Micro-Sized Fluid Metering Pump

Abstract

A motor-driven fluid pump has a positive displacement rotary pumping element with an offset circular cam carried outwardly of the element, the cam being rotated with the pumping element by contact with pistons carried radially by the pumping element. Ends of the pistons are spherical and bear directly on the cam's inner surface. During breaking in of each pump, the piston ends wear a single concave groove in the inner surface of the cam, which helps to stabilize the pistons. The pump maintains a constant mass flow rate for a given input command by adjusting for fluid type, measured fluid operating temperature, and changing motor speed. The pump also maintains a constant flow output for its life by adjusting for internal wear; it also predicts its remaining life by comparing its current motor speed for a given flow against the maximum allowable motor speed.

Description

REFERENCE TO PRIOR APPLICATIONS

The benefits of priority of Provisional Application No. 61/627,291, filed Oct. 7, 2011, and of regular utility application no. 13629989, filed Sep. 28, 2012, are hereby claimed.

FIELD OF THE INVENTION

The present invention relates to pumps and pertains particularly to motor driven micro-sized fluid metering pumps. The disclosed pump can be operated at high rotary speeds, has the capability to electronically set and maintain required flow regardless of fluid temperature, and has system condition (“health”) monitoring features. It is particularly suited for use in remotely-piloted “drone” aircraft.

BACKGROUND OF THE ART

Motor driven fuel pumps have found uses in the fuel systems of internal combustion and gas turbine engines. Typically, these motor driven fuel pumps contain a rotary positive displacement pumping element, a DC motor, and an electronic motor controller.

The higher the rotary speed of the DC motor the more flow a pump can produce for a given size. The rotary speed of positive displacement pumping elements is limited by the allowable sliding velocities of the chosen pumping element material(s). Miniature high speed positive displacement pumps often use costly hardened materials for wear resistance.

A pump's electronic motor controller transmits pulse-width-modulation (PWM) signals to the DC motor to set the pump speed and thus its discharge flow. Analog electronic controllers are typically used to create PWM whose minimum and maximum duty cycles are determined by resistor values and cannot easily be adjusted to meet a range of flow requirements unless bulky potentiometers are used.

Many small vehicles, such as Unmanned Aerial Vehicles, operate in large ambient and fuel temperature ranges and also have a need to maximize their vehicle's range or mission duration. To accomplish this, their propulsion engines require a pump with good flow metering capability, and it would be desirable to maintain a constant burn flow to their engines regardless of ambient and internal fuel temperature variations.

Unmanned Aerial Vehicle manufacturers also want to reduce overall system cost and to improve system reliability and mission readiness. A pump that incorporates health monitoring features and automatically adjusts for internal wear to maximize its useful life is desirable.

Embodiments disclosed include an electric motor driven, positive displacement rotary pump that is capable of achieving approximately twice the rotary speed of state-of-the-art positive displacement rotary pumps, using less costly materials than current pumps, with the capability of meeting multiple flow requirements, using common hardware parts, maintains a constant mass flow rate, and has health monitoring and flow compensation capabilities. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a high power density, motor driven, positive displacement pump, using common hardware elements, to accommodate a wide variety of engine fuel flow requirements. The pump is capable of maintaining a constant mass flow rate. It can communicate its health information at periodic intervals during operation as well as provide specific desired information at any time by replying to a query command.

In one aspect, the invention provides a means of decreasing the weight and size of a rotary piston pump by minimizing the relative velocity and Hertzian contact stress between the pump pistons and the pump cam. The cam is attached to, and is free to spin on, a rotating element bearing that is rotated by the pistons as they contact it. This contact generates a concave groove in the cam inner surface which conforms to the pistons' spherical radius tips. By allowing the eccentric circular cam to spin with the rotor and decreasing the Hertzian contact stress between the pistons and the cam, high and variable rotational rotor speeds can be achieved without substantial wear of the cam or piston faces.

In another aspect, the invention provides a means of adjusting the motor speed to provide the minimum and maximum flows required, by modifying two variables set within the pump's microprocessor code. The pump contains a temperature sensor that measures the fluid temperature, which is fed back into the microprocessor. The microprocessor then adjusts the speed of the motor to account for fluid type, density, and viscosity so that a constant mass flow rate can be maintained for a given input command.

In another aspect, the invention provides a means of communicating the remaining life of the pump back to the vehicle and to ground control so that is can be replaced at the appropriate maintenance interval. The microprocessor monitors the output flow electrical signal against the expected flow electrical signal and adjusts the motor speed accordingly. This intelligence allows the pump to compensate for internal wear and compares the required motor speed against the maximum allowable motor speed. This ratio is then used to predict the remaining life of the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a pump assembly according to one embodiment of the invention;

FIG. 2 is an exploded view of a pumping element according to one embodiment of the invention;

FIG. 3 is an exploded view of the installation of the rolling element bearing and cam, according to one embodiment of the invention;

FIG. 4 is a plan view of a cam eccentric to a manifold and showing pistons contacting said cam, according to one embodiment of the invention;

FIG. 5 is a diagram depicting the uploading of the motor-driven pump firmware into the microprocessor, according to one embodiment of the invention;

FIG. 6 is a chart depicting a flow set-up calibration procedure of one embodiment of the invention;

FIG. 7 is a block diagram depicting the calibration procedure for the motor-driven fluid pump health monitoring system, according to one embodiment of the invention.

FIG. 8 is a block diagram depicting the motor-driven fluid pump health monitoring and flow compensation system according to an embodiment of the invention.

FIG. 9 is a sectional view through the central rotor and the cam ring, showing the concave groove formed in the cam ring by sliding action of the pistons during break-in of the pump.

While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as included within the spirit and scope of the invention, as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

An exploded view of a motor-driven fluid pump 100 according to one embodiment of the invention is shown in FIG. 1. In this embodiment the motor-driven fluid pump 100 includes three main sub-assemblies, a positive displacement pumping element 102, a driving motor 104, and an electronic control module 106. The electronic control module 106 receives an external flow demand input signal that is sent electrically to a microprocessor 108. The microprocessor 108 transmits a pulse-width-modulation signal that causes motor 104 to rotate. Motor 104 drives or rotates the positive displacement pumping element 102.

The positive displacement pumping element 102, according to an embodiment of the invention, is depicted in more detail in FIG. 2. The positive displacement pumping element 102 includes a stationary manifold 110 which consists of a fluid film bearing 120 that supports the rotor 112 as it rotates. The rotor 112 has radially oriented chambers that contain and support pistons 114 for radial movement as they rotate and traverse or engage the cam surface.

FIG. 3 shows the installation of a rolling element bearing 116 and a cam 118 according to an embodiment of the invention. Rolling element bearing 116 has a diameter 138 that fits onto and is located by manifold 110 diameter 136. The stroke of each of the pistons 114 is determined by the eccentricity between diameter 136 and that of the manifold 110 fluid film bearing 120. Cam 118 diameter 142 fits onto and is located by the rolling element bearing 116 diameter 140.

FIG. 4 illustrates a cross section depicting the rotational mechanics of the cam 118 according to an embodiment of the invention. The rotor 112 which contains the pistons 114 is rotated about the center point 144 of the manifold 110 fluid film bearing 120. The rolling element bearing 116 diameter 140 (FIG. 3), along with the cam 118, rotate about the center shown by point 146. When the rotor 112 is rotated, the pistons 114 are initially centrifugally loaded against the inner cylindrical surface 147 of cam 118. To minimize contact stress and side loading the pistons 114 have a spherical radius machined on their end surfaces that contact the cam 118 cylindrical surface 147. Due to the eccentricity between the center of the cam 118 and the center of the rotor 112, the pistons 114 stroke radially outward between 0° and 180° (inlet arc) and are pushed radially inward by the cam 118 surface 147 between 180° and 0° (discharge arc). The manifold 110 contains an inlet flow port 122 and an outlet flow port 124. As the pistons 114 move radially outwardly fluid is drawn in behind them via the inlet flow port 122. As the pistons 114 move radially inwardly fluid is expelled via the outlet flow port 124. Because the expelled fluid is usually being forced through a downstream orifice, pressure is generated in the outlet flow port 124 area. This discharge pressure creates an additional radial hydraulic force between the pistons 114 and cam 118 while traveling in the discharge arc. The centrifugal and hydraulic forces exerted by the piston 114 cause the cam 118 to rotate about its center point 146. As a result, the relative rotational surface velocity between the pistons 114 and cam 118 is kept to a minimum, whereas, in prior art pumps the cam 118 is stationary.

The material hardness of pistons 114 is higher than the material hardness of cam 118, so during breaking in of the motor-driven fluid pump 100 the spherical radius ends on pistons 114 generate a concave groove 119 into the inner surface of cam 118. When motor-driven fluid pump 100 is initially started, the Hertzian contact stress between pistons 114 and cam 118 exceeds the allowable value for the chosen cam 118 material. As a result, a concave groove 119 matching the profile of the spherical radius of the heads of the pistons 114 is generated within the inner surface of the cam 118, as in FIG. 9. As the depth of the concave groove 119 increases, the surface contact area between the pistons 114 and cam 118 increases, thereby lowering the Hertzian contact stress. Once the Hertzian contact stress reaches the allowable value of the material of cam 118, which equates to a predetermined concave groove depth, generation of the concave groove 119 stops. Pistons 114 riding in the concave groove 119 are more dynamically stable inside rotor 112 than without the groove.

The combination of a low relative velocity and a low Hertzian contact stress equates to a lower surface wear factor on pistons 114 and cam 118, which thereby increases the durability and useful life of the motor-driven fluid pump 100 as well as having the capability to obtain higher rotational speeds than prior art pumps. The end result is that the motor-driven fluid pump 100 has a higher power density than prior art micro fluid pumps, because a higher flow rate is generated for a given pump volume.

FIG. 5 depicts the motor-driven fluid pump 100 firmware code 130 being uploaded and burned to the microprocessor 108 according to an embodiment of the invention. The firmware code 130 contains:

• • a) A variable that allows selection of the motor-driven fluid pump 100 operating fluid;
  • b) A parameter that monitors the temperature of the motor-driven fluid pump 100 operating fluid;
  • c) The equation of the fluid viscosity versus temperature for the designated motor-driven fluid pump 100 operating fluid;
  • d) The equation of the fluid density versus temperature for the designated motor-driven fluid pump 100 operating fluid;
  • e) A set of variables that determine the duty cycle of the pulse-width-modulation signal being sent to the motor-driven fluid pump 100 motor 104;
  • f) An algorithm that varies the speed of motor 104 based upon the temperature of the fluid; and.
  • g) An algorithm that calculates the remaining life of the motor-driven fluid pump 100 based upon the operating speed history of the motor 104.

Vehicles such as Unmanned Aerial Vehicles need the capability to operate their engines on a multitude of fuels and over extreme temperature ranges without sacrificing performance or mission range. For any set condition, the mass flow rate of prior art motor-driven fluid pumps is not constant over varying operating temperatures and fluid types because they lack the intelligence to adjust their motor RPM for fluid density and viscosity automatically.

FIG. 6 presents a chart depicting the motor-driven fluid pump 100 flow calibration procedure according to an embodiment of the invention. With the motor-driven fluid pump 100 connected to a test stand that is capable of reading fluid flow, and the designated pumping fluid at a known temperature, the highest input command electrical signal corresponding to the maximum required flow rate is provided. Variable 132, which is set within firmware code 130, is adjusted until the RPM of motor 104 provides the required maximum flow rate. With the input command electrical signal then set to the minimum required flow rate, variable 134 located within firmware code 130 is adjusted. Once variables 132 and 134 are set, the motor-driven fluid pump 100 will maintain a constant mass flow rate for a given input command regardless of fluid temperature.

Prior art pumps do not have the flexibility to set their required minimum and maximum flow rates by simply modifying two software variables (132 and 134). Typically the PWM signal going to their motor 104 is adjusted by modifying the resistance in their electronic control module 106.

A block diagram depicting the motor-driven fluid pump 100 logic scheme used to set up the constant mass flow rate according to an embodiment of the invention is shown in FIG. 7. A temperature sensor 126 which is located within the motor-driven fluid pump 100 measures the motor-driven fluid pump 100 fluid operating temperature and transmits an electrical signal proportional to the measured temperature to the microprocessor 108.

FIG. 8 is a block diagram depicting the motor-driven fluid pump 100 health monitoring and flow compensation system according to an embodiment of the invention. A system flow or pressure sensor is required downstream of the motor-driven fluid pump 100 along with a system capability to transmit and receive signals via serial communication. The serial communication protocol resides in the motor-driven fluid pump 100 electronic control module 106 and can be an RS-232, RS-422 or RS-485 device. Once the motor-driven fluid pump 100 microprocessor 108 firmware code 130 has been uploaded and the flow versus input command signal is set as described in FIG. 6, the health monitoring and flow compensation system operates as follows;

• • a) An aircraft fluid type 8-bit serial code signal is transmitted through communication protocol to the microprocessor 108. The microprocessor 108 looks up the serial code in its firmware 130 and sets corresponding fluid density and viscosity algorithms.
  • b) The microprocessor 108 firmware code 130 monitors and compares the flow/pressure output feedback signal being transmitted against the embedded expected signal range or tolerance for the set point variable 132.
  • c) The microprocessor 108 firmware code 130 constantly monitors fluid temperature feedback signal and motor 104 rotary speed.
  • d) The microprocessor 108 firmware code 130 has embedded in it the maximum permissible speed for the set point established in variable 132.

As the positive displacement pumping element 102 components wear, internal leakage occurs between the discharge and inlet pressures, and so the output flow for a given motor 104 RPM decreases. As flow output decreases the flow/pressure sensor feedback signal will become out of tolerance of the expected signal and the microprocessor 108 will increase motor 104 RPM to move feedback signal back into the expected signal range. The microprocessor 108 firmware code 130 compares the new required motor 104 RPM against the maximum permissible motor 104 RPM and calculates the remaining life by using the equation shown below:

$\mathrm{Life}:=\frac{\mathrm{Max}N-\mathrm{Adj}N}{\mathrm{Max}N-\mathrm{Cal}N}·\mathrm{MTBF}$

Where:

Parameter	Description	Units
MaxN	Maximum permissible motor speed	RPM
CalN	Motor speed required at variable set point	RPM
	132 during calibration
AdjN	Motor speed required from pump wear	RPM
MTBF	Pump useful life	Hours
Life	Remaining pump life	Hours

When queried by the system, the remaining pump life will be transmitted to the engine system via an 8-bit serial code.

Prior art pumps do not have the capability to transmit their remaining life to the vehicle by comparing their current motor 104 speed against their maximum allowable motor 104 speed.

Many variations may be made in the invention as shown and in its manner of use without departing from the principles of the invention as described herein and/or as claimed as our invention. Minor variations will not avoid use of the invention.

Claims

1. A motor-driven fluid pump having a fixed manifold connected into a fluid flow control system which generates a flow demand signal, the pump comprising: the cam rotates on rolling element bearings carried on said manifold, and the cam is rotated with the pumping element solely by contact with outer ends of the plurality of pistons, said spherical ends bearing against the concave inner surface portion of the cam.

a positive displacement rotary pumping element driven by said motor;

a plurality of pistons carried radially within the pumping element;

a rotating cam carried radially outwardly of the pumping element and eccentrically thereto, and having a concave radius portion within a circular inner surface;

an electronic motor control module connected to receive said flow demand signal from the fluid flow control system, and wherein

2. A motor-driven pump as defined in claim 1, wherein the pump further comprises a microprocessor chip having means in said chip for receiving a remote fluid type signal from the fluid flow control system and for sensing at least one of fluid flow, fluid pressure, and fluid temperature.

3. A motor-driven pump as defined in claim 2, wherein the microprocessor chip is programmed to set the speed of the motor to provide the required fluid flow control system range for a given fluid type.

4. A motor-driven pump as defined in claim 3, wherein the fluid flow control system range is set by entering two numerical values in the microprocessor programming code, the values corresponding to motor speed set via a graphical user interface.

5. A motor-driven pump as defined in claim 4, wherein the minimum fluid flow is set by a value equating to the minimum flow input demand signal and the maximum fluid flow is set by another value equating to the maximum flow input demand signal.

6. A motor driven pump as defined in claim 1, further comprising means for transmitting a measure of the pump's remaining useful life to a fluid flow control system.

7. A motor-driven pump as defined in claim 2, wherein the microprocessor has programmed in it a means for calculating remaining useful pump life by comparing required motor speed against maximum permissible motor speed.

8. A motor-driven pump as defined in claim 1, wherein the pump delivers flow over the entire fluid flow control system range by having said electronic motor control module receiving a proportional flow demand input signal.

9. A motor-driven pump as defined in claim 1, wherein the pistons are fabricated from a higher hardness material than that of the rotating eccentric cam

10. A motor-driven pump as defined in claim 9, wherein the pistons generate a concave groove in the inner surface portion of the cam conforming to said pistons' spherical ends by physical contact between the pistons and an interior surface of the cam caused by centrifugal and hydraulic forces exerted on the pistons as the rotor spins during said motor-driven pump's break-in procedure.

11. A motor-driven pump as defined in claim 10, wherein the depth of concave groove in the inner surface portion of the cam is determined by a Hertzian contact stress between said pistons and the cam interior surface.

12. A motor driven pump as defined in claim 11, wherein outer ends of said pistons fit into and operate within said concave groove in said cam inner surface.