An Improved Pump

Field

The present disclosure is directed towards an improved pump. The improved pump disclosed herein has been found particularly suitable for applications in the field of liquid chromatography.

Background

Over the past decade there has been an enormous amount of work done in the area of microfluidic systems and miniaturised platforms for use in deployable analytical instrumentation. A growing area of interest is the application of such platforms to portable high-performance liquid chromatography (HPLC) and ion chromatography (IC) systems. For example, HPLC and IC systems have many applications, for example: environmental applications (such as the rapid large scale chemico-nutrient monitoring of river systems), forensics, agricultural (such as improved milk quality and content assessments), etc. . The columns in HPLC systems have high peak efficiencies and give good performance, however, they require pumping systems that can equally operate within a high performance envelop, particularly with regard to pressure and flow rate stability.

Many of the peripheral devices that constitute such an analytical system (detectors, injectors, analytical columns, etc.) have been successfully miniaturised. However, the task of manufacturing a micro pumping system capable of pumping volumes in the pL - mL range at high back pressures has, to date, presented a formidable technical barrier to the development of suitable portable instrumentation.

The most common type of pump used in liquid chromatography is a cam driven pump. However most commercially available cam driven pumping systems operate using special cam profiles to give the required piston profile. This means that the pump is often excessively large. A cam driven system also requires the use of a very powerful drive motor, further increasing the size, weight, and electrical power requirements of the pumping system.

Other pump types, such as peristaltic pumps, and other piston pumps, are known. However, they are not very suitable in providing the performance and accuracy (flow rate stability, high pressure operation, etc) required for use in liquid chromatography. In particular, it has not proved possible to obtain stable flow rates under the high back pressures which are exhibited by modern commercially available analytical columns required for chromatography.

A further example is an axial piston pump. Several micro-scale axial piston pumps currently exist. These pumps are capable of providing flow rates in the required range, meeting the pressure demands, and taking up a small footprint area.

One type of axial piston is a swashplate driven axial piston pump. Many references and published papers directed to swashplate driven axial piston pumps exist in the literature. For example, “Fatigue Life Improvement Of The Roller Swashplate Bearing Of An Axial Swashplate Type Piston Pump” by Du, Hongliu et al; “Calculating Method for the Leakage Between Slipper and Swashplate in Spherical Swashplate Type Axial Piston Pump with Conical Cylinder” by Chen, Yong qin et al; “The Slipping Shoe Motion Trajectory Analysis and Simulation of Spherical Swashplate Type Axial Piston Pump” by Chen Yong-qin et al; “A Kinematic Analysis on Piston Rod Mechanism is Swashplate Type Hydraulic Axial Piston Motor/Pump Using Constant Velocity Joint” by Kim, Sung-Dong et al; “Predicting The Behaviour Of Slipper Pads In Swashplate-Type Axial Piston Pumps” by Harris, RM et al; “Study On Vibrational-Energy Transmission Characteristics From Cylinder To Swashplate Within An Axial Piston Pump” by Qiu, Zl et al; “Dynamic Analysis Of An Axial Piston Pump Swashplate Control” by Zeiger, G et al; and “Torque On The Swashplate Of An Axial Piston Pump” by Zeiger, G et al. Example patent publications which disclose swashplate driven axial piston pumps include WO2015/15140033 and DE102013200718.

Axial piston pumps have not proved to be an ideal fit for HPLC and IC systems since the operation of such pumps demands the use of many pistons in order to provide a stable, pulseless, flow. As such, pump dead volume is excessive as a result of the number of cylinders used.

EP0309596 discloses a pumping apparatus for delivering liquid at a high pressure and at a selectable flow rate. The pumping apparatus includes the pistons coupled to ball-screw drives which translate the rotary motion of the spindles into a linear motion of the pistons. EP3091 29 discloses a metering pump joined to the stepping motor through an eccentric mechanism for converting a revolving motion of the stepping motor into a reciprocating motion of a plunger, and makes a constant liquid delivery. JP 2001263253 discloses a non-pulsative pump that include three plungers driven by a motor via a slider crank mechanism. The angular velocity of the motor is controlled by a motor driving device so that the total delivery quantity of the three plungers becomes constant. US 6227807 discloses a single fluid pump means having a single linear moving piston actuated by a rod and a crankshaft within a single cylinder for pumping a fluid through said single cylinder. WO 2004/104418 discloses a method for controlling a diaphragm or piston pump that is actuated by a cam driven by an electric motor and via a ram or connecting rod, for enabling quantities of metered media to be delivered at the most constantly rate possible.

However, none of the above mentioned cited documents disclose pumps that are suitable for High Performance Liquid Chromatography (HPLC). They do not address the challenges required in such systems, such as very stable flow, very high pressures, small footprint etc). In particular, none of the above mentioned pumps are able to obtain stable flow rates under the high back pressures which are exhibited by modern commercially available analytical columns required for chromatography. Although, each of the above discloses maintaining a constant flow rate in the pump, but none of them discloses a mechanism to negate the variations in flow rate in real-time.

The larger benchtop based pumps can pump at high pressures with small flow rate or pressure fluctuation. However, these types of systems are usually based around a cam design, requiring powerful motors to operate, they take up a large footprint and are heavy. Hence, there is a need for a pump that is compact with a small footprint, and that can operate at high pressures (>100bar) and have excellent flow rate stability with very low %RSD of pressure at constant flow rates.

The present disclosure is directed towards providing a pump which overcomes the problems with prior art pumps set out above.

Summary

The present invention, as set out in the appended claims, is directed towards a reciprocating pump comprising: a motor and drive shaft; a piston; a mechanical element operatively connected to the drift shaft and the piston, wherein the mechanical element is configured to convert rotational motion into reciprocating linear motion of the piston; and a varying means configured to vary the rotational speed of the motor by modulating the frequency of a drive or pulse signal such that the piston has a constant stroke velocity.

The invention provides a small, lightweight, pulseless pump, which exhibits a low dead volume and is capable of high pressures (e.g. over 20,000 kPa) for use in portable/deployable HPLC and IC applications which can be easily deployed to sites of interest where some sort of chemical analysis (by LC) needs to be performed. The invention provides portable and deployable pump design that is small and compact, which can deliver the same power/potential as a benchtop pump. The pump uses a swashplate with a shallow angle, which makes it possible to pump at high pressures using a much smaller motor since the braking torque on the motor is significantly reduced. The swashplate mechanism itself is very compact, and combined with the small drive motor and pump heads the overall footprint is a fraction of a benchtop system.

By using the frequency modulated swashplate design, it is possible to obtain a small, compact pumping system, which can provide high pumping pressures with very low flow rate noise. Also, by using such mechanism, the piston diameter can be reduced from 1/8" to 1/16", to produce pressures in excess of 1000bar making the system suitable for portable UHPLC applications.

Preferably, the varying means comprises: a rotary encoder operatively coupled to the driveshaft, wherein: the rotary encoder provides a feedback signal representative of the angular position of the driveshaft (and thus the piston(s)); and the varying means varies the rotational speed of the driveshaft based on the feedback signal.

Preferably, the varying means comprises: a controller configured to modulate the speed of the motor based on the feedback signal from the rotary encoder, thereby varying the speed of the of the motor.

The pump comprises a flow meter to detect the rate of fluid flow to or from the pump, wherein the controller is configured to calibrate the modulation (or more specifically the rate of change of modulation) of the signal to the motor based on the detected flow rate.

Preferably, the pump comprises less than four pistons. More preferably, the pump comprises two pistons.

Preferably, the mechanical element being driven by the driveshaft is a swash plate.

In an aspect of the present invention, there is provided a reciprocating pump that includes a motor and a drive shaft operably coupled to the motor, a piston, a swashplate operatively connected to the drive shaft and the piston, wherein the rotational motion of the swashplate is converted into reciprocating linear motion of the piston, and a varying means configured to vary the rotational speed of the swashplate, by modulating the frequency of a drive or pulse signal that drives the motor, such that the piston has a constant stroke velocity to enable a linear flow of liquid with constant pressure.

Preferably, the varying means comprises an encoder operatively coupled to the motor or drive shaft and that generates a first feedback signal representative of an angular position of the swashplate or linear position of the piston, a flow rate sensor that generates a second feedback signal representative of a rate of fluid flow to or from the pump, a pressure sensor that generates a third feedback signal representative of a pressure inside the pump, and a controller operably connected to the encoder, pressure sensor, flow rate sensor, and the motor to form a closed loop control system, wherein the controller is configured to modulate the frequency of the drive or pulse signal that drives the motor based on the first, second and third feedback signals, thereby varying the rotational speed of the swashplate.

Preferably, the encoder comprises a linear position sensor or a rotary encoder to calculate a position of the piston.

Preferably, the swashplate is mounted on the drive shaft at a shallow angle to reduce a length of piston stroke, reduce swept volume of corresponding cylinder, and reduce dead volume within the pump.

Preferably, the varying means modulates the frequency of the drive or pulse signal sinusoidally, and wherein the rotational speed of the swashplate varies sinusoidally in phase with a linear position of the piston, such that the sinusoidal variation of the rotational speed of the swashplate cancels out the sinusoidal variation of the piston velocity, resulting in a linear stroke velocity of the piston. Preferably, the swashplate rotates at a maximum speed at the start and end of piston strokes, and rotates at a minimum velocity when the piston stroke is halfway between start and end.

In another aspect of the present invention, there is provided a method of operating a reciprocating pump comprising a motor, a drive shaft and a piston. The method includes the steps of connecting a swashplate to the drive shaft and the piston, operably driving the swashplate by the motor, and configuring the swashplate to convert corresponding rotational motion into reciprocating linear motion of the piston, and varying the rotational speed of the swashplate by modulating the frequency of a drive or pulse signal that drives the motor, such that the piston has a constant stroke velocity to enable a linear flow of liquid with constant pressure.

The present disclosure is also directed to a method of operating a reciprocating pump. For the purposes of this method, the pump comprises: a motor for rotating a drive shaft; a piston(s) for pumping a fluid; and a mechanical element operatively connected to the axle and the piston, wherein the mechanical element is configured to convert rotational movement of the axle into reciprocating linear movement of the piston, and the method comprises: varying the rotational speed of the drive shaft such that the piston stroke has a constant velocity.

Preferably, the method comprises monitoring the angular position of the drive shaft wherein the rotational speed of the motor is varied based on the angular position of the drive shaft.

Optionally, the method comprises determining the flow rate from the pump to ensure a constant and consistent flow rate.

In another embodiment there is provided a method of operating a reciprocating pump comprising a motor, a drive shaft and a piston, the method comprising the steps of: connecting a mechanical element to the drift shaft and the piston; configuring the mechanical element to convert rotational motion into reciprocating linear motion of the piston; and varying the rotational speed of the motor by modulating the frequency of a drive or pulse signal such that the piston has a constant stroke velocity.

The present disclosure also provides a computer readable storage medium for storing instructions which, when executed by a processing means, cause a pump to perform a method as set out above.

Brief Description of the Drawings

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:-

FIG.1 is a graph of the piston displacement of an axial piston pump driven (a), at a constant velocity, and (b). by a frequency modulated signal - both are plotted against time;

FIG. 2 is a graph of the frequency of the drive signal provided to the motor, modulated sinusoidally to give a constant piston velocity;

FIG. 3 is one example of a pump in accordance with the present disclosure;

FIG. 4 is a function block diagram of a typical pump flow control for frequency modulated operation, according to one embodiment of the invention; and

FIG. 5 is a block diagram of a closed loop control system for controlling the flow rate of the pump of FIG.3, in accordance with an embodiment of the present invention

Detailed Description of the Drawings

Axial piston pumps offer exceptional pumping power in a highly compact design, making them a promising technology for use in portable analytical instrumentation. However, as noted above, the operation of such pumps demands the use of many pistons to provide a close to pulseless flow. A swashplate driven axial piston pump uses a rotating angled plate (a swashplate) to convert axial rotation into horizontal displacement. Pistons, oriented parallel to the axis of rotation of the swashplate, contact it directly or via rollers, and as the swashplate is rotated the pistons move back and forth, the piston/rollers staying in contact with the swashplate at all times. Piston return may be achieved through a simple spring return.

For chromatography applications, a highly constant flow of liquid is required. Figure 1 is a graph of the piston displacement of an axial piston pump driven (a), at a constant velocity, and (b) by a frequency modulated signal - both are plotted against time. However, as shown in figure 1 , since the linear displacement of a piston in a swashplate driven axial piston pump is achieved by rotation of the swashplate, the piston velocity corresponds to a sinusoidal pattern. As a result, the volume of liquid driven by such a piston over time will not be linear and will result in significant pressure pulsations within the system.

Given that the piston displacement profile of axial piston pumps is not constant, axial piston pumps with a small number of pistons are wholly unsuitable for use in liquid chromatography (where a constant liner flow is required). Typically, this is overcome by using a high number of pistons to smooth the flow profile to being approximately linear.

However, as each piston exerts a force on the swashplate, increasing the number of pistons unfortunately increases the forces acting on the swashplate (and thus increases the braking torque on the motor, reducing the responsiveness of the pump). Furthermore, each additional piston requires an additional cylinder. This increases the volume of liquid required to be held in the pump as it is run - this volume is known as ‘dead volume’. However, mobile phase gradients are often used in liquid chromatographic methods. Since flow rates can be very small (from pL to mL per minute) it is important to minimise the volume of liquid between the pump inlets and the chromatographic column. This can be achieved by minimising the dead volume of the pump. If the dead volume of the pump is too high, it will be impractical to use gradients because it will take too long for the mobile phase to equilibrate as it passes from the pump inlet through the pump, injector, and onto to the head of the column.

The present invention is directed towards modifying a swashplate driven axial piston pump to overcome these problems. In particular, a pump in accordance with the present disclosure uses less than four pistons. By minimising the number of pistons used, dead volume is reduced. Further, a swashplate with a shallow angle is used, giving a short stroke to each piston. Reducing the length of the piston stroke reduces the volume of piston chamber, and reduces the swept volume of the cylinders. The combination of these features minimise the dead volume of the pump, and making it suitable for gradient separations where rapid equilibration of solvent composition is required. Additionally, the forces acting on the swashplate itself are reduced. As a result, the pump can pump liquid at very high pressures. Suitably, the pump is an axial piston pump driven by a servo motor. There is a lower limit to the swashplate angle however, as the pistons require enough swept volume to overcome the volumes within the microfluidic check valves during each stroke. The upper limit to the swashplate angle is limited by dead volume within the pump head (large swept volumes) if using gradients and the increased braking torque on the motor, i.e. lower pressures, higher electrical load.

A further adaptation is the use of a thrust bearing between the swashplate and the pistons. A thrust bearing is a particular type of rotary bearing. Like other bearings, they permanently rotate between parts, but they are designed to support a predominantly axial load. The thrust bearings make it possible for the pistons to contact the swash plate directly, since vertical displacement of the swashplate with respect to the axis of motion of the pistons is reduced to a manageable level.

While these adaptations significantly reduce the dead volume of the pump, they result in a design that has a highly sinusoidal flow profile and thus a highly variant piston velocity, causing a non-linear flow of liquid and pressure pulsation within the column.

The present disclosure overcomes this problem by adding a further sinusoidal distortion by way of modulating the signal driving the motor. By carefully selecting this further distortion such that has a frequency matching the difference between the maximum and minimum piston velocities, and by providing this sinusoidal distortion out of phase with the sinusoidal variation in piston velocity, the two cancel each other out, resulting in a linear piston speed.

In a preferred embodiment, the rotational velocity of the swashplate is varied sinusoidally over time from a minimum speed to a maximum speed. By varying the rotational velocity of the swashplate so that the speed varies sinusoidally in phase with the position of the piston(s) it is possible for the sinusoidal variation of the rotational velocity of swashplate to cancel out the sinusoidal variation of the piston velocity. The timing of this variation is designed such that the swashplate is at its maximum speed at the start of a piston stroke (i.e. when the maximum volume of liquid is in the piston chamber) and end of a piston stroke (i.e. when the piston is fully extended into the piston chamber and the minimum volume of liquid is in the piston chamber) and the swashplate is at its minimum velocity when the piston stroke is half-way between these extremes.

Preferably, the sinusoidal variation in the rotational speed of the swashplate is achieved by driving the swashplate with a stepper or servo motor which in turn is driven by a frequency modulated signal or using a controller. As a result, the rotational velocity of the swashplate also varies sinusoidally making it possible to turn the otherwise sinusoidal piston velocity into a constant velocity. As a result, the stroke pattern has a saw tooth pattern. As the velocity of a piston is constant during a piston stroke, a constant flow rate can be obtained with no pressure pulsations.

Figure 2 is a graph of the frequency of the drive signal provided to the motor, modulated sinusoidally to give a constant piston velocity. The straight line represents the displacement of the piston plotted against time. The curve signal shows an indicative signal with changing frequency for driving the rotation of the swash plate, the latter being modulated with a sinusoidal frequency modulation synchronised with piston position.

Figure 3 shows a pump in accordance with one embodiment of the present invention. The pump is provided with a motor 10. A power source (not shown) provides power to the motor 10. A drive circuit (not shown) provides either a pulsed or PWM drive signal to the motor 10 which is modulated sinusoidally as described above.

Feedback through a rotary encoder 11 can be used to modulate the rotational speed of a swashplate 12. This modulation is configured to cancel out sinusoidal variations in the velocity of at least one or more pistons 13 due to the pistons being driven by the swashplate 12. This results in a very precise linear motion pattern for the pistons 13, resulting in a smooth, pulseless flow of liquid from the pump.

Preferably, the motor 10 is a servomotor, and is provided with an inbuilt rotary encoder or external encoder as shown in 11 . The rotary encoder 11 converts the angular position or motion to analogue or digital output signals. Alternatively the rotary encoder 11 is located on the swashplate shaft. In this configuration, the rotary encoder 11 detects the angular position of the swashplate 12. The output of the rotary encoder 11 is provided as feedback to a drive/control circuit.

In particular, the output of the rotary encoder is provided to a controller (not shown) which controls the speed of the motor. The controller modulates the drive signal provided to the motor based on the output of the rotary encoder.

The motor 10 can be connected to the swashplate 12 in the swashplate assembly 14 by an optional gearbox 15. The swashplate assembly 14 comprises a swashplate 12 and a swashplate retaining plate 16. The swashplate retaining plate 16 is held in place by one or more resilient members. As shown in figure 3, the one or more resilient members can be a plurality of springs. Preferably four springs are used. The swashplate retaining plate 16 is used to prevent the pistons 13 rotating around the axis of that swashplate 12 and to hold the pistons 13 in fixed proximity to the swashplate 12. Preferably a low number of pistons is used. Preferably, four or two pistons are used. Ideally, as shown in figure 3, two pistons are used. The pistons 13 drive liquid in pump head 17 out of the pump.

Through providing a closed loop feedback frequency modulated driven swashplate with a short piston stroke (shallow swashplate), the pump disclosed is particularly advantageous. The design means that only two pistons with a shallow stroke are required, thus providing the pump with a smaller dead volume than prior art solutions. The pump’s low dead volume makes it ideal for use with separations employing mobile phase gradients.

Furthermore, the uniform piston velocity results in displacement profile that exactly matches that of the desired saw tooth pattern required for a constant liquid flow rate, resulting in pulseless flow of liquid from only two pistons. Such a design is extremely compact and lightweight. The high axial forces of the pistons translate to low braking torque forces on the motor given the shallow angle of the swashplate. As a result, the pump of the present disclosure does not require an overly powerful motor to drive it, even when operating at high pressures.

Figure 4 is a functional block diagram of a typical pump flow control for frequency modulated operation, according to one embodiment of the invention. A controller 20, or a varying means, can output a modulated drive signal output to the motor 21 . The drive signal can be selected based on one or more inputs. One of the inputs can be from a measured encoder position module 22. A second input can be provided from a flow control module 23 which can output control information based on measured flow rate and a user defined flow rate setpoint. FIG.5 is a block diagram illustrating a closed loop control system 500 for controlling a flow rate in the pump shown in FIG.3, in accordance with an embodiment of the present invention.

The closed loop control system 500 includes a linear position sensor 502 that generates a first feedback signal representative of an angular position of the mechanical element or linear position of the piston, for example piston 13. The linear position sensor 502 may be similar to the rotary encoder 11 . The flow rate sensor 504 generates a second feedback signal representative of a rate of fluid flow to or from the pump. The pressure sensor 506 generates a third feedback signal representative of a pressure within the pump.

The microcontroller 508 is operably connected to the linear position sensor 502, pressure sensor 504, flow rate sensor 506, and the stepper motor 510, wherein the microcontroller 508 is configured to modulate the frequency of the drive or pulse signal that drives the motor 510 based on the first, second and third feedback signals, thereby varying the rotational speed of the swashplate 12 to obtain a constant stroke velocity of the piston 13, and a linear flow of liquid with constant pressure. As explained earlier, the microcontroller 508 varies the rotational speed of the swashplate 12 sinusoidally in phase with a linear position of the piston 13, such that the sinusoidal variation of the rotational speed of the swashplate 12 cancels out the sinusoidal variation of the piston velocity, resulting in a linear stroke velocity of the piston 13.

Those skilled in the art will note that the above examples are intended to be illustrative. Many variations can be made by those skilled in the art without departing from the scope of the present disclosure. For example, the pump of the present disclosure has been described with reference to liquid chromatography including ion chromatography, however the pump could also be used in many biomedical applications where high pressure and smooth, continuous flow are required. Due to the design of the drive mechanism, the pump head and pistons are easily and quickly interchangeable, making the design suitable for use with disposable/interchangeable pump heads and pistons. This opens up many applications for the technology including uHPLC applications with pressures exceeding 100 Mpa (>1000 bar).

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.