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&#39;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 
       [0001]    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 
       [0002]    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 
       [0003]    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. 
         [0004]    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. 
         [0005]    A pump&#39;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. 
         [0006]    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&#39;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. 
         [0007]    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. 
         [0008]    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 
       [0009]    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. 
         [0010]    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&#39; 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. 
         [0011]    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&#39;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. 
         [0012]    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 
         [0013]      FIG. 1  is an exploded view of a pump assembly according to one embodiment of the invention; 
           [0014]      FIG. 2  is an exploded view of a pumping element according to one embodiment of the invention; 
           [0015]      FIG. 3  is an exploded view of the installation of the rolling element bearing and cam, according to one embodiment of the invention; 
           [0016]      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; 
           [0017]      FIG. 5  is a diagram depicting the uploading of the motor-driven pump firmware into the microprocessor, according to one embodiment of the invention; 
           [0018]      FIG. 6  is a chart depicting a flow set-up calibration procedure of one embodiment of the invention; 
           [0019]      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. 
           [0020]      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. 
           [0021]      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. 
       
    
    
       [0022]    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 
       [0023]    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 . 
         [0024]    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. 
         [0025]      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 . 
         [0026]      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. 
         [0027]    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. 
         [0028]    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. 
         [0029]      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 .       
 
         [0037]    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. 
         [0038]      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. 
         [0039]    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 . 
         [0040]    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 . 
         [0041]      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 .       
 
         [0046]    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: 
         [0000]    
       
         
           
             Life 
             := 
             
               
                 
                   
                     Max 
                      
                     
                         
                     
                      
                     N 
                   
                   - 
                   
                     Adj 
                      
                     
                         
                     
                      
                     N 
                   
                 
                 
                   
                     Max 
                      
                     
                         
                     
                      
                     N 
                   
                   - 
                   
                     Cal 
                      
                     
                         
                     
                      
                     N 
                   
                 
               
               · 
               MTBF 
             
           
         
       
     
       Where: 
       [0047]      
         [0000]                                    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.
 
         [0048]    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. 
         [0049]    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.