' is "closed" by a magnetic coupling formed by the magnetic fingers 545' and 555' ofthe buffers 540' and 550', which are at a raised-up position. The magnetic coupling is formed because that the fingers 545' are magnetised by the outer pole ofthe electromagnet while the fingers 555' by the inner pole, so these fingers are attracted to one another until they bend and their tips contact one another.The effects of this magnetic coupling is to increase the electromagnet's self-induction, therefore reduce the electric current passing through its coil at this particular moment. Since at this moment the distance between the electromagnet 530' and the piston is large, the electromagnet has no important influence to the piston's movement. In other words, electric power consumption by the electromagnet 530' is saved here when it is lest productive.

As shown in Fig. 6B, here the electric currents to the electromagnets are reversed, the piston 560 is driven downwards by the forces of the electromagnets and the buffers 540 and 550, etc. During the piston's downward stroke, the lower shunt magnet 571' contacts the buffer 550' first to break the magnetic coupling between the fingers 545' and 555'. Once they are separated, the fingers 545' are attracted to the lower edge of piston's outer pole piece to form a more effective magnetic connection between the outer pole of the electromagnet 530' and the outer pole of the piston, and the fingers 555' are attracted to the shunt magnet 571'.

Such contacts form closer connection between the lower branch of piston's magnetic circuit and the magnetic circuit of the electromagnet 530', causing stronger attraction between the piston and the electromagnet 530'.

On the other hand, once the piston has moved away from the top electromagnet 530, it makes enough room for the fingers 545 and 555, attracted to one another by their magnetic polarity, to bend until they touch one another. When this happens, the magnetic flux from the electromagnet 530 is effectively "turned off" from the piston 560.

It is clear from the above description that by incorporating the magnetic coupling mechanisms, each of the electromagnets is automatically "turned off" when it no longer has effective contribution in driving the piston, therefore the consumption of electric energy at such ineffective moments is significantly reduced and the compressor's overall energy efficiency is improved. Furthermore, the arrangement also improves general magnetic connection between the piston and the electromagnet by reducing the average size of the air gaps between them, i.e. increasing the magnetic flux concentration and the effectiveness of the magnetic connection.This is shown by the facts that when in operation the piston has a stroke length S shown at the left side of Fig. 6A, while the maximum air gap between the shunt magnet 571' and the magnetic fingers 555' is merely g, shown in Fig. 6A, which is much smaller compared with S. In addition, each ofthe buffer mechanisms 540, 550, 540' and 550' has a movable range of d, which is slightly less than a half of the stroke length S. That is to say, for most of time in operation the piston 560, including the shunt 570, is in direct contact with at least one of the buffers which serves as extension of piston's magnetic poles to make interactions between the piston and electromagnets more effective.

Figs. 9A to 10B show a piston 600 according to a fifth preferred embodiment of the present invention. This piston can be used in the compressor of the previous embodiment to replace the piston 560, and it is suitable when the compressor's capacity and therefore dimensions are large. Figs. 9A and 9B are cross-sectional views taken along the central axis of the piston showing the positional changes of its different parts during its operation. Fig.

10A is the cross-sectional view taken along the plane C-C in Fig. 9A, showing details of a magnet disc 610, and Fig. 10B is the cross-sectional view taken along the plane D-D in Fig.

9A, showing details of a magnet ring 640'.

As shown in Figs. 9A and 9B, the piston 600 has an annular permanent magnet disc 610 sandwiched between protection layers 620 and 620'. An outer sliding pole piece 630 engages the outer periphery of the disc 610 and an inner sliding pole piece 660 engages its inner periphery. The inner pole piece 660 carries a movable magnetic shunt 650 which works in a similar way as the magnetic shunt 570 of the previous embodiment.

More specifically, as also shown in Fig. 10A, the disc 610 has a number of inner channels 612 and outer channels 613 formed in axial direction on its inner and outer periphery.

The protective layers 620 and 620' on two sides of the disc are made by moulding plastic or rubber material to cover both sides and they are connected to each other via these channels to form an integrated body with the disc 610. This structure provides good protection to the brittle disc and also defines angled valve holes 611 covered by a valve member 621.

Now back to Fig. 9A, the outer pole piece 630 includes a cylinder member 631 made of a soft magnetic material, which is arranged to have sliding engagement with the outer periphery of the disc 610 fitted with two sliding seals 614. Two permanent magnet rings 640 are each fitted to an end of the cylinder 631, to provide a secondary magnet in each of the two circuit branches ofthe piston's magnetic circuit. A seal 634 is clamped between each ring 640 or 640' and the cylinder 631. A pair of springs 633 and 633' are each fitted between a ring 640 and a seal 614, to keep the disc 610 biased relative to the rings 640.

The inner pole piece 660 includes a magnetic cylinder 662 slidingly engaging the inner periphery of the disc 610, two magnetic end caps 661 each fitted to an end of the cylinder 662, and a non-magnetic filling stuff 663, such as gasified plastics, filed in the cylinder 662.

The magnetic shunt 650 includes two permanent magnet caps 651 and 651' connected to each other by a non-magnetic tube 652, which is slidingly supported on the inner pole piece 660 by the filling stuff 663 and sealed by seals 664. A pair of springs 653 and 653' keep the shunt 650 biased relative to the pole piece 660. The magnet cap 651 or 651' is shaped to have a leading surface providing pole face matching with that of an electromagnet as shown in Figs.

6A and 6B, and a back pole face matching that of the cap 661 or 661'. The magnet caps 651 and 651' form the secondary magnets at the other end of the piston's two circuit branches.

The polarities of the main and secondary magnets are arranged, as shown in Figs. 9A and 9B, to make each secondary magnet being attracted by the main magnet 610.

When in operation, the disc 610 would slide along the cylinder 631 towards one axial end, under the effects of the push-and-pull force of the two electromagnets. Fig. 9A shows the disc at its lower end position, where the lower protection member 620' engages both the magnet ring 640' and the magnet cap 651' to define a smooth piston face matching that of the pole faces of the lower electromagnet (not shown). At this position, the inner pole of the disc 610 is magnetically connected to the magnet cap 651' via the cylinder 662 and the cap 661' of the inner pole piece 660, so is the outer pole to the magnet ring 640' via the cylinder 631 and the spring 633'. That is to say, from the viewpoint of the lower electromagnet, the piston 600 as a whole behaves as a powerful permanent magnet.

As shown in Fig. 9B, once the electric currents in the two electromagnets are reversed in direction, the piston assembly is forced to move upwards, as indicated by an arrow A to the left. At the same time, since the disc 610 is no longer attracted by the lower electromagnet, it is forced by the springs 633 and 633' to move upwards along the cylinder 631. Similarly, the inner pole piece 660 is also forced upwards by the springs 653 and 653' until the upper edge ofthe cap 661' engages the disc 610 and carries the same to move with it. Since the inner pole piece 660 is very light and it is not subjected to any significant fluid resistance to its movement, it can move much faster than the disc 610, so as to effectively switch the disk's main magnetic flux from its lower branch to its upper branch, as described before in relation to the previous embodiment. Again, these springs can be arranged to provide more downward biasing forces for balancing the piston's movements. Also the cushioning and magnetic coupling for the piston 600 can be the same.

In operation, the disc 610 with its protection layers 620 and 621' can slide a distance S1 along the outer pole piece 630, in additional to the piston's stroke length S, as shown in Fig. 6A. That is to say, the piston's effective stroke length is increased without increasing the pole pieces' movements, so the piston has better agility in response to the driving forces by the electromagnets. The reduced movements of the outer pole piece 630, which contributes to a very large portion of the piston's total weight due to the steel cylinder 631, ensure that the overall energy efficiency is improved.

To keep the piston's moving parts well lubricated, a number of small holes are made through the cylinder 631, so that lubricant can enter the gap between the outer periphery of the disc 610 and the inner surface of the cylinder 631. Furthermore, a fabric layer 615 is sandwiched between each side face of the disc 610 and a corresponding protection layer 620 or 620' to provide capillary passages for the lubricant to enter the gap between the inner periphery of the disc 610 and the inner pole piece 660, so the disc is well lubricated for its movements relative to the inner and outer pole pieces.

Finally, an additional feature is that the leading pole face of the magnet rings 640 and 640' has a number of channels 641 and 641', which match the channels 536 formed on the outer pole face 531 in Fig. 6B. During the operation, these channels have the effects of causing the piston to rotate so that they can align with one another, in a way similar to the toothed patterns in the magnetic core of a stepping motor. By arranging the channels on the two rings 640 and 640' at staggered annular positions, they will ensure, together with the effects of the angled valve holes 611, continued rotation of the piston 600 during its reciprocating movements, so as to improve its lubrication and reduce its wear. The channels 641 are filled by a non-magnetic material to define a smooth piston face.

Industrial Applicability It is not difficult to understand that the reciprocating mechanisms disclosed in the present application can be used in many different applications. Due to the generally tubular structure of the compressor/pump of the present invention, they are especially advantageous in meeting the needs of an existing system design or hardware pipeline because such a compressor or pump can be easily fitted into an existing system at any position that a driving force is needed, without any need to re-design the system or change the existing hardware. It is also advantageous that the standard models can be connected serially or in parallel to meet a wide varieties of requirements to output pressures and/or flow rates.Furthermore, the tubular structure of the compressor makes it easy for heat dissipation, therefore ideal for applications in refrigeration and air-conditioning.

The above description of the preferred embodiments is made by way of examples, which also indicates that different features described above such as the polarity and/or general shape of the poles of the magnets or the flow direction of the valve arrangements, can be modified or changed by combining the described features in different ways. For example, the cross-sectional shape of the arrangement is described as cylindrical, but it can be replaced by other generally symmetrical polygonal shapes, especially when the number of sides of a polygon is large, therefore the term "cylindrical" should be interpreted accordingly.

Furthermore, as described above, the pistons with permanent magnets have the advantage of reduced weight, therefore high efficiency. Obviously, electromagnets can be used on piston to replace the permanent magnets and the whole arrangement would still work. Using electromagnet on the piston has an advantage that the magnetism will not be affected by temperature changes, therefore it may be desirable for applications in a relatively high temperature environment. On the other hand, when the piston carries an electromagnet, the two driving magnets flitted to the housing can use permanent magnets so as to reduce the total weight of the motor/compressor as a whole. Needless to say, many other changes, modifications or adjustments can be made by replacing equivalent parts of the described examples following the general concept of the present invention and all such changes should be within the scope ofthe present invention.

Claims

1. A reciprocating motor comprising: two driving members with magnetic pole means arranged coaxially opposing each other along a common axis thereof; a reciprocating member with magnetic pole means disposed movably between and coaxial with the pole means of said two driving members; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and said reciprocating member have complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means are arranged to form a pushand-pull driving force pair along said axis in response to an electric input of said energising means, so as to cause said reciprocating member to reciprocate.

2. A reciprocating compressor/pump comprising: a cylindrical housing defining an inner space with two driving members with magnetic pole means arranged coaxially opposing each other along a common axis; a reciprocating member with magnetic pole means forming a piston, being disposed in said inner space movable between and coaxial with said two driving members; valve means for forming one-way fluid passage into and out of said inner space; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and said reciprocating member have complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means are arranged to form a pushand-pull driving force pair along said axis in response to an electric input of said energising means so as to cause said reciprocating member to reciprocate.

3. An apparatus according to claim 1 or claim 2, wherein said two driving members are electromagnets and said reciprocating member has permanent magnet means.

4. An apparatus according to claim 1 or claim 2, wherein the pole means of said reciprocating member is formed by electromagnetic means.

5. An apparatus according to any of the preceding claims, wherein said inner and outer poles of each said driving member and said reciprocating member are annular with a substantially conical pole face so that the opposing poles of the driving members and that of the reciprocating member match one another along said axis.

6. An apparatus according to any of the preceding claims, wherein said reciprocating member carries two magnets, each said magnet forms the inner and outer poles facing one of said two driving members, and said two permanent magnets are magnetically insulated from each other by a non-magnetic member.

7. An apparatus according to any of claims 1 to 5, wherein said reciprocating member carries two magnets, one of which forms the inner pole of the reciprocating member and the other forms the outer pole, and said two magnets are magnetically insulated from each other by a non-magnetic member.

8. An apparatus according to any of claims 1 to 5, wherein said reciprocating member has a magnet with radially directed poles and inner and outer pole pieces providing pole faces facing said axial direction.

9. An apparatus according to claim 8, wherein said magnet is arranged slidingly movable relative to said outer pole piece.

10. An apparatus according to claim 9, wherein said magnet is biased relative to said outer pole piece by elastic means.

11. An apparatus according to any of claims 8 to 10, further comprising secondary permanent magnet means fitted to each axial end of said outer pole piece.

12. An apparatus according to any of claims 8 to 11, wherein said magnet is arranged slidingly movable relative to said inner pole piece.

13. An apparatus according to any of claims 8 to 12, wherein said reciprocating member has a magnetic shunt mechanism axially movable relative to said magnet, so as to regulate its magnetic flux to said two driving members.

14. An apparatus according to claim 13, wherein said movable shunt mechanism is biased by elastic means and is movable relative to said inner pole piece.

15. An apparatus according to claim 13 or claim 14, further comprising secondary permanent magnet means fitted to each axial end of said shunt mechanism

16. An apparatus according to any of preceding claims, further comprising buffer means to prevent direct impacts between said reciprocating member and the driving members at each end position ofthe reciprocating movement thereof.

17. An apparatus according to claim 16, wherein said buffer means include sealing means for forming a closed space at said end position for trapping fluid therein to provide fluid cushioning to said reciprocating member.

18. An apparatus according to claim 16 or claim 17, wherein said buffer means further include spring means for elastic cushioning to said reciprocating member.

19. An apparatus according to any of claim 16 to 18, wherein said buffer means further include means for magnetic cushioning to said reciprocating member.

20. An apparatus according to any of claim 10 to 19, wherein said elastic means, shunt mechanism and/or buffer means are adapted to balance said reciprocating member's axial movements.

21. An apparatus according to any of preceding claims, wherein said magnetic pole means of the driving members and of the reciprocating member are arranged in a manner to keep the magnetic flux therebetween within the space between the two opposing driving members.

22. An apparatus according to any of preceding claims, further comprising magnetic coupling means associated with the two driving members, which coupling means has movable members for selectively adjusting each driving member's magnetic flux.

23. An apparatus according to any of preceding claims, fiuther comprising sensor means for detecting the position of said reciprocating member relative to at least one of said driving members and means for adjusting current to said energising means accordingly.

24. An apparatus according to any of preceding claims, further comprising movable means for supporting at least one of said driving members and biasing means for adjusting the position of said supporting means in response to the output load of said reciprocating member.

25. An apparatus according to any of preceding claims, further comprising a lubricant circulating arrangement having capillary means for circulating a lubricant liquid.

26. An apparatus according to any one of claims 2 to 23, wherein said piston has fluid passage means for causing it to rotate during its reciprocating movements.

27. An apparatus according to claim 26, wherein said piston has magnetic means for causing it rotate during its reciprocating movements.

28. An apparatus according to any of preceding claims, wherein said energizing means is connected to a DC supply and comprises switching means which reverse said DC supply in response to the movement of said reciprocating member.

29. An assembly comprising a plurality of reciprocating compressors/pumps according to claim 2 or any one of claims 3-28 in combination with claim 2, said compressors/pumps being connected in series so as to build up a high output pressure.

30. A reciprocating motor constructed substantially as described herein with reference to any of Figs. 1 and 2, Fig. 3, Figs. 4 and 5, Figs. 6A to 8B or Figs. 9A to 10B of the accompanying drawings.

31. A compressor constructed substantially as described herein with reference to any of Figs. 1 and 2, Fig. 3, Figs. 4 and 5, Figs. 6A to 8B, or Figs. 9A to 10B ofthe accompanying drawings.

31. A compressor constructed substantially as described herein with reference to any of Figs. 1 and 2, Fig. 3, Figs. 4 and 5, Figs. 6A to 8B, or Figs. 9A to 10B of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A reciprocating motor comprising: two driving members with magnetic pole means arranged coaxially opposing each other along a common axis thereof; a reciprocating member with magnetic pole means disposed movably between and coaxial with the pole means of said two driving members; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and said reciprocating member has complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means are arranged to form a pushand-pull driving force pair along said axis in response to an electric input of said energising means, so as to cause said reciprocating member to reciprocate.

2. A reciprocating compressor/pump comprising: a cylindrical housing defining an inner space with two driving members with magnetic pole means arranged coaxially opposing each other along a common axis; a reciprocating member with magnetic pole means forming a piston, being disposed in said inner space movable between and coaxial with said two driving members; valve means for forming one-way fluid passage into and out of said inner space; and means for energising said driving members and/or reciprocating member to generate an alternating electromagnetic field therebetween; wherein said pole means of each said driving member include an outer pole of a first polarity and an inner pole of a second polarity located within the area defined by said outer pole, and said reciprocating member has complementary outer and inner poles facing that of the two driving members; and wherein the opposing pole means are arranged to form a pushand-pull driving force pair along said axis in response to an electric input of said energising means so as to cause said reciprocating member to reciprocate.

6. An apparatus according to any of claims 1 to 3 or 5, wherein said reciprocating member carries two magnets, each said magnet forms the inner and outer poles facing one of said two driving members, and said two permanent magnets are magnetically insulated from each other by a non-magnetic member.

7. An apparatus according to any of claims 1 to 3 or 5, wherein said reciprocating member carries two magnets, one of which forms the inner pole of the reciprocating member and the other forms the outer pole, and said two magnets are magnetically insulated from each other by a non-magnetic member.

8. An apparatus according to any of claims 1 to 3 or 5, wherein said reciprocating member has a magnet with radially directed poles and inner and outer pole pieces providing pole faces facing said axial direction.

15. An apparatus according to claim 13 or claim 14, further comprising secondary permanent magnet means fitted to each axial end of said shunt mechanism 16. An apparatus according to any of preceding claims, further comprising buffer means to prevent direct impacts between said reciprocating member and the driving members at each end position ofthe reciprocating movement thereof.

23. An apparatus according to any of preceding claims, further comprising sensor means for detecting the position of said reciprocating member relative to at least one of said driving members and means for adjusting current to said energising means accordingly.