GB2179738A - Fluid level responsive apparatus - Google Patents

Abstract

A system for controlling the level of bilge water in a bilge tank includes an acoustic transmitter (2) and an acoustic receiver (4) mounted in spaced relationship on an acoustically inert pad (6) to monitor a space at a predetermined level in a bilge tank. A circuit (10 to 18) monitors the output of the receiver (4) when the transmitter is energised and generates a pump actuating signal for actuating a bilge pump (20) when bilge water enters the space. When the level of bilge water drops so that bilge water is no longer present in the space the pump actuating signal is discontinued and the pump (20) is switched OFF. Time delay means allow for level changes due to the pitching action of the boat to be ignored, so that the pump does not work out of water. Repeated start-stop operation out of the water is also prevented. <IMAGE>

Description

SPECIFICATION Fluid level responsive apparatus This invention relates to fluid level responsive apparatus in the form of a fluid level sensor, or in the form of an electric circuit comprising such a sensor and being adapted to be connected to, for the purpose of controlling, an electric pump motor. The invention also includes within its scope a said circuit including a said motor, and also a said circuit including a said motor and having coupled to the latter a pump, the arrangement being such that the motor can, under the control of a said circuit, be switched on an off so as, when said sensor and a hydraulic circuit comprising said pump are installed in, or a relation to, a vessel, the level of a fluid in said vessel can be maintained substantially at or below a predetermined value.

Liquid level sensors presently available are mainly of two types, viz.: a) float switches which rely on a float and arm to operate the switch. These have the disadvantages of: moving parts tending to wear, jam or stick, bearings which are subject to corrosion, puncturing of the float, and sensitivity to orientation of mounting. They are also inherently bulky.

(b) pressure switches which comprise a moving membrane over a sealed chamber.

These switches have the disadvantages of: tendency for the pressure chamber to puncture, fatigue of the membrane, mechanical distortion, the chamber being susceptible to temperature variations giving spurious signals, and moving parts which may jam.

It is an object of the present invention to provide an improved fluid level sensor which does not suffer from the aforesaid disadvantages, and liquid level responsive apparatus incorporating such an improved sensor.

According to the present invention there is provided a fluid level responsive apparatus comprising an acoustic transmitter and receiver mounted in spaced relationship in an environment arranged to receive a fluid, means for monitoring the output of the receiver when the transmitter is operational, to generate a trigger signal in response to a predetermined change in the acoustic characteristics in said environment in the vicinity of the transmitter and receiver resulting from the fluid occupying the environment in said viscinity being displaced by a different fluid.

According to the present invention there is further provided a liquid level detection system for controlling the level of bilge water in a bilge tank, the system comprising an acoustic transmitter and receiver mounted in spaced relationship to monitor a space at a predetermined level in the bilge tank, a pump for pumping bilge water from the bilge tank, and a control circuit for monitoring the output of the receiver when the transmitter is energised, said control circuit being responsive to a change in the acoustic characteristics of said space resulting from bilge water entering the space to switch said pump ON and being responsive to the subsequent change in the acoustic characteristics of the space resulting from the bilge water leaving the space to switch said pump OFF.

According to the present invention there is still provided a liquid level detection system for controlling the filling of a tank with a liquid, the system comprising an acoustic transmitter and a receiver mounted in spaced relationship to monitor a space in said tank at a predetermined level to which said tank is required to be filled, a pump for pumping liquid into said tank, and a control circuit for monitoring the output of the receiver when the transmitter is energised, said control circuit being responsive to a change in the acoustic characteristics of said space resulting from the liquid entering said space to inhibit the pumping action of said pump and thereby preventing the liquid entering said tank from exceeding said predetermined level.

One form of liquid level responsive sensor and apparatus embodying the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a block diagram and Figure 2 a circuit diagram of the apparatus.

Referring to Fig. 1, acoustic transmitter and receiver are arranged in a space or vessel in which the liquid level is to be controlled, so that acoustic waves may propagate through this liquid from the transmitter to the receiver.

Circuits using known techniques may be incorporated to measure any acoustic parameter of the liquid through which the propagation occurs and thereby appropriate discrimination between different liquids may be accomplished.

Examples of these parameters are acoustic impedance, frequency response, velocity of acoustic waves, and phase relationship between transmitted and received signals.

Measurements may be made in pulsed or continuous form. This allows not only the detection of the medium entering the space but also identification of its nature.

The block diagram of Fig. 1 refers to a circuit suitable for monitoring the water in a bilge tank of a boat to operate a bilge pump.

A transmitter 2 and receiver 4 are mounted in spaced relationship on an acoustically inert pad 6 of a material such as, for example, silicone rubber. The pad 6 is positioned to monitor a space at a predetermined level in the bilge tank. The transmitter 2 and receiver 4 are preferably of a piezo-ceramic material to allow oscillation in the ultrasonic frequency range.

An amplifier 8 couples the transmitter 2 and receiver 4, and the acoustic coupling and amplifier gain are arranged to be less that unity until bilge water enters that space and the consequent change in acoustic impedance makes the total (viz. acoustic plus amplifier) loop gain become greater than unity and oscillation occurs.

The output of the amplifier 8 is fed through a rectifier 10 to a peak follower current 16 with a rapid time delay. The output of the peak follower circuit 16 is connected to a Schmitt trigger circuit which in turn controls a motorised pump 20.

The rectifier circuit 10 also feeds a rapid peak follower circuit 12. A NOR gate 14 has one input connected to the output of the rapid peak follower circuit 12 and a second output connected to the output of the Schmitt trigger circuit 18. The output of the NOR gate 14 controls the reset input of the peak follower circuit 16.

The operation of the circuit will be described in more detail in conjunction with the circuit diagram of Fig. 2.

The frequency of oscillation is partly dependent on the state of the water entering the space and can be used to monitor the water for sludge, oil or other impurities.

The electronics may be pulsed, or phase signals monitored, using standard techniques.

It would be appreciated that the apparatus, and more especially the sensor, can be made to have the following advantages over liquid level responsive equipment of the prior art, viz.:- total encapsulation in inert materials; entire submersibility; no jamming or wear by reason of the solid state construction; no danger from puncturing; capability of being mounted in any orientation; small size; very fast response; and ability to withstand hostile environment such as for example high or low temperatures or corrosive liquids.

In its application of the bilge section of relatively small boats, and the fairly adverse environment consisting of water borne oil, sludge, and general debris splashing and swirling around, the function of the apparatus is to sense a sustained increase in such polluted water level within the bilge and to switch on a pump in response thereto. The supply to the pump motor is maintained until the water level drops below the height of the sensor. The switch is positioned just above the pump; the supply to the pump motor is from 12 or 24 volt batteries, the apparatus being left switched on in unattended boats to protect them from filling and consequently sinking.

In practice, although the volume of such polluted water in the bilge may be fairly constant (when observed over a short period of time), owing to the pitching action of the vessel, the level does not remain constant. Therefore, operation of the pump motor switch must be inhibited if it is this pitching movement has caused the level to rise over the sensor, since the pump will otherwise be working out of water and will burn out. This condition is met by introducing a delay between the sensing of the level rise and the actuation of the pump motor switch. This time delay is required to be bi-directional, i.e. the pump is activated after a short delay (5 seconds) from sensing the increased water level and de-activated after a longer delay (15 seconds) from the time the water level fell below the threshold to allow the pump to drain the rest of the water.Since this time delay is bi-directional and because the turn-on delay is shorter than the turn-off delay, sustained splashing of water on the sensor as the boat rocks will cause a gradual "pumping" of the charge held on a timing capacitor of the apparatus, resulting in eventual actuation of the pump. This is undesirable from the pump's point of view, in that repeated start-stop operation out of the water may cause possible burn out. Herice a further logical mechanism is included to complete the algorithm:-"lF there is no "water sense" signal AND the motor is not yet actuated THEN RESET the timing capacitor"; Operation of the bilge pump is therefore inhibited until there is a rise in water level above the sensing threshold for longer than the "turnon" time delay.

Referring now to Fig. 2, two piezo-ceramic discs are mounted facing each other in close proximity. They are acoustically de-coupled from each other by mounting them in a silicone rubber moulding. One disc is connected to the input of a high gain, small signal amplifier and acts as a microphone. The other identical disc is connected to the output of the aforementioned amplifier and acts as an ultrasonic loudspeaker. Thus a feedback-loop has been created. The gain of the interposed amplifier is limited so as to keep the total loop gain below unity when the device is in the relatively high acoustic impedance of air. Placing the device into water raises the total loop gain above unity by virtue of the now low acoustic impedance presented by the water and so oscillation is sustained until the device is removed from the water. The frequency of oscillation is governed by the resonant frequency of the piezo-disc assembly and in this case is approximately 80kHz.

TOR 1 and associated resistors R1, R2 and R3 form a high-gain common-emitter amplifier. R1 is the base-bias resistor for TR1 and determines the operating-point of the amplifier. R1 may be connected between R2 and the positive rail to provide a higher gain if necessary but only at the expense of operating point stability. The junction of RXTAL (receiving crystal), R1 and R2 is a summing node for the current through R1 and RXTAL. R2 serves to prevent loading of RXTAL. R3 is the collector load resistor and also determines the gain and operating-point of this stage. TR2 and TR10 are configured as a complimentary driver stage for the TXTAL. D7 gives a suitable offset for biasing TR2 and TR10. R4 is a bleed resistor ensuring the conduction of TR2 at all times, and thus enables the onset of oscillation when acoustic feedback occurs.R5 and C1 are needed to decouple the supply for this highly sensitive amplifier from supply-borne transients caused primarily by the pump motor.

C2 and D1 provide rebiasing of the output signal signal from TR2 emitter. D1 prevents the a.c. signal at C2 going negative i.e. below 0 volts. D2 provides rectification of the a.c.

signal and pumps the resulting positive signal into C3, thus forming a d.c. level at the junction of D2 and C3 which follows closely the peak level of the a.c. waveform. This signal is the "WATER SENSE" signal and is applied to one input of the 2-input NOR gate formed by TR3, TR4, R6, R7, R8, and R9. R6 and R7 provide suitable base impedances for TR3 but in addition provide a discharge path for C3 when the a.c. waveform at TR2 emitter disappears. If TR3 is turned ON, i.e. "WATER SENSE" signal HIGH, then its collector will be LOW, thereby turning TR5 OFF which releases the reset on the time delay capacitor C5. Similarly, if the pump is activated TR4 will be turned ON (via base resistor R9) again releasing the reset condition on C5.

D3 points the way to the time delay circuit formed by R11, R10, and C5. D3 performs the rectification of the a.c. waveform from C2.

The resulting positive pulses from the cathode of D3 are applied to C5 via R 11. The pulses "pump" up C5 in a similar fashion to the peak-follower formed by D2 and C3. The exception is that this time rate of flow of charge is restricted by R 11. This means that the voltage on C5 takes a finite time to build up, i.e.

it is a delayed response to the onset of oscillation in the amplifier section. The discharge path for C5 is primarily through R11 and R10 although when a RESET condition occurs TR5 provides a rapid discharge path via R12.

The resulting d.c. voltage at C5 is applied to the schmitt trigger formed by R13, TR6, TR7, R14, R16 and R15. Fig. 2 shows the action of this device. R13 provides a suitable high-impedance input for the schmitt.

When the voltage on C5 is below the upper threshold level TR6 is OFF. The collector of TR6 is therefore pulled HIGH due to the action of R14. Since the base of TR7 is connected to the collector of TR6 it is forced onto conduction and TR7 is therefore said to be ON.

The voltage at the emitters of TR6 and TR7 in this condition is approximately 4% of the supply voltage and represents the upper threshold level.

When the voltage on C5 exceeds this upper threshold TR6 will turn ON. The collector of TR6 now goes LOW and TR7 is turned OFF.

The voltage at the emitters of TR6 and TR7 now falls rapidly under these conditions and further reinforces the ON state of TR6. This voltage is now 0.5% of supply voltage and represents the lower threshold voltage.

In order to allow the schmitt to return to its original condition, (TR6=OFF; TR7=ON), the voltage on C5 must now fall until it is below the lower threshold voltage. The difference between the upper and lower threshold voltage is termed the hysteresis of the Schmitt Trigger. N.B. In practice, the upper and lower thresholds are higher than indicated due to the volt drop across base resistor R13 and the base-emitter junction of TR6.

When the schmitt trigger output (TR7 collector) is HIGH TR8 is turned ON. TR8 in turn switches TR4 ON (input to NOR gate) and TR9 ON (motor output). D4 and D5 provide a volt drop of approximately 1.2 volt which ensures that TR9 and TR4 are OFF whenever TR7 is ON. This is necessary because TR7 collector never falls completely to 0 volts when ON. This danger is heightened when operating a 24 volt supply.

Finally, D6 provides reversed supply protection to the device and C4 provides supply decoupling.

It will be appreciated that, whilst the invention has been described in the context of a bilge pump and its associated sensing and control equipment, it may also find application in the protection of plant and machinery which may be susceptible to damage if allowed to run dry, such as for example lubrication and cooling systems involving the use of pumps.

The invention can also be used to control the filling of containers instead of emptying them. For example, the pump can be used to pump oil into a road vehicle oil tanker. In this case the sensor is mounted in the top of the tank and the pump controlled to switch the pump OFF when the level of the oil reaches the sensor. This prevents the tanker from being over filled.

Claims (15)

1. A fluid level responsive apparatus comprising an acoustic transmitter and receiver mounted in spaced relationship in an environment arranged to receive a fluid, means for monitoring the output of the receiver when the transmitter is operational, to generate a trigger signal in response to a predetermined change in the acoustic characteristics in said environment in the vicinity of the transmitter and receiver resulting from fluid the occupying the environment in said viscinity being displaced by a different fluid.

2. Apparatus according to Claim 1 wherein said monitoring means is arranged to monitor the acoustic impedence of said environment in said visicinity.

3. Apparatus according to Claim 1 wherein said monitoring means is conditioned to detect changes in the frequency of the acoustic signal transmitted from the transmitter to the receiver.

4. Apparatus according to Claim 1 wherein said monitoring means is conditional to detect changes in the phase of the acoustic signal transmitted from the transmitter to the receiver.

5. Apparatus according to Claim 1 wherein said monitoring means is conditioned to detect changes in the velocity of the acoustic signal transmitted from the transmitter to the receiver.

6. Apparatus according to any preceeding claim wherein said monitoring means comprises an amplifier connected to receive the output of the receiver and providing an output signal to drive the transmitter, the gain of the amplifier being such that the loop gain is less than unity when said first mentioned fluid occupies said space and greater than unity when said different fluid occupies said space.

7. Apparatus according to any one of Claims 1 to 6 including delay means for delaying the generation of said trigger signal until a first predetermined period has elapsed after said predetermined change has occurred, and for sustaining said trigger signal for a second predetermined period after said predetermined change has disappeared.

8. Apparatus according to Claim 7 wherein said first predetermined period is substantially longer than said second predetermined period.

9. Apparatus according to any preceding claim including a fluid pump coupled to said environment and operable to cause the fluid in said environment to be displaced by a different fluid, said pump being deactivated by said monitoring means when the fluid in said viscinity is displaced by said different fluid.

10. Apparatus according to Claim 9 wherein said pump is actuated by the monitoring means when said different fluid in said viscinity is displaced by said first mentioned fluid.

11. Apparatus according to any one of Claims 1 to 8 including a fluid pump coupled to said environment, said fluid pump pumping said different fluid from said environment for as long as said trigger signal is sustained.

12. A liquid level detection system for controlling the level of bilge water in a bilge tank, the system comprising an acoustic transmitter and receiver mounted in spaced relationship to monitor a space at a predetermined level in the bilge tank, a pump for pumping bilge water from the bilge tank, and a control circuit for monitoring the output of the receiver when the tranmitter is energised, said control circuit being responsive to a change in the acoustic characteristic of said space resulting from bilge water entering the space to switch said pump ON and being responsive to the subsequent change in the acoustic characteristics of the space resulting from the bilge water leaving the space to switch said pump OFF.

13. A system according to Claim 12 including a delay circuit for delaying the switching of said pump ON and OFF, the delay in switching said pump ON being substantially longer than the delay in switching said pump OFF.

14. A liquid level detection system for controlling the filling of a tank with a liquid, the system comprising an acoustic transmitter and a receiver mounted in spaced relationship to monitor a space in said tank at a predetermined level to which said tank is required to be filled, a pump for pumping liquid into said tank, and a control circuit for monitoring the output of the receiver when the transmitter is energised, said control circuit being responsive to a change in the acoustic characteristics of said space resulting from the liquid entering said space to inhibit the pumping action of said pump and thereby preventing the liquid entering said tank from exceeding said predetermined level.

15. A liquid level detection system substantially as hereinbefore described with reference to the acompanying drawings.