Abstract:
A liquid dispensing system increases the precision, reliability, and safety of dispensing medications and other liquids. An electronic brake grounds a power terminal of a pump motor to absorb the kinetic energy of the motor and other mechanically coupled components. The electronic brake reduces overage and overage variation in the dispensing of liquids at very little cost or complexity. A watchdog circuit monitors a controller heartbeat signal and disables the motor in the absence of a regular beat. The watchdog circuit along with redundant power switches greatly reduce the possibility of motor runaway in the event of component failure. A controller begins or ends each dispensing cycle with a diagnostic which determines whether the liquid dispensing system is fully operational. If not, the controller may disable the motor, emit an alarm tone, and display an alarm message.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates generally to pumping control systems. More specifically, the invention relates to systems for controlling dispensing of medications to more accurately control dispensing of medication. 
     2. The Relevant Art 
     Medical science often requires that liquids be administered to a patient in a variety of situations. These liquids include simple intravenous feeding solutions, saline solutions for providing pressure to the eye during ocular surgery, contrast media infused to enhance imaging abilities, blood administered during transfusions, and nutrient solutions, medications, chemotherapy solutions, or the like delivered via intervenal or enteral means. In virtually all these applications, reliable and precise delivery of liquids is critical to successful treatment of the patient. In some applications, improper delivery of the liquids such as overrun may be life threatening to the patient. 
     Systems that are reliable and precise are often costly to design, manufacture and test. Indeed, the relative cost of medical care continues to increase often due to the increased cost of medical technology. Accordingly, within the health care industry there is a high need for low-complexity technology that is effective and reliable. 
     Overrun is particularly dangerous in medical applications in that even when detected, excessive delivery of a liquid often cannot be reversed. Overrun may be caused by hardware failures such as electronic switches being stuck in a certain state. Controller failures are also a problem in that processors are particular sensitive to environmental factors such as temperature and static. The firmware associated with a controller may contain logic errors or bad memory cells resulting in runaway programs that may “crash”. Runaway or crashing programs may leave control circuitry stuck in a certain state such as in a pumping state where liquid is being delivered at a high rate. 
     In addition to reliability to prevent overrun and other errors associated with component failure, liquid dispensing systems need to be precise. Precise delivery facilitates the adjustment of dosages, to rates and levels that are optimum for treatment of the patient. Precision also facilitates consistency over time and between various devices and systems, a highly desirable feature in dosage systems. 
     Stepper motors are often used in applications that require precision. Unfortunately stepper motors require complex control signals that must be properly phased to advance the motor. Stepper motors are also often costly and difficult to test. 
     Standard motors such as induction motors are typically low cost but suffer from lack of precision in that motors often freewheel after power is cut off. Freewheeling results in liquid overage and overage variability in that the duration and speed of freewheeling is affected by environmental and usage factors such as temperature and motor speed. 
     Accordingly, what is needed is a liquid dispensing control system for standard motors that is low-cost, and reliable, and that eliminates freewheeling and prevents overrun in the event of system failures. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available liquid dispensing systems. Accordingly, it is an overall object of the present invention to provide an improved method, apparatus and system for dispensing liquids that overcomes many or all of the above-discussed shortcomings in the art. 
     In particular, it is an object of the present invention to provide a control system for a liquid dispensing system that is low-cost, reliable, eliminates or reduces freewheeling, and substantially prevents overrun in the event of system failures. 
     These and other objects of the invention are realized in a control system for a liquid dispensing system which includes an electronic brake to stop motor freewheeling, power and ground switches to reliably control the motor, and a watchdog circuit to monitor the controller and disable the motor in the event of system failures. 
     In accordance with a first aspect of the invention, the electronic brake grounds a power terminal of the motor in response to a stop signal. Grounding the power terminal creates a braking effect as kinetic energy from mechanical inertia is converted to electromagnetic energy within the windings of the motor. The brake in turn absorbs the electromagnetic energy to greatly reduce or eliminate freewheeling. In essence, the motor momentarily acts as a generator allowing the brake to quickly absorb and stop the kinetic energy present in the dosage delivery system. 
     In accordance with a second aspect of the invention, redundant switches, one to connect to power and the other to connect to ground, ensure that the failure of a switch does not result in the motor being stuck in a running state. 
     In accordance with a third aspect of the invention, a watchdog circuit monitors a controller heartbeat signal to ensure that the controller is reliably providing a valid ‘beat’ at an acceptable rate. If so, the watchdog asserts an enable signal which activates one of the switches that control power to the motor. If the required conditions are not met, the watchdog de-asserts the enable signal placing a switch in a non-conductive state, thus disabling the motor and preventing the possibility of overrun. 
     In accordance with a fourth aspect of the invention, a controller starts or ends each dispensing cycle with a diagnostic which determines whether the liquid dispensing system is fully operational. If not, the controller may perform shutdown operations such as disabling the motor along with error indication operations such as emitting an alarm tone and displaying an alarm message. 
     To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein in the preferred embodiments, an apparatus, method and system for delivery of liquids is described that is reliable, precise and low-cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the manner in which the advantages and objects of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a block diagram of a dosage delivery system in accordance with the present invention; 
     FIG. 2 a  is a block diagram highlighting control circuitry associated with the motor in accordance with the present invention; 
     FIG. 2 b  is a schematic diagram showing a first embodiment of a brake in accordance with the present invention; 
     FIG. 2 c  is a schematic diagram showing a second embodiment of a brake in accordance with the present invention; 
     FIG. 2 d  is a schematic diagram showing a third embodiment of a brake in accordance with the present invention; 
     FIG. 3 a  is a schematic diagram showing one embodiment of a power switch in accordance with the present invention; 
     FIG. 3 b  is a schematic diagram showing a first embodiment of a ground switch in accordance with the present invention; 
     FIG. 3 c  is a schematic diagram showing a second embodiment of a ground switch in accordance with the present invention; 
     FIG. 4 is a schematic diagram and associated timing diagram depicting one embodiment of a watchdog in accordance with the present invention; 
     FIG. 5 a  is a flow chart of a liquid dispensing method in accordance with the dosage delivery system of the present invention; 
     FIG. 5 b  is a flow chart of a diagnostic method in accordance with the dosage delivery system of the present invention; and 
     FIG. 5 c  is a flow chart of a stop method in accordance with the dosage delivery system of present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. Additionally, it should be understood that not all embodiments made in accordance with aspects of the present invention will necessarily achieve all objects of the invention. 
     FIG. 1 shows a dosage delivery system  100  of the present invention. In the depicted embodiment the dosage delivery system  100  includes a motor  102 , a pump  104 , and a brake  106 . The motor  102  is mechanically coupled to the pump  104  via a coupler  108 . In one embodiment, the coupler  108  is a screw-type drive shaft tangential to an eccentric gear of the pump  104 . In the preferred embodiment, the motor  102  is a DC motor, the pump  104  is a peristaltic pump and the coupler  108  provides a very low gear ratio to increase precision. A peristaltic pump is preferred in that it facilitates precise delivery of liquids at a low cost. 
     The motor  102  drives the pump  104  which in turn pumps a liquid  109  from a reservoir  110  to a delivery device  112 . The liquid  109  may travel through a supply tube  114  to the pump  104  and further through a delivery tube  116  to the delivery device  112 . In certain embodiments, for example with some peristaltic pumps, the supply tube  114  and the delivery tube  116  are the same tube. 
     In the depicted embodiment, the motor  102  is electrically connected to a power terminal  118  and a ground terminal  120 . The power terminal  118  and the ground terminal  120  provide a DC voltage to the motor  102  as controlled by a power switch  122  and a ground switch  124 . When the power switch  122  and the ground switch  124  are both in a conductive state, DC power is supplied to the motor  102  from a power bus  126  and a ground bus  128 . The motor  102  drives the pump  104 . When either switch is in a non-conductive state no power is supplied to the motor  102 . Requiring both switches to be in a conductive state prevents the possibility of runaway pumping in the event of the failure of a switch. 
     Due to a variety of factors, removing the supply of power to the motor  102  may not result in immediate stoppage of the pump  104  resulting in overage in the delivery of the liquid  109 . For example, the motor  102  may have inductive energy stored within various windings. The motor  102 , the pump  104 , and the coupler  108  may have some mechanical inertia. The amount of overage may be inconsistent and dependent on a variety of electrical, mechanical and environmental factors that vary from system to system and are highly dependent upon usage patterns. 
     To address the problem of overage and inconsistent overage, the brake  106  of the present invention more quickly stops the motor  102  resulting in greater delivery precision. In the depicted embodiment, the brake  106  grounds the power terminal  118 . Grounding the power terminal  118  removes any electromagnetic energy present within the windings of the motor  102 . More particularly, grounding the power terminal  118  creates a braking effect as mechanical kinetic energy from inertia is converted to electromagnetic energy within the windings of the motor  102  which is absorbed by the brake  106 . In essence, the motor  102  momentarily acts as a generator allowing the brake  106  to quickly absorb and stop the kinetic energy present in the dosage delivery system  100 . 
     In the depicted embodiment, the brake  106  receives a stop signal  130  and a enable signal  132 . In several of the Figures, the ‘STOP!’ label corresponding to the stop signal  130  is appended with an exclamation point indicating that the signal is asserted with a low voltage rather than a high voltage. In addition to the brake  106 , the stop signal  130  is also received by the power switch  122 , while the enable signal  132  is also received by the ground switch  124 . Using separate signals to control the power switch  122  and the ground switch  124  increases the reliability of the present invention. In the depicted embodiment, the stop signal  130  is provided by a controller  134  while a watchdog  136  provides the enable signal  132 . 
     As depicted in FIG. 1, the controller  134  provides a heartbeat signal  140  to the watchdog  136 . The heartbeat signal indicates that the controller is in a healthy active state. The watchdog  136  monitors the heartbeat signal  140  to ensure that the controller  134  is reliably providing a valid ‘beat’ at an acceptable rate. If so, the watchdog asserts the enable signal  132 . If the required conditions are not met, the watchdog  136  de-asserts the enable signal  132  placing the ground switch  124  in a non-conductive state, thus disabling the motor  102  and the pump  104 . 
     In addition to providing the stop signal  130  and the heartbeat signal  140 , the controller  134  receives signals from a number of sensors  142 . The sensors  142 , include for example pressure sensors, temperature sensors, motor sensors and the like. The sensors  142  provide the controller  134  with information that is useful and essential to ensure that the dosage delivery system  100  is operating accurately and reliably. In the preferred embodiment, the sensors  142  include a dosage sensor that provides information on rotational movement in the motor  102  or the pump  104 . The dosage sensor may be an emitter-detector pair aligned perpendicular to a rotational surface with alternating dark and light regions. 
     The controller  134  may also provide signals to control a visual indicator  143 , such as an LCD display, and an audio indicator  144 , for example a speaker. The visual indicator  143  and the audio indicator  144  facilitate the rendering of information such as battery levels, fluid levels, flow rates, error conditions, alarms and the like. 
     FIGS. 2 a - 2   d  contain several schematic block diagrams of control circuitry associated with the motor  102 . As depicted in FIG. 2 a , the brake  106  is essentially a grounding switch which is activated under control of the stop signal  130  and the enable signal  132 . The stop signal  130  and the enable signal  132  also control the power switch  122  and the ground switch  124 . In the preferred embodiment, the brake  106  need only be activated when the enable signal  132  is asserted (indicating that the motor  102  is potentially on), followed by assertion of the stop signal  130 . In this scenario, asserting the stop signal places the power switch  122  in a non-conductive state, thereby cutting off power to the motor  102 . Furthermore, the brake  106  is placed in a conductive state thereby absorbing the kinetic energy of the mechanical components as previously described. 
     FIGS. 2 b ,  2   c , and  2   d  show various embodiments of the brake  106  that are placed in a conductive state when both the enable signal  132  and the stop signal  130  are asserted. Referring to FIG. 2 b , a pair of isolation resistors  202   a  and  202   b  isolate the enable signal  132  and the stop signal  130  from circuitry internal to the brake  106 . The isolation resistor  202   a  allows an asserted enable signal  132  to provide current to either an anding transistor  204  or a darlington pair  206  depending on the state of the stop signal  130 . When the stop signal  130  is not asserted, the anding transistor  204  absorbs any current provided by the asserted enable signal  132  thus turning off the darlington pair  206  and placing the brake  106  in a non-conductive state. 
     When the enable signal  132  is not asserted (i.e. at a low voltage), current through the isolation resistor  202   a  pulls the input the transistor  206   a  toward ground resulting in little or no current being drawn from the power terminal  118 . Likewise, when the stop signal is not asserted (i.e. at a high voltage) the anding transistor  204  is turned on which also results in the input of the transistor  206   a  being pulled toward ground. The combination of the anding transistor  204  and the isolation resistor  202   a  effectively create an ‘AND’ gate. Only by asserting both the enable signal  132  and the stop signal  130  simultaneously will the brake  106 , as depicted in FIG. 2 a , absorb current (other than leakage current) from the power terminal  118 . 
     The transistors  206   a  and  206   b  that comprise the darlington pair  206  facilitate a potentially large current draw on the power terminal  118 . A large current draw allows the brake  106  to absorb considerable energy from the power terminal  118  and therefore the motor  102 . A leakage resistor  208  absorbs any leakage current from the transistor  206   a.    
     Referring to FIGS. 2 c  and  2   d , alternative embodiments of the brake  106  replace the darlington pair  206  and the leakage resistor  208  with a single transistor  210  or a FET  212 . Otherwise, the embodiments depicted in FIGS. 2 c  and  2   d  are identical to the embodiment depicted in FIG. 2 b . Using the single transistor  210  may be preferred in situations where little current need be absorbed from the power terminal  118  and the motor  102 . The FET  212  may be desirable in applications requiring the brake  106  to provide good isolation (i.e. a very low leakage current) when in a non-conductive state. FIGS. 3 a - 3   c  show several schematic block diagrams which focus on the power switch  122  and the ground switch  124 . Referring to FIG. 3 a , one embodiment of the power switch  122  includes an isolation resistor  302 , an inverting transistor  304 , a pull-up resistor  306  and a FET  308 . The various components of the power switch  122  work together to place the power switch  122  in a conductive or non-conductive state depending on whether the stop signal  130  is asserted. 
     When the stop signal  130  is asserted (i.e. at a low voltage), the isolation resistor  302  is pulled toward ground thus turning off the inverting transistor  304 . In this condition, the pull-up resistor  306  raises the voltage on the gate of the FET  308 . In the preferred embodiment, the FET  308  is a p-channel MOSFET and a high voltage places the FET  308  in a non-conductive state thus cutting off power to the power terminal  118  and disabling the motor  102 . 
     In those situations in which the stop signal  130  is not asserted (i.e. at a high voltage), the isolation resistor  302  pulls the gate of the inverting transistor  304  toward power causing the inverting transistor  304  to turn on. In this condition, the inverting transistor  304  pulls the voltage on the gate of the FET  308  toward ground. In the preferred embodiment, the FET  308  is a p-channel MOSFET and a low voltage places the FET  308  in a conductive state thus providing power from the power bus  126  to the power terminal  118 , and thereby the motor  102 . 
     Referring to FIG. 3 b , a first embodiment of the ground switch  124  is comprised solely by a FET  310  which is preferably an n-channel MOSFET. When the enable signal  132  is asserted (i.e. at a high voltage) the FET  310  is placed in a conductive state. In a conductive state, the ground terminal  120  and the ground bus  128  are electrically tied together allowing a return or grounding path for current in the motor  102 . In contrast, when the enable signal  132  is not asserted (i.e. at a low voltage) the FET  310  is placed in a non-conductive state, the ground terminal  120  and the ground bus  128  are electrically isolated resulting in no return or grounding path for current in the motor  102 . As used within the dosage delivery system  100  depicted in FIG. 1, not asserting the enable signal  132  effectively disables the motor  102 . 
     Referring to FIG. 3 c , a second embodiment of the ground switch  124  similar to FIG. 3 b , includes a transistor  312  and an isolation resistor  313 . When the enable signal  132  is asserted (i.e. at a high voltage), the transistor  312  is turned on allowing current to flow from the ground terminal  120  to the ground bus  128 . When the enable signal  132  is not asserted (i.e. at a low voltage) the transistor  312  is turned off thereby electrically isolating the ground terminal  120  and the ground bus  128 . 
     Referring to FIG. 4, the watchdog  136  may include a pair of timers  402   a  and  402   b . The timers  402   a  and  402   b  detect whether the heartbeat signal is reliably providing a valid ‘beat’ at an acceptable rate. As depicted, the timers  402   a  and  402   b  are essentially RS flip-flops that are reset when the voltage on the threshold input is greater than a threshold voltage  403   a  and set when the voltage on the trigger input is less than a trigger voltage  403   b . In one embodiment, the timers  402   a  and  402   b  are  555  timers, the threshold voltage  403   a  is ⅔rds the supply voltage, and the trigger voltage  403   b  is ⅓rd the supply voltage. The timer  402   a  detects a valid beat from the heartbeat signal  140 , while the timer  402   b  ensures that the detected beats occur at an acceptable rate. Timing is controlled by RC circuits external to the timers  402   a  and  402   b.    
     A rising edge of the heartbeat signal  140  is passed by a high pass filter consisting of a high pass capacitor  404  and a discharging resistor  406 . The rising edge produces a voltage on an input  408  sufficient enough to reset the output of the timer  402   a  and thereby provide a beat detected signal  410  (the beat detected signal  410  is asserted with a low voltage). The discharging resistor  406  eventually bleeds off the input  408  resulting in de-assertion of the beat detected signal  410 . A diode  412  provides input protection to the timer  402   a.    
     The beat detected signal  410  is tied to the trigger input of the timer  402   b . Asserting the beat detected signal  410  sets the output of the timer  402   b  thereby asserting the enable signal  132 . In addition to asserting the enable signal  132 , timer  402   b  discontinues discharging a timing capacitor  416 . The enable signal  132  remains set until current from a charging resistor  414  charges a timing signal  418  beyond a certain threshold which in one embodiment is ⅔rds the supply voltage. 
     Continuing to refer to FIG. 4, the timing capacitor  416  and the charging resistor  414  determine the length of a heartbeat window  420 . As long as the beat detected signal  410  is asserted within the heartbeat window  420 , the enable signal  132  will remain asserted. The enable signal  132  is used within the dosage delivery system  100  depicted in FIG. 1 to enable and disable the motor  102  via the ground switch  124 . As depicted in FIG. 1, the enable signal  132  also enables the brake  106 . 
     Referring to FIG. 5 a , a liquid dispensing method  500  of the present invention is shown that may be performed in conjunction with the dosage delivery system  100 . The liquid dispensing method  500  starts  502 , and proceeds immediately to a diagnostic  504 . In one embodiment, the diagnostic  504  ascertains whether the dosage delivery system  100  is functioning properly. If not, the method proceeds to a halt  506 . 
     In conducting the halt  506 , the liquid dispensing method  500  may perform shutdown operations such as disabling the motor  102 , as well as error indication operations such as emitting an alarm tone through the audio indicator  144  and displaying an alarm message through the visual indicator  143 . If the diagnostic determines that the system is functioning properly, the liquid dispensing method  500  continues to an activate  508 . 
     The activate  508 , activates the dispensing of liquid. In one embodiment, activating the dispensing of liquid includes de-asserting the stop signal  130 , providing a valid beat on the heartbeat signal  140 , and sensing that the motor  102  is active. The activate  508  is followed by a wait  510 . The wait  510  delays the execution of a stop  512  until an appropriate amount of liquid  109  has been dispensed. The wait  510  may be performed in a variety manners including without limitation scheduling a timer interrupt, polling a timer, polling a dispensing meter, waiting for a hardware signal and the like. In the preferred embodiment the wait  510  includes monitoring a dosage sensor that provides information on the amount of liquid being dispensed. In one embodiment, the wait  510  also includes providing a valid beat on the heartbeat signal  140  at regular intervals. 
     The stop  512  generally stops all activity commenced by the activate  508 . In particular the stop  512  discontinues dispensing of the liquid  109 . In one embodiment, the stop  512  asserts the stop signal  130  and discontinues providing a valid beat on the heartbeat signal  140 . In the preferred embodiment, asserting the stop signal  130  causes the brake  106  to ground the power terminal  118  resulting in electromagnetic braking of the pump  104 . After the stop  512 , the liquid dispensing method  500  proceeds to a sleep  514 . 
     The sleep  514  delays further processing of the liquid dispensing method  500  until further processing is needed. In one embodiment, the sleep  514  includes placing the controller  134  in a low-power standby mode, scheduling a timer interrupt, and resuming normal processing in response to the timer interrupt. After the sleep  514  is completed, the liquid dispensing method  500  returns to the diagnostic  504 . Assuming the diagnostic  504  yields favorable results as described above, the liquid dispensing method  500  may loop indefinitely. 
     Referring to FIG. 5 b , a diagnostic method  520  may be performed in accordance with the dosage delivery system  100  and the liquid dispensing method  500 . In one embodiment the diagnostic method  520  is performed as the diagnostic  504  of the liquid dispensing method  500 . The diagnostic method  520  tests critical elements of the dosage delivery system  100 . In the preferred embodiment, the diagnostic method  520  tests circuitry used to control the motor  102 . 
     The diagnostic method  520  commences with a start  522 , followed by an enable power switch  524 . In the preferred embodiment, the enable power switch  524  occurs by discontinuing a valid beat on the heartbeat signal  140 , and de-asserting the stop signal  130 . De-asserting the stop signal  130  should place the power switch  122  in a conductive state and thereby provide power to the motor  102 . However, discontinuing a valid beat on the heartbeat signal  140  causes the enable signal  132  to be de-asserted which should result in the ground switch  124  being placed in a non-conductive state and the motor  102  being disabled. 
     The diagnostic method  520  proceeds from the enable power switch  524  to a first motor test  526  which ascertains if the motor  102  is actually running. If so, a problem exists in the control circuitry associated with the motor  104  and the diagnostic method  520  proceeds to an error  528  and terminates. In one embodiment, the error  528  performs error indication operations such as emitting an alarm tone through the audio indicator  144  and displaying an alarm message through the visual indicator  143 . 
     As depicted in FIG. 5 b , the diagnostic method  520  proceeds from the first motor test  526  to an enable ground switch  530 . In the preferred embodiment, the enable ground switch  530  occurs by asserting the stop signal  130  and providing a valid beat on the heartbeat signal  140 . Providing a valid beat on the heartbeat signal  140  should result in the ground switch  124  being placed in a conductive state thus providing a return path for power to the motor  102 . However, asserting the stop signal  130  should result in the power switch  122  being placed in a non-conductive state and the motor  104  being disabled. 
     The enable ground switch  530  is followed by a second motor test  532  which in one embodiment is identical to the first motor test  526  and ascertains whether the motor  102  is running. If so, a problem exists in the control circuitry associated with the motor  102  and the diagnostic method  520  proceeds to the error  528  and terminates. Otherwise, the diagnostic method  520  proceeds to an end  534  where the method terminates. 
     FIG. 5 c  shows one embodiment of a stop method  540  in accordance with the dosage delivery system  100  and the liquid dispensing method  500 . In one embodiment, the stop method  540  is performed as the stop  512  step of the liquid dispensing method  500 . In the preferred embodiment, the stop method  540  facilitates stopping the motor  102  by converting kinetic energy to electromagnetic energy in the motor  102  and absorbing the electromagnetic energy with the brake  106 . 
     The stop method  540  begins with a start  542  and proceeds concurrently to a brake  544 , and a disable motor  546 . The brake motor  544  and the disable motor  546  need not occur simultaneously. In the preferred embodiment, the staging of the brake motor  544  and the disable motor  546  is tuned to result in the least variation in stoppage of the motor  102 . 
     In the preferred embodiment, the brake motor  544  engages the brake  106  to absorb electromagnetic energy from the windings of the motor  102 . In one embodiment, engaging the brake  106  grounds the power terminal  118 . In that same embodiment, the disable motor  546  places the power switch  122  in a non-conductive state thus isolating the power bus  126  from the power terminal  118  and cutting off power to the motor  102 . After completion of the brake motor  544  and the disable motor  546 , the stop method  540  proceeds to an end  548  whereupon the method terminates. 
     In conjunction with the stop method  540 , the liquid dispensing method  500 , and the dosage delivery system  100 , it is worth mentioning the advantage of cutting off power and absorbing electromagnetic energy from the windings of the motor  102 . Cutting power and absorbing electromagnetic energy from the windings of the motor  102  results in efficient stoppage of the motor  102  including the dissipation of kinetic energy in the dosage delivery system  100 . Tuning the relative timing of engaging the brake  106  and disabling the motor  102  facilitates reducing overage in the delivery of the liquid  109 . Furthermore, the faster stoppage achieved by these methods helps minimize system and usage dependent variations in overage. The result is increased precision for the dosage delivery system  100 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the preceding description. All changes, which come within the meaning and range of equivalency of the claims, are to be embraced within their scope.