Double-acting membrane pump

Abstract: Double-acting membrane pump. The pump comprises two membranes coupled to one another, pumping being carried out alternately in a first pump chamber and in a second pump chamber. With this arrangement the control of the movement of the membranes is carried out in such a way that it is possible not only to control the frequency and the stroke but that, in addition, there is the possibility of incorporating a pause of controllable duration in the rest position of the membrane pump.

Description: The present invention relates to a double-acting membrane pump according to the preamble of claim 1. A double-acting membrane pump of this type is disclosed in both European Application 0 386 610 and US Patent 4 818 191. Pumps of this type are used, inter alia, for filling packs and other containers of all manner of types. One application is the filling of paint cans. The pump disclosed in the general prior art is provided with a control device which, like the pump, operates pneumatically. This control device comprises a pneumatic reciprocating piston which alternately brings the plenum chamber behind the first membrane and the plenum chamber behind the second membrane under pressure. Change-over from the one position to the other position of the control piston is determined by the relevant membranes coupled to one another reaching the extreme position. With this device it is, basically, not possible to restrict the stroke length of the membranes. That is to say, problems arise with the metering accuracy in the final stage of filling packs, for example on a weight basis. This can be avoided by constructing the pump with a relatively small capacity or by connecting various pumps in parallel, but it will be understood that none of these solutions are practicable. The reciprocating piston of the device according to the general prior art moves to and fro with a specific frequency, which is dependent on the medium pumped and, to a lesser extent, on the pressure of the compressed air. This frequency cannot be influenced during the filling cycle. After all, changing the air pressure supplied gives rise to the risk that the pump comes to a standstill. This too is a disadvantage because, when filling containers to a specific fill weight or volume, in the first instance material can be supplied at high speed whereas at a later stage it is necessary to work at a lower speed. This can be avoided by incorporating an air speed control upstream. With this arrangement the pump still has to execute complete strokes, which leads to the said poor meterability. It is proposed to install a metering valve downstream of the double-acting membrane pump. Although a construction of this type is, as such, able to realise the aims described above, it is clear that the costs increase appreciably as a result and that further installation space is necessary, which is a disadvantage in certain applications. In the construction described in US Patent 4 818 191 an alternating valve is present, which can be actuated electrically and with which pressure can be applied alternately to the one and then to the other membrane. As a result of the electrical control, accurate control of the amplitude and frequency of the membrane movement can be obtained. However, control problems arise when metering highly fluctuating amounts per unit time. The movement executed by the membrane will, after all, follow a sinusoidal path and when dispensing very small amounts it is particularly difficult to obtain a sufficiently accurate controlled outward movement of the membranes in order to realise the desired metering accuracy. A solution to this problem has been found by the provision of a valve or shut-off device which has various passages. That is to say, a pre-setting of the valve is achieved, by which means the metering accuracy can be set. Although a construction of this type is reasonably adequate, such a construction has the disadvantage that the price of such valves is particularly high. WO 92/19867A discloses a membrane pump, comprising two halves which are not connected to each other, i.e. this pump is not double-acting. FR 2518660 A shows a double-acting membrane pump being controlled by a control valve having two discrete positions. The aim of the present invention is to provide a metering system with which it is possible to meter highly fluctuating amounts of fluid without this resulting in an excessive rise in the cost price of the double-acting membrane pump and the control thereof. This aim is achieved with a membrane pump as described above having the characterising features of claim 1 and a method according to claim 5. The insight on which the invention is based is not to allow the one stroke of the one membrane to follow directly on the preceding stroke of the other membrane. A controllable pause can be inserted using the valve according to the invention. As a result, it is also possible to allow only one membrane to carry out work. With this construction the sinusoidal path described above is no longer necessary for metering very small amounts. It is possible to give a number of 'square wave pulses'. It will be understood that a very small amount of material can still be metered with a readily controllable relatively large amplitude of the membrane. Moreover, it will be understood that bringing the movement to rest can be carried out relatively simply. This is, for example, possible by fitting two control valves, one control valve always serving to provide the connection between the pressurised fluid and the pressure side of the membrane, whilst the valve has an on/off position. If both valves are controlled such that they are in the off position, there will be no pressure acting on any side of the membrane whatsoever. As a result, the membrane pump is in the rest position. A control of this type can be achieved relatively simply using the microprocessors currently available. For further equalising of the pressure it is possible to install a buffer downstream of the membrane pump outlet for the fluid to be pumped. It is likewise possible to use a three-position valve which can be set to a fixed position in each of the three positions. The first position is then, for example, the control for the one membrane, the second position the control for the second membrane and the third position a position in which the pressure on both membranes is relieved. The various features must be differentiated from the construction which is shown in the abovementioned European Patent Application 0 386 610. In the latter construction the position in which both connections to the membrane pump between the working fluid and the source of the working fluid are interrupted is not controllable and constitutes solely an uncontrollable fractional part of the alternating stroke of the alternating valve concerned. With the device according to the invention it is, moreover, possible to control and to vary the stroke length very accurately. It is also possible to determine the frequency independently of the progression in the movement of the membranes. The "flow" can be controlled by varying the frequency. As the membrane pump according to the invention is, moreover, of double-acting construction, a continuous, non-pulsating, output flow of fluid can be obtained, the volume of which can be accurately controlled. As a result, a container can, for example, be filled accurately on the basis of weight or fill height. This control valves preferably have a throttle function. To this end, the control valves can be of rapidly pulsing design. By, for example, actuating a membrane for a specific time, such a membrane executes a stroke to one side. If the relevant air pulse is sufficiently large, the membrane will execute the complete stroke. In the case of a smaller air pulse, the membrane will execute a short stroke. It is possible to reverse the direction of movement without the end position having to be reached. The delivery of the pump can be controlled by controlling the duration of the air pulses and the time at which these are applied. With short air pulses, the membranes of the pump will execute only short strokes, as a result of which a low flow can be realised. As a result of the double character of the pump, a pulsing action of this type is not reflected in the metered amount of fluid. In contrast to the "sliding valve" used in the general prior art, reversal of the stroke movement, after which the other pump chamber becomes effective for pressurising fluids, does not automatically take place after the end position is reached. With the installation according to the invention it is possible to insert a "pause". In this way a very accurate final metering for filling containers of all manner of types can be achieved. The invention will be explained in more detail below with reference to an illustrative embodiment shown in the drawing. In the drawing the single figure shows a conventional membrane pump provided with modifications, and a controller coupled thereto, according to the invention. The pump shown in the single figure is indicated in its entirety by 1. This pump is controlled by a controller indicated in its entirety by 2. Pump 1 consists of a first pump chamber 3 and a second pump chamber 4. These chambers are separated by, respectively, a first membrane 5 and a second membrane 6 from a first plenum chamber 11 and a second plenum chamber 12. The membranes are connected to one another via a coupling rod 13. Plenum chambers 11 and 12 are provided with openings 7. In this way plenum chamber 11 is connected to line 14 and plenum chamber 12 to line 15. Line 14 is connected to the first control valve 8, whilst line 15 is connected to second control valve 9. These lines are supplied with air via a line 16. Valves 8 and 9 are controlled by a microprocessor 17. The latter is provided with an input 18 which can be connected to a further microprocessor, which, for example, can be controlled by a weighing unit. The common inlet of the membrane pump is indicated by 19 and the common outlet by 20. Ball valves 21 are present. A membrane pump of this type is generally known in the prior art. For example, the pumps are marketed by Wilden, model M.025 and model M1. The apparatus described above functions as follows. Depending on the signal on line 18, microprocessor 17 controls control valves 8 and 9 in a pulsing manner with a specific frequency. With this arrangement it is possible that a long pulse is emitted so that the membrane will travel over a relatively long path before the function of the valves 8 and 9 is reversed so that the working direction of the membranes is reversed. When a short pulse is given, the stroke of the membrane concerned will likewise be shorter. It is also possible to supply a series of pulses to a membrane. In this way, throttle device 10 can be (partially) replaced. It will be understood that in this way in the first instance a large volume of fluid can be pumped, whilst at the end of the pumping cycle the speed of the double-acting membrane pump can be appreciably reduced without this resulting in a pulsating fluid flow. An example of actuating the valves will be given below: At 100 % pump speed, control valve 8 opens for 250 milliseconds. Valve 9 then opens for 250 milliseconds, whilst air bleeds from valve 8, etc. At 50 % actuation, valve 8 opens for 125 milliseconds and is vented for 125 milliseconds without valve 9 being actuated (pause), after which valve 9 is opened for 125 milliseconds and is then vented, etc. At 20 % actuation, valve 8 opens for 50 milliseconds and is vented for 200 milliseconds without valve 9 being actuated (pause), after which valve 9 is opened in the same way. In this way very regular discharge behaviour is achieved without a large number of pulses in the pump. Although the invention has been described above with reference to a preferred embodiment, it must be understood that numerous modifications can be made thereto without going beyond the scope of the present application. For instance, it is possible to use a single control valve instead of the two control valves shown here, which will result in further savings in cost. In that case, said single valve is constructed as an alternating valve (optionally with three positions). This and other alternative embodiments obvious to a person skilled in the art are considered to fall within the scope of the present claims.

Claims: Double-acting membrane pump (1) comprising a first pump chamber (3) in which a first membrane (5) is arranged and a second pump chamber (4) in which a second membrane (6), coupled to the first membrane, is arranged, each pump chamber being provided with a connection (7) for working fluid and a control device being coupled to each of said connections, for the metered supply of working fluid, which control device comprises at least one electrically controlled pulsing control valve (8, 9) for applying or relieving the pressure on one of the connections to the relevant pump chamber, characterised in that said at least one control valve is constructed such that said at least one valve has a position in which the pressure on both connections to the pump chambers is relieved at the same time in a manner controllable as a function of time by the control device. Membrane pump according to Claim 1, containing two on/off control valves. Membrane pump according to Claim 1, containing a three-way valve, wherein one position is a position in which the pressure is relieved on both connections. Membrane pump according to one of the preceding claims, characterised in that the control device comprises a microprocessor. Method for metering the amount dispensed by a double-sided pneumatically driven membrane pump, comprising control of the frequency and the stroke of each of the membranes, characterised in that the duration of the 0 position of the membranes is controlled.

Inventors: PIERIK JOHANNES, PIERIK, JOHANNES
Applicants: WIJK ENGINEERING B V VAN, VAN WIJK ENGINEERING B.V.
Application year: 1995
Main classification: F04B43
IPC codes: F04B43