Abstract:
The present invention is a means and method for providing constant output flow from a fluid pump. The invention does this by controlling the radial speed of the pump motor during discreet segments of the motor&#39;s 360° angular/radial path through a revolution of the pump. The electric pump motor is controlled throughout the 360° radial path by employing a control means for controlling the speed of actuation of the radial steps of a stepper motor throughout the 360° path of rotation of the stepper motor. Control means for controlling the speed of the discreet steps of the stepper motor comprises at least a memory means, a counting means and an amplification means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a constant flow fluid pump and in particular to means and method for reduction of pressure and flow pulsations in a fluid pump by selectively controlling the rotational speed of the pump driving motor at any one of a predetermined number of discrete rotational steps around the 360° periphery of the driving motor rotation. 
     2. Description of the Related Art 
     There are many applications for analyzing blood and other fluids for which it is important to move the fluid to be examined at a uniform rate through testing/analyzing equipment, such as a flow cytometer. These fluids are usually driven by a constant pressure source. However the application of a constant pressure to a fluid may not result in a constant flow if the resistance to flow changes. For constant flow, the force pumps for driving these fluids are either of the diaphragm or reciprocating piston type of positive displacement pump that is actuated by an electric motor. 
     A problem with positive displacement pumps is that the rotary displacement of the electric motor must be converted to a linear displacement in order to activate the pump and thereby pump the fluid, i.e., both the diaphragm and the reciprocating piston are driven by a powered rod of some type that receives its linear motion by means of a reciprocating crankshaft. Whether it be a diaphragm pump or a reciprocating piston pump, the linear actuated rod must have its power converted from the rotary motion of the motor by means of a crankshaft/driving rod arrangement. It is well known that the output of a rotary motor driving a rod through a crankshaft arrangement, has a sinusoidal displacement output. The driving rod experiences displacement variations ranging from a minimum of zero at both top dead center and bottom dead center of its rotation through the crankshaft journal to a maximum displacement midway between top dead center and bottom dead center. It is also well understood in the Art that other parameters of the output pump also experience the same sinusoidal variation through the 360° rotation of the driving motor through the crankshaft/driving rod arrangement. For example, it is well-known that the pressure and the flow output of both a diaphragm and a reciprocating piston pump consist of a half-rectified sine wave. If the pump is driving a purely resistive load, the pressure and flow will be in phase and have their maximum value when the crank of the pump is in the middle of its upstroke, at 90° away from top dead center (TDC). After the pump passes TDC, the flow and pressure go to zero for a purely resistive load until the crank reaches bottom dead center (BDC). 
     Positive displacement pumps of the leadscrew drive type can provide a constant flow independent of resistance. However they must be refilled during the downstroke, during which time there is no output flow. Dual acting positive displacement pumps of the leadscrew drive type operate in tandem, so that as one pump is supplying fluid, the other pump is refilling. However these types of double acting pumps are expensive and complex. 
     A flow cytometer requires a pulseless flow of sheath fluid to obtain precise particle measurements. Present flow cytometers, in order to compensate for the pressure/flow variation described above, use one of two methods known in the Art to apply a pulseless flow of sheath fluid. The first is the use of a pressurized tank of sheath fluid that will even out the pulsations and the second is the use of a compliant member such as for example compressing a static volume of air through a flexible membrane. The problems with these two compensation methods is that the tank must have a very small height to prevent pressure variations from occurring as the tank empties and the tank must be sturdy enough to withstand pressure of 5-10 PSI, and that a constant pressure source doesn&#39;t provide a constant flow if the resistance to flow changes. Furthermore, the sheath fluid becomes saturated with air, which may be released as micro bubbles at the flow cell, causing the detection of false particles. The second method is equally problematic in the use of flow cytometry as well as other fluids analytical instruments in that the compliant member often is large and unwieldy and sometimes several compliant members are necessary to smooth pulsations in the flow of sheath fluid. Accordingly, it would be desirable to have a fluid pump driven by an electric motor through a crankshaft/driving rod arrangement that would have as close to a constant pressure and fluid output as possible through the 360° rotational driving range of motion of the electric motor. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention is a means and method for controlling the output flow of a fluid pump. The invention does this by controlling the radial speed of the pump motor during discreet segments of the motor&#39;s 360° angular/radial path through a revolution of the pump. The electric pump motor is controlled throughout the 360° radial path by employing a control means for controlling the speed of actuation of the radial steps of a stepper motor throughout the 360° path of rotation of the stepper motor. Control means for controlling the speed of the discreet steps of the stepper motor comprises at least a memory means, a counting means and an amplification means. The memory means may be an EPROM or other memory device for storing a series of numbers, each number representing selective speeds for which the stepper motor rotates to desired positions. The counting means which may be for example a binary counter is for retrieving the discreet numbers from the memory means for each selective position speed. The amplifying means, which may be for example a bipolar constant current driver, takes the output control signal from the counting means and amplifies it and conditions it so that it is suitable for energizing the stepper motor to rotate at the selected discreet speed necessary to achieve the selected discreet position along the 360° rotation of the stepper motor output shaft. 
     The motor/pump combination of the invention provides a constant or near constant flow during the upstroke of the pump. Of course, every pump outputs no flow during the downstroke time. The constant flow motor/pump of the invention compensates for this by keeping the down stoke time to {fraction (1/30)} of the upstroke time and provides a flow interruption filter to suppress this interruption of the flow during the upstroke. This flow interruption filter includes two compliant lengths of tubing separated by a resistive orifice. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention may be understood and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of the exemplary embodiments, taken with the accompanying drawings in which: 
     FIG. 1 is a graph of flow rate verses time for the prior art sinusoidal output of a rotary positive displacement pump driven by an electric motor; 
     FIG. 2 is a pictorial view of a rotary positive displacement pump driven by a stepper motor, constructed according to the teachings of the invention; 
     FIG. 2A is an enlarged top plan view of the reset sensing means with rotating flag shown in FIG. 1; 
     FIG. 3 is a block diagram of the control means for controlling the angular velocity/speed of the stepper motor of FIG. 2; 
     FIGS. 4A and 4B are a schematic view of the wiring diagram for the control means for controlling the angular velocity/speed of the stepper motor of FIG. 2; 
     FIG. 5 is a graph of the non-sinusoidal angular velocity (in half steps/second verses motor position in half steps) of the rotary positive displacement pump stepper motor constructed according to the teachings of the invention; 
     FIG. 6 is a graph of flow rate verses time of the rotary positive displacement pump driven by the stepper motor, all constructed according to the teachings of the invention; 
     FIG. 7 is a schematic drawing of the output filter apparatus that is placed in series with the output flow or the rotary positive displacement pump of the invention to minimize the flow interruption when the motor is turning the pump in the downstroke positions; and 
     FIG. 8 is a graph of flow rate verses time after the flow output from the rotary positive displacement pump of the invention passes through the filter apparatus of FIG.  7 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings and to FIG. 1 in particular there is shown a graph  10  of the prior art sinusoidal output of a rotary positive displacement pump driven by an electric motor. Both the pressure and volume output of the pump are shown as a half rectified sine wave, which graph replicates the actual output experienced by displacement pumps of the prior art. 
     Referring now to FIGS. 2 and 2A there are shown pictorial views of a rotary positive displacement pump unit  20  constructed according to the teachings of the invention. Pump unit  20  includes positive displacement pump  22 , mechanically coupled with and driven by stepper motor  24 , having mounting bracket  26  disposed there between. Stepper motor  24  is adapted for connection to outside electrical power by means of electrical connector  28 . Electrical power supplied to stepper motor  24  is conditioned by a control means of the invention so as to control the output of pump  22  to replicate as closely as possible a constant step function of both pressure and flow (volume per unit time). A full rotation of motor shaft  29  is sensed by sensing means  30 , which may be for instance an optical proximity sensor, when shaft flag  31  passes there through. 
     Referring now to FIG. 3 there is shown a block diagram  30  of the control means for controlling the angular velocity/speed of the stepper motor  24  of FIG. 2 through 400 half step output positions. Likewise referring now to FIGS. 4A and 4B there are shown a corresponding schematic view of the electronic circuit and wiring diagram for the control means for controlling the angular velocity/speed of the stepper motor of FIG.  2 . FIGS.  3  and  4 A,B have a one to one correspondence between their respective blocks in FIG.  3  and the directly corresponding individual electronic circuits shown in phantom boxes in FIGS. 4A and 4B. Block diagram  30  includes counter means  32 , memory means  34 , conversion means  36 , amplification means  38  and reset means  42 . Likewise electronic circuit and wiring diagram  50  includes counter means circuit  52  which corresponds directly with counter means block  32  of block diagram  30 ; memory means circuit  54  which corresponds directly with memory means block  34  of block diagram  30 ; conversion means circuit  56  which corresponds directly with conversion means block  36  of block diagram  30 ; amplification means circuit  58  which corresponds directly with amplification means block  38  of block diagram  30 ; and reset means circuit  62  which corresponds directly with reset means block  42  of block diagram. Of course both electronic circuit  50  and block diagram  30  control stepper motor  24  which is shown only in block diagram  30 . 
     Description/Function of Stepper Motor Control System 
     Referring now to FIGS. 3 and 4, there will be described the description/function of the individual blocks of block diagram  30  and their directly corresponding individual circuits of electronic circuit  50 . 
     Counter Means: The counter means  32  counts the number of half steps taken by the motor  24 , and sends this count to the Memory Means  34 . The counter advances 400 steps during 1 revolution, and then is reset by the Reset Means  42 . 
     Memory Means: The memory means  34  contains 400 numbers, each number representing the velocity of the motor  24  at each of the 400 steps. The specification of which of the 400 numbers to access is provided by the counter means  32  and the number is output to the conversion means  36 . 
     Conversion Means: The conversion means  36  takes the number from the memory means  34  representing the motor velocity, and loads the number into an internal conversion means counter. The internal conversion means counter then counts down to zero at a constant rate. When the internal conversion means counter reaches zero, a pulse is output to the Counter Means  32  to advance the counter means  32  by 1, and to the Amplifier Means  38  to advance the stepper motor  24  by half a step. Note that larger numbers provided to the Conversion Means  36  represent slower velocities, since the time required to count to zero is longer. 
     Amplifier Means: The pulse from the Conversion Means  36  causes the amplifier means  38  to advance the stepper motor  24  by half a step. Reset Means: Once per revolution, an optical sensor  30  which may be mounted on the stepper motor  24  sends a signal to the Counter Means  32  when flag  31  passes there through (FIGS. 2 and 2A) that resets the count. If the stepper motor  24  has missed a step during the prior revolution, the counter means will be resynchronized with the actual position of the stepper motor at this point. 
     Referring now to FIG. 5 there is shown a graph  70  of the non-sinusoidal angular velocity output of the rotary positive displacement pump  22  driven by a stepper motor  24 , constructed according to the teachings of the invention. Steps  0 - 99  are the second half of the upstroke, steps  100 - 199  are the first half of the downstroke, steps  200 - 299  are the second half of the downstroke, and steps  300 - 399  are the first half of the upstroke. To maintain a constant upstroke velocity between steps  0 - 99 , the motor&#39;s rotational velocity must become very large as it approaches the end of the upstroke. This requires a very large acceleration. The torque required to achieve this acceleration must be compared to the torque available from the motor. At some point near the end of the upstroke, the acceleration will be limited by the torque available from the motor. With moderately sized stepper motors, this limitation occurs at around step  95 . So between steps  0 - 95 , a constant flow profile is maintained, and between steps  96 - 100 , the flow profile is torque limited. 
     The downstroke time, from steps  100 - 299 , must be minimized. Therefore, to accomplish this, acceleration is maximized between steps  100 - 199 , and deceleration is maximized between steps  200 - 299 . The maximum available motor torque is determined at each step, and then the maximum acceleration or deceleration is determined. With a moderately sized stepper motor, the downstroke requires a period of about 58 milliseconds. 
     Lastly, to maintain a constant upstroke velocity between steps  300 - 399 , the motor must be rapidly decelerated as it enters the beginning of the upstroke to maintain a constant upstroke velocity. Again, the torque required to achieve this deceleration must be compared to the torque available from the motor. The deceleration is torque limited between steps  300 - 305 , and a constant flow profile is maintained between steps  306 - 399 . The process is then repeated starting with step  0 . 
     The speed of the individual steps as shown on graph  5  in half steps per second will be calculated by means of a stepper pump profile calculation. 
     Stepper Pump Profile Calculation 
     Calculate 2nd half upstroke (400 steps/revolution, so 100 steps for second half upstroke) 
     For i=0 to 99 do 
     Steps/sec=A/sin(B*i) [where A and B are constants representative of and derived from the stroke volume of the pump  72  (FIG. 2) and desired flow rate to the load  82  (FIG.  7 ), respectively] 
     Calculate torque required for next step acceleration 
     Compare to torque available from motor at this speed 
     Limit acceleration if necessary 
     Convert steps/sec to counter counts 
     Write counter counts to profile file and store in Memory Means  34   
     Calculate 1st half downstroke 
     For i=100 to 199 do 
     Calculate maximum torque available from motor at steps/sec 
     Subtract torque required to overcome friction to determine torque available for acceleration 
     Calculate maximum increase in steps/sec using torque available for acceleration 
     Calculate new steps/sec 
     Convert steps/sec to counter counts 
     Write counter counts to profile file and store in Memory Means  34   
     Calculate 2nd half downstroke 
     For i=200 to 299 do 
     Steps/sec[i]=steps/sec[400−i] for mirror image of 1st half downstroke 
     Write counter counts to profile file and store in Memory Means  34   
     Calculate 1st half upstroke 
     For i=300 to 399 do 
     Steps/sec[i]=Steps/sec[400−i] for mirror image of 2nd half upstroke 
     Write counter counts to profile file and store in Memory means  34 . 
     When the pump has been driven by the motor at the motor velocity shown in FIG. 5, the flow rate verses time of the graph shown in FIG. 6 is the resulting output. Please note the flat, constant step function portions  80  that represent the upstroke portion of the motor/pump rotation and the interruptions  82  to this constant step function  80  that represent the downstroke portions of the motor/pump rotation. The teachings of the invention endeavor to minimize these interruptions  82 , with the ideal condition of eliminating these downstroke interruptions altogether. This is accomplished according to the teachings of the invention by the use of a filtering means. 
     Referring now to FIG. 7, there is shown a schematic drawing of the output filter apparatus  100  that is placed in series with the output flow of the rotary positive displacement pump of the invention to minimize the flow interruption when the motor is turning the pump in the downstroke portion of the cycle. 
     Pump  72 &#39;s output is fed through two elastic members  74 ,  78  respectively, and a resistive member  76 , which together with load  82 , comprise filter apparatus  100 . Elastic members  74 , 78  may be for instance, 0.132 diameter Silastic tubing and resistive member  76  may be for instance, a 0.007 inch orifice.  80  represents the output flow after passing through the elastic and resistive members,  82  represents the load and  84  is atmosphere. 
     FIG. 6 shows the output of pump  72  without the benefit of Filter apparatus  100  and FIG. 8 shown the output of pump  72  with filter apparatus  100 . It is evident that the flow  80  is nearly constant (to approximately 0.2% of the flow rate&#39;s original amplitude). 
     In conclusion, what has been disclosed is means and methods for changing the analog motion of an electric motor to a digital representation both in the profile and in the actual performance, i.e., the positioning, of the electric motor by means of the motor control system of the invention coupled with a standard stepper motor that is available in the art. The flow profile shown in FIG. 6 has a constant flow during the upstroke, but no flow during the downstroke. Although the downstroke time is only {fraction (1/30)} of the upstroke time in this embodiment of the invention, this interruption of flow is not desirable in flow applications, for which the invention was developed. In order to suppress this interruption of the flow rate, compliant tubing and a resistive orifice can be added to the output of the pump. FIG. 7 illustrates the filter configuration used to suppress the interruptions of flow shown in FIG.  6 . This filter consist of two compliant lengths of tubing separated by a resistive orifice. In this particular embodiment of the invention, the flow rate decreases by only 0.2% during the downstroke, or conversely, the flow remains at 99.8% of its desires output during the downstroke, i.e. approximating the ideal conditions of eliminating the downstrokes altogether.