GB2248939A - Capacitive switch for an automatic pump - Google Patents

Abstract

The switch comprises first and second capacitive plates 5, 6 which define therebetween a gap 9 into which fluid can enter. Circuitry which senses the changed capacitance resulting from the presence of fluid between the plates and changes the state of a switch from an inoperative state in which the pump is turned off to an operative state in which the pump is turned on. The outer plate 6 may be grounded to screen the inner plate. The circuitry includes a capacitive bridge which oscillates until the capacitance between the plates 5, 6 exceeds a predetermined value, and a timing capacitor which changes, after oscillation ceases, to a threshold value to operate the switch. The switch may be used to control a bilge pump. <IMAGE>

Description

SWITCH FOR AN AUTOMATIC PUMP This invention relates to a switch for an automatic pump, particularly but not exclusively to a bilge pump capable of turning on or off automatically in response to the presence or absence of fluid.

Such pumps are known and are in use on most commercial sea going vessels to ensure that action is taken to remove bilge water as soon as the fluid is detected by the pump and without the need for manual intervention. In one such pump, the switch which is operable to turn the pump on and off is responsive to the passage of an ultrasonic signal through the fluid to be detected. This signal which is transmitted between two plates has different properties when travelling through air and through the fluid to be detected. The difference between these properties is utilised to turn on the switch when fluid is present. It is a paramount requirement of automatic bilge pumps that they should be wholly reliable, that is that they should turn on immediately that bilge fluid is detected and that they should in all other circumstances remain off.In the case of the pump which operates according to the ultrasonic sensing technique described above, it has been found that the vibration of the pump itself when operating can cause the switch erroneously to turn off. The result of course is that bilge fluid will continue to rise in the bilge. Furthermore, vibration of the engine of the boat itself at certain speeds can cause the switch to turn the pump on, with the result that the power supply on the boat is unnecessarily drained. Furthermore, operation of the pump when no fluid is present can cause damage to the pump.

In another automatic bilge pump, the inherent resistance of water is detected to make a connection between two plates and thereby activate a switch to turn the pump on when water is detected. This pump too suffers from reliability problems since scum from the bilge fluid can build up within the switch and create a connection even in circumstances when there is no bilge fluid present. Hence, the pump will be turned on when it is not required.

Hence, these pumps and other known automatic pumps do not satisfy the criteria of complete reliability of operation.

According to the present invention there is provided a switch operable to actuate a pump in response to detection of a fluid the switch comprising first and second capacitive plates defining therebetween a gap into which fluid when present can enter and sensing circuitry for sensing the change in capacitance resulting from the presence of fluid between the capacitive plates and for changing the state of the switch between an inoperative state in which the pump is turned off and an operative state in which the pump is turned on.

Generally the dielectric between the capacitive plates in the absence of fluid is air. As the dielectric constant of water is about 80 times that of air there is a large increase in capacitance when water is present between the capacitive plates. This renders the switch extremely reliable, particularly in circumstances where the fluid consists predominantly of water which is the case where bilge pumps are to be used. However, the pump is also suitable for other fluids since the dielectric constant of most fluids is several times that of water.

In one embodiment the capacitive plates are arranged coaxially, with one surrounding the other. In this case, the outer plate can be at ground potential to screen the inner plate. However, any other suitable arrangement of capacitive plates may be adopted.

Preferably, the sensing circuitry includes a capacitive bridge having a reference capacitor and an oscillator which is connected to oscillate unless the value of the capacitance between the first and second capacitive plates exceeds a threshold value related to the value of the capacitance of the reference capacitor. The output of the oscillator can be connected to a timing capacitor so that when oscillation ceases, the timing capacitor is allowed to charge to a voltage sufficient to operate the switch.

Preferably, the sensing circuitry includes a comparator arranged to compare the voltage on the timing capacitor with a reference voltage and to operate the switch when the former exceeds the latter.

Preferably the switch is an electromagnetic relay.

Preferably the sensing circuitry includes first timing means for determining the delay between first detecting a fluid and turning on the switch, and a second timing means for determining the delay between ceasing to detect a fluid and turning off the switch. The first timing means incorporates the timing capacitor referred to hereinabove.

To protect the switch from damage under a wide range of voltage supply conditions, the sensing circuitry can include a square wave oscillator arranged to drive the switch and whose duty cycle varies depending on the value of the voltage supply value to supply on average a constant voltage to the switch.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a perspective view from the outside of a switch for use in a pump according to the present invention; Figure 2 is a section through the switch of Figure 1; and Figure 3 is a circuit diagram of sensing circuitry for the switch.

Figure 1 shows a casing 1 for a switch, the casing being formed of a polypropylene moulding and having two lugs 2,3 for mounting the switch. Reference numeral 4 designates a power supply cable which houses not only two power supply lines but also two relay contacts for the switch. As can be seen from Figure 2, the casing houses first and second capacitive plates designated by reference numerals 5 and 6 in Figure 2. These are arranged coaxially within the casing so that fluid, when present, enters the gap 9 between the capacitive plates 5,6 and thereby forms the dielectric of the capacitor. The outer plate 6 is arranged outside the inner plate 5 so as to screen the inner plate 5 and is connected to ground. To avoid clogging and surface tension effects, a fairly wide spacing should be selected between the capacitive plates 5,6. This means that the capacitance to be sensed should generally be within the range 5 to 10 picofarads in air in order to keep the casing to a reasonable size. The casing is placed in an area prone to flooding and is fully submersible. The casing also houses sensing circuitry arranged on a printed circuit board 7 which actuates the pump when fluid is sensed between the capacitive plates. Reference numeral 8 designates a screening plate.

The sensing circuitry will now be described with reference to Figure 3. The inner capacitive plate 5 is connected in a two pointed bridge circuit of which a reference capacitor C2 is connected between the positive input 20 of an operational amplifier U1A and its output 21. There is also connected between the output 21 of the operational amplifier U1A and its negative input 22 a resistor R6. The negative input 22 is also connected by a resistor R7 to a voltage reference line which is selected as being half the regulated supply voltage l/2Vcc by a potential divider formed from resistors R4 and R5.The operational amplifer U1A is thus connected as an oscillator with an oscillation frequency determined by a resistor R3 connected between the positive capacitive plate 5 and the resistor R7, the resistors R7 and R6, the capacitor C2 and the capacitance of the capacitor formed by the capacitive plates 5,6. The oscillator so formed will oscillate continuously until the capacitive value of the capacitor formed by the capacitive plates 5,6 exceeds a threshold value determined by the reference capacitor C2 and the resistive potential divider network constituted by resistors R6,R7. It will be apparent that the value of C2 is selected so as to be greater than the value of the capacitance between the capacitive plates 5,6 when the dielectric therebetween is air by a factor determined by the potential divider comprising resistors R6,R7.While the oscillator is oscillating, the minimum voltage on the output of the operational amplifier U2A is zero and the voltage on a timing capacitor C1, which is connected between ground and one node of a resistor R2 the other node of which is connected to the supply voltage Vcc, is prevented from rising due to the clamping action of a diode D3 connected between the output of the operational amplifier and the timing capacitor C1. Reference numeral R1 designates a current limiting resistor connected between the capacitor C1 and the diode D3. When the oscillator stops, the output of the operational amplifier U1A self biases to 1/2Vcc and thus removes the clamping action of the diode D3 from the capacitor allowing it to charge up via the resistor R2.The speed at which the capacitor C1 charges up is naturally dependent on its relationship with the value of the resistor R2 so that the time taken to reach a specific voltage at the node of resistor R1 and capacitor C1 is dependent on the RC time constant R2.C1. This controls the delay between sensing fluid and turning on the switch, this delay being for example 5 seconds. The voltage at the node of the capacitor C1 and the resistor R1 is fed to the negative input 23 of a comparator U2A. The comparator U2A has as its output 25 an open collector transistor. The positive input 24 of the comparator U2A receives a reference voltage which is 1/2Vcc determined by the potential divider formed of resistors R4 and R5.Once the voltage on the timing capacitor C1 exceeds the reference voltage the output transistor of the comparator U2A conducts and via a resistor R16 shorts the negative input 26 of a second comparator U2B to ground. The second comparator U2B has as its output an open collector transistor and has as a reference voltage on its positive input 27 a value of 1/2Vcc.

The output 28 of the comparator U2B is connected to the base of a transistor Q2. The state of the transistor Q2 controls a power relay 29. When the output transistor of the comparator U2B is OFF, the base of the transistor Q2 is driven by current from a square wave oscillator circuit designated generally by reference numeral 31 and described in more detail hereinafter. In these circumstances the coil of the relay 29 is driven and the switch is ON.

When fluid is no longer present between the capacitive plates 5 and 6, the oscillator formed primarily of the operational amplifier U1A in the two pointed capacitive bridge will commence oscillating so that the sensing capacitor C1 will cease to remain charged. As the charge on the capacitor C1 reduces below a threshold value, the output transistor of the comparator U1B ceases to conduct and its collector floats.

This in turn causes the output transistor of the comparator U2B to conduct so that the base of the transistor Q2 is shorted to ground, the coil of the relay ceases to be driven and the switch turns off. However, the output transistor of the second comparator U2B will remain off under the influence of charge stored in a capacitor C6 for a time determined by the relationship between the size of capacitor C6 and a resistor R9. This CR combination controls the "switch off" time, i.e. the delay between "dry out" and turning off the switch. For example, this delay can be 20 seconds.

The relay is a high current electromagnetic relay which has an advantage over semiconductor transistor relays in that it does not require a power drop for its operation. The relay will continue to hold the switch in its operative position until the transistor Q2 is turned off.

The circuit shown in Figure 3 also includes circuitry denoted generally by the dotted line marked 30 for power regulation purposes. As this circuit does not form part of the present invention it will not be described further herein.

The circuit also includes a component VDR1 to protect against spikes in the power supply.

The circuit is such that it can be mounted on a small printed circuit board and contained within the casing 1 of the switch.

The square wave oscillator circuit which provides the drive current for the relay coil while the switch is on will now be described.

The duty cycle of the oscillator circuit is related to the supply voltage so as to allow operation over a wide input voltage range without damaging the relay coil. The components of this oscillator circuit are contained in the box designated generally by reference numeral 31. The square wave oscillator comprises an operational amplifier U1B whose positive input 32 is connected via a feedback resistor R12 to the output 34 of the operational amplifier U1B. The negative input 33 is connected to ground via a resistor R8 and a capacitor C4 connected in parallel. The negative input 33 is also connected to the positive supply rail via a resistor R14.

There is a further resistor Rll connectd as a feedback resistor between the output 34 of the operational amplifier U1B and its negative input 33. Together these components form a relaxation oscillator.

As the comparator U2B has an open collector output, the relay 29 may be turned off under the control of this operational amplifier U2B, irrespective of the state of the operational amplifier U1B of the square wave oscillator.

The DC bias for the oscillator circuit is a combination of 1/2Vcc, half the regulated supply voltage, derived from the potential divider formed from resistors R4 and R5 and fed to the negative input 32 of the operational amplifier U1B via a resistor R13 and half the input supply voltage derived from the potential divider formed from resistors R14 and R8. When the output 34 of the operational amplifier U1B has just gone high, the capacitor C4 charges exponentially towards a voltage defined by the combination of resistors R14, Rll and R8. This process continues until the voltage at the node 36 of capacitor C4 reaches the voltage on the positive input 32 which is defined by the resistor network comprising R4,R5,R13,R12 and the state of the output 34 of the operational amplifier U1B.

At this point the circuit regenerates, and the output 34 goes low.

After the output 34 of the operational amplifier U1B has just gone low, the capacitor C4 discharges exponentially towards a voltage defined by the combination of resistors R14,Rll and R8. This process continues until the voltage at the node 36 of capacitor C4 decays to the voltage on the positive input 32 which is defined by the resistor network comprising R4,R5,R13,R12 and the state of the output 34 of the operational amplifier.

The DC conditions and resistor values determining the charge and discharge voltage levels, the target voltages to which capacitor C4 is being charged towards and the resistance of the charge and discharge paths is so arranged that the duty cycle of the relaxation oscillator changes with varying input supply voltage. The values are so chosen that at approximately 12v DC input voltage the duty cycle is 100% (i.e. the relaxation oscillator stops oscillating and the output of the operational amplifier U1B assumes a high voltage level), and at 24v DC input voltage the relaxation oscillator oscillates with an approximate duty cycle of 50%. Thus the mean drive current into the relay coil is kept constant over this voltage range.

Claims (9)

CLAIMS:

1. A switch operable to actuate a pump in response to detection of a fluid, the switch comprising first and second capacitive plates defining therebetween a gap into which fluid when present can enter and sensing circuitry for sensing the change in capacitance resulting from the presence of fluid between the capacitive plates and for changing the state of the switch between an inoperative state in which the pump is turned off and an operative state in which the pump is turned on.

2. A switch as claimed in claim 1, wherein the dielectric between the capacitive plates in the absence of fluid is air.

3. A switch as claimed in claim 1 or 2, wherein the capacitive plates are arranged coaxially, with an outer one of the capacitive plates surrounding an inner one of the capacitive plates.

4. A switch as claimed in claim 3, in which the outer plate is at ground potential to screen the inner plate.

5. A switch as claimed in any preceding claim, wherein the sensing circuitry includes a capacitive bridge having a reference capacitor and an oscillator which is connected to oscillate unless the value of the capacitance between the first and second capacitive plates exceeds a threshold value related to the value of the capacitance of the reference capacitor.

6. A switch as claimed in claim 5, wherein the output of the oscillator is connected to a timing capacitor so that when oscillation ceases the timing capacitor is permitted to charge to a voltage sufficient to operate the switch.

7. A switch as claimed in claim 6, which includes a comparator arranged to compare the voltage on the timing capacitor with a reference voltage and to operate the switch when the former exceeds the latter.

8. A switch as claimed in any of claims 5 to 7, wherein the sensing circuitry comprises a square wave oscillator arranged to drive the switch and whose duty cycle varies depending on the value of the voltage supply thereby to supply on average a constant voltage to the switch.

9. A switch substantially as hereinbefore described with reference to or as shown in the accompanying drawings.