Electric motor driven pulsatile pressure system

Abstract

An apparatus for generating a controlled pressure pulsatile flow includes: a fluid flow path extending from a reservoir to an application; a primary pump disposed in the fluid flow path downstream of the reservoir; a reference pump; a variable speed electric motor coupled to the reference pump; a bleed orifice connected in fluid communication with the reference pump; a pressure relay device configured to communicate a reference pressure from the reference pump to the fluid flow path; and an electronic controller configured to selectively supply electric power to the electric motor so as to generate a pulsatile fluid flow in the fluid flow path.

Description

ELECTRIC MOTOR DRIVEN PULSATILE PRESSURE SYSTEM BACKGROUND

[0001] Electronic pressure regulators (EPRs) are common in industry and convert asignal (typically electrical such as current or voltage) into pressure in a fluid, typically air or inert gas. Serial signals are also now common. Also known as I / P or E / P transducers, several methods are used, such as double valve (fill / exhaust), piezo, or electromagnetic coil actuated valves. EPRs are the dominant method of generating a pilot pressure to other valves and instruments, and also have many other uses to dispense gasses or pressurize devices.

[0002] In the hydraulic industry, other methods of generating liquid pressure aretypically used. These include modulating liquid pressure from a pump via a proportional pressure valve. Because hydraulic pressure systems costs several times more than EPRs for gases, the two markets tend to be quite separate.

[0003] Disadvantages of Prior Art

[0004] EPRs obviously require a compressed gas source, which is convenient inindustrial and laboratory settings, but requires the use of additive compressors or compressed gas cylinders when automated processes are needed outside of these settings. To use a pilot-operated valve, therefore, it typically requires a pressurized gas source, the EPR, and the device to be actuated or piloted.

[0005] For applications requiring pulsatile pressure, such as a portable medicalapplication emulating the pressure of the arterial system, EPRs are a known solution when used in combination with a diaphragm back pressure regulator. However, the bulky nature of these several components, along with their cost, and other disadvantages such as noise and power consumption are a significant issue. SUMMARY

[0006] A pulsatile pressure generating system incorporates an electric driven pump tomove fluid. The speed of the pump is continuously variable according to varyingelectric power (such as DC voltage for example). A continuous bleed orifice is carefully chosen to drain excess pressure from the system, thereby providing for the maximum rate of pressure descent. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figures in which:

[0008] FIG. 1 is a schematic diagram of an apparatus for generating a controlledpressure flow;

[0009] FIG. 2 is a graph showing fluid pressure versus pump flow for varying orificesizes;

[0010] FIG. 3 is a graph showing applied pump motor voltage and a resultingpressure waveform at the application;

[0011] FIG. 4 is a graph showing applied pump motor voltage and a resultingpressure waveform at the application;

[0012] FIG. 5 is a graph showing applied pump motor voltage and a resultingpressure waveform at the application;

[0013] FIG. 6 is a schematic diagram of an alternative apparatus for generating acontrol pressure flow;

[0014] FIG. 7 is a diagram showing a control pressure system incorporating 2reference pump;

[0015] FIG. 8 is a diagram showing an apparatus for providing reversible output froma reference pump in the first configuration;

[0016] FIG. 9 is a diagram showing an apparatus for providing reversible output froma reference pump in a second configuration; and

[0017] FIG. 10 is a graph showing the additive effects of two independently operatingreference pumps. DETAILED DESCRIPTION

[0018] FIG. 1 illustrates a controlled pressure generating apparatus 10 in which anelectric driven reference pump is used to move a fluid which provides a reference or pilot pressure to a pressure regulator The pressure regulator is an example of a device which relays or communicates a pressure to a process fluid. The speed of the reference pump is continuously variable according to varying electric power. Methods of varying supplied electric power to control speed of the motor depend on the type of motor used. Examples include varying DC voltage supplied to a DC motor, using a pulse width modulated motor controller to vary the average current supply to a motor, a stepper-type motor, a motor with encoder position feedback, or varying the frequency of electric current supplied to a synchronous motor.. A continuous bleed orifice is provided to drain excess pressure from the apparatus, thereby providing for the maximum rate of pressure descent. Orifice is herein defined as any fluid component that allows the reference fluid to vent from the system, such that higher reference pressures yield a higher vent flow than lower reference pressures. Adjustable valves and lengths of tubing are other suitable devices known in the art may substitute for the orifice function.

[0019] In a preferred embodiment, air or inert gas is the reference fluid. Also, in apreferred embodiment, the pressure range is less than 1 bar. In an example which is the focus for medical arterial emulation systems, pressure is in the range of arterial pressures found in humans and animals (0-2 psig approximately).

[0020] While pulsatile performance is a key feature of this invention, the conceptsdescribed herein are applicable to both steady state and pulsatile applications.

[0021] The example system 10 of FIG. 1 includes a reservoir 12 containing a fluid"F". A primary pump 14 is located downstream of the reservoir 12. The reservoir 12 may be positioned at a higher elevation than the primary pump 14 in order to provide a gravity head for the fluid F. The pump 14 may be a nonpositive displacement type, such as a centrifugal pump, or it may be a positive displacement type, such as aperistaltic pump. If a positive displacement type pump is used, the apparatus 10 may be provided with a expansion chamber (such as a device with compliance that can expand to dampen pressure spikes, or a gas space that compresses to dampen pressure spikes) (not shown) to reduce the strength of pressure pulses generated during operation of the primary pump 14.

[0022] An application "A", for example a human or animal organ to be perfused withfluid F, is positioned downstream of the primary pump 14. An appropriate fluid flow path is defined from the reservoir 14 through the pump 14 and the application A. For example, flexible tubing may be used to interconnect the various components for this purpose. An outlet line 16 extends downstream of the application A. Fluid exiting the outlet line 16 may be discarded, collected in a container (not shown), or recirculated back into the reservoir 12.

[0023] A pilot actuated back pressure regulator 18 is provided as part of the apparatus10. In the illustrated example, The back pressure regulator 18 includes an inlet port 20 communicating with a process surface 22, an outlet port 24 communicating with the process surface 22, and a flexible diaphragm 26 disposed in a reference chamber 28. Back pressure regulator 18 is arranged such that space referred to as a "dome" 30 is formed on the side of the diaphragm 26 opposite the process surface 22. The dome 30 is in flow communication with a reference port 32.

[0024] The inlet port 20 is coupled in fluid communication with the output flow of theprimary pump 14 being supplied to application A.

[0025] In operation, the reference port 32 is supplied with a fluid in a referencepressure. When the pressure from the primary pump 14 (process pressure) is less than the reference pressure, the diaphragm 26 seals against the process surface 22, blocking flow between the inlet port 20 and the outlet port 24. When the process pressure exceeds the reference pressure, the diaphragm 26 lifts away from the process surface 22, permitting flow of process fluid from the inlet port 20 to the outlet port 24 . The regulator 18 thus diverts process fluid away from the application A. Fluidexiting the outlet port 24 may be recirculated back into the reservoir 12 as illustrated, collected in a container (not shown), or discarded.

[0026] It is noted that the relationship of the process pressure exceeding the referencepressure, resulting in venting of fluid and decrease of process pressure, may occur as a result either of increase of the pressure output by the pump 14, or by a reduction in the reference pressure.

[0027] A reference pump 34 driven by an electric motor 36 is coupled in flowcommunication with the reference port 32 of the back pressure regulator 18. The speed of the motor 36, and accordingly the flow rate of the reference pump 34, is continuously variable according to varying electric power (such as DC voltage for example). Any type of motor with controllable speed may be used, such as an AC motor, DC motor, or a stepper motor. A continuous bleed orifice 38 is provided in the line connecting the reference pump 34 to the back pressure regulator 18.

[0028] One or more sensors are provided to measure physical parameters of theapparatus 10 and generate a signal representative thereof. One option is a pressure transducer 40 configured to measure a fluid pressure in the flow path downstream of the primary pump 14 and upstream of the application A. Another option is a pressure transducer 41 configured to measure a reference pressure supplied to the regulator 18.

[0029] An electronic controller 42 is provided. This is operable to receive one ormore inputs, such as a pressure signal from pressure transducer 40, and provide variable voltage or current to the motor 36.

[0030] An explanation of the theoretical function is provided for the case of air. Insteady state cases, there is no other consumption of the air other than the bleed orifice 38 (say, in case of a pilot actuated valve application). Therefore, the pressure versus pump flow curve follows direction from the flow curve from an orifice of a fixed size. This theoretical relationship is shown in FIG.2, for two hypothetical bleed orifice diameters.

[0031] As the voltage increases to the DC motor 36, the amperage and thereforepower increases, as does the RPM of the motor 36. As the exemplary pump (discussed below) is a positive displacement diaphragm pump, the air displacement is approximately proportional to the RPM. Therefore, as the voltage increases, the RPM of the motor 36 increases, the flow rate of the pump 34 increases, and the resulting system pressure increases as provided in the chart. Decreases in the voltage follow decreases in each of the variables mentioned above.

[0032] For compressible fluids, the rate of pressure ascent and descent is alsoimpacted by the system volume. Therefore, the rate of pressure ascent is proportional to the flow of the pump rate divided by the system volume. The rate of descent is proportional to the bleed rate (at the current pressure) divided by the system volume (assuming the pump 34 can quickly be de-energized).

[0033] This apparatus can be used for controlling higher pressures as well. Acompressible gas control system can be adapted at higher pressures, such as 10 bar(g) and up to 100 bar(g), though for compressible fluids the time constant might be longer due to the compressibility of the gas (but also considering the pump capacity).

[0034] For non-compressible fluids, such as aqueous, oils, or hydraulic oils, thecontrol system can be implemented for both lower and higher pressures up to the limit of high pressure pumps (for example, 20,000 psig).

[0035] Demonstration System

[0036] A test system was constructed as a demonstration of the principles describedherein. An example of a suitable reference pump includes miniature gas diaphragm pumps (DC motors with brushes, operating typically 24VDC maximum but which rotate at much lower voltages such as 6 VDC), which are common in medical and instrumentation applications. Pumps with minimal rotational inertia are preferred where rapid changes in pressure are desired.

[0037] For the purpose of demonstration, a non-limited example of such a pump is theMicro Diaphragm Pump developed for ink-jet printing technology. Model: JYY(B)-Y-10-1. Power: 3W±5%. Flow: 100-200ml / min Uses: liquid & air. Voltage: 24V DC (max)

[0038] In the demonstration, voltages below 10VDC were used to run the pump atlower pressures, and up to about 17 VDC for the higher pressures.

[0039] In the demonstration system, a 0-150 psig differential pressure cell from E+Hwas used. A small brass metering valve was used (in place of an orifice) to create a very small bleed. A DC motor driver was used as the power amplifier to deliver power to the motor, though less than 2 watts were required for the demonstration.

[0040] FIG. 6 illustrates an alternative embodiment of a controlled pressuregenerating apparatus 110 in which an electric driven reference pump is used to move a fluid which provides a reference pressure that is overlaid on a pressure generated by a primary pump. The speed of the reference pump is continuously variable according to varying electric power (such as DC voltage for example). A continuous bleed orifice is provided to drain excess pressure from the apparatus, thereby providing for the maximum rate of pressure descent.

[0041] The example system 110 of FIG. 6 includes a reservoir 12 containing a fluid"F". A primary pump 114 is located downstream of the reservoir 12. The reservoir 12 may be positioned at a higher elevation than the primary pump 114 in order to provide a gravity head for the fluid F. The pump 114 may be a nonpositive displacement type, such as a centrifugal pump, or it may be a positive displacement type, such as a peristaltic pump (illustrated). The pump 114 is driven by a motor 115.

[0042] An application "A", for example a human or animal organ to be perfused withfluid F, is positioned downstream of the primary pump 114. An appropriate fluid flow path is defined from the reservoir 114 through the pump 114 and the application A. For example, flexible tubing may be used to interconnect the various components for this purpose. An outlet line 16 extends downstream of the application A. Fluid exiting the outlet line 16 may be discarded, collected in a container (not shown), or recirculated back into the reservoir 12.

[0043] A pressure conditioning chamber 118 is provided as part of the flow path ofthe apparatus 110. This includes a bellows 126 or other compressible member which isolates the process fluid from a reference space 128 defined within the conditioning chamber 118. The pressure conditioning chamber 118 is an example of a device which relays or communicates a pressure to a process fluid. Specifically, fluid pressure applied to the reference page 128 will be communicated through the bellows 126 to the process fluid. Note: a compressible member is defined here as a fluid containing member that can expand and contract its internal volume in order to minimize differential pressure differences between the internal and external surfaces. While a bellows is shown, one skilled in the art may design other geometries, including geometries that minimize air bubble entrapment. Suitable materials include medically suitable elastomers (such as silicone) and polymers (such as polyolefins).

[0044] A reference pump 34 driven by an electric motor 36 is coupled in flowcommunication with the reference space 128 of the pressure conditioning chamber 118. The speed of the motor 36, and accordingly the flow rate of the reference pump 34, is continuously variable according to varying electric power (such as DC voltage for example). A continuous bleed orifice 38 is provided in the line connecting the reference pump 34 to the back pressure regulator 18.

[0045] One or more sensors are provided to measure physical parameters of theapparatus 10 and generate a signal representative thereof. One option is a pressure transducer 40 configured to measure a fluid pressure in the flow path downstream of the primary pump 14 and upstream of the application A. Another option is a pressure transducer 41 configured to measure a reference pressure supplied to the pressure conditioning chamber 118.

[0046] An electronic controller 42 is provided. This is operable to receive one ormore inputs, such as a pressure signal from pressure transducer 40 _, and provide variable voltage or current to the motor 36, and to provide variable voltage or current to the motor 115.

[0047] Control Strategies

[0048] In one embodiment, a pressure control system can include a pressure sensor inthe fluid loop, shown as 41. A high speed closed loop controller, such as PID, can be used to provide excellent control of pressure. In this sense, the pump / bleed / sensor combination can be substituted for a compressor / EPR combination directly (just as an EPR is normally fitted with an internal pressure sensor and a high-speed PID loop).

[0049] In another example, this system can be used to pilot a valve on a process fluidin the fluid flowpath, such that feedback may be received from the process pressure sensor 40 or flow (or other variable such as temperature). In this way, the cost of the local pressure sensor on the pump / bleed system can be eliminated.

[0050] In another preferred embodiment, both a local pressure sensor (41) and a moreremote system pressure sensor (40) could be present, with a cascade or other PID control strategy utilized to provide both very high speed local control (of 41 to desired pressure) and also highly responsive system pressure readings remote at the process application (40).

[0051] PID control strategies excel when working with steady state or continuouslyvariable setpoints. However, for repeating waveforms such as medical arterial applications, PID is not the only approach. Some non-limiting examples of control strategies are provided to illustrate the invention.

[0052] Simple PID Control

[0053] When a pressure waveform is desired, similar to that of an arterial pressurewaveform, the desired waveform can be supplied as the Set Point (SP) of the PID loop, a pressure sensor can be supplied as the Process Value (PV) of the loop, and the Control Output of the PID loop (CO) drives the motor speed control. The tuning paraments can be adjusted to provide for acceptable control even without any custom algorithm tweaks.

[0054] Referring to FIG. 3, a simple PID is implemented as follows. The solid line isthe Set Point (0-100% or arbitrary units). The resulting pressure is provided on thedashed line (0-1 psig). The relative voltage supplied to the motor is represented by the dash-dot line (where 0-1 is proportional to approximately 0-16 VDC).

[0055] Optimum tuning results from having a very high integral gain value inaddition to a significant proportional value.

[0056] Simple Waveform Adaptation

[0057] In FIG. 4, a simple square wave pattern of voltage is provided (dashed) andthe resulting pressure is shown in the solid line. A time delay and a significant rounding of the rising waveform can be observed, which are artifacts of delays from digital-to-analog and analog-to-digital conversion, the delay of spinning up the inertia of the motor, and finally the delay of compressing the volume of the air volume. In FIG.5, an example of a simple waveform adaptation is provided. To overcome some of the aforementioned delays, the voltage waveform is exaggerated at the beginning with a time decay (dashed). The resulting pressure waveform (solid line) is a bit more symmetrical.

[0058] One skilled in the art could make numerous adaptations to the curve shape tofurther optimize the resulting waveform. Further, by optimizing the selection pump (capacity), power supply, system volume, and bleed orifice further, it would be readily possible to make sharper waveforms or to extend the pressures to much higher levels. Note: these waveforms are arbitrarily selected to demonstrate the capability of the system, and are not necessarily waveforms selected by a cardiologist to provide optimum perfusion, though one skilled in the art could easily adapt these to meet the physiological requirements (Mean Arterial Pressure, Pulse Pressure, etc.) of actual organs.

[0059] Combination Of PID And Waveform Adaptation

[0060] A simple waveform pattern repeating constantly would not provide the neededcontrol for a medical application such as an organ arterial emulation. For example, over time many variables drift, changing the organ resistance and capacitance to flow, supply reservoir levels change, tubing arrangements are changed (elevation, canbecome crimped), and even tubing lengths and diameters can change from procedure to procedure. Therefore, a highly preferred embodiment consists of a combination of waveform adaptation and a long-term monitoring algorithm (such as or similar to a PID). Details follow.

[0061] In the aforementioned example, a waveform is selected that provides thegeneral waveform shape that is desired for the application. Such as the dashed line in FIG.5 with the decaying upper value. The system monitors the resulting maximum and minimum pressures in the waveform (FIG.5, solid line) and compares these values with the target max and minimum values. A simple correction algorithm can be provided which scales the selected waveforms so that the max and min remain close to their target values, even as longer term shifts continue (such as mentioned above, changes in supply reservoir level or system resistance). In a more specific example, a simple PID could be tuned where each iteration of the pulse wave initiates a calculation based on the max pressure (PV) compared with the desired max (SP). The tuner of the system can provide for more or less damping (i.e. to correct over a number of cycle trends, or respond quickly to the last few cycles). The same can be repeated for the minimum, whereby the waveform is scaled according to simply algebraic formulas according to the longer-term adaptation. Such a control loop is unique from other control loops in that it is aware of the cyclic nature and filters out the max and min variables separately for scaling treatment. Other variables could be controlled in a similar manner. For example, the rate of pressure ascent or descent could be controlled, especially considering an example where the rates are less sudden as provided in the patterns above. (Note: many arterial emulations require much more rounded and stable pressures, especially those organs that are supplied by arteries and even veins further from the defined pulses of the heart). The defined waveform can utilize sloped upward and downward sections and utilize exponential decays instead of linear segments. These and other variables can be controlled using feedback loops as described in the preceding paragraph if desired.

[0062] Other Control Methods

[0063] Many other control strategies are known in the process industries and can beused to control the inventive schematic of this invention. Further, a supervisory control function may be provided by a Machine Learning algorithm or a neural network or other type of learning AI.

[0064] Advantages

[0065] The apparatus described herein provides a simple pulsatile pressure generatingsystem for fluids that consumes very low energy, is quiet, and is capable of generating various pressure waveforms.

[0066] Optionally, the addition of further sources of pressure, e.g., pumps may beemployed in controlling the desired waveform by overlapping summative waveforms with constructive or destructive manners. One example of each constructive or destructive addition may be using two pumps to build pressure faster than one pump alone or using the opposing side of the pump with negative flow to reduce pressure quickly. The usage of two or more pumps / sources of pressure may be used in an overlapping fashion to yield the desired waveform by methods such as two pumps with peak ramp speeds being offset to produce a notch in the waveform. FIG.10 shows an example of this effect, where the lower dashed line represents pressure versus time of the first pump output, the lower solid line represents pressure versus time of a second pump output, and the upper solid line represents the summed output of both pumps.

[0067] FIG. 7 illustrates an example where two reference pumps 34, each driven byits own motor 36, are coupled in the flowpath with a pressure relay device as described above, labeled 218 generically. Each pump 34 may be controlled independently by controller 42. This arrangement may be substituted for either of the single pump configurations shown in FIG.1 or FIG.6.

[0068] Any of the reference pumps described herein may be selectively configured tosupply positive pressure or negative pressure (suction / vacuum). One means by which this may be accomplished is illustrated in FIGS.8 and 9. The pump 34 produces a one-way flow from its inlet 33 to its outlet 35, as a consequence of its internalvalving. In the configuration shown in FIG.8, the inlet 33 and outlet 35 are connected to ports of a four-port rotary valve with “double L” configuration 250. When the valve 250 is in the position shown in FIG.8, the inlet 33 is connected in flow communication with the pressure relay device 218 and the outlet 35 is connected to a vent (e.g. atmosphere). Operation of the reference pump 34 with the valve 250 in this position will lower the pressure in the pressure relay device 250.

[0069] When the valve 250 is in the position shown in FIG. 9, the inlet 33 isconnected in flow communication with the vent (e.g. atmosphere) and the outlet 35 is connected to the pressure relay device 218. Operation of the reference pump 34 with the valve 250 in this position will increase the pressure in the pressure relay device 218. The valve 250 may be operated by a solenoid or other suitable actuator (not shown), driven by the controller 42.

[0070] Optionally, the system described herein may implement multiple applicationsof independent waveform controls via a spanning algorithm. One further application is the use of an algorithm with commands and control to articulate several channels such as an arterial and venous ends of an organ during perfusion. An algorithm may be employed if the effects from one channel overlap into the waveform of another to give rise to controls that give multiple independent controls for multiple channels. The control may implement controllable time and pressure domains, incorporating additional features such as customizable beats per minute for a physiological setup.

[0071] The foregoing has described a pulsatile pump apparatus. All of the featuresdisclosed in this specification, and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive.

[0072] Each feature disclosed in this specification may be replaced by alternativefeatures serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0073] The invention is not restricted to the details of the foregoing embodiment(s).The invention extends, or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

WHAT IS CLAIMED IS:

1. An apparatus for generating a controlled pressure pulsatile flow, comprising: a fluid flow path extending from a reservoir to an application; a primary pump disposed in the fluid flow path downstream of the reservoir; a reference pump; a variable speed electric motor coupled to the reference pump; a bleed orifice connected in fluid communication with the reference pump; a pressure relay device configured to communicate a reference pressure from the reference pump to the fluid flow path; and an electronic controller configured to selectively supply electric power to the electric motor so as to generate a pulsatile fluid flow in the fluid flow path.

2. The apparatus of claim 1, wherein the pressure relay device comprises a diaphragm pressure regulator, and the reference pump is connected to a reference port of the diaphragm pressure regulator.

3. The apparatus of claim 2, wherein the diaphragm pressure regulator is positioned upstream of the application.

4. The apparatus of claim 3, wherein the diaphragm pressure regulator is configured to divert fluid flow away from the application.

5. The apparatus of claim 4, wherein the primary pump is a peristaltic pump.

6. The apparatus of claim 1, further comprising a fluid pressure sensor configured to sense a pressure between the primary pump and the application and to provide a signal to the electronic controller.

7. The apparatus of claim 3, further comprising a fluid pressure sensor configured to sense the reference pressure r and to provide a signal to the electroniccontroller.

8. The apparatus of claim 1, where the electronic controller is configured in a high speed control loop which adapts the electric power in real time as compared with the error between desired and actual pressure.

9. The apparatus of claim 6, wherein the high speed control loop is a PID loop.

10. The apparatus of claim 1, wherein the reference pump is a diaphragm pump.

11. The apparatus of claim 1, wherein the pressure relay device comprises a pressure conditioning chamber including a compressible member, and the reference pump is connected to a reference space of the pressure conditioning chamber.

12. The apparatus of claim 1, wherein: two or more reference pumps are provided; each of the reference pumps are connected in fluid communication with the pressure relay device; and the electronic controller is configured to independently control the 2 or more reference pumps.

13. The apparatus of claim 12 wherein at least one of the reference pumps is coupled to a valve movable between a first position in which an outlet of the reference pump is coupled to the pressure relay device, and a second position in which an inlet of the reference pump is coupled to the pressure relief device.

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