GB2354557A

GB2354557A - Reciprocating Electromagnetic Pump

Info

Publication number: GB2354557A
Application number: GB9921976A
Authority: GB
Inventors: Ernest James Bransden
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-09-16
Filing date: 1999-09-16
Publication date: 2001-03-28
Anticipated expiration: 2019-09-16
Also published as: GB2354557B; GB9921976D0

Abstract

A solenoid operated positive displacement pump 10 which includes a moving permanent magnet 35 fixable to a reciprocating shaft 31, a Hall effect switch 61 for detecting the position of the permanent magnet 35 and an electronic switching means in circuit relationship with the solenoid 41 and the Hall effect switch 61 for controlling current flow through the solenoid 41 in response to the actuation of the Hall effect switch 61. The position at which the piston completes its discharging or discharging stroke is adjustable by adjusting the position of a magnetic shield 70 in a space between the Hall effect switch 61 and the moving permanent magnet 35 so that the pump operation can be optimised once the pump has been assembled. The electronic switching means comprises a MOSFET which is in circuit relationship with a switching delay circuit including a resistor, capacitor, and a diode.

Description

2354557 Reciprocatinq Electromagnetic Pump The invention relates to the

field of electromagnetic pumps and in particular to a solenoid operated positive displacement fluid pump.

Common types of electromagnetic fluid pumps include a reciprocating piston which mechanically actuates an electrical switch for controlling current flow through a solenoid. Actuation of the electrical switch at the end of the pumping stroke energises the solenoid which returns the piston to the beginning of its pumping stroke and in so doing compresses a spring. The pumping stroke is effected when the solenoid deenergizes and the piston moves to discharge the pump under the force of the compressed spring. The operating life of these kinds of pumps is partly determined by the reliability of the mechanical contacts of the electrical switch.

In Patent GB2077515A there is disclosed a solenoid operated reciprocating fluid pump which includes a fixed permanent magnet system having an incomplete magnetic circuit, a Hall effect switch for detecting the position of a reciprocating piston, and an electronic switching circuit in circuit relationship with a solenoid for controlling the current flow through the solenoid in response to the actuation of the Hall effect switch. Actuation of the Hall effect switch is achieved by the magnetic field of the piston closing the magnetic circuit of the fixed permanent magnet. The electronic switching circuit includes a transistor arrangement in circuit relationship with the solenoid and the solenoid is energised and cleenergized by the Hall effect switching a transistor on and off, respectively. Discharging of a capacitor and resistor in circuit relationship with the gate of the transistor controls the period for which the solenoid remains energised. A limitation of this type of pumping system is that the position at which the 2 piston actuates the Hall effect switch cannot be easily adjusted for optimum pump operation once the pump has been assembled.

In the highly competitive field of electromagnetic fuel pumps there is need to make further improvements to electromagnetic pumps and provide an electromagnetic pump operated with a solid state switching device which has certain characteristics of a pump operated with a mechanical switch and mechanical toggle, including in particular, no loss of electrical power in conditions of no demand and full retained pressure and a strong clicking sound on pumping. Furthermore, there is a need to provide a pump control means that is easily adapted for use with existing positive displacement pumps including reciprocating piston pumps and other types of reciprocating pumps, such as diaphragm pumps.

According to one aspect of the invention, there is provided a solenoid operated positive displacement pump comprising a pumping housing and a pump chamber having an inlet port and an outlet port between which is defined a flow path through the pumping chamber, a reciprocable means including a permanent magnet section, and a pumping section arranged to vary the volume of the pumping chamber as the reciprocable body is reciprocated, a guide member in which the reciprocable body is free to reciprocate, biasing means for urging the reciprocable body in a first direction to effect a charging or discharging stroke, a solenoid for moving the reciprocable body in a direction opposite to that in which the biasing mean urges the reciproable body, a magnetic field sensing device disposed adjacent a path of movement of the permanent magnet, an electronic switching means for deenergizing or energising the solenoid in response to the magnetic field sensing device activating or deactivating, wherein a magnetic shield is positioned between the magnetic field sensing device and the permanent magnet and adjusting means is provided for adjusting the position of the shield to accordingly adjust the position at which the

3 permanent magnet deactivates or activates the magnetic field sensing device.

According to another aspect of the invention, there is provided a method of operating a solenoid actuated positive displacement pump having a pumping chamber, a reciprocable means, a permanent magnet fixed to the reciprocating body, biasing means for displacing the reciprocable body, a guide member in which the reciprocable body is free to reciprocate, a solenoid for moving magnetic material fixed to the reciprocable body, a magnetic fields sensing device with an adjustable magnetic shield, and electronic switching means and comprising the steps of energising the solenoid in order to displace the reciprocable body in one direction against the force of the biasing means, denergizing the solenoid in order to allow the reciprocable body to displace in the opposite direction under the force of the biasing means, varying the 'Volume of the pumping chamber upon displacing the reciprocating body in either the one direction or the opposite direciton, passing the magnetic field of the permanent magnet through a magnetic field sensing device when the reciprocable body is substantially displaced, deactivating an electronic switch in order to denergize the solenoid in response to the magnetic field sensing device activating or deactivating, activating an electronic switch in order to energise the solenoid in response to the magnetic field sensing device deactivating or activating, positioning a magnetic shield between the magnetic field sensing device and the solenoid, and adjusting the position at which the permanent magnet activates or deactivates the magnetic field sensing device by adjusting the position of the magnetic shield.

According to the invention, the position of the reciprocating body at the end of the charging stroke or the discharging stroke can be adjusted for optimum pump performance by adjusting the position of the magnetic shield. The position of the reciprocating body at the end of the charging stroke can 4 be adjusted according to the invention by configuring the magnetic field sensing device to activate or deactivate at the end of the charging stroke. Similarly, the position of the reciprocating body at the end of the discharging stroke can be adjusted according to the invention by configuring the magnetic field sensing device to activate or deactivate at the end of the discharging stroke. Typically, the biasing means comprises a resilient member and the reciprocable body comprises a shaft and a permanent magnet which is either integral with the shaft or fixable to the shaft. Preferably, the shaft also includes a diaphragm which is fixed to or integral with one end of the shaft and the surface of the diaphragm and the lower portion of the pumping housing defines a chamber in which are spaced apart the inlet port and outlet ports.

According to a feature of an embodiment of the invention, the magnetic field sensing device is activated by the magnetic field of the permanent magnet in combination with the magnetic field of the solenoid and the electronic switching means deenergizes the solenoid in response to the magnetic field sensing device activating.

According to another feature of an embodiment of the invention, the magnetic field sensing device is a Hall effect device. Preferably, the Hall effect device is a self-regulated logic level Hall effect switch.

According to another feature of an embodiment of the invention, the solenoid moves the reciprocable body to charge the pump and the resilient member moves the reciprocable body to discharge the pump. In this particular case, the pump may also be designed so that the fluid pressure in the chamber resists movement of the reciprocating body when it is subject to the force of the biasing resulting in low electrical power loss under the conditions of no demand and full retained pressure Alternatively, the solenoid moves the reciprocable body to discharge the pump and the resilient member moves the reciprocable body to charge the pump.

According to yet another feature of an embodiment of the invention, the electronic switching means for controlling the solenoid includes a power MOSFET which is connected in series with the solenoid, the solenoid and power MOSFET being connected between opposite poles of an electrical power source such that the MOSFET substantially blocks current flow through the solenoid coil when a gate voltage of the MOSFET is switched to a voltage sufficient to deactivate the MOSFET in response to the Hall effect device activating. Alternatively, the electronic switching circuit may be configured such that the MOSFET blocks current flow through the solenoid coil in response to the Hall effect device activating. N-channel or Pchannel MOSFETS may be used according to the electronic circuit configuration selected.

According to yet another feature of an embodiment of the invention the electronic switching m6ans may also include a switching delay circuit that allows the action of the pump to substantially imitate that of a mechanical toggle. The switching delay circuit typically comprises an R-C network connected to the gate of the MOSFET for delaying the switching of the gate voltage, a capacitor of the R-C network being connected between the gate and one pole of the electrical power source such that the capacitor charges and discharges in response to the Hall effect device activating or deactivating. The switching delay circuit may also include a diode connected in parallel with a resistance of the R-C network in order that the resistance is bypassed when discharging the capacitor in response to the Hall effect device activating, the resistance being defined by at least one resistor placed between the Hall effect device and the gate of the MOSFET.

According to yet another feature of an embodiment of the invention, the electronic switching means also includes a Zener diode connected across the source and drain of the MOSFET for protecting the MOSFET against back emf of the solenoid and voltage transients.

6 According to yet another feature of an embodiment of the invention, a Zener diode is connected across the Hall effect device and a resistor is connected in series with the Zener diode and one pole of the electrical power source in order that the electronic switching means and Hall effect 5 device is voltage regulated and protected against back emf of the solenoid. A capacitor may also be connected across the electronic switching circuit for clecoupling and suppressing radio frequency signals.

One major advantage of the electromagnetic pump of the present invention is that the position of the reciprocating piston at which it completes either its charging stroke or discharging stroke can be adjusted for optimum pumping performance after the pump has been assembled. The electromagnetic pump of the present invention alleviates problems of burning, wear and fire hazards which are often associated with switching contacts and mechanical to'ggles in mechanically switched electromagnetic pumps. Using a pumping configuration according to an embodiment of the invention which enables the magnetic field of the solenoid to supplement that of the permanent magnet for activation of the Hall effect switch, allows a relatively weaker and smaller permanent magnet to be adopted and the pumping system to be made more compact. The self-regulated logic level Hall effect device switch according to a feature of the preferred embodiment of the invention enables fast and precise control of an electronic switch, such as a MOSFET. Also, the R-C delay circuit prevents the pumping system buzzing and vibrating and improves electrical efficiency and the high back emf clamping of the MOSIFIET as opposed to the solenoid, allows precise control of the solenoid, thus enabling a rapid change in movement of the shaft to be achieved and the clicking noise characteristic of the mechanical pump to be preserved.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which; 7 Figure 1 is sectional view of an electromagnetic pump according to the present invention, Figure 2 is a block diagram of the electromagnetic operating system according to a preferred embodiment of the invention, Figure 3 (a) is a schematic side view of the Hall effect switch, the magnetic shield, and the permanent magnet of the pump shown in Figure 1, illustrating the position at which the permanent magnet activates the Hall effect switch when the magnetic shield is positioned out of the space between the Hall effect switch and permanent magnet.

Figure 3 (b) is the same view as Figure 3 (a) but with the magnetic shield positioned partially in the space between the Hall effect switch and the permanent magnet.

Figure 3 (c) is the same view as Figure 3 (a) but with the magnetic shield fully located in the Space between the Hall effect switch and the permanent magnet.

Figure 4 is a circuit diagram of the electronic switching circuit in circuit relationship with the solenoid and Hall effect switch according to an embodiment of the present invention.

Figure 5 is a circuit diagram of the power source connected to the electronic switching circuit shown in Figure 4 but with the negative pole connected directly to the circuit and the positive pole connected to the circuit through a common positive ground.

Referring to Figure 1 of the accompanying drawings, there is shown a solenoid operated positive displacement pump 10 according to the preferred embodiment of the invention including a housing 20 and a reciprocable body 30. The reciprocable body 30 comprises a shaft 31 which is free to reciprocate within a cylindrical guide member 40 disposed within the housing 20. The cylindrical member 40 is rigid and made from a non magnetic material, such as nylon or PTFE. One end section of the shaft 31 8 extends through an opening 33 at the top of the housing 20 and is attached to a permanent magnet 35 by means of a threaded carrier 34. A flexible diaphragm 36 is fixed at the other end of the shaft 31 and has its periphery secured (in a manner not shown) to an inner wall of the housing 20, The diaphragm is free to flex up and down as the shaft 31 reciprocates. The opposite surface of the diaphragm 36 to that from which the shaft extends together with a lower portion of the housing 20 define an enclosed pump chamber 21 which includes an inlet port 22 and an outlet port 23 located on opposing sides of the pump chamber 21 and between which is defined a flow path. The inlet port 22 and outlet port 23 accommodate one way flow valves 24 and 25, respectively. Fixable above the pump chamber 21 is a solenoid 41 circumscribing the cylindrical guide 40. A slug 38, 39 formed from a magnetically permeable material, such as for example mild steel or soft iron, is fixedly attached'to the shaft above the diaphragm such that the slug is able to enter within the space that the coil of the solenoid circumscribes when the shaft reciprocates. In the present embodiment, the slug 38,39 defines shaft arms 38, 39. One end of a spring 50 is attached to a support member 51 disposed within the housing 20 above the diaphragm 36 and the other end of the spring 50 is attached to one of the shaft arms 38, 39. The spring 50 is substantially expanded at the end of the charging stroke. Spaced support posts 63 extend from an upper end of the housing 20 upwardly as shown in Fig. 1 and support a printed circuit board 62 to which is fixed a Hall effect switch 61. The N pole of the permanent magnet 35 faces the Hall effect switch 61, which is fixed sufficiently close to the permanent magnet 35 so that the magnetic field at the Hall effect switch 61 exceeds the Hall effect switch 61 threshold at the end of the charging stroke. One side of an magnetic shield 70 is pivotably mounted to one of the support posts 63. The magnetic shield 70 is pivotable about the post in a horizontal plane perpendicular to the axis of 9 motion of the reciprocating body and includes a clamping means (not shown) of a known configuration which is operable to enable rotation of the magnetic shield into and out of the magnetic field of the permanent magnet in the space between the Hall effect switch 61 and the permanent magnet 35 and clamping of the magnetic shield to the post in order that the shield may be manually fixable in a selected position in the horizontal plane in which it is displaceable. The magnetic shield 70 is made of a magnetic material so that it is capable of acting as a shield to a magnetic field, such as for example mild steel or soft iron, possibly zinc plated to prevent corrosion.

An electronic switching circuit 80, as shown in Figure 4, arranged on the printed circuit board 62 receives the output from the Hall effect switch 61 and electrical power from an external power source 87, such as a battery. One pole of the 'external source is connected to the switching circuit 80 and the other pole is connected to a ground common to both the switching circuit 80 and pump chamber 20. The output from the electronic switching circuit 80 is connected to the solenoid winding 41. Details of the electrical system set up and the electrical circuitry are illustrated in Figures 2 and 4, respectively. An n-channel MOSFET 95 is used as the power switch and the source 101 of the n-channel MOSFET 95 is connected to the end of the solenoid winding 41is closest to the diaphragm and the drain 102 of the MOSFET 95 is connected to the negative pole 86 of the external power source 87. The other end of the solenoid 41 is connected to the positive pole 85 of the external power source 87 and the gate 100 of the n channel MOSFET 95 is connected to the output electrode 61b of the Hall effect switch 61 through a first resistor 81 and a second resistor 82 in series with the first resistor. A heat sink is attachable to the MOSFET (not shown). The Hall effect switch 61 is a self-regulating logic level type and is activated when the magnetic flux density at the Hall effect switch switch is deactivated when the magnetic flux density at the Hall effect switch 61 decreases below the Hall effect switch threshold. The output of the Hall effect switch 61 is also connected to the positive pole 85 of the external power source through a third resistor 83 in series with a fourth resistor 84. The positive electrode 61a of the Hall effect switch 61 is connected to the positive pole 85 of the external power source 87 through the fourth resistor 84 and the negative electrode 61c is connected to the negative pole 86 of the external power source. A first Zener diode 88 is placed across the positive electrode 61a and negative electrode 61c of the Hall effect switch 61 and second Zener diode 89 is placed across the source 101 and drain 102 of the MOSFET 95. A diode 90 is placed in parallel with the second resistor 82 and a first capacitor 91 is connected between the gate of the MOSFET 95 and the negative pole 86 of the external power source. A second capacitor 92 is connected between the poles of the external power source 87 for decoupling and suppressing radio frequency signals. The negative pole 86 of the external source 87 is connected to the electronic switching circuit through a negative common ground 93 and the positive pole 85 directly to the electronic switching circuit. Alternatively, linkable connections can be used to connect the positive pole 85 of the external source to the electronic switching circuit through a positive common ground 94 and the negative pole 86 directly to the electronic switching circuit as shown in Fig. 5. A fuse link for over current protection is also provided.

The operation of the pump shall now be discussed with reference to Figures 1 and 2 and the circuit diagram of Figure 4. At the end of the discharging stroke the permanent magnet 35 is furthest away from Hall effect switch 61 and the magnetic flux density at the Hall effect switch 61 is below the Hall effect switch threshold. As a consequence, the Hall effect switch 61 is in a deactivated state and the output of the Hall effect switch 61 remains at the voltage set by the first, second, third and fourth resistors 81, 82, 83, and 84. The charge across the first capacitor 91 is sufficient to ensure that the voltage at the gate is higher than the threshold voltage of the MOSIFIET, which makes the MOSFET fully conductive and permits current to flow through the solenoid. Current flow through the solenoid generates a magnetic field that pulls the slug 38,39 towards and into the solenoid which completes the magnetic field at the end of the solenoid and moves the diaphragm 36 to charge the pump and expand the spring 50. As the permanent magnet 35 approaches the Hall effect switch 61, the magnetic flux density at the Hall effect switch 61 increases above the Hall effect switch 61 threshold activating the Hall effect switch which switches the output from the voltage set by the first, second, third and fourth resistors 81, 82, 83 and 84 to negative through the negative electrode 61c.

The juncture of the first and third resistors 81 and 83 is pulled down to negative voltage and the first capacitor 91 discharges through the diode 90 and the first resistor 81 decreasing the voltage at the gate of the MOSFET.

When the gate voltages decreases below the voltage threshold of the MOSFET, the MOSFET becomes non-conductive and the current flow through the solenoid coil is prevented. The logic level operation of the Hall effect device ensures that the output voltage changes rapidly which allows precise control of the switching of the MOSFET and limits power loss. The second Zener diode 89 across the MOSFET protects the MOSFET from the back emf generated by the solenoid as it is cleenegized without slowing the pump. Also, the first Zener diode 88 in conjunction with the second resistor 84 ensures that the Hall effect switch 61 and the switching circuit are voltage regulated and protected against the back emf of the solenoid. The field of the solenoid no longer exists and the force exerted on the shaft arm

39 by the spring 50 contracting pulls the shaft and diaphragm 36 in a direction to reduce the volume of the pumping chamber to discharge fluid 12 direction to reduce the volume of the pumping chamber to discharge fluid from the pump chamber 21. Preferably, the force exerted on the shaft arm 39 by the spring 50 is insufficient to move the diaphragm 36 against the fluid pressure in the pump chamber when the fluid pressure is high as a result of no pump demand. In this way, the pump stops rather than free runs on no demand, e.g. when a float actuated carburettor needle valve of a carburettor supplied from the outlet port 23 is closed as a consequence of the fuel level in the carburettor being sufficiently high. As the permanent magnet 35 moves back away from the Hall effect switch 61, the magnetic field density at the switch decreases below the Hall effect switch 61 threshold deactivating the Hall effect switch 61. The voltage at the juncture of first and third resistors 81 and 83 is switched from negative voltage to the voltage set by the first, second, third and fourth resistors 81, 82, 83, and 84 by virtue of the Hall effect output being disconnected from the negative voltage. The first capacitor 91 begins recharging through the first, second, third and fourth resistors 81, 82, 83, and 84 and after a delay the gate voltage increases above the threshold voltage making the MOSIFIET conductive. The cylce then repeats itself. The delay in recharging the first capacitor 91 and making the MOSIFET conductive ensures that the diaphragm 36 is pulled sufficiently back by the spring 50 so as to adequately discharge the pump. The delay period may be adjusted by adjusting the time constant for charging the first capacitor 91, which is set by the first capacitor 91 and the first, second, third and fourth resistors 81, 82, 83 and 84. In practice, the value of the fourth resistor 84 is low relative to the values of the first, second and third resistors and so the effect of the fourth resistor on the charging and discharging of first capacitor 91 is insignificant. When the Zener voltage of the first Zener diode 88 is exceeded, a voltage drop is produced across the fourth resister 84 without 13 the voltage across the Hall effect switch rising and without damage to the first Zener diode.

The operation of the adjustable magnetic shield 70 will now be discussed with reference to Figures 3 (a) (b) and (c). Figures 3(a) to 3(c) show views of the position of the permanent magnet 35 at which the Hall effect switch 61 is activated for different shield positions. In Figure 3(a) the magnetic shield 70 is positioned out of the space between the Hall effect switch 61 and the permanent magnet 35, the Hall effect switch 61 is directly exposed to the magnetic field of the permanent magnet 35 and so the permanent magnet 35 to Hall effect switch 61 distance D1 at which activation of the Hall effect switch 61 occurs is relatively long. However, when the magnetic shield 70 is positioned partially in the space as illustrated in Figure 3(b), the Hall effect switch 61 is partially shielded from the magnetic field and the magnetic field intensity at the Hall effect switch 61 is reduced. The reduction of the intensity of the field must be compensated by moving the permanent magnet 35 closer to the Hall effect switch 61 to a distance D2 before the Hall effect switch 61 will activate. As indicated in Figure 3(c), the more the Hall effect switch 61 is shielded the shorter the permanent magnet 35 to Hall effect switch 61 distance D3 required for activation of the Hall effect switch 61. Accordingly, the position at which the pump reaches the end of the charging stroke and begins the discharge stroke can therefore be adjusted and optimised by adjusting the position at which the permanent magnet 35 activates the Hall effect switch 61 using the magnetic shield 70.

In accordance with another embodiment of the invention, the solenoid operated positive displacement pump has a pumping configuration similar to that illustrated in Figure 1 but with the solenoid disposed closer to the Hall effect switch 61. The solenoid position is selected so that when the solenoid is energised, the magnetic field generated by the solenoid

14 interacts with the Hall effect switch 61 but with an intensity that is insufficient to activate the Hall effect switch 61. As the permanent magnet 35 moves towards the Hall effect switch 61, the magnetic field of the permanent magnet 35 and the magnetic field generated by the solenoid combine at the Hall effect switch 61 and the intensity of the combined magnetic field increases above the Hall effect switch threshold activating the Hall effect switch 61. As before, the output of the Hall effect switch 61 switches from the voltage set by first, second, third and forth resisitors 82, 83, 84 to negative through negative electrode 61c. The first capacitor 91 discharges through diode 90 and the first resistor 81 and the gate voltage of the MOSFET decreases below the voltage threshold of the MOSIFIET making the MOSIFIET non-conductive. Current flow through the solenoid is impeded, the solenoid is deenergized, and the shaft is pulled back under the force of the contracting spring 50. 'The Hall effect switch 61 is deactivated as the magnetic field intensity at the switch decreases below the switch threshold either as a result of the solenoid deenergizing or, if this is not sufficient to deactivate the Hall effect switch, as a result of the permanent magnet 35 moving away from the Hall effect switch 61. The delay in recharging capacitor 91 and making the MOSIFET conductive ensures that the Hall effect switch is not immediately reactivated upon reenergizing the solenoid and that the diaphragm 36 is pulled sufficiently back by the spring 50 so as to adequately discharge the pump.

In accordance with yet another embodiment of the invention, the solenoid operated positive displacement pump has a pumping configuration that differs from the preferred embodiment in that the MOSFET is p- channel rather than n-channel, the drain of the MOSIFET is connected to the positive pole 85 of the external power source, the solenoid is connected between the source of the MOSIFIET and the negative pole 86 of the external power source, and the first capacitor 91 is connected to thepositive pole rather than the negative pole of the external power source. Also, the rigid member to which one end of the spring 50 is attached is disposed above rather than below the shaft arm. In this configuration, when the Hall effect switch 61 is activated by the permanent magnet 35 the output of the Hall effect switch 61 changes from the voltage set by the first, second, third, and fourth resistors 81, 82, 83, 84 to negative through electrode 61c, as before. The first capacitor 91 discharges through first resistor 81 and the diode and the gate voltage at the MOSFET decreases below the threshold voltage of the MOSIFIET making the MOSIFET conductive. The solenoid is energised and the magnetic field generated by the solenoid repels the slug 38, 39 away from the solenoid moving the diaphragm 36 and shaft 31 in a direction to reduce the volume of the pump chamber to discharge the pump and expand the spring 50. As the permanent magnet 35 is moved away from the Hall switch, the Hall effect switch 61 is deactivated and the voltage at the juncture of first and third resistors 83 and 81 is switched to the voltage set by the first, second, third and fourth resistors 82, 83, and 84. The first capacitor 91 recharges through first, second, and third resistors 81, 82, 83 and after a delay the gate voltage increases above the gate threshold of the MOSIFIET, making the MOSIFIET non-conductive. The delay in recharging the first capacitor 91 and making the MOSFET conductive ensures that the diaphragm 36 is pushed sufficiently displaced by the solenoid so as to adequately discharge the pump. The solenoid is denergized and the shaft is pulled back under the force of the contracting spring 50 towards the Hall effect switch 61 moving the diaphragm 36 to charge the pump. The cycle then repeats itself.

It is not intended that the invention be restricted to the arrangements of the pump, magnetic fields, magnetic field sensing devices, biasing means, magnetic shield and electronic switching circuits illustrated and described herein. It would be obvious to a skilled man in the art that

16 the invention may be applied to other pump configurations, such as reciprocating pistons, or used with reciprocating bodies other than the diaphragm and shaft arrangement. It is also recognised that those skilled in the art will be capable of designing other switching circuits to perform the same basic function as those described herein without departing from the scope of the invention. In particular, circuitry using transistors rather than MOSIFETS may be adopted. It would also be recognised that the permanent magnet and Hall effect switch may be applied in other configurations in conjunction with the same or similar electronic switching circuits without departing from the scope of the invention. Magnetic field sensing devices other than a Hall effect switch may be used, like for example reed switches or other Hall effect sensors. It would be obvious to those skilled in the art that other types of magnetic shield adjusting means than may be applied that are capable of adjusting the vertical and horizontal position of the shield in the space between the Hall effect switch and the permanent magnet.

17

Claims

1 A solenoid operated positive displacement pump comprising:

a pumping housing and a pump chamber having an inlet port and an outlet port between which is defined a flow path through the pumping chamber; a reciprocable means or body including a permanent magnet section, and a pumping section arranged to vary the volume of the pumping chamber as the reciprocable body is reciprocated; a guide member in which the reciprocable body is free to reciprocate; biasing means for urging the reciprocable body in a first direction to effect a charging or discharging stroke; a solenoid for moving the reciprocable body in a direction opposite to that in which the biasing mean urges the reciproable body; a magnetic field sensing device disposed adjacent a path of movement of the permanent magnet; an electronic switching means for deenergizing or energising the solenoid in response to the magnetic field sensing device activating or deactivating; wherein a magnetic shield is positioned between the magnetic field sensing device and the permanent magnet and adjusting means is provided for adjusting the position of the shield to accordingly adjust the position at which the permanent magnet deactivates or activates the magnetic field sensing device.

18

2. An apparatus according to claim 1, wherein the end of the charging or discharging stroke can be adjusted for optimum pump performance by adjusting the postion of the magnetic shield.

3. An apparatus according to claim 1 or 2, wherein the position of the reciprocating body at the end of the charging stroke is adjustable by configuring the magnetic field sensing device to activate or deactivate at the end of the charging stroke.

4. An apparatus according to claim 1 or 2, wherein the position of the reciprocating body at the end of the discharging stroke is adjustable by configuring the magnetic field sensing device to activate or deactivate at the end of the discharging stroke.

5. An apparatus according to any preceding claim, wherein the magnetic field sensing device is activated by the magnetic field of the permanent magnet in combination with the magnetic field of the solenoid and wherein the electronic switching means deenergizes the solenoid in response to the magnetic field sensing device activating.

6. An apparatus according to any preceding claim, wherein the magnetic field sensing device is a Hall effect device.

7. An apparatus according to any preceding claim, wherein the biasing 25 means comprises a resilient member.

8. An apparatus according to any preceding claim, wherein the reciprocable body includes a shaft and a permanent magnet, the 19 permanent magnet being either integral with the shaft or fixable to the shaft.

9. An apparatus according to claim 8, wherein the reciprocable body further comprises a diaphragm fixable to or integral with one end of the shaft, and wherein a surface of the diaphragm and the lower portion of the pumping housing defines a chamber, the inlet port and outlet ports being spaced apart in the chamber.

10. An apparatus according to claim 9, wherein the fluid pressure in the chamber resists movement of the reciprocable body, the reciprocable body being subject to the force of the biasing means.

11. An apparatus according to any preceding claim, wherein the solenoid moves the reciprocable body to charge the pump, and the resilient member moves the reciprocable body to discharge the pump.

12. An apparatus according to any one of claims 1 to 10, wherein the solenoid moves the reciprocable body to discharge the pump, and the resilient member moves the reciprocable body to charge the pump.

13. An apparatus according to any preceding claim, wherein the electronic switching means includes a switching delay circuit that allows the action of the pump to substantially imitate the action of a mechanical toggle.

14. An apparatus according to any preceding claim 6 to 13, wherein the electronic switching means includes a power MOSFET connected in series with the solenoid, the solenoid and power MOSFET being connected between opposite poles of an electrical power source such that the MOSIFIET substantially blocks current flow through the solenoid coil when a gate voltage of the MOSIFIET is switched to a voltage sufficient to deactivate the MOSFET in response to the Hall effect device activating [or deactivating].

15. An apparatus according to claim 14, wherein the electronic switching means is configured such that the MOSIFIET blocks current flow through the solenoid coil in response to the Hall effect device activating.

16. An apparatus according to claim 14 or 15, wherein the MOSIFET is an nchannel or p-channel type MOSFET.

17. An apparatus according to claim 13, 14, 15 or 16, wherein the electronic switching means also includes a switching delay circuit comprising an R-C network connected to the gate of the MOSIFET for delaying the switching of the gate voltage, a capacitor in the R-C network being connected between the gate and one pole of the electrical power source such that the capacitor charges or discharges in response to the Hall effect device activating.

18. An apparatus according to clam 17, wherein the switching delay circuit also includes a diode connected in parallel with a resistance in the R-C network in order that the resistance is bypassed when discharging the first capacitor in response to the Hall effect device activating, the resistance of the R-C network being defined by at least one resistor placed between the Hall effect device and the gate of the MOSFET.

21

19. An apparatus according to any one of claims 14 to 18, wherein the electronic switching means also includes a Zener diode, the Zener diode being connected across the source and drain of the MOSIFIET for protecting the MOSIFIET against back emf of the solenoid and voltage transients.

20. An apparatus according to any one of claims 14 to 19, wherein another Zener diode is connected across the Hall effect device and another resistance is connected in series therewith and one pole of the electrical power source in order that the electronic switching means and Hall effect device is voltage regulated and protected against back emf of the solenoid.

21. An apparatus according to any preceding claim, wherein a capacitor is connected across the electronic switching means, for clecoupling and suppressing radio frequency signals.

22. An apparatus according to any one of claims 6 to 21, wherein the Hall effect device is a logic level self-regulating Hall effect switch.

23. A method of operating a solenoid actuated positive displacement pump having a pumping chamber, a reciprocable means or body, a permanent magnet fixable to the reciprocating body, biasing means for displacing the reciprocable body, a guide member in which a reciprocable body is free to reciprocate, a solenoid for moving magnetic material fixed to the reciprocable body, a magnetic fields sensing device with an 22 adjustable magnetic shield, and electronic switching means and comprising the steps of; energising the solenoid in order to displace the reciprocable body in one direction against the force of the biasing means; denergizing the solenoid in order to allow the reciprocable body to displace in the opposite direction under the force of the biasing means; varying the volume of the pumping chamber upon displacing the reciprocating body in either the one direction or the opposite direction; passing the magnetic field of the permanent magnet through a magnetic field sensing device when the reciprocable body is substantially displaced; deactivating an electronic switch in order to denergize the solenoid in response to the magnetic field sensing device activating or deactivating; activating an electronic switch in order to energise the solenoid in response to the magnetic field sensing device deactivating or activating; positioning a magnetic shield between the magnetic field sensing device and the solenoid; and adjusting the position at which the permanent magnet activates or deactivates the magnetic field sensing device by adjusting the position of the magnetic shield.

24. A method according to claim 23, including the steps of activating the magnetic field sensing device when the flux density of the magnetic field of the permanent magnet in combination with the magnetic field of the solenoid exceeds the magnetic field sensing device threshold; and deenergizing the solenoid in response to the magnetic field sensing device activating.

25. A method according to claim 23 or 24, including the step of discharging the pump by displacing the reciprocable body under the force of the biasing means.

26. A method according to claim 23, 24 or 25, including the steps of subjecting the reciprocable body to the pressure of the fluid in the chamber, the pressure of the fluid resisting movement of the reciprocable body when the reciprocable body is subjected to the force of the biasing means.

27. A method according to any one of claims 23 to 25, wherein the magnetic sensing device is a Hall effect device.

28. A method according to claim 27, wherein the step of deactivating the electronic switch comprises switching the gate voltage of a MOSFET connected to a voltage supply from a gate voltage sufficient to activate the MOSFET to a voltage sufficient to deactivate the MOSFET by means of the Hall effect device.

29. A method according to claim 28, including the step of delaying the switching of the gate voltage of the MOSFET using an R-C network connected to the Hall effect device and the MOSFET, the capacitor of the R-C network discharging the gate via a resistance in the R-C network when the Hall effect device is activated and charging the gate via the resistance or another resistance in the R-C network when the Hall effect device is deactivated.

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30. A method according to claim 29, wherein the discharging step includes discharging the capacitor through one part of the resistance in the R-C network and a diode, the diode bypassing the other part of the resistance in the R-C network.

31. A method according to claim 28, 29 or 30, including the step of blocking with a Zener diode the MOSFET from voltage transients and the back emf of the solenoid.

32. A method according to any one of claims 27 to 31, including the steps of stabilising the voltage supply and protecting the Hall effect device against transients with another Zener diode and resistor; and decoupling and suppressing radio frequency signals using a capacitor.

33. A method according to any one of claims 27 to 32, wherein the Hall effect device is a self regulated logic level Hall effect switch.

34. A solenoid operated positive displacement pump constructed substantially as hereinbefore described with reference to Figures 1 to 4 of 20 the accompanying drawings.

35. A method of operating a solenoid actuated positive displacement pump constructed substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.