GB2295254A - Method of controlling the emptying of a vessel - Google Patents

Abstract

The method comprises the steps of (i) comparing a first sensor signal to a reference value to indicate whether or not there is liquid at or proximate a minimum level (L1) in a vessel (10), (ii) comparing a second sensor signal from a sensor at a maximum level (L2) with the first sensor signal to indicate that liquid is at or proximate both levels, and where those steps indicate that liquid is at or proximate both levels then removing liquid from the vessel until there is an indication that the liquid is at or proximate the minimum level. <IMAGE>

Description

METHOD OF SENSING LIQUID LEVEL The present invention relates to a method of sensing liquid level, particularly, but not exclusively a method of sensing the level of condensate in the sump of a compressor fed pneumatic system using a capacitive effect.

In a compressor fed pneumatic system, for example a pneumatic drill, the process of compression of the air causes some components of the air used to condense out as liquid droplets. In particular, water and oil vapour, both of which may exist in the compressed air, may condense within the system. A sump is usually provided for collection of such condensate. When the sump is full, the contents thereof must be emptied and one method of emptying involves the use of a manually operated tap.

The tap may be key operated to restrict access thereto to authorised operators. However, such a method is not satisfactory because an operator cannot ascertain accurately the condensate level within the sump. As a result, the sump may either overfill, which may cause damage to the pneumatic system, or the operator may clear the sump too frequently which is inefficient. In each emptying, some compressed air is likely to escape with the condensate, which is potentially dangerous to an operator. The need for a system for detecting liquid level within the sump has therefore been recognised.

One system for detecting the liquid level within the sump has been proposed which makes use of the dielectric properties of the condensate. At least the side walls of the sump are constructed of electrically conductive material. An upright liquid-impermeable closed tube of an insulating material is arranged within the sump. Two capacitor plates are arranged within the tube, one at a desired minimum level of condensate within the sump, and the other at a desired maximum level of condensate within the sump. The system operates by measuring the capacitance between the sidewall and the respective plates.

Sensing the presence of liquid adjacent the upper plate will cause opening of a solenoid operated drain valve, which will allow condensate to flow from the sump.

Sensing of the presence of the liquid adjacent the lower plate will cause the solenoid operated valve to close.

It can be seen that the upper plate/sidewall capacitor is "normally dry" and the lower plate/sidewall capacitor is "normally wet". As a result, the system senses the presence of the liquid adjacent the upper plate when the capacitance between the upper plate and the sidewall increases by a certain amount; and the system senses the liquid level at the lower plate when the capacitance between the lower plate and sidewall decreases by a certain amount.

A particular difficulty with using a capacitive sensor of this type in such a system arises from the fact that the composition of the condensate in such a pneumatic system can range from pure water to pure mineral oil, including all intermediate emulsions. Relative to air, such an oil has a dielectric constant of approximately 2, and water has a dielectric constant of approximately 80. Hence, it can be seen that the dielectric constant of the condensate in such a system will vary greatly depending on the composition of the condensate.

If the capacitive sensors are sensitised sufficiently to detect a change in capacitance appropriate to the presence of oil, then if water were actually present in the sump, the sensors would register the appearance of the liquid level rather too early, thus the valve would open when the sump was only partially full, and close when the sump was partially empty. The amount of condensate discharged by such a system diminishes and the solenoid valve opens and closes very frequently, reducing the life thereof.

Various attempts to solve the problem set out above have been proposed. The sensors may be replaced by a float switch. However, that includes mechanical components which are less reliable than the electrical components used in the capacitive sensors. Alternatively the diminished discharge period may be prolonged by a timer circuit. However, that may cause the sump to exhaust completely of compensate, which allows compressed air to vent through the solenoid valve which can be hazardous.

EP-A-0081826 describes a system which measures the presence of liquid at two points in a vessel prior to emptying of the vessel. The system then establishes a suitable time period for emptying the vessel without air loss through the discharge valve. Should the pressure change or the condensate production diminish, the time established by the device may be incorrect and air spillage can occur.

An object of the present invention is to provide a method of sensing liquid level independently of the dielectric constant of the liquid.

According to one aspect of the invention there is provided a method of sensing the level of a liquid in a liquid collection vessel between a minimum level and a maximum level, the method comprising the steps of:providing a first sensor associated with the minimum level, comparing a first sensor signal to a reference value to indicate whether or not there is liquid at or proximate the minimum level, providing a second sensor associated with the maximum level and, where there is an indication that there is liquid at or proximate the minimum level, comparing a second sensor signal to the first sensor signal to indicate that liquid is at or proximate both levels and where the comparison between the first and second signals indicates liquid at or proximate both levels, removing liquid from the collection vessel until there is an indication that the liquid is no longer at or proximate the minimum level.

According to another aspect of the invention there is provided apparatus for sensing the level of a liquid in a liquid collection vessel between a minimum level and a maximum level comprising:a first sensor associated with the minimum level, means operative to compare a first sensor signal to a reference value to indicate whether or not there is liquid at or proximate the minimum level, a second sensor associated with the maximum level and means operative to compare a second sensor signal with said first sensor signal to indicate that liquid is at or proximate both levels and means for removing liquid from the collection vessel operative when liquid is at or proximate both levels until there is an indication that liquid is no longer at or proximate the minimum level.

Preferably, each of the first and second signals is dependent on the dielectric constant of fluid within said vessel at or proximate said minimum level and maximum level respectively. In that way, the sensors can distinguish between the case where the fluid is gas, having one dielectric constant, and the case where the fluid is liquid, having another dielectric constant.

Preferably at least said first sensor and preferably also the second sensor are capacitive sensors. Where the first sensor is capacitive, the first sensor may comprise two plates, one of which may comprise a wall of the vessel. Where the second sensor is capacitive, the second sensor may comprise two plates, one of which may comprise a wall of the vessel. Preferably, the wall of the vessel is grounded.

Preferably, the or each capacitive sensor comprises signal generating means operative to provide a current to the plates. In that case, the sensor may compare the rate of change of voltage across the plates to which the current is provided.

Preferably, the plates of the first sensor are parallel.

Where the second sensor is capacitive, the plates of the second sensor may be parallel.

Preferably said first and second sensors are substantially identical.

The first and second signals are preferably related to the capacitance of the first and second capacitive sensors respectively.

The indication that there is liquid at or proximate the minimum level preferably occurs when said first sensor signal is equal to the reference value.

The indication that there is liquid at or proximate both levels preferably occurs when said second sensor signal and the first sensor signal have the same value.

The step of removing the liquid preferably includes operating drainage means to allow the liquid to drain from the vessel. In such a case, the drainage means is preferably a solenoid operated valve which may be arranged in a base of the vessel.

An embodiment of the invention will now be described by way of example with reference to the drawings in which: Fig 1 shows a schematic diagram of a system operative in accordance with the invention and Fig 2 shows a circuit diagram of comparators and a latch as illustrated in Fig 1.

A sump 10 is provided in a pneumatic system 12 for collection of condensate therefrom. The sump 10 has a substantially cylindrical sidewall 14 and a circular base 16. The sidewall 14 is constructed of a conductive metallic material, and is connected to ground 18.

Coaxially within the sump 10 there is provided an inner tube 20.

The inner tube 20 is sealed to prevent ingress of condensate from the sump 10 to the interior of the inner tube. The inner tube 20 is constructed from an electrically insulating and liquid impermeable material.

The sump 10 includes a drainage outlet 22 in the base 16, the drainage outlet 22 being openable by a solenoidoperated valve 24.

Two substantially identical lower and upper capacitor plates 26, 28 respectively are provided within the inner tube 20. The lower plate 26 is arranged at a point on the longitudinal axis of the inner tube 20 substantially at a level L1 of the sump 10, which is the minimum level of condensate permitted within the sump 10. The other, upper, plate 28 is arranged at a point on the longitudinal axis of the inner tube 20 substantially at a level L2 of the sump, which is the maximum level of condensate permitted within the sump 10.

Each plate 26, 18 is constructed of a conductive metallic material, and is arranged to form a capacitance between the plate and the sidewall 14. It will be appreciated that if the level of condensate within the sump is adjacent the plate, the condensate will act as a dielectric.

A first comparator 30 is provided, operative to compare capacitances. The comparator 30 receives two inputs, one from the lower plate 26 and the other from a reference capacitor 32. The reference capacitor 32 is selected to have a higher capacitance than the capacitance between the lower plate 26 and the sidewall 14 with no condensate therebetween, and a higher capacitance than the capacitance between upper plate 28 and the sidewall with no condensate therebetween.

A second comparator 34 is provided, operative to compare capacitances. The comparator 34 receives two inputs, one from the upper plate 28 and the other from the lower plate 26. The comparators 30, 34 supply signals dependent upon the result of the comparison.

A latch 36 is provided which receives the comparator signals. The latch 36 controls the solenoid valve 24.

The solenoid valve 24 is normally closed. The latch 36 opens the solenoid valve 24 when the lower comparator 30 indicates that the capacitance between the lower plate 26 and the sidewall 14 is equal to the capacitance of the reference capacitor 32 and when the upper comparator 34 indicates that the capacitance between the upper plate 28 and the sidewall 14 is equal to the capacitance between lower plate 26 and the sidewall 14. The latch 36 closes the solenoid valve when the lower comparator 30 indicates that the capacitance between the lower plate 26 and the sidewall 14 is less than the capacitance of the reference capacitor 32. In this way, the sump 10 may fill with condensate until the upper comparator 34 indicates that a dielectric material (ie. condensate) has been interposed between both the upper plate 28 and the sidewall 14, and the lower plate 26 and the sidewall 14.

At this point, the solenoid valve 24 is opened, condensate being allowed to drain out of the sump 10 until the lower comparator 30 indicates that dielectric material (ie. condensate) has been removed from between the lower plate 26 and the side wall 14.

As a further improvement, the latch 36 could be operative to close the solenoid valve 24 only when the upper comparator 34 indicates that the capacitance between the upper plate 28 and the sidewall 14 is equal to the capacitance between the lower plate 26 and the sidewall 14. Such a step would act in the same way as the above described steps, in that the solenoid valve is closed dependent on a comparison between two capacitive sensors which experience the same variations in the dielectric constant of their dielectric material.

Figure 2 illustrates the comparators 30,34 and the latch 36 in more detail. The first comparator 30 comprises a first D-type flip-flop 40. Terminals D and CK of the first flip-flop 40 are held at ground 41 which may comprise, for example, a casing for the system. A first potentiometer 42 comprises first and second terminals 44,46 and a brush 48. The first terminal 44 is connected to an input S of the first flip-flop 40 and the second terminal 46 is connected to an input R of the first flipflop 40. An astable multivibrator 50 supplies a square wave signal to the brush 48 of the potentiometer 42.

The lower plate 26 is connected to the S input and the reference capacitor 32 is connected to the R input of the flip-flop 40. In that way, the resistance of the portion of the first potentiometer 42 between the brush 48 and the first terminal 44, and the capacitance of the lower plate 26, form an RC network through which the square wave is applied to the S input of the flip-flop 40.

Similarly, the resistance of the portion of the first potentiometer 42 between the brush 48 and the second terminal 46, and the capacitance of the reference capacitor 32, form an RC network through which the square wave is applied to the R input. The time constant associated with each RC network results in the square wave input being modified by the time it reaches the S and R inputs. In each case, the rising and falling edges of the square wave are somewhat ramped. The higher the capacitance of the lower plate 26, the greater the degree of ramping. The position of the brush 48 relative to the first and second terminals 44,46 of the first potentiometer 42 may be adjusted to vary the relative values of the time constants of the two RC networks.

If the RC networks have a different time constants, the square waves will be ramped to differing degrees.

Accordingly, the signals will cross the threshold value for activating the first flip-flop 40 at different times.

If the time constant associated with the S input is lower than the time constant associated with the R input, then the signal at the S input will lead the signal at the R input.

In the present embodiment, a "high" logic signal at the S input will cause a "high" output an output terminal Q of the first flip-flop 40. A "high" logic signal at the R input will cause a "low" output at the output terminal Q. Those are the well known results of S and R inputs of flip-flops. However, under normal operation of the S and R inputs, while one of the inputs is "high", the other should be "low". In the present arrangement it is clear that the square wave is applied to both inputs in phase, disregarding the delays introduced by the time constants.

Accordingly, for part of the cycle of the square wave, both the S input and the R input will be at a "high" level. Although those input conditions are considered normally to produce an ambiguous output, in fact under those conditions, the output at Q is "high". Although not illustrated, the first flip-flop 40 also comprises an inverted output Q. In the particular case where both S and R are high, the inverted output Q is also high.

When the falling edge of the square wave is applied, one or other of the S and R inputs will experience the falling edge last, because of the unequal time constants of the RC networks at the S and R inputs. In a first case, the time constant at the S input is shorter than that at the R input and so the signal at the S input falls to zero more quickly than that at the R input. The transient "high" signal at the R input is registered by the first flip-flop 40 and causes the output Q to fall to a "low" signal.

Accordingly, when the time constant at the S input is shorter than that at the R input, the first comparator 30 outputs a square wave signal.

In the second case where the time constant at the R input is shorter than that at the S input, the S input will experience the falling edge of the square wave from the multivibrator 50 after the R input. In that case, the output Q will remain "high". Accordingly, the output Q is at a constant "high" level under the second set of circumstances.

The time constant at the S input varies with the capacitance of the lower plate 26. Thus, the first comparator 30 may be calibrated to set a threshold for the comparison of the capacitance of the lower plate 26 relative the reference capacitor 32. As the capacitance of the lower plate 26 rises, for instance through the proximity of a dielectric such as water or oil, the time constant at the S input becomes longer until it is longer than the time constant at the R input. The output Q consequently achieves a constant "high" level.

The second comparator 34 is constructed similarly to the first comparator 30. The second comparator 34 comprises a second D type flip-flop 60. The D and CK terminals of the second flip-flop 60 are held at ground 61. A second potentiometer 62 comprises first and second terminals 64,66 and a brush 68. The first terminal 64 is connected to the S input of the second flip-flop 60 and the second terminal 66 is connected to the R input of the second flip-slop 60. The square wave signal of the astable multivibrator 50 is supplied to the brush 68 of the potentiometer 62. The upper plate 28 is connected to the S input and the lower plate 26 is connected to the P input. As described in relation to the first comparator 30, RC networks are consequently set up at the S and R inputs of the second flip flop 60.

The output from the second comparator 34 depends on the capacitances at the lower and upper plates 26, 28.

Varying capacitance causes switching between the two output states (oscillatory and steady state) as described in relation to the first comparator 30.

The output Q from each of the first and second comparators 30, 34 are fed to the latch 36 which is operative as described above.

The method of maintaining the level of condensate within two limits overcomes the problem of the varying dielectric constant of the condensate, by comparing the capacitances at the respective levels, thus cancelling the effect of any variation in dielectric constant.

The method and apparatus is particularly advantageous in that the quantity of liquid drained from the sump 10 on opening of the solenoid valve 24 does not vary substantially even though in one case the liquid or the major portion of the liquid may be water and in another case may be oil.

It is important to ensure that the drainage outlet 22 remains charged with liquid to prevent the escape of compressed air through the valve 24 during drainage of the sump 10. Therefore, the reference capacitor 32 is calibrated such that the valve 24 will always close in time to leave the drainage outlet 22 charged with liquid whatever the dielectric constant of the liquid concerned.

Claims (21)

1. A method of sensing the level of a liquid in a liquid collection vessel between a minimum level and a maximum level, said method comprising the steps of providing a first sensor associated with the minimum level, comparing a first sensor signal to a reference value to indicate whether or not there is liquid at or proximate the minimum level, providing a second sensor associated with the maximum level and, where there is an indication that there is liquid at or proximate the minimum level, comparing a second sensor signal to the first sensor signal to indicate that liquid is at or proximate both levels and where the first and second signals comparison indicates liquids at or proximate both levels, removing liquid from the collection vessel until there is an indication that the liquid is no longer at or proximate the minimum level.

2. A method according to Claim 1 wherein the first and second signals are related to the capacitance of the first and second capacitance sensors respectively.

3. A method according to Claim 1 or 2 wherein the indication that there is liquid at or proximate the minimum level occurs when said first sensor signal is equal to the reference value.

4. A method according to any preceding claim wherein the indication that there is liquid at or proximate both levels occurs when said second sensor signal is equal to the first sensor signal.

5. A method according to any preceding claim wherein the step of removing the liquid includes operating drainage means to allow the liquid to drain from the vessel.

6. A method according to Claim 5 wherein the drainage means is a solenoid operated valve.

7. Apparatus for sensing the level of a liquid in a liquid collection vessel between a minimum level and a maximum level comprising a first sensor associated with the minimum level, means operative to compare a first sensor signal to a reference value to indicate whether or not there is liquid at or proximate the minimum level, a second sensor associated with the maximum level and means operative to compare a second sensor signal with said first sensor signal to indicate that liquid is at or proximate both levels and means for permitting removal of liquid from the collection vessel operative when liquid is at or proximate both levels until there is an indication that liquid is no longer at or proximate the minimum level.

8. Apparatus according to Claim 7 wherein each the first and second signals is dependent on the dielectric constant of fluid within said vessel at or proximate said minimum level and maximum level respectively.

9. Apparatus according to Claim 7 or 8 wherein said first sensor is a capacitive sensor.

10. Apparatus according to Claim 9 wherein the first sensor comprises two plates.

11. Apparatus according to Claim 10 where one of said plates of said first sensor comprises a wall of the vessel.

12. Apparatus according to Claim 10 or 11 wherein the plates of the first sensor are parallel.

13. Apparatus according to any one of Claims 9 to 12 wherein said second sensor is a capacitive sensor.

14. Apparatus according to Claim 13 wherein the second sensor comprise two plates.

15. Apparatus according to Claim 14 wherein one of said plate of said second sensor comprises a wall of the vessel.

16. Apparatus according to Claim 14 or 15 wherein the plates of the second sensor are parallel.

17. Apparatus according to any one of Claims 13 to 16 wherein the first and second signals are related to the capacitance of the first and second capacitive sensors respectively.

18. Apparatus according to any one of Claims 7 to 17 wherein the wall of the vessel is grounded.

19. Apparatus according to any one of Claims 7 to 18 wherein said first and second sensors are substantially identical.

20. A method of sensing the level of a liquid in a liquid collection vessel substantially as described herein with reference to the accompanying drawings.

21. Apparatus for sensing the level of a liquid in a liquid collection vessel substantially as described herein with reference to the accompanying drawings.