Patent Publication Number: US-2019189322-A1

Title: Electromagnetic actuator

Description:
FIELD OF THE INVENTION 
     The present invention relates to electromagnetic actuators, and more particularly but not exclusively to electromagnetic actuators that are able to operate silently. It also concerns combination of a valve with an electromagnetic actuator. 
     BACKGROUND OF THE INVENTION 
     In known electromagnetic actuators, an armature including a permanent magnet is switchable between two stable rest positions using a pair of coils. Some examples of actuators of this form are described in GB-A-2466102. 
     Pinch valves are able to control the flow of a fluid through tubing without the valve entering the flowpath. The valve controls the fluid flow by pinching the tubing. Pinch valves are therefore particularly suitable for applications where contamination of the fluid is undesirable, such as medical applications, chemical process engineering and food technology for example. 
     Known electromagnetically operated pinch valves use solenoid type actuators. Such actuators are switchable between positions in which the tubing is either pinched or unpinched. A solenoid actuator is biased towards one position and when energised, it moves to and is retained in the other position. When the energising current is removed, the actuator then reverts to its original position, under the influence of the biasing force. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electromagnetic actuator comprising:
         an armature comprising a permanent magnet;   two coils; and   two coil cores, each of which extends within (or through) a respective coil;   wherein the armature is moveable relative to the coils between first and second stable rest positions by passing a current through at least one of the coils, and   in each rest position, the armature is closer to one of the coil cores than the other of the coil cores and is spaced by a gap from the closer coil core.       

     Accordingly, the armature may not contact a coil core (and preferably is not brought into contact with any other part of the actuator) as it moves into each rest position. 
     This avoids the generation of a clicking noise as the armature switches into either rest position. In known bistable actuators, an armature is accelerated into contact with a coil core in each stable rest position, causing a loud clicking noise to be produced. In many applications at which the present actuator is directed, such as medical environments, frequent generation of clicking sounds may be undesirable. 
     The actuator may have only two stable rest positions and therefore constitute a bistable actuator. In some embodiments, there may be two end-of-travel positions which are stable rest positions. In some actuators embodying the invention, there may is also be one or more further stable rest positions located between the two end-of-travel positions which may be selected by suitable control of the actuator. 
     The actuator preferably includes two resilient components, wherein one of the resilient components is compressed by movement of the armature towards each stable rest position (or more particularly, in some embodiments, during the final part of the movement of the armature towards and into each stable rest position). 
     The resilient component may be able to store potential energy as the armature moves into each of its first and second rest positions. When the armature is switched towards its other position, this potential energy is released and acts to accelerate the armature towards the other position. The resilient component may be in the form of a single resilient part or alternatively it may be an assembly of parts. 
     In a preferred embodiment, the actuator includes a push rod which extends along a central axis of the actuator and is moveable relative to the coils along the central axis. Preferably, the armature is axially slideable along the push rod. The actuator configuration may permit relative movement between the armature and the push rod in order to reduce the impulse exerted by the push rod in response to forces acting on the armature. This may be beneficial when the push rod is acting on a viscous fluid for example, as smoother application of force to such a fluid may provide a smoother switching action. 
     More preferably, one end of each resilient component may be fixed in position relative to the push rod so as to be unable to move relative thereto. For example, it may be mounted in a fixed position on the push rod or bear against another component which is fixed in position relative to the push rod. Alternatively, one end of each resilient component may bear against an internal fixed surface of the actuator. The armature may be arranged to exert a compressive force against the other end of one of the resilient components by sliding along the push rod as it moves towards a stable rest position. This arrangement may reduce the impulse exerted by the push rod in response to forces acting on the armature, as the resilient component may act to absorb some of the kinetic energy of the armature. 
     One of the resilient components may exert a force on the armature as it moves into each rest position which prevents the armature from contacting the coil core (or preferably any part of the actuator) that it is moving towards. This may avoid generation of a clicking noise as the armature moves into each rest position which would otherwise be generated by the impact of the armature on a coil core. Each rest position may be an equilibrium position defined by a balance between (a) the force exerted on the armature in the axial direction by the resilient component and (b) the magnetic forces exerted on the armature in the axial direction, which may predominantly be due to attraction of its permanent magnet to the closer of the two coil cores. 
     The permanent magnet may be polarised in a radial direction relative to the central axis of the actuator. Preferably it has an annular configuration. 
     In some embodiments, each coil core extends from one end thereof to an opposite end through a respective coil. At least part of the armature may be located around one of the ends of the closer coil core in a plane perpendicular to the central axis of the actuator when the actuator is in each stable rest position. This configuration may serve to reliably hold the armature in each stable rest position due to a strong magnetic attraction between the armature and a radially adjacent portion of the coil core. 
     Preferably, at least part of the armature is located between the ends of the closer coil core in each stable rest position. This configuration may serve to reliably hold the armature in each stable rest position due to a strong magnetic attraction between the armature and the two ends of the coil core. 
     In further embodiments, the actuator may include:
         an actuator housing; and   a pivotal axis in a fixed position relative to the housing,   wherein the armature is mounted for rotation about the pivotal axis between the first and second stable rest positions.       

     As the armature of the actuator may swing between positions in which it is located between (but does not come into contact with) the poles of the coil cores, the noise level associated with actuation of the actuator can be reduced. 
     In each of the first and second positions, the poles of the coil core adjacent to the permanent magnet may be spaced from the permanent magnet in a direction parallel to the pivotal axis of the armature. The actuator may be configured in this way such that the permanent magnet does not contact the poles in each of the first and second positions. This serves to avoid generation of noise due to the armature impacting on the coil core as it moves into each position. 
     The armature may be held in each of the first and second positions due to interaction of the magnetic flux generated by the permanent magnet and the respective coil core in each position. The armature may be securely held in each position, without electrical energy being consumed by passing a current through either of the coils. 
     In preferred embodiments, the armature comprises a main body comprising the permanent magnet, a sub-assembly body, and a resilient coupling which resiliently couples the sub-assembly body to the main body. This resilient coupling may comprise the two resilient components referred to above. 
     The armature&#39;s main body and sub-assembly body may be able to pivot relative to each other about the pivotal axis. This configuration provides reduced inertia, faster reaction, reduced power consumption and full magnetic force (torque) available outside the actuator. 
     The sub-assembly body may be pivotable between first and second end-of-travel positions. 
     In some embodiments, one of the coils of the actuator has a greater number of turns than the other coil. This may be beneficial in applications where a greater switching force is required when switching in one direction relative to the opposite direction. 
     The present invention further provides a pinch valve comprising an actuator as described herein, a pinching mechanism which includes or is mechanically coupled to the armature of the actuator; and a supporting base for carrying a resilient tube to be selectively pinched by the pinching mechanism when the armature moves into one of its stable rest positions. 
     The armature may include a tubing engaging portion (or be mechanically coupled to a component) which defines a profile in a plane perpendicular to the pivotal axis, and the profile may be shaped such that the profile is closer to the base when the armature is in the first position than when the armature is in the second position. Thus, when the armature is in its first position, the tubing engaging portion pinches the tubing between itself and the base. This may substantially close the tubing. When the armature is in the second position, the tubing engaging portion is spaced further from the base such that the tubing is open to allow fluid to flow through it. 
     By employing an actuator as described herein in a pinch valve, the level of noise generated by operation of the pinch valve may be reduced. 
     The armature may include a first base engaging portion which engages the supporting base when the armature is in its first position. The first base engaging portion and the base may be configured such that, as the armature swings into its first position, the first based engaging portion is brought into contact with the upper surface of the supporting base at an acute angle, such that they are brought into contact in a smooth and gradual manner. The contact between the first base engaging portion and the supporting base may assist in retaining the armature firmly and reliably in the first position. 
     The first base engaging portion may comprise a first roller. Provision of a roller having a rotational axis parallel to the pivotal axis of the armature in this manner may further assist in providing a smooth action as the first based engaging portion is brought into contact with the supporting base. 
     The first roller may define part of the profile of the tubing engaging portion of the armature. This serves to reduce frictional resistance when the armature is brought into contact with the tubing, as the roller is able to roll over the tubing, providing a smoother engagement between them. The roller may comprise a hub with radially extending flanges on each side thereof. The hub may engage the upper surface of the tubing and the flanges may engage the supporting base on either side of the tubing when the armature is in the first position. 
     Furthermore the armature may include a second base engaging portion which engages the supporting base when the armature is in its second position. The second base engaging portion may comprise a second roller. These features may provide similar benefits to the first roller as discussed above. The characteristics of the actuator may therefore be similar as it moves into the first and second positions, respectively. 
     In preferred embodiments, the pinch valve includes a removable cartridge, wherein the cartridge includes a length of resilient tubing and the supporting base of the valve which carries the tubing. The length of tubing with which the pinch valve interacts may become worn and degrade over time. Provision of a removable cartridge including the length of tubing facilitates easy replacement of the tubing. This may also ensure that tubing of sufficient quality is used when the tubing is replaced, to reduce the risk of premature failure of the tubing in the valve. 
     In preferred embodiments, armatures are provided in the form of assemblies that move relative to the coils of the actuator and comprise two parts which are coupled together using one or more resilient couplings. One of these parts comprises a permanent magnet which generates magnetic forces within the actuator. This part is coupled by the resilient coupling(s) to a second part which transfers mechanical force generated by the actuator onto other devices or components. This second part may be a push rod for example or an armature sub-assembly in accordance with preferred embodiments described herein. In these configurations, the part comprising the permanent magnet is able to accelerate relatively quickly. This may mean that a relatively short current pulse is sufficient to be applied to the coils to switch the actuator, thereby reducing energy consumption. The second part then follows the first part in response to forces exerted on the second part by the first part via the resilient coupling(s). In this way, the second part may move more slowly than the first part. Thus, whilst the first part may switch relatively quickly, the second part can switch from one stable rest position to another more slowly. This may be particularly beneficial when using the actuator to exert a force on a viscous fluid. Such a fluid will generate a lower reactive, resistive force if this force is exerted more gradually, with a lower impulse. This leads to more gentle operation, consuming less energy and exerting less stress on components of the actuator. 
     This two part configuration is also beneficial in that the resilient coupling provides internal energy recycling as energy is stored in the resilient coupling in each stable rest position. This provides rapid acceleration of the first part out of each stable rest position giving a fast actuator response whilst providing reduced energy consumption and providing substantial transfer of magnetic forces generated by the actuator to outside the actuator. 
     The invention further provides an actuation system including an actuator as described herein and an actuator controller for selectively sending electrical control pulses to the coils of the actuator to switch the armature between the first and second positions. 
     Furthermore, the present disclosure provides such an actuation system which includes a valve mechanically coupled to the armature of the actuator so that the valve is operable by the actuator. The valve may take various forms. It may be a pinch valve or an inline valve, for example. Preferably, the valve is substantially silent when switching between its open and closed configurations. 
     The invention may also provide a gas supply control system comprising an actuation system as described herein and a gas supply controller, wherein the gas supply controller comprises the actuator controller. 
     The gas supply controller of the gas supply control system may be configured to receive a pressure signal from a pressure sensor. The signal may be dependent on or indicative of the gas pressure within a mask worn by a patient, for example. Alternatively, or additionally, the gas supply controller may be configured to receive an oxygen level signal which is dependent on or indicative of the oxygen level in the blood of a patient. For example, the signal may be generated by an oxygen saturation sensor which is responsive to the level of oxygen saturation in a patient&#39;s blood. The controller may be arranged to adjust the timing of the switching actions of the actuator between its rest positions in order to open and close the valve, and/or adjust the pressure of the gas being supplied to the valve, having regard to the pressure signal and/or the oxygen level signal, with the aim of maintaining the oxygen level in the patient&#39;s blood between upper and lower thresholds. 
     The present invention also provides a gas supply control system comprising a pinch valve as described herein, a pressure sensor and a gas supply controller, wherein the pressure sensor is configured to generate a pressure signal in response to a gaseous pressure sensed by the sensor, the gas supply controller is communicatively coupled to the pressure sensor so as to receive the pressure signal and also communicatively coupled to the pinch valve, and wherein the gas supply controller is configured to send control signals to the pinch valve in response to the pressure signal, which control signals cause the pinch valve to switch the armature of the pinch valve between its first and second positions. Such a system may be used for example to control a supply of oxygen to a patient. 
     The pressure sensor may be responsive to the breathing of the patient, and the pressure signal may be employed to control the oxygen supply in time with the patient&#39;s breathing cycle. 
     As the actuator of the valve is able to select either a first or second position and be held in either position without consuming electrical energy, the valve may be switched to be continuously open as a failsafe default mode, in a medical environment for example. 
     As consumption of electrical energy by the valve actuator is minimised, it may be particularly suitable for portable applications using a battery as a source of power. 
     It is common for existing oxygen supply systems to provide a continuous flow of oxygen. A large proportion of the oxygen is not inhaled by the patient and is wasted. The present system allows an on-demand metered supply of oxygen thereby reducing the amount of oxygen wastage substantially. 
     Provision of a pressure sensor for detecting the start of an intake of breath by a patient means that the timing of the dose of oxygen may be provided automatically, without requiring manual adjustment. 
     The gas supply controller may be configured to generate a switching rate signal dependent on (or providing a value corresponding to) the rate of switching of the valve. This information may be valuable to medical professionals to give an indication of the respiratory rate of the patient. Preferably, the system may include a display to which the signal is outputted to cause the display to show an indication dependent on the switching rate signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example and with reference to the accompanying schematic drawings wherein: 
         FIGS. 1 to 3  are perspective cross-sectional views of a linear actuator embodying the present invention with its armature shown in three different positions; 
         FIG. 4  is a cross-sectional side view of another linear actuator embodying the invention in combination with a pinch valve mechanism; 
         FIGS. 5 and 6  are perspective views of a pinch valve embodying the present invention with its armature in two different positions; 
         FIG. 7  is an enlarged view of part of  FIG. 6 ; 
         FIGS. 8 and 9  are perspective views and  FIG. 10  is a side view of a further pinch valve embodying the invention which includes rollers; 
         FIGS. 11, 13 and 14  are side views of a further pinch valve configuration embodying the invention; 
         FIGS. 12, 15 and 16 to 18  are perspective views of the pinch valve shown in  FIGS. 11, 13 and 14 ; 
         FIG. 19  corresponds to  FIG. 14  and has additional annotations; 
         FIGS. 20 and 21  are schematic graphs of torque and pinch forwards respectively, for the pinch valve of  FIG. 19 ; 
         FIG. 22  is a diagram illustrating generation of a pinch force using a valve configuration of  FIG. 19 ; 
         FIG. 23  is a plan view of magnetic components of a pinch valve embodying the invention; 
         FIGS. 24 and 25  are perspective cross-sectional views of a linear actuator embodying the invention in combination with an inline valve, with the armature of the actuator in two different positions; 
         FIG. 26  is a block diagram of a gas supply control system embodying the present invention; 
         FIG. 27  is a block diagram of another gas supply system embodying the present invention; and 
         FIG. 28  is a side view of a patient mask for use in a gas supply control system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 to 3  are cross-sectional views of a compact linear actuator  200  according to an embodiment of the invention. An elongate, linear push rod  202  extends along a central, longitudinal axis  204  of the actuator. The push rod is slidable along this axis relative to a coaxial cylindrical actuator housing  206 . 
     An annular armature  208  extends around the push rod and is coaxially mounted relative to the push rod. The armature comprises a permanent magnet  210  which is provided between radially inner and outer pole pieces  212  and  214 . Permanent magnet  210  is polarised in a radial direction. 
     The armature also includes a magnet mount  216 . The magnet mount extends radially inwardly from the inner pole piece  212  towards the push rod  202 . The outer circumferential surface of the magnet mount  216  is mounted in a fixed position on an inner circumferential surface of the inner pole piece  212 . A circular aperture  218  is is defined by a radially inwardly facing surface of the magnet mount  216 . The push rod  212  extends through the aperture  218  and is able to move relative to it. In this way, the armature  208  and the push rod  202  are able to slide relative to each other in an axial direction. 
     A pair of resilient components  220 ,  222  is provided. In the embodiment illustrated, they are in the form of coil springs, although it will be appreciated that other forms of resilient device may be used. One end of each resilient component bears against a respective one of two opposite, transversely extending side faces  224  and  226  of the magnet mount  216 . The other end of each resilient component is mounted in a fixed position on the push rod or retained in a fixed position relative to the push rod. In the embodiment illustrated, the outer ends of the coil springs  220  and  222  are in engagement with respective discs  228  and  230  which are mounted in fixed positions on the push rod  202 . Each resilient component is compressed between one of the fixed discs at one end, the magnet mount at its other end. 
     The actuator includes two wire coils  232  and  234 . They are located towards opposite ends of the cylindrical housing  206 . A central axis of each coil is coaxial with the central axis  204  of the actuator. The coils are wound in respective opposite directions. 
     A coil core  236  extends through coil  232  and the coil core  238  extends through coil  234 . One end of each coil core extends axially inwardly to form respective annular, inner pole pieces  240  and  242 . The opposite end of each coil core extends radially outwardly, past axially outwardly facing surfaces of the respective coils, radially beyond the respective coils, and then axially inwardly to form outer annular pole pieces  244  and  246 . In this way, the coil cores form inner and outer pole pieces which are radially spaced apart in a common transverse plane, with a coil core extending between each pair of pole pieces, through the centre of the respective coils. 
     A handle  248  is provided at one end of the push rod  202 . In the embodiment illustrated, this is in the form of a disc which is integrally formed with the push rod. This facilitates manual switching of the actuator if required. 
     Each coil core  236  and  238  is fixedly mounted on a radially inwardly facing surface of the housing  206 , at axially opposite ends of the housing. A pair of seals  250  and  252  extend radially inwardly between a respective coil core and the push rod  202  at opposite ends of the push rod to close each end of the housing around the push rods. Each coil core forms a respective radially extending end face of the actuator housing. 
     The linear actuator of  FIGS. 1 to 3  has two stable rest positions. A first stable rest position is shown in  FIG. 1 . In the configuration of  FIG. 1 , the armature  208  is located closer to the coil core  236 . Also, the push rod  202  is located in its left-most position. A second stable rest position is shown in  FIG. 3 , in which the armature is located closer to the coil core  238  and the push rod is in its right-most position relative to the housing. In each rest position, the armature is attracted towards the adjacent coil core by magnetic forces generated by the permanent magnet  210 . Magnetic flux generated by the magnet links with the coil core to form a magnetic circuit. This magnetic force urges the armature toward the adjacent coil core. In each stable rest position, this force is balanced by an opposing mechanical force generated by one of the resilient components  220  and  222 . In each rest position, one of the resilient components may be compressed to a greater extent than the other, or one may be compressed and the other substantially uncompressed. 
     As the armature moves into each rest position, one of the resilient components will be compressed when the armature moves relative to the push rod, when further movement of the push rod is constrained externally or due to one of the discs  228  or  230  contacting an interior surface of the actuator. 
     In each rest position, the armature is spaced from the adjacent coil core by a gap. It does not therefore come into contact with the adjacent coil core as it moves into each stable rest position. This avoids generation of a clicking noise as the armature moves into each stable rest position. 
     The armature may be separated from the adjacent coil core by a fluid filled gap. The is fluid may be a gas such as air. 
     The inner and outer poles of each coil core may be spaced apart in the radial direction such that the distance between them is greater than the radial extent of the annular armature formed by permanent magnet  208  and pole pieces  212  and  214 . In this case, in each stable rest position, the armature may overlap with one or both of the poles of the coil core to some extent in the axial direction. 
       FIG. 2  shows the actuator in an unstable, neutral configuration, with the armature located mid-way between the coil cores. In practice, the armature will not rest in this position, but move through it when switching from one stable rest position to the other. 
     The armature may be switched from one stable rest position to another by briefly energising one or both of the coils. 
     For example, the actuator may be switched from the configuration of  FIG. 1  to that of  FIG. 3  by applying a brief current pulse to the coil  232 . This pulse should be sufficient to reduce the magnetic attraction between the armature and the coil core to such an extent that resilient component  220  pushes the armature away from the starting rest position, towards the stable rest position of  FIG. 3 . The pulse should also be sufficient to ensure that the armature is reliably attracted into the other stable rest position by magnetic forces acting between the permanent magnet and the Opposite coil core  238 . Application of a suitable pulse to the coil  234  will result in the armature being switched back in a similar manner to its original stable, rest position as shown in  FIG. 1 . 
     An alternative linear actuator configuration is shown in  FIG. 4 , in combination with a pinch valve mechanism. The same reference numerals are used to denote the same or similar features of this actuator  300  to those of the actuator  200  shown in  FIGS. 1 to 3 . 
     In this embodiment, the armature  216  is mounted in a fixed position on the push rod is  202 . The outer ends of the resilient components  220  and  222  bear against respective coil cores  236  and  238 . Coil  232  is shown as having a greater number of windings than coil  234 . This is so that the actuator is able to provide a greater force when switching in one direction then the other (that is, to close the valve in the embodiment illustrated). The operation of the actuator  300  is similar to that of actuator  200 . 
     One end of the push rod  202  is coupled to a pinching mechanism  302 . The mechanism includes a cam member  304  which is mounted for rotation about a pivot  306  of the pinching mechanism. A coupling  308  is pivotally mounted on the cam member  304  for rotation relative to the cam member about a second pivot  310 . The coupling  308  is connected to one end of the push rod  202 . 
     The pinching mechanism  302  includes a pinching element  312  which has a wheel  314  rotatably mounted thereon. The wheel  314  acts as a cam follower, following a cam surface  316  formed on the cam member  304 . The pinching element  312  is in engagement with a tube  318  carried by a support member  320 . The cam member is shaped so as to convert linear movement of the push rod  202  caused by actuator  300  into movement of the pinching element  312  in a direction perpendicular to the longitudinal axis  322  of the tube  318 . In some cases, it may be preferable for the pinching element to move in a direction perpendicular to the tube, rather than the motion of the pinching element having a component in an axial direction, to minimise wear on the tube and axial pulling forces on the tube. 
     In  FIG. 4 , actuator  300  is shown in its neutral, unstable condition, with the armature located midway between the coil cores  236  and  238 . 
     The pinching element  312  is biased upwardly towards the cam member by an arm  324  to keep the cam follower wheel  314  in contact with cam surface  316 . In one stable rest position of the actuator, the tube  318  is open. When the actuator moves into its other stable rest position, the pinching element is moved downwardly, towards the tube, pinching the tube between the pinching element and the supporting body  320 , so as to close the tube. 
     Another pinch valve embodying the invention will now be described with reference to  FIGS. 5 to 7 . The valve  2  is provided within a housing  4 . The housing is only shown partly in  FIG. 5  for the purposes of clarity in the drawings. The valve includes an actuator  6 . 
     The actuator has an armature  8 . The armature has an armature body  10  which includes a permanent magnet  12 . The armature also includes a sub-assembly body  14 . A resilient coupling in the form of a pair of leaf springs  16  resiliently couples the sub-assembly body to the armature body. It will be appreciated that this resilient coupling may be implemented in other ways. A torsion bar or torsion spring may be used for example. 
     A pivot  20  is mounted in a fixed position on the housing  4 . A pivotal axis  22  extends axially through the centre of the pivot  20 . The armature body and the sub-assembly body are each mounted on the pivot for rotation about the pivotal axis. 
     The actuator includes two coils  30  and  32 . Each coil is wound around a respective coil core  34  and  36 . Each coil core comprises a pair of arms  38  and  40  on either side of the coil. The arms extend transversely with respect to the pivotal axis. A pair of poles  42 ,  44  is provided at the distal ends of each pair of arms. 
     In the configuration shown in  FIG. 5 , the permanent magnet of the armature is located between the poles  44 . This represents a stable rest position of the armature.  FIG. 6  shows a second stable rest position, in which the permanent magnet  12  is located between the other pair of poles  42 . 
     The valve includes a length of resilient tubing  50  which is carried by a supporting base  52 . 
     A pair of rounded flanges  60  extend in a direction which is transverse with respect to the pivotal axis, and away from the sub-assembly body. In the configuration shown in  FIG. 5 , it can be seen that the flanges  60  are in contact with the supporting base  52  is on either side of the tubing  50 . In this position, the tubing is not compressed by the sub-assembly body and fluid is able to flow through freely through the tubing. 
     In the other armature position shown in  FIGS. 6 and 7 , a tubing engaging portion  62  of the sub-assembly body is pinching the tubing located between it and the supporting base  52  to substantially close the tubing. The tubing engaging portion  62  provides a convex surface curved in a plane perpendicular to the pivotal axis of the actuator to enable it to be brought smoothly into engagement with the tubing, with a view to reducing wear of the tubing. 
     The permanent magnet is orientated with its poles aligned in a direction parallel to the pivotal axis. In each of its rest positions, magnetic flux generated by the magnet links with the adjacent coil core so that the armature is retained in a stable rest position by magnetic forces generated by the magnet. 
     In order to switch the armature from one stable rest position to the other, the coil associated with the current rest position is energised to repel the magnet, and the magnet pulls itself towards the other coil core. In this way, the pinch valve can be switched to open or close the tubing  50  using a single coil driving technique. 
       FIGS. 8 to 10  show another pinch valve embodying the invention which is similar to that of  FIGS. 5 to 7 . A first roller  70  is provided instead of the tubing engaging portion  62 , and a second roller  72  is provided instead of the flanges  60 , as part of the sub-assembly  14 . Each roller comprises a pair of flanges which are brought into contact with the supporting base  52  as the armature moves into a respective stable end position. The diameter of the hub of the second roller  72  which extends between its flanges is dimensioned such that it does not compress the tubing. The first roller  70  is dimensioned so as to pinch the tubing to the desired extent as it is brought into contact with it. 
     In  FIG. 9 , the coil core  34  is shown in outline only so that the permanent magnet  12  is more clearly visible. 
     is  FIGS. 11 to 18  show side and perspective views of a further pinch valve embodying the invention which is similar to that shown in  FIGS. 8 to 10 . In  FIG. 17 , one of the coils and part of its core are omitted so that the armature configuration is more easily visible. In  FIG. 18 , the armature body is omitted so that the sub-assembly and resilient couplings are more readily visible. 
       FIGS. 12 and 13  show an intermediate configuration in which the armature  10  is in a neutral, unstable position, midway between the two stable rest positions. In this position, the two resilient couplings, in this case in the form of leaf springs  16 , are both uncompressed, or slightly compressed, each to the same extent. 
     In  FIG. 19 , the distance “R” is marked which corresponds to the distance between the centre of one pair of pole pieces and the pivotal axis. A distance “X” is marked which is the distance that the centre of the first roller is offset from a perpendicular line extending through the pivotal axis from the supporting base. The ratio R/X is equal to the leverage ratio for this configuration. Preferably, R is several times greater than X in order to provide a substantial pinching force. 
       FIG. 20  is a schematic plot of the torque exerted on the armature against angle. In this graph, 0 and 30° correspond to the central position and one end of travel of the armature, respectively. It represents the torque attributable to the magnetic forces generated by the permanent magnet alone, without the coils being energised. 
       FIG. 21  is a plot of pinch force against armature position by way of example. A substantial pinch force may be generated in a valve embodying the present invention to reliably close tubing compressed by the pinch valve. 
     This arrangement depicted in  FIG. 19  is shown diagrammatically in  FIG. 22 . With reference to the dimensions labelled in  FIG. 22 : 
         Pf=T*Rm/Rr* sin(α)
 
       where: 
     Pf=pinching force; 
     T=torque acting on the armature; 
     Rm=the distance between the centre of the permanent magnet and the pivotal axis of the armature; 
     Rr=the distance between the centre of the pinching roller and the pivotal axis; and 
     α=the angle subtended between a line between the pinching roller axis and the pivotal axis of the armature and a perpendicular line through the pivotal axis from the supporting base. 
     It can be seen that: 
     
       
         
           
             
               
                 
                   Pf 
                   = 
                     
                    
                   
                     T 
                     * 
                     
                       Rm 
                       / 
                       Rr 
                     
                     * 
                     
                       ( 
                       
                         Rx 
                         / 
                         Rr 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     T 
                     * 
                     
                       Rm 
                       / 
                       Rx 
                     
                   
                 
               
             
           
         
       
     
       FIG. 23  is a diagram showing a plan view of an actuator embodying the invention. The permanent magnet  12  is located between the poles  42  of the coil core associated with the coil  30 . The rectangle in the centre of the diagram represents the pivot of the armature. A plurality of black arrows have been superimposed on the diagram to represent magnetic flux generated by the permanent magnet. The size of each arrow is proportional to the magnetic field strength at its location and the direction of the arrow shows the direction of the magnetic flux at its location. It can be seen that the magnetic flux generated by the magnet acts to securely retain the magnet between the adjacent poles. 
       FIGS. 24 and 25  show a linear actuator  200  similar to that depicted in  FIGS. 1 to 3  in combination with an inline valve  400 . In some applications, it may be preferable to use an inline valve rather than a pinch valve. 
     The valve body  402  is mounted onto one end of the housing of the actuator  200 . The body  402  defines a fluid inlet port  404  and a fluid outlet port  406 . The inlet and outlet ports are coupled via inlet and outlet conduits  410  and  412  to a cylindrical bore  408  is which is also defined by the valve body. A sleeve  414  is provided which is a close sliding fit within the bore  408 . A fluid-tight seal is provided between the outer surface of the sleeve and the bore on each side of the inlet  410  by seals  416  and  418 . 
     The diameter of the bore  408  is increased around the location where the inlet meets the bore to form a chamber  420  which extends around the outside of the sleeve  414 . 
     An opening or a pair of openings  422  and  424  is/are defined by the wall of the sleeve which extend radially through the sleeve. The sleeve is attached to one end of the push rod  202  of the actuator or integrally formed with the push rod. Movement of the push rod by the actuator therefore moves the sleeve along bore  408  of the valve. 
     In one stable rest position of the actuator as shown in  FIG. 24 , the openings coincide with the chamber  420 . Thus, in this valve configuration, the valve is open as its inlet port is in fluid communication with its outlet port via the inlet conduit  410 , chamber  420 , openings  422 ,  424 , the interior of sleeve  414  and the outlet conduit  412 . 
     In the other stable rest position of the actuator as shown in  FIG. 25 , the sleeve has been axially displaced by the actuator within bore  208 , so that the openings  422 ,  424  are no longer within chamber  420 . The valve is then closed as the sleeve has closed the fluid path between the inlet and outlet ports. 
     The axial extent of the chamber  420  is preferably greater than that of the opening(s) in the wall of the sleeve. This allows a greater degree of tolerance in the axial positions of the sleeve that will allow fluid to flow through the valve. This may be advantageous as the stable rest positions of the actuator at each end of the travel of its push rod are dependent on spring forces, rather than contact between rigid moving and stationary parts of the actuator. As a result, there may be some variation in the location of the push rod when the actuator has been switched to open the valve which is accommodated by the relative axial dimensions of the chamber and the opening(s). 
       FIG. 26  is a block diagram representing a gas supply control system incorporating a pinch valve embodying the present invention, in a preferred use as an Intelligent Medical Oxygen System (IMOS). The pinch valve  2  is represented schematically in the drawing and is arranged to control the flow of gas through tubing  50 . 
     The pinch valve includes a cartridge. In preferred configurations the cartridge is removable. The cartridge includes a length of resilient tubing  82  and the supporting base  52  of the pinch valve. The cartridge may be removed from the pinch valve and replaced with a new cartridge as and when required due to wear of the tubing  82  by the valve. This modular cartridge construction ensures that the replacement of the tubing can be achieved reliably, without requiring a high degree of skill or training. 
     A connector  90  is provided to connect the tubing  50  to a supply of oxygen. This supply is in turn fed through an oxygen pressure regulator  92  before being fed through the pinch valve  2 . 
     A flow indicator  94  monitors the flow rate of the gas passed through the pinch valve along tubing  50 . 
     The supply of oxygen is then fed to a mask  96  for use by a patient. A pressure sensor tubing  98  is coupled to the interior of the mask at one end and to a pressure sensor  100  at the other. 
     The pressure sensor  100  is configured to generate an electrical pressure signal along output  102 . This signal is communicatively coupled to a gas supply controller  104  which includes an actuator controller  106 . 
     The pressure signal is received by a preamplifier  108 , the output of which is coupled to an analogue-to-digital converter  110 . The output from converter  110  is inputted into a microprocessor  112  of the actuator controller. A display  114  is communicatively coupled to the microcontroller  112 . The microcontroller outputs control signals on lines  116  and  118  which are coupled to drivers  120  and  122  for respective coils  32  and  30  of the actuator. 
     The pressure sensor  100  is able to detect a pressure drop within the mask  96  indicative of a patient beginning an intake of breath. The microcontroller may respond to signals derived from the output of the pressure sensor to open and close the pinch valve as appropriate to feed an appropriate supply of oxygen to the mask along tubing  50  in time with the breathing cycle of the patient. Alternatively, or in addition, the microcontroller may respond to signals derived from the output of the pressure sensor by adjusting the oxygen pressure regulator  92  so as to alter the pressure of the oxygen fed supplied to the valve  2 . 
       FIG. 27  is a further block diagram of a gas supply control system embodying the invention. It illustrates a number of possible modifications that may independently be made to a system exemplified by the configuration shown in  FIG. 26 . The same reference numerals are used to identify the same or similar features in  FIG. 27 . 
     A gas supply control system embodying the invention may optionally include an oxygen saturation sensor  130 . As shown in  FIG. 27 , such a sensor may be communicatively coupled to the microcontroller  112 . The microcontroller may be configured to adjust the control signals sent to the actuator having regard to signals received from the sensor  130  with a view to maintaining the level of oxygen in a patient&#39;s blood within predetermined thresholds. Alternatively, or additionally, the microcontroller may adjust the pressure of the gas being supplied to the valve by adjusting the oxygen pressure regulator  92  having regard to signals received from the sensor  130 . 
     The microcontroller  112  may be communicatively coupled to a transmitter  132 . This allows the system to transmit data wirelessly to a receiver on another device or a network (via a Bluetooth link for example) so that information regarding the current and/or historical operation of the system can be monitored and/or recorded for subsequent use. 
     Instead of the pinch valve assembly shown in  FIG. 26 , an alternative valve assembly  134  may be used. For example, the valve assembly may consist of an actuator as described herein and an inline valve. This combination may be of the form shown in  FIGS. 24 and 25  for example. 
       FIG. 28  is a side view of a mask which is adapted for use in the systems of  FIGS. 26 and 27 . In addition to an inflow tubing  50 , it also includes a pressure sensor tubing  98  which is coupled to the interior space defined by the mask.