Patent Publication Number: US-2011056710-A1

Title: Method of Operating a Servo Motor in a Fire-Extinguishing System

Description:
BACKGROUND 
     Modern fire fighting apparatus use a foam proportioning system (FPS) to extinguish fires with a water-foamant solution. A constant concentration of a water-foamant solution is desired for the most effective fire-extinguishing properties. Generally, the FPS can include additive pumps, which can be driven by different power sources including, for example, electric motors or hydraulic motors. For high flow rates, hydraulic motors are used due to excessive power requirements of an equivalent electric motor. The hydraulic pressure driving the hydraulic motor often varies over the period of the fire-fighting operation. As a result, hydraulic motors are less suitable for low-volume flows, because a steady stream of water-foamant solution can be difficult to provide. In addition to the hydraulic motor in the FPS, a direct current (DC) electric motor is often used to provide the low-volume flow rates. 
     SUMMARY 
     Some embodiments of the invention provide a method of controlling a motor that drives a pump for injecting foamant into a stream of water in a fire extinguishing system. The method includes providing an over-voltage circuit that can dissipate transient voltages of at least about 150 volts, providing a relay between low voltage circuits in the fire extinguishing system and the battery power source, and turning off all electronics in the fire extinguishing system and de-energizing the relay when an over-voltage condition is detected. 
     Some embodiments of the invention provide a method including providing a motor including an increased torque constant in order to decrease a peak current required by the motor, achieving a first continuous operating point with the decreased peak current, and altering a back electromagnetic force constant in the motor in order to achieve a second continuous operating point. 
     Some embodiments of the invention provide a method including monitoring a signal generated by the flow sensor substantially continuously, transmitting the signal from the flow sensor to the integrated controller on the motor, controlling a speed of the motor shaft based on the signal from the flow sensor, and injecting foamant into the stream of water at low flow rates less than about 30 percent of a maximum output of the foam pump without stopping and starting the motor shaft in order to optimize mixing of foamant and water. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fire-extinguishing system including a servo motor and having a foamant injection point upstream of a flow meter according to one embodiment of the invention. 
         FIG. 2  is a schematic diagram of a fire-extinguishing system including the servo motor and having a foamant injection point downstream of a flow meter according to another embodiment of the invention. 
         FIG. 3  is a schematic diagram of a fire-extinguishing system including the servo motor and having a foamant injection point upstream of a water pump according to yet another embodiment of the invention. 
         FIG. 4A  is a perspective view of the servo motor according to one embodiment of the invention. 
         FIG. 4B  is a cross-sectional view of the servo motor of  FIG. 4A . 
         FIG. 5  is a schematic diagram of a controller for use with any one of the fire-extinguishing systems of  FIGS. 1 ,  2 , and  3 . 
         FIG. 6  is a schematic block diagram of electrical components for use with any one of the fire-extinguishing systems of  FIGS. 1 ,  2 , and  3  according to some embodiments of the invention. 
         FIG. 7  is a schematic block diagram of a load dump protection system according to one embodiment of the invention. 
         FIG. 8  is flowchart of a load dump protection method according to one embodiment of the invention. 
         FIG. 9  is a flowchart of a power management control of the servo motor according to one embodiment of the invention. 
         FIGS. 10A through 10D  are schematic graphs of various pulse shapes according to some embodiments of the invention. 
         FIG. 11  is a flowchart of a current fold back protection method according to one embodiment of the invention. 
         FIG. 12  is a schematic block diagram of a rectification bridge according to one embodiment of the invention. 
         FIG. 13  is a flow chart of an operation of the rectification bridge of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is presented to enable a person skilled in the art to make and use embodiments of the invention. Various modifications to the illustrated embodiments will be readily apparent to those skilled in the art, and the generic principles herein can be applied to other embodiments and applications without departing from embodiments of the invention. Thus, embodiments of the invention are not intended to be limited to embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. The following detailed description is to be read with reference to the figures, in which like elements in different figures have like reference numerals. The figures depict selected embodiments and are not intended to limit the scope of embodiments of the invention. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of embodiments of the invention. 
     The following description refers to elements or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/feature is directly or indirectly connected to another element/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/feature is directly or indirectly coupled to another element/feature, and not necessarily mechanically. Thus, although the schematic shown in  FIG. 5  depicts one example arrangement of processing elements, additional intervening elements, devices, features, or components may be present in an actual embodiment (assuming that the functionality of the system is not adversely affected). 
     The invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. 
     In accordance with the practices of persons skilled in the art of computer programming, the invention may be described herein with reference to symbolic representations of operations that may be performed by the various computing components, modules, or devices. Such operations are sometimes referred to as being computer-executed, computerized, software-implemented, or computer-implemented. It will be appreciated that operations that are symbolically represented include the manipulation by the various microprocessor devices of electrical signals representing data bits at memory locations in the system memory, as well as other processing of signals. The memory locations where data bits are maintained are physical locations that have particular electrical, magnetic, optical, or organic properties corresponding to the data bits. 
       FIG. 1  illustrates a fire-extinguishing system  1  according to one embodiment of the invention. The fire-extinguishing system  1  can be stationary (e.g., a sprinkler system of a building) or mobile (e.g., installed on a fire truck). In other embodiments, the fire-extinguishing system  1  can be used to help prevent fires by protecting buildings or by providing exposure protection. The fire-extinguishing system  1  can include a foam proportioning system (FPS)  2 , a water tank  4 , a water pump  6 , a flow meter  8 , a controller  10 , and a display  12 . The water pump  6  can receive water from the water tank  4  and/or other sources (e.g., a lake, a stream, or a municipal hydrant). The water can be fed through a hose or other conduit  14  to the inlet of the water pump  6 , which can be driven by a suitable motor or engine, such as an electrical motor, an internal combustion engine, or a hydraulic motor. The water pump  6  can be a high-pressure, high-flow rate pump. The outlet of the water pump  6  can be connected by a suitable conduit  16  to the flow meter  8 . The flow meter  8  can generate a signal transmitted via a line  18  that is proportional to the volume flow rate of the total flow through the conduit  16 . The FPS  2  can introduce an amount of foamant into the water stream to create a water-foamant solution at a desired concentration rate. The term “foamant” as used herein and in the appended claims can include any one or more of the following: liquid chemical foams, concentrates, water additives, emulsifiers, gels, and additional suitable substances. 
     Downstream of the flow meter  8 , the pumped water can be routed to a discharge manifold  20 . In one embodiment, a single discharge line (e.g., a single fire hose or a sprinkler head) can be connected to the discharge manifold  20 . Other embodiments can include two or more discharge lines configured to dispense the water-foamant solution at substantially equal concentrations. In some embodiments, the fire-extinguishing system  1  can include two or more individual discharge lines with one discharge line dispensing the water-foamant solution at a different concentration than another discharge line. 
     As also shown in  FIG. 1 , the FPS  2  can include a foam pump  22 , a servo motor  24 , and a foam tank  26 . The foam pump  22  can be a positive displacement pump or any other suitable type of pump. For example, the foam pump  22  can be a plunger pump, a diaphragm pump, a gear pump, or a peristaltic pump. The foam tank  26  can store a supply of foamant, which can be in liquid form. In some embodiments, the foam tank  26  can include a float mechanism  28  or another suitable type of level-sensing device. The float mechanism  28  can generate a signal transmitted via a line  30  to the controller  10 . The signal can indicate that the amount of the foamant remaining in the foam tank  26  has dropped below a preset level. The foam tank  26  can be coupled by a hose or other suitable conduit  32  to an inlet of the foam pump  22  so that the foamant can be gravity-fed to the foam pump  22 . However, in other embodiments, the foamant can be drawn against gravity into the foam pump  22 . In some embodiments, the conduit  32  can be at least somewhat flexible to compensate for vibrations of the foam pump  22 , reducing the risk of a fatigue rupture. In some embodiments, the FPS  2  can include a second flow meter (not shown) that can measure the amount of foamant being injected into the stream of water. In some embodiments, the second flow meter can measure the amount of foamant injected rather than or in addition to calculating the amount of foamant injected based on the displacement of the foam pump  22 . 
     The foam pump  22  can include different cylinders with varying piston size and/or stroke to adapt to a wide range of flow rates. The amount of the foamant drawn from the foam tank  26  and pumped through the conduit  32  can be proportional to the stroke volume of each cylinder and to the speed at which the foam pump  22  is driven by the servo motor  24 . 
     In some embodiments, the rotor shaft angle of the servo motor  24  can be used to calculate the position of a piston (not shown) of the foam pump  22 . Under normal operating conditions, the calculated position of the piston of the foam pump  22  can be used to alter a rotor shaft speed of the servo motor  24 . The use of a calculated piston position to alter the rotor speed is disclosed in U.S. Pat. No. 6,979,181 issued to Kidd, the entire contents of which is herein incorporated by reference. If the position, of the piston is close to finishing a stroke in either direction (i.e., the movement of the piston is about to change to the opposite direction), the controller  10  can increase the rotor shaft speed by an increment. Conversely, when the piston is moving in a single direction without an imminent direction change, the rotor shaft speed can be decreased by an increment by the controller  10 . As a result, foamant can be introduced in a more steady manner and power peaks of the servo motor  24  can be leveled off, reducing its power consumption and heat generation. In this manner, smoother and higher flow rates over extended periods of time can be achieved. 
     In some embodiments, the display  12  can serve as a user interface to allow communication with the controller  10  via a line  34 . The display  12  can communicate a concentration of the water-foamant solution selected by the user to the controller  10 . The controller  10  can include the selected concentration of the water-foamant solution to calculate a foam-flow rate at which the foamant should be injected into the stream of water. In order to achieve the necessary foam-flow rate, the controller  10  can send a corresponding speed signal to the servo motor  24  via a line  36 . If the servo motor  24  operates the foam pump  22  at its maximum speed, the servo motor  24  can continue to run at the maximum speed, even if the flow rate through the conduit  16  requires a higher foam flow rate, thereby decreasing the selected concentration of the water-foamant solution. In some embodiments, the display  12  can also receive information regarding the status of the fire-extinguishing system  1  and other operating information from the controller  10  via a line  38  (e.g., current flow rates of water or foamant, the amount of total water or total foamant that was pumped during the current fire-fighting operation, etc.). 
     The controller  10  can communicate with the servo motor  24 . In some embodiments, the servo motor  24  can transmit to the controller  10  the rotor shaft speed signal via the line  36 , a current signal via a line  40 , a temperature signal via a line  42 , and a rotor shaft angle signal via a line  44 . In some embodiments, the rotor shaft speed can be transmitted to the controller  10  (via line  36 ) and the rotor shaft torque can be calculated by the controller  10  based on the current signal received on the line  40 . The controller  10  can operate the servo motor  24  based on the received signals and/or user input. 
     As further shown in  FIG. 1 , the FPS  2  can include a shut-off valve  46 , a line strainer  48 , a conduit  50 , a first check valve  52 , and a second check valve  54 . The shut-off valve  46  and the line strainer  48  can be positioned along the conduit  32 . The shut-off valve  46  can allow flushing of the foam pump  22  without having to drain the foam tank  26 . The shut-off valve  46  can either be manually or electrically operated. Downstream of the shut-off valve  46 , the line strainer  48  can prevent unwanted particles, such as dirt and sand, from reaching the inlet of the foam pump  22 . In some embodiments, the line strainer  48  can be used to supply water for flushing residual foamant from the foam pump  22 . Flushing the foam pump  22  can help the FPS  2  be more reliable, because residual foamants can otherwise corrode the metal components of the foam pump  22 . 
     The conduit  50  can couple an outlet of the foam pump  22  to the conduit  16  carrying the stream of water. The first check valve  52  can be positioned along the conduit  50  and can prevent water from reaching the foam pump  22 . The second check valve  54  can connect the conduit  50  to the conduit  16 . The second check valve  54  can prevent foamant from flowing into the water pump  6  and any additional equipment upstream of the water pump  6  (e.g., the water tank  4 ). If no foamant is introduced during a fire-fighting operation, the second check valve  54  can prevent a backflow of water into the water pump  6 , so that the water can be forced to exit through the manifold  20 . In some embodiments, an injector fitting (not shown) can connect the conduit  50  with the conduit  16 . The injector fitting can introduce the foamant coming from the conduit  50  into substantially the center of a cross section of the conduit  16 . The injector fitting can result in enhanced mixing of the foamant with the stream of water. 
     In some embodiments, the FPS  2  can include a selector valve  56 , which can be either manually or electrically operated. In some embodiments, the selector valve  56  can be hydraulic or pneumatic. In a first position, the selector valve  56  can be used to route foamant from the foam tank  26  out a spigot  58  for priming of the FPS  2 , for calibration of new additives, for drain-down of the foam tank  26 , and/or for flushing of the FPS  2 . The controller  10  can provide a simulated control mode for calibrating the FPS  2 . The calibration of the FPS  2  can be based on parameters stored in the controller  10  to facilitate the calibration process, In some embodiments, signals from specific sensors (e.g., the flow meter  8 ) can be ignored for calibration purposes while the foam pump  22  can be fully operational. Over a certain time period, the pumped foamant can be collected in a measuring cup at the spigot  58  and can be compared to the desired flow rate. The user can adjust parameters (e.g., the speed of the foam pump  22 ) until a desired accuracy of the FPS  2  is achieved. In a second position, the selector valve  56  can route the foamant being pumped by the foam pump  22  through the conduit  50  and into the conduit  16 . 
     In some embodiments, the selector valve  56  can be an electric calibration injection valve that can be used to automatically prime the FPS  2 . When the foam pump  22  starts before the FPS  2  is primed, there will be some air in the lines. When the pistons of the foam pump  22  are pushing air, the torque profile of the motor rotor shaft (as discussed below) is different than when the foam pump  22  is pushing only foamant. In order to prime the FPS  2 , the controller  10  can monitor the torque profile when the foam pump  22  is started and the controller  10  can automatically open the electric calibration injection valve in order to purge the air from the FPS  2 . The electric calibration injection valve can be left open until the controller  10  determines that the torque profile has changed to indicate that the foam pump  22  is only pushing foamant and therefore the FPS  2  is primed. Once the FPS  2  is primed, the controller  10  can automatically close the electric calibration injection valve. 
     In some embodiments, rather than or in addition to the foam tank  26 , one or more off-board foam sources can be coupled to the FPS  2  (e.g., for situations in which the foam tank  26  does not store a sufficient amount of foamant). The off-board foam sources can be any one or more of an off-board tote (e.g., typically a five gallon bucket of foamant), a second stationary foam tank, or a mobile trailer with a foam tank. An off-board foam source can be coupled to the FPS  2  with an off-board pick-up line that can be typically 10 to 20 feet long and can be filled with air before being primed. In order to prime the off-board pick-up line, the controller  10  can monitor the torque profile of the motor rotor shaft when the foam pump  22  is started. As long as the torque profile indicates that air is being pulled through the off-board pick-up line, the controller  10  can operate the foam pump  22  at a higher speed. Once the torque profile indicates only foamant is being pulled through the off-board pick-up line, the foam pump  22  can automatically slow down to a normal speed for foamant injection. Conversely, the controller  10  can also determine when the off-board foam source is running out of foamant. The controller  10  can indicate on the display  12  that the off-board foam source is running low. In some embodiments, the controller  10  can calculate how much longer (e.g., in minutes) the FPS  2  can be operated until the off-board foam source will run out of foamant. The display  12  can indicate that the foamant is low and the display  12  can indicate a remaining time period (e.g., a number of minutes) that the FPS  2  can continue to operate. The controller  10  can calculate the remaining time period by taking into account the current flow rate of the foamant through the foam pump  22 . Once the controller  10  has determined that the off-board foam source is substantially empty, the controller  10  can automatically shut down the FPS  10 . 
     Similarly, in some embodiments, the controller  10  can determine how much longer the FPS  2  can be operated until the foam tank  26  will run out of foamant. The level sensor  28  in the foam tank  26  can give a general indication that the foamant is running low. The display  12  can indicate that the foamant is low and the display I 2  can also indicate a remaining time period (e.g., a number of minutes) that the FPS  2  can continue to operate. The controller  10  can calculate the remaining time period by taking into account the current flow rate of the foamant through the foam pump  22 . Once the controller  10  has determined that the foam tank  26  is substantially empty, the controller  10  can automatically shut down the FPS  10 . 
     In some embodiments, the fire-extinguishing system  1  can include a compressed air foam system (CAFS). A compressor of the CAFS can provide pressurized air to a nozzle of the discharge lines connected to the manifold  20 . The compressed air can further enhance the effectiveness of the foamant. 
       FIG. 2  illustrates a fire-extinguishing system  1  according to another embodiment of the invention. While the flow meter  8  of  FIG. 1  measures the total flow rate (i.e., the water flow rate plus any foamant), the flow meter  8  of  FIG. 2  only measures the flow rate of the water. In some embodiments, multiple flow meters can be used to measure flow rates of the water through various points in the system  1 . 
       FIG. 3  illustrates a fire-extinguishing system  1  according to yet another embodiment of the invention in which the water pump  6  can pump a water-foamant solution. The outlet of the foam pump  22  can be connected to the conduit  14  upstream of the water pump  6 . As a result, the flow meter  8  can measure the total flow rate. The foamant can be introduced into the stream of water at a lower pressure, because the stream of water in the conduit  14  is at a lower pressure than in the conduit  16 . 
       FIG. 4A  illustrates a perspective view of the servo motor  24  according to one embodiment of the invention. The servo motor  24  can include a housing  60 , a heat sink  62 , a stand  64 , and connectors  66 . The heat sink  62  can include ribs  68 , which can be positioned around a perimeter of the housing  60 . The stand  64  can be used to securely mount the servo motor  24  in a suitable location. The connectors  66  can be used to supply power to the servo motor  24 . In some embodiments, the controller  10  can be housed within the servo motor  24 . In some embodiments, the controller  10  can include a digital signal processor (DSP)  70 . In some embodiments, the DSP  70  can be coupled to the housing  60  of the servo motor  24 . The DSP  70  can include a connector  72 , which can enable the DSP  70  to connect to additional electronic equipment of the fire-extinguishing system  1 . In some embodiments, the connector  72  can be used to supply power to the DSP  70 . 
       FIG. 4B  illustrates a cross-sectional view of the servo motor  24  according to one embodiment of the invention. The servo motor  24  can include a rotor shaft  74 , one or more rotors  76 , and a stator  78 . The rotor shaft  74  can be coupled to the housing  60  with one or more bearings  80  enabling the rotor shaft  74  to rotate with respect to the housing  60 . The rotor shaft  74  can include a first end  82  and a second end  84 . The first end  82  can include a coupling  86 , which can enable the servo motor  24  to connect to the foam pump  22 . The second end  84  can extend beyond the housing  60 . In some embodiments, the second end  84  can extend into the DSP  70 . The second end  84  can include projections  88 . A sensor  90  can be positioned adjacent to the second end  84 . The sensor  90  can include an encoder and/or a resolver. The sensor  90  can measure the position and/or speed of the rotor shaft  74 , as disclosed in U.S. Pat. Nos. 6,084,376 and 6,525,502 issued to Piedl et al., the entire contents of which are herein incorporated by reference. 
     In some embodiments, the rotor  76  can be a permanent-magnet rotor. The rotor  76  can be positioned inside the stator  78 . The stator  78  can include a stator core  92  and stator windings  94 . In some embodiments, the rotor  76  can rotate to drive the rotor shaft  74 , while the stator core  92  and the stator windings  94  can remain stationary. The connector  66  can extend into the housing  60  toward the rotor shaft  74 . The connectors  66  can be coupled to the stator  78 . 
     In some embodiments, the sensor  90  can be built into the motor housing  60  to accurately indicate the position and/or speed of the rotor shaft  74 . In other embodiments, the sensor  90  can be included in the DSP  70 . In some embodiments, the rotor shaft speed of the servo motor  24  can be substantially continually monitored via a feedback device, such as an encoder, resolver, hall effect sensors, etc. In other embodiments, the rotor shaft speed of the servo motor  24  can be measured without a physical sensor (e.g., by extracting information from a position of the rotor shaft  74 ). 
     The term “servo motor” generally refers to a motor having one or more of the following characteristics: a motor capable of operating at a large range of speeds without over-heating, a motor capable of operating at substantially zero speed and retaining enough torque to hold a load in position, and/or a motor capable of operating at very low speeds for long periods of time without over-heating. The term “torque” can be defined as the measured ability of the rotor shaft to overcome turning resistance. Servo motors can also be referred to as permanent-magnet synchronous motors, permanent-field synchronous motors, or brushless electronic commutated motors. 
     The servo motor  24  can be capable of precise torque control. The output torque of the servo motor  24  can be highly responsive and substantially independent of the rotor  76  position and the rotor shaft  74  speed across substantially the entire operating speed range. In some embodiments, the current draw of the servo motor  24  can be sent to the DSP  70  over the line  40  and can be used to compute the torque necessary to drive the servo motor  24 . 
     The use of the servo motor  24  can simplify the actuation and control of the FPS  2 , as opposed to a conventional DC electric motor having to rely on pulse width modulation (PWM) control for low flow/concentration rates (e.g., flow rates less than about  30  percent of a maximum output of the foam pump  22 , or in one embodiment, about  0 . 01  GPM to about  5  GPM). As a result, the servo motor  24  can enable a smooth injection of the foamant into the water stream. In some embodiments, an operating pressure of the stream of water can be between about  80  PSI and about  800  PSI. In some embodiments, the use of the servo motor  24  can allow a smooth injection of the foamant even at low rotations per minute (RPM), which can result in an optimized mixing of the foamant into the water stream. Some embodiments of the invention improve the accuracy of the foamant/water mixture or ratio, which can improve the efficacy of the system and can provide a safer system for use by fire fighters. 
     In some embodiments including the CAFS, the servo motor  24  can eliminate or at least substantially reduce a so-called “slugging” or “slug-flow effect.” First, conventional DC electric motors operated by pulse width modulation can result in pressure variations in the foam pump  22 , which can be caused by the pulsing of the DC electric motors. Second, conventional DC electric motors operated by pulse width modulation can result in a poor mixing of the air with the foamant-water solution possibly forming air pockets inside the conduit  16  and/or the manifold  20 . The formation of the air pockets can be exacerbated by an uneven injection of the foamant resulting from the pressure variations of the foam pump  22 . The air pockets can induce a slugging of the discharge line connected to the manifold  20 . The slugging can move the discharge line making it harder for an operator to control the discharge line. In some embodiments, the smooth injection of the foamant resulting from the use of the servo motor  24  can substantially reduce the poor mixing and/or the air pockets inside the conduit  16  and/or the manifold  20  thereby substantially weakening or even eliminating the “slug-flow effect.” 
     The controller  10  can be external to the servo motor  24  or housed inside the servo motor  24 . As shown in  FIG. 5 , the controller  10  can include the digital signal processor (DSP)  70 , a micro-processor  100 , and a memory  102 . The memory  102  can include random access memory (RAM), read only memory (ROM), and/or electrically erasable programmable read only memory (EEPROM). In some embodiments, the controller  10  can include an analog/digital (A/D) converter and/or a digital/analog (D/A) converter in order to process different input signals and/or to interface with peripherals. In some embodiments, the DSP  70 , the micro-processor  100 , and the memory  102  can be included in a single device, while in other embodiments, the DSP  70 , the micro-processor  100 , and the memory  102  can be housed separately. In some embodiments, the DSP  70  and/or the memory  102  can be positioned inside or near the servo motor  24 , while the micro-processor  100  and/or the memory  102  can be included with the display  12 . 
     In some embodiments, the micro-processor  100  can provide an auto-start feature for the FPS  2 , as disclosed in U.S. Pat. No. 7,318,482 issued to Arvidson et al., the entire contents of which is herein incorporated by reference. When selected by the user, the display  12  can transmit the auto-start user input to the micro-processor  100  via the line  34 . With the auto-start feature selected, the foam pump  22  can be automatically activated, if the flow meter  8  indicates a positive flow rate and no error can be detected by the micro-processor  100 . If the flow meter  8  indicates no flow (which can be referred to as “zero flow cut-off”) or an error is detected, the controller  10  can stop the injection of foamant. 
       FIG. 6  illustrates the connections between the electrical components and/or electronic equipment of the fire-extinguishing system  1  according to one embodiment of the invention. The measured flow rate of either total flow or water flow can be transmitted to the micro-processor  100  via the line  18 . When a positive flow rate is detected, the micro-processor  100  can read a user input regarding the desired foamant concentration via the line  34 . Based on the desired concentration, the micro-processor  100  can compute a base speed at which the servo motor  24  can operate the foam pump  22 . In some embodiments, the micro-processor  100  can use the desired concentration and the flow rate signal from the line  18  to compute the base speed. 
     The DSP  70  can receive the base speed from the micro-processor  100  for the desired concentration of the water-foamant solution and the measured flow rate via a line  104 . After initializing the addition of foamant (when the servo motor  24  is not running), the base speed can be transmitted directly to the servo motor  24  over the line  36 . Once the servo motor  24  is running, the DSP  70  can process one or more of the following signals from the servo motor  24 : the current draw of the servo motor  24 , the speed of the rotor shaft  74 , the angle of the rotor shaft  74 , and temperature of the servo motor  24 . Any suitable combination of these signals or additional signals can be used by the DSP  70  and/or the micro-processor  100  to modify the base speed to provide closed-loop control. 
     In some embodiments, the actual speed of the rotor shaft  74  of the servo motor  24  can be transmitted back to the DSP  70  via the line  36 , which can transmit the signals to the micro-processor  100  via the line  104 , if the foam tank level sensor  28  does not indicate a low foamant level and no other error can be detected within the fire-extinguishing system  1 . If a low foamant level signal is sent to the micro-processor  100  via the line  30  or an error is communicated by the DSP  70  to the micro-processor  100  via a line  106 , the micro-processor  100  can send a command to the DSP  70  to stop the servo motor  24 . 
     In some embodiments, the calculated torque of the rotor shaft  74  can be transmitted to the micro-processor  100  via a line  108 . With the actual speed of the rotor shaft  74  and the calculated torque of the rotor shaft  74 , the micro-processor  100  can compute the flow rate of the foamant. The newly-computed flow rate can be compared to the previous flow rate required to provide the desired concentration, and a new base speed can be computed by the micro-processor  100 . 
     In some embodiments, the rapid compute time of the controller  10  can allow for several evaluations of foamants and modifications of base speed per pump cycle. This can result in rapid adjustments to varying parameters (e.g., the water flow rate), while helping to provide a substantially uninterrupted and smooth flow of the water-foamant solution at precise concentrations. In some embodiments, the controller  10  can determine the viscous properties of the foamant that is being pumped by the foam pump  22 . In some embodiments, the controller  10  can automatically compensate for different foamants having different viscosities or for a single type of foamant having a different viscosity depending on the current operating temperature of the FPS  2 . The controller  10  can take into account the change in viscosity feedback so that the water-foamant solution can continue to be provided with a precise concentration. In some embodiments, more than one foam tank  26  can be coupled to the FPS  2 . The controller  10  can automatically determine that different types of foamant are stored in the different foam tanks  26 . The controller  10  can automatically operate the foam pump  22  to achieve precise concentrations in the water-foamant solution for each particular type of foamant. 
     As shown in  FIG. 6 , the servo motor  24  can be powered by an external power source  110 . The rotor shaft  74  speed signal can be sent from the DSP  70  via the line  36  to a power amplifier  112 , which can be connected to the external power source  110 . Depending on the rotor shaft  74  speed signal received from the DSP  70 , the power amplifier  112  can provide the appropriate power (e.g., the appropriate current draw) to the servo motor  24 . In some embodiments, the power amplifier  112  can supply the servo motor  24 , the controller  10 , and additional electrical components and/or electronic equipment with power. 
     In some embodiments, the fire-extinguishing system  1  can include a load dump protection circuit  114 . In some embodiments, the load dump protection circuit  114  can be part of the power amplifier  112 . The load dump protection circuit  114  can prevent an over-voltage peak from causing damage to the controller  10 , the servo motor  24 , and other electrical components and/or electronic equipment. In some embodiments, the load dump protection circuit  114  can protect the electrical components and/or electronic equipment of the fire-extinguishing system  1  from an under-voltage condition and/or a wrong polarity of the external power source  110 . In some embodiments, the load dump protection circuit  114  can disconnect the electrical components and/or electronic equipment of the fire-extinguishing system  1 , if the voltage of the external power source  110  is negative, below a minimum, or above a specified level. 
       FIG. 7  illustrates the load dump protection circuit  114  according to one embodiment of the invention. The load dump protection  114  can include a sensing circuit  116 , a relay contact  118 , a relay coil  120 , a capacitor  122 , a first diode  124 , a second diode  126 , and a current source  128 . The relay coil  120  can be connected to the sensing circuit  116 . The relay coil  120  can energize and de-energize the relay contact  118 . Before the relay contact  118  closes, the current source  128  can charge the capacitor  122  with a limited current to enable a “soft start.” Once the capacitor  122  is charged to the correct level, the current source  128  and the second diode  126  can be bypassed by the relay contact  118  enabling the high currents of normal operation to flow. 
     The first diode  124  and the second diode  126  can prevent damage to the sensing circuit  116  and/or other electronic equipment of the fire-extinguishing equipment  1 , if the voltage supplied from the external power supply  110  has the wrong polarity. For example, if the external power supply  110  is a battery, which is being disconnected for maintenance and/or repair procedures, the first diode  124  and the second diode  126  can prevent damage to the electronic equipment of the fire-extinguishing system  1 , if the battery is re-connected incorrectly. 
     In some embodiments, the sensing circuit  116  can withstand an over-voltage peak. The sensing circuit  116  can also rapidly detect the over-voltage peak or an under-voltage condition. The sensing circuit  116  can detect the over-voltage peak or the under-voltage condition substantially independent of a power status of the servo motor  24  and/or the controller  10 . In some embodiments, the sensing circuit  116  can detect the over-voltage peak or the under-voltage condition even if the servo motor  24  and/or the controller  10  are not running. The sensing circuit  116  can de-energize the relay contact  118  through the relay coil  120 . As a result, all of the internal power supplies of the fire-extinguishing system  1  can be switched off almost immediately. In some embodiments, the current source  128  can charge the capacitor  122  with the limited current before the relay contact  118  is re-energized again. The sensing circuit  116  can re-energize the relay contact  118  and can re-connect all internal power supplies once no over-voltage conditions, such as over-voltage peaks, or under-voltage conditions are being detected. In some embodiments, the relay contact  118  can be re-energized once no over-voltage conditions or under-voltage conditions are being detected and the capacitor  122  is charged to the correct level. Once the relay contact  118  is re-energized, the second diode  126  and the current source  128  can be bypassed by the relay contact  118  to enable the supply of normal operating currents. For example, if the fire-extinguishing system  1  includes a fire truck, welding being performed on the fire truck for repairs, maintenance, or equipment installation can result in over-voltage peaks traveling through the fire truck. The load dump protection circuit  114  can help prevent damage to the electronic equipment of the fire-extinguishing system  1  possibly caused by the over-voltage peaks. 
       FIG. 8  is a flow chart describing a load dump protection method  200  according to one embodiment of the invention. In some embodiments, the sensing circuit  116  can sense (at step  202 ) a voltage U supply , If the voltage U supply  is less than a maximum threshold U max  but higher than a minimum threshold U min  (at step  204 ), the sensing circuit  116  can sense (at step  202 ) the voltage U supply  again. If the voltage U supply  is higher than the maximum threshold U max  or below the minimum threshold U min  (at step  204 ), the sensing circuit  116  can disconnect (at step  206 ) the electronic equipment of the fire-extinguishing system  1  including the controller  10 , the servo motor  24 , and/or other electronics substantially before the over-voltage condition or the under-voltage condition can cause damage to the electronic equipment of the fire-extinguishing system  1 . In some embodiments, the sensing circuit  116  can disengage the relay contact  118  to disconnect the electronic equipment of the fire-extinguishing system  1 . Once disconnected, the sensing circuit  116  can continue to sense (at step  208 ) the voltage U supply  until the voltage U supply  has dropped below the maximum threshold U max  or has risen above the minimum threshold U min  (at step  210 ). The sensing circuit  116  can re-connect (at step  212 ) the electronic equipment before the load dump protection method  200  is restarted (at step  202 ). In some embodiments, the relay contact  118  can be re-energized in order to re-connect the electronic equipment of the fire-extinguishing system  1 . 
     In some embodiments, the controller  10  can provide drive diagnostics for the FPS  2 , which can be downloaded for further processing. A technician can use the drive diagnostics to analyze any errors of the FPS  2 . The drive diagnostics can include error messages specifically for the servo motor  24 . In some embodiments, the controller  10  can be capable of detecting an interrupted connection between components of the FPS  2  and can send an error signal to the controller  10 . In one embodiment, the following types of errors can be communicated to the DSP  70  and/or the micro-processor  100 : one or more components of the servo motor  24  exceed threshold temperatures, the servo motor  24  requires a higher current for the operation than a threshold current (which can be referred to as “current fold back”), and the servo motor  24  is experiencing a stall condition. 
     In some embodiments, the servo motor  24  can generate heat, especially at high RPM, (i.e., for high concentration rates of the water-foamant solution and/or high flow rates of the water stream). The servo motor  24  can include passive heat controls, such as heat sinks, vent holes, etc. In some embodiments, as shown in  FIG. 9 , the servo motor  24  can use a power management control method  300  to actively prevent over-heating. In some embodiments, the duty cycle of the current supplied to the servo motor  24  can be altered to prevent over-heating. 
       FIG. 9  illustrates the power management control method  300  according to one embodiment of the invention. In some embodiments, the DSP  70  can measure (at step  302 ) a temperature T motor  of the servo motor  24 . The DSP  70  can measure the temperature of any component of the servo motor  24 . In some embodiments, the DSP  70  can measure the temperature of multiple components. The DSP  70  can determine (at step  304 ) if the temperature T motor  is approaching a maximum temperature T max  (i.e., if the temperature T motor  is within a range ε). The maximum temperature T max  can be stored in the memory  102 , and if multiple components of the servo motor  24  are monitored by the DSP  70 , the maximum temperature T max  can be component specific. If the maximum temperature T max  does not approach the temperature T motor , the controller  10  can operate the servo motor  24  with the computed speed to fulfill the foamant flow rate and/or injection pressure at  306 . The DSP  70  can restart (at step  302 ) the power management control method  300  by measuring the temperature T motor . 
     If the temperature T motor  approaches the maximum temperature T max , the DSP  70  can determine (step  308 ) whether the maximum temperature T max  has been exceeded. If the maximum temperature T. has been exceeded, the servo motor  24  can be shut down (at step  310 ) and the DSP  70  can start a timer (at step  312 ). The timer can be set for a time period long enough to allow the servo motor  24  to cool. In some embodiments, the timer can be set for a time period of about one minute. After the timer has been started (at step  312 ), the DSP  70  can continue to monitor (at step  314 ) the temperature T motor  of the servo motor  24 . If the temperature T motor  has dropped below the maximum temperature T max , the DSP  70  can determine whether the timer has expired (at step  316 ). Once the timer has expired (at step  314 ), the DSP  70  can restart (at step  318 ) the servo motor  24  and can measure (at step  302 ) the temperature T motor  again. 
     If the temperature T motor  is below the maximum temperature T max  but within the range ε, the DSP  70  can shut down (at step  320 ) the servo motor  24  for a first time interval TI 1 . The DSP  70  can turn on (at step  322 ) the servo motor  24  for a second time interval TI 2 . In some embodiments, the first time interval TI 1  and/or the second time interval TI 2  can be a default value and/or a previously stored value in the controller  10 . In some embodiments, the servo motor  24  can run continuously during the second time interval TI 2 , while in other embodiments, the servo motor  24  can be pulsed with a certain frequency F pulse . The temperature T motor  can be compared (at step  324 ) to a previously-stored temperature T prev . In some embodiments, the temperature T prev  can be a default value during initialization (i.e., if no temperature has been previously stored in the memory  102  since the last power-up of the servo motor  24 ). If the temperature T prev  is lower than the temperature T motor , the DSP  70  can increase (at step  326 ) the first time interval TI 1 , decrease (at step  328 ) the second time interval TI 2 , and/or decrease (at step  330 ) the frequency F pulse . The DSP  70  can store (at step  332 ) the temperature T motor  as the temperature T prev in the memory  102 . The DSP  70  can operate (at step  334 ) the servo motor  24  with the first time interval TI 1  and the second time interval TI 2  resulting in a pulsing of the servo motor  24 . In some embodiments, the pulse frequency resulting from the first time interval TI 1  and the second time interval TI 2  can be substantially lower than the frequency F pulse , at which the servo motor  24  can be operated during the second time interval TI 2 . In some embodiments, the frequency F pulse . can be less than about 20 kilohertz. 
     If the temperature T motor  is not higher than the temperature T prev  (at step  324 ), the DSP  70  can determine (at step  336 ) whether the temperature T prev  is higher than the temperature T motor . If the temperature T prev  is higher than the temperature T motor,  the DSP  70  can decrease (at step  338 ) the first time interval TI 1 , increase (at step  340 ) the second time interval TI 2 , and/or increase (at step  342 ) the frequency F pulse . The DSP  70  can store (at step  332 ) the temperature T motor  as the temperature T prev  in the memory  102 . The DSP  70  can pulse (at step  334 ) the servo motor  24  with the first time interval TI 1  and the second time interval TI 2 . If the temperature T prev  is substantially equal to the temperature T motor , the servo motor  24  can be pulsed (t step  334 ) with the first time interval TI 1  and the second time interval TI 2 . After step  334 , the DSP  70  can restart (at step  302 ) the power management control  300 . 
     In some embodiments, the power management control method  300  can be self-adapting and can learn the optimal values for at least one of the first time interval TI t , the second time interval TI 2 , and the frequency F pulse . As a result, the servo motor  24  can operate at high RPM over prolonged periods of time before having to shut down due to an over-temperature condition. In some embodiments, the power management control method  300  can adjust at least one of the first time interval TI 1 , the second time interval TI 2 , and the frequency F pulse  over a short period of time while enabling the FPS  2  to deliver the maximum foamant flow rate without exceeding the maximum temperature T max . In some embodiments, the period of time in which the power management control method  300  learns the optimal values for pulsing the servo motor  24  can be within about 10 rotations of the rotor shaft  74 . 
     In some embodiments, the operation of the servo motor  24  with the frequency F pulse  can result in power losses in the servo motor  24  itself, the controller  10 , and/or the power amplifier  112 . The power losses can increase the temperature of the respective component and/or equipment. In some embodiments, the frequency F pulse  can be used to determine a physical location of the power losses. In some embodiments, the frequency F pulse  can be increased to reduce the power losses in the servo motor  24  in order to assist with the power management control method  300  in preventing the servo motor  24  from overheating. As a result, the increase frequency F pulse  can increase the power losses in the controller  10  and/or the power amplifier  112 . To prevent overheating of the controller  10  and/or the power amplifier  112 , the frequency F pulse  can be decreased in order to limit the power losses. As a result, the decreased frequency F pulse  can be used to increase the power losses in the servo motor  24 . 
     In some embodiments, the power management control method  300  can be used to adjust the frequency F pulse  to balance the power losses. In some embodiments, the power management control method  300  can vary the frequency F pulse  in order to prevent overheating of the servo motor  24  and/or any other electronic equipment of the fire-extinguishing system  1 . In some embodiments, the power management control method  300  can determine a certain frequency F pulse  depending on an operation point and/or condition of the servo motor  24 . In some embodiments, varying the frequency F pulse  can maximize the overall system efficiency of the FPS  2 . 
       FIGS. 10A through 10D  illustrate various tailored pulse shapes  400  according to some embodiments of the invention. The tailored pulse shapes  400  can include a step pulse shape  402  ( FIG. 10A ), a linear ramp pulse shape  404  ( FIG. 10B ), a polynomial pulse shape  406  ( FIG. 10C ), and a trigonometric pulse shape  408  ( FIG. 10D ). In some embodiments, a beginning and/or an end of a pulse can be tailored in order to derive the tailored pulse shapes  400 . The polynomial pulse shape  406  can be approximated by any suitable higher polynomial and/or rational function. The trigonometric pulse shape  408  can be approximated by any trigonometric function including sine, cosine, tangent, hyperbolic, arc, and other exponential functions including real and/or imaginary arguments. 
     In some embodiments, the power management control method  300  can use the tailored pulse shapes  400 . The tailored pulse shapes  400  can be adjusted to minimize the mechanical wear of the servo motor  24 . In some embodiments, the tailored pulse shapes  400  can minimize mechanical stresses being transferred from the servo motor  24  onto the FPS  2  and/or additional components of the fire-extinguishing system  1 . For example, the tailored pulse shapes  400  can minimize a mechanical stress on the foam pump  22  and connecting conduits. The tailored pulse shapes  400  can be adjusted to optimize the amount of work output for the amount of power supplied to the servo motor  24 . In some embodiments, the tailored pulse shapes  400  can be modified to lower a thermal shock of the servo motor  24 . Heat generated by the servo motor  24  at a high RPM (e.g., high foamant flow rates and/or high water flow rates) can be reduced so that the servo motor  24  can continue to operate at the high RPM over prolonged periods of time without shutting down due to an over-temperature condition and/or changing the first time interval TI 1 , the second time interval TI 2 , and/or the frequency F pulse . 
       FIG. 11  is a flow chart describing a current fold back protection method  500  according to some embodiments. The current fold back protection method  500  can prevent the servo motor  24  from drawing a high current that would damage the servo motor  24 . The current fold back protection method  500  can optimize the operation of the servo motor  24 . In some embodiments, the current fold back protection method  500  can maximize an output of the FPS  2 . The current fold back protection method  500  can be performed by the controller  10 . In some embodiments, the DSP  70  can perform the current fold back protection method  500 . The controller  10  can sense (at step  502 ) the rotor shaft speed. The controller  10  can sense (at step  504 ) the rotor shaft torque and/or an actual phase current I phase  supplied to the servo motor  24 . In some embodiments, the controller  10  can compute the rotor shaft  74  torque with the phase current I phase . The controller  10  can compute (at step  506 ) a maximum motor phase current I motor,max , which can be the highest allowable current being supplied without damaging the servo motor  24  and/or the controller  10 . In some embodiments, the maximum motor phase current I motor,max  can vary with the speed of the rotor shaft  74 . In some embodiments, the controller  10  can multiply the speed of the rotor shaft  74 , the torque of the rotor shaft  74 , and an efficiency parameter of the servo motor  24  in order to compute the maximum motor phase current I motormax . 
     If the phase current I phase  is less than the maximum motor phase current I motor,max  (at step  508 ), the controller  10  can compute (at step  510 ) a difference Δ between a continuous current limit I cont  and the phase current I phase . The continuous current limit I cont  can be the maximum current at which the servo motor  24  can substantially continuously run without resulting in an over-temperature of the servo motor  24  and/or the controller  10 . In some embodiments, the continuous current limit I cont  can be based on an overall thermal capacity of the fire-extinguishing system  1 . The continuous current limit I cont  can be stored in the memory  102 . 
     If the continuous current limit I cont  is larger than the phase current I phase , the difference Δ is positive and can be used to optimize (at step  512 ) the operation of the servo motor  24 , for example to increase an injection pressure of the FPS  2 . If the difference Δ is negative, the controller  10  can determine (at step  514 ) whether the continuous current limit I cont  can be exceeded. To determine whether the continuous current limit I cont  can be exceeded, the controller  10  can evaluate a history of supplied currents to operate the servo motor  24  and/or the difference Δ. In some embodiments, the history of supplied currents to operate the servo motor  24  can include computing a root mean square (RMS) value of the supplied current and/or squaring the supplied current and multiplying the time. 
     If the continuous current limit I cont  can be exceeded, the controller  10  can operate (at step  516 ) the servo motor  24  with the phase current I phrase . If the continuous current limit I cont  may not be exceeded, the controller  10  can operate (at step  518 ) the servo motor  24  with the continuous current limit I cont . If the phase current I phase  is larger than the maximum motor phase current I motor,max  (at step  508 ), the servo motor  24  can be operated with the maximum motor phase current I motor,max  (at step  520 ). At step  522 , the controller  10  can store either one of the phase current I phrase , the continuous current limit I cont , and the maximum motor phase current I motor,max , which has been supplied to the servo motor  24 , in the memory  102 . The controller  10  can then restart the current fold back protection method  500  by sensing (at step  502 ) the speed of the rotor shaft  74 . 
     If the phase current I phrase  is limited to the maximum motor phase current I motor,max  or the continuous current limit I cont , the servo motor  24  can be operated with the maximum motor phase current I motor,max  (at step  520 ) or the continuous current limit I cont  (at step  518 ). Operating the servo motor  24  at the maximum motor phase current I motor,max  or the continuous current limit I cont  can prevent damage to the servo motor  24 . Due to the maximum motor phase current I motor,max  and/or the continuous current limit I cont  being lower than the current draw necessary to operate the servo motor  24 , operating the servo motor  24  at the maximum motor phase current I motor,max  or the continuous current limit I cont  can result in a stall of the servo motor  24 . The controller  10  can detect the stall of the servo motor  24 . In one embodiment, the angle of the rotor shaft  74  of the servo motor  24  can be used to identify a stall condition of the servo motor  24 . Other embodiments of the invention can use the speed of the rotor shaft  74  of the servo motor  24  to detect a stall condition of the servo motor  24 . Once a stall condition has been detected, the servo motor  24  can attempt to operate again after a certain time interval. In some embodiments, the time interval can be about one second so that the servo motor  24  can drive the foam pump  22  again substantially immediately after the stall condition has been removed. 
     A power stage rating of the servo motor  24  and/or the controller  10  can be determined by a continuous operating current and a peak operating current. The continuous operating current can influence the heat generated by the servo motor  24  and/or the controller  10 . The peak operating current can determine the power rating of the servo motor  24  and/or the controller  10 . In some embodiments, the servo motor  24  can be designed to achieve a specific torque constant. Multiple parameters can influence the torque constant. In some embodiments, the torque constant can depend on the number of windings  94 , the number of poles of the rotor  76 , the pattern of the windings  94 , the thickness of the wire used for the windings  94 , the material of the wire, the material of the stator  78 , and numerous other parameters. In some embodiments, the temperature of the servo motor  24  can influence the torque constant. As a result, the torque constant can vary because the temperature of the servo motor  24  can change significantly over the course of a fire-fighting operation. In some embodiments, the DSP  70  can include a mapping procedure to compensate for the temperature variation and the resulting change in the torque constant. As a result, the torque of the rotor shaft  74  that is necessary to drive the servo motor  24  can be accurately computed over a large range of temperatures. 
     The torque constant can be stored in the memory  102 . In some embodiments, the torque constant can be accessed by the DSP  70 . In some embodiments, the DSP  70  can compute the torque of the rotor shaft  74  that is necessary to drive the servo motor  24  based on the torque constant and the current draw of the servo motor  24 . The torque constant can influence the peak operating current. In some embodiments, a large torque constant can result in a low power stage rating of the servo motor  24 . In some embodiments, the high torque constant can reduce the peak operating current. In some embodiments, the peak operating current can be reduced from about  110  Amperes to about  90  Amperes. In some embodiments, the heat generation during peak operation of the servo motor  24  can be reduced by increasing the torque constant. In some embodiments, the large torque constant can lengthen a time period during which the servo motor  24  can operate at peak operating current without overheating. 
     In some embodiments, the servo motor  24  can be driven with high torque values down to substantially zero RPM. As a result, the FPS  2  can introduce the foamant into the water stream of the fire-extinguishing system  1  with superior accuracy and/or substantially superior mixing efficiency. The high torque values can be achieved by an increased back electromotive force (BEMF) constant of the servo motor  24 . In some embodiments, the BEMF constant can be proportional to the torque constant. The increased BEMF constant can reduce the current necessary to drive the servo motor  24 . As a result, the servo motor  24  can achieve a certain torque of the rotor shaft  74  at the reduced current. The increased BEMF constant can reduce power losses in the controller  10  and/or other electronic equipment of the fire-extinguishing system  1 . In some embodiments, the BEMF constant can be related to the highest viscosity of the foamant to be intended to be used in the fire-extinguishing system  1 . In some embodiments, the BEMF constant can be at least 3.5 volts root mean square per thousand RPM (VRMS/KPRM) for a DC bus voltage of about 12 volts. In some embodiments, the BEMF constant can be at least about 46 VRMS/KPRM for a DC bus voltage of about 160 volts. In some embodiments, the ratio of the BEMF constant to a voltage driving the servo motor  24  can be constant. 
     In some embodiments, the high BEMF constant can reduce the maximum speed of the rotor shaft  74  at which the servo motor  24  can be driven. In some embodiments, the BEMF constant and the maximum speed of the rotor shaft  74  of the servo motor  24  can be directly proportional. For example, if the BEMF constant is doubled, the maximum speed of the rotor shaft  74  of the servo motor  24  can be halved. In some embodiments, the BEMF constant can be a compromise between a low speed requirement, a high speed requirement, and a thermal load requirement of the servo motor  24 . In some embodiments, the low speed requirement of the servo motor  24  can dictate a certain BEMF constant, which can result in the servo motor  24  not being able to fulfill the high-speed requirement in order to fulfill a specific foamant flow rate and/or injection pressure of the FPS  2 . 
     In some embodiments, the servo motor  24  can use a phase angle advancing technique for the supplied power in order to increase the maximum speed of the rotor shaft  74 . In some embodiments, a phase angle can be advanced by supplying a phase current at an angle increment before the rotor  76  passes a BEMF zero crossing firing angle. In some embodiments, the phase angle advancing technique can retard the phase angle by supplying the phase current at the angle increment after the rotor  76  has passed the BEMF zero crossing firing angle. In some embodiments, the phase angle advancing technique can influence the BEMF constant. In some embodiments, advancing the phase angle can decrease the BEMF constant. 
     In some embodiments, the servo motor  24  can be optimized to a certain injection pressure and/or desired foamant flow rate range for the fire-extinguishing system  1 . In one embodiment, the servo motor  24  can drive the foam pump  22  without the phase angle advancing technique to result in a foamant flow rate of about 2 to about 4 gallons per minute (GPM) and an injection pressure of about 400 pounds per square inch (PSI). In this embodiment, the phase angle advancing technique can increase the foamant flow rate to about 5 GPM, which can be delivered at the injection pressure of about 150 PSI. In some embodiments, the increment of the phase angle advancing technique can be related to the speed of the rotor shaft  74 . In one embodiment, the increment can be about +/−45 electrical degrees. 
     In some embodiments, the torque necessary to drive the servo motor  24  can be an indication of the viscosity of the foamant. As a result, the flow rate of the foamant can be precisely calculated. The micro-processor  100  can also use the torque of the rotor shaft  74  that is calculated by the DSP  70  to identify the foamant being added to the water stream. The calculated torque of the rotor shaft  74  can be compared with calibration values stored in the memory  102  of the controller  10 . The auto-calibration feature of the FPS  2  can allow foamants to be interchanged without repeating the calibration that is usually necessary to obtain accurate flow rates. 
     In some embodiments, the servo motor  24  can be operated with a direct current (DC) power supply (e.g., a battery of a fire truck). In other embodiments, the servo motor  24  can be operated with an alternating current (AC) power supply (e.g., a generator or alternator of a fire truck or a mains power supply in a building). 
     In some embodiments, the FPS  2  and/or the servo motor  24  can be powered by external power sources  110  providing different voltages. The voltages can include one or more of 12 Volts, 24 Volts, 48 Volts, 120 Volts, and 240 Volts. In some embodiments, the stator windings  94  of the servo motor  24  can be adapted to a specific voltage. In some embodiments, the stator windings  94  can be adapted so that the servo motor  24  can operate with more than one power source (e.g., with a DC power supply or an AC power supply). Other embodiments can include different input power stages that allow the servo motor  24  to selectively operate with different voltages and/or power sources. For example, if the fire-extinguishing system  1  is used as a stationary unit for a sprinkler system in a building, the servo motor  24  operating the foam pump  22  can be driven by the 120 Volts AC mains power supply. If mains power is lost, the fire-extinguishing system  1  can automatically switch to a 12 Volts DC battery power supply to continue the fire-extinguishing operation. 
       FIG. 12  illustrates a rectification bridge  600  according to one embodiment of the invention. The rectification bridge  600  can be used to operate the servo motor  24  with an AC power supply. The rectification bridge  600  can include two or more transistors  602 , an AC bus  604 , and a DC bus  606 . The AC bus  604  can connect to the external power supply  110 . The DC bus  606  can be used to supply power to the servo motor  24 . The transistors  602  can each include an intrinsic diode  608 . In some embodiments, the transistors  602  can include metal oxide semiconductor field effect transistors (MOSFETs). In some embodiments, the transistors  602  can be N-type MOSFETs, while in other embodiments, the transistors  602  can be P-type MOSFETs. In some embodiments, the transistors  602  can include a first transistor  610 , a second transistor  612 , a third transistor  614 , and a fourth transistor  616  configured in an H-bridge. 
     In some embodiments, the controller  10  can sense an incoming current I AC  at a first location  618  on the AC bus  604 . In other embodiments, the controller  10  can sense the incoming current I AC  at a second location  620  along with a third location  622  of the rectification bridge  600 . Sensing the incoming current I AC  of the rectification bridge  600  can result in a much higher level of electrical noise immunity instead of, for example, sensing voltages. If the incoming current I AC  is below a threshold current I limit , the intrinsic diodes  608  can be used to rectify the incoming current I AC . If the incoming current I AC  is above the threshold current I limit , the transistors  602  can be used to rectify the incoming current I AC . To rectify the incoming current I AC , the transistors  602  can be turned on by control signals from the controller  10 . The rectification bridge  600  can provide the correct timing for the switching of the transistors  602 . In some embodiments, the control current can prevent a discharge of the DC bus  606  and/or a shortening of the AC bus  604 . By sensing I AC  instead of sensing voltages, the control circuitry can have a much higher level of electrical noise immunity. 
     In some embodiments, a voltage drop across the transistors  602  can be lower than a voltage drop across the intrinsic diodes  608 . As a result, the switching of the transistors  602  can limit the power losses of the rectification bridge  600 , if the incoming current I AC  exceeds the threshold current I limit . In some embodiments, the threshold current I limit  can be low enough to prevent the rectification bridge  600  from overheating due to the power losses of the intrinsic diodes  608 , but high enough to provide substantial immunity to interference and noise on the AC bus  604 . The rectification bridge  600  can have much lower power losses than a conventional rectification bridge including diodes only. As a result, the use of the rectification bridge  600  can enable a higher efficiency and an operation in higher ambient temperatures. In some embodiments, the rectification bridge  600  can limit the power losses to about  30  Watts at an ambient temperature of about 70° C. (160° F.). In some embodiments, the threshold current I limit  can include hysteresis to increase an immunity to the noise on the AC bus  604 . 
       FIG. 13  illustrates a rectification method  700  according to one embodiment of the invention. The incoming current I AC  can be sensed (at step  702 ). If the absolute value of the incoming current I AC  is below the current threshold I limit  (at step  704 ), the intrinsic diodes  608  can rectify the incoming current I AC  and the rectification method  700  can be restarted (at step  702 ) with sensing the incoming current I AC . If the absolute value of the incoming current I AC  is above the current threshold I limit  (at step  704 ), the controller  10  can determine (at step  706 ) whether the incoming current I AC  is negative. If the incoming current I AC  is positive, the controller  10  can supply (at step  708 ) the control current to the transistors  602 . In some embodiments, the controller  10  can use the first transistor  610  and the fourth transistor  616 , which can be positioned diagonally across from one another in the rectification bridge  600 . If the incoming current I AC  is negative, the controller  10  can supply (at step  710 ) the control current to the transistors  602 . In some embodiments, the controller  10  can use the second transistor  612  and the third transistor  614 , which can be positioned diagonally across from one another in the rectification bridge  600 . After the step  708  and/or the step  710 , the rectification method  700  can be restarted by sensing the incoming current I AC  so that the intrinsic diodes  608  can be substantially immediately used for the rectification, if the incoming current I AC  drops below the current threshold I limit . 
     Although the fire-extinguishing system  1  is described herein as having only a single FPS  2 , the fire-extinguishing system I can include two or more additive supply systems. Foamants can be introduced into one or several water supplies and individual flow rates can be monitored by a single controller  10 , but can alternatively be monitored by two or more controllers. In some embodiments, the fire-extinguishing system  1  can include other additive supply systems powered by non-electric motors (e.g., hydraulic motors). 
     It will be appreciated by those skilled in the art that while the invention has been described above in connection with particular embodiments and examples, the invention is not necessarily so limited, and that numerous other embodiments, examples, uses, modifications and departures from the embodiments, examples and uses are intended to be encompassed by the claims attached hereto. The entire disclosure of each patent and publication cited herein is incorporated by reference, as if each such patent or publication were individually incorporated by reference herein. Various features and advantages of the invention are set forth in the following claims.