Abstract:
A variable capacitor for sensing the level of a liquid. The capacitor provides a readable capacitance that varies with respect to the level of the liquid. A pump control system implementing the capacitive sensor to control the level of a liquid by activating and deactivating the pump depending on the level of the liquid. Methods relating to varying capacitance of a capacitive sensor and controlling a pump based on the level of a liquid. A pump controller for controlling the level of a liquid in a reservoir includes a controller and a capacitor. The capacitor is adapted to provide an activation signal to the controller when the liquid in the reservoir reaches a first predetermined level relative thereto. Additionally, the capacitor is adapted to provide a trigger signal to the controller when the liquid in the reservoir reaches a second predetermined level relative thereto. Based on the trigger signal, the controller determines when to deactivate the pump.

Description:
RELATED APPLICATION 
       [0001]    This application claims benefit to U.S. Provisional Application No. 60/919,059 filed Mar. 19, 2007, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a capacitive sensor and, more particularly, to a method and apparatus for controlling a pump using same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Sensors are needed for a variety of applications. For example, pump applications, such as sump, dewatering, sewage, utility, effluent and grinder pumps, can use sensors to determine when the pump should be turned on and/or turned off. Conventional sump pumps generally include a pump having a mechanical switch connected to a float mechanism for controlling a liquid level in a reservoir. The float mechanism is disposed within the reservoir and adapted to travel on the surface of the liquid as the liquid rises and falls. Typical float mechanisms are mechanically connected to the switch and according to the position of the float relative to the pump, the switch controls power to the pump. 
         [0004]    In one configuration, the mechanical connection between the switch and the float includes a flexible tether. As the float travels up or down on the surface of the liquid in the reservoir, the orientation of the flexible tether relative to the switch changes. Another typical form of a float mechanism includes one or more rods or interconnected linkages. Similar to the tether, the rods or linkages are configured to allow the float to travel freely with the rising or falling of the surface of the liquid in the reservoir. In either of these configurations, once the float reaches a predetermined upper limit, the tether, rod, or linkage transfers a mechanical force to flip the switch, thereby completing the circuit and activating the pump. Conversely, when the liquid level and the float reach a predetermined lower limit, the tether, rod, or linkage transfers a mechanical force to the switch in an opposite direction, thereby interrupting the circuit and deactivating the pump. 
         [0005]    A shortcoming of the above-described sump pump float switch mechanisms is that they are inclined to experience mechanical failure. Sometimes mechanical failure occurs due to a deterioration of the mechanical connection between the float and the switch. Other times, the mechanical failure may occur due to objects in the reservoir that restrict or hinder the proper operation of the float mechanism. 
         [0006]    A further known sump pump switching mechanism includes a resistance switching mechanism. Resistance switching mechanisms include a pair of electrodes exposed in the liquid in the reservoir. As the level of the liquid in the reservoir changes relative to the electrodes, the electrical resistance between the two electrodes changes. Based on the change in resistance between the two electrodes, a controller activates or deactivates the pump. A shortcoming of resistance type switch mechanisms is that the electrodes are exposed to the liquid and tend to be vulnerable to corrosion. Once corroded, the electrodes fail to generate accurate resistances that the controller expects and the controller fails to operate properly. 
         [0007]    A still further known sump pump switching mechanism includes a capacitance switching mechanism. Capacitance switching mechanisms generally include a controller, an upper capacitor having two electrodes, and a lower capacitor having two electrodes. The upper and lower capacitors operate substantially independent of each other. When the level of the liquid reaches the upper capacitor, the controller detects a capacitance across both capacitors and activates the pump. The controller continues to activate the pump as the level of the liquid in the reservoir drops. Once the level of the liquid drops below the lower capacitor, the controller detects no capacitance across the lower capacitor and deactivates the pump. One shortcoming of such capacitance-based switching mechanisms is the reliance on multiple capacitors. Failure of one of the upper and lower capacitors may detrimentally affect the proper operation of the entire sump pump. 
         [0008]    In other known sump pump applications, magnetic switching mechanisms, such as Hall Effect sensors or switches, are used to detect water levels and operate a pump. For example, in some applications, a float is used to raise a magnet to an upper magnetic sensor at which point the pump is turned on. When the water level drops the float descends down to a lower magnetic sensor at which point the pump is turned off. A shortcoming of such magnetic sensors is that they again require moving parts and are inclined to experience mechanical failure, such as that discussed above with respect to tethers. 
         [0009]    Accordingly, it has been determined that a need exists for an improved sensor and method and apparatus for controlling a pump using same which overcome the aforementioned limitations and which further provide capabilities, features and functions, not available in current sensors and pumps. 
       SUMMARY OF THE INVENTION 
       [0010]    In one form the present invention provides a variable capacitor having first and second electrodes and a dielectric connecting the first and second electrodes to form a capacitor having a readable capacitance. The dielectric includes a first part made of an insulative material and a second part made of a liquid that changes levels with respect to the insulative material which causes a change in the capacitance of the capacitor. Thus, the changing liquid level with respect to the insulative material provides a variable capacitor capable of producing a plurality of different capacitances. 
         [0011]    In another form, the invention provides a capacitive sensor having a capacitor at least partially immersed in a liquid having a level that changes in relation to the capacitor, with the capacitor having a variable capacitance depending on the level of the liquid for providing a capacitance reading associated with the liquid level as mentioned above, and a circuit connected to the capacitor to determine the capacitance of the capacitor. Thus, the level of the liquid within which the capacitor is immersed may be determined based on the capacitance of the capacitor and the sensor may be used with a number of different pieces of equipment that are to be operated in response to changing liquid levels. 
         [0012]    For example, one aspect of the present invention provides a pump controller for controlling the level of a liquid in a reservoir. The pump controller includes a controller and a capacitor. The capacitor is adapted to provide a first capacitance to the controller when the liquid in the reservoir reaches a first predetermined level relative thereto. Additionally, the capacitor is adapted to provide a second capacitance to the controller when the liquid in the reservoir reaches a second level relative thereto. Based on the second capacitance, the controller determines when to deactivate the pump. 
         [0013]    One advantage of this form of the present invention is that it requires no moving parts that may suffer mechanical failure. The apparatus serves as a solid state sensor that detects liquid level to control activation and deactivation of the pump. Another advantage of this form of the present invention is that the capacitor may be wholly contained within the pump controller. Thus, the electrodes of the capacitor do not have to be exposed to the liquid in the reservoir and, therefore, would not be vulnerable to corrosion such as the electrodes in prior known resistance-based devices. A further advantage of this pump controller is that it includes a single capacitor in communication with the controller. This overall design reduces the number of electrical, mechanical, or electro-mechanical components that may suffer failure, makes it easier to assemble the sensor and can reduce cost associated with assembly and/or material costs for the apparatus. 
         [0014]    In another form, the controller determines a run-time based on the second capacitance detected by the controller for which the pump should be activated to move a predetermined amount of the liquid out of the reservoir. For example, the controller may determine the flow rate of the liquid out of the reservoir based on the difference in capacitance readings from the time the pump was activated (e.g., the first capacitance reading) to the time the second capacitance reading was taken and calculate how much longer the pump needs to remain operating at that flow rate in order to lower the liquid level in the reservoir to a desired level. 
         [0015]    In another form, the controller may be configured to deactivate the pump upon detecting the second capacitance from the capacitor. For example, the controller may be setup to regularly, or even continually, monitor the capacitance reading from the capacitor and shut off the pump once a predetermined capacitance value has been reached because the predetermined capacitance value is indicative of the fact the liquid level in the reservoir has dropped to a desired level. In one form, the apparatus includes a power source generating an alternating current and the controller is configured to detect the capacitance of the capacitor (or data associated with same) each time the alternating current is at a zero-crossing. In another form, the apparatus continually monitors the capacitance reading from the capacitor (or data associated with same). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will be explained in exemplary embodiments with reference to drawings, in which: 
           [0017]      FIG. 1  is a side view of a first embodiment of a sump pump system disposed within a reservoir and incorporating a sensor unit in accordance with one form of the present invention; 
           [0018]      FIG. 2  is a side cross-sectional view of the sensor unit of the first embodiment of the sensor unit depicted in  FIG. 1 ; 
           [0019]      FIG. 3  is a block diagram of the pump control of  FIG. 1 ; 
           [0020]      FIG. 4A  is a detailed schematic diagram of a pump control circuit using the sensor unit depicted in  FIGS. 1-3 ; 
           [0021]      FIG. 4B  is an enlarged schematic cross-sectional view of a the capacitor of the control circuit of  FIG. 4A ; 
           [0022]      FIG. 5  is a flowchart of a general operation process of the sensor unit depicted in  FIGS. 1-3 ; 
           [0023]      FIG. 6  is a flowchart of a process of controlling a level of a liquid in a reservoir in accordance with one form of the present invention; 
           [0024]      FIG. 7  is a flowchart of a process of controlling a level of a liquid in a reservoir in accordance with another form of the present invention; 
           [0025]      FIG. 8  is a side view of an alternate embodiment of a sump pump disposed within a reservoir and incorporating an integrated sensor unit according to the principles of the present invention; 
           [0026]      FIG. 9  is a perspective view of an alternate embodiment of a sump pump incorporating an integrated sensor unit in accordance with the present invention, with a portion of the outer housing shown in transparent to illustrate the internal components therein; 
           [0027]      FIG. 10  is a top cross-sectional view of the embodiment of  FIG. 9 ; 
           [0028]      FIG. 11  is a top cross-sectional view of an alternate embodiment of the sump pump of  FIG. 9  with the integrated sensor unit mounted in a slot of the pump housing; 
           [0029]      FIG. 12  is an alternate embodiment of a sensor unit in accordance with the invention, showing the sensor unit connected to a discharge pipe rather than the pump housing; 
           [0030]      FIG. 13  is a perspective view of yet another embodiment of the pump sensor and configuration for the pump and pump sensor in accordance with the invention; 
           [0031]      FIGS. 14A , B and C are perspective, front and rear elevational views of the sensor illustrated in  FIG. 13 ; 
           [0032]      FIG. 14D  is a cross-sectional view of the sensor of  FIGS. 14A-C  taken along line  14 D- 14 D of  FIG. 14B ; 
           [0033]      FIGS. 15A-C  are top, front and rear elevational views of a piggyback switch cord in accordance with the invention; 
           [0034]      FIG. 15D  is a wiring schematic for the piggyback switch cord of  FIGS. 15A-C ; 
           [0035]      FIG. 16  is an enlarged perspective view of a sensor circuit board in accordance with the invention illustrating a heat sink connected to the circuit board via a circuit component; and 
           [0036]      FIG. 17  is a perspective view of a dual pump system with a primary pump system incorporating a sensor unit in accordance with the invention and a battery-powered back-up pump system; the dual pump system includes a wireless or wired alert system including a receiver for informing the user of the status of the system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]      FIG. 1  depicts a sump pump system  10  disposed within a reservoir  26 . The sump pump system  10  includes a pump  12 , a sensor or sensor unit  14 , and a discharge pipe  16 . In general, the sensor unit  14  monitors the level of a liquid  34  within the reservoir  26  and serves as a switch for activating and deactivating the pump  12  based on that level. When the level of the liquid  34  reaches a predetermined upper limit, which is identified by reference numeral  30  in  FIG. 1 , the sensor unit  14  activates the pump  12 . Upon activation, the pump  12  begins moving the liquid  34  up and out of the reservoir  26  via the discharge pipe  16 . This begins to lower the level of the liquid  34  in the reservoir  26 . Once the level of the liquid  34  reaches a predetermined lower limit, which is identified by reference numeral  32  in  FIG. 1 , the sensor unit  14  deactivates the pump  12 . The details of the sump pump system  10  will now be discussed in more detail with continued reference to the figures. 
         [0038]      FIG. 1  depicts the sensor unit  14  including a power cord, such as piggy-back cord  22 , having an originating end  22   a  fixed to the sensor unit  14  and a terminal end  22   b  connected to a plug  24 . The piggy-back plug  24  has a standard three-prong male connector  24   a  and a standard three-point female receptacle  24   b . The pump  12  includes a power cord  18  having an originating end  18   a  fixed to the pump  12  and a terminal end  18   b  connected to a plug  20 . The plug  20  has a standard three-prong male connector  20   a . Upon installation, the male connector  24   a  of the piggy-back plug  24  of the sensor unit  14  is disposed within a standard 115 VAC-230 VAC electrical outlet, which is identified by reference numeral  28  in  FIG. 1 . Additionally, the male connector  20   a  of the plug  20  of the pump  12  is disposed within the female receptacle  24   b  of the piggy-back plug  24  of the sensor unit  14 . Thus, the electrical outlet  28 , the sensor unit  14 , and the pump  12  are electrically connected in series with one another. So configured, electrical current provided by the electrical outlet  28  will only power the pump  12  when the sensor unit  14  operates as a closed switch, completing the circuit and enabling current to pass therethrough. Additionally, this configuration enables the sensor unit  14  and the pump  12  to be constructed independent of each other. An advantage of this independence is that the pump  12  and/or the sensor unit  14  may be replaced or purchased independently of the other. Meaning, the sensor unit  14  could be adapted to operate with nearly any available pump so long as the plugs are interconnectable. 
         [0039]      FIG. 2  depicts a more detailed view of the sensor unit  14  of the sump pump system  10  depicted in  FIG. 1 . As stated above, the sensor unit  14  includes a power cord  22  terminating in a piggy-back plug  24 . Additionally, as depicted in  FIG. 2 , the sensor unit  14  includes a housing  36 , a reference electrode  38 , a detection electrode  40 , and a circuit board  42 . In the form illustrated, the housing  36  is a hollow, generally L-shaped box including a base portion  36   a  and an upper portion  36   b  extending generally perpendicularly from the base portion  36   a . The base portion  36   a  is box-shaped and has a generally square side cross-section defined by a bottom wall  35 , a first side wall  37 , a second side wall  39 , and a top wall  41 . Additionally, the base portion  36   a  includes an opening in the top wall  41  receiving the originating end  22   a  of the power cord  22 , which is electrically connected to the circuit located on circuit board  42 , and preferably a strain relief  23 . The upper portion  36   b  of the housing  36  is also box-shaped and has a generally elongated rectangular side cross-section defined by a top wall  43 , a first side wall  45 , and a second side wall  47 . 
         [0040]    The detection electrode  40  is disposed wholly within the upper portion  36   b  of the housing  36  and is situated directly above the reference electrode  38 . A lower portion of the reference electrode  38  is disposed within the base portion  36   a  of the housing  36  and an upper portion of the reference electrode  38  is disposed within the upper portion  36   b  of the housing  36 . The reference and detection electrodes  38 ,  40  each include a conductor, such as a metal plate. More specifically, in the embodiment illustrated, the detection electrode  40  includes a thin metal plate  40   a  having upper and lower biased portions  44   a ,  44   b . In the form illustrated, the upper and lower biased portions  44   a ,  44   b  include metallic foil rings. The foil rings  44   a ,  44   b  enable the detection electrode  40  to provide a non-linear output across its length. For example, capacitance generated between the electrodes  38 ,  40  is larger when the level of the liquid  34  in the reservoir  26  is near one of the foil rings  44   a ,  44   b  than when it is near the center of the detection electrode  40 . Additionally, the reference and detection electrodes  38 ,  40  are electrically connected to the circuit on the circuit board  40  with wires  48  and  50 , respectively. 
         [0041]    With reference to the block diagram provided in  FIG. 3 , the sump pump system  10  and, more particularly, the circuit board  42  includes a power supply  52 , a capacitive sensor  54 , a controller, such as microprocessor  58 , an AC switch, such as solid state relay (SSR)  60 , and signaling circuitry  70 . The microprocessor  58  detects capacitance from the capacitive sensor  54  upon receipt of a signal delivered by the signaling circuitry  70 , as will be described in more detail below. The microprocessor  58  then activates the pump  12  via the SSR  60  when the capacitance detected by the capacitive sensor  54  indicates that the liquid  34  in the reservoir  26  has reached the predetermined upper limit  30 , as identified in  FIGS. 1 and 2 . 
         [0042]    Referring now to FIGS.  3  and  4 A-B, the pump control circuit on circuit board  42  will be described in more detail. In the form illustrated, the pump control includes a power supply  52 , a capacitive sensor  54 , including a capacitor  33  and a capacitive sensing integrated circuit (IC)  57 , a controller  58  and an AC switch  60  for actuating the pump (not shown). The power supply  52  includes an AC power source or input (e.g., 115-230 VAC) (not shown), a voltage divider  62 , a rectifier  64 , a zener diode  66 , a capacitor C 7 , and a voltage regulator  68 . The voltage divider  62  includes a plurality of resistors R 9 , R 10 , R 11  and R 68  and the rectifier  64  includes two diodes D 1  and D 3 . Together, the voltage divider  62 , the rectifier  64  and the zener diode  66  step the AC voltage down to a rough or pulsating DC voltage, which in turn is filtered or smoothed out by the capacitor C 7  and the voltage regulator  68  to generate a 5 VDC output. This 5 VDC output is supplied to various components of the circuit including, among other items, the capacitive sensor  54  and the microprocessor  58 . 
         [0043]    The signaling circuitry  70  comprises a line brought off of the AC input to the microprocessor (pin  5 ) through a current limiting resistor R 8  to tell the processor when the input voltage signal is low enough to back bias the rectifier diodes. This tells the microprocessor to take a measurement reading from the capacitive sensor IC when there is a high impedance between the power line and reading circuitry, which minimizes the effects of stray capacitance tied to the two sensor plates  38  and  40  isolated by the dielectric layer  71 . Thus, when the signaling circuitry  70  monitors the voltage from the power supply  52  and informs the microprocessor  58  when a zero-crossing of the voltage input signal occurs, the input voltage signal is low enough to back bias the diodes D 1  and D 3  of the rectifier  64  so that the microprocessor  58  can take an accurate reading from the capacitor  33 . 
         [0044]    The capacitor  33  includes the reference electrode  38 , the detection electrode  40 , a dielectric wall  71 , and a capacitive sensing integrated circuit (IC), such as capacitance-to-digital converter  57 , which is connected to the capacitor  33  so that the controller  58  can read and process the capacitance of capacitor  33  at the zero-crossings of the AC supply. It should be understood, however, that in alternate embodiments, a controller may be selected which can read and process data directly from the capacitor  33 , if desired. 
         [0045]    With reference to  FIG. 4A , the dielectric wall  70  includes the first side wall  45  of the housing  36  of the sensor unit  14 , as described above with reference to  FIG. 2 . The dielectric wall  70  serves to isolate the reference and detection electrodes  38 ,  40  from the liquid  34  in the reservoir  26 , thereby creating capacitor  33 . In a preferred form, the electrodes  38 ,  40  are positioned flush against the dielectric as illustrated in  FIG. 4B  so as to avoid air gaps between the dielectric and the electrodes  38 ,  40 . In this form, the electrodes may be attached to the dielectric with epoxy so no air gaps will exist between the capacitor electrodes and the dielectric, which would otherwise negatively affect the performance of the capacitive sensor. In another form, the electrodes  38 ,  40  are encased in the insulative material of the dielectric, which also would eliminate air gaps between the electrodes and the dielectric. The reference electrode  38  is electrically connected to circuit ground and the detection electrode  40  is electrically connected to the capacitive sensing IC  57 , as depicted schematically in  FIG. 4A . The level of the liquid  34  in the reservoir  26  alters the performance of the side wall  45  and ultimately the value of capacitance generated by the capacitor  33 . Thus, in this way, the dielectric is made up in part by the side wall  45  and in part by the liquid  34  so that the capacitance of capacitor  33  varies in relation to the liquid level of liquid  34 . 
         [0046]    In the form illustrated in  FIG. 2 , the side wall  45  is made of a polymer, such as plastic, and the housing  36  is filled with a protective material, such as a potting compound, to protect the capacitor  33  and other electronic circuit components from exposure to the liquid within which the capacitor  33  is immersed. The housing is first partially filled with the potting compound before the circuit board is inserted. Then, after the circuit board is inserted, the housing is filled with additional potting compound to fully protect the circuit components. The potting compound used to fill the housing after the circuit board is inserted may be the same potting compound as the first, or it may be of a different composition. For example, a second, different potting compound may be used for certain applications, such as sewage applications, where external conditions dictate the use of different materials. A small piece of foam may be used to hold the circuit board against the inside wall of the housing while the potting compound cures. This method has also been found effective to keep air from being trapped between the electrodes  38 ,  40  and the dielectric. However, as mentioned above, in a preferred form the electrodes  38 ,  40  are either epoxied to the dielectric wall  45  or encased in the dielectric wall to eliminate air gaps. In this form, the capacitance generated by the reference and detection electrodes  38 ,  40  varies from approximately 1 picofarad (pF) with the level of the liquid  34  in the reservoir  26  being located at the predetermined lower limit  32  of the detection electrode  40  to approximately 11 pF at the predetermined upper limit  30  of the detection electrode  40 . As will be discussed more thoroughly below, the microprocessor  58  reads the capacitance generated by the reference and detection electrodes  38 ,  40  from the capacitive sensing IC  57 . When the capacitance indicates that the level of the liquid  34  has reached the predetermined upper limit  30 , the microprocessor  58  actuates the AC switch or SSR  60 , which activates the pump  12 . 
         [0047]    The SSR  60  includes an opto-triac  74  and an AC solid state switch, such as a triac  76 , or an alternistor. The switch  76  is electrically connected between the AC power supply  52  and the pump  12 , and the opto-triac  74  is electrically connected between switch  76  and the microprocessor  58 . The opto-triac  74  provides a zero voltage switch for triggering the switch  76  and, in the form illustrated, the switch  76  performs substantially the same function as two thyristors such as silicon controlled rectifiers (SCRs) wired in inverse parallel (or back-to-back). Thus, the opto-triac  74  drives the switch  76  and isolates or protects the microprocessor  58  and the other digital circuitry from the non-rectified AC signal that passes through the switch  76  when the pump  12  is activated. Additionally, the switch  76  allows both the positive and negative portions of the AC signal to be passed through to operate the pump  12 . 
         [0048]      FIG. 5  depicts a flowchart of a general operational process performed by the microprocessor  58  of the sump pump system  10 . First, when the level of the liquid  34  in the reservoir  26  reaches the predetermined upper limit  30 , the microprocessor  58  detects the existence of an activation capacitance (e.g., equal to or above a predetermined capacitance) from the capacitor  33  of the sensor unit  14  at block  501 . The microprocessor  58  then activates the pump  12  at block  502  to begin moving the liquid  34  out of the reservoir  26 . Meanwhile, the microprocessor  58  continues detecting the capacitance generated by capacitor  33 . Once the level of the liquid in the reservoir  26  falls to the lower limit  32  shown in  FIGS. 1 and 2 , the microprocessor  58  will detect the existence of a sample or trigger capacitance (which may be equal to or below a predetermined capacitance or alternatively a random capacitance) from the capacitor  33  at block  503 , resulting in the microprocessor  58  deactivating the pump  12  at block  504 . For example, in one form, the trigger capacitance is a predetermined value of capacitance and the microprocessor  58  simply deactivates the pump  12  when the trigger capacitance was detected. In another form, however, the trigger capacitance is either a predetermined capacitance value or a random capacitance value that simply allows the microprocessor  58  to calculate the flow rate of the liquid  34  evacuating the reservoir  26  so that the microprocessor  58  can determine how long the pump  12  should remain operating. This process will be discussed in greater detail below with reference to the various embodiments described with reference to  FIGS. 6 and 7 . 
         [0049]      FIG. 6  depicts a detailed flowchart of a process  600  performed by the microprocessor  58  for activating and deactivating the pump  12  according to the present invention. The process  600  controls the level of the liquid  34  in the reservoir  26  by utilizing a sump pump system  10  such as that described above. First, the microprocessor  58  receives a zero-crossing signal from the signaling circuitry  70  at block  601 . Substantially immediately thereafter, the microprocessor  58  detects a capacitance generated by the capacitor  33  at block  602 . Specifically, in the form of the sump pump system  10  discussed above, the capacitance is generated between the reference and detection electrodes  38 ,  40  of the capacitor  33  and detected and translated to digital data by the capacitance-to-DC converter  57  so that the microprocessor  58  can process the digital data and determine whether to activate or deactivate the pump  12 . 
         [0050]    After the microprocessor  58  detects the capacitance, it determines whether the detected capacitance is equal to a predetermined upper limit capacitance at block  603 . The predetermined upper limit capacitance corresponds to a capacitance generated by the electrodes  38 ,  40  when the level of the liquid  34  in the reservoir  26  is at the predetermined upper limit  30  shown in  FIGS. 1 and 2 . In the event the detected capacitance is equal to the upper limit capacitance, the microprocessor  58  activates the pump  12  at block  604  to move the liquid  34  out of the reservoir  26  via the discharge pipe  16 . Specifically, in the form of the sump pump system  10  discussed above, the microprocessor  58  triggers or turns on the opto-triac  74  and the opto-triac  74  triggers or turns on the switch  76 . This closes the circuit between the AC power supply and the pump  12  allowing the alternating current to travel directly to the pump  12  to operate the pump  12 . Once the microprocessor  58  activates the pump  12 , it waits to receive another zero-crossing signal from the signaling circuitry  70  at block  601  and repeats the process  600  accordingly. 
         [0051]    Alternatively, if the microprocessor  58  determines at block  603  that the capacitance detected at block  602  is not equal to the predetermined upper limit capacitance, the microprocessor  58  determines whether the detected capacitance is less than or equal to a trigger capacitance at block  605 . In this form of the process  600 , the trigger capacitance is equal to a predetermined lower limit capacitance, which corresponds to a capacitance generated by the electrodes  38 ,  40  when the level of the liquid in the reservoir  26  is at the predetermined lower limit  32  shown in  FIGS. 1 and 2 . If the detected capacitance is greater than the lower limit capacitance, the microprocessor  58  returns to receiving zero-crossing signals from the signaling circuitry  70  at block  601 . Alternatively, however, if the detected capacitance is less than or equal to the lower limit capacitance, the microprocessor  58  deactivates the pump  12  at block  606  and then returns to receiving zero-crossing signals from the signaling circuitry  70  at block  601 . The process  600  thereafter repeats itself. 
         [0052]      FIG. 7  depicts a detailed flowchart of an alternative process  700  performed by the microprocessor  58  for activating and deactivating the pump  12 . The process  700  controls the level of the liquid  34  in the reservoir  26  utilizing a sump pump system  10  such as that described above. First, the microprocessor  58  receives a zero-crossing signal from the signaling circuitry  70  at block  701 . Substantially immediately thereafter, the microprocessor  58  detects a capacitance generated by the capacitor  54  at block  702 . Specifically, in the form of the sump pump system  10  discussed above, the capacitance is generated between the reference and detection electrodes  38 ,  40  and stored by the capacitance sensing IC  57 . Therefore, the microprocessor  58  detects or reads the capacitance from the IC  57 . 
         [0053]    After the microprocessor  58  detects the capacitance, it determines whether the detected capacitance is equal to a predetermined upper limit capacitance at block  703 . The predetermined upper limit capacitance corresponds to a capacitance generated by the electrodes  38 ,  40  when the level of the liquid  34  in the reservoir  26  is at the predetermined upper limit  30  shown in  FIGS. 1 and 2 . In the event the detected capacitance is equal to the upper limit capacitance, the microprocessor  58  activates the pump  12  at block  704  to move the liquid  34  out of the reservoir  26  via the discharge pipe  16 . Specifically, in the form of the sump pump system  10  discussed above, the microprocessor  58  triggers or turns on the opto-triac  74  and the opto-triac  74  triggers or turns on the switch  76 . This completes the circuit between the AC power supply and the pump  12  and allows the alternating current provided by the power supply to operate the pump  12 . Once the microprocessor  58  activates the pump  12 , it waits to receive another zero-crossing signal from the signaling circuitry  70  at block  701  and proceeds accordingly. 
         [0054]    Alternatively, if the microprocessor  58  determines at block  703  that the capacitance detected at block  702  is not equal to the predetermined upper limit capacitance, the microprocessor  58  determines whether the detected capacitance is less than or equal to a predetermined trigger capacitance at block  705 . The predetermined trigger capacitance is equal to a capacitance generated by the reference and detection electrodes  38 ,  40  when a surface of the liquid in the reservoir  26  is at a predetermined location below the upper limit  30  illustrated in  FIGS. 1 and 2 , but above the lower limit  32  illustrated in  FIGS. 1 and 2 . In one embodiment of the present invention, the trigger capacitance is measured when the surface of the liquid  34  in the reservoir  26  is approximately 1 inch below the upper limit  30 . However, such trigger capacitance may be measured at virtually any location along the detection electrode  40  that is below the upper limit  30  and above the lower limit  32 . 
         [0055]    Nevertheless, if the microprocessor  58  determines at block  705  that the detected capacitance is not less than or equal to the trigger capacitance, the microprocessor returns to receiving zero-crossing signals from the signaling circuitry  70  at block  701 . Alternatively, however, if the microprocessor  58  determines at block  705  that the detected capacitance is less than or equal to the trigger capacitance, it calculates a run-time at block  706 . 
         [0056]    The run-time is the amount of time that it took to pump down the liquid  34  in the reservoir  26  from the upper limit  30  to the predetermined location between the upper and lower limits  30 ,  32 . The microprocessor  58  determines this run-time by monitoring the time that passed between when the microprocessor  58  determined the capacitance to be equal to the predetermined upper limit capacitance and when the microprocessor determined the capacitance to be equal to the trigger capacitance. In one form of the process  700 , this determination may be made by using an internal clock in the microprocessor  58  to determine how much time has lapsed between the start of the pump and/or detection of the predetermined upper limit capacitance and detection of the trigger capacitance. However, it should be appreciated that the microprocessor  58  may determine this run-time in any effective manner which allows the microprocessor  58  to calculate the flow rate of the liquid  34  being moved out of the reservoir  26 . 
         [0057]    After determining the run-time at block  706 , the microprocessor  58  calculates a total run-time at block  707 . The total run-time is a factor of the run-time and corresponds to how long the pump  12  should remain activated to lower the level of the liquid  34  in the reservoir  26  to the predetermined lower limit  32  or some other desired level. In one form, the total run-time determined at block  707  is five times the run-time determined at block  706 . Therefore, after the total run-time passes, the microprocessor  58  deactivates the pump  12  at block  708  and returns to receiving subsequent zero-crossing signals from the signaling circuitry  70  at block  701  and the process repeats itself accordingly. 
         [0058]    While the above-described process  700  has been described as including a determination of a run-time and a total run-time, an alternate form of the process may include a determination of a flow rate at which the level of the liquid  34  drops between the microprocessor  58  detecting the upper limit capacitance and the trigger capacitance. In such a case, the microprocessor  58  would deactivate the pump  12  only after the pump  12  has removed a predetermined volume of liquid  34  out of the reservoir  26 . 
         [0059]    Additionally, it should be appreciated that while the above-described processes  600  and  700  have been described as including a series of actions described according to a sequence of blocks or steps, the present invention is not intended to be limited to any specific order or occurrence of those actions. Specifically, the present invention is intended to include variations in the sequences at which the above-described actions are performed, as well as additional or supplemental actions that have not been explicitly described, but could otherwise be successfully implemented. 
         [0060]    Furthermore, in a preferred embodiment of the processes  600 ,  700  described above, the microprocessor  58  is programmed to activate the pump  12  for a minimum of four seconds and a maximum of sixteen seconds. Additionally, the microprocessor  58  is programmed to insure deactivation of the pump  12  for a minimum of one second between activation and deactivation. It should be appreciated, however, that such specific activation and deactivation periods are merely exemplary and that the microprocessor  58  may be programmed to accommodate various different sizes, models and configurations of pumps  12  and, therefore, these timings may also be changed to satisfy the desired conditions for any given application. 
         [0061]    Referring now to  FIGS. 8 and 9 , alternative embodiments of systems are shown using a sensor in accordance with the invention. For convenience, features of the alternate embodiments illustrated in  FIGS. 8-9  that correspond to features already discussed with respect to the embodiment of  FIGS. 1-7  are identified using the same reference numerals in combination with the prefix “1” merely to distinguish one embodiment from the other, but otherwise such features are similar. In this form, sump pump system  110  includes a pump  112  powered by a motor  184 , a sensor unit  114 , and a liquid discharge pipe  116 . Unlike the sump pump system  10  described above, the pump  112  and the sensor unit  114  are an integral unit sharing a common power cord  118 . The power cord  118  includes an originating end  118   a  fixed to the sensor unit  114  and a terminal end  118   b  connected to a plug  120 . The plug  120  is adapted to be electrically connected to a standard electrical outlet  122 , similar to that described above with reference to the first embodiment of the sump pump system  10 . Therefore, while the electrical connection between the sensor unit  14  and the pump  12  described in accordance with the first embodiment of the sump pump system  10  was achieved externally via the different cords, the same electrical connection is made in the sump pump system  110  of this alternative embodiment internally. Specifically, the sensor unit  114  and the pump  112  are hard-wired together and constructed as a single operational unit. Otherwise all features, characteristics and functions are generally the same as described above regarding the first embodiment and will not be described in detail again. 
         [0062]    In the form illustrated, the capacitor is disposed in the housing  136  of the pump  112  and uses an outer wall of the housing  136  as part of the dielectric and the liquid level of liquid  134  with respect to the housing  136  to affect the dielectric performance and capacitance of the variable capacitor of capacitive sensor  114 . Thus, when the liquid level of liquid  134  raises or lowers with respect to housing  136 , a corresponding change in capacitance will be detected by sensor  114 . When the detected capacitance is equal to or greater than the capacitance associated with the predetermined upper limit  130 , the pump will be activated to evacuate liquid out of the reservoir  126  until the liquid  134  has dropped below a desired lower limit  132 . 
         [0063]    In the forms illustrated in  FIGS. 9-11 , the sensor  114  is disposed in the outer wall of the housing  136  and at least a portion of the outer housing is shown in transparent so that the internal components and sensor  114  can be seen therein. In one form shown in  FIGS. 9 and 10 , the sensor  114  may be molded directly into the housing wall  136 . Alternatively, the sensor  114  may be coupled to the housing by fitting into a slot  186  formed in the housing wall  136 . The sensor  114  may have an arcuate configuration to match the curvature of the housing wall  136 , as shown in  FIG. 10 , or it may have a flat configuration, as shown in  FIG. 11 . The configurations described above are merely examples in accordance with the present invention, and other configurations are contemplated, as would be apparent to those skilled in the art. 
         [0064]    Another embodiment of the pump sensor is illustrated in  FIG. 12  and, for convenience, features of this embodiment that correspond to features already discussed with respect to the embodiment of  FIGS. 1-11  are identified using the same reference numeral in combination with the prefix “2” merely to distinguish one embodiment from the other, but otherwise such features are similar. In the form illustrated, the capacitive sensor  214  is shown connected to the discharge pipe  216  via a mounting bracket  280 . The bracket  280  allows the sensor  214  to be positioned at any desired location on the discharge pipe  216 , which allows the operator to determine how much liquid he or she wishes to maintain in the reservoir (not shown). For example, if an operator wishes to maintain a larger amount of liquid in the reservoir, the operator may slide the sensor  214  up the discharge pipe  216  and away from the pump (not shown) so that the predetermined upper limit for the liquid level is reached more slowly. Conversely, if the operator wishes to maintain less liquid in the reservoir, the operator may slide the sensor  214  down the discharge pipe  216  closer to the pump so that the predetermined upper limit for the liquid level is reached faster. In this way, the bracket  280  further allows the operator or installer to account for reservoirs or pits of different sizes and configurations. 
         [0065]    An alternate housing  282  is also used for the sensor  214 . In the form illustrated, the housing  282  forms more of an elongated sleeve with a longitudinal axis running generally parallel to the pipe  216 . In this drawing the housing  282  is shown as being partially transparent so that the circuit board  242  and power cord end  222   a  of piggyback cord  222  are visible through the housing  282 . In a preferred form, however, the housing  282  will be opaque and filled with a suitable potting material for protecting the circuit and circuit components on circuit board  242  from exposure to the liquid in which the sensor  214  is immersed. With this configuration, the length of the housing may be selected based on the pump application. For example, if a longer level sensor plate is desired so that the capacitor may track a larger range of liquid levels, the housing  282  can be elongated to accommodate the larger level sensor plate. 
         [0066]    Yet another embodiment of the sensor and configuration for the pump and sensor are illustrated in FIGS.  13  and  14 A-D. As has been done before, features of this embodiment that correspond to features already discussed with respect to the embodiment of  FIGS. 1-11  are identified using the same reference numeral in combination with the prefix “3” merely to distinguish one embodiment from the other, but otherwise such features are similar. In the form illustrated, the sensor  314  is connected to the pump  312  via a plurality of mounting brackets  380 . Although a hollow housing  336  is illustrated so that the circuit board  345  may be seen, the housing  336  will preferably be filled with a potting material to protect the circuit and components on the circuit board  345  from the liquid in which the sensor  314  will be disposed. 
         [0067]      FIGS. 15A-D  illustrate one form of a piggyback power cord  422  for use with the embodiments illustrated herein and provide a wiring schematic for same. It should be understood, however, that alternate forms of piggyback cords may be provided so long as these cords allow the pump control disclosed herein to complete the circuit between the pump and the power source when a desired liquid level has been reached to activate the pump and break the circuit between the pump and power supply to deactivate the pump. 
         [0068]    Although the embodiments illustrated thus far have had the level sensor plate (e.g.,  30 , etc.) of capacitor  33  located on top and the reference plate (e.g.,  32 ) of capacitor  33  located below the level sensor plate, it should be understood that in alternate embodiments, the level sensor plate may be located below the reference plate. Such a configuration may be particularly advantageous in applications wherein a very minimal amount of liquid is to be monitored and/or maintained. For example, by placing the level sensor plate in the bottom of the capacitive sensor, liquids may be monitored and maintained much closer to the bottom of the pump and/or the bottom surface of the reservoir. In some applications, however, such a configuration will not be desired due to high contamination levels in the liquid causing deposits and/or foaming on the surface of the housing of the sensor opposite the level sensor plate or due to residual surface moisture lingering or being present on the surface of the housing of the sensor opposite the level sensor plate. 
         [0069]    These and other concerns may also provide grounds for taking the sampling capacitance at a position slightly below the upper limit and/or well above the bottom of the level sensor plate and calculating a run-time for the pump to operate rather than trying to detect exactly when the liquid has dropped to a desired level on the level sensor plate. For example, if the lower portion of the level sensor plate contains residual surface moisture, this moisture may affect the readings of the capacitor (e.g.,  33 ) and cause the pump control to continue to operate as if the liquid level has not dropped to the desired level on the level sensor plate because the residual water is affecting the capacitance reading of the capacitor. 
         [0070]    In light of the foregoing, it should be understood that additional and/or supplemental features and processes are intended to be within the scope of the present invention. For example, the sensor unit  14  may include noise filtering components in order to ensure that the sensor unit  14  operates properly and efficiently. In another alternative form, a temperature sensor may be connected to the SSR  60  in order to limit the run-time of the pump  12 . The temperature sensor may monitor the temperature of the opto-triac  74  and/or the switch  76  and, if the device gets too hot, direct the microprocessor  58  to deactivate the pump. 
         [0071]    In a preferred form shown in  FIGS. 12 and 16 , a portion of the switch  76  discussed above, which is illustrated as triac  876  in these figures, is mounted to the circuit board  842  and another portion is mounted to a heat sink, such as a copper plate  844 , to prevent the switch  876  from overheating. The heat sink is attached to the triac  876  using a surface mount reflow process, which can be undertaken at the same time that the other circuit components are being soldered to the circuit board. This process eliminates a separate process step as well as reduces labor time. In effect, the thermal metallization of the switching device  876  is operable as a thermal and mechanical bridge between the heat sink  844  and the circuit board  842 . The heat sink is effectively connected to the circuit board  842  by the triac  876 , which also eliminates the need for separate mounting hardware to mount the heat sink, thereby increasing production efficiency. The copper plate  844  is sized such that it has a relatively large surface area to effectively dissipate heat through the potting and sensor housing (not shown) and into the external environment. Preferably, the heat sink is located near the lower end of the housing so that it is more likely to be located below the liquid level. This way, heat produced by the circuit is transferred to the liquid. As a result, heat may be dissipated through the housing much more effectively, because liquid is a much better thermal conductor than air. 
         [0072]    It should be noted that different applications and conditions may require the sensor and related components to be manufactured from different materials. For example, the materials used for the power cord and the potting for standard applications (such as sump applications) were found to be less suited for sewage applications. PVC or thermoplastic jackets used on power cords in testing were found to fail tests required to obtain sewage rating under applicable UL requirements. Upon experiment, it was found that rubber or thermoset jackets were preferable to PVC for sewage applications. In addition, the protective material, such as potting, used to protect the electric circuitry of the sensor in standard applications was less suited for sewage applications. However, no potting material suitable for a sewage application could be found that had the desirable flammability rating to meet UL requirements. Therefore, after much experimentation, it was found that using two different potting compounds arranged in layers was effective to meet both flammability and sewage requirements. Therefore, in a preferred form for sewage applications or other applications with similar conditions, the sensor electrical components are first covered with a first potting compound, and then a second potting compound is disposed on at least a portion of the first potting compound. The first potting material is preferably a flame retardant compound, such as EL-CAST FR resin mixed with 44 hardener, manufactured by United Resin. The second potting compound, which forms an outer layer disposed on the first, is preferably an acid-resistant potting compound, such as E-CAST F-28 resin mixed with LB26X92A hardener, also manufactured by United Resin. Thus, in a preferred form, the sensor housing is partially filled with the flame retardant potting compound, and then the second, acid resistant compound is poured into the housing such that the second layer is formed having an approximate thickness in the range of about ⅛ to ¼ inch. As mentioned above, in another form, the second potting compound may be the same composition as the first potting compound. In yet other forms, one or more protective materials effective to protect circuit components may be used as alternatives to one or more potting compounds, as would be apparent to one skilled in the art. 
         [0073]    In one example of a typical sump application, the capacitive sensor may be implemented in a conventional battery back-up system. The purpose for the battery back-up in this instance is to allow the pump to continue to pump fluid even when main power is out in a residence or commercial facility. Thus, if the power did go out, the battery back-up system would supply power to the pump so that fluid could be evacuated in order to prevent flooding. Such systems also often include alarms that alert individuals to unusual pump operation, such as high water conditions, continuous running of the pump, overheating pumps, low battery, etc. These alert systems can be hard wired between the pump system and a display or can be wirelessly connected using a transmitter and receiver setup. Typically, the hard wired systems use telephone cable  922  (see  FIG. 17 ) for connecting the pump system to the display and the wireless systems use radio frequency transmitters and receivers. In alternate embodiments, however, other types of cable may be used to hard wire the alert system and other types of convention wireless transmission techniques can be used such as infrared, Bluetooth, etc. In yet other embodiments the wireless system may be connected to a network, such as a LAN or WAN network, so that alerts can be sent via a local area network such as a server or a wide area network such as the Internet. 
         [0074]    In another embodiment illustrated in  FIG. 17 , the capacitive sensor may be used in a dual pump system  900 , such as one having primary and backup pump systems  902 ,  904 . The primary pump system  902  may include a first pump  906  acting as the primary pump, a liquid level sensor, such as a capacitive sensor  908  as described in detail above, and a wired or wireless transmitter for communication with a remote receiver  910  of the pump system  900 . The backup pump system  904  includes a second pump  912  acting as a backup, in case of either the failure of the first pump  906  or a power outage as discussed above. The secondary pump  912  is preferably battery-operated, such as a 24-volt direct current (DC) pump. The backup pump system may also include a battery bank or back-up  914  for powering the secondary pump  912 , a battery charger  916 , a float switch  918 , a transmitter  920  and a backup pump controller. The backup system  904  may operate by turning on the secondary pump  912  whenever the liquid level triggers the float switch  918 , which is normally placed above the regular high liquid level setting of the primary pump  906 . Thus, the backup pump  912  is triggered whenever the liquid rises high enough to trigger the float switch  918 , which occurs when the primary pump  906  is not pumping liquid at a sufficient flow rate, such as when the primary pump  906  lacks power or is inoperable, clogged, frozen, etc. 
         [0075]    The pump system  900  may include an alert system, which includes the remote receiver  910 . The remote receiver  910  may be wired or wireless, and is operable to receive information about the status of the system  900  from one or more transmitters of the system and indicate to the user various system conditions, such as when the primary pump  906  has no power or the liquid sensor (such as the capacitive sensor  908 ) is sensing a high water level, when the backup pump  912  is running or inoperable, when the battery  914  is low, or when the float switch  918  is sensing high liquid level. In addition, the receiver  910  may indicate when its own battery power is low or dead, or when the receiver  910  has lost AC power. The features described above are meant for illustrative purposes only, as one of ordinary skill in the art would contemplate the numerous applications in which the capacitive sensor described above could be implemented. 
         [0076]    In addition, the capacitive sensor discussed herein may be implemented with pumps having known features such as cast iron impellers, top suction intakes, carbon/ceramic shaft seals, and stainless steel motor housing and impeller plates. Further, the sensor may be implemented with pump systems having features such as automatic battery recharging, battery fluid and charge monitors, and controls to automatically run the pump periodically to ensure operation. 
         [0077]    Finally, it should be appreciated that the foregoing merely discloses and describes examples of forms of the present invention. It should therefore be readily recognizable from such description and from the accompanying drawings that various changes, modifications, and variations may be made without departing from the spirit and scope of the present invention. For example, although the drawings show the capacitor and sensor discussed herein being used in a sump pump application, it should be understood that such a capacitor and sensor may be used in a variety of different applications and with a variety of different pieces of equipment including, but not limited to, dewatering, sewage, utility, pool and spa equipment, wired or wireless back-up pump systems, well pumps, lawn sprinkler pumps, condensate pumps, non-clog sewage pumps, effluent and grinder pump applications, water level control applications, as well as other non-pump related applications requiring liquid level control.