Method for controlling a magnetic valve and method for dispensing or aspirating a volume of liquid as well as corresponding dispenser/pipetting apparatus

Abstract

A method for controlling a magnetic valve and particularly a method for dispensing and/or aspirating a volume of liquid as well as a corresponding dispenser/pipetting apparatus is disclosed. The method for controlling a magnetic valve has measuring a capacitance at the magnetic valve and determining a position of a plunger based on the measured capacitance. The method for dispensing or aspirating a volume of liquid has controlling a flow of a system fluid by a magnetic valve located between a pressure source and a dispenser/pipetting tip, dispensing or aspirating a volume of liquid through an exterior opening of the tip dependent on the flow of the system fluid, wherein controlling the flow and determining a flow time in dependence of the volume of liquid to be dispensed or aspirated, and controlling the magnetic valve is held open for the duration of the flow time.

Claims

1. A method for controlling a magnetic valve (3), comprising a solenoid coil (13) and a mobile anchor forming a plunger (14), wherein the plunger (14) is moveable between a closed position (P.sub.c) and an open position (P.sub.o), the method comprising the steps of: applying an opening current as a driving current to the solenoid coil (13) to drive the plunger (14) from the closed position (P.sub.c) to the open position (P.sub.o); measuring a capacitance of the magnetic valve (3) by applying a measuring voltage to the solenoid coil (13); determining a position (P) of the plunger (14) based on the capacitance measured of the magnetic valve (3).

2. The method of claim 1, further comprising the step of: holding the plunger (14) at a predetermined position (P) for a predetermined holding time (T.sub.h) by applying a holding current as the driving current to the solenoid coil (13), the holding current having a smaller amplitude than the opening current.

3. The method of claim 1, further comprising the step of: applying a closing current as the driving current to the solenoid coil (13) to drive the plunger (14) from the open position (P.sub.o) to the closed position (P.sub.c), the closing current having an opposite polarity to the opening current.

4. The method of claim 1, wherein the magnetic valve (3) further comprises a stationary anchor coaxially arranged adjacent to the mobile anchor, electrically connected to a metallic housing (17) of the magnetic valve (3), wherein the solenoid coil (13) and the housing (17) are connected to a capacitance measurement unit (11) for measuring the capacitance.

5. The method of claim 1, wherein the measuring voltage is applied as an alternating current signal, a pseudo random noise signal or an exponential function shaped signal.

6. The method of claim 5, wherein the measuring voltage has a different frequency than the driving current.

7. The method of claim 6, wherein the measuring voltage has a different amplitude than a driving voltage associated with the driving current.

8. The method of claim 5, wherein the measuring voltage is superimposed on the driving voltage associated with the driving current.

9. The method of claim 1, wherein the driving current is applied as a pulse width modulated signal having a duty cycle consisting of an active phase and a passive phase, wherein the driving current is zero during the passive phase.

10. The method of claim 9, wherein the measuring voltage is only present during the passive phase of the duty cycle of the pulse width modulated signal.

11. The method of claim 1, further comprising the step of: adjusting the driving current in dependence of the position (P) of the plunger (14).

12. The method of claim 1, further comprising at least one of the following steps: determining an opening time (T.sub.o) of the magnetic valve (3) as a time interval between the plunger (14) leaving the closed position (P.sub.c) and reaching the open position (P.sub.o); determining a closing time (T.sub.c) of the magnetic valve (3) as a time interval between the plunger (14) leaving the open position (P.sub.o) and reaching the closed position (P.sub.c), and further comprising at least one of the following steps: adjusting the driving current and the closing current in dependence of the opening time (T.sub.o) and/or the closing time (T.sub.c); providing a fault indication in dependence of the opening time (T.sub.o) and/or the closing time (T.sub.c).

13. The method according to claim 12, comprising adjusting or calibrating a holding time (T.sub.h) of the magnetic valve (3) in dependence of the determined opening time (T.sub.o) and/or closing time (T.sub.c).

14. The method according to claim 1, comprising monitoring an operation of the magnetic valve (3) based on the position (P) of the plunger (14).

15. A method for dispensing or aspirating a volume of liquid (8) comprising the steps of: applying a pressure from a pressure source (1.sub.+, 1.sub.−) to a system fluid (9); controlling a flow of the system fluid (9) by means of a magnetic valve (3) located between the pressure source (1.sub.+, 1.sub.−) and a tip (6) of a dispenser or pipette; dispensing or aspirating the volume of liquid (8) through an exterior opening (7) of the tip (6) dependent on the flow of the system fluid (9), the system fluid (9) being in fluid communication with the liquid (8) to be dispensed or aspirated, wherein controlling the flow comprises: determining a flow time in dependence of the volume of liquid (8) to be dispensed or aspirated; controlling the magnetic valve (3) by applying an opening current as a driving current to a solenoid coil (13) to drive a plunger (14) from a closed position (P.sub.c) to an open position (P.sub.o), measuring a capacitance of the magnetic valve (3) by applying a measuring voltage to the solenoid coil (13), determining a position (P) of the plunger (14) based on the measured capacitance, and holding the plunger (14) at a predetermined position (P) for a predetermined holding time (T.sub.h) by applying a holding current as the driving current to the solenoid coil (13), the holding current having a smaller amplitude than the opening current, wherein—the predetermined holding time (T.sub.h) is set to the flow time.

16. The method of claim 15, wherein a restrictor (4), such as a capillary, for restricting the flow is interconnected between the pressure source (1.sub.+, 1.sub.−) and the tip (6), the restrictor (4) having a flow resistance which is at least twice that of the exterior opening (7) of the tip (6).

17. A dispenser/pipetting apparatus comprising: a pressure source (1.sub.+, 1.sub.−) adapted to apply a pressure to a system fluid (9), the system fluid (9) being in fluid communication with a liquid (8) to be dispensed or aspirated; a magnetic valve (3); comprising a solenoid coil (13) and a mobile anchor forming a plunger (14), wherein the plunger (14) is moveable between a closed position (P.sub.c) and an open position (P.sub.c); a dispenser or pipetting tip (6) with an exterior opening (7) through which the liquid (8) can be dispensed or aspirated; a capacitance measurement unit (11) adapted to measure a capacitance of the magnetic valve (3); and a control unit (12) adapted to control a flow of the system fluid (9) by means of the magnetic valve (3), which is located between the pressure source (1.sub.+, 1.sub.−) and the tip (6), wherein the control unit (12) is adapted to: determine a flow time in dependence of a volume of liquid (8) to be dispensed or aspirated; and control the magnetic valve (3) by applying an opening current as a driving current to the solenoid coil (13) to drive the plunger (14) from the closed position (P.sub.c) to the open position (P.sub.o), measuring a capacitance of the magnetic valve (3) by applying a measuring voltage to the solenoid coil (13), determining a position (P) of the plunger (14) based on the measured capacitance, and holding the plunger (14) at a predetermined position (P) for a predetermined holding time (T.sub.h) by applying a holding current as the driving current to the solenoid coil (13), the holding current having a smaller amplitude than the opening current, wherein—the predetermined holding time (T.sub.h) is set to the flow time.

18. The apparatus according to claim 17, wherein the magnetic valve (3) further comprises a stationary anchor coaxially arranged adjacent to the mobile anchor, electrically connected to a metallic housing (17) of the magnetic valve (3), wherein the solenoid coil (13) and the housing (17) are connected to the capacitance measurement unit (11) for measuring the capacitance.

19. The apparatus of claim 17, further comprising a restrictor (4) adapted to restrict the flow and interconnected between the pressure source (1.sub.+, 1.sub.−) and the tip (6), the restrictor (4) having a flow resistance which is at least twice that of the exterior opening (7) of the tip (6).

20. An automated liquid handling system comprising the apparatus of claim 17.

21. The method of claim 6, wherein the frequency of the measuring voltage is higher than 1 kHz.

22. The method of claim 6, wherein the frequency of the measuring voltage is higher than 10 kHz.

23. The method of claim 6, wherein the frequency of the measuring voltage is between 100 kHz and 1 MHz.

24. The method of claim 11, wherein driving current is at least one of one of the opening current, the holding current and the closing current, and wherein the amplitude of the driving current is adjusted.

25. The method of claim 11, wherein the driving current is a pulse width modulated signal, and wherein a pulse width of the driving current is adjusted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is further explained below by means of non-limiting specific embodiments and with reference to the accompanying drawings, which show the following:

(2) FIG. 1 a dispenser/pipetting apparatus according to the present invention;

(3) FIG. 2 a schematic representation of an embodiment of a magnetic valve according to the present invention a) in an open position, and b) in a closed position; and

(4) FIG. 3 exemplary graphs of the progression over time of: a) a driving voltage applied to the solenoid coil of a magnetic valve as a PWM signal together with an interleaved measuring voltage, b) a driving voltage applied to the solenoid coil as a PAM signal together with a superimposed measuring voltage, and c) the position of a plunger of a magnetic valve resulting from applying the drive voltage to the solenoid coil.

DETAILED DESCRIPTION OF THE INVENTION

(5) FIG. 1 depicts a dispenser/pipetting apparatus according to the present invention. A positive and a negative pressure source 1.sub.+, 1.sub.− selectable by a pressure switching means 2 apply a pressure to a system fluid 9. Instead of employing a separate positive and a negative pressure source 1.sub.+, 1.sub.− a single pressure source with an adjustable pressure, such as a (plunger) pump, may be used. The system fluid 9 is in fluid communication with a liquid 8 to be dispensed or aspirated via a pipetting/dispenser tip 6, which is typically attached to a pipetting tube 5. In case the system fluid 9 is not a gas, such as air, the liquid 8 is separated from the system fluid 9 by air 10 (or another gas) in order to avoid contact (and therewith e.g. contamination of the liquid 8) with the system fluid 9. In the case of a dispenser the liquid 8 to be dispensed can itself be used as the system fluid 9 (and the interjacent air 10 is not necessary). A magnetic valve 3 is located in the path of the system fluid 9 in order to regulate its flow. Furthermore, a restriction 4, such as a capillary, may be located between the magnetic valve 3 and the tip 6. However, the restriction 4 may also be arranged between the pressure source 1.sub.+, 1.sub.− and the magnetic valve 3, or may even form part of the magnetic valve 3, e.g. it may be integrated into the magnetic valve 3. The purpose of the restrictor 4 is to restrict the flow of the system fluid 9 by exerting a certain desired flow resistance, whereby the flow resistance is typically chosen to be at least double that of the exterior opening 7 of the tip 6.

(6) In case liquid 8 is to be dispensed, a higher pressure than the pressure currently being exerted on the liquid 8 (e.g. above atmospheric pressure) is applied to the system fluid 9, i.e. the positive pressure source 1.sub.+ is selected by the pressure switching means 2. The pressure switching means 2 may comprise two valves, one at the output of the positive pressure source 1.sub.+, which is open when dispensing, and one at the output of the negative pressure source 1.sub.−, which is closed when dispensing. This higher pressure becomes effective on the liquid 8 to be dispensed from the tip 6 as soon as the magnetic valve 3 is opened and stays effective until the magnetic valve 3 is closed again. The amount (i.e. the volume) of liquid 8 which is dispensed is dependent on the level of the pressure being applied and the length of time during which the pressure is applied, i.e. the time during which the magnetic valve 3 is open. If the magnetic valve 3 opens and closes essentially instantaneously (i.e. the opening and closing time are negligible), the holding time T.sub.h during which the magnetic valve 3 is held open determines the volume of liquid 8 which is dispensed if the pressure is maintained at a constant level. Therefore, given the amount of liquid 8 to be dispensed the control unit 12 determines the necessary holding time T.sub.h for dispensing the desired volume of liquid 8, and controls the magnetic valve 3 accordingly, as will be described in more detail below.

(7) In case liquid 8 is to be aspirated, a lower pressure than the pressure currently being exerted on the liquid 8 (e.g. below atmospheric pressure) is applied to the system fluid 9, i.e. the negative pressure source 1.sub.− is selected by the pressure switching means 2, e.g. the valve at the output of the negative pressure source 1.sub.− is opened and the one at the output of the positive pressure source 1.sub.+ is closed. Again, this lower pressure becomes effective on the liquid 8 to be aspirated into the tip 6 as soon as the magnetic valve 3 is opened and stays effective until the magnetic valve 3 is closed again.

(8) As previously indicated the amount/volume of liquid 8 dispensed/aspirated varies based on manufacturing tolerances, aging and mechanical wear of the magnetic valve as well as environmental influences such as temperature and humidity, operating conditions such as pressure, liquid properties and supply voltage. All these influences can lead to changes of the switching behaviour of the magnetic valve 3, e.g. the opening and closing times are increased, and therefore are no longer negligible (or at least take on other values than the initial ones over time). Furthermore, the flow of the system fluid 9 decreases over time, for instance when the magnetic valve 3 is hindered from fully opening, e.g. due to clogging. This will reduce the amount/volume of liquid 8 that is dispensed/aspirated during a fixed holding time T.sub.h determined by the control unit 12 and used to control the magnetic valve 3. Consequently, the holding time T.sub.h should be adjusted (or calibrated) based on an appropriate feedback signal, as will be explained in the following.

(9) FIG. 2 schematically shows a magnetic valve 3 comprising a solenoid coil 13 and a mobile anchor forming a plunger 14. In order to open the magnetic valve 3 an opening current is applied as a driving current to the solenoid coil 13, which drives the plunger 14 from its closed position P.sub.c (shown in FIG. 2 b)) to an open position P.sub.o (shown in FIG. 2 a)). As soon as the magnetic valve 3 is open system fluid 9 may flow through the passage 16 of the magnetic valve 3, thereby transferring the pressure from the positive or negative pressure source 1.sub.+, 1.sub.− to the liquid 8 to be dispensed or aspirated.

(10) Typically, a closing spring 15 is arranged at the mobile anchor to apply a closing force that pushes the plunger 14 back towards the closed position P.sub.c. Therefore, once the magnetic valve 3 has been opened a holding current needs to be applied as a driving current to the solenoid coil 13 to counteract the closing force of the closing spring 15. This holding current is maintained during the holding time T.sub.h determined by the control unit 12 such that the desired amount/volume of liquid 8 is dispensed/aspirated. Once the holding time T.sub.h has passed the magnetic valve 3 is closed again. This can simply be achieved by no longer driving the solenoid coil 13 and letting the closing spring 15 close the magnetic valve 3, or alternatively when no closing spring 15 is employed applying a closing current having the opposite polarity to the opening and holding current as a driving current to the solenoid coil 13 such that the plunger 14 is moved to the closed position P.sub.c again. A closing current can also be employed together with the closing spring 15, for example to decelerate (i.e. apply a breaking action on) the plunger 14 before reaching the closed position P.sub.c in order to avoid strong impacts and thus increase the working lifespan of the magnetic valve 3.

(11) In an open loop control system the driving currents are preset to certain levels for opening, holding open and closing the magnetic valve 3. Likewise, the holding time T.sub.h is preselected depending on the desired amount of liquid 8 to be dispensed (e.g. based on a lookup table).

(12) To allow closed loop control of the magnetic valve 3 a feedback signal must be available. This is made possible by the present invention by measuring a capacitance at/of the magnetic valve 3. The measured capacitance changes depending on the position P of the plunger 14 as indicated by the large capacitor illustrated in FIG. 2 a) when the magnetic valve 3 is open and the mobile anchor is largely retracted within the solenoid coil 13 (plunger 14 in the open position P.sub.o) and the smaller capacitor illustrated in FIG. 2 b) when the magnetic valve 3 is closed and the mobile anchor is less retracted within the solenoid coil 13 (plunger 14 in the closed position P.sub.c).

(13) When the magnetic valve 3 for instance comprises a stationary anchor that is coaxially arranged adjacent to the mobile anchor and electrically connected to a metallic housing 17 of the magnetic valve 3, the solenoid coil 13 and the housing 17 can be connected to a capacitance measurement unit 11 for measuring the capacitance. Measuring the capacitance can be achieved by applying a measuring voltage to the solenoid coil 13 in addition to the driving current, e.g. by superimposing the measuring signal onto the driving signal. Thereby, the measuring signal may be an alternating current signal, a pseudo random noise signal or an exponential function shaped signal. Furthermore, the measuring signal and the driving signal may have different frequencies so that they can be easily separated from one another, e.g. by means of a highpass filter.

(14) To achieve closed loop control of the magnetic valve 3 the applied driving current can be adjusted dependent on the position P of the plunger determined based on the measured capacitance. This allows to achieve targeted opening, holding and closing times T.sub.o, T.sub.h, T.sub.c despite manufacturing tolerances, aging and mechanical wear of the magnetic valve as well as environmental influences such as temperature and humidity, operating conditions such as pressure, liquid properties and supply voltage.

(15) FIG. 3 displays graphs of two different examples for driving the magnetic valve 3. FIG. 3 a) shows a driving voltage u.sub.d applied as a pulse width modulated (PWM) signal having a duty cycle consisting of an active phase (u.sub.d>0 VDC) and a passive phase (u.sub.d=0 VDC). In this example the measuring voltage in the form of a sinusoidal signal is only present during the passive phase of the duty cycle of the PWM signal, i.e. the driving and measuring signals are interleaved. In the alternative example shown in FIG. 3 b) the driving voltage u.sub.d is applied as a pulse amplitude modulated (PAM) signal. In this example the measuring voltage in the form of a sinusoidal signal is superimposed on the PAM driving signal and continuously present. The graph depicted in FIG. 3 c) shows the position P of the plunger 14 over time dependent on the driving voltage u.sub.d. During the opening time T.sub.o the opening voltage is applied to the solenoid coil 13 (e.g. for two duty cycles of the PWM signal in FIG. 3 a)) to drive the plunger 14 from the closed position P.sub.c to the open position P.sub.o. Subsequently, the plunger 14 is held in the open position P.sub.o by applying the holding voltage to the solenoid coil 13 during the holding time T.sub.h (e.g. for four duty cycles of the PWM signal in FIG. 3 a)). Finally, the plunger 14 is forced back to the closed position P.sub.c, in this case by setting the closing voltage to 0 V during the closing time T.sub.c (e.g. for two duty cycles of the PWM signal in FIG. 3 a)) and leaving it to the closing force of the closing spring 15 to retract the plunger 14. The desired amount of liquid 8 to be dispensed or aspirated can then be precisely controlled by the control unit 12 by appropriately adjusting the driving voltage u.sub.d dependent on the position of the plunger 14 determined based on the capacitance measured at the magnetic valve 3.

LIST OF REFERENCE SYMBOLS

(16) 1.sub.+ positive pressure source 1.sub.− negative pressure source 2 pressure switching means 3 magnetic valve 4 restriction, e.g. capillary 5 pipetting tube 6 dispenser/pipetting tip 7 tip opening 8 liquid 9 system fluid 10 air 11 capacitance measurement unit 12 control unit 13 solenoid coil 14 plunger (mobile anchor) 15 closing spring 16 passage 17 housing P.sub.c position of the plunger P.sub.o closed position of the plunger t time T.sub.c closing time T.sub.h holding time T.sub.o opening time u.sub.d driving voltage