US 5,457,626 AGrant
Bimodal Liquid Chromatography Pump Employing Artificial Intelligence Logic Feedback Control
Issue Date:1995-10-10
•9 Claims
•7 Drawing Sheets
Abstract
Flowrate in a precision pump used for liquid chromatography employs a digital control system incorporating artificial intelligence. The pump system operates in a default flow mode, wherein real-time pressure feedback is not used to control motor speed, or in pressure mode, wherein motor speed is controlled by the pump system pressure point. The artificial intelligence commands mode changes to pressure mode when the constant displacement flow measurement time is within a threshold relative to commanded flowrate, and when the higher pressure piston is being measured. Flow mode pressure ripple is minimized by monitoring pressure points and commanding motor speed change at appropriate positions during the motor cam rotation. Pressure mode uses the higher pressurized piston as a reference for constant displacement flow measurement and provides accurate flowrate even if one piston is leaky. In pressure mode, the artificial intelligence monitors for intake cycle oscillation and optimizes a highest system pressure gain dynamically. By determining the duration of hydraulic intake and activating proportioning only during the constant intake flow portion of an intake cycle, constant flowrate proportioning is provided. The present invention compensates for a variety of system and environmental variables including leaky valves, air bubbles, a leaky cylinder head, pressure changes, and variations in anticipated compliance and can maintain a flowrate constant within about .+-.1% without using real-time pressure feedback.
Metadata
Assignee
- Dionex Corporation
Inventor
- David A. Wolze
Application Information
Application Number:US 2999889
Filing Date:1994-09-01
Priority Date:1994-09-01
Art Unit:236
Classifications
IPC:
G06F 1546B01D 1508
Field of Search:
364395210417148;152;160;166;180;509;510900;906101;198.2;656;657;65843;45
Patent Drawings (7 sheets)
Description
Field of the Invention
This invention relates to precision control of flowrate and accurate proportioning of solvents in an analytical procedure, and more specifically to flowrate control in liquid chromatography analysis.
Background of the Invention
Many applications require the controlled mixing and/or delivery of fluid eluents, liquid chromatography ("LC") for example. In LC, a flowing stream of liquid solvent in a mobile phase carries a liquid sample containing components to be analyzed. A precision pump mechanism causes the liquid solvent to pass through a chromatography column typically packed with ion exchange particles in a stationary phase. While passing through the column, components within the liquid sample are differentially adsorbed and desorbed from the stationary phase. These individual components then elute from the column at different times and are separately detected and quantified as they flow through a detector. In this fashion, analytical information is provided as to the constituents present in the liquid sample.
Even more effective separations result from high performance liquid chromatography systems ("HPLC"), wherein mixtures of solvents are used as the mobile phase. When the components of the mixture are held constant, an isocratic mode results. By contrast, gradient chromatography results when the composition of the liquid changes over time while being pumped to the column, for example, a composition going from 100% water to 100%, methanol.
FIG. 1 depicts a generic liquid chromatography system that may be operated in an HPLC mode, wherein differential analysis may be provided It is the purpose of precision pump mechanism 10 to deliver a liquid solvent via an output port 12 at a constant flowrate to a column 14 or other downstream analytical apparatus. In the HPLC mode, there are at least two sources of liquid input, 16A, 16B, each of which may contain different constituents.
In FIG. 1, liquid input sources 16A, 16B are coupled to a proportioning valve 17 whose operation is controlled by the digital control system 28. Through a "T-connector", valve 17 outputs liquid to inlet check valves 32A and 32B. Of course in non-differential analysis, there is a single liquid input source, e.g., 16A or 16B.
In FIG. 1, a rotary motor mechanism 18 receives an input voltage from a driver amplifier 20, and outputs rotary shaft motion in response to the input voltage. A tachometer mechanism 22 senses the rotary speed of the motor mechanism shaft 24 and provides this information as an input to the driver amplifier 20 in a closed feedback loop configuration.
As is known in the art, affixed to motor shaft 24 is a disk 26A that contains precisely located slots through which light may pass and be sensed by a sensor unit 26B that typically includes light emitting diode and detector pairs. At the motor shaft 24 rotates with speed .omega., the light sensors detect a digital pattern of light and no-light, which information is coupled to a digital control system 28. Other precision mechanisms for detecting rotary shaft speed and position could of course be used.
Digital control system 28 contains a control panel (not shown) permitting an operator to set a desired liquid flowrate at the output port 12. The output control signal from digital control system 28 is then provided as an additional input to the driver amplifier 20.
A mechanism 30 translates the rotary motion of shaft 24 to a reciprocating back-and-forth motion that is mechanically coupled to at least two piston heads (or "ends") 34A, 34B associated with surrounding cylinders (not shown). Where, as shown, two piston heads are used, they reciprocate 180.degree. out of phase. One piston intakes liquid while the other piston exhausts or outputs liquid, the intake cycle being shorter than the exhaust cycle. This two piston configuration uses a point of cross-over during which both pistons are pumping simultaneously so as to maintain system pressure without a dead zone. In this pressure mode of operation, to maintain constant system pressure and therefore constant flowrate at cross-over requires approximately halving motor speed. At cross-over, one piston is ending its exhaust cycle while the remaining piston is commencing its exhaust cycle. However, the prior art cannot accurately predict when during the system cycle motor speed should be halved to avoid significant pressure fluctuations.
In non-precision, non-proportioning applications a single reciprocating end may be used. In contrast to parallel-coupled 180.degree. out-of-phase ends, it is also known in the art to use a single motor that operates two series-coupled lead-screw heads to pump eluent to a column.
In response to the reciprocating motion provided by mechanism 30, ends 34A, 34B cause liquid from the respective liquid inputs 16A, 16B, after passing through respective unidirectional intake and exhaust check valves 32A, 32B, and 36A, 36B to be mixed at a "T"-connector proportioning valve 38. The thus-mixed liquids pass through a pressure transducer 40, through output port 12 and to the first stage of the downstream analytical apparatus, e.g., liquid chromatography column 14. As shown in FIG. 1, the output pressure measured by transducer 40 is coupled as an input to the digital control system 28 in an attempt to regulate the flowrate of the liquid exiting output port 12. Total flowrate is indirectly derived from the measured output pressure.
The real time use of output pressure data from transducer 40 to control flowrate in pump 10 is termed a pressure mode of operation. Pressure mode operation minimizes pressure ripple, and is advantageous for ion detection by conductivity or other detector. Such detection is relatively sensitive to pressure variations. However, pressure mode operation is disadvantageous where viscosity changes rapidly, or where other dynamic conditions are presented (e.g., column switching, injection, high speed operation). In the presence of such dynamically changing conditions, the lagging behind of feedback information precludes pump 10 from responding with sufficient speed to achieve the correction.
Under certain steady state conditions with normal pump speeds, even prior art pressure feedback can reduce pressure ripple effectively. However, under dynamic conditions, due to viscosity changes, high speed operation, or rapid change in system parameters as in switching columns or during an injection, the present invention reduces cross-over ripple, whereas prior art techniques cannot.
Unfortunately, prior art pump 10 cannot operate in a flow mode, wherein output flowrate is regulated without making real time use of output pressure data. Were it possible, a flow mode operation would provide superior flowrate performance in the presence of rapidly changing viscosity gradients.
The analysis application and the flowrate at output port 12 determine the liquid pressure. In practice, standard bore piston heads or ends 34A, 34B operate at 1 piston stroke every six second, which delivers approximately 1 ml/minute, whereas so-called microbore pistons deliver approximately 250 .mu.l/minute operating at the same piston stroke rate. Pressure at outlet port 12 is typically in the range 300 psi to 5,000 psi and is determined substantially by the column 14 or other apparatus downstream from the outlet port 12 of the pump 10.
In liquid chromatography, a detector (not shown) detects separated components in the eluent (solvent) passing through column 14 by outputting a peak signal. These signal peaks are then integrated as a function of time to arrive at meaningful analytical information. It is therefore important that the flowrate of the liquid at output port 12 and passing through the column 14 be a constant.
Unfortunately, prior art pump systems such as shown in FIG. 1 cannot reliably maintain a desired flowrate within an acceptable tolerance. For example, a leaky valve or piston seal will cause the pump 10 to output at less than the desired flowrate commanded by digital control system 28. Even if the valves and seals are not leaky, output flowrate can vary due to a differential change in compliance, e.g., variations in material expansion under pressure.
Further, production variations in manufacturing tolerances associated with cylinder heads, plastic or stainless steel fluid-carrying tubing, valves and other components can cause undesired deviations in the output flowrate, as can the presence of a bubble within the fluid flow path within pump 10. For example, a piston head attempting to compress liquid might in fact be compressing an air bubble, with the result that less liquid is pumped, with an attendant drop in output flowrate at port 12. Prior art control systems do not recognize such conditions, cannot distinguish between the output of each piston head, and cannot, for example, cause the higher pressure-delivering piston head to compensate for the remaining, lower-pressure, piston head.
In addition, the compliance (e.g., expandability) of the tubing, piston seals, and components comprising pump 10 may change with time, or material changes may be made, such as substituting stainless steel tubing for plastic tubing. If for whatever reason fluid-coupling components within pump 10 expand, the flowrate at outlet port 12 will be less than what digital control system 28 (and the system operator) believe to be present. Further, ambient environmental pressure or temperature, as well as changes in the liquid viscosity or compressibility can alter the output flowrate. Maintaining a constant output flowrate during chromatography using a gradient eluent is especially challenging because viscosity of the liquid mixture can rapidly vary by a factor of three.
A persistent problem in attempting to run with eluents of rapidly changing gradients is that in a conventional pressure mode system, there will always be several motor revolution cycles lagging of the necessary corrective action, while viscosity continues to change. Also, as viscosity changes, the inherent system pressure changes and the prior pressure control system undesirably changes flowrate (e.g., motor speed) to compensate for the change in pressure. Further, the prior art's reliance upon pressure parameters alone has a deleterious effect as pressure and flowrate tend to counter each other when attempting to make adjustments.
Thus, prior art systems cannot control flowrate to better than about 10% to 15% during rapid gradient change, although the error will be consistent for repeated runs. Thus, because of its relative consistency, the prior art can compensate for flowrate variation in repeated runs. Alternatively, a syringe-type lead screw system capable of only a single flowrate will be employed in the prior art during fast gradient analyses.
To recapitulate, existing pump systems operate in pressure mode, and cannot adequately compensate for variations in equipment, viscosity, pressure, compliance, temperature, the presence of air bubbles, and other variables likely to be experienced. Such systems measure and make real time use of total pressure indirectly derive flowrate. However, the total pressure under measurement will reflect leakage flow components, compliance-variable components, in addition to the actual eluent fluid flow. Simply stated, the prior art cannot distinguish or even recognize the various error components that contribute to undesired variations in output flowrate.
Thus, there is a need for a control mechanism that can control output flowrate in a precision pump despite the presence of variables including a leaky valve, an air bubble, a leaky cylinder head, variations in pressure, and compliance variations. Preferably such mechanism should permit pump system operation in flow mode, in pressure mode, and in an intelligent combination of each mode.
Such mechanism should provide acceptable flowrate control even if only one piston head is operating according to specification, wherein the correctly operating piston head compensates for the remaining piston head. In addition, such mechanism should maintain a pressure characteristic substantially free of peaks or dips over the approximate range 300 psi to 5,000 psi.
Further, such mechanism should provide experimenters with a constant fluid sample characteristic that is independent of system variables such as compliance, system fabrication tolerances, and ambient environmental changes, including transient changes. Such mechanism should reduce cross-over pressure ripple despite dynamic flow conditions, and should control flowrate even during a rapidly changing viscosity gradient experiment to within about .+-.5% or better.
The present invention provides such a control mechanism.
Summary of the Present Invention
The present invention provides a precision pump used for liquid chromatography and the like with bimodal operation, flow mode and pressure mode. In flow mode, a constant flowrate is maintained by operating the motor at a chosen one of two speeds without using real-time pressure feedback. As such, flow mode is especially advantageous for pressure-sensitive runs, e.g., those involving varying viscosity. In pressure mode, a constant flowrate is maintained by continuously adjusting motor speed in response to real-time pressure data. As such, pressure mode is advantageous for experiments dictating minimum pressure ripple.
The present invention regulates flowrate in such a pump system by providing a digital control system that incorporates artificial intelligence to regulate the pump motor speed.
In a first aspect, the pump system can operate bimodally, in a flow mode or in a pressure mode. The system user can manually select the mode of operation, and the pump system can automatically switch between modes as required to maintain constant flowrate. In the default flow mode, the pump motor operates at a constant speed without using real-time pressure feedback data to control motor speed. This is in stark contrast to the prior art.
Flow mode is implemented by a fuzzy logic routine controlling the digital control system, wherein pressure ripple is minimized by precisely varying the rotational speed of the pump motor between one of two speeds. By monitoring output pressure and encoder disk-derived motor position, applicant's algorithm predicts when the motor speed should be increased or decreased during motor driven cam rotation. Decreasing motor speed (preferably by 50%) during piston overlap relative to the speed during piston intake stroke results in a substantially constant flowrate. Because real-time output pressure data is not used to control flowrate, flow mode is especially useful in experimental runs wherein sudden viscosity changes or gradients occur in the liquid being pumped.
When the user selects pressure mode, the pump system automatically changes from the default flow mode to pressure mode when three conditions are met. These conditions are that motor speed is within a preferably 3% threshold of the commanded flowrate, the piston being measured is at a higher pressure than the remaining piston, and two cam cycles have elapsed. In entering pressure mode, the higher pressurized piston will now be used as the reference piston for monitoring system pressure. By contrast, in the prior art the monitored piston is arbitrarily selected without regard to piston pressure. Motor'speed is measured by angular displacement on an encoder disk mounted on the motor shaft. This measurement occurs during a constant displacement portion of the pump cycle wherein one piston is pressurized and operating in steady-state.
Pressure mode uses the higher pressurized piston as a reference for constant displacement flow measurement, with the operating pressure set point updated every other piston cycle. In this manner, flowrate is accurately controlled even when the system includes a leaky piston. In pressure mode, if flow rate deviates by more than a desired tolerance, the control system automatically returns the pump system to default flow mode. Reinitialization then occurs, and after two cycles if the desired flowrate regime is again attained, pressure mode is re-entered.
In yet another aspect, the artificial intelligence functions in pressure mode to determine the optimum highest system pressure gain, independently of piston head type or system application. During intake, pressure is monitored for the presence of oscillation. If oscillation is detected, pressure gain is decremented until a highest stable pressure gain is reached. This ability to dynamically adjust pressure gain on the fly advantageously permits changing-from microbore to standard bore operation, as well as compensating for system variables including compliance.
In this manner, flowrate is controlled during flow mode operation, which is commonly used for analysis involving rapidly changing eluent viscosity or gradients, or other varying system parameters. However, flowrate is also controlled for pressure mode operation, a mode wherein pressure ripple is especially minimized. Pressure mode is commonly used for chromatography runs that use detectors that are highly sensitive to pressure variations, e.g., ion chromatography with suppressed conductivity suppression detection, or HPLC with amperometry detection.
In either mode, the present invention can compensate for a variety of system and environmental variables including leaky valves, air bubbles, a leaky cylinder head, pressure changes, and variations in anticipated compliance. In either mode, a flowrate can be established and maintained within about .+-.1%, even when one of the pistons is leaky.
In a third aspect, the present invention determines the onset and duration of hydraulic intake and permits proportioning during the constant intake flow portion of an intake cycle. In this fashion, proportioning occurs with constant flowrate over a wide pressure regime, thus providing a more accurately mixed output eluent.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.
Brief Description of the Drawings
FIG. 1 depicts a digitally controlled precision pump suitable for liquid chromatography, according to the prior art;
FIG. 2A depicts a fuzzy-logic digitally controlled precision pump suitable for liquid chromatography, according to the present invention;
FIG. 2B is a block diagram showing information flow to the digital signal processing CPU, according to the present invention;
FIG. 3 schematically depicts a cam mechanism used to generate reference tick counts for use with the present invention;
FIGS. 4A-4C depict pressure ripple and its reduction by advancing or regarding pressure points, according to the present invention;
FIG. 5 depicts intake flow into the piston heads and the constant intake flow portion thereof used by the present invention;
FIG. 6A is a block diagram showing the two feedback stage implementation of flow mode, according to the present invention;
FIG. 6B is a block diagram showing the three feedback stage implementation of pressure mode, according to the present invention.
Detailed Description of the Preferred Embodiment
The present invention is shown in FIG. 2A as a digital control system 128 including an artificial intelligence or fuzzy logic unit 150 and control unit 138. FIG. 2A. FIG. 2B is a more detailed block diagram showing implementation within digital control system 128.
In FIG. 2A, like element reference numerals refer to components that can be identical to what was described with respect to the prior art pump system 10 shown in FIG. 1. In FIG. 2A, a pump system 100 includes a digital control system 128 with a digital signal processor central processing unit ("CPU") 130, a set of algorithms comprising artificial intelligence or fuzzy logic 150, attendant memory 132, an interface CPU 136 located within a control panel unit 138 that includes operator usable keys, trackball and other input devices 134, and a monitor 140. In the preferred embodiment, monitor 140 can present a Windows.TM. or similar format menu permitting the system user to change pump parameters and to monitor pump performance.
Using a monitor displayed menu or a mechanical switch, system 128 permits the system user to select whether pump system 100 should operate in a flow mode or in a pressure mode, and to command the desired flowrate through output port 12. These commands may be issued using the input devices 134. If desired, system 128, control panel 134 and display 136 may include a personal computer or a work station unit, wherein the algorithms implementing artificial intelligence 150 may be loaded or stored.
With reference to FIG. 2A, DSP CPU 130 preferably is an Analog Devices embedded digital signal processing unit number ASDP 2105 that communicates with other portions of digital control system 128 through a parallel bus. Analog system input signals such as a tachometer voltage from tach 22, output pressure from transducer 40, motor current from motor 18 are digitized by an analog-to-digital converter and presented to the parallel data bus. Also presented are an optional vacuum degas pressure that allows system 100 to degas the input fluids before a run. A 1.00 V reference voltage is also presented for diagnostic purposes.
In FIG. 2B, signals to and from the interface CPU 136 travel by bus and a backplane bus. Various signals are bus-coupled to the motor drive circuit, shown generically in FIG. 2A as amplifier driver 20, which outputs a signal that determines the rotational speed .omega. of motor 18. Other signals from DSP CPU 130 are coupled out to solenoid drivers that can electrically operation the various check valves shown in FIG. 2A.
In the preferred embodiment, interface CPU 136 is an Intel 80186 single board computer unit used primarily to handle user input and user display tasks. The architecture and use of DSP CPU 130 and interface CPU 136 (or equivalent components) are well known in the art. Motor 18 preferably is a Yasgowa model Minertia gear motor, with a .+-.12 V control input voltage range, although other motors may be used instead. In the preferred embodiment, applicant's artificial intelligence (see APPENDIX 1) comprises 1,000 words each comprising 24 bits or 3 bytes. The algorithm is stored in 3 KB of flash memory associated with the interface CPU 136. The algorithm could, however, be stored in the digital control system 128, or stored elsewhere.
As shown by FIG. 2A, one input to the digital control system 128 is the output from a high resolution incremental encoder disk 126 (to be described further with respect to FIG. 3). As best seen in FIG. 3, the encoder disk 126 is mounted to the shaft of motor 18, as is a cam 160.
Among the many advantages provided by the present invention is that the rotational speed of motor 18 may be precisely regulated over a range of about 256:1, and that this range may be changed by changing the pistons and cylinders to a different size (e.g., standard bore of 100 .mu.l/cylinder displacement, and microbore of 25 .mu.l/cylinder displacement). The ability to change cylinders will permit a standard bore system flowrate of 0.04 ml/minute to 10 ml/minute, and a microbore flowrate of 0.01 ml/minute to 2.5 ml/minute.
In the preferred embodiment shown in FIG. 2A, mechanism 30 employs a cam 160 (described more fully with respect to FIG. 3), to translate rotational motor action to reciprocating motion that drives first and second cylinder piston heads or ends 34A, 34B. However, the present invention may also be practiced where motor 18 drives a mechanism 30 that includes a pair of lead screw mechanisms.
Those skilled in the art will appreciate that such lead screw mechanisms would receive liquid through valves 32A, 32B and deliver the pumped liquid through outlet valves 36A, 36B, a "T"-connector proportioning valve 38, through a pressure transducer 40 to the column or other load 14. It is also understood that the various valves shown in FIGS. 2A and 3 preferably are solenoids operating under control of digital control system 120. Because lead screw mechanisms are well known in the art, no further description of their use with the present invention will be given.
As shown in FIG. 2A, pump system 100 also uses a pressure transducer 40 to monitor pressure at the pump output port 12, which pressure data is used by digital control system 128 to rapidly derive flowrate on a short term basis. A pressure transducer is because inexpensive real time basis flowrate measurement devices do not exist for the low flowrate regimes used in liquid chromatography at not available. Further, because pump system 100 may operate at pressures exceeding 5,000 psi, transducer 40 provides a mechanism for monitoring the safety of the equipment being used, and can also provides meaningful data concerning pressure leaks to digital control system 120.
With reference to FIG. 3, a high resolution incremental encoder disk 26A' is firmly locked to a cam 160 and the shaft 34 of rotary mechanism 18. Cam 160 is associated with a suitable mechanism 30 for translating rotational to reciprocal motion.
In pressure mode, the present invention derives flowrate from a constant displacement flow measurement, and adjusts the system by changing the pressure set point once per cam revolution in a sampled feedback configuration (see FIG. 6B). The pressure set point is the pressure at which pump system 100 is operating.
In flow mode, the present invention derives flowrate from a similar constant displacement flow measurement, and adjusts the system by changing the motor speed set point twice per cam revolution (see FIG. 6A).
In FIG. 3, cam 160 is shown at a cross-over point for piston head 34B, which is at the end of the exhaust stroke for piston 34B and beginning of its intake stroke. Because the cam is not symmetrical, piston 34A will also remain in the exhaust portion of its cycle before commencing its intake stroke.
Thus, FIG. 3 depicts an overlap condition, wherein both piston heads have simultaneously moved outward in an exhaust stroke. Some overlap is necessary to prevent a pressure decrease during the intake stroke of any given piston and there is a concomitant decrease in motor speed to provide constant pressure. It will be appreciated that pump system 100 is at any time in either an intake regime or an overlap regime, which in the preferred embodiment represent, respectively, 151.degree. and 29.degree. of cam rotation. The exhaust stroke represents 180.degree. plus 29.degree., or 209.degree. and is not descriptive of a unique event.
If pressure is examined during overlap, a slight increase or decrease in pressure is observed, depending upon the extent and timing of the overlap. The prior art attempts to design a pump system 10 with sufficient overlap that, at maximum pressure (e.g., perhaps 5,000 psi), there will be almost perfect cancellation in pressure with only a small pressure or flowrate ripple exhibited. But at lower pressures, a pressure peak is present at overlap, which must be compensated for by the control system. Alternatively, some prior art designs are optimized for a nominal pressure, above which there will be a pressure dip and below which there will be a pressure peak at overlap. However the compromises, prior art pump systems exhibit pressure peaks or dips during experiments, with result noise and poor performance. Unfortunately, no prior art systems can maintain a pressure characteristic free of peaks or dips over the range 300 psi to 5,000 psi.
With further reference to FIG. 3, the cross-over point of piston 34B does not represent a stable time for digital control system 128 to sample system parameters as the piston pressure and motor speed are changing. By contrast, stable data is sampled when one of the pistons is halfway through its exhaust stroke, whereat the piston will be fully pressurized, pressure and motor speed will be constant, and fluid will flow linearly.
APPENDIX 1, attached, is a computer printout in assembly language code of applicant's fuzzy logic algorithm 150 for digital control system 128. With respect to APPENDIX 1, pages 39-42 contain code setting up procedures used for applicant's flow mode (see page 42 therein), and pressure mode (see page 42 therein, continuing to page 43). At APPENDIX 1, pages 39-43, code implementing applicant's positive displacement flow control is set forth, which code is used in both flow mode and pressure mode operation.
The present invention measures the time for the last fourth of the intake cycle, wherein pressure and motor speed are stable, to indirectly determine flowrate. If control system 128 is set for one motor stroke every six seconds, the cam should require 1.50 seconds to traverse one-fourth the stroke. Assume that the cam took X% too long for the quarter stoke. In pressure mode, control system 128 will increase the pressure set point by the same X% to compensate during the next cam revolution. However in flow mode, control system 128 will increase the motor speed set point by the same X% to compensate during next piston stroke. In this fashion, the pressure set point is directly altered once per cam revolution in pressure mode, or once per piston stroke in flow mode, as needed, to indirectly alter motor speed in a sampled feedback configuration.
In pressure mode, the size of the constant displacement angle preferably small, e.g., less than 45.degree.. But in flow mode, the pressure feedback cannot compensate for system compliance, resulting in a longer term effect. Thus, the constant displacement measurement angle preferably is wider to cover almost the entire intake stroke. This permits adequately accounting for the compliance. A good comprise for operating in both pressure and flow mode was found to be about 135.degree., e.g., from 070 hex to 1F0 hex.
As shown in FIG. 3, disk 26A' carries a ring of 1024 precisely defined, equi-spaced slots 170 that will define a first information channel (channel A), and also carries a single index slot 180 defining a second information channel, channel B. While a number different than 1024 may be used, 1024 slots provides adequate resolution to control pump 150 both in LC and HPLC modes of operation. These slots are used with two light emitting detector ("LED") and sensor pairs 26B to communicate to applicant's fuzzy logic module 150 the absolute position of the motor rotary shaft 24, and thus the absolute position of cam 160.
With reference to APPENDIX 1, page 34, in a manner known to those skilled in the art of digital circuit design, the detected light from the first information channel is coupled to up- and/or down-counters that may be reset with the output from the detected light from the second information channel. In FIG. 3, the "0" reference position is denoted as "TIC 0" and within the digital control system 128 is identified as hex-based count 1F0.sub.h.
In FIG. 3, head 34A and 34B are respectively coupled to piston shafts 190A, 190B whose distal ends include rollers (not shown) that contact the perimeter of the rotating cam 160. Piston head 34B, shown within cylinder 180B, is shown in a position terminating its exhaust cycle and beginning its intake cycle. Conversely, since it is 180.degree. out of phase, piston head 34A, operating within cylinder 180A, is terminating its intake cycle and commencing its exhaust cycle. As is common in the art, a minimum of two heads are used in precision applications to ensure there is no dead zone with respect to refilling the cylinders with fluid during the intake/exhaust strokes. While the present invention will function with more than two heads, doing so adds to the cost of the overall pump system.
As shown in APPENDIX 1, page 34, part of the code defines index 180 and correlate its location relative to the cam 160, permitting determination of the point whereat a piston crosses on a trough as cam 160 rotates. The point where a piston crosses, at the end of the intake stroke, will be deemed "0", as shown by "TICK 0" on FIG. 3, which point will also be the start of overlap in that both heads will then move outward.
In the present invention, an edge-sensing down counter is set to the cam encoder offset, which is a number of counts (or "TICKS") from the encoder index slot 180 to the zero point on the cam mechanism 160. This offset is set to the cam encounter alignment factor value, e.g., to 180, which is the number of ticks from the index to the point where cross-over begins. When the down counter decrements to 0, it sets a second tick-synchronized counter to 0, from which the second counter increments with each tick. As reflected by APPENDIX 1, only the first nine bits are read on the second (up) counter, but all sixteen bits are read on the down-counter. Thus, when the rotating cam is at angular position 1FF hex, the next TICK will reset the up-counter to zero. The down-counter, of course, does not reset until the index 180 is read, at which point the down-counter is reset to the cam encoder offset value as provided by the system user.
At the digital control system control panel 134, the system user may enter a cam encoder offset number, which permits compensating for production variations associated with the mechanism itself. The use of two counters advantageously permits the routine to function, even if the index is missed after synchronization is acquired, even if the encoder misses counts, or has extra counts due to signal glitches. In the preferred .embodiment, the down-counter is a 16-bit device, and the up-counter is a 9-bit device.
Thus, the digital control system 128 knows how many ticks on channel A exist between the index mark on channel B to the Zero point ("TICK 0") whereat one of the pistons crosses the zero point. This cam encoder offset value is also referred to as the alignment value. As such, the first part of applicant's code tracks where at a given point in time the cam mechanism is relative to index 0, the end of the intake stroke, with respect to piston head position.
It is important to appreciate that when the pump system is first started (or if the user desires), the routine commands a flow mode (as contrasted to pressure mode). Flow mode results in that there is no real time reliance upon output pressure data feedback, and motor 18 is operating at a constant rotational speed. Although transducer 40 provides pressure data, real time control of the speed of motor 18 is not dependent upon such data. By contrast, in a pressure mode of operation, pump 100 can make real time use of output pressure data.
Reference is now made to APPENDIX 1, pages 43-44. As will be described, fuzzy logic 100 intelligently permits pump system 100 to commence operation in a default flow mode. In default, system 100 is initialized to attain the system working pressure for the user-selected system flowrate. The user may wish to remain in flow mode, especially if runs involving varying viscosity are to be performed. On the other hand, pressure mode may be selected from the control panel if the user wishes to attain the advantage of lower pressure ripple.
However, even if pressure mode is user selected, the system will first default to flow mode and switch to pressure mode only after flowrate is determined to be established within a desired threshold, preferably .+-.3% of a nominal value, and the system operating pressure has been achieved. Whenever the threshold flowrate value is exceeded, the control system automatically reverts to flow mode operation until the flowrate is again maintained within the desired threshold tolerance. As such, system 100 can autonomously switch back and forth between flow mode, a mode best suited for compensating for rapid viscosity changes, and pressure mode, a mode best suited to reduce pressure ripple.
In the set up procedures at pages 36-37 of APPENDIX 1, the execution code at page 38, and the get data code at pages 44-47, a learning algorithm portion of applicant's fuzzy logic 150 is described, as well as a positive displacement flow control. By way of background, assume that the motor 18 is running at an arbitrary speed .omega.1 during the intake stroke, wherein one piston, e.g., 34A is exhausting, and that system 100 is functioning ideally with no leakage, compliance, or other variations.
As shown in FIG. 3, at cross-over, both pistons move outward in exhaust with both piston head areas pumping liquid. Thus, to maintain constant flowrate with both pistons exhausting, the motor speed must be reduced to approximately 0.5 .omega.1. In practice, reduction may readily be controlled between about 0.4 .omega.1 to 0.6 .omega.1. APPENDIX 1 sets forth the constant displacement control code at pages 39-42 and 36-39, with applicable fuzzy logic at pages 44-47.
Applicant's fuzzy logic 150 permits maintaining constant flowrate in a flow mode (e.g., without any pressure feedback) by operating motor 18 at a constant full speed during the intake stroke, and halving the speed during overlap. By definition, overlap starts when a piston crosses the center of the notch in cam 160, which notch is located approximately opposite position 1FF hex in FIG. 3. Overlap ends when the opposite piston moves over the cam nose, which nose is at the right-hand TICK 88 in FIG. 3. This .flow mode operation controls output port 12 flowrate within about .+-.1% with very little ripple.
In practice, it is difficult to predict where in the motor cycle overlap actually commences and ends, during which time the motor speed should be halved, relative to an ideal motor speed during intake. For example, if motor 18 is optimized for 4,000 psi pressure such that motor speed is halved at two specific points during cam rotation, the pump 100 will exhibit substantial ripple at other than 4,000 psi. Further, even at 4,000 psi, if a change in pressure, liquid viscosity occurs, or a bubble is present, there will be substantial ripple in the output flowrate.
It is understood that during overlap, both cylinders are exhausting. However, one cylinder will be almost fully exhausted, whereas the other cylinder will just be starting its exhaust stroke. For the almost fully exhausted cylinder, the corresponding outlet check valve (e.g., valve 36A for cylinder 34A) will have been open, whereas the outlet check valve for the new cylinder just starting its exhaust stroke will now be opening.
The present invention recognizes that pistons 34A and 34B will never be identical in practice. For example, if examined on a diagnostic display, each piston would exhibit its own characteristic pressurization points, also called "p-points".
Pressurization points are numbers that represent where, in TICK count, a piston first becomes pressurized and thus active. With reference to FIG. 3, this will be where in the cam stroke (in terms of TICK location) the cylinder that is just beginning its exhaust stroke will actually start to pump liquid.
Pressurization-point characteristics can differ piston-to-piston due to production variations, seal variations, as well as the presence of bubbles, among other variables. Pressurization point numbers can range from 0-76 TICKS, with a wide discrepancy between pressurization numbers from cylinder to cylinder implying a leaky or otherwise defective cylinder or head.
A relatively larger pressurization point implies more compressibility, perhaps due to the presence of a bubble. A larger pressurization point number would thus indicate that the piston goes farther into the stroke to attain pressure within that cylinder. A relatively smaller pressurization point number implies less compressibility, and thus a higher pressure. At a higher pressure set point (e.g., system operating pressure), the number of TICKS required to pressurize a cylinder increases with a positive co-efficient. Thus, under ideal conditions, the pressurization point will be increased with increasing system pressure.
Applicant's fuzzy logic analytically determines the pressurization point for each cylinder using data from transducer 40 and from the disk encoder 126. With reference to FIG. 3, the pressurization point is the number of ticks relative to TICK 0 (e.g., from start of overlap) to the angular position whereat the rotational speed of motor 18 is slowed. As noted, the slowed velocity will be fixed at about 50% of the intake motor velocity within the cross-over, with a maximum pressurization-point occurring where overlap ends. In flow mode, motor speed is one of two fixed values, whereas in pressure mode, as overlap is entered the motor speed is momentarily reduced to 50% of nominal flow mode speed, and the pressure servo is reset at the pressure point. In contrast to flow mode, motor speed can continuously vary during pressure mode.
Thus, fuzzy logic 150 predicts for each piston when overlap occurs, permitting mechanism 30 to regulate the speed of each piston to achieve good flow mode operation. In flow mode, output flowrate will be relatively consistent as a function of motor speed. By flow mode monitoring pressurization-points and the constant displacement for each piston, applicant's digital control system 128 provides a useful diagnostic tool. Normally, the characteristics of each piston should be within some nominal range of each other (typically a few percent), and a widely deviating detected characteristic is indicative of a piston problem. This control system diagnostic capability simply does not exist in prior art pump systems.
A portion of applicant's fuzzy logic 150 predicts when the exhaust check valve will start moving for the piston that is going into the exhaust stroke, for example valve 36A for piston 34A. This prediction is based upon count ticks of detected cam slots 170. As was noted with respect to FIG. 3, cam index 180 tick 0 represents the start of overlap. In an ideal system (e.g., non-compressible liquid, no bubbles, and no system compliance deviation), the 0 tick would consistently represent the point at which the motor speed should be halved.
For example, at 300 psi, the pressurization-point display would show a number close to tick 0, perhaps 5 or 6. But at 4,000 psi, the p points would be higher, perhaps 70. In this manner, system compliance can be reflected by the pressurization-point readings for the different piston heads, materials, seals, fluids, and so on. For example, the presence of a bubble in the system Will increase compliance, and the piston cylinder containing the bubble will exhibit higher pressurization-points than the other fluid path.
Thus, in a flow mode operation, without using pressure feedback, the present invention can compensate for variables such as fluid compressibility variations, compliance, leakage, a bubble. Not only can the present invention detect such variables, but they may be identified as to where in the system they are occurring. Further, in the case of a bubble, applicant's fuzzy logic 150 can actually approximate the bubble size, and discern the value of the pressurization-point relative to the other portion of the system.
Applicant's use of pressurization-point information permits flow mode operation of pump system 100, wherein a constant flowrate is maintained without having to use data from pressure transducer 40 for real time compensation of the speed of motor 18. Because it ignores real time pressure feedback data, flow mode operation permits pump system 100 to maintain flowrate even during sudden viscosity changes, or during analytical gradient runs. Such viscosity changes appear to have only a second effect upon flowrate, which effect is adequately compensated for by moving pressurization-points. Thus, in the preferred embodiment, flow mode is the default mode of operation.
Although flow rate mode advantageously is relatively insensitive to viscosity gradients, pressure mode under steady state conditions has the advantage of running a pump system 100 with less ripple. In applications such as ion detection or conductivity detection, the reduced pressure variation provided in pressure mode is advantageous.
To provide the best of both worlds, pump system 100 will automatically switch from flow mode to pressure mode operation whenever two conditions are met: (a) the constant displacement flow measurement time is within a threshold, preferably 3%, of the commanded flowrate, and (b) the piston being measured is at a higher pressure than the remaining piston. Condition (a) is determined mathematically by comparing the number of tick degrees and associated time to the commanded flowrate speed. Condition (b) is simply a matter of comparing pressures within each piston, since in flow mode both pistons are monitored. Typically transition from default flow mode to pressure mode occurs within about four piston strokes, which is two cam 360.degree. cycles. Applicable code appears in APPENDIX 1 at pages 43 and 44.
Once transition from flow mode to pressure mode occurs, the piston determined in flow mode to provide higher pressure (e.g., the piston associated with less leakage) is the only piston referenced with regard to the constant displacement during pressure mode. Thus, if in flow mode piston 34A was determined to have a higher pressure than piston 34B, upon entering pressure mode only the constant displacement of piston 34A will be considered, and piston 34B will be ignored. Please see APPENDIX 1, pages 45 and 46.
Thus, in stark contrast to the prior art, this discrimination between pistons permits system 100 to function properly, and compensate for the occurrence of a defective piston. Understandably, the higher pressure of the two (or more) pistons will be the piston that will better operate at the constant motor speed associated with pressure mode operation.
Assume then that the conditions for exiting flow mode have been met, and pump system 100 is now in pressure mode, with piston 34A having been determined in flow mode to be the higher pressurized piston. In pressure mode, each time the cam makes one revolution, the time required for the cam to move through the last 45.degree. (e.g., 170.sub.h to 1F0.sub.h) w2) of the intake stroke of piston 34A is measured by a counter within digital control system 128. This 45.degree. region corresponds to the arc between points A and B on came 160 in FIG. 3.
With reference to APPENDIX 1, page 44, the measured time is compared to the commanded flow time and the control system decides whether to continue in pressure mode or to change to flow mode. In the preferred embodiment, if the flowrate error exceeds 3%, flow mode operation is resumed, using parameters that were in effect when pressure mode was entered. Once back in flow mode, the preferred embodiment causes four more cam strokes to occur to re-equilibrate the pump to begin anew to meet the two conditions necessary to leave flow mode. On the other hand, if the error is less than 3%, pressure mode continues. The pressure set point is changed to maintain the proper time and flow rate.
Applicant's pressurization-point algorithm uses artificial intelligence or fuzzy logic routine, wherein instead of using a model and formulas, measurements are made of a group or an event, and are placed in one of two membership sets. A decision is then based on which membership set the information is in, the decision being where to put the pressurization-point.
In the preferred embodiment, the pressurization-point algorithm is a core element in implementing the flow mode, but is also active during pressure mode operation. The pressurization-point algorithm is denoted as AI (for artificial intelligence) in APPENDIX 1.
The summary of the AI routine appears at pages 44 to 47 of APPENDIX 1 herein. The AI routine requires five different break points or tick counts, based on the zero tick count noted in FIG. 3. With further reference to FIG. 3, while there are 1024 ticks defining the perimeter of disk 126, the tick count need only go from count 1 to 511 to be able to pick up the opposite piston. As such, count 0 can mean that either piston 34A or 34B is starting its overlap point, other means being used within the code to track the piston.
With reference to FIG. 3, the tick count is relative to tick 0 and can be for either piston 34A or 34B. There is a mechanical overlap that goes from tick count 0 to a count corresponding to 29.degree. (overlap), which is approximately count 88. However, there also exists a hydraulic overlap that represents the pressurization-point count, which can vary from 0 to 29.degree..
A portion of the code within logic 150 is directed to the process of determining at what point in the travel of cam 126 rotational speed of motor 18 should be slowed to reduce and control cross-over. Proper control of crossover reduces overlap ripple, and thus pressure and flowrate ripple. Reducing cross-over is used in both flow mode and in pressure mode operation.
In pressure mode, at higher flowrates system delays and other factors can prevent the system from responding to the pressure peak caused by the overlap. One advantage of the applicant's algorithm is that the feedback is augmented such that the cross-over ripple is reduced even at higher pump speeds and under dynamic system conditions.
Under many steady state conditions with normal pump speeds, even prior art pressure feedback can reduce pressure ripple effectively. However, under dynamic conditions, due to viscosity changes, high speed operation, rapid change in system parameters as in switching columns or during an injection, the present invention reduces cross-over ripple, whereas prior art techniques cannot.
With reference to APPENDIX 1, pages 33-52, 1FF hex represents the end of the intake stroke stoke and is next to the 0 tick shown in FIG. 3 (however, the 0 tick is associated with piston 34B, whereas 1FF is associated with piston 34A), while 1F0 hex represents tick count 88, the cross-over point for piston 34B.
In the preferred embodiment, as shown at pages 45-47 in APPENDIX 1, an average intake pressure is formed. This value is formed by averaging pressure when one piston is in intake and the other one piston is late within the exhaust stroke but before overlap (e.g., before the first piston also begins exhausting). Pressure samples are taken for each count tick and are averages on a piston as the system goes into overlap at a time period surrounding the pressurization point.
Window 1 is an overlap pressure average window, and is centered about the pressurization-point number. Window 1 begins a fixed integer m, e.g., m=8, and ends a fixed integer n, e.g., n=24, after the p-value. Thus if the pressurization-point=20, the start of the window 1 averaging will be 20-8 or 12, and the end of the window 1 averaging will be 20+24=48. As such, the width of the window is constant (e.g., 32 tick width) and window relative position, with reference to p-position, is constant (e.g., a constant offset from the p-position).
An average of pressure taken during this window 1 period is compared with an average taken within the stable, good pressure portion of the stroke wherein one piston was running smoothly (window 2). The reference window 2 sample will always be taken just before tick 0, e.g., while the new piston is still in its intake stroke and thus in a non-overlap, steady-state portion of the cam cycle.
As shown in FIGS. 4A, 4B, 4C, reference window 2 is static in the sense it always starts and ends at the same point in the cam travel. In practice, the angular width of window 2 may be from about 12.5.degree. to about 135.degree. of cam rotation, although applicant has found 12.5.degree. to be a good compromise value that tends to equalize flowrate for pressure mode and for flow mode. With respect to FIG. 3, if window 2 were 90.degree. of cam travel, it would start at 0F0 hex (TICK 240) and would end at 1F0 hex (TICK 496). More preferably, window 2 is about 12.5.degree. of cam travel, and would start at 1D0 hex (TICK 464) and extend to 1F0 hex (TICK 496). As such, reference window 2 may be fixed in size at 32 TICKS, since it is known where on the cam travel the good performance data should occur.
For reference window 2, pressure is averaged by sampling the running pressure at each TICK from TICK 1D0 hex to 1F0 hex, and then dividing the resulting sum by 32. Please see APPENDIX 1, page 49, AVTIME routine, wherein this is set up. The OVAVG routine at pages 50-51 in APPENDIX 1 executes the averaging operation.
By contrast, the overlap pressure average window 1 is centered about the predicted pressurization point as predicted by the fuzzy logic algorithm (see APPENDIX 1) using output pressure information from transducer 40. The window 1 sample will occur just past tick 0 and, as shown in FIGS. 4A-4C, may move about, in terms of TICK location, in contrast to the static reference window 2. In FIG. 4A, for example, window 1 is centered about a predicted pressurization point of 15. As shown in APPENDIX 1, page 47, in the preferred embodiment, window 1 and window 2 are each 32 TICKS wide, although the windows need not be equal in size.
With reference to APPENDIX 1, pages 46-47, the routine is shown that provides a one stroke delay to the p-point update (referred to as a "modifier" in the code listing). This routine also assigns the start and stop points for window 1. The OVFLW routine picks up the left-right flag (LRFLG), which it uses to assign the correct p-point to the appropriate piston. This is handled by controlling an address pointer in the OVFLW routine. In APPENDIX 1 at page 47, window 1 is set up, and starts 8 TICKS before the p-point value, with a start point limit of 0 established by the routine. Thus, if the p-point is 4, the start point will be 0, not -4. Similarly, the stop or end point of window 1 is set to 24 TICKS after the p-point value, with no maximum limit imposed by the routine.
Comparing these two window 1, window 2 averages, fuzzy logic 150 decides whether to retard or to advance the pressurization point to vary the rotational speed of motor 18.
With reference to FIGS. 4A, 4B, and 4C, FIGS. 4A and 4B show, respectively, a pressure dip with an associated pressurization-point number of 25, and a pressure peak with an associated pressurization-point number of 15. However, by suitable retarding or advancing, a nearly ripple-free pressure curve is arrived at in FIG. 4C with an associated pressurization-point value of 20. As noted, the pressurization-point represents the number of ticks from 0 to where motor speed is halved within the cross-over region.
In FIG. 4A, a pressure dip is shown, representing a ripple component. According to the present invention, an average pressure measurement in taken within window 1, associated with an overlap transient piston region, and within an equal-sized window 2, associated with the last part of the stable intake piston stroke. In FIG. 4A, the pressure in window 1 is too low, which implies that motor 18 is slowly down too soon. Thus, it is desired to delay the slow down of motor 18 to advance the pressurization-point (e.g., to move it rightward in the figure) to a value higher than the p=15 shown.
By contrast, FIG. 4B shows a pressure peak, wherein too high an average pressure occurs during window 1 relative to the average pressure read within window 2. The excess pressure in window 1 implies that the motor 18 is being sped-up too soon, the correction for which is to retard (e.g., move leftward in the figure) the pressurization-point from the p=25 value shown.
In FIG. 4C, motor 18 is commanded by control system 128 at approximately the correct time because there is a cancellation of a pressure peak with a corresponding pressure dip. A comparison between the average pressure readings within windows 1 and 2 is essentially even. When the comparison is essentially even, applicant's algorithm by default will advance the pressurization-point. This default advance is desired because if the system is operating at low motor speed and the feedback system can respond quickly enough to compensate and remove the ripple transients, the pressurization-point is increased to the overlap limit point, e.g., the end of overlap. This, in essence, eliminates the adjustment, since the motor speed is to be increased at end of overlap, thus cancelling the motor speed change. Eliminating the adjustment reverts to a prior art mode, namely slow speed with steady pressure.
The above-described technique wherein averages are taken between tightly coupled window 1, window 2 samples resulted from many months of experimentation wherein no alternative technique for arriving at a suitable correction under actual operating conditions was discovered. For example, attempting to use the pressure at the pressurization-point to make correction leads to difficulties under various conditions due to the delay between the actual and sensed pressure.
Further, system gain must be reasonably low to prevent oscillation when motor speed is running fast, which further complicates the p-value correction process. Even attempting to compensate by more than one tick for relatively large differences between window 1 and window 2 data was considered and rejected in that the absolute value of the window 1 peak magnitude is not readily related to the number of corrective ticks. In short, the above-described window-average, wherein the sample window 1 moves about with the pressurization-point appears to represent the best solution discovered by applicant for practical applications.
In pressure mode with the pump 100 running under steady-state conditions, pressure gain is not known. Pressure gain is the amount of real-time correction that the motor speed is commanded by control system 128 to take for a given pressure variation. Further, no adequate model is known from which pressure gain may be derived in pressure mode, apparently due to large compliance changes. The inability to predict pressure mode pressure gain precludes swapping piston heads 34A, 34B in the prior art, for example, to change from a standard pump to a microbore pump system.
In the present invention, logic 150 includes an algorithm that sets the pressure gain to an optimum value each time pressure mode is entered. The relevant code sections appear at APPENDIX 1, pages 49 to 50 (see intime routine).
According to the present invention, pressure gain is initially set to a default high level such that under most conditions, pump system 100 may begin to oscillate, especially under conditions of low pressure and low compliance. The relevant algorithm monitors pressure and checks for oscillation during the intake portion of the pump cycle. Oscillation is detected by looking for a pressure peak greater than 3% above a set point, followed by a pressure dip exceeding 3% less than the set point, the set point being determined by the pump operation. Of course, the presence of oscillation could be confirmed in a reverse sensing direction as well, e.g., a dip followed by a peak.
Oscillation monitoring is only during intake as too many changes may occur simultaneously during overlap that could generate a pressure ripple that interferes with such monitoring. Also, since both pistons are pumping, the pressure gain is different during overlap.
If oscillation is detected during intake, logic unit 150 decrements system pressure gain by a small amount. If oscillation continues to be detected during intake, the pressure gain continues to be reduced until eventually no oscillation is monitored during intake. In this manner, a highest stable system pressure gain is ascertained and dynamically maintained on the fly. By contrast, in the prior art, pressure gain was manually preset using toggle switches to program in known compensation. However the compensation known to provide suitable pressure gain when the pump system was first used did not necessarily continue to provide good compensation as the pump aged, or the piston head materials were changed.
The above-described ability to optimize pump pressure gain on the fly permits great flexibility in manufacturing pump system 100. The materials and tolerances with which the pump system is fabricated may now be changed with great leeway. Further, piston heads and head sizes, as well other components may be field replaced and logic unit 150 will continue to maintain the requisite flowrate tolerance by automatically optimizing pump pressure.
With reference to APPENDIX 1, pages 47-49, FIG. 5 depicts intake flow into the piston heads as a function of time. Using piston head 34A as an example, at time 88, exhaust check value 36A closes, intake check valve 32A opens and liquid from source 16A is drawn into the cylinder as the piston head withdraws. When the check valve 32A opens, intake flow begins and continues for a period of time until the piston starts forward, closing check value 32B and opening exhaust valve 36A at tick 0, when overlap commences. While the time duration of the intake flow can vary with a pump system configuration, the duration is typically on the order of perhaps 5 seconds at about 1 ml/minute.
FIG. 5 demonstrates, however, that there is not an instantaneous change from zero flow to full intake flow and vice versa. When the mechanical start of intake commences, intake flow will remain zero while system compliance is taken up, and after additional delay the hydraulic start of intake will commence. While the mechanical start of intake is known from the encoder disk (albeit somewhat inaccurately known), hydraulic start of intake is an unknown. At hydraulic start of intake a gradual transition up to constant intake flow occurs. At the end of intake, there will similarly be a gradual transition down to zero intake flow. These gradual transitions each represent perhaps 2% of the total intake time period (about 7 TICKS), or perhaps 0.1 seconds. The slow transition results from delay in opening the input check valves, from the hydraulics associated with the acceleration of the liquid entering the piston cylinder to reach a maximum velocity, among other factors.
It will be appreciated that during the transition time, for example at TICK X, the intake flow and thus the flowrate are changing and are unknown. However, at TICK Y, intake flow is stabilized and the flowrate will be a known constant.
It is evident that when running a gradient or a proportioned isocratic mixture, the gradual transition characteristics shown in FIG. 5 will result in poor analytical results. In a mixture, the component liquids are proportioned into the cylinders during the intake stroke. However, during the curved portion of the intake, the flowrate is unknown, which uncertainty causes a variability in the composition of the piston volume that will be pumped and provided through the output port 12. By contrast, proportioning during over later than TICK Y will take place when intake flow and thus flowrate are constant.
The prior art attempts to cope with the flowrate transition by imposing a fixed time delay from start of intake before commencing proportioning. However, the fixed time delay will work well only within a limited pressure regime, for example approximately 2,000 psi. However, when using a prior art system at another pressure, perhaps 500 psi, the fixed time delay no longer ensures that proportioning occurs during the fixed intake flow regime. Even if an assortment of toggle switches are provided to change the delay according to historical data, changes that may have been effective with a prior art pump system was new will not necessarily be effective when the pump has aged, or components have been changed.
By contrast, the present invention dynamically correctly compensates for the transitioning intake flow independently of the pressure. Since pump 100 does not include a sensor to discern start of hydraulic flow, empirical methods are provided by logic 150. The empirical data were generated over several actual chromatography runs at several pressures for each of several heads, and an appropriate compensation factor was noted as the pressure was changed.
A proper compensating factor depends upon the length of the intake hydraulic stroke, and a knowledge of when the hydraulic stroke actually starts. The intake stroke length is needed to control portioning Assume, for example, that the control system CPU commands a 75%:25% proportioning, e.g.,input from liquid source 16A for 75% of the intake cycle, and from liquid source 16B for 25%. It is therefore necessary that check valve 36B be opened for 25% of the tick length of the stable portion of the hydraulic intake cycle, and check valve 36A for 75% of the tick length. (It will be appreciated from FIG. 5 that since TICK error can accumulate from the mechanical start, that more accurate proportioning will occur by flowing the larger here 75%, proportion of the liquid first and last in the intake cycle, and the smaller, here 25%, in the center.)
With reduced pressure, the start of hydraulic flow will move leftward in FIG. 5, e.g., have a smaller number TICK assignment, and thus have a positive co-efficient. However, when pressure increases, the stroke length decreases, which gives a negative co-efficient. Applicant's algorithm causes the location of TICKS Y and Y' to move about as pressure changes, to maintain a constant intake flow regime for proportioning.
This dynamic, on the fly compensation uses data from output pressure transducer 40 for a reference. The portion of logic unit 150 providing this correction also makes use of the window 2 reference pressure described with respect to FIGS. 4A, 4B and 4C. This dual use is possible because the window 2 reference pressure information is derived at the end of the intake stroke. This information is then used to set up the end of the next intake stroke.
Thus the CPU associated with digital control system 128 issues a command for the start of each intake stroke, before which command the window 2 reference data will have already been processed. From the reference 2 average data, the intake stroke length and corresponding number of TICKS may be derived for each check valve breakpoint (e.g., when the valve is to start to move).
The preferred embodiment has been described with respect to a liquid chromatography pump system 100 manufactured by assignee Dionex Corporation, Sunnyvale, Calif., and marketed as Dionex Model DX500/GP40 gradient pump. However, those skilled in the art will recognize that the present invention may be used to control a precision pump in applications unrelated to liquid chromatography.
Those skilled in the art will appreciate that the default flow mode is implemented as a two stage servo system such as depicted in FIG. 6A, and comprises a constant displacement flow servo loop and a motor speed servo loop. With reference to FIG. 6A, flow set point data that is time sampled from the encoder disk 126 is input to the constant displacement loop, which loop is updated once per piston stroke. This first loop outputs a motor tachometer set point as input to the motor speed servo loop, which is coupled to applicant's digital control system fuzzy logic 150 (see APPENDIX 1). In flow mode, the fuzzy logic algorithm updates once per piston stroke, but delays by one piston stroke (see APPENDIX 1, page 45-47). The digital signal processing code for the operation of the overlap speed adjust routine appears in APPENDIX 1 at page 38. This portion of the routine provides adjustment for both flow and pressure modes of operation.
This inner motor speed servo loop compares the motor tachometer speed with the motor drive voltage. As has been described, the fuzzy logic sets the time at which the rotational speed of motor 18 is changed (by approximately 2:1) to vary between first and second motor speeds to accommodate overlap. The output of the second loop is the mechanical rotation of shaft 24, which is coupled to mechanism 30. Thus, acting together these two loops regulate flowrate without real-time use of pressure output data during flow mode.
Referring to APPENDIX 1, page 38, applicant's adjust routing is set forth, wherein the first few instructions take the nominal motor speed set point (e.g., the motor operating set point during intake), and multiplies this speed by a STRTSP coefficient. The product yields a new set point for motor speed operation during the actual pump overlap period. When the overlap period is completed the routine is run again to restore the set point to the intake nominal value.
By contrast, the pressure mode operation is a three stage servo system as shown by FIG. 6B. In pressure mode, the motor speed tachometer set point is input to an inner motor speed servo as was above-described. This tachometer set point is controlled by the output from a pressure servo, and the pressure set point is determined by constant displacement by a constant displacement flow servo.
The constant displacement flow servo updates once per two piston strokes, whereas the fuzzy logic algorithm updates once per piston stroke but delays one piston stroke (see APPENDIX 1, page 36-39). It will be appreciated that in pressure mode, instead of a constant motor speed (as in flow mode), the pressure set point at which the system is running controls the motor speed set point.
In pressure mode, using well known techniques, the motor speed set point is determined by a servo algorithm that includes a filter (see APPENDIX 1, page 58). In this algorithm, the error is the input and the output is the output and the algorithm operates recursively. The next time that it is run, the new error value becomes the error, whereas the old error value becomes the delayed error. Further, the new output value is the output, and the old output value becomes the delayed output. The next time after that, the old delayed values are discarded and replaced by the old values.
The result is that motor speed may vary widely as the pressure servo attempts to maintain constant pressure. In pressure mode, applicant's OVADJ routine initializes the pressure servo each time that it runs. The first two routine steps force the motor speed to the flow mode overlap or intake value, and the next two steps clear the pressure servo delayed error value to minimize transients. After this routine runs, the pressure servo is again free to change the motor speed to maintain the pressure setpoint, even during overlap.
Modifications and variations may be made to the disclosed embodiments without departing from the subject and spirit of the invention as defined by the following claims. ##SPC1##
Claims
What is claimed is:
1. A bimodal pump system for delivering a liquid at an output port with a user-commanded constant flowrate in either a flow mode or a pressure mode, comprising: a motor with a rotating shaft; a cam and encoder disk attached to said shaft; position means for determining rotational position of said cam using said encoder; a mechanism translating rotation of said shaft to reciprocating motion; a first pump cylinder containing a first piston head and characterized by a first pressurization number, switcheably coupleable to a source containing at least a portion of said liquid; said first piston head coupled to said mechanism; a second pump cylinder containing a second piston head and characterized by a second pressurization number that may equal said first pressurization number, switcheably coupleable to a source containing at least a portion of said liquid; said second pump piston coupled to said mechanism; wherein said first piston head and said second piston head are moved by said mechanism through a pump cycle having an intake stroke portion, during which said first piston head is intaking said liquid and said second piston head is exhausting said liquid, and a overlap stroke portion during which said first piston head and said second piston head are both exhausting said liquid; means for monitoring fluid pressure at said output port; control means including means for determining from said position means and from said monitoring means when during said pump cycle said overlap stroke occurs, and for reducing rotational speed of said motor during said overlap stroke relative to rotation speed during said intake stroke, said control means having a first input coupled to an output of said means for monitoring, a second input coupled to an output of said means for determining, and an output coupled to a drive input of said motor; wherein in a default flow mode, a substantially constant flowrate is maintained without using real-time output pressure information.
2. The pump system of claim 1, wherein said control means further includes means for identifying which of said first pump cylinder and said second pump cylinder is operating at higher pressure.
3. The pump system of claim 2, wherein said means for identifying so identifies using said first pressurization number and said second pressurization number.
4. The pump system of claim 2, wherein said control means further includes means for determining from said positioning means whether constant displacement flow measurement time for said first or second piston head is within a desired threshold of a said user-commanded flowrate, and for changing operation of said pump system from said flow mode to said pressure mode operation if said constant displacement flow measurement time is referenced to a higher pressure operating said first or second pump cylinder.
5. The pump system of claim 1, further including means for permitting a user to command a chosen one of said flow mode and pressure mode operation of said pump system.
6. The pump system of claim 1, wherein said control means further includes means for dynamically determining and dynamically maintaining a highest stable pressure gain within said pump system.
7. The pump system of claim 6, wherein said means for dynamically determining so establishes said highest stable pressure gain each time pressure mode is entered.
8. The pump system of claim 6, wherein said means for dynamically determining decrements pressure gain within said pump system when excessive oscillation is detected during a said intake stroke portion of a said pump cycle.
9. The pump system of claim 1, wherein said control means includes means for determining start and end of said intake stroke portion, and further includes means for causing a proportioning of said liquid to occur only between start and end of said intake stroke portion; wherein proportioning occurs during a constant flowrate.